Fast heterogeneous N<sub>2</sub>O<sub>5</sub> uptake and ClNO<sub>2</sub> production in power plant and industrial plumes observed in the nocturnal residual layer over the North China Plain

Abstract

Abstract. Dinitrogen pentoxide (N 2 O 5 ) and nitryl chloride (ClNO 2 ) 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 NO x recycling upon photolysis of ClNO 2 . To better understand the roles of N 2 O 5 and ClNO 2 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 ClNO 2 plumes were frequently observed in the nocturnal residual layer with a maximum mixing ratio of 2.1 ppbv (1 min), whilst N 2 O 5 was typically present at very low levels (< 30 pptv), indicating fast heterogeneous N 2 O 5 hydrolysis. Combined analyses of chemical characteristics and backward trajectories indicated that the ClNO 2 -laden air was caused by the transport of NO xrich plumes from the coal-fired industry and power plants in the NCP. The heterogeneous N 2 O 5 uptake coefficient (γ ) and ClNO 2 yield (φ) were estimated from steady-state analysis and observed growth rate of ClNO 2 . 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 N 2 O 5 reactions dominated the nocturnal NO x 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 N 2 O 5 reactivity and chlorine activation in the NCP, and their unique and universal roles in fine aerosol formation and NO x transformation, and thus their potential impacts on regional haze pollution in northern China.

ClNO2 Yields From Aircraft Measurements During the 2015 WINTER Campaign and Critical Evaluation of the Current Parameterization

“…A previous methods comparison of data from winter 2011 in Colorado also showed similarity between Methods 2 and 3, with an average ϕ (ClNO 2 ) value of 0.05 ± 0.15 using Method 2 (Riedel et al, ) and a mode of ~0.06 derived from a box model similar to the one used here (Wagner et al, ). An additional comparison of Method 2 and the right‐hand side of (calculated from the steady state approach and observed ClNO 2 production rates) by Z. Wang, Wang, Tham, et al () found absolute agreement within 0.03 for the campaign average ϕ (ClNO 2 ) value at a ground site in northern China during summer 2017.…”

“…In this section, four methods are applied to WINTER data in an attempt to provide a direct comparison and evaluation of methods commonly used in past literature. Due to the difference in aircraft and ground‐based observational data, ϕ (ClNO 2 ) derivations using steady state‐derived in (used by Tham et al, ; X. Wang, Wang, Xue, et al, ; Z. Wang, Wang, Tham, et al, ) and the ratio of ClNO 2 to p NO 3 − production rates (used by Wang et al, ) could not be compared to box model results. Each additional method tested here is described below and assumes that heterogeneous production is the only source of ClNO 2 , and that ClNO 2 is stable overnight.…”

“…Regardless of particle chloride differences, both predicted median values that were factors of 2 to 4.3 larger than other methods, and outside the range of uncertainties associated with Methods 1 and 3 (Figure , right). All previous field studies to have made this comparison between data‐based methods and the laboratory‐based parameterization have shown an overprediction in ϕ (ClNO 2 ) by the parameterization (Riedel et al, ; Tham et al, ; Thornton et al, ; Wagner et al, ; X. Wang, Wang, Xue, et al, ; Z. Wang, Wang, Tham, et al, ). ClNO 2 yields derived from ambient seawater samples by Ryder et al () have also resulted in values lower than the parameterized equivalents.…”

“…In addition, these field‐derived ClNO 2 yields are lower than those predicted by the laboratory‐derived parameterization, which are based on aerosol chloride and water concentrations alone. This disagreement is found in every study that has made the comparison (Riedel et al, ; Ryder et al, ; Tham et al, ; Thornton et al, ; Wagner et al, ; Z. Wang, Wang, Tham, et al, ; X. Wang, Wang, Xue, et al, ), which suggests that the current mechanistic understanding of ClNO 2 production may be complicated by the presence of additional aerosol‐phase components or an undefined loss process that consumes ClNO 2 (e.g., Roberts et al, ). As ClNO 2 formation continues to be incorporated into 3‐D models (e.g., Sherwen et al, ), further investigation into the source(s) of these field‐model discrepancies is required to better understand and improve the predictive capabilities of ClNO 2 formation in the wintertime RL.…”

Summertime fine particulate nitrate pollution in the North China Plain: Increasing trends, formation mechanisms, and implications for control policy

“…Previous air pollution control in China 10 primarily focused on the reduction of anthropogenic emissions of sulfur dioxide (SO 2 ), given the dominant contributions of sulfate (SO 4 2-) to the PM 2.5 and acid depositions (Hao et al, 2000 and2007). In the last decade, about 75% reduction of SO 2 emissions in China has successfully resulted in decreases in the ambient levels of both SO 2 and aerosol SO 4 2-in fast-developing regions including the NCP Li et al, 2017). In comparison,…”