Elucidating Decade-Long Trends and Diurnal Patterns in Aerosol Acidity in Shanghai

Aerosol acidity is a critical factor affecting atmospheric chemistry. Here, we present a study on annual, monthly, and daily variations in PM_2.5 pH in Shanghai during 2010–2020. With the effective control of SO₂ emissions, the NO₂/SO₂ ratio increased from 1.26 in 2010 to 5.07 in 2020 and the NO₃⁻/SO₄²⁻ ratio increased from 0.68 to 1.49. Aerosol pH decreased from 3.27 in 2010 to 2.93 in 2020, regardless of great achievement in reducing industrial SO₂ and NOx emissions. These findings suggest that aerosol acidity might not be significantly reduced in response to the control of SO₂ and NOx emissions. The monthly variation in pH values exhibited a V-shape trend, mainly attributable to aerosol compositions and temperature. Atmospheric NH₃ plays the decisive role in buffering particle acidity, whereas Ca²⁺ and K⁺ are important acidity buffers, and the distinct pH decline during 2010–2016 was associated with the reduction of Ca²⁺ and K⁺ while both temperature and SO₄²⁻ were important drivers in winter. Sensitivity tests show that pH increases with the increasing relative humidity in summer while it is not sensitive to relative humidity in winter due to proportional increases in <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mrow><mi mathvariant="normal">H</mi></mrow><mrow><mi mathvariant="normal">a</mi><mi mathvariant="normal">i</mi><mi mathvariant="normal">r</mi></mrow><mrow><mo>+</mo></mrow></msubsup></mrow></semantics></math></inline-formula> and aerosol liquid water content (ALWC). Our results suggest that reducing NOx emissions in Shanghai will not significantly affect PM_2.5 acidity in winter.