Weihao Wang

SO2 Initiates the Efficient Conversion of NO2 to HONO on MgO Surface

Nitrous acid (HONO) is an important source of hydroxyl radical (OH) that determines the fate of many chemically active and climate relevant trace gases. However, the sources and the formation mechanisms of HONO remain poorly understood. In this study, the effect of SO2 on the heterogeneous reactions of NO2 on MgO as a mineral dust surrogate was investigated. The reactivity of MgO to NO2 is weak, while coexisting SO2 can increase the uptake coefficients of NO2 on MgO by 2-3 orders of magnitude. The uptake coefficients of NO2 on SO2-aged MgO are independent of NO2 concentrations in the range of 20-160 ppbv and relative humidity (0-70%RH). The reaction mechanism was demonstrated to be a redox reaction between NO2 and surface sulfite. In the presence of SO2, NO2 was reduced to nitrite under dry conditions, which could be further converted to gas-phase HONO in humid conditions. These results suggest that the reductive effect of SO2 on the heterogeneous conversion of NO2 to HONO may have a significant contribution to the unknown sources of HONO observed in polluted areas (for example, in China).

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%).

Revisiting nitrous acid (HONO) emission from on-road vehicles: A tunnel study with a mixed fleet

ABSTRACT Nitrous acid (HONO) is an important precursor of OH radicals in the atmosphere. In urban areas, emissions from vehicles are the main source of air pollutants, including reactive nitrogen. Previously reported emission ratios of HONO (HONO/NOx) from vehicles were measured in the late 1990s and need to be updated due to the significant changes in emission control technologies. We measured the emission ratio of a fleet of vehicles (38% diesel on average) from March 11 to 21, 2015, in a road tunnel in Hong Kong. The emission ratio of 1.24% (±0.35%) obtained is greater than the commonly adopted 0.8% or 0.3%. The elevated emission ratio is found to be related to the presence of vehicles equipped with diesel particle filters (DPFs). Positive correlation between HONO and black carbon (BC) shows that HONO and BC were emitted together, while the lack of correlation or even anticorrelation between HONO/NOx and BC indicates that the BC-mediated conversion of NO2 to HONO in the dark was insignificant in the immediate vicinity of the emission sources. Implications: Vehicular emission is a key source for HONO in the urban atmosphere. However, the most commonly used emission ratio HONO/NOx in modeling studies was measured more than 15 years ago. Our tunnel study suggests that a mixed fleet nowadays has a higher emission ratio, possibly because of the diesel particle filter (DPF) retrofit program and the growing share of Euro IV or more advanced diesel vehicles. Our study also provides new insight into the role of black carbon in HONO formation from vehicles.

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.

Photochemical Smog in Southern China: A Synthesis of Observations and Model Investigations of the Sources and Effects of Nitrous Acid

Recent studies have revealed potentially important effects of additional source(s) of hydroxyl radicals on the atmosphere’s oxidative capacity and, in turn, the production of secondary air pollutants. In this paper, we give an overview of our recent efforts in investigating the sources and effects of nitrous acid (HONO) on ozone and some secondary aerosols in southern China by combining field measurements and model simulations. Beginning in 2011, a series of field measurements of HONO were conducted at five sites, with diverse land use and different effects of emission sources. We observed the seasonal characteristics, emission ratios, heterogeneous production, and made simulations with a chemical transport model for the photochemical effects of HONO. The key findings are as follows. The derived emission ratios from vehicles exhibited wide variability and were mostly higher than the more uniform value of 0.8% reported in the literature. Larger nocturnal heterogeneous conversion rates of NO2 to HONO were observed when air masses were passing over sea surfaces, compared with land surfaces. Widely reported daytime sources of HONO also exist in Hong Kong. Moreover, the revised WRF-Chem model with comprehensive HONO sources significantly improved the simulations of the observed HONO, which enhanced regional hydroxyl radicals, O3, and PM2.5 by 10–20, 8–15, and 10–15% over urban areas in the Pearl River Delta region, respectively. Our studies highlight the importance of considering HONO sources when simulating secondary pollutants in polluted atmospheres.