Nighttime NOx loss and ClNO2 formation in the residual layer of a polluted region: Insights from field measurements and an iterative box model

Double-Edged Role of VOCs Reduction in Nitrate Formation: Insights from Observations during the China International Import Expo 2018

Aerosol nitrate (NO3-) constitutes a significant component of fine particles in China. Prioritizing the control of volatile organic compounds (VOCs) is a crucial step toward achieving clean air, yet its impact on NO3- pollution remains inadequately understood. Here, we examined the role of VOCs in NO3- formation by combining comprehensive field measurements conducted during the China International Import Expo (CIIE) in Shanghai (from 10 October to 22 November 2018) and multiphase chemical modeling. Despite a decline in primary pollutants during the CIIE, NO3- levels increased compared to pre-CIIE and post-CIIE─NO3- concentrations decreased in the daytime (by -10 and -26%) while increasing in the nighttime (by 8 and 30%). Analysis of the observations and backward trajectory indicates that the diurnal variation in NO3- was mainly attributed to local chemistry rather than meteorological conditions. Decreasing VOCs lowered the daytime NO3- production by reducing the hydroxyl radical level, whereas the greater VOCs reduction at night than that in the daytime increased the nitrate radical level, thereby promoting the nocturnal NO3- production. These results reveal the double-edged role of VOCs in NO3- formation, underscoring the need for transferring large VOC-emitting enterprises from the daytime to the nighttime, which should be considered in formulating corresponding policies.

Changes in nitrate accumulation mechanisms as PM2.5 levels increase on the North China Plain: A perspective from the dual isotopic compositions of nitrate

Nitrate (NO₃^-) has become recognized as the most important water-soluble ion in fine particulate (PM_2.5), and has been proposed as a driving factor for regional haze formation. However, nitrate formation mechanisms are still poorly understood. In this study, PM_2.5 samples were collected from September 2017 to August 2018 in Shijiazhuang, a city located on the North China Plain, and NO₃^-concentration, δ¹⁸O-NO₃^- and δ¹⁵N-NO₃^- values in PM_2.5 were analyzed. NO₃^- concentrations increased as PM_2.5 levels increased during both polluted and non-polluted days over the entire year. δ¹⁸O-NO₃^- values during cold months (63.5-103‰) were higher than those during warm months (50.3-85.4‰), these results suggested that the nitrate formation pathways shifted from the NO₂ + OH (P_OH) in warm months to the N₂O₅ + H₂O (P_N2O5) and NO₃ + VOCs (P_NO3) pathways in cold months. Especially during cold months, δ¹⁸O-NO₃^- values increased from 65.2-79.9‰ to 80.7-96.2‰ when PM_2.5 increased from ∼25 to >100 μg/m³, but when PM_2.5 > 100 μg/m³, there were relatively small variations in δ¹⁸O-NO₃^-. These results suggested that nitrate formation pathways changed from P_OH to P_N2O5 and P_NO3 pathways when PM_2.5 < 100 μg/m³, but that P_N2O5 and P_NO3 dominated nitrate production when PM_2.5 > 100 μg/m³. Higher δ¹⁵N-NO₃^- values in warm months (-11.8-13.8‰) than in cold months (-0.7-22.6‰) may be attributed to differences in NO_x emission sources and nitrogen isotopic fractionation among NO_x and NO₃^-. These results provide information on the dual isotopic compositions of nitrate to understand nitrate formation pathways under different PM_2.5 levels.

Field Determination of Nitrate Formation Pathway in Winter Beijing

Particulate nitrate (pNO₃^-) has often been found to be the major component of fine particles in urban air-sheds in China, the United States, and Europe during winter haze episodes in recent years. However, there is a lack of knowledge regarding the experimentally determined contribution of different chemical pathways to the formation of pNO₃^-. Here, for the first time, we combine ground and tall-tower observations to quantify the chemical formation of pNO₃^- using observationally constrained model approach based on direct observations of OH and N₂O₅ for the urban air-shed. We find that the gas-phase oxidation pathway (OH+NO₂) during the daytime is the dominant channel over the nocturnal uptake of N₂O₅ during pollution episodes, with percentages of 74% in urban areas and 76% in suburban areas. This is quite different from previous studies in some regions of the US, in which the uptake of N₂O₅ was concluded to account for a larger contribution in winter. These results indicate that the driving factor of nitrate pollution in Beijing and different regions of the US is different, as are the mitigation strategies for particulate nitrate.

