Global chemical model analysis of biomass burning and lightning influences over the South Pacific in austral spring
Amanda C. Staudt, Daniel J. Jacob, Jennifer A. Logan
Division of Engineering and Applied Sciences and Department of Earth and Planetary Sciences, Harvard University, 29 Oxford St., Cambridge, MA 02138
David Bachiochi, T. N. Krishnamurti
Department of Meteorology, Florida State University, Tallahassee, FL 32306
Nathalie Poisson
Agence de l'Environnement et de la Maitrise de L'Energie, Paris, France
Submitted to J. Geophys. Res.
A global 3-D model of tropospheric chemistry driven by assimilated meteorological observations is used to examine the sources of O3, CO, and nitrogen oxides (NOx=NO+NO2) in the South Pacific troposphere during the NASA Pacific Exploratory Mission in the Tropics (PEM-Tropics A) in September-October 1996. Aircraft observations up to 12 km during that mission revealed considerable biomass burning influence on O3 and CO, both in terms of elevated pollution layers and in terms of regional enhancements. The model reproduces the long-range transport of biomass burning effluents from southern Africa and South America in the westerly subtropical flow over the South Pacific. Meteorological conditions in 1996 were particularly favorable for this transport. Africa and South America make comparable contributions to the biomass burning pollution over the South Pacific; the contribution from Australia and Indonesia is much less. Biomass burning dominates the supply of NOx in the lower troposphere over the South Pacific (through long-range transport and decomposition of peroxyacetylnitrate), but lightning dominates in the upper troposphere. Observations in PEM-Tropics A and elsewhere indicate low HNO3/NOx concentration ratios and an imbalance in the chemical budget of NOx in the upper troposphere. We reproduce these observations and show that they reflect the subsidence of primary lightning NOx injected to the uppermost troposphere, rather than any fast chemistry recycling HNO3 to NOx. We find that biomass burning and lightning made similar contributions to O3 production over the South Pacific during PEM-Tropics A. Biomass burning plumes sampled in PEM-Tropics A contained little NOx, and the O3 enhancements observed in these plumes originated from production over the source continents rather than over the South Pacific.
The PEM-Tropics A aircraft mission over the South Pacific in September-October 1996 (see Figure 1 for flight tracks) revealed ubiquitous biomass burning pollution plumes, typically a few kilometers in vertical extent and hundreds of kilometers in north-south extent [Hoell et al., 1999]. This finding demonstrated that biomass burning, which is maximum in the austral tropics during the July-November dry season [e.g. Hao and Liu, 1994], influences the most remote regions of the tropical troposphere. Of concern is how chemical reactions involving biomass burning effluents in the tropical troposphere affect O3, a greenhouse gas, and OH, the main atmospheric oxidant, relative to natural conditions where emissions from lightning and the biosphere dominate the chemistry. In this paper, we use a global 3-D chemical transport model driven by assimilated meteorological observations to examine the origins of biomass burning pollution plumes observed in PEM-Tropics A and to assess their effect on the chemistry of the Pacific troposphere.
The biomass burning plumes were characterized by elevated O3, CO, hydrocarbons, PAN, and methyl halides combined with low relative humidity and C2Cl4 [Blake et al., 1999]. Dibb et al. [1999a] concluded that the biomass burning plumes had been convectively processed near their source region because they were depleted in biomass burning aerosol tracers such as elemental C, NH4+ and K+, and then did not experience subsequent scavenging during transport because they had elevated 210Pb, a decay product of 222Rn which attaches to aerosols. Elevated concentrations of HNO3 and H2O2, which are also removed efficiently by wet scavenging, confirm that the plumes had not recently experienced convection and had traveled long enough for significant photochemical processing [Dibb et al., 1999b; Cohan et al., 1999]. Thus, the typical plume formed when continental air polluted by biomass burning was convected to the upper troposphere and then carried long distances during which it experienced subsidence and additional photochemical activity [Schultz et al., 1999]. Hydrocarbon concentration ratios measured in the plumes implied a transport time of 1-2 weeks from the point of emission [Blake et al., 1999; Dibb et al., 1999b]. Back trajectories computed along the flight tracks by Board et al. [1999] show that most plumes were transported to the South Pacific by strong westerly winds at subtropical latitudes. The trajectories extend back to southern Africa or South America within 10 days, consistent with the photochemical age estimates.
Southern Africa and Brazil are the two dominant biomass burning source regions of the austral tropics [Hao and Liu, 1994]. The TRACE-A aircraft mission, conducted in September-October, 1992 over these source regions and the South Atlantic [Fishman et al., 1996a], provided considerable information on the composition and near-field transport of biomass burning pollution. Thompson et al. [1996] found that South American pollution is frequently convected to the upper troposphere and then advected across the South Atlantic, while African pollution is transported below 4-5 km in the prevailing easterlies towards the South Atlantic. The air above the South Atlantic slowly recirculates around a semi-permanent center of high pressure [Krishnamurti et al., 1993] allowing for the formation of an O3 maximum observable from satellites [Fishman et al., 1996b]. Some southern African pollution is exported southeastward to the Indian Ocean where it can accumulate by circulation around the semi-permanent Mascarene high situated above Madagascar [Garstang et al., 1996; Tyson et al., 1996]. Air is episodically flushed from these accumulation regions by westerly moving mid-latitude frontal systems [Diab et al., 1996; Chatfield et al., 1998]. In this paper, we will show that this flushing played a major role in the formation of the biomass burning plumes observed over the South Pacific during PEM-Tropics A.
The large-scale distribution of chemical species over the South Pacific was clearly affected by biomass burning outflow. Pollutants observed during PEM-Tropics A show a strong longitudinal gradient across the South Pacific troposphere with elevated concentrations at 2-8 km in the western Pacific [Blake et al., 1999; Singh et al., 2000]. The South Pacific Convergence Zone (SPCZ), which stretched along a northeast-southwest axis from 140°E at the equator to 150°W at 20°S [Fuelberg et al., 1999], was an effective barrier for horizontal mixing; significantly higher median concentrations of biomass burning tracers were observed to the south of the SPCZ compared to the north [Gregory et al., 1999; Talbot et al., 1999]. Thus, the equatorial South Pacific troposphere (0-10°S) exhibited little biomass burning influence while air to the south had considerably more [Fenn et al., 1999; Gregory et al., 1999; Schultz et al., 1999].
