May 2007: The importance of injection heights of biomass burning emissions in boreal forest regions

Our study of the effects of the intense forest fires of 1998 in Siberia, Alaska, and Northern Canada on tropospheric composition in the northern hemisphere highlights the importance of injection heights to our understanding of the effects of biomass burning emissions from boreal regions [Leung et al., 2007]. Because fires in the boreal forests are extremely energetic, a large fraction of the emissions from these fires are immediately lofted into the free troposphere. Surface data alone does not allow us to constrain both the magnitude of the fire emissions and their injection altitude. GEOS-Chem reproduces both surface and column anomalies at most stations when a large fraction of boreal forest fire emissions are released in the free troposphere. In particular, the model matches the large CO anomaly in August in northern Japan (Hokkaido) only when emissions are released above the boundary layer. One important consequence of lofted fires is a higher rate of ozone production at sites far downwind of the fires.

FIGURE 1. Comparison of the observed and simulated anomaly in CO mixing ratios for 1998, with three scenarios for the injection height and emissions from KAS05. Emissions are injected in the boundary layer only (KAS05.BL, solid), distributed throughout the troposphere (KAS05.D1, dot-dashed) and with 40% in the boundary layer and 60% at 3-5 km (KAS05.D2, dashed). The symbols (*) are the difference between monthly CO in 1998 and the mean CO for the same month for 1992-1997. The lines are the difference between the simulation with 1998 emissions and the baseline simulation with mean fire emissions.

FIGURE 2. Comparison of the observed and simulated anomaly in the CO column for 1998, with three scenarios for the injection height and emissions from KAS05. Emissions are injected in the boundary layer only (KAS05.BL, solid), distributed throughout the troposphere (KAS05.D1, dot-dashed) and with 40% in the boundary layer and 60% at 3-5 km (KAS05.D2, dashed). The symbols (*) are the difference between monthly CO in 1998 and the mean CO for the same month for 1996-2001 (excluding 1998). The lines are the difference between the simulation with 1998 emissions and the baseline simulation with mean fire emissions.

More details may be found in Leung et al 2007.