Stable oxygen isotope constraints on nitrate formation in Beijing in springtime

Rapid accumulation of aerosol nitrate (NO₃^-) contributes to haze pollution; however, studies quantifying NO₃^- formation mechanisms remain scarce. To explore aerosol nitrate formation pathways, total suspended particulate (TSP) samples were collected in Beijing during the spring of 2013, and the concentration of NO₃^- and δ¹⁸O- NO₃^- value were analyzed. The NO₃^- concentrations on polluted days (PD) were higher than those on non-polluted days (NPD). Furthermore, higher δ¹⁸O- NO₃^- values were observed on PD (86.8 ± 8.1‰) as compared with NPD (73.7 ± 11.0‰) suggest that more nitrate was produced by pathways with relative high δ¹⁸O-HNO₃ values during PD. Based on the calculated δ¹⁸O-HNO₃ values from different formation pathways and the observed δ¹⁸O- NO₃^- values, the possible fractional contributions of HNO₃ formed via various pathways to TSP NO₃^- were estimated using the Bayesian isotope mixing model. The δ¹⁸O- NO₃^- constrained calculations suggest that the pathways of N₂O₅ + H₂O/Cl^-, NO₃ + VOCs, and ClNO₃ + H₂O possibly contributed 53%-89% to nitrate production during PD. During NPD, the NO₂ + OH pathway produced 37%-69% of the NO₃^-. Using the δ¹⁸O- NO₃^- value combined with the isotope mixing model is a promising approach for exploring NO₃^- formation pathways.

Wintertime N2O5 uptake coefficients over the North China Plain

The heterogeneous hydrolysis of dinitrogen pentoxide (N₂O₅) plays an important role in regulating NO_x. The N₂O₅ uptake coefficient, γ(N₂O₅), was determined using an iterative box model that was constrained to observational data obtained in suburban Beijing from February to March 2016. The box model determined 2289 individual γ(N₂O₅) values that varied from <0.001 to 0.02 with an average value of 0.0046 ± 0.0039 (and a median value of 0.0032). We found the derived winter γ(N₂O₅) values in Beijing were relatively low as compared to values reported in previous field studies conducted during winter in Hong Kong (average value of 0.014) and the eastern U.S. coast (median value of 0.0143). In our study, field evidence of the suppression of γ(N₂O₅) values due to pNO₃^- content, organics and the enhancement by aerosol liquid water content (ALWC) is in line with previous laboratory study results. Low ALWC, high pNO₃^- content, and particle morphology (inorganic core with an organic shell) accounted for the low γ(N₂O₅) values in the North China Plain (NCP) during wintertime. The field-derived γ(N₂O₅) values are well reproduced by a revised parameterization method, which includes the aerosol size distribution, ALWC, nitrate and organic coating, suggesting the feasibility of comprehensive parameterization in the NCP during wintertime.

Fast heterogeneous loss of N2O5 leads to significant nighttime NOx removal and nitrate aerosol formation at a coastal background environment of southern China

Nitrate radical (NO₃) and dinitrogen pentoxide (N₂O₅) play crucial roles in the nocturnal atmosphere. To quantify their impacts, we deployed a thermal-dissociation chemical ionization mass spectrometry (TD-CIMS), to measure their concentration, as well as ClNO₂ at a coastal background site in the southern of China during the late autumn of 2012. Moderate levels of NO₃, N₂O₅ and high concentration of ClNO₂ were observed during the study period, indicating active NO_x-O₃ chemistry in the region. Distinct features of NO₃, N₂O₅ and ClNO₂ mixing ratios were observed in different airmasses. Further analysis revealed that the N₂O₅ heterogeneous reaction was the dominant loss of N₂O₅ and NO₃, which showed higher loss rate compared to that in other coastal sites. Especially, the N₂O₅ loss rates could reach up to 0.0139 s^-1 when airmasses went across the sea. The fast heterogeneous loss of N₂O₅ led to rapid NO_x loss which could be comparable to the daytime process through NO₂ oxidization by OH, and on the other hand, to rapid nitrate aerosol formation. In summary, our results revealed that the N₂O₅ hydrolysis could play significant roles in regulating the air quality by reducing NO_x but forming nitrate aerosols.

ClNO2 Production from N2O5 Uptake on Saline Playa Dusts: New Insights into Potential Inland Sources of ClNO2

Nitryl chloride (ClNO₂), formed when dinitrogen pentoxide (N₂O₅) reacts with chloride-containing aerosol, photolyzes to produce chlorine radicals that facilitate the formation of tropospheric ozone. ClNO₂ has been measured in continental areas; however, the sources of particulate chloride required to form ClNO₂ in inland regions remain unclear. Dust emitted from saline playas (e.g., dried lakebeds) contains salts that can potentially form ClNO₂ in inland regions. Here, we present the first laboratory measurements demonstrating the production of ClNO₂ from playa dusts. N₂O₅ reactive uptake coefficients (γ_N2O5) ranged from ∼10^-3 to 10^-1 and ClNO₂ yields (φ_ClNO2) were >50% for all playas tested except one. In general, as the soluble ion fraction of playa dusts increases, γ_N2O5 decreases and φ_ClNO2 increases. We attribute this finding to a transition from aerosol surfaces dominated by silicates that react efficiently with N₂O₅ and produce little ClNO₂ to aerosols that behave like deliquesced chloride-containing salts that generate high yields of ClNO₂. Molecular bromine (Br₂) and nitryl bromide (BrNO₂) were also detected, highlighting that playas facilitate the heterogeneous production of brominated compounds. Our results suggest that parameterizations and models should be updated to include playas as an inland source of aerosol chloride capable of efficiently generating ClNO₂.