Supplementary material to "Mutual promotion effect between aerosol particle liquid water and nitrate formation lead to severe nitrate-dominated particulate matter pollution and low visibility"

Abstract. As has been the case in North America and Western Europe, the SO2 emissions substantially reduced in North China Plain (NCP) in recent years. A dichotomy of reductions in SO2 and NOx concentrations result in the frequent occurrences of nitrate (pNO3−)-dominated particulate matter pollution over NCP. In this study, we observed a polluted episode with the nitrate mass fraction in non-refractory PM1 (NR-PM1) up to 44 % during wintertime in Beijing. Based on this typical pNO3−-dominated haze event, the linkage between aerosol water uptake and pNO3− formation, further impacting on visibility degradation, have been investigated based on field observations and theoretical calculations. During haze development, as ambient relative humidity (RH) increased from ~ 10 % up to 70 %, the aerosol particle liquid water increased from ~ 1 μg/m3 at the beginning to ~ 75 μg/m3 at the fully-developed haze period. Without considering the water uptake, the particle surface area and the volume concentrations increased by a factor of 4.1 and 4.8, respectively, during the development of haze event. Taking water uptake into account, the wet particle surface area and volume concentrations enhanced by a factor of 4.7 and 5.8, respectively. As a consequence, the hygroscopic growth of particles facilitated the condensational loss of dinitrogen pentoxide (N2O5) and nitric acid (HNO3) to particles contributing pNO3−. From the beginning to the fully-developed haze, the condensational loss of N2O5 increased by a factor of 20 when only considering aerosol surface area and volume of dry particles, while increasing by a factor of 25 considering extra surface area and volume due to water uptake. Similarly, the condensational loss of HNO3 increased by a factor of 2.7~2.9 and 3.1~3.5 for dry and wet aerosol surface area and volume from the beginning to the fully-developed haze period. Above results demonstrated that the pNO3− formation is further enhanced by aerosol water uptake with elevated ambient RH during haze development, in turn, facilitating the aerosol taking up water due to the hygroscopicity of nitrate salt. Such mutual promotion effect between aerosol particle liquid water and nitrate formation can rapidly degrade air quality and halve visibility within one day. Reduction of nitrogen-containing gaseous precursors, e.g., by control of traffic emissions, is essential in mitigating severe haze events in NCP.