Ozone pollution in the Southeast US involves complex chemistry driven by emissions of anthropogenic nitrogen oxide radicals (NO ≡ NO + NO) 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 SEACRS 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 from the US Environmental Protection Agency (EPA) is too high. This finding is based on SEACRS observations of NO and its oxidation products, surface network observations of nitrate wet deposition fluxes, and OMI satellite observations of tropospheric NO columns. Our results indicate that NEI NO emissions from mobile and industrial sources must be reduced by 30-60%, dependent on the assumption of the contribution by soil NO emissions. Upper tropospheric NO from lightning makes a large contribution to satellite observations of tropospheric NO that must be accounted for when using these data to estimate surface NO emissions. We find that only half of isoprene oxidation proceeds by the high-NO pathway to produce ozone; this fraction is only moderately sensitive to changes in NO emissions because isoprene and NO emissions are spatially segregated. GEOS-Chem with reduced NO 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 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.

[1] Many prior studies clearly document episodic Asian pollution in the western U.S. free troposphere. Here, we examine the mechanisms involved in the transport of Asian pollution plumes into western U.S. surface air through an integrated analysis of in situ and satellite measurements in May-June 2010 with a new global high-resolution ($50 Â 50 km 2 ) chemistry-climate model (GFDL AM3). We find that AM3 with full stratosphere-troposphere chemistry nudged to reanalysis winds successfully reproduces observed sharp ozone gradients above California, including the interleaving and mixing of Asian pollution and stratospheric air associated with complex interactions of midlatitude cyclone air streams. Asian pollution descends isentropically behind cold fronts; at $800 hPa a maximum enhancement to ozone occurs over the southwestern U.S., including the densely populated Los Angeles Basin. During strong episodes, Asian emissions can contribute 8-15 ppbv ozone in the model on days when observed daily maximum 8-h average ozone (MDA8 O 3 ) exceeds 60 ppbv. We find that in the absence of Asian anthropogenic emissions, 20% of MDA8 O 3 exceedances of 60 ppbv in the model would not have occurred in the southwestern USA. For a 75 ppbv threshold, that statistic increases to 53%. Our analysis indicates the potential for Asian emissions to contribute to high-O 3 episodes over the high-elevation western USA, with implications for attaining more stringent ozone standards in this region. We further demonstrate a proof-of-concept approach using satellite CO column measurements as a qualitative early warning indicator to forecast Asian ozone pollution events in the western U.S. with lead times of 1-3 days. Citation: Lin, M., et al. (2012), Transport of Asian ozone pollution into surface air over the western United States in spring,

Abstract. Formic acid (HCOOH) is one of the most abundant acids in the atmosphere, with an important influence on precipitation chemistry and acidity. Here we employ a chemical transport model (GEOS-Chem CTM) to interpret recent airborne and ground-based measurements over the US Southeast in terms of the constraints they provide on HCOOH sources and sinks. Summertime boundary layer concentrations average several parts-per-billion, 2–3× larger than can be explained based on known production and loss pathways. This indicates one or more large missing HCOOH sources, and suggests either a key gap in current understanding of hydrocarbon oxidation or a large, unidentified, direct flux of HCOOH. Model-measurement comparisons implicate biogenic sources (e.g., isoprene oxidation) as the predominant HCOOH source. Resolving the unexplained boundary layer concentrations based (i) solely on isoprene oxidation would require a 3× increase in the model HCOOH yield, or (ii) solely on direct HCOOH emissions would require approximately a 25× increase in its biogenic flux. However, neither of these can explain the high HCOOH amounts seen in anthropogenic air masses and in the free troposphere. The overall indication is of a large biogenic source combined with ubiquitous chemical production of HCOOH across a range of precursors. Laboratory work is needed to better quantify the rates and mechanisms of carboxylic acid production from isoprene and other prevalent organics. Stabilized Criegee intermediates (SCIs) provide a large model source of HCOOH, while acetaldehyde tautomerization accounts for ~ 15% of the simulated global burden. Because carboxylic acids also react with SCIs and catalyze the reverse tautomerization reaction, HCOOH buffers against its own production by both of these pathways. Based on recent laboratory results, reaction between CH3O2 and OH could provide a major source of atmospheric HCOOH; however, including this chemistry degrades the model simulation of CH3OOH and NOx : CH3OOH. Developing better constraints on SCI and RO2 + OH chemistry is a high priority for future work. The model neither captures the large diurnal amplitude in HCOOH seen in surface air, nor its inverted vertical gradient at night. This implies a substantial bias in our current representation of deposition as modulated by boundary layer dynamics, and may indicate an HCOOH sink underestimate and thus an even larger missing source. A more robust treatment of surface deposition is a key need for improving simulations of HCOOH and related trace gases, and our understanding of their budgets.