Effects of NH 3 and alkaline metals on the formation of particulate sulfate and nitrate in wintertime Beijing.

Sulfate and nitrate from secondary reactions remain as the most abundant inorganic species in atmospheric particle matter (PM). Their formation is initiated by oxidation (either in gas phase or particle phase), followed by neutralization reaction primarily by NH ₃ , or by other alkaline species such as alkaline metal ions if available. The different roles of NH ₃ and metal ions in neutralizing H ₂ SO ₄ or HNO ₃ , however, are seldom investigated. Here we conducted semi-continuous measurements of SO ₄ ^2- , NO ₃ ^- , NH ₄ ⁺ , and their gaseous precursors, as well as alkaline metal ions (Na ⁺ , K ⁺ , Ca ²⁺ , and Mg ²⁺ ) in wintertime Beijing. Analysis of aerosol acidity (estimated from a thermodynamic model) indicated that preferable sulfate formation was related to low pH conditions, while high pH conditions promote nitrate formation. Data in different mass fraction ranges of alkaline metal ions showed that in some ranges the role of NH ₃ was replaced by alkaline metal ions in the neutralization reaction of H ₂ SO ₄ and HNO ₃ to form particulate SO ₄ ^2- and NO ₃ ^- . The relationships between mass fractions of SO ₄ ^2- and NO ₃ ^- in those ranges of different alkaline metal ion content also suggested that alkaline metal ions participate in the competing neutralization reaction of sulfate and nitrate. The implication of the current study is that in some regions the chemistry to incorporate sulfur and nitrogen into particle phase might be largely affected by desert/fugitive dust and sea salt, besides NH ₃ . This implication is particularly relevant in coastal China and those areas with strong influence of dust storm in the North China Plain (NCP), both of which host a number of megacities with deteriorating air quality.
Competing Interests: Declaration of competing interest The authors declare no competing financial interest.
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