Qinyi Li

Observations of Nitryl Chloride and Modeling its Source and Effect on Ozone in the Planetary Boundary Layer of Southern China

Nitryl chloride (ClNO2) plays potentially important roles in atmospheric chemistry, but its abundance and effect are not fully understood due to the small number of ambient observations of ClNO2 to date. In late autumn 2013, ClNO2 was measured with a chemical ionization mass spectrometer (CIMS) at a mountain top (957 m above sea level) in Hong Kong. During 12 nights with continuous CIMS data, elevated mixing ratios of ClNO2 (>400 parts per trillion by volume) or its precursor N2O5 (>1000 pptv) were observed on six nights, with the highest ever reported ClNO2 (4.7 ppbv, 1 min average) and N2O5 (7.7 ppbv, 1 min average) in one case. Backward particle dispersion calculations driven by winds simulated with a mesoscale meteorological model show that the ClNO2/N2O5-laden air at the high-elevation site was due to transport of urban/industrial pollution north of the site. The highest ClNO2/N2O5 case was observed in a later period of the night and was characterized with extensively processed air and with the presence of nonoceanic chloride. A chemical box model with detailed chlorine chemistry was used to assess the possible impact of the ClNO2 in the well-processed regional plume on next day ozone, as the air mass continued to downwind locations. The results show that the ClNO2 could enhance ozone by 5–16% at the ozone peak or 11–41% daytime ozone production in the following day. This study highlights varying importance of the ClNO2 chemistry in polluted environments and the need to consider this process in photochemical models for prediction of ground-level ozone and haze.

Nighttime NO x loss and ClNO 2 formation in the residual layer of a polluted region: Insights from field measurements and an iterative box model

The heterogeneous reaction of dinitrogen pentoxide (N2O5) on aerosols is an important sink of nitrogen oxides (NOx) in the polluted boundary layer, and the production of nitryl chloride (ClNO2) can have significant effects on the atmospheric oxidative capacity. However, the heterogeneous loss of N2O5 and the formation of ClNO2 are still not well quantified, especially in China. In a previous study, we measured ClNO2 and N2O5 concentrations in several air masses at a high-elevation site in Hong Kong, and found the highest levels ever reported at one night. The present study employed an iterative box model to investigate five N2O5/ClNO2-laden nights. We first estimated the N2O5 uptake coefficient and ClNO2 yield and then calculated the relative importance of N2O5 heterogeneous reactions to NOx loss and the accumulated ClNO2 production over the entire night. The average uptake coefficient was 0.004±0.003, and the average yield was 0.42±0.26. As the air masses aged, the accumulated ClNO2 reached up to 6.0ppbv, indicating significant production of ClNO2 in the polluted air from the Pearl River Delta. ClNO2 formation (N2O5+Cl-), N2O5 hydrolysis (N2O5+H2O), and NO3 reactions with volatile organic compounds (NO3+VOCs) consumed 23%, 27%, and 47% of the produced NO3, respectively, as the average for five nights. A significant portion of the NOx in the air masses (70%±10%) was removed during the night via NO3 reactions with VOCs (~40%) and N2O5 heterogeneous loss (~60%).

Abundance and origin of fine particulate chloride in continental China

Particulate chloride can be converted to nitryl chloride (ClNO2) through heterogeneous reactions with dinitrogen pentoxide (N2O5), and photolysis of ClNO2 affects atmospheric oxidative capacity. However, the characteristics and sources of chloride, especially those with an anthropogenic origin, are poorly characterized, which makes it difficult to evaluate the effects of ClNO2 on radical chemistry and air quality in polluted regions. Aerosol composition data from the literature were compiled to derive the spatial distributions of particulate chloride across China, and hourly aerosol composition data collected at a highly polluted inland urban site in eastern China and at a coastal site in southern China were analysed to gain further insights into non-oceanic sources of chloride. The results show that particulate chloride is concentrated mainly in fine particles and that high chloride loadings are observed in the inland urban areas of northern and western China with higher Cl-/Na+ mass ratios (2.46 to 5.00) than sea water (1.81), indicative of significant contributions from anthropogenic sources. At the inland urban site, the fine chloride displays distinct seasonality, with higher levels in winter and summer. Correlation analysis and positive matrix factorization (PMF) results indicate that coal combustion and residential biomass burning are the main sources (84.8%) of fine chloride in winter, and open biomass burning is the major sources (52.7%) in summer. The transport of plumes from inland polluted areas leads to elevated fine chloride in coastal areas. A simulation with WRF-Chem model confirmed a minor contribution of sea-salt aerosol to fine chloride at the inland site during summer with winds from the East Sea. The widespread sources of chloride, together with abundant NOx and ozone, suggest significant ClNO2 production and subsequent enhanced photochemical processes over China.

