Abstract
Abstract. The dependence of aerosol acidity on particle size, location, and altitude over Europe during a summertime period is investigated using the hybrid version of aerosol dynamics in the chemical transport model PMCAMx. The pH changes more with particle size in northern and southern Europe owing to the enhanced presence of non-volatile cations (Na, Ca, K, Mg) in the larger particles. Differences of up to 1–4 pH units are predicted between sub- and supermicron particles, while the average pH of PM1−2.5 can be as much as 1 unit higher than that of PM1. Most aerosol water over continental Europe is associated with PM1, while coarse particles dominate the water content in the marine and coastal areas due to the relatively higher levels of hygroscopic sea salt. Particles of all sizes become increasingly acidic with altitude (0.5–2.5 units pH decrease over 2.5 km) primarily because of the decrease in aerosol liquid water content (driven by humidity changes) with height. Inorganic nitrate is strongly affected by aerosol pH with the highest average nitrate levels predicted for the PM1−5 range and over locations where the pH exceeds 3. Dust tends to increase aerosol pH for all particle sizes and nitrate concentrations for supermicron range particles. This effect of dust is quite sensitive to its calcium content. The size-dependent pH differences carry important implications for pH-sensitive processes in the aerosol.
Highlights
Acidity is an aerosol property of central importance driving gas–particle partitioning and heterogeneous chemistry (Pye et al, 2020). pH affects the formation of semi-volatile particulate matter and the nitrogen cycle by modulating HNO3
Higher pH values for all particle sizes are predicted over the Atlantic due to the presence of sea salt and the systematically higher RH and liquid water content – all of which act to reduce aerosol acidity
High pH is predicted for parts of central and northern Europe due to the corresponding ammonia and the effect of the alkaline dust particles
Summary
Acidity is an aerosol property of central importance driving gas–particle partitioning and heterogeneous chemistry (Pye et al, 2020). pH affects the formation of semi-volatile particulate matter and the nitrogen cycle by modulating HNO3–NO−3 and NH3–NH+4 gas–particle partitioning (Meskhidze et al, 2003; Guo et al, 2017; Nenes et al, 2020). The nitrate partitioning to the aerosol phase is favored when pH exceeds a threshold value (between 1.5 and 3) that depends logarithmically on liquid water content and temperature (Meskhidze et al, 2003; Guo et al, 2016; Nenes et al, 2020). For lower pH values (below 1.5 to 2), formation of aerosol nitrate is not favored and remains in the gas phase as HNO3 Between these pH value limits, a sensitivity window (of 1 to 1.5 pH units) exists in which nitrate can be found either as gas or as aerosol (Vasilakos et al, 2018; Nenes et al, 2020).
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