Effect of ammonia on fine-particle pH in agricultural regions of China: comparison between urban and rural sites

Particle acidity is a fundamental property that affects atmospheric particulate chemistry. Synchronous online monitoring was performed at two urban sites, Zhengzhou (U-ZZ) and Anyang (U-AY), and three rural sites, Anyang (R-AY), Xinxiang (R-XX), and Puyang (R-PY) in Henan Province, during a haze episode to investigate the pH value and its driving factors in the agricultural regions of China. The pH values of particles calculated by ISORROPIA-II model at rural sites were slightly higher than those at urban sites, with the median (min-max) values of 5.2 (4.8-6.9, R-PY), 5.1 (4.7-6.5, R-AY), 4.9 (4.1-6.8, R-XX), 4.8 (3.9-5.9, U-AY), and 4.5 (3.8-5.2, U-ZZ). Sensitivity tests show that TNHx (total ammonium, gas + aerosol) followed by total sulfate were the most important factors that influenced the predicted pH. Generally, particle pH increased with a cation increase and decreases in anions, temperature, and relative humidity. Similar pH values (similar to 3.0) at the required NHx concentrations for the five sites indicated that the presence of excess NHx was likely important for the lower acidity of PM2.5 during the severe haze episodes in this region. Moreover, the concentrations of excess NHx may drive the higher pH values at rural sites, because of the higher excess NHx concentrations at rural sites than those at urban sites. The underlying influence of regional transport on local particle pH cannot be neglected, as it differs the chemical components of PM2.5 and meteorological conditions. Air masses transported from rural and agricultural regions may enhance the particle pH value in urban aerosols given the high pH of particles and high ammonia levels. These results suggest that ammonia is urgently needed to be involved in the regional strategy for the improvement of air quality in China.