Abstract
Abstract. As has been the case in North America and western Europe, the SO2 emissions have substantially reduced in the North China Plain (NCP) in recent years. Differential rates of reduction in SO2 and NOx concentrations result in the frequent occurrence of particulate matter pollution dominated by nitrate (pNO3-) over the NCP. In this study, we observed a polluted episode with the particulate nitrate mass fraction in nonrefractory PM1 (NR-PM1) being up to 44 % during wintertime in Beijing. Based on this typical pNO3--dominated haze event, the linkage between aerosol water uptake and pNO3- 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 ∼10 % to 70 %, the aerosol particle liquid water increased from ∼1 µg m−3 at the beginning to ∼75 µg 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 N2O5 over particles. 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 when considering extra surface area and volume due to water uptake. Furthermore, aerosol liquid water favored the thermodynamic equilibrium of HNO3 in the particle phase under the supersaturated HNO3 and NH3 in the atmosphere. All the above results demonstrated that pNO3- 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.
Highlights
Aerosol particle hygroscopicity plays an important role in air quality deterioration and cloud formation (Yu and Luo, 2009; Fitzgerald, 1973; Kreidenweis and Asa-Awuku, 2014; Wang and Chen, 2019; McFiggans et al, 2006) and can directly influence aerosol measurements (Chen et al, 2018a)
In atmospheric environments influenced by anthropogenic activities, particulate secondary inorganic compounds are often dominated by particulate sulfate and nitrate (Heintzenberg, 1989), which originate from the oxidation of sulfur dioxide (SO2) and nitrogen oxides (NOx) via multiple chemical pathways (Calvert et al, 1985; Cheng et al, 2016; Wang et al, 2016; Gen et al, 2019a, b)
Nitrate was the major contributor to the secondary inorganic components and accounted for up to ∼ 44 % of nonrefractory PM1 (NR-PM1) mass, while sulfate contributed for ∼ 12 % on average
Summary
Aerosol particle hygroscopicity plays an important role in air quality deterioration and cloud formation (Yu and Luo, 2009; Fitzgerald, 1973; Kreidenweis and Asa-Awuku, 2014; Wang and Chen, 2019; McFiggans et al, 2006) and can directly influence aerosol measurements (Chen et al, 2018a). Increased aerosol particle liquid water could accelerate secondary inorganic and organic aerosol formation by decreasing the kinetic limitation of mass transfer of gaseous precursors and providing more of a medium for multiphase reactions (Mozurkewich and Calvert, 1988; Cheng et al, 2016; Wang et al, 2016; Ervens et al, 2011; Kolb et al, 2010).
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