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We present a novel method, the Gaussian observational model for edge to center heterogeneity (GOMECH), to quantify the horizontal chemical structure of plumes. GOMECH fits observations of short-lived emissions or products against a long-lived tracer (e.g., CO) to provide relative metrics for the plume width ( w i / w CO ) and center ( b oxidation processes in smoke plumes sampled during FIREX-AQ (Fire Influence on Regional to Global Environments and Air Quality, a 2019 wildfire smoke study). An analysis of 430 crosswind transects demonstrates that nitrous acid (HONO), a primary source of OH, is narrower than CO ( i / w CO ). To validate GOMECH, we investigate OH and NO 3 oxidation processes in smoke plumes sampled during FIREX-AQ (Fire Influence on Regional to Global Environments and Air Quality, a 2019 wildfire smoke study). An analysis of 430 crosswind transects demonstrates that nitrous acid (HONO), a primary source of OH, is narrower than CO ( w HONO / w CO = 0.73-0.84 ± 0.01) and maleic anhydride (an OH oxidation product) is enhanced on plume edges ( w maleicanhydride / w CO = 1.06-1.12 ± 0.01). By contrast, NO 3 production [P(NO 3 )] occurs mainly at the plume center ( w P(NO 3 ) / w CO = 0.91-1.00 ± 0.01). Phenolic emissions, highly reactive to OH and NO 3 , are narrower than CO ( w phenol / w CO = 0.96 ± 0.03, w catechol / w CO = 0.91 ± 0.01, and w methylcatechol / w CO = 0.84 ± 0.01), suggesting that plume edge phenolic losses are the greatest. Yet, nitrophenolic aerosol, their oxidation product, is the greatest at the plume center ( w nitrophenolicaerosol / w CO = 0.95 ± 0.02). In a large plume case study, GOMECH suggests that nitrocatechol aerosol is most associated with P(NO 3 ). Last, we corroborate GOMECH with a large eddy simulation model which suggests most (55%) of nitrocatechol is produced through NO 3 in our case study.

HONO is an important component of reactive nitrogen (N r ) and precursors of OH radical. However, the source and removal of HONO are not clear. Here, measurements of HONO (May 18-31, 2018) were conducted in Xi'an and Xianyang simultaneously for the first time. The relationship between HONO and other N r (such as NO and NO 2 ) in two cities was analyzed. The mixing ratio of HONO in Xi'an was 1.2 ± 0.8 ppbv, and that in Xianyang was 1.2 ± 1.1 ppbv. The nighttime HONO mixing ratio was higher in Xianyang, while the daytime HONO was higher in Xi'an. Compared with the contribution from heterogeneous process of NO 2 , direct emissions and homogeneous processes (NO + OH) were less important for nocturnal HONO formation in these two cities. The relative contribution of heterogeneous process in Xianyang was more important than that in Xi'an. The reaction of NO 2 upon aerosols surface was identified as an important source of HONO for two sites. The conversion of NO 2 on the other surfaces might attend the heterogeneous formation of HONO in Xianyang site. Daytime HONO budget analysis indicated that there was an additional unknown formation process of HONO at two sites. The net OH production rate from HONO (from 08:00 to 17:00) was 1.6 × 10 7 and 1.3 × 10 7 molecule/(cm 3 s) for Xian and Xianyang, 5.2 and 3.5 times higher than from O 3 photolysis. Besides, a dust storm appeared during this observation period, and the impact of local emission and transport processes was separately analyzed. The sources, characteristics, and effects of HONO identified in this study laid a foundation for further research on HONO and air pollution in the Guanzhong area., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Nitrogen dioxide (NO 2 ) is an air pollutant discharged from combustion of human activities. Nitrous acid (HONO), measured as NO 2 , is thought to impact respiratory function more than NO 2 . HONO and NO 2 have an equilibrium relationship, and their reaction is affected by climate conditions. This study was conducted to discuss the extent of HONO contained in NO 2 , depending on the level of urbanization. Whether climate conditions that promote HONO production enhanced the level of NO 2 measured was investigated using time series analysis. Climate and outdoor air pollution data measured in April 2009-March 2017 in urban (Tokyo, Osaka, and Aichi) and rural (Yamanashi) areas in Japan were used for the analysis. Air temperature had a trend of negative associations with NO 2 , which might indicate the decomposition of HONO in the equilibrium between HONO and NO 2 . The associations of relative humidity with NO 2 did not have consistent trends by prefecture: humidity only in Yamanashi was positively associated with NO 2 . In high relative humidity conditions, the equilibrium goes towards HONO production, which was observed in Yamanashi, suggesting the proportion of HONO in NO 2 might be low/high in urban/rural areas.

