Global O3-CO Correlations in a Chemistry and Transport Model During July-August: Evaluation with TES Satellite Observations and Sensitivity to Input Meteorological Data and Emissions

We examine the capability of the Global Modeling Initiative (GMI) chemistry and transport model to reproduce global mid-tropospheric (618hPa) O₃-CO correlations determined by the measurements from Tropospheric Emission Spectrometer (TES) aboard NASA's Aura satellite during boreal summer (July-August). The model is driven by three meteorological data sets (fvGCM with sea surface temperature for 1995, GEOS4-DAS for 2005, and MERRA for 2005), allowing us to examine the sensitivity of model O₃-CO correlations to input meteorological data. Model simulations of radionuclide tracers (²²²Rn, ²¹⁰Pb, and ⁷Be) are used to illustrate the differences in transport-related processes among the meteorological data sets. Simulated O₃ values are evaluated with climatological ozone profiles from ozonesonde measurements and satellite tropospheric O₃ columns. Despite the fact that three simulations show significantly different global and regional distributions of O₃ and CO concentrations, all simulations show similar patterns of O₃-CO correlations on a global scale. These patterns are consistent with those derived from TES observations, except in the tropical easterly biomass burning outflow regions. Discrepancies in regional O₃-CO correlation patterns in the three simulations may be attributed to differences in convective transport, stratospheric influence, and subsidence, among other processes. To understand how various emissions drive global O₃-CO correlation patterns, we examine the sensitivity of GMI/MERRA model-calculated O₃ and CO concentrations and their correlations to emission types (fossil fuel, biomass burning, biogenic, and lightning NO_x emissions). Fossil fuel and biomass burning emissions are mainly responsible for the strong positive O₃-CO correlations over continental outflow regions in both hemispheres. Biogenic emissions have a relatively smaller impact on O₃-CO correlations than other emissions, but are largely responsible for the negative correlations over the tropical eastern Pacific, reflecting the fact that O₃ is consumed and CO generated during the atmospheric oxidation process of isoprene under low NO_x conditions. We find that lightning NO_x emissions degrade both positive correlations at mid-/high- latitudes and negative correlations in the tropics because ozone production downwind of lightning NO_x emissions is not directly related to the emission and transport of CO. Our study concludes that O₃-CO correlations may be used effectively to constrain the sources of regional tropospheric O₃ in global 3-D models, especially for those regions where convective transport of pollution plays an important role.

Ozone and carbon monoxide budgets over the Eastern Mediterranean

The importance of the long-range transport (LRT) on O3 and CO budgets over the Eastern Mediterranean has been investigated using the state-of-the-art 3-dimensional global chemistry-transport model TM4-ECPL. A 3-D budget analysis has been performed separating the Eastern from the Western basins and the boundary layer (BL) from the free troposphere (FT). The FT of the Eastern Mediterranean is shown to be a strong receptor of polluted air masses from the Western Mediterranean, and the most important source of polluted air masses for the Eastern Mediterranean BL, with about 40% of O3 and of CO in the BL to be transported from the FT aloft. Regional anthropogenic sources are found to have relatively small impact on regional air quality in the area, contributing by about 8% and 18% to surface levels of O3 and CO, respectively. Projections using anthropogenic emissions for the year 2050 but neglecting climate change calculate a surface O3 decrease of about 11% together with a surface CO increase of roughly 10% in the Eastern Mediterranean.

A Large Underestimate of Formic Acid from Tropical Fires: Constraints from Space-Borne Measurements

Formic acid (HCOOH) is one of the most abundant carboxylic acids and a dominant source of atmospheric acidity. Recent work indicates a major gap in the HCOOH budget, with atmospheric concentrations much larger than expected from known sources. Here, we employ recent space-based observations from the Tropospheric Emission Spectrometer with the GEOS-Chem atmospheric model to better quantify the HCOOH source from biomass burning, and assess whether fire emissions can help close the large budget gap for this species. The space-based data reveal a severe model HCOOH underestimate most prominent over tropical burning regions, suggesting a major missing source of organic acids from fires. We develop an approach for inferring the fractional fire contribution to ambient HCOOH and find, based on measurements over Africa, that pyrogenic HCOOH:CO enhancement ratios are much higher than expected from direct emissions alone, revealing substantial secondary organic acid production in fire plumes. Current models strongly underestimate (by 10 ± 5 times) the total primary and secondary HCOOH source from African fires. If a 10-fold bias were to extend to fires in other regions, biomass burning could produce 14 Tg/a of HCOOH in the tropics or 16 Tg/a worldwide. However, even such an increase would only represent 15-20% of the total required HCOOH source, implying the existence of other larger missing sources.