Abstract
High levels of ammonia (NH3) have been suggested to elevate ambient particle pH levels to near neutral acidity (pH = 7), a condition that promotes rapid SO2 oxidation by NO2 to form aerosol sulfate concentration consistent with “London fog” levels. This postulation is tested using aerosol data from representative sites around the world to conduct a thorough thermodynamic analysis of aerosol pH and its sensitivity to NH3 levels. We find that particle pH, regardless of ammonia levels, is always acidic even for the unusually high NH3 levels found in Beijing (pH = 4.5) and Xi’an (pH = 5), locations where sulfate production from NOx is proposed. Therefore, major sulfate oxidation through a NO2-mediated pathway is not likely in China, or any other region of the world (e.g., US, Mediterranean) where the aerosol is consistently more acidic. The limited alkalinity from the carbonate buffer in dust and seasalt can provide the only likely set of conditions where NO2-mediated oxidation of SO2 outcompetes with other well-established pathways. The mildly acidic levels associated with excessive amounts of ammonia can promote high rates of SO2 oxidation through transition metal chemistry, this may be an alternative important aerosol chemical contributor to the extreme pollution events.
Highlights
PH is a fundamental particle property that affects aerosol formation, composition, toxicity and nutrient delivery[1,2,3,4,5,6]
Et al.[8] and Cheng, et al.[9] argue that very high levels of NH3 from intense agriculture can sufficiently elevate pH in fine mode aerosol (PM1 and PM2.5) to promote rapid sulfate formation from NO2 oxidation of SO2. We explore this by carrying out a thorough thermodynamic analysis with the ISORROPIA-II model[18] for conditions of aerosol- and gas-phase constituents that
To understand the major drivers of aerosol acidity, we explore pH levels for aerosol of increasing chemical complexity, and its sensitivity to NH3 levels found throughout the world; we focus on two well-characterized “extremes” of anthropogenic influence: the relatively clean southeastern US and the heavily polluted regions of Beijing and Xi’an, China
Summary
PH is a fundamental particle property that affects aerosol formation, composition, toxicity and nutrient delivery[1,2,3,4,5,6].
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