10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Aerosol Liquid Water Driven by Anthropogenic Inorganic Salts: Implying Its Key Role in the Haze Formation over North China Plain
ZHIJUN WU, Yu Wang, Keding Lu, Song Guo, Limin Zeng, Yuanhang Zhang, Min Hu, Peking University, China
Abstract Number: 420 Working Group: Air Quality in Megacities: from Sources to Control
Abstract The aerosol liquid water content (ALWC) exhibits a profound impact on both air quality degradation and climate change. In this study, the ALWC was calculated based on (1) a thermodynamic model using long-term filter-based chemical composition data and (2) measured high-temporal-resolution size-resolved hygroscopic growth factors. The ALWC increased from 2% to 74% accompanying with the secondary inorganic faction in dry PM2.5 rose from 24% to 55% and ambient relative humidity (RH) increased from 15% to 83% in the atmosphere over Beijing. During haze episodes, simultaneously elevated ambient RH levels and anthropogenic secondary inorganic mass concentrations resulted in an abundant ALWC. The condensed water could act as an efficient medium for multiphase reactions, thereby facilitating the transformation of reactive gaseous pollutants into particles and accelerating the formation of heavy haze. In particular, a greater ALWC is observed during the nighttime relative to the daytime. Unexpectedly, the secondary inorganic fraction in PM2.5 increased with an increase in the ambient RH, which is a meteorological parameter independent of anthropogenic activities, indicating the presence of a feedback mechanism driven by Henry’s law and thermodynamic equilibrium. As a particle uptakes anthropogenic compounds such as SO2 and N2O5, its inorganic mass increases, thereby driving an increase in the water uptake. Then, a greater ALWC reduces the aerosol aqueous solution concentration, allowing for a greater uptake of inorganic compounds and completing the feedback mechanism. The ALWC was well correlated with the mass concentrations of both nitrate and sulfate, indicating that both nitrate and sulfate salts play key roles in determining the ALWC. Coincident with a significant reduction in SO2 emissions throughout China, nitrates will become a dominant anthropogenic inorganic salt driving the ALWC. Thus, the abundance of the ALWC and its effects on the aerosol chemistry and climate should be reconsidered.