Relatively little attention has been paid to photochemical production of O3 in the aged biomass burning plumes sampled in PEM-Tropics A. On the basis of calculations with a local photochemical model as well as with a global 3-D model driven by general circulation model (GCM) meteorology, Schultz et al. [1999] concluded that half of the ozone observed in the 0-12 km column over the South Pacific during PEM-Tropics A was photochemically produced within the region at a rate limited by the supply of nitrogen oxide radicals (NOx=NO+NO2) in the plumes and in the large-scale atmosphere where the plumes disperse. The other half was transported into the region and included a dominant contribution from biomass burning. Although biomass burning is a large source of NOx, the lifetime of NOx against oxidation is short (ranging from less than 1 day in the lower troposphere to about 10 days in the upper troposphere) so that chemical recycling of NOx from its reservoir species nitric acid (HNO3) and peroxyacetylnitrate (PAN) is necessary to sustain NOx levels sufficient for continued O3 production. Observed NO concentrations were generally low over the South Pacific, ranging from a few pptv near the surface to 50-150 pptv in the upper troposphere. On the basis of local photochemical model calculations, Schultz et al. [2000] concluded that chemical recycling from the HNO3 and PAN reservoirs could not account for the observed NOx levels. Previous studies of the NOx budget in other regions of the free troposphere had found a similar imbalance between chemical sources and sinks; fast heterogeneous conversion from HNO3 to NOx on sulfate aerosols [Chatfield, 1994; Fan et al., 1994] or on soot [Hauglustaine et al., 1996; Lary et al., 1997] was proposed as an explanation. However, there is no experimental support for such fast heterogeneous reactions of HNO3 [Jacob, 2000].
Although techniques such as back trajectories, the age of air, and local photochemical modeling can give a general idea about the origin of biomass burning pollution over the South Pacific and its chemical evolution, a three-dimensional model can better resolve the coupling between transport, convection, and photochemistry. We have interfaced the Harvard chemical tracer model with assimilated meteorological data from the Florida State University (FSU) global spectral model to create a global three-dimensional model of atmospheric chemistry and transport for the PEM-Tropics A period. The Harvard/FSU model is better suited to studies of the PEM-Tropics A data than the 3-D GCM-based model used by Schultz et al. [1999] because it simulates actual dynamical conditions for the period of observations and has an improved treatment of tropical dynamics. We use the model to investigate the origins of the biomass burning pollution plumes sampled during PEM-Tropics A, to examine how dispersal of biomass burning pollution modifies O3 and NOx concentrations over the South Pacific on a larger scale, and to compare the roles of biomass burning and lightning in that context.
The Harvard/FSU model is described in section 2, followed by a model evaluation with PEM-Tropics A and selected other observations in section 3. The mechanisms for long-range transport of biomass burning plumes over the South Pacific are examined in section 4 and the contribution of each source region to the biomass burning pollution in the southern hemisphere is quantified. The factors which affect the budgets of NOx and O3 over the South Pacific are addressed in sections 5 and 6, respectively. Conclusions are offered in section 7.
We have interfaced the Harvard chemical transport model for O3-NOx-hydrocarbon chemistry [Wang et al., 1998; Horowitz and Jacob, 1999; Bey et al., 2000] with assimilated meteorological fields produced by FSU for July 22 - October 6, 1996. The FSU model has been used for many applications including hurricane tracks [Krishnamurti et al., 1989] and passive tracer studies investigating transport in the South Atlantic region [Krishnamurti et al., 1993, 1996]. An emphasis on accurately simulating tropical dynamics during the period of the PEM-Tropics A mission makes the FSU assimilated meteorological fields particularly well suited for our application. The physical initialization procedure in the FSU model is based on reverse algorithms which minimize the absolute value of the difference between the model-generated rainfall and the observed values, coupled with a Newtonian relaxation of the model dynamic variables and temperature towards the ECMWF analysis [Krishnamurti et al., 1991]. The precipitation rates used in the physical initialization process are obtained from a linear multiple regression relationship between the Laplacian of outgoing longwave radiation (OLR) derived precipitation rates and the Laplacian of SSM/I precipitation rates [Gairola and Krishnamurti, 1992]. The resulting relationship generates sharpened precipitation rate estimations from given OLR data by accounting for the incorrect interpretation of cirrus overcast associated with tropical convection. The OLR based precipitation rates are developed using twice daily outgoing longwave radiation from NOAA satellites as proposed by Krishnamurti et al. [1983].
FSU provides sixteen variables which are used to drive the dynamic and radiative processes of the Harvard/FSU model. These variables include surface pressure, sensible heat flux, momentum flux, and mixed layer height, as well as vertically resolved temperature, relative humidity, zonal and meridional velocities, convective cloud mass flux, convective and large-scale precipitation, number of dry convective events, cloud optical depth, and cloud fractional area. The data are archived every 2 hours at a horizontal resolution of T42 (~2.8°) on 14 sigma vertical levels centered at 0.99, 0.95, 0.9, 0.85, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1, 0.07, 0.03. The data are converted from the Gaussian grid to a Cartesian grid with a resolution of 4° latitude by 5° longitude, which is used in the chemical tracer model together with the original 14 sigma vertical levels.
The Harvard/FSU model is initialized in mid-July with chemical tracer fields produced by the Harvard/GISS II chemical transport model [Horowitz and Jacob, 1999] for all species except O3 which is initialized with tropospheric climatological values for July as prepared by Logan [1999]. Processes of advection, convection, emissions, chemistry and deposition take place in the model on a one hour time step. The model is allowed to run for six weeks before the output is examined. This initialization period is sufficiently long for the processes of interest here, although it implies that simulated concentrations of long-lived tracers (such as O3 and CO) inherit a significant background from the specified initial conditions.
The model advection and convection schemes have been adapted from a current version of the Irvine/Harvard/GISS chemical transport model [Wild and Prather, 2000]. Tracer advection is simulated with a second order moments scheme [Prather, 1986]. Mixing of air and tracers by dry and wet convection is parameterized. For dry convection, the Harvard/FSU model redistributes tracer mass uniformly over all layers and fractions of layers up to the local mixed layer height. This adjustment generally affects the bottom 3-5 layers in the daytime. Wet convection is driven by subgrid convective vertical velocities supplied by FSU. These velocities are converted to fluxes using a convective cloud area which is parameterized as a function of grid scale convective precipitation following Slingo [1987]. Starting at the bottom of each column, if there is a net upwards cloud mass flux out of a gridbox, the amount of air that must be moved out of the box to conserve mass balance is entrained and placed into a "pipe." The air in the pipe is carried upwards until encountering a gridbox with no net upward flux, where the updraft air is detrained. Compensatory large-scale subsidence outside the pipe allows the grid column to retain its original air mass distribution.
