Comparisons of measured nitrous acid (HONO) concentrations in a pollution period at urban and suburban Beijing, in autumn of 2014

Additional HONO and OH Generation from Photoexcited Phenyl Organic Nitrates in the Photoreaction of Aromatics and NOx

HONO acts as a major OH source, playing a vital role in secondary pollutant formation to deteriorate regional air quality. Strong unknown sources of daytime HONO have been widely reported, which significantly limit our understanding of radical cycling and atmospheric oxidation capacity. Here, we identify a potential daytime HONO and OH source originating from photoexcited phenyl organic nitrates formed during the photoreaction of aromatics and NOx. Significant HONO (1.56-4.52 ppb) and OH production is observed during the photoreaction of different kinds of aromatics with NOx (18.1-242.3 ppb). We propose an additional mechanism involving photoexcited phenyl organic nitrates (RONO2) reacting with water vapor to account for the higher levels of measured HONO and OH than the model prediction. The proposed HONO formation mechanism was evidenced directly by photolysis experiments using typical RONO2 under UV irradiation conditions, during which HONO formation was enhanced by relative humidity. The 0-D box model incorporated in this mechanism accurately reproduced the evolution of HONO and aromatic. The proposed mechanism contributes 5.9-36.6% of HONO formation as the NOx concentration increased in the photoreaction of aromatics and NOx. Our study implies that photoexcited phenyl organic nitrates are an important source of atmospheric HONO and OH that contributes significantly to atmospheric oxidation capacity.

A comprehensive observation on the pollution characteristics of peroxyacetyl nitrate (PAN) in Beijing, China

Peroxyacetyl nitrate (PAN) is a typical secondary photochemical product in the atmospheric environment with significant adverse effects on human health and plant growth. In this study, PAN and other pollutants, as well as meteorological conditions were observed intensively from August to September in 2022 at a typical urban sampling site in Beijing, China. The mean and maximum PAN concentrations during the observation period were 1.00 ± 0.97 ppb and 4.84 ppb, respectively. Severe photochemical pollution occurred during the observation period, with the mean PAN concentration about 3.1 times higher than that during the clean period. There was a good positive correlation between O3 and PAN, and their correlation was higher during the O3 exposure period than that during the clean period. The simulated results by box-model coupled with the Master Chemical Mechanism (MCM v3.3.1) showed that the O3-related reactions were the largest sources of OH radicals during O3 exposure period, which was conducive to the co-contamination of PAN and O3. Acetaldehyde (CH3CHO) and methylglyoxal (MGLY) were the largest OVOCs precursors of peroxyacetyl radicals (PA), with the contributions to the total PA generated by OVOCs about 67 % - 83 % and 17 % - 30 %, respectively. The reduction of emissions from liquefied petroleum gas (LPG) and solvent usage has the highest reduction effect on PAN and O3, followed by the control of gasoline vehicle exhaust emissions. This study deepens the understanding of the PAN photochemistry in urban areas with high O3 background conditions and the impact of anthropogenic activities on the photochemical pollution. Meanwhile, the findings of this study highlight the necessity of strengthening anthropogenic emissions control to effectively reduce the co-contamination of PAN and O3 in Beijing in the future.

Atmospheric HONO formation during and after the Spring Festival holidays in a coastal city of China

Nitrous acid (HONO) is an important source of hydrogen oxides (HOx), which affects air quality, the atmospheric oxidation capacity, and human health. Here, we present ambient measurements of the HONO concentrations in Zhuhai, a coastal city in Southern China, from February 7 to March 15, 2021. The campaign was classified into two periods during (P1) and after (P2) the Spring Festival holidays. The average HONO mixing ratio during P2 (1.19 ± 0.85 ppbv) was much higher than that during P1 (0.24 ± 0.18 ppbv), likely due to the contribution of homogeneous HONO formation. During nighttime, the heterogeneous conversion rate during P2 (0.0089/hr) was considerably higher than that during P1 (0.0057/hr), suggesting a higher heterogeneous NO₂ conversion potential. However, the heterogeneous NO₂ conversion was the dominant way during P1 with a high percentage of 88%, while comparable ratios of heterogeneous and homogeneous formation were found (54% vs. 46%) during P2, indicating that the homogeneous formation was also important during P2. During daytime, homogeneous reaction was the major known pathway, with a contribution of 16% during P1 and 27% during P2, leaving large unknown HONO sources which reasonably correlated with the photo-enhanced NO₂ conversion. Two case scenarios were additionally explored, showing that there might be a primary emission source during one scenario (February 17-18) and vehicle emissions might be the major unknown HONO source for another scenario (March 3-5). The results suggest that large unknown daytime sources still exist which need more future ambient and laboratory studies.

