In an environment with many local, remote, persistent, and episodic sources of pollution, meteorology is the primary factor that drives periods of unhealthy air quality and reduced visibility. The 2016 Korea-UnitedStatesAirQuality(KORUS-AQ)fieldstudyprovidesauniqueopportunitytoexaminethe impactofmeteorologyontherelativeinfluenceoflocalandtransboundarypollution.MuchoftheKORUS-AQ campaign can be grouped into four distinct research periods based on observed synoptic meteorology, includingaperiodofcomplexaerosolverticalprofilesdrivenbydynamicmeteorology,stagnationundera persistent anticyclone, low-level transport and haze development, and a blocking pattern. These episodes areexaminedusingadiversearchiveofground,airborne,andsatellitedata.Whilefrontalboundaries are recognized as the primary mechanism driving pollution transport in eastern Asia, results show that they are not always related to sustained periods of hazardous air quality and reduced visibility at the surface.Significantlong-rangetransportofpollutionanddustwasconstrainedtoafewshortevents, suggesting that the majority of pollutants sampled during KORUS-AQ originated from local sources. A severeregionalpollutionepisodeisexaminedindetail,featuringdensehazeandsignificantsecondary particle formation within a shallow moist boundary layer. Observations during KORUS-AQ also highlight a rapid,40ppbvincreaseinozonepollutionasastrongseabreezefronttraversedtheSeoulMetropolitan Area. Representativeness of meteorology and pollution conditions measured by KORUS-AQ is considered by comparison with climatology. This analysis is an essential step toward improved local and regional forecasting of air quality and visibility.

[1] Airborne measurements of CH 2 O were acquired employing tunable diode laser absorption spectroscopy during the 2001 Transport and Chemical Evolution Over the Pacific (TRACE-P) study onboard NASA's DC-8 aircraft. Above $2.5 km, away from the most extreme pollution influences and heavy aerosol loadings, comprehensive comparisons with a steady state box model revealed agreement to within ±37 pptv in the measurement and model medians binned according to altitude and longitude. Likewise, a near unity slope (0.98 ± 0.03) was obtained from a bivariate fit of the measurements, averaged into 25 pptv model bins, versus the modeled concentrations for values up to $450 pptv. Both observations suggest that there are no systematic biases on average between CH 2 O measurements and box model results out to model values $450 pptv. However, the model results progressively underpredict the observations at higher concentrations, possibly due to transport effects unaccounted for in the steady state model approach. The assumption of steady state also appears to contribute to the scatter observed in the point-by-point comparisons. The measurement-model variance was further studied employing horizontal flight legs. For background legs screened using a variety of nonmethane hydrocarbon (NMHC) tracers, measurement and model variance agreed to within 15%. By contrast, measurement variance was $60% to 80% higher than the model variance, even with small to modest elevations in the NMHC tracers. Measurement-model comparisons of CH 2 O in clouds and in the lower marine troposphere in the presence of marine aerosols suggest rather significant CH 2 O uptake by as much as 85% in one extreme case compared to expectations based on modeled gas phase processes.

[1] Aerosol data collected near Asia on the DC-8 aircraft platform during TRACE-P has been examined for evidence of uptake of NO 3 À and SO 4 = on dust surfaces. Data is compared between a sector where dust was predominant and a sector where dust was less of an influence. Coincident with dust were higher mixing ratios of anthropogenic pollutants. HNO 3 , SO 2 , and CO were higher in the dust sector than the nondust sector by factors of 2.7, 6.2, and 1.5, respectively. The colocation of dust and pollution sources allowed for the uptake of NO 3 À and nss-SO 4 = on the coarse dust aerosols, increasing the mixing ratios of these particulates by factors of 5.7 and 2.6 on average. There was sufficient nss-SO 4 = to take up all of the NH 4 + present, with enough excess nss-SO 4 = to also react with dust CaCO 3 . This suggests that the enhanced NO 3 À was not in fine mode NH 4 NO 3 . Particulate NO 3 À (p-NO 3 À ) constituted 54% of the total NO 3 À (t-NO 3 À ) on average, reaching a maximum of 72% in the dust sector. In the nondust sector, p-NO 3 À contributed 37% to t-NO 3 À , likely due to the abundance of sea salts there. In two other sectors where the influence of dust and sea salt were minimal, p-NO 3 À accounted for <15% of t-NO 3 À .

The Geostationary Coastal and Air Pollution Events (GEO-CAPE) mission was recommended by the National Research Council's (NRC's) Earth Science Decadal Survey to measure tropospheric trace gases and aerosols and coastal ocean phytoplankton, water quality, and biogeochemistry from geostationary orbit, providing continuous observations within the field of view. To fulfill the mandate and address the challenge put forth by the NRC, two GEO-CAPE Science Working Groups (SWGs), representing the atmospheric composition and ocean color disciplines, have developed realistic science objectives using input drawn from several community workshops. The GEO-CAPE mission will take advantage of this revolutionary advance in temporal frequency for both of these disciplines. Multiple observations per day are required to explore the physical, chemical, and dynamical processes that determine tropospheric composition and air quality over spatial scales ranging from urban to continental, and over temporal scales ranging from diurnal to seasonal. Likewise, high-frequency satellite observations are critical to studying and quantifying biological, chemical, and physical processes within the coastal ocean. These observations are to be achieved from a vantage point near 95°–100°W, providing a complete view of North America as well as the adjacent oceans. The SWGs have also endorsed the concept of phased implementation using commercial satellites to reduce mission risk and cost. GEO-CAPE will join the global constellation of geostationary atmospheric chemistry and coastal ocean color sensors planned to be in orbit in the 2020 time frame.

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