Abstract
Abstract. pH is a fundamental aerosol property that affects ambient particle concentration and composition, linking pH to all aerosol environmental impacts. Here, PM1 and PM2. 5 pH are calculated based on data from measurements during the California Research at the Nexus of Air Quality and Climate Change (CalNex) study from 15 May to 15 June 2010 in Pasadena, CA. Particle pH and water were predicted with the ISORROPIA-II thermodynamic model and validated by comparing predicted to measured gas–particle partitioning of inorganic nitrate, ammonium, and chloride. The study mean ± standard deviation PM1 pH was 1.9 ± 0.5 for the SO42−–NO3−–NH4+–HNO3–NH3 system. For PM2. 5, internal mixing of sea salt components (SO42−–NO3−–NH4+–Na+–Cl−–K+–HNO3–NH3–HCl system) raised the bulk pH to 2.7 ± 0.3 and improved predicted nitric acid partitioning with PM2. 5 components. The results show little effect of sea salt on PM1 pH, but significant effects on PM2. 5 pH. A mean PM1 pH of 1.9 at Pasadena was approximately one unit higher than what we have reported in the southeastern US, despite similar temperature, relative humidity, and sulfate ranges, and is due to higher total nitrate concentrations (nitric acid plus nitrate) relative to sulfate, a situation where particle water is affected by semi-volatile nitrate concentrations. Under these conditions nitric acid partitioning can further promote nitrate formation by increasing aerosol water, which raises pH by dilution, further increasing nitric acid partitioning and resulting in a significant increase in fine particle nitrate and pH. This study provides insights into the complex interactions between particle pH and nitrate in a summertime coastal environment and a contrast to recently reported pH in the eastern US in summer and winter and the eastern Mediterranean. All studies have consistently found highly acidic PM1 with pH generally below 3.
Highlights
Ambient aerosol particles affect human health and climate (Lim et al, 2012; IPCC, 2013), and have many other environmental effects
For the WINTER study, which included measurements over coastal and marine areas, we found that PM1 pH was accurately predicted with only particle-phase SO24−, NO−3, and NH+4, whereas sea salt components had some, but generally small, effects on the prediction of particle pH (Guo et al, 2016)
During the first half of the campaign, 15 to 29 May, daily maximum T was below 26 ◦C and PM1 (AMS) and PM2.5 (PILS-IC) SO24−, NO−3, and NH+4 showed a general decreasing trend (PM2.5 NH+4 data were not available in this period)
Summary
Ambient aerosol particles affect human health and climate (Lim et al, 2012; IPCC, 2013), and have many other environmental effects. Particle pH is linked to all of these by altering the fundamental aerosol properties of particle mass and chemical composition. H. Guo et al.: Fine particle pH and gas–particle phase partitioning of inorganic species. PH directly affects particle mass and composition through altering the partitioning of both semi-volatile inorganic and organic acids between particle and gas phases (Guo et al, 2016). PH affects the nitrogen cycle through gas–particle partitioning of HNO3–NO−3 and NH3–NH+4 , impacting deposition patterns due to large differences in gas versus particle dry deposition rates (Huebert and Robert, 1985; Duyzer, 1994; Schrader and Brummer, 2014). Particle pH is linked to adverse health impacts, both directly and indirectly. Metal mobility affects nutrient distributions with important impacts on photosynthesis productivity (Duce and Tindale, 1991; Meskhidze et al, 2003; Nenes et al, 2011; Ito and Xu, 2014; Myriokefalitakis et al, 2015; Li et al, 2017), carbon sequestration and ocean oxygen levels (Ito et al, 2016)
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