The Harvard chemistry, deposition, and emissions schemes [Wang et al., 1998; Horowitz and Jacob, 1999] are employed in this new model with only a few adaptations. Eighty species including 24 transported tracers are included to represent tropospheric O3-NOx-hydrocarbon chemistry. The chemical mechanism is that of Horowitz et al. [1998] with minor updates as discussed in Bey et al. [2000], and is integrated with a fast Gear solver [Jacobson and Turco, 1994]. The reaction rates for HNO3+OH and for NO2+OH are from Portmann et al. [1998]. Heterogeneous chemistry follows the recommendations of Jacob [2000] using a monthly mean sulfate aerosol field calculated by Chin et al. [1996] which is assumed to be wet or dry depending on the relative humidity. The mean aerosol surface area in the model over the South Pacific is 0.7 mm2 cm-3. Photolysis rates are computed with the code of Wild et al. [2000] which includes accurate radiative transfer through clouds. Dry deposition of O3, HNO3, NO2, CH2O, H2O2, PAN and other organic nitrates is calculated as described in Jacob et al. [1993], although friction velocities and Monin-Obukhov lengths are provided directly by FSU rather than derived from a parameterization. Wet deposition of soluble tracers (HNO3 and H2O2) is performed using the method developed by Balkanski et al. [1993]. A scavenging efficiency of 40% km-1 in wet convective updrafts, as recommended by Liu et al. [2000], is employed.
Emissions of anthropogenic and biogenic NOx, CO, and NMHCs are mostly as described by Wang et al. [1998]. Table 1 shows the emissions during the PEM-Tropics A time-period for NOx and CO. Emissions of NOx from lightning (Figure 2) are parameterized following Price and Rind [1992] and distributed vertically following Pickering et al. [1998]. They are adjusted to yield a global source of 5.7 Tg N yr-1, within the accepted range for a global source of 1-12 Tg N yr-1 [Huntrieser et al., 1998]. Analyses of fire counts observed by satellite indicate that emissions during September 1996 were not substantially different from the long-term mean [Olson et al., 1999; Duncan et al., 2000]. We therefore use the climatological biomass burning emissions described by Wang et al. [1998], with minor adjustments in the spatial distribution of emissions over Africa and Australia to reflect the location of fires during September 1996 [Olson et al. 1999]. The distribution of biomass burning emissions is shown in Figure 2 for NOx.
The fluxes from the stratosphere of O3, NOx, and HNO3 contribute to the budgets of these species in the troposphere. The FSU model does not have the vertical resolution needed for an accurate simulation of stratospheric dynamics. We therefore use a cross-tropopause flux boundary condition for O3 at 100 hPa altitude as described by Wang et al. [1998] with an annual global flux of 400 Tg/year distributed by latitude and month of the year. We also use a flux of 0.48 Tg N yr-1 of NOy computed by Wang et al. [1998] which is transported across the tropopause as NOx and HNO3 with a molar ratio of 1:4.
Horizontal transport is generally well described in GCMs which use assimilated meteorological observations, but vertical transport is more difficult to capture because it must be inferred from a convergence of the horizontal winds and it includes a major contribution from subgrid convection. Simulation of long-range transport of biomass burning pollution in our model depends critically upon vertical transport both over land, where emitted gases are vented to the free troposphere, and over oceans where these gases subside. We evaluated the representation of vertical transport in the Harvard/FSU model with simulations of 222Rn and CO to test continental convection and CH3I to test marine convection.
Radon-222 is emitted from soils and decays radioactively with a lifetime of 5.5 days, making it useful as a tracer of continental convection [Jacob and Prather, 1990; Mahowald et al., 1995]. The 222Rn concentrations simulated by the model (not shown) compare favorably with results from a recent intercomparison of global 3-dimensional models [Jacob et al., 1997]. In particular, concentrations of ~20x10-21 mol/mol are simulated at 300 hPa over the tropical continents indicating rapid transport from the boundary layer to the upper troposphere. Continental convection in the Harvard/FSU model tends to distribute tracer through the column such that concentrations decrease monotonically from the surface, rather than showing a "C" profile with enhanced concentrations in the upper troposphere. This tendency of convection to create a well-mixed column above the source region is problematic for simulating distinct biomass burning plumes in the model, as shown below.
Other tracers with surface sources, such as CO, can also be used to evaluate vertical transport [Allen et al., 1997; Mahowald et al., 1995]. Observations of CO over southern Africa and South America during the TRACE-A aircraft mission are particularly relevant to our problem. Results are shown in Figure 3. Because TRACE-A took place during a different year than simulated by the Harvard/FSU model, comparison of model results with observations is not well constrained. The model captures the mean vertical shape of the observed CO profile over Africa. It does not reproduce the C-shaped profile over eastern Brazil, though the observations there were targeted to sample convective outflow in the upper troposphere [Pickering et al., 1996] and may not be representative. We also see from Figure 3 that the model underestimates NO in the upper troposphere over both southern Africa and eastern Brazil. This underestimate could be due to insufficient convective transport not adequately diagnosed by the 222Rn simulation [Penner et al., 1998]. It may also be caused by insufficient production of NOx from lightning in the model.
Marine convection in the model was evaluated with CH3I, a species whose main source is from oceanic emissions. Measurements of CH3I concentrations were made by Blake et al. [1999] during PEM-Tropics A. Atmospheric loss of CH3I is by photolysis with a lifetime of about 4 days in the tropics [Cohan et al., 1999]. CH3I provides a good test of marine boundary layer (MBL) venting because its ocean source is fairly uniform, its lifetime is short, and its loss process is simple and well-defined. The MBL concentration of CH3I, constrained by observations compiled by Hsu et al. [1999], is specified as a boundary condition in the lowest layer of the model (0-240 m). Photolysis rate constants are calculated using absorption cross section data from Roehl et al. [1997]. Figure 4 compares calculated and observed vertical profiles of CH3I during PEM-Tropics A. The model simulates successfully the MBL depth, the sharp concentration gradient between the MBL and the free troposphere, and the lack of a mean vertical gradient at higher altitudes which indicates that convective outflow is distributed vertically throughout the free troposphere. We conclude that MBL ventilation and marine convection are well reproduced in the model to within the constraints afforded by CH3I. This will be an important consideration in comparing simulated and observed vertical profiles of O3.
Figure 5 shows maps of September mean model fields at 500 hPa for CO, NOx, HNO3, PAN, and O3. Figure 6 shows comparison with the PEM-Tropics A observations averaged over the regions in Figure 1 and Figure 7 compares simulated and observed O3:CO correlations in the middle troposphere over the South Pacific. The simulated profiles in Figure 6 include both the means for the entire region (short dashes) and the means as sampled along the flight tracks and for the specific flight days (long dashes). Although two aircraft were used during PEM-Tropics A (the DC-8 and the P3-B), we focus our model evaluation on the DC-8 because it surveyed a larger domain, it had a higher ceiling (12 km altitude), and its suite of observations included HNO3 and PAN.
