Seasonal variation of aerosol acidity in Nagoya, Japan and factors affecting it

Abstract

Aerosol acidity plays an important role in atmospheric chemistry, but it is difficult to measure directly. To estimate aerosol acidity or in situ pH (pHISF), the forward mode of thermodynamic equilibrium models (Extended Aerosol Inorganic Model IV, E-AIM IV) was applied in this study of data obtained from continuous observations of NH3, HNO3, and ionic constituents of fine particles (<1.9 μm) conducted in Nagoya during 2017–2018. We investigated the seasonal variation of pHISF and the factors affecting it. Measured acidity ([H+]meas), as calculated from pH measurements of extracted samples, showed seasonal variation with a high in summer and low in winter. The results were correlated with the SO42− concentration (R2 = 0.67). The estimated pHISF was 3.68–5.37, with the average of 4.44 ± 0.39. The highest value appeared during a typhoon period when strong southern winds brought huge amounts of sea salt particles (Na+ and Cl−). The seasonal variation of pHISF, which was low in summer and high in winter, is closely correlated with ambient temperature (R2 = 0.83), increasing temperatures by 2.5 °C and decreasing the pH unit by 0.1.Various sensitivity tests conducted using E-AIM IV were applied to assess factors affecting pHISF in Nagoya by changing them one-by-one while fixing the other factors. According to analysis related to temperature effects on pHIS, temperature dependence on the aerosol liquid water content was an important moderator. Results show that the temperature effect was apparent only in cases below the threshold SO42− concentration (ca. 126 nmol m−3 in case of Nagoya). These results were consistent with those obtained from an initial analysis based on observations indicating that the major factors affecting pHIS of fine aerosols in Nagoya were air temperature and SO42− concentration in the particles. Keywords: Ambient air temperature, E-AIM IV model, In situ pH, SO42− concentration