Abstract
Abstract. The acidity of atmospheric particulate matter regulates its mass, composition, and toxicity and has important consequences for public health, ecosystems and climate. Despite these broad impacts, the global distribution and evolution of aerosol particle acidity are unknown. We used the comprehensive atmospheric multiphase chemistry–climate model EMAC (ECHAM5/MESSy Atmospheric Chemistry) to investigate the main factors that control aerosol particle acidity and uncovered remarkable variability and unexpected trends during the past 50 years in different parts of the world. Aerosol particle acidity decreased strongly over Europe and North America during the past decades while at the same time it increased over Asia. Our simulations revealed that these particle acidity trends are strongly related to changes in the phase partitioning of nitric acid, production of sulfate in aqueous aerosols, and the aerosol hygroscopicity. It is remarkable that the aerosol hygroscopicity (κ) has increased in many regions following the particle pH. Overall, we find that alkaline compounds, notably ammonium and to a lesser extent crustal cations, regulate the particle pH on a global scale. Given the importance of aerosol particles for the atmospheric energy budget, cloud formation, pollutant deposition, and public health, alkaline species hold the key to control strategies for air quality and climate change.
Highlights
Aerosol particle acidity is a central property of atmospheric particulates that influence clouds, climate, and air quality, including impacts on human health (Raizenne et al, 1996; Lelieveld et al, 2015)
It affects the partitioning of semivolatile acids between the gas and particle phases (Guo et al, 2016, 2017, 2018; Nenes et al, 2020), secondary organic aerosol (SOA) formation (Xu et al, 2015; Marais et al, 2016), the solubility of trace metals in aerosol particles (Oakes et al, 2012) associated with their toxicity (Fang et al, 2017) and nutrient capacity (Jickells et al, 2005), the activation of halogens that act as oxidants (Saiz-Lopez and von Glasow, 2012), the conversion of sulfur dioxide (Seinfeld and Pandis, 2006; Cheng et al, 2016), the particle hygroscopic growth and lifetime (Metzger et al, 2006; Abdelkader et al, 2015; Karydis et al, 2017), and atmospheric corrosivity (Leygraf et al, 2016)
The inevitable decrease in the particle pH hindered the partitioning of nitric acid into the particulate phase and the sulfate production in the aerosol aqueous phase; the aerosol hygroscopicity increased over Asia following a reverse correlation with the particle pH
Summary
Aerosol particle acidity is a central property of atmospheric particulates that influence clouds, climate, and air quality, including impacts on human health (Raizenne et al, 1996; Lelieveld et al, 2015). Most atmospheric chemistry models do not consider crustal elements (e.g., Ca2+, Mg2+, K+) and Na+ in sea salt These species affect the ion balance by influencing the phase partitioning of nitrate and ammonium, especially in areas where aeolian dust is abundant (Karydis et al, 2016). The pH calculations are performed online with the ISORROPIA II thermodynamic equilibrium model (Fountoukis and Nenes, 2007)
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