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. As has been the case in North America and western Europe,the SO2 emissions have substantially reduced in the North China Plain (NCP) inrecent years. Differential rates of reduction in SO2 and NOx concentrations result in the frequent occurrence of particulate matter pollution dominated by nitrate( p NO 3 - ) over the NCP. In thisstudy, we observed a polluted episode with the particulate nitrate massfraction in nonrefractory PM 1 (NR-PM 1 ) being up to 44 % duringwintertime in Beijing. Based on this typical p NO 3 - -dominated hazeevent, the linkage between aerosol water uptake and p NO 3 - enhancement, further impacting on visibility degradation, has beeninvestigated based on field observations and theoretical calculations.During haze development, as ambient relative humidity (RH) increased from ∼10  % to 70 %, the aerosol particle liquid waterincreased from ∼1 µg m−3 at the beginning to ∼75 µg m−3 in the fully developed haze period. Theaerosol liquid water further increased the aerosol surface area and volume,enhancing the condensational loss of N2O5 over particles. From thebeginning to the fully developed haze, the condensational loss of N2O5 increased by a factor of 20 when only considering aerosolsurface area and volume of dry particles, while increasing by a factor of 25 whenconsidering extra surface area and volume due to water uptake. Furthermore,aerosol liquid water favored the thermodynamic equilibrium of HNO3 inthe particle phase under the supersaturated HNO3 and NH3 in theatmosphere. All the above results demonstrated that p NO 3 - isenhanced by aerosol water uptake with elevated ambient RH during hazedevelopment, in turn facilitating the aerosol take-up of water due to thehygroscopicity of particulate nitrate salt. Such mutual promotion betweenaerosol particle liquid water and particulate nitrate enhancement canrapidly degrade air quality and halve visibility within 1 d. Reductionof nitrogen-containing gaseous precursors, e.g., by control of trafficemissions, is essential in mitigating severe haze events in the NCP.

atmospheric chemistry and physics

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