In addition to the aircraft observations, O3 was measured by sondes launched from Samoa, Tahiti, and Easter Island on several days during the PEM-Tropics A mission. The mean September 1996 sonde profiles are compared to the mean model profiles at the same locations in Figure 8. Individual model profiles for the same days as the sondes were launched are shown by the dotted lines in Figure 8.
Model results for CO show concentrations of 100-200 ppbv near the source regions of South America and Africa, comparable to observations during TRACE-A (Figure 3). Above most of the South Pacific, the simulated concentrations are 55-65 ppbv (Figure 5), with little vertical or horizontal structure. These concentrations and relatively low variances are consistent with PEM-Tropics A measurements in the equatorial South Pacific and eastern South Pacific regions, where the observed standard deviation is only about 5 ppbv (Figure 6). Simulated concentrations over the western South Pacific increase from equatorial to subtropical latitudes due to westerly subtropical transport from the African and South American continents [Fuelberg et al., 1999]. This gradient is qualitatively consistent with the PEM-Tropics A observations [Schultz et al., 1999; Blake et al., 1999], but the observations over the southwestern Pacific are considerably more variable and show a mid-tropospheric bulge of 15 ppbv that the model does not properly capture (Figure 6).
The O3:CO correlations in Figure 7 clearly illustrate that the model underestimates CO over the southwest Pacific due to its inability to reproduce the extreme concentrations observed in the plumes. Simulated CO concentrations rarely exceed 70 ppbv, while the observations extend up to 120 ppbv. The problem does not appear to reflect an underestimate of biomass burning emissions. Doubling these emissions in the model increases CO concentrations above the subtropical South Pacific by 5-8 ppbv throughout the column but still does not reproduce the mid-tropospheric bulge in the southwest region; it also causes or aggravates overestimates of CO and O3 above the eastern South Pacific. We attribute the model underestimate of CO over the subtropical South Pacific to excessive vertical mixing during convective transport over the tropical continents, as discussed in section 3.1. An alternative hypothesis would be that the model does not maintain the vertical structure of the plumes during horizontal transport. To test this hypothesis, we conducted model experiments with a passive tracer artificially inserted in a narrow altitude band over southern Africa. We found that the vertical structure was preserved during the 1-2 weeks of tracer transport to the South Pacific. The Prather [1986] advection algorithm used here is known to be successful in preserving strong gradients with little numerical diffusion.
Model results show a large enhancement of CO over the southeastern equatorial Pacific due to outflow of biomass burning pollution from South America (Figure 5). This enhancement produces in the mean model fields a south-north gradient of increasing concentrations north of Easter Island, where DC-8 flight 8 was conducted. The model overestimates by 5-10 ppbv the CO concentrations measured in that flight above 4 km (Figure 6). The simulations of HNO3, O3, and PAN on the same flight show similar biases, as also shown in Figure 6. It may be that the model does not properly account for the blocking effect of the Andes on the flow from South America to the South Pacific. It may also be that aircraft observations from flight 8 are not representative. Easterly outflow from South America with CO concentrations greater than 100 ppbv was observed during PEM-Tropics A on several P3-B flights offshore from Guayaquil, Ecuador [Blake et al., 1999; Logan et al., 2000]. Examination of the observations from DC-8 flight 8 indicates CO concentrations of 80-90 ppbv in the lower troposphere at the northernmost latitude of the flight (7°S, 110°W), and back trajectory analysis show that this enhancement was due to South American outflow [Blake et al., 1999].
The model simulates NOx concentrations in excess of 100 pptv over the tropical continents due to contributions from biomass burning, lightning, and soil (Figure 3). Minimum concentrations are simulated for the equatorial South Pacific marine boundary layer where NOx is 5-10 pptv and NO is as low as 1-2 pptv. Such low values were observed during PEM-Tropics A and contribute to low O3 in the MBL. The model tends to underestimate NOx concentrations in the upper troposphere, both over the tropical continents (as shown in comparison to the TRACE-A observations in Figure 3) and over the South Pacific (Figure 6). This consistent bias might suggest that the model underestimates the source of NOx from lightning. However, simple scaling of the lightning source would cause the model to overestimate both HNO3 and O3 concentrations observed in PEM-Tropics A. A more likely explanation is that the FSU meteorological fields underestimate the frequency of very deep convective events discharging lightning NOx to the uppermost troposphere. As we will see below, this explanation would also account for the model overestimate of O3 in the upper troposphere.
Model results for HNO3 (Figure 5) show minimum concentrations at the ITCZ due to frequent scavenging and a general increase in concentration poleward. Enhancements in HNO3 are associated with outflow from NOx source regions. The model underestimates significantly the large mid-tropospheric concentrations observed over the subtropical South Pacific region (Figure 6). Nitric acid, like CO, was elevated in numerous plumes sampled by the aircraft.
Also shown in Figure 5 are model results for PAN, which provided the dominant reservoir for NOx in the lower and middle troposphere during PEM-Tropics A [Schultz et al., 1999, 2000]. Model concentrations for the PEM-Tropics A region are as low as 2 pptv near the surface, consistent with observations (Figure 6). In the upper troposphere, PAN ranges between 20 pptv in the equatorial region where air depleted in PAN is frequently transported to the upper troposphere by convection, to 100 pptv south of 15°S latitude where colder temperatures and slower vertical mixing allow PAN to have a longer lifetime. Above the subtropical South Pacific, the modeled PAN compares well with observations. The simulation is less successful above the eastern South Pacific, where the model predicts upper tropospheric PAN nearly a factor of 2 greater than observed, probably due to easterly export of biomass burning pollution from South America in the model (see section 3.2.1).
Mean simulated O3 concentrations (Figure 5) in the tropical southern hemisphere reach a maximum of 50-70 ppbv in the upper troposphere over South America and Africa, comparable to observations during TRACE-A, although the model is about 10 ppbv too low (Figure 3). The aircraft observations during PEM-Tropics A show a gradient of increasing concentrations from the equatorial to the subtropical South Pacific, which the model captures well (Figure 6), but also a gradient of decreasing concentrations from west to east, which the model does not capture and which results in a model overestimate for the eastern South Pacific region. As pointed out previously, the model overestimate over the latter region could reflect excessive easterly outflow of biomass burning pollution form South America, but because the aircraft observations are for only one flight it is hard to be conclusive. The model simulates well in the mean the sonde observations at Easter Island, but it shows occasional pollution events at 1-2 km altitude that are not seen in the observations (Figure 8).
