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Key Role of NO3 Radicals in the Production of Isoprene Nitrates and Particulate Nitrooxyorganosulfates in Beijing
JACQUELINE HAMILTON, Daniel Bryant, Peter Edards, Archit Mehra, Thomas J. Bannan, Alfred Mayhew, James Hopkins, James Lee, Mike Newland, Bin Ouyang, Andrew Rickard, University of York
Abstract Number: 601
Working Group: Urban Aerosols
Abstract
Poor air quality is the biggest environmental factor contributing to premature mortality globally. As the Earth’s population has grown, the number of people living in urban areas has increased rapidly from 751 million in 1950 to 4.2 billion in 2018. By 2030, the UN estimates there will be 43 megacities (> 10 million inhabitants), with most of them located in developing countries in Africa, Asia and Latin America. Since many of these locations are situated in the tropics, high average temperatures can lead to significant emissions of biogenic volatile organic compounds (BVOC) to the urban atmosphere, in particular isoprene. Beijing, China, is a well-studied megacity, with significant air quality issues related to particle pollution and ozone (O3) production. Beijing experiences high average summertime temperatures (ca. 30 C) and has a high percentage of urban green space (> 41 % urban green space), which can lead to significant amounts of isoprene being emitted. Photochemical oxidation of isoprene in the presence of high levels of anthropogenic pollutants, in particular nitrogen oxides (NOx) and sulfur dioxide (SO2), can lead to enhanced secondary organic aerosol (SOA) production.
A key uncertainty in understanding SOA production from isoprene is the role of isoprene nitrates (IsN). IsN are formed in chain terminating reactions, during oxidation by hydroxyl radicals (OH) in the presence of NO, or by nitrate radicals (NO3). IsN can lead to significant secondary organic aerosol (SOA) production and they can act as reservoirs of atmospheric nitrogen oxides. Recent observations in a boreal forest by Liebmann et al. (2019) under low-NO conditions indicated that daytime alkyl nitrate production from NO3 chemistry can dominate over OH under certain conditions.
As part of UK-China Atmospheric Pollution and Human Health in a Chinese Megacity (APHH-China) program during the summer of 2017, we collected a comprehensive suite of gas- and particle-phase chemical observations. We estimated the rate of production of IsN from the reactions of isoprene with OH and NO3 radicals during the summertime in Beijing. Unusually low NO concentrations during the afternoon resulted in NO3 mixing ratios of ca. 2 pptv at approximately 15:00, which we estimate to account for around a third of the total IsN production in the gas phase (Hamilton et al., 2021). Heterogenous uptake of IsN produces nitrooxyorganosulfates (NOS). Offline filter samples were analysed using high resolution mass spectrometry coupled to chromatography to identify isoprene NOS for comparison to IsN levels in the gas phase. Two mono-nitrated NOS were correlated with particulate sulfate concentrations and appear to be formed from sequential NO3 and OH oxidation. Di-and tri-nitrated isoprene related NOS, formed from multiple NO3 oxidation steps, peaked during the night. This work highlights that NO3 chemistry can play a key role in driving biogenic-anthropogenic interactive chemistry in Beijing with respect to the formation of IsN during both the day and night. Further work will compare the production of NOS in other megacities and laboratory experiments to determine the role of NO3 in forming IsN in urban areas.