Observation and Simulation of Vertically Resolved Nitrous Acid (HONO) in Autumn over Urban Beijing, China

Abstract. Nitrous acid (HONO) is a key precursor of atmospheric hydroxyl radicals (OH) and significantly influences the formation of secondary pollutants, making it essential for understanding and controlling air pollution. While many studies have focused on its formation mechanisms, few have explored the impact of anthropogenic activities variation on HONO formation. Therefore, we investigated the impact of anthropogenic activities variation on HONO formation based on a comprehensive observation conducted in urban Beijing during autumn and winter of 2022. During clean periods with a 53 % drop in Traffic Performance Index, HONO, CO, and NO2 levels decreased by 2–3 times compared to polluted periods and significantly lower than previously reported wintertime levels in Beijing. Source apportionment revealed that NO2 heterogeneous reaction on ground was the dominant HONO source across all periods. Vehicle emissions contributed more to HONO during clean periods, suggesting that reduced anthropogenic activities has a stronger influence on secondary HONO formation. NO3- photolysis contributed more to HONO during polluted periods, due to higher NO3- fractions in PM2.5 under more polluted conditions. Despite including all known formation pathways in the model, unidentified HONO sources still remain. This is strongly associated with intense solar radiation and high OH concentrations at daytime, as well as elevated NH3 concentrations at nighttime. Emission reduction simulations further revealed that a 50 % NOx reduction during polluted periods could lower HONO by up to 38.4 %, directly demonstrating that reducing anthropogenic activities significantly suppresses HONO formation and provides a scientific basis for the development of air pollution control strategies.

Investigating the sources of atmospheric nitrous acid (HONO) in the megacity of Beijing, China

Abstract. Nitrous acid (HONO), an important precursor of the hydroxyl radical (OH), plays a key role in atmospheric chemistry, but its sources are still debated. The production of HONO on aerosol surfaces or on ground surfaces in nocturnal atmospheres remains controversial. The vertical profile provides vertical information on HONO and NO2 to understand the nocturnal HONO production and loss. In this study, we report the first high-resolution (<2.5 m) nocturnal vertical profiles of HONO and NO2 measured from in situ instruments on a movable container that was lifted on the side wiring of a 325 m meteorological tower in Beijing, China. High-resolution vertical profiles revealed the negative gradients of HONO and NO2 in nocturnal boundary layers, and a shallow inversion layer affected the vertical distribution of HONO. The vertical distribution of HONO was consistent with stratification and layering in the nocturnal urban atmosphere below 250 m. The increase in the HONO ∕ NO2 ratio was observed throughout the column from the clean episode to the haze episode, and relatively constant HONO∕NO2 ratios in the residual layer were observed during the haze episode. Direct HONO emissions from traffic contributed 29.3 % ± 12.4 % to the ambient HONO concentrations at night. The ground surface dominates HONO production by heterogeneous uptake of NO2 during clean episodes. In contrast, the HONO production on aerosol surfaces (30–300 ppt) explained the observed HONO increases (15–368 ppt) in the residual layer, suggesting that the aerosol surface dominates HONO production aloft during haze episodes, while the surface production of HONO and direct emissions into the overlying air are minor contributors. Average dry deposition rates of 0.74±0.31 and 1.55±0.32 ppb h−1 were estimated during the clean and haze episodes, respectively, implying that significant quantities of HONO could be deposited to the ground surface at night. Our results highlight the ever-changing contributions of aerosol and ground surfaces in nocturnal HONO production at different pollution levels and encourage more vertical gradient observations to evaluate the contributions from varied HONO sources.

Atmospheric nitrous acid (HONO) in an alternate process of haze pollution and ozone pollution in urban Beijing in summertime: Variations, sources and contribution to atmospheric photochemistry

Nitrous acid (HONO), a key precursor of hydroxyl radicals (OH), is one of the factors affecting atmospheric chemistry and air quality. Currently, the proposed sources of HONO are not able to fully explain observed HONO concentrations. In this study, a comprehensive field observation of HONO was conducted in the autumn of 2021 in urban Beijing. The box model using a default Master Chemical Mechanism (MCM) was unable to reproduce the observed HONO concentrations with a normalized mean bias (NMB) of −92.8%. The NMB improved to −46.1% after the inclusion of seven additional HONO formation pathways. Several factors like vehicle emission factor (1.23%) and nocturnal NO2 heterogeneous uptake coefficient on the ground surface (8.25 × 10−6) were calculated based on observational data. The enhancement factor for nocturnal NO2 heterogeneous conversion was established as a function of relative humidity (RH) and incorporated into the model, which compensated for the missing nocturnal HONO sources and well-reproduced the observed HONO concentrations, with an NMB of −5.1%. The major source of HONO at night was found to be the heterogeneous reaction of NO2 on the ground surface, contributing up to 85.6%. During the daytime, it was the homogeneous reaction of NO with OH, accounting for 41.8%. The daytime primary source of OH was mainly the photolysis of HONO, which constituted 73.6% and therefore promoted the formation of secondary pollutants and exacerbated haze events.

Abstract. Nitrous acid (HONO) is a key precursor of atmospheric hydroxyl radicals (OH) and significantly influences the formation of secondary pollutants, making it essential for understanding and controlling air pollution. While many studies have focused on its formation mechanisms, few have explored the impact of variations in anthropogenic activities on HONO formation. Therefore, we investigated the impact of variations in anthropogenic activities on HONO formation based on comprehensive observations conducted in urban Beijing during autumn and winter of 2022. During clean periods with a 53 % drop in Traffic Performance Index, HONO, CO, and NO2 levels decreased by 2–3 times compared to polluted periods and significantly lower than previously reported wintertime levels in Beijing. Source apportionment revealed that NO2 heterogeneous reaction on ground was the dominant HONO source across all periods. Vehicle emissions contributed more to HONO during clean periods, suggesting that reducing anthropogenic activities has a stronger influence on secondary HONO formation. Particulate nitrate (pNO3) photolysis contributed more to HONO during polluted periods, due to higher pNO3 fractions in PM2.5 under more polluted conditions. Despite including all known formation pathways in the model, unidentified HONO sources still remained. This was strongly associated with intense solar radiation and high OH concentrations at daytime, as well as elevated NH3 concentrations at nighttime. Emission reduction simulations further revealed that a 50 % NOx reduction during polluted periods could lower HONO by up to 46.3 %, directly demonstrating that reducing anthropogenic activities significantly suppresses HONO formation and provides a scientific basis for the development of air pollution control strategies.

Inconsistent capacity of potential HONO sources to enhance secondary pollutants: Evidence from WRF-Chem modeling.