The vertical distribution of O3 concentrations over the South Pacific is in general consistent with observations, as shown in Figures 6 and 8. The most prominent discrepancy is an inability to capture the very low concentrations observed in the uppermost troposphere (10-15 km altitude) over Tahiti and to a lesser extent over Samoa. These low values must represent deep convective injection of boundary layer air, with detrainment above the ceiling of the aircraft; there is some indication of a decrease of O3 concentrations at the highest altitudes in the aircraft observations over the subtropical South Pacific (Figure 6), but it is far less pronounced than in the sonde data. Insufficient accounting of deep convection events and of the associated subsidence in the model could be a major factor behind the underestimate of NOx concentrations and of the NOx/HNO3 concentration ratio, as discussed further in section 5.
The model has more success simulating the observed mid-tropospheric bulge of O3 concentrations over the subtropical South Pacific than it did for CO (Figure 7). This mid-tropospheric bulge is also apparent in the sonde observations over Tahiti and in the corresponding model profiles (Figure 8). The high extremes in the CO observations are more muted for O3, presumably because of the strong nonlinearity in net O3 production [Lin et al., 1988].
An ubiquitous biomass burning influence on O3 over the South Pacific during PEM-Tropics A is apparent in the correlation observed between O3 and CO in the ensemble of the observations (Figure 7). At 4-8 km, the two species are strongly correlated (R2=0.63) with a slope of 1.2 mol mol-1, consistent with biomass burning plumes over a week old [Mauzerall et al., 1998]. Considering the remoteness from sources, the coherence of this relationship, as well as that between CO and other tracers of continental influence [Blake et al., 1999], is remarkable. The model reproduces the positive correlation between O3 and CO in the mid-troposphere (slope=3.2 mol mol-1, R2=0.41). The slope is higher in the model, which does not capture the relatively low DO3/DCO enhancement ratios observed in the strongest plumes.
The Harvard/FSU model simulates several events of westerly outflow from southern Africa and South America which lead to elevated CO and O3 in the tropical South Pacific troposphere. We show here one example of a model plume which was observed on September 18, 1996 by the DC-8 aircraft on flight 12 as it flew from Tahiti to Christchurch, New Zealand (Figure 9) and by an O3 sonde launched from Tahiti (Figure 8) on the same day. Ozone concentrations of 60-80 ppbv, and occasionally higher, were observed in the mid-troposphere for nearly the entire flight. Profiles of other species measured during ascents and descents of the plane confirmed the biomass burning origin of the elevated O3 (enhanced CO and ethane, no C2Cl4 enhancement).
The transport history of the September 18 plume is depicted in Figure 10 which shows the evolution of the free tropospheric CO column in the model for the September 9-18 period. Column abundances allow us to follow the plume even as it spreads or meanders vertically. Superimposed are the locations of high and low pressure centers and the mean horizontal air mass fluxes at 500 hPa, the altitude of maximum outflow from southern Africa to the Indian Ocean. On September 9, a strong South American outflow to the South Atlantic is induced by a mid-latitude cyclone located at 50°W and 45°S, to the southwest of the subtropical South Atlantic High. As the low pressure system migrates eastward in the subsequent days, this export pathway becomes less important. African biomass burning pollution is also exported to the Atlantic free troposphere, as was documented extensively in TRACE-A [Thompson et al., 1996; Mauzerall et al., 1998; Chatfield et al., 1998]. Some African biomass burning pollution also travels southeastward to the Indian Ocean and circulates around the Mascarene High. By September 11, this southeastward outflow has intensified due to a cut-off low that develops southwest of the Mascarene High, creating a strong pressure gradient. On September 12, the Mascarene High shifts northeastward and the cut-off low moves southeastward to rejoin the westerly wave flow at midlatitudes. These motions leave a wide region of low pressure extending from Africa to Australia which facilitates rapid eastward transport from September 12-17 of the South American and African biomass burning pollution which had accumulated in both high pressure systems. The export on September 14 shown in Figure 10 is representative of this flow. By September 18, the pollution has reached the South Pacific where it was observed over Tahiti. The plume in the model is corroborated by column aerosol abundances measured by the Earth Probe TOMS satellite; a large aerosol export event from southern Africa to the Indian Ocean took place on September 10-14 [Logan et al., 2000; Chatfield et al., 2000]. The location and timing of the aerosol outflow corresponds quite well with the model enhancements in CO.
Despite biomass burning emissions during 1996 being close to the climatological average [Olson et al., 1999; Duncan et al., 2000], the meteorological conditions that led to the September 18 plume were anomalous. Figure 11 shows the mean September 1996 surface pressure anomalies relative to the climatology; these anomalies are caused by the September 9-18 period. The low pressure located southwest of the Mascarene High was well developed, allowing efficient outflow from Africa and the South Atlantic. In addition, a sustained area of low pressure extended from Africa to Australia at southern mid-latitudes, rather than alternating highs and lows, allowing rapid and uninterrupted westerly transport to the South Pacific. Average September windspeed at 500 hPa over the Indian Ocean was 50-60% greater than the climatology [see also Logan et al., 2000]. Indeed, the mean O3 concentrations observed at 4-12 km altitude by sondes launched from Samoa during September 1996 are 5-10 ppbv higher than the average September O3 concentrations measured during 1986-1990 and 1995-1999 (long-dashed lines Figure 8). At Tahiti, where sondes have been launched only since 1995, a similar enhancement of 5-10 ppb is found at 4-8 km altitude. Unfortunately, two of the three sondes launched from Tahiti during September 1996 (on the 16th and the 18th) were affected by the large plume described above, making comparisons difficult.
In order to identify the contribution of different sources to CO observed above the South Pacific, we conducted a tagged tracer simulation in which CO from different sources was carried as separate tracers [Bey et al., 2000; Staudt et al., 2000]: (1) biomass burning from South America (180°W-25°W), (2) biomass burning from Africa (25°W-70°E), (3) biomass burning from Australia and Indonesia (70°E-180°E), (4) other anthropogenic sources (fossil fuel and biofuel), and (5) oxidation of CH4 and isoprene. Biomass burning emissions from Africa and South America are of similar magnitude, as shown in Table 1. Emissions from Indonesia and Australia are a factor of 3 less. Loss of the five CO tracers by reaction with OH is calculated using daily mean OH fields archived from the full chemistry simulation. When the tagged CO fields are summed, the total CO field is identical to that obtained from the full chemistry simulation. The tagged CO run was initialized in mid-July with output from a 1 year run by the GEOS-CHEM model [Bey et al., 2000] with tracers defined in the same way.
