# Mutual promotion between aerosol particle liquid water and particulate nitrate enhancement leads to severe nitrate-dominated particulate matter pollution and low visibility

As has been the case in North America and western Europe, the inline-formulaSO2 emissions have substantially reduced in the North China Plain (NCP) in recent years. Differential rates of reduction in inline-formulaSO2 and inline-formulaNOx concentrations result in the frequent occurrence of particulate matter pollution dominated by nitrate (inline-formula $M4inlinescrollmathmlchemp{\mathrm{normal NO}}_{normal 3}^{-}$ 32pt16ptsvg-formulamathimg80301589dd125f3933fea08fd04566b2 acp-20-2161-2020-ie00001.svg32pt16ptacp-20-2161-2020-ie00001.png ) over the NCP. In this study, we observed a polluted episode with the particulate nitrate mass fraction in nonrefractory PMinline-formula1 (NR-PMinline-formula1) being up to 44 % during wintertime in Beijing. Based on this typical inline-formula $M7inlinescrollmathmlchemp{\mathrm{normal NO}}_{normal 3}^{-}$ 32pt16ptsvg-formulamathimg97eee6abb7442c3dca576c5397fcf50b acp-20-2161-2020-ie00002.svg32pt16ptacp-20-2161-2020-ie00002.png -dominated haze event, the linkage between aerosol water uptake and inline-formula $M8inlinescrollmathmlchemp{\mathrm{normal NO}}_{normal 3}^{-}$ 32pt16ptsvg-formulamathimgbf993075addbdcb9c5e94dd00bb6267c acp-20-2161-2020-ie00003.svg32pt16ptacp-20-2161-2020-ie00003.png enhancement, further impacting on visibility degradation, has been investigated based on field observations and theoretical calculations. During haze development, as ambient relative humidity (RH) increased from inline-formula∼10 % to 70 %, the aerosol particle liquid water increased from inline-formula∼1inline-formulaµg m−3 at the beginning to inline-formula∼75inline-formulaµg m−3 in the fully developed haze period. The aerosol liquid water further increased the aerosol surface area and volume, enhancing the condensational loss of inline-formulaN2O5 over particles. From the beginning to the fully developed haze, the condensational loss of inline-formulaN2O5 increased by a factor of 20 when only considering aerosol surface area and volume of dry particles, while increasing by a factor of 25 when considering extra surface area and volume due to water uptake. Furthermore, aerosol liquid water favored the thermodynamic equilibrium of inline-formulaHNO3 in the particle phase under the supersaturated inline-formulaHNO3 and inline-formulaNH3 in the atmosphere. All the above results demonstrated that inline-formula $M19inlinescrollmathmlchemp{\mathrm{normal NO}}_{normal 3}^{-}$ 32pt16ptsvg-formulamathimg90de1192fbdc093890377c47914f9681 acp-20-2161-2020-ie00004.svg32pt16ptacp-20-2161-2020-ie00004.png is enhanced by aerosol water uptake with elevated ambient RH during haze development, in turn facilitating the aerosol take-up of water due to the hygroscopicity of particulate nitrate salt. Such mutual promotion between aerosol particle liquid water and particulate nitrate enhancement can rapidly degrade air quality and halve visibility within 1 d. Reduction of nitrogen-containing gaseous precursors, e.g., by control of traffic emissions, is essential in mitigating severe haze events in the NCP.

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Wang, Yu / Chen, Ying / Wu, Zhijun / et al: Mutual promotion between aerosol particle liquid water and particulate nitrate enhancement leads to severe nitrate-dominated particulate matter pollution and low visibility. 2020. Copernicus Publications.

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