How aerosol pH responds to nitrate to sulfate ratio of fine-mode particulate
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RESEARCH ARTICLE
How aerosol pH responds to nitrate to sulfate ratio of fine-mode particulate Yansheng Cao 1,2 & Zhongyi Zhang 1,2 & Hongwei Xiao 1,2 & Yajun Xie 1,2 & Yue Liang 1,2 & Huayun Xiao 1,2 Received: 31 December 2019 / Accepted: 17 June 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Aerosol acidity (pH), one of key properties of fine-mode particulate (PM2.5), depends largely on nitrate and sulfate in particle. The mass contribution of nitrate relative to sulfate in PM2.5 has tended to increase in many regions globally, but how this change affects aerosol pH remains in debate. In this way, we measured PM2.5 ionic species and oxygen isotopic composition of nitrate in the eastern China, and predicted aerosol pH using the ISORROPIA-II model. When nitrate to sulfate molar ratio increases and thus PM2.5 is gradually enriched in ammonium nitrate (NH4NO3), aerosol pH tends to increase. The oxidation of nitrogen dioxide (NO2) by hydroxyl radical is responsible for most of nitrate formation (generally above 60%). These indicate that nitrate formation through gas-to-particle conversion involving ammonia and nitric acid results in increasing aerosol pH with increasing molar ratio of nitrate to sulfate. Conversely, aerosol pH is expected to decrease with increasing relative abundance of nitrate as ammonia emissions are lowered. Our research concludes that it should be considered to reduce aerosol NH4NO3 by reducing the precursors of nitric oxide and ammonia emissions, to substantially improve the air quality (i.e., reduce PM2.5 levels and potential nitrate deposition) in China. Keywords Aerosol pH . Nitrate to sulfate ratio . Particle nitrate . Nitrate formation
Introduction Aerosol pH depends largely on nitrate (NO3−) and sulfate (SO42−) in particle, which is formed through the oxidation of nitric oxide (NOx) and sulfur dioxide (SO2) in the atmosphere, respectively. Driven by the emission control policies, the molar ratio of NO3− to SO42− (NO3−/SO42−) in PM2.5 in China has tended to increase since 2000 (Wang et al. 2011). Moreover, the ratio is expected to maintain growing in the Yansheng Cao and Zhongyi Zhang contributed equally to this work. Responsible editor: Gerhard Lammel Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11356-020-09810-0) contains supplementary material, which is available to authorized users. * Huayun Xiao [email protected] 1
Jiangxi Province Key Laboratory of the Causes and Control of Atmospheric Pollution, East China University of Technology, Nanchang 330013, China
2
School of Water Resources and Environmental Engineering, East China University of Technology, Nanchang 330013, China
predictable future based on the current emission control policies and implementation status (Wang et al. 2011). The trend of increasing NO3−/SO42− ratio is not limited to China and has been observed in other parts of the world, such as the USA and Canada (Cheng and Zhang 2017; Lawal et al. 2018; Murphy et al. 2017). This trend has re
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