Figure 12 shows the column abundance of CO from each source during September. Oxidation of methane and isoprene (not shown) contributes a 40-60% background. Fossil and biofuel emissions account for 14% of southern hemispheric CO during September, African biomass burning emissions for 18%, South American biomass burning emissions for 10%, and Australian and Indonesian biomass burning emissions for 5%. The large African contribution reflects the early start of the biomass burning season there (June), allowing CO to accumulate by September. The relatively large contribution from fossil and biofuels combustion must reflect in major part transport from the northern hemisphere as well as emissions in the southern hemisphere. Approximately equal amounts of CO in the tropical Pacific (165°E-90°W, 0-30°S) are contributed by fossil and biofuels (14%), South American biomass burning (13%), and African biomass burning (14%), while burning in Indonesia and Australia contributes much less (5%). The PEM-Tropics A observations did not show obvious signatures from fossil fuel pollution, in contrast to the biomass burning plumes. Since the fossil fuel source is mostly from the northern hemisphere, it would presumably contribute an enhancement to the CO background rather than distinct plumes.
We now turn our attention to the budget of NOx during PEM-Tropics A. The largest primary sources of NOx in the southern hemisphere during austral spring are biomass burning and lightning (Table 1 and Figure 2). The influence of these sources on NOx concentrations in the South Pacific upper troposphere is shown in Table 2 where model concentrations from the standard simulation are compared to those from simulations with either of these two primary sources turned off. Lightning has a much larger impact on NOx concentrations than biomass burning because of the large source over the western equatorial Pacific and Indonesia (Figure 2). Intense lightning activity was observed in this region during the PEM-Tropics A period [Fuelberg et al., 1999]. With a lifetime ranging from less than 1 day in the lower troposphere to about 10 days in the upper troposphere, most NOx from biomass burning that reaches the South Pacific region will have been converted into its reservoir species, PAN and HNO3. In fact, the simulation without biomass burning has nearly the same NOx and NO concentrations as in the standard simulation, but PAN concentrations are considerably less. The strong correlation between PAN and CO in the observations (R2=0.78 for 0-30°S, 165°E-105°W, 6-12 km altitude) and the weak correlation between NO and CO (R2=0.08 for the same region) is consistent with these source attributions in the model. The large contributions to PAN from biomass burning reflects the emission of hydrocarbons together with NOx from the fires.
Local photochemical model calculations by Schultz et al. [2000] constrained with the ensemble of observations in PEM-Tropics A showed that the chemical loss rate (LNOx) of the expanded NOx family (NOx=NO + NO2 + NO3 + 2 N2O5 + HNO2 + HO2NO2) could be balanced in the lower troposphere by the local chemical production rate (PNOx) from recycling of PAN and HNO3 (mostly PAN). Above 6 km altitude, however, LNOx exceeded PNOx by a factor ranging from 1.9 (when NOx loss via N2O5 hydrolysis was neglected) to 2.4 (when it was included). Another way to diagnose the chemical imbalance in the NOx budget is the HNO3/NOx concentration ratio. In the upper troposphere, most of the imbalance between NOx sources and sinks results from cycling with HNO3, thus an accurate simulation of the HNO3/NOx ratio indicates that the chemical production and loss rates are properly accounted for in the model. The observed ratio is 1.7 (Table 2), while photochemical steady state models compute the ratio to be 3-5 [Schultz et al., 2000]. The discrepancy between steady state simulated and observed HNO3/NOx concentration ratios is consistent with the excess of LNOx over PNOx and implies that either primary sources remain important in the NOx budget of the South Pacific upper troposphere or that our understanding of NOx chemistry is incomplete.
We investigate the observed chemical imbalance using the Harvard/FSU 3-D model. Median concentrations for the reactive nitrogen species in the model compare well with those observed in the upper troposphere (Table 2). The HNO3/NOx ratio of 1.9 in the 3-D model is similar to the observed ratio of 1.7. The 3-D model has a chemical imbalance in the NOx budget as diagnosed by LNOx/PNOx of 1.6 (Table 3), compared to the point model value of 1.9-2.4 as given by Schultz et al. [2000]. Because the 3-D model can reproduce both the observed HNO3/NOx ratio and the point model chemical imbalance of LNOx/PNOx, we conclude that this chemical imbalance is due to transport of primary NOx into the region rather than to missing chemistry converting HNO3 to NOx.
Examination of the NOx budget terms in the 3-D model for the upper troposphere over the South Pacific (Table 3) gives further insight into the role of transport from primary sources located outside the South Pacific region. The model indicates that 63% of the NOx source in the South Pacific upper troposphere is due to chemical recycling from the reservoir species HNO3 and PAN, 28% is due to transport of NOx into the region, and 9% is due to in situ emissions. Most of the primary NOx transported into the region in the model is from lightning over the western equatorial Pacific and Indonesia (Figure 2). When lightning sources are turned off in the 3-D model (Table 3), the transport of primary NOx into the region decreases by more than a factor of 5 and the photochemical imbalance is reduced by 50%. The longitudinal gradient of LNOx/PNOx in the upper troposphere (Figure 13) supports the importance of westerly transport of this primary source of NOx; in both the 3-D model and in the point model, LNOx/PNOx decreases from 3-4 in the western tropical Pacific to 1.3 in the eastern tropical Pacific (with the exception of anomalously high values on the Easter Island flight).
Schultz et al. [2000] concluded that primary NOx from lightning was not responsible for the NOx chemical budget imbalance in their point model calculations because the greatest imbalances were at low relative humidities, while air masses affected by recent convection have high relative humidity [Cohan et al., 1999]. Low relative humidity, however, is consistent with a NOx source from air masses that have subsided after encountering convection and lightning. Subsidence likely plays an important role in sustaining the chemical imbalance in the NOx budget. Because the lifetime of NOx increases with altitude, a primary source at high altitudes will take longer to reach chemical equilibrium. In the tropics, we expect most of the NOx from lightning to be emitted above the 12 km ceiling of the DC-8.
We find in the model that transport of primary NOx from biomass burning makes relatively little contribution to the imbalance in the chemical NOx budget of the upper troposphere. In fact, as shown in Table 3, turning off biomass burning emissions in the model slightly increases the imbalance. Biomass burning NOx originates from further away than lightning NOx, it travels at lower altitudes, and it has a short photochemical lifetime due to the concurrent abundance of hydrocarbons in the fire plumes. All these factors would contribute to bringing NOx from biomass burning into chemical steady state for airmasses sampled over the South Pacific.