Atmospheric oxidizing capacity in autumn Beijing: Analysis of the O3 and PM2.5 episodes based on observation-based model

Atmospheric oxidizing capacity (AOC) is the fundamental driving factors of chemistry process (e.g., the formation of ozone (O₃) and secondary organic aerosols (SOA)) in the troposphere. However, accurate quantification of AOC still remains uncertainty. In this study, a comprehensive field campaign was conducted during autumn 2019 in downtown of Beijing, where O₃ and PM_2.5 episodes had been experienced successively. The observation-based model (OBM) is used to quantify the AOC at O₃ and PM_2.5 episodes. The strong intensity of AOC is found at O₃ and PM_2.5 episodes, and hydroxyl radical (OH) is the dominating daytime oxidant for both episodes. The photolysis of O₃ is main source of OH at O₃ episode; the photolysis of nitrous acid (HONO) and formaldehyde (HCHO) plays important role in OH formation at PM_2.5 episode. The radicals loss routines vary according to precursor pollutants, resulting in different types of air pollution. O₃ budgets and sensitivity analysis indicates that O₃ production is transition regime (both VOC and NO_x-limited) at O₃ episode. The heterogeneous reaction of hydroperoxy radicals (HO₂) on aerosol surfaces has significant influence on OH and O₃ production rates. The HO₂ uptake coefficient (γHO₂) is the determining factor and required accurate measurement in real atmospheric environment. Our findings could provide the important bases for coordinated control of PM_2.5 and O₃ pollution.

Formation mechanisms and atmospheric implications of summertime nitrous acid (HONO) during clean, ozone pollution and double high-level PM2.5 and O3 pollution periods in Beijing

Nitrous acid (HONO) is a key precursor of the hydroxyl radicals (OH) and has a significant impact on air quality. Nowadays, the source of HONO is still controversial due to its complex formation mechanisms, which is widely explored in extensive field and laboratory studies. In this study, the pollution characteristics and source contribution of HONO under different air quality conditions in summer in Beijing were analyzed. The observation periods were classified as three typical periods: clean, ozone pollution, and double high pollution (co-occurrence of high PM_2.5 and O₃ concentrations). The average concentrations of observed HONO were 0.38 ± 0.35 ppb, 0.21 ± 0.18 ppb, 0.26 ± 0.20 ppb and 0.54 ± 0.45 ppb during the whole, clean, ozone and double high periods, respectively. The elevated HONO levels at night were attributed to vehicle emissions and the RH-dependent heterogeneous conversion of NO₂ to HONO. The average emission ratio (HONO/NO_x) was 0.85 % ± 0.38 %, and the mean value of calculated nocturnal NO₂ to HONO conversion frequency was 0.0076 ± 0.0031 h^-1. Based on daytime HONO budget analysis, the largest potential source of HONO was the homogeneous reaction of NO and OH (0.33 and 0.34 ppb h^-1), followed by the unknown source (0.11 and 0.21 ppb h^-1) during clean and ozone periods, while the unknown source (0.49 ppb h^-1) played the predominant role during double high period. The unknown sources of HONO could be attributed to the photo-enhanced heterogeneous conversion of NO₂ and the photolysis of particulate nitrate. Furthermore, the photolysis of ozone (0.17, 0.34 and 0.44 ppb h^-1) was the major contributor to primary OH during three typical periods. HONO photolysis contributed considerable amounts of primary OH (0.32 ppb h^-1) during double high period. These results are helpful to further understand the linkage between HONO and air quality variation.