In this study, the formation of nitrate aerosol from 16 to 24 December 2015 in the Guanzhong basin, China is simulated using the WRF-Chem model. The predicted near-surface O 3 , NO 2 , and fine particulate matters (PM 2.5 ) in the basin and inorganic aerosols and nitrous acid (HONO) in Xi'an are generally in good agreement with the observations. Sensitivity studies show that the heterogeneous HONO sources play an appreciable role in the nitrate formation in the basin, contributing 9.2% of nitrate mass concentrations during heavy haze days. Nitrate formation is also affected by sulfate due to their competition for ammonia, particularly in urban areas. A 50% decrease in SO 2 emissions enhances the nitrate concentration by 6.2% during heavy haze days on average in the basin, and a 50% increase in SO 2 emission reduces the nitrate concentration by 9.7%. The roles of HONO and sulfate competition in nitrate formation are strongly modulated by ammonia. Agricultural emissions predominate the nitrate level in the basin (93.5%), but the non-agricultural sources cannot substantially influence nitrate formation (3.7%-14.6%). Reducing agricultural emission is an effective control strategy to mitigate nitrate pollution in the basin., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Characteristics and transformation of nitrous acid (HONO) and particulate nitrite were investigated with high time-resolution field measurements at an urban site in Ji'nan, China from Nov. 2013 to Jan. 2014. During the sampling period, averages of 0.35ppbv HONO and 2.08μgm(-3) fine particulate nitrite were observed. HONO and particulate nitrite exhibited similar diurnal variation patterns but differed in the time at which concentration peaks and valleys occurred. Elevated nocturnal HONO concentration peaks were mainly associated with primary emissions from vehicle exhaust and secondary formation via heterogeneous reactions of NO2. In fresh air masses dominated by vehicle emissions, the average HONO/NOx ratio was 0.58%. The nocturnal heterogeneous reactions of NO2 contributed to about half of the elevated HONO concentration peaks, with the conversion rates in the range of 0.05% to 0.96%h(-1). Meanwhile, a large amount of particulate nitrite, which greatly exceeded the concentration of the gas-phase HONO, was also produced through the heterogeneous reactions of NO2. The large yields of particulate nitrite were facilitated by abundant ammonia and particulate cations in urban Ji'nan. Notably, in the daytime, particulate nitrite acted as a potential source of HONO, especially in conditions of low humidity and acidic aerosols, which possibly has subsequent effects on photochemistry in the boundary layer., (Copyright © 2015 Elsevier B.V. All rights reserved.)

Nitrous acid (HONO) is a crucial precursor of tropospheric hydroxyl radicals but its sources are not fully understood. Soil is recognized as an important HONO source, but the lack of measurements of soil-atmosphere HONO exchange flux (FHONO) has led to uncertainties in modeling its atmospheric impacts and understanding the reactive nitrogen budget. To address this, we conduct long-period FHONO measurements over agricultural fields under fertilized (FHONO-NP) and non-fertilized (FHONO-CK) treatments. Our results show that nitrogen fertilizer use causes a remarkable increase in FHONO-NP and it exhibits distinct diurnal variations, with an average noontime peak of 152 ng N m-2 s-1. The average FHONO-NP within three weeks after fertilization is 97.7 ± 8.6 ng N m-2 s-1, around two orders of magnitude higher than before fertilization, revealing the remarkable promotion effect of nitrogen fertilizer on HONO emissions. We also discuss other factors that influence soil HONO emissions, such as meteorological parameters and soil properties/nutrients. Additionally, we estimate the HONO emission factor of 0.68 ± 0.07% relative to the applied nitrogen during the whole growing season of summer maize. Accordingly, the fertilizer-induced soil HONO emission is estimated to be 0.06 and 0.16 Tg N yr-1 in the North China Plain (NCP) and China, respectively, representing a significant reactive nitrogen source. Furthermore, our observations reveal that soil emissions sustain a high level of daytime HONO, enhancing the atmospheric oxidizing capacity and aggravating O3 pollution in the NCP. Our results indicate that in order to effectively mitigate regional air pollution, future policies should consider reactive nitrogen emissions from agricultural soils. [ABSTRACT FROM AUTHOR]