We now examine the production and loss of O3 over the South Pacific and how it is affected by emissions from biomass burning and lightning. The mean net production of O3 in the model averaged over the PEM-Tropics A time period is shown in Figure 14. Production and loss rates for the tropical South Pacific, shown in Table 4, are within 20% of values computed by a point photochemical model constrained with in situ observations [Schultz et al., 1999]. The South Pacific troposphere is a region of net photochemical destruction of O3 with a 0-12 km column loss of 1.5x1011 molecules cm-2 s-1. Net loss in the marine boundary layer is as high as 3 ppbv day-1. Transition from net loss to net production takes place at 6 km altitude. Net production of up to 2 ppbv day-1 is found at 6-12 km over the western Pacific, decreasing to less than 1 ppbv/day over the eastern Pacific.
We find that O3 production is not enhanced in biomass burning plumes as compared to background air masses in the model, reflecting low concentrations of NOx; there is no significant correlation between simulated CO concentrations and net O3 production in the middle and upper troposphere over the South Pacific (Figure 15). This finding is consistent with results from point photochemical model calculations [Logan et al., 2000]. We conclude that the majority of O3 enhancements from biomass burning in the model are generated in the source regions. Turning off emissions of NOx from lightning causes a 17% reduction in column O3 production over the South Pacific, compared to a 14% reduction when biomass burning emissions are turned off. We conclude that biomass burning and lightning made contributions of similar magnitude to the tropospheric O3 production over the South Pacific during PEM-Tropics A.
We have used a global 3-D model of tropospheric chemistry driven by assimilated meteorological observations to examine the origins of biomass burning pollution plumes observed during PEM-Tropics A and their implications for the regional chemical budgets of NOx and ozone. Extensive evaluation of model results with aircraft and sonde observations during PEM-Tropics A, and with earlier observations from the TRACE-A aircraft campaign over Brazil and southern Africa, shows that the model captures major features of biomass burning influence over the southern tropics in general and over the South Pacific in particular. It does not capture the layered structure of biomass burning plumes observed during PEM-Tropics A, a flaw which we attribute to excessive column mixing during deep continental convection. There is also some indication that the model has insufficient convection to the uppermost troposphere, with implications in particular for subsidence of lightning NOx over the aircraft flight tracks (ceiling 12 km altitude). The model reproduces the strong correlation between O3 and CO observed at 4-8 km over the South Pacific. This correlation illustrates the regional impact of aged biomass burning emissions on the South Pacific troposphere.
The model simulation of biomass burning plumes demonstrates how horizontal advection associated with westerly moving mid-latitude cyclones facilitates efficient export of continental-scale airmasses from biomass burning source regions and from high pressure systems located over the South Atlantic and Madagascar. Because of the arrangement of these mid-latitude low pressure systems during September 1996, we find that the South Pacific probably was more heavily influenced by biomass burning pollution than in other years. Indeed, O3 sonde measurements in the mid-troposphere at Samoa and Tahiti in September, 1996 were 5-10 ppbv higher than the mean September sonde observations.
A simulation with tagged tracers indicates that 14% of the CO above the tropical South Pacific originated from biomass burning over Africa, 13% from biomass burning over South America, 5% from biomass burning over Indonesia and Australia, and 14% from fossil fuel combustion. Fossil fuel emissions are predominantly located in the northern hemisphere and probably contribute to the CO background rather than distinct plumes. Plumes with an urban origin were not observed during PEM-Tropics A.
Lightning was the most important source of NOx in the upper troposphere over the South Pacific during PEM-Tropics A, while decomposition of PAN from biomass burning was more important in the lower troposphere. We showed that the previously reported chemical imbalance in the NOx budget of the upper troposphere is due to subsidence of primary NOx emitted by lightning over the Indonesian warm pool, rather than to missing chemistry converting HNO3 to NOx. When lightning NOx emissions are turned off in the 3-D model, the imbalance is decreased by 50%. Emissions of NOx from biomass burning do not contribute to the imbalance because they originate from further away, are transported in the mid-troposphere where the NOx lifetime is shorter, and are more photochemically active due to concurrent hydrocarbon emissions.
The lower troposphere over the South Pacific is a region of net O3 destruction, with net loss rates up to 3 ppbv/day. At about 6 km, a transition to net O3 production takes place, with net production of 1-2 ppbv/day in the upper troposphere. The net O3 production in biomass burning plumes is not found to be enhanced relative to the residual background air masses in the model because NOx in these plumes is low. Biomass burning and lightning are found to make comparable contributions to O3 production over the tropical South Pacific.
This work was supported by the NASA Global Tropospheric Chemistry Program. Amanda Staudt was supported by a National Science Foundation Graduate Fellowship. We would like to express our appreciation to G. Gardner her help in model development, to M. Prather for the use of an updated chemical transport model driver and Fast-J, and to M. Schultz for point photochemical model results and helpful comments.
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Map of PEM-Tropics A flight tracks for the DC-8 aircraft. Observations from PEM-Tropics A are averaged over three regions for model evaluation: (a) Subtropical South Pacific, (b) Equatorial South Pacific, and (c) Eastern South Pacific. Observations from TRACE-A are averaged over two regions for model evaluation: (d) Eastern Brazil and (e) Southern Africa.
Distribution of NOx emissions during the PEM-Tropics A time period (August 28 - October 5, 1996) from (a) lightning and (b) biomass burning.
Comparison of simulated (dashed line) and observed (solid line) vertical profiles of CO, NO, and ozone concentrations over Southern Africa and Eastern Brazil. The observations were taken during TRACE-A and have been averaged over the regions identified in Figure 1. The error bars indicate one standard deviation. Model values are averages for August 28 - October 6, 1996. The number of observations in each vertical interval is indicated on the right of each plot.
Comparison of simulated (dashed line) and observed (solid line) profiles of CH3I concentrations over the South Pacific during PEM-Tropics A. The observations have been averaged over the regions identified in Figure 1. The error bars indicate one standard deviation.
Monthly mean simulated concentrations for September, 1996 of CO, NOx, HNO3, PAN, and O3 at 500 hPa.
Comparison of mean simulated (dashed line with triangles) and observed (solid line) vertical profiles of CO, NO, HNO3, PAN, and O3 for the PEM-Tropics A period and for the regions identified in Figure 1. Error bars indicate one standard deviation. The long dashed line with crosses is the average of model values sampled along the flight tracks and for the flight days. The number of observations in each vertical interval is indicated on the right of each plot.
Scatterplots of O3 versus CO concentrations as observed on the DC-8 during PEM-Tropics A (grey triangles) and as sampled along the flight tracks in the Harvard/FSU model (black crosses) for a South Pacific region extending from 0-25°S, 165°E-105°W, and 4-8 km altitude.