Heterogeneous N 2 O 5 uptake coefficient and production yield of ClNO 2 in polluted northern China: Roles of aerosol water content and chemical composition

Heterogeneous uptake of dinitrogen pentoxide (N2O5) and production of nitryl chloride (ClNO2) are important nocturnal atmospheric processes that have significant implications for the production of secondary pollutants. However, the understanding of N2O5 uptake processes and ClNO2 production remains limited, especially in China. This study presents a field investigation of the N2O5 heterogeneous uptake coefficient (γ (N2O5)) and ClNO2 production yield (φ) in a polluted area of northern China during the summer of 2014. The N2O5 uptake coefficient and ClNO2 yield were estimated by using the simultaneously measured ClNO2 and total nitrate in 10 selected cases, which have concurrent increases in the ClNO2 and nitrate concentrations and relatively stable environmental conditions. The determined γ (N2O5) and φ values varied greatly, with an average of 0.022 for γ (N2O5) (±0.012, standard deviation) and 0.34 for φ (±0.28, standard deviation). The variations in γ (N2O5) could not be fully explained by the previously derived parameterizations of N2O5 uptake that consider nitrate, chloride, and the organic coating. Heterogeneous uptake of N2O5 was found to have a strong positive dependence on the relative humidity and aerosol water content. This result suggests that the heterogeneous uptake of N2O5 in Wangdu is governed mainly by the amount of water in the aerosol, and is strongly water limited, which is different from most of the field observations in the US and Europe. The ClNO2 yield estimated from the parameterization was also overestimated comparing to that derived from the observation. The observation-derived φ showed a decreasing trend with an increasing ratio of acetonitrile to carbon monoxide, an indicator of biomass burning emissions, which suggests a possible suppressive effect on the production yield of ClNO2 in the plumes influenced by biomass burning in this region. The findings of this study illustrate the need to improve our understanding and to parameterize the key factors for γ (N2O5) and φ to accurately assess photochemical and haze pollution.

Combined impacts of nitrous acid and nitryl chloride on lower-tropospheric ozone: new module development in WRF-Chem and application to China

Nitrous acid (HONO) and nitryl chloride (ClNO2) – through their photolysis – can have profound effects on the nitrogen cycle and oxidation capacity of the lower troposphere. Previous numerical studies have separately considered and investigated the sources/processes of these compounds and their roles in the fate of reactive nitrogen and the production of ozone (O3), but their combined impact on the chemistry of the lower part of the troposphere has not been addressed yet. In this study, we updated the WRFChem model with the currently known sources and chemistry of HONO and chlorine in a new chemical mechanism (CBMZ_ReNOM), and applied it to a study of the combined effects of HONO and ClNO2 on summertime O3 in the boundary layer over China. We simulated the spatial distributions of HONO, ClNO2, and related compounds at the surface and within the lower troposphere. The results showed that the modeled HONO levels reached up to 800–1800 ppt at the surface (0–30 m) over the North China Plain (NCP), the Yangtze River Delta (YRD), and the Pearl River Delta (PRD) regions and that HONO was concentrated within a 0–200 m layer. In comparison, the simulated surface ClNO2 mixing ratio was around 800–1500 ppt over the NCP, YRD, and central China regions and was predominantly present in a 0–600 m layer. HONO enhanced daytime ROx (OH+HO2+RO2) and O3 at the surface (0– 30 m) by 2.8–4.6 ppt (28–37 %) and 2.9–6.2 ppb (6–13 %), respectively, over the three most developed regions, whereas ClNO2 increased surface O3 in the NCP and YRD regions by 2.4–3.3 ppb (or 5–6 %) and it also had a significant impact (3–6 %) on above-surface O3 within 200–500 m. The combined effects increased surface O3 by 11.5, 13.5, and 13.3 % in the NCP, YRD, and PRD regions, respectively. Over the boundary layer (0–1000 m), the HONO and ClNO2 enhanced O3 by up to 5.1 and 3.2 %, respectively, and their combined effect increased O3 by 7.1–8.9 % in the three regions. The new module noticeably improved O3 predictions at ∼ 900 monitoring stations throughout China by reducing the mean bias from −4.3 to 0.1 ppb. Our study suggests the importance of considering these reactive nitrogen species simultaneously into chemical transport models to better simulate the formation of summertime O3 in polluted regions.