Abstract
Abstract. Dinitrogen pentoxide (N2O5) and nitryl chloride (ClNO2) are key species in nocturnal tropospheric chemistry and have significant effects on particulate nitrate formation and the following day's photochemistry through chlorine radical production and NOx recycling upon photolysis of ClNO2. To better understand the roles of N2O5 and ClNO2 in the high-aerosol-loading environment of northern China, an intensive field study was carried out at a high-altitude site (Mt. Tai, 1465 m a.s.l.) in the North China Plain (NCP) during the summer of 2014. Elevated ClNO2 plumes were frequently observed in the nocturnal residual layer with a maximum mixing ratio of 2.1 ppbv (1 min), whilst N2O5 was typically present at very low levels (< 30 pptv), indicating fast heterogeneous N2O5 hydrolysis. Combined analyses of chemical characteristics and backward trajectories indicated that the ClNO2-laden air was caused by the transport of NOx-rich plumes from the coal-fired industry and power plants in the NCP. The heterogeneous N2O5 uptake coefficient (γ) and ClNO2 yield (ϕ) were estimated from steady-state analysis and observed growth rate of ClNO2. The derived γ and ϕ exhibited high variability, with means of 0.061 ± 0.025 and 0.28 ± 0.24, respectively. These values are higher than those derived from previous laboratory and field studies in other regions and cannot be well characterized by model parameterizations. Fast heterogeneous N2O5 reactions dominated the nocturnal NOx loss in the residual layer over this region and contributed to substantial nitrate formation of up to 17 µg m−3. The estimated nocturnal nitrate formation rates ranged from 0.2 to 4.8 µg m−3 h−1 in various plumes, with a mean of 2.2 ± 1.4 µg m−3 h−1. The results demonstrate the significance of heterogeneous N2O5 reactivity and chlorine activation in the NCP, and their unique and universal roles in fine aerosol formation and NOx transformation, and thus their potential impacts on regional haze pollution in northern China.
Highlights
Nitrogen oxides (NOx = NO + NO2) play central roles in the oxidative capacity of the atmosphere and photochemical air pollution
The low N2O5 mixing ratios observed during most of the nights suggest a rapid loss of N2O5, which is consistent with the observed high aerosol surface area (Sa), which varied from ∼ 100 to 7800 μm2 cm−3 with a mean value of 1440 μm2 cm−3
The results revealed the frequently elevated ClNO2 mixing ratios and efficient ClNO2 yields (0.46 ± 0.24) resulting from power plant and industrial plumes in the residual layer
Summary
Nitrogen oxides (NOx = NO + NO2) play central roles in the oxidative capacity of the atmosphere and photochemical air pollution. Dinitrogen pentoxide (N2O5) is an important reactive intermediate in the oxidation of NOx and exists in rapid thermal equilibrium with nitrate radical (NO3) formed via the reaction between NO2 and O3. The heterogeneous hydrolysis of N2O5 has been recognized as a key step in nocturnal NOx removal and can affect regional air quality by regulating the reactive nitrogen budget and nitrate aerosol formation (e.g., Brown et al, 2006; Abbatt et al, 2012). The heterogeneous reaction of N2O5 on and within atmospheric aerosols, fog, or cloud droplets produces soluble nitrate (HNO3 or NO−3 ) and nitryl chloride (ClNO2) when chloride is available in the aerosols (Finlayson-Pitts et al, 1989). Wang et al.: Fast heterogeneous N2O5 uptake and ClNO2 production in power plant and industrial plumes probability ( known as the uptake coefficient γN2O5 ), and it can be described by the following expression when the gasphase diffusive effect is negligible
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