October 2007
Observing the diurnal evolution of NO_x-chemistry and emissions from space

Since September 2004, the SCIAMACHY and OMI satellite sensors are taking concurrent measurements of tropospheric NO2 columns. The different local overpass times of SCIAMACHY (10:00 hrs), and OMI (13:30) offer the opportunity to examine the consistency between the two instruments under tropospheric background conditions and the effect of different observing times. We find that the two instruments agree well over remote regions, but also that SCIAMACHY observes higher tropospheric NO2 columns at 10:00 than OMI at 13:30 over the fossil fuel source regions in northern midlatitudes. In contrast, SCIAMACHY observes lower tropospheric NO2 columns than OMI over biomass burning regions in the tropics. Using a global 3-D chemical transport model (GEOS-Chem), we show that the 10:00-13:30 decrease in tropospheric NO2 column over fossil fuel source regions can be explained by photochemical loss, dampened by the diurnal cycle of anthropogenic emissions that has a broad daytime maximum. The observed 10:00-13:30 NO2 column increase over tropical biomass burning regions points to a sharp midday peak in emissions, and is consistent with a diurnal cycle of emissions derived from geostationary satellite fire counts.

The figures above show the monthly mean (August 2006) tropospheric NO2 columns observed by SCIAMACHY at 10:00 hrs (upper left) and OMI at 13:30 hrs (lower left) on days that both instruments took a measurement at the same location under cloud-free conditions. The right panel shows the absolute differences between the two measurements, where red colors indicate higher NO2 at 10:00 than 13:45, and blue colors indicate the reverse.

For more information see Boersma et al. [2007].