Despite implementation of strict emission mitigation measures since 2013, heavy haze with high levels of secondary aerosols still frequently engulfs the Guanzhong Basin (GZB), China, during wintertime, remarkably impairing visibility and potentially causing severe health issues. Although the observed low ozone (O 3) concentrations do not facilitate the photochemical formation of secondary aerosols, the measured high nitrous acid (HONO) level provides an alternate pathway in the GZB. The impact of heterogeneous HONO sources on the wintertime particulate pollution and atmospheric oxidizing capability (AOC) is evaluated in the GZB. Simulations by the Weather Research and Forecast model coupled with Chemistry (WRF-Chem) reveal that the observed high levels of nitrate and secondary organic aerosols (SOA) are reproduced when both homogeneous and heterogeneous HONO sources are considered. The heterogeneous sources (HET-sources) contribute about 98% of the near-surface HONO concentration in the GZB, increasing the hydroxyl radical (OH) and O 3 concentration by 39.4% and 22.0%, respectively. The average contribution of the HET-sources to SOA, nitrate, ammonium, and sulfate in the GZB is 35.6%, 20.6%, 12.1%, and 6.0% during the particulate pollution episode, respectively, enhancing the mass concentration of fine particulate matters (PM 2.5) by around 12.2%. Our results suggest that decreasing HONO level or the AOC becomes an effective pathway to alleviate the wintertime particulate pollution in the GZB. [Display omitted] • High levels of secondary aerosols have been measured in winter in the Guanzhong Basin (GZB). • High HONO levels provide an alternative pathway for wintertime photochemistry. • Heterogeneous HONO deteriorates the wintertime particulate pollution in the GZB. • Heterogeneous HONO contributes about 12.2% to the wintertime PM 2.5 level in the GZB. • Heterogeneous HONO plays a more important role in the daytime PM 2.5 levels than nighttime. Capsule: The heterogeneous HONO sources deteriorate the wintertime particulate pollution in the Guanzhong Basin, with the PM 2.5 contribution of about 12.2%. [ABSTRACT FROM AUTHOR]

Nitrogenous air pollutants including nitrogen dioxide (NO), nitric acid (HNO), nitrate (NO), ammonia (NH), ammonium (NH), and nitrous acid (HONO) were characterized at an urban forested (UF) site in Hiroshima and at a suburban forested (SF) site in Fukuoka, western Japan, using an annular denuder system for 1 year from May 2006 to May 2007 to compare the concentrations and chemical species of atmospheric nitrogenous pollutants between UF and SF sites. The proximity of the urban area was reflected in higher NO concentrations at the UF site than at the SF site. NO was more oxidized at the SF site because it is farther from an urban area than the UF site, which was reflected in higher concentrations of HNO at the SF site than the UF site. HNO and acidic sulfate is neutralized by NH, existing as ammonium nitrate (NHNO) and ammonium sulfate [(NH)SO] at the UF site. At the SF site, acidic sulfate is neutralized by NH, existing as (NH)SO, but NHNO, had scarcely formed at the SF site. A much higher HONO concentration was observed at the UF site than at the SF site, especially in winter and spring at night, which could be explained by higher NO concentrations at the UF site because of its proximity to an urban area and stagnant meteorological conditions. Atmospheric HONO determination was critical in evaluating the possibility of damage to trees in UF areas. [ABSTRACT FROM AUTHOR]

Soot particles produced by incomplete combustion processes are one of the major components of urban air pollution. Chemistry at their surfaces lead to the heterogeneous conversion of several key trace gases; for example NO2 interacts with soot and is converted into HONO, which rapidly photodissociates to form OH in the troposphere. In the dark, soot surfaces are rapidly deactivated under atmospheric conditions, leading to the current understanding that soot chemistry affects tropospheric chemical composition only in a minor way. We demonstrate here that the conversion of NO2 to HONO on soot particles is drastically enhanced in the presence of artificial solar radiation, and leads to persistent reactivity over long periods. Soot photochemistry may therefore be a key player in urban air pollution. [ABSTRACT FROM AUTHOR]

Abstract: In the summers of 2002–2003, acidic gases, ammonia and water-soluble ions in PM10 were measured in Beijing. The mean concentrations of HCl, HONO, HNO3, SO2 and NH3 are 0.6, 3.6, 1.9, 14.1 and 16.6μgm−3, respectively, and 2.2, 14.6, 19.3 and 8.9μgm−3 for in PM10. The concentrations of secondary ions in PM10 are found to have strong dependence on the pathway of trajectories. The most frequent southerly air flow is connected with high concentrations of secondary water-soluble ions during summertime. Other trajectories with northwest and north direction lead to lower concentrations of secondary ions. Hebei and Shandong Provinces and the Tianjin Municipality are the main source areas for sulfate as identified by Potential Source Contribution Function. This result emphasizes that the non-Beijing sources play an important role in the sulfate mass concentration in the urban atmosphere of Beijing and validates conclusions based on model calculations for the region. [Copyright &y& Elsevier]

