[1] Ozone measurements from the OMI and MLS instruments on board the Aura satellite are used for deriving global distributions of tropospheric column ozone (TCO). TCO is determined using the tropospheric ozone residual method which involves subtracting measurements of MLS stratospheric column ozone (SCO) from OMI total column ozone after adjusting for intercalibration differences of the two instruments using the convective-cloud differential method. The derived TCO field, which covers one complete year of mostly continuous daily measurements from late August 2004 through August 2005, is used for studying the regional and global pollution on a timescale of a few days to months. The seasonal and zonal characteristics of the observed TCO fields are also compared with TCO fields derived from the Global Modeling Initiative's Chemical Transport Model. The model and observations show interesting similarities with respect to zonal and seasonal variations. However, there are notable differences, particularly over the vast region of the Saharan desert.

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With the advent of many newly developing 3-D chemistry and transport models it is advantageous to have such a dataset for evaluating the performance of the models in relation to dynamical and photochemical processes controlling the ozone distributions in the troposphere and stratosphere. The OMI/MLS gridded ozone climatology data are made available to the science community via the NASA Goddard Space Flight Center ozone and air quality website http://ozoneaq.gsfc.nasa.gov/.

This paper studies the significance of large‐scale transport and pyrogenic (i.e., biomass burning) emissions in the production of tropospheric ozone in the tropics. Using aerosol index (AI) and tropospheric column ozone (TCO) time series from 1979 to 2000 derived from the Nimbus‐7 and Earth Probe Total Ozone Mapping Spectrometer measurements, our study shows significant differences in the seasonal and spatial characteristics of pyrogenic emissions north and south of the equator in the African region and Brazil in South America. Notwithstanding these differences, most of the observed seasonal characteristics are well simulated by the GEOS‐CHEM global model of tropospheric chemistry. The only exception is the northern African region where modeled and observed TCO differ significantly. In the Indonesian region the most significant increase in TCO occurred during September–December 1997, following large‐scale forest and savanna fires associated with the El Niño induced dry condition. The increase in TCO extended over most of the western Pacific well outside the burning region and was accompanied by a decrease in the eastern Pacific resembling a west‐to‐east dipole about the dateline. These features are well simulated in the GEOS‐CHEM model which suggests that both the biomass burning and changes in meteorological conditions during the El Niño period contributed almost equally to the observed increase in TCO in the Indonesian region. During 1997 the net increase in TCO integrated over the tropical region between 15°N and 15°S was about 6–8 Tg (1 Tg = 1012 g) over the mean climatological value of about 77 Tg. The GEOS‐CHEM model suggests that most of this increase may have been caused by biomass burning in the Indonesian region since dynamical components of El Niño induced changes in TCO tend to cancel out in the area‐averaged data. In addition to biomass burning, the interannual variability in the area‐averaged column ozone in the tropics is influenced by a number of factors including the quasi‐biennial oscillation and solar cycle.

We estimate the tropospheric column ozone using a forward trajectory model to increase the horizontal resolution of the Aura Microwave Limb Sounder (MLS) derived stratospheric column ozone. Subtracting the MLS stratospheric column from Ozone Monitoring Instrument total column measurements gives the trajectory enhanced tropospheric ozone residual (TTOR). Because of different tropopause definitions, we validate the basic residual technique by computing the 200‐hPa‐to‐surface column and comparing it to the same product from ozonesondes and Tropospheric Emission Spectrometer measurements. Comparisons show good agreement in the tropics and reasonable agreement at middle latitudes, but there is a persistent low bias in the TTOR that may be due to a slight high bias in MLS stratospheric column. With the improved stratospheric column resolution, we note a strong correlation of extratropical tropospheric ozone column anomalies with probable troposphere‐stratosphere exchange events or folds. The folds can be identified by their colocation with strong horizontal tropopause gradients. TTOR anomalies due to folds may be mistaken for pollution events since folds often occur in the Atlantic and Pacific pollution corridors. We also compare the 200‐hPa‐to‐surface column with Global Modeling Initiative chemical model estimates of the same quantity. While the tropical comparisons are good, we note that chemical model variations in 200‐hPa‐to‐surface column at middle latitudes are much smaller than seen in the TTOR.

The NASA Goddard Space Flight Center (GSFC) two‐dimensional (2‐D) model of stratospheric transport and photochemistry has been used to predict ozone changes that have occurred in the past 20 years from anthropogenic chlorine and bromine emissions, solar cycle ultraviolet flux variations, the changing sulfate aerosol abundance due to several volcanic eruptions including the major eruptions of El Chichón and Mount Pinatubo, solar proton events (SPEs), and galactic cosmic rays (GCRs). The same linear regression technique has been used to derive profile and total ozone trends from both measurements and the GSFC model. Derived 2‐D model ozone profile trends are similar in shape to the Solar Backscattered Ultraviolet (SBUV) and SBUV/2 trends with highest percentage decreases in the upper stratosphere at the highest latitudes. The general magnitude of the derived 2‐D model upper stratospheric negative ozone trend is larger than the trends derived from the observations, especially in the northern hemisphere. The derived 2‐D model negative trend in the lower stratosphere at middle northern latitudes is less than the measured trend. The derived 2‐D model total ozone trends are small in the tropics and larger at middle and high latitudes, a pattern that is very similar to the Total Ozone Mapping Spectrometer (TOMS) derived trends. The differences between the derived 2‐D model and TOMS trends are generally within 1–2% in the northern hemisphere and the tropics. The derived 2‐D model trends are generally more in southern middle and high latitudes by 2–4%. Our 2‐D model predictions are also compared with the temporal variations in total ozone averaged between 65°S and 65°N over the TOMS observing period (1979–1993). Inclusion of anthropogenic chlorine and bromine increases, solar cycle ultraviolet flux variations, and the changing sulfate aerosol area abundance into our model captures much of the observed TOMS global total ozone changes. The model simulations predict a decrease in ozone of about 4% from 1979 to 1995 due to the chlorine and bromine increases. The changing sulfate aerosol abundances were computed to significantly affect ozone and result in a maximum decrease of about 2.8% in 1992 in the annually averaged almost global total ozone (AAGTO) computed between 65°S and 65°N. Solar ultraviolet flux variations are calculated to provide a moderate perturbation to the AAGTO over the solar cycle by a maximum of ±0.6% (about 1.2% from solar maximum to minimum). Effects from SPEs are relatively small, with a predicted maximum AAGTO decrease of 0.22% in 1990 after the extremely large events of October 1989. GCRs are computed to cause relatively minuscule variations of a maximum of + 0.02% in AAGTO over a solar cycle.