Higher temperatures caused by increasing greenhouse gas concentrations are predicted to exacerbate photochemical smog if precursor emissions remain constant. We perform a statistical analysis of 21 years of ozone and temperature observations across the rural eastern U.S. The climate penalty factor is defined as the slope of the ozone/temperature relationship. For two precursor emission regimes, before and after 2002, the climate penalty factor was consistent across the distribution of ozone observations. Prior to 2002, ozone increased by an average of ∼3.2 ppbv/°C. After 2002, power plant NOx emissions were reduced by 43%, ozone levels fell ∼10%, and the climate penalty factor dropped to ∼2.2 ppbv/°C. NOx controls are effective for reducing photochemical smog and might lessen the severity of projected climate change penalties. Air quality models should be evaluated against these observations, and the climate penalty factor metric may be useful for evaluating the response of ozone to climate change.

Tropospheric ozone samples collected during a twelve‐month period in urban air show an enrichment of about 9% in the heavy isotope 50O3 consistent with predictions from laboratory measurements. The enhancement of about 7% observed in 49O3 is still within the uncertainty of the expected value. These measurements confirm that the isotope effect, repeatedly found in laboratory experiments, is also produced in the atmosphere during the ozone formation process.

[1] In situ measurements of trace gases and aerosol optical properties were made in March 2005 at Xianghe (39.798°N, 116.958°E, 35 m), a rural site about 70 km southeast, and generally downwind of the Beijing metropolitan area. High pollutant levels were observed during the experiment, with CO (1.09 ± 1.02 ppmv, average ± standard deviation), SO 2 (17.8 ± 15.7 ppbv), NO y (26.0 ± 24.0 ppbv), aerosol scattering coefficients (b sp , (468 ± 472) Â 10 À6 m À1 ), and aerosol absorption coefficients (b ap , (65 ± 75) Â 10 À6 m À1 ) all much higher than observed at some rural sites in the United States. O 3 (29.1 ± 16.5 ppbv) was relatively low during this study, suggesting inactive photochemical processes. Strong synoptic fluctuations in pollutant levels were detected every 4-5 days during the experiment, as cold fronts passing over the region drastically reduced the ground-level pollution. Very little precipitation was measured during the whole observational period, implying pollutant uplift and transport by rain-free cold fronts and dry convection. The observed CO/NO y ratio agrees better with inventories. Further analysis suggests that such comparisons may shed some light on the quality of emission inventories, but quantification of any error requires more extensive measurements over longer period and larger areas, as well as direct characterization of emission sources, especially mobile sources and small boilers. Using black carbon (BC)/CO ratio from the experiment, BC emissions from China are estimated at about 1300 Gg (10 9 g)/yr, but could be as high as 2600 Gg/yr.

[1] The meteorological mechanisms for lofting trace gases and aerosols out of the planetary boundary layer (PBL) into the free troposphere are key to understanding local air pollution problems as well as regional and global atmospheric chemistry and climate issues. Over the North American continent, convective storms and lifting in warm conveyor belts transport pollutants into the free troposphere. Little is known about the vertical distribution of pollutants and dust over east Asia, and the processes leading to transport, transformation, and removal of these species remain uncertain. To provide insight into these mechanisms, we report on eight flights based out of Shenyang in NE China as part of the U.S./China EAST-AIRE project conducted in April 2005. We evaluate profiles of trace species, along with back trajectories and satellite data, in the meteorological context of cyclonic systems. The warm-sector PBL air ahead of a cold front was highly polluted, while in the free troposphere concentrations of trace gases and aerosols were lower, but well above background; we measured $300 ppb CO, $2 ppb SO 2 , $70 ppb O 3 , and $ 8 Â 10 À5 m À1 aerosol scattering between $1000 and 4000 m altitude. Satellite observations indicate that the entire plume contained almost 10 5 tons of SO 2 and that the gas decayed with a lifetime of 3-5 d. Roughly the same mass of aerosol was transported into the free troposphere. Over the east Asian continent, dry convection appears to dominate with warm conveyor belts first coming into play as the cyclonic systems move off the coast.

[1] From 1997 to 2003, airborne measurements of O 3 , CO, SO 2 , and aerosol properties were made during summertime air pollution episodes over the mid-Atlantic United States (34.7-44.6°N, 68.4-81.6°W) as part of the Regional Atmospheric Measurement, Modeling, and Prediction Program (RAMMPP). Little diurnal variation was identified in the CO, SO 2 , and Å ngström exponent profiles, although the Å ngström exponent profiles decreased with altitude. Boundary layer O 3 was greater in the afternoon, while lower free tropospheric O 3 was invariant at $55 ppbv. The single scattering albedo increased from morning to afternoon (0.93 ± 0.01À0.94 ± 0.01); however, both profiles decreased with altitude. A cluster analysis of back trajectories in conjunction with the vertical profile data was used to identify source regions and characteristic transport patterns during summertime pollution episodes. When the greatest trajectory density lay over the northern Ohio River Valley, the result was large O 3 values, large SO 2 /CO ratios, highly scattering particles, and large aerosol optical depths. Maximum trajectory density over the southern Ohio River Valley resulted in little pollution. The greatest afternoon O 3 values occurred during periods of stagnation. North-northwesterly and northerly flow brought the least pollution overall. The contribution of regional transport to afternoon boundary layer O 3 was quantified. When the greatest cluster trajectory density lay over the Ohio River Valley ($59% of the profiles), transport accounted for 69-82% of the afternoon boundary layer O 3 . Under stagnant conditions ($27% of the profiles), transport only accounted for 58% of the afternoon boundary layer O 3 . The results from this study provide a description of regional chemical and transport processes that will be valuable to investigators from the Baltimore, New York, and Pittsburgh EPA Supersites.