Simultaneous measurements of gaseous species and fine-mode, particulate inorganic components were performed at the University of Seoul, Seoul in Korea. In the simultaneous measurements, a certain level of nitrous acid (HONO) was observed in the gas-phase, indicating possible heterogeneous HONO production on the surface of the ambient aerosols. On the other hand, high particulate nitrite (NO2−) concentrations of 1.41(±2.26)μg/m3 were also measured, which sometimes reached 18.54μg/m3. In contrast, low HONO-to-NO2 ratios of 0.007(±0.006) were observed in Seoul. This indicates that a significant fraction of HONO is dissolved in atmospheric aerosols. Around the Seoul site, sufficient alkalinity may have been provided to the atmospheric aerosols from the excessive presence of NH3 in the gas-phase. Due to the alkaline particulate conditions (defined in this study as a particle pH >∼3.29), the HONO molecules produced at the surface of the atmospheric aerosols appeared to have been converted into particulate nitrite, thereby preventing their further participation in the atmospheric O3/NOy/HOx photochemical cycles. It was estimated that a minimum average of 65% of HONO was captured by alkaline, anthropogenic, urban particles in the Seoul measurements. [Copyright &y& Elsevier]

The major objective of this paper is to provide insights to sources and sinks of nitrous acid in urban areas, and their seasonal dependency on meteorology, photochemistry and long range transport. With this aim, nitrous acid (HONO) mixing ratios and other compounds were measured in Ashdod (south of Tel Aviv, Israel), a typical Mediterranean urban area. Statistical data analysis revealed the expected correlation between HONO and nitrogen oxides during the autumn campaign when HONO sources appeared to be traffic-, harbor-, and industry-related. Conversely, during summer HONO and NO2 were no longer correlated: NO2 at nighttime was probably deposited onto surfaces, soil and plants, whereas HONO at daytime was likely destroyed photolytically contributing to the OH concentration. Photolysis was expected to be the dominant HONO sink at daytime, especially during the summer period. Using modeled photolytical HONO lifetimes we estimate the magnitude of heterogeneous and/or organic electron transfer source reactions of HONO as 6–8 ppbv/h. [ABSTRACT FROM AUTHOR]

Abstract: Measurements of snow surface—atmosphere exchange fluxes of nitrous acid at Ny-Ålesund, Svalbard during springtime confirmed earlier findings that no significant emissions occurred from alkaline snow surfaces at this marine high Arctic location. On 29 April 2004 air arriving from the North Pole brought both ozone depleted air and significant snowfall. On that day we observed HONO emission fluxes of up to 40nmolm−2 h−1. These emissions were made possible by the combination of two independent phenomena. The acidic snow allowed the transport of NO3 − ions to photochemically reactive areas and the release of produced HONO from the snow interstitial air; while the absence of ozone likely increased the HONO/NO fraction over that of NO2 in the snow interstitial air. [Copyright &y& Elsevier]

In recent years, high fine particulate (PM2.5) pollution episodes with high ozone (O3) levels have been observed in Shanghai from time to time. However, their occurrence and characteristics remain poorly understood. Meanwhile, as a major precursor of tropospheric hydroxyl radical (OH) that initiates the formation of hydroperoxyl and organic peroxy radicals, HONO would inevitably affect the formation of O3, but its role in the formation of O3 during the double high-level PM2.5 and O3 pollution episodes remains unclear. In this study, the characteristics of the double high pollution episodes and the role of HONO in O3 formation in these episodes were investigated based on field observation in urban Shanghai from 2014 to 2016. Results showed that high PM2.5 pollution and high O3 pollution could occur simultaneously. The cases with data of double high O3 and PM2.5 concentrations accounted for about 1.0% of the whole sampling period. During the double high pollution episodes, there still existed active photochemical processes, while the active photochemical processes at high PM2.5 concentration were conductive to the production and accumulation of O3 under a VOC-limited regime and a calm atmospheric condition including high temperature, moderately high relative humidity, and low wind speed, which in turn enhanced the conversions of SO2 and NO2 and the formation and accumulation of secondary sulfate and nitrate aerosols and further promoted the increase of PM2.5 concentration and the deterioration of air pollution. Further analysis indicated that the daytime HONO concentration could be strongly negatively correlated with O3 concentration in most of the double high pollution episodes, revealing the dominant role of HONO in O3 formation during these pollution episodes. This study provides important field measurement-based evidence for understanding the significant contribution of daytime HONO to O3 formation, and helps to clarify the formation and coexistence mechanisms of the double high-level O3 and PM2.5 pollution episodes. [ABSTRACT FROM AUTHOR]