Why do Models Overestimate Surface Ozone in the Southeastern United States?

Ozone pollution in the Southeast US involves complex chemistry driven by emissions of anthropogenic nitrogen oxide radicals (NO_x ≡ NO + NO₂) and biogenic isoprene. Model estimates of surface ozone concentrations tend to be biased high in the region and this is of concern for designing effective emission control strategies to meet air quality standards. We use detailed chemical observations from the SEAC⁴RS aircraft campaign in August and September 2013, interpreted with the GEOS-Chem chemical transport model at 0.25°×0.3125° horizontal resolution, to better understand the factors controlling surface ozone in the Southeast US. We find that the National Emission Inventory (NEI) for NO_x from the US Environmental Protection Agency (EPA) is too high. This finding is based on SEAC⁴RS observations of NO_x and its oxidation products, surface network observations of nitrate wet deposition fluxes, and OMI satellite observations of tropospheric NO₂ columns. Our results indicate that NEI NO_x emissions from mobile and industrial sources must be reduced by 30-60%, dependent on the assumption of the contribution by soil NO_x emissions. Upper tropospheric NO₂ from lightning makes a large contribution to satellite observations of tropospheric NO₂ that must be accounted for when using these data to estimate surface NO_x emissions. We find that only half of isoprene oxidation proceeds by the high-NO_x pathway to produce ozone; this fraction is only moderately sensitive to changes in NO_x emissions because isoprene and NO_x emissions are spatially segregated. GEOS-Chem with reduced NO_x emissions provides an unbiased simulation of ozone observations from the aircraft, and reproduces the observed ozone production efficiency in the boundary layer as derived from a regression of ozone and NO_x oxidation products. However, the model is still biased high by 8±13 ppb relative to observed surface ozone in the Southeast US. Ozonesondes launched during midday hours show a 7 ppb ozone decrease from 1.5 km to the surface that GEOS-Chem does not capture. This bias may reflect a combination of excessive vertical mixing and net ozone production in the model boundary layer.

Evaluation of short-term Ogawa passive, photolytic, and federal reference method sampling devices for nitrogen oxides in El Paso and Houston, Texas

Passive Sampling Devices (PSDs) have been successfully used by government and academic agencies to monitor common ambient air pollutants such as ozone and nitrogen dioxide (NO(2)). Most PSD studies have involved long-term (e.g. bi-weekly or monthly) sampling. But the Passive Ozone Network of Dallas (POND) studies of 1998 and 1999 showed that high quality 24-hour and 12-hour data using the Ogawa PSD could be collected for ambient ozone concentrations. This paper presents an evaluation of short-term passive sampling results for nitrogen oxides (NO(x)) in El Paso and Houston, Texas, using the Ogawa PSD. The Ogawa NO(x) PSDs were compared to both Federal Reference Method (FRM) monitors and a photolytic converter, with the photolytic converter designed to report closer concentrations to "true" NO(x) by more effectively limiting the interferences of other nitrogen species. Overall, good agreement was noted for all three monitor types in both cities, supporting the potential use of lower cost Ogawa PSDs for large multi-site episodic NO(x)/NO(2)/NO saturation screening studies. This evaluation was conducted during two separate six week periods of the cooler winter months so additional testing of the Ogawa PSDs during different seasons is recommended.