Laboratory experiments have been performed with O and O2 in their ground electronic states to study the distribution of all possible ozone isotopes formed. Results show that with respect to 48O3 the two symmetric molecules 17O17O17O and 18O18O18O are depleted, in good agreement with standard recombination theory. A large enrichment of about 18% is found in the asymmetric molecule 16O17O18O, while all others carry about 2/3 of that. A comparison with past laboratory and stratospheric ozone isotope measurements leads to the following conclusion: There is a standard enrichment which resides in asymmetric molecules only. It will lead to an enrichment of stratospheric 49O3 and 50O3 of 8 to 9%; this has been actually observed in recent balloon experiments. Occasionally, the enrichments in the stratosphere are larger, reaching 40% at certain altitudes. Only when ozone was formed in an electric discharge process have larger enrichments been measured in laboratory experiments, affecting both symmetric and asymmetric molecules. The results provide an important connection between numerous laboratory studies and stratospheric measurements.

Trends in the composition of the lower atmosphere (0–1500 m altitude) and surface air quality over the Baltimore/Washington area and surrounding states were investigated for the period from 1997 to 2011. We examined emissions of ozone precursors from monitors and inventories as well as ambient ground-level and aircraft measurements to characterize trends in air pollution. The US EPA Continuous Emissions Monitoring System (CEMS) program reported substantial decreases in emission of summertime nitrogen oxides (NO_x) from power plants, up to ∼80% in the mid-Atlantic States. These large reductions in emission of NO_x are reflected in a sharp decrease of ground-level concentrations of NO_x starting around 2003. The decreasing trend of tropospheric column CO observed by aircraft is ∼0.8 Dobson unit (DU) per year, corresponding to ∼35 ppbv yr⁻¹ in the lower troposphere (the surface to 1500 m above ground level). Satellite observations of long-term, near-surface CO show a ∼40% decrease over western Maryland between 2000 and 2011; the same magnitude is indicated by aircraft measurements above these regions upwind of the Baltimore/Washington airshed. With decreasing emissions of ozone precursors, the ground-level ozone in the Baltimore/Washington area shows a 0.6 ppbv yr⁻¹ decrease in the past 15 yr. Since photochemical production of ozone is substantially influenced by ambient temperature, we introduce the climate penalty factor (CPF) into the trend analysis of long-term aircraft measurements. After compensating for inter-annual variations in temperature, historical aircraft measurements indicate that the daily net production of tropospheric ozone over the Baltimore/Washington area decreased from ∼20 ppbv day⁻¹ in the late 1990s to ∼7 ppbv day⁻¹ in the early 2010s during ozone season. A decrease in the long-term column ozone is observed as ∼0.2 DU yr⁻¹ in the lowest 1500 m, corresponding to an improvement of ∼1.3 ppbv yr⁻¹. Our aircraft measurements were conducted on days when severe ozone pollution was forecasted, and these results represent the decreasing trend in high ozone events over the past 15 yr. Back trajectory cluster analysis demonstrates that emissions of air pollutants from Ohio and Pennsylvania through Maryland influence the column abundances of downwind ozone in the lower atmosphere. The trends in air pollutants reveal the success of regulations implemented over the past decades and the importance of region-wide emission controls in the eastern United States

h i g h l i g h t sObservations of ozone are higher over the Chesapeake Bay than areas upwind on land. Dry deposition rates, boundary layer depth, and photolysis play an integral role. Model resolution plays a role in determining accurate surface ozone concentrations. Observations of total reactive nitrogen are much lower than model simulations. a b s t r a c tAir quality models, such as the Community Multiscale Air Quality (CMAQ) model, indicate decidedly higher ozone near the surface of large interior water bodies, such as the Great Lakes and Chesapeake Bay. In order to test the validity of the model output, we performed surface measurements of ozone (O 3 ) and total reactive nitrogen (NO y ) on the 26-m Delaware II NOAA Small Research Vessel experimental (SRVx), deployed in the Chesapeake Bay for 10 daytime cruises in July 2011 as part of NASA's GEO-CAPE CBODAQ oceanographic field campaign in conjunction with NASA's DISCOVER-AQ air quality field campaign. During this 10-day period, the EPA O 3 regulatory standard of 75 ppbv averaged over an 8-h period was exceeded four times over water while ground stations in the area only exceeded the standard at most twice. This suggests that on days when the Baltimore/Washington region is in compliance with the EPA standard, air quality over the Chesapeake Bay might exceed the EPA standard. Ozone observations over the bay during the afternoon were consistently 10e20% higher than the closest upwind ground sites during the 10-day campaign; this pattern persisted during good and poor air quality days. A lower boundary layer, reduced cloud cover, slower dry deposition rates, and other lesser mechanisms, contribute to the local maximum of ozone over the Chesapeake Bay. Observations from this campaign were compared to a CMAQ simulation at 1.33 km resolution. The model is able to predict the regional maximum of ozone over the Chesapeake Bay accurately, but NO y concentrations are significantly overestimated. Explanations for the overestimation of NO y in the model simulations are also explored.