Climatology of aerosol pH and its controlling factors at the Melpitz continental background site in central Europe

Abstract. Aerosol acidity has importance for the chemical and physical properties of atmospheric aerosol particles and for many processes that affect their transformations and fate. Here, we characterize trends in aerosol pH and its controlling factors over the period of 2010 – 2019 at the Melpitz research station in eastern Germany, a continental background site in central Europe. Aerosol liquid water content (ALWC) decreased by 50 % during the analyzed time period in response to decreasing sulfate and nitrate. Aerosol pH exhibited an increase of 0.06 units per year, a trend that was distinct from other regions. Seasonal analysis showed strong variability in factors controlling aerosol pH. Temperature, the most important factor driving pH variability overall, was most important in summer (responsible for 51 % of pH variability) and less important during spring and fall (22 % and 27 %, respectively). NH₃, the second most important factor contributing to pH variability overall (29 %), was most important during winter (38 %) and far less important during summer (15 %). Aerosol chemistry in Melpitz is influenced by the high buffering capacity contributed by NH₄⁺/NH₃ and, to a lesser degree, NO₃^-/HNO₃. Thermodynamic analysis of the aerosol system shows that secondary inorganic aerosol formation is most frequently HNO₃ limited, suggesting that NO_x controls would be more effective than NH₃ controls in reducing PM mass concentrations. However, the non-linear response of gas-phase HNO₃ and aerosol NO₃^- to NO_x emissions in the region highlights the challenge associated with PM reductions needed to attain new air quality standards in this region.