Anthropogenic Control Over Wintertime Oxidation of Atmospheric Pollutants

Abstract

During winter in the midlatitudes, photochemical oxidation is significantly slower than in summer and the main radical oxidants driving formation of secondary pollutants, such as fine particulate matter and ozone, remain uncertain, owing to a lack of observations in this season. Using airborne observations, we quantify the contribution of various oxidants on a regional basis during winter, enabling improved chemical descriptions of wintertime air pollution transformations. We show that 25-60% of NO x is converted to N 2 O 5 via multiphase reactions between gas-phase nitrogen oxide reservoirs and aerosol particles, with~93% reacting in the marine boundary layer to form >2.5 ppbv ClNO 2 . This results in >70% of the oxidizing capacity of polluted air during winter being controlled by multiphase reactions and emissions of volatile organic compounds, such as HCHO, rather than reaction with OH. These findings highlight the control local anthropogenic emissions have on the oxidizing capacity of the polluted wintertime atmosphere.Plain Language Summary During summer, rapid transformations of primary pollutants, those emitted directly into the atmosphere, into secondary pollutants, such as particulate matter and ozone, are driven by reactions with the hydroxyl radical, formed in the atmosphere when sunlight strikes ozone in the presence of water vapor. During winter, when there is less sunlight and water vapor, production of this radical is lower. Yet the conversion of primary pollutants into secondary pollutants still occurs rapidly, pointing to a misunderstanding in the chemical processes that drive this conversion during winter. Using aircraft data collected across the northeast United States during the winter of 2015, we show that reactions with radicals arising from atypical precursors, such as nitryl chloride, account for more than 70% of the reactions that directly emitted pollutants undergo. We show that during winter, the formation of these radicals is tied to human activities. Our data provide critical constraints for improving the descriptions of chemical processes in air quality models, which will help guide improved air quality policy. Other regions of the world, such as China, Europe, and northern India, also experience this seasonal chemical shift in the atmosphere. Our findings, therefore, have global scale implications for understanding wintertime pollution transformations and transport.

Wintertime Formaldehyde: Airborne Observations and Source Apportionment Over the Eastern United States

“…The Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER) mission was designed to understand the processes controlling atmospheric composition in the wintertime boundary layer of the Eastern U.S. A recent analysis of these observations has revealed that HCHO can be as much as 50% of the daily integrated radical source (Haskins et al, 2019). Furthermore, Jaeglé et al (2018) found a bias of −46% in GEOS-Chem simulated HCHO concentrations relative to WINTER observations at altitudes < 1 km.…”

Winter observations of ClNO&lt;sub&gt;2&lt;/sub&gt; in northern China: Spatiotemporal variability and insights into daytime peaks

“…The highest winter concentrations of ClNO2 to date were observed in southern China, with a maximum level of 4.7 ppbv at a mountain top in Hong Kong in aged urban/industrial plumes from the Pearl River Delta (PRD) (Wang et al, 2016) and 8.3 ppbv during a severe pollution episode within the PRD (Yun et al, 2018 and ppbv found by Yun et al, 2018), which contrasts the high concentrations of PM2.5 and low concentrations of O3 over northern China during the cold winter. The winter mixing ratios of ClNO2 in the US and Europe range from approximately 0.3 ppbv in urban California (Mielke et al, 2016) and urban Manchester (Priestley et al, 2018), respectively, to 1.3 ppbv in the outflow of coastal urban areas (Riedel et al, 2013;Haskins et al, 2019). In general, the winter concentrations of ClNO2 over northern China were comparable to or slightly higher than those observed in the US and Europe.…”

“…Subsequent studies demonstrated the worldwide ubiquity of ClNO2 and confirmed its significant role in photochemistry (Thornton et al, 2010;Mielke et al, 2011;Phillips et al, 2012;Edwards et al, 2013;Bannan et al, 2015;Wild et al, 2016;Wang et al, 2016;Bannan et al, 2019;Eger et al, 2019). The role of ClNO2 in the radical budget could be more important than that of OH in winter, because OH production is reduced in winter owing to lower concentrations of O3 and H2O vapor in this season (Haskins et al, 2019). A limited number of winter observations of ClNO2 have been conducted on various platforms, including on aircrafts over northern Europe (Priestley et al, 2018) and the eastern US (Haskins et al, 2019), on a tall tower in Boulder, USA (Riedel et al, 2013), on a mountain top in Hong Kong (Wang et al, 2016), and at ground sites in Alberta, Canada (Mielke et al, 2016) and Heshan, China (Yun et al, 2018).…”

“…The role of ClNO2 in the radical budget could be more important than that of OH in winter, because OH production is reduced in winter owing to lower concentrations of O3 and H2O vapor in this season (Haskins et al, 2019). A limited number of winter observations of ClNO2 have been conducted on various platforms, including on aircrafts over northern Europe (Priestley et al, 2018) and the eastern US (Haskins et al, 2019), on a tall tower in Boulder, USA (Riedel et al, 2013), on a mountain top in Hong Kong (Wang et al, 2016), and at ground sites in Alberta, Canada (Mielke et al, 2016) and Heshan, China (Yun et al, 2018). These studies found high ClNO2 mixing ratios of up to 7.7 ppbv (Yun et al, 2018) in winter and a contribution of ClNO2 to Cl liberation of up to 83 % (Priestley et al, 2018).…”

Significant Decrease in Wet Deposition of Anthropogenic Chloride Across the Eastern United States, 1998–2018

“…The broad influence of chlorine chemistry stems from the high reactivity of Cl atoms with many gas phase species, including methane (Atkinson, 1997), many volatile organic compounds (VOCs) (Simpson et al, 2015), dimethyl sulfide (Chen et al, 2017; Hoffmann et al, 2016), and mercury (Horowitz et al, 2017). Cl atoms can even be the dominant early‐morning oxidative agent in the winter, serving to increase OH production by enhancing formaldehyde through Cl atom‐initiated VOC oxidation and enhancing O 3 production from NO x recycling (Haskins et al, 2019).…”