1. Retrieval of tropospheric NO2 from GOME
Monthly mean tropospheric NO2 columns for July 1996 from (top) the GOME satellite instrument and (bottom) the GEOS-CHEM model. Both have been filtered for cloud fractions less than 20%. Tropospheric columns from GOME are derived with a three-step process. First, slant columns are determined by spectral fitting. Second, stratospheric and tropospheric columns are separated using data over the central Pacific. Third, slant columns are converted to vertical columns by using the LIDORT radiative transfer model and relative vertical NO2 distributions from the GEOS-CHEM model.

2. Mapping reactive hydrocarbon emissions from space using GOME satellite observations of HCHO columns
Formaldehyde (HCHO) columns retrieved from GOME provide a proxy for reactive hydrocarbon emissions and enable mapping of these emissions from space. Integrated analysis of the GOME data together with in situ surface and aircraft observations of HCHO over North America using the GEOS-CHEM model provides confidence in the quality of the retrieval.
Top panel: HCHO columns retrieved from a GOME orbit on July 7, The maximum is attributed to high isoprene emission in the Ozarks Plateau. Bottom panel: reactive hydrocarbon emissions in the eastern U.S. in July 1996 derived from the GOME observations. High emissions in the southeast are due to isoprene. The GOME data indicate that the current BEIS2 inventory used by EPA greatly underestimates isoprene emissions.

3. Tropospheric ozone maximum over the Middle East
Top panel: climatology of observed ozone at 400 hPa in July from ozonesondes and MOZAIC aircraft (circles) with corresponding GEOS-CHEM model results for 1997 (contours). Bottom panel: GEOS-CHEM tropospheric ozone columns for July A Middle East maximum of free tropospheric ozone in July is apparent in the observations and in the model. It is due in the model to lightning outflow from the Indian monsoon and easterly transport of Asian pollution, with strong subsidence in the Middle East. Further confirmation and study of this maximum should have important implications for our understanding of tropospheric ozone and its role in climate forcing.

4. Trends in carbon monoxide, 1988-1997
Observed
GEOS-CHEM
Concentrations of CO in at three NOAA/CMDL surface stations. Observations (black lines) are compared to GEOS-CHEM model results (red lines). Barrow, Alaska, shows a downward trend in both the model and observations due to decreasing fossil fuel emissions from Russia. At Mace Head, Ireland, the model captures the highly variable nature of the observations subjected alternately to relatively clean air from the Atlantic Ocean and to polluted air from the European continent. At Mauna Loa, Hawaii, the spring maximum is largely defined by long-range transport of Asian fossil fuel and biomass burning emissions.