Ozone concentration profiles above Samoa, Tahiti, and Easter Island during PEM-Tropics A. The solid line shows the mean values from ozone sondes launched over the course of the PEM-Tropics A campaign, with error bars indicating one standard deviation. The long-dashed lines are the mean values from ozone sondes launched from 1995-1999 at Tahiti and from 1986-1990 and 1995-1999 at Samoa. The dashed line with symbols is the mean model profile at the same location. Dotted lines are model profiles for the days on which ozone sondes were launched.
Vertical concentration profiles observed at 26°S and 164°W on September 18, 1996 during DC-8 Flight 12 between Tahiti and Christchurch, New Zealand.
Simulated column CO above the boundary layer (700 hPa - 85 hPa) in mol m-2 for four days leading up to the large biomass burning plume event observed between Tahiti and Christchurch on September 18, 1996. Also shown are the corresponding daily mean centers of high and low surface pressure and air mass flux vectors at 500 hPa.
Mean pressure anomaly (hPa) for September 1996 relative to a 1958-1999 climatology from the NOAA NCEP/NCAR reanalysis.
Simulated September mean columns of CO (mol m-2) contributed by fossil and biofuel combustion, biomass burning in South America, biomass burning in Africa, and biomass burning in Australia and Indonesia.
Ratio of 24 hour average loss to production rates of NOx (LNOx/PNOx) computed in a photochemical point model constrained with observations of NO, HNO3, PAN, and other species from PEM-Tropics A [Schultz et al., 2000]. Values for individual points (representing ~3 minute averages in the observations) are plotted for 10-30°S as a function of longitude. The dashed line shows the 3-D model results averaged for 10-30°S and August 28 - October 5, 1996.
Simulated net production of O3 (ppbv/day) averaged over 10-30°S for the PEM-Tropics A time period (August 28 - October 5, 1996) plotted versus longitude and altitude.
Net production of O3 (ppbv/day) plotted versus CO concentration (ppbv) as sampled in the model along the PEM-Tropics A flight tracks for a South Pacific region defined as 10-30°S, 165°E-105°W, and 4-8 km altitude.
Table 1: Global emissions for CO and NOx (Aug. 28, 1996-Oct. 6, 1996)
|
Southern Hemisphere |
Northern Hemisphere | |
|---|---|---|
|
CO (Tg CO) | ||
|
Fossil fuel combustion |
2.1 (3%) |
38 (60%) |
|
Biofuel |
2.9 (4%) |
11 (18%) |
|
Biomass burning |
63 (93%) |
14 (22%) |
|
South America |
29 |
1.2 |
|
Africa |
26 |
8.8 |
|
Indonesia/Australia |
8.0 |
3.5 |
|
NOx (Tg N) | ||
|
Fossil fuel combustion |
0.3 (6%) |
3.9 (65%) |
|
Biomass burning |
2.9 (71%) |
0.6 (10%) |
|
Soil |
0.3 (8%) |
0.6 (10%) |
|
Lightning |
0.6 (15%) |
0.8 (14%) |
|
Aircraft |
0.004 (<1%) |
0.06 (1%) |
Table 2: Concentrations of NOy species in the upper troposphere over the South Pacific
|
Species |
DC-8 observations |
Harvard/FSU model standard run |
Harvard/FSU model no lightning |
Harvard/FSU model no biomass burning |
|---|---|---|---|---|
|
NO, pptv |
26 (12-56) |
22 (16-37) |
14 (12-19) |
22 (16-36) |
|
NOx, pptv |
34 (15-69) |
34 (37-52) |
21 (18-25) |
33 (26-51) |
|
HNO3, pptv |
58 (24-99) |
78 (51-96) |
40 (26-55) |
74 (48-91) |
|
HNO3/NOx, mol/mol |
1.7 (1.2-4.0) |
1.9 (1.1-2.6) |
1.6 (1.1-2.4) |
1.8 (1.0-2.6) |
|
PAN, pptv |
40 (23-88) |
76 (49-110) |
65 (45-96) |
51 (34-75) |
Median and interquartiles for a region defined as 6-12 km, 0-30°S, 165°E-105°W and for the PEM-Tropics A time period of August 28 - October 5, 1996. Statistics in the observations are for the ensemble of the data averaged over HNO3 measurement intervals. Statistics in the model are for the ensemble of time-averaged concentrations in all gridboxes with the region.
Table 3: NOx budget for southern tropical Pacific upper troposphere
|
Standard Simulation |
No Lightning NOx |
No Biomass Burning | |
|---|---|---|---|
|
Sources (Mmol day-1): | |||
|
Net transport into region |
15 (28%) |
3 (10%) |
16 (32%) |
|
Emissions (lightning + aircraft) |
5 (9%) |
0.4 (2%) |
5 (11%) |
|
Chemical production (PNOx) | |||
|
HNO3 -> NOx |
16 (29%) |
7 (27%) |
15 (31%) |
|
PAN -> NOx |
18 (34%) |
16 (58%) |
12 (26%) |
|
other reactants -> NOx |
1 (2%) |
1 (3%) |
- |
|
Total |
55 |
27 |
48 |
|
Sinks (Mmol day-1): | |||
|
Chemical loss (LNOx) | |||
|
NOx -> HNO3 |
36 (67%) |
16 (55%) |
34 (73%) |
|
NOx -> PAN |
17 (31%) |
13 (45%) |
11 (25%) |
|
NOx -> other products |
1 (2%) |
- |
1.0 (2%) |
|
Total |
54 |
29 |
46 |
|
Accumulation (Mmol day-1) |
1 |
-2 |
2 |
Budget terms are model averages for a region defined as 6-12 km, 0-30°S, 165°E-100°W and for the PEM-Tropics A time period of August 28 - October 5, 1996.
Table 4: Tropospheric O3 budget over the South Pacific during PEM-Tropics A
|
PO3 |
LO3 |
(P-L)O3 | |
|---|---|---|---|
|
Point modela | |||
|
0-10 S |
1.4 |
3.2 |
-1.8 |
|
10-30 S |
1.8 |
3.0 |
-1.2 |
|
Harvard/FSU modelb | |||
|
0-10 S |
1.8 |
3.3 |
-1.5 |
|
10-30 S |
2.0 |
3.5 |
-1.5 |
Budget terms (1011 molecules cm-2 s-1) are calculated for a region defined as 165°E-105°W, 0-30°S, 0-12 km altitude and for the PEM-Tropics A time period of August 28 - October 5, 1996. Gross production rates PO3 and gross loss rates LO3 are calculated for the odd oxygen family (Ox=O3 + O + NO2 + HNO4 + 2*NO3 + 3*N2O5 + HNO3 + PAN + HNO3). Considering that O3 generally accounts for over 90% of Ox, the budgets of O3 and Ox can be viewed as equivalent. (P-L)O3 is the net production rate.
a. Median values from Schultz et al. [1999]
b. Average values