Atmospheric chemistry of nitrous acid and its effects on hydroxyl radical and ozone at the urban area of Beijing in early spring 2021

The atmospheric chemistry of nitrous acid (HONO) has received extensive attention because of its significant contribution to hydroxyl (OH) radicals. Heterogeneous reaction of NO₂ is an important HONO source, and its reaction mechanism is affected by many factors, such as concentration of gaseous NO₂, surface adsorbed water, relative humidity and temperature. Although laboratory studies have confirmed the effect of temperature on heterogeneous reaction of NO₂, there are few field observations reporting about it. We have conducted a field observation in the early spring 2021 when the temperature ranges widely (-0.1-24.7 °C). Concentrations of HONO and related pollutants at the urban area of Beijing are obtained. The hourly averaged HONO concentration reaches 4.87 ppb with a mean value of 1.48 ± 1.09 ppb. Combined with box model and RACM2 mechanism, we found an optimal temperature (∼10 °C) existing for heterogeneous reaction of NO₂ during this measurement. When considering the promotion effect of optimal temperature, the contribution of heterogeneous reaction of NO₂ to HONO can increase by 10%. This result will provide essential information for developing an accurate model of HONO chemistry in the atmosphere especially for certain periods or regions with temperature changing largely. Moreover, heterogeneous reaction of NO₂ is the vital source of HONO, contributing 63-76% to simulated HONO during this measurement. Note that HONO photolysis is the most important formation pathway of OH radicals, and ambient HONO concentration is the obbligato constraint for evaluating atmospheric oxidation by model simulations.

Effect of Different Combustion Processes on Atmospheric Nitrous Acid Formation Mechanisms: A Winter Comparative Observation in Urban, Suburban and Rural Areas of the North China Plain

Atmospheric nitrous acid (HONO) is a dominant precursor of hydroxyl (OH) radicals, and its formation mechanisms are still controversial. Few studies have simultaneously explored effects of different combustion processes on HONO sources. Hereby, synchronous HONO measurement in urban (BJ), suburban (XH) and rural (DBT) areas with different combustion processes is performed in the North China Plain in winter. A box model is utilized to analyze HONO formation mechanisms. HONO concentration is the highest at the DBT site (2.51 ± 1.90 ppb), followed by the XH (2.18 ± 1.95 ppb) and BJ (1.17 ± 1.20 ppb) sites. Vehicle exhaust and coal combustion significantly contribute to nocturnal HONO at urban and rural sites, respectively. During a stagnant pollution period, the NO+OH reaction and combustion emissions are more crucial to HONO in urban and rural areas; meanwhile, the heterogeneous reaction of NO₂ is more significant in suburban areas. Moreover, the production rate of OH from HONO photolysis is about 2 orders of magnitude higher than that from ozone photolysis. Consequently, vehicle exhaust and coal combustion can effectively emit HONO, further causing environmental pollution and health risks. It is necessary to expand the implementation of the clean energy transition policy in China, especially in areas with substantial coal combustion.

Formation mechanisms of nitrous acid (HONO) during the haze and non-haze periods in Beijing, China

As an important precursor of hydroxyl radical (OH), nitrous acid (HONO) plays a significant role in atmospheric chemistry. Here, an observation of HONO and relevant air pollutants in an urban site of Beijing from 14 to 28 April, 2017 was performed. Two distinct peaks of HONO concentrations occurred during the observation. In contrast, the concentration of particulate matter in the first period (period Ⅰ) was significantly higher than that in the second period (period Ⅱ). Comparing to HONO sources in the two periods, we found that the direct vehicle emission was an essential source of the ambient HONO during both periods at night, especially in period Ⅱ. The heterogeneous reaction of NO₂ was the dominant source in period Ⅰ, while the homogeneous reaction of NO with OH was more critical source at night in period Ⅱ. In the daytime, the heterogeneous reaction of NO₂ was a significant source and was confirmed by the good correlation coefficients (R²) between the unknown sources (P_unknown) with NO₂, PM_2.5, NO₂ × PM_2.5 in period Ⅰ. Moreover, when solar radiation and OH radicals were considered to explore unknown sources in the daytime, the enhanced correlation of P_unknown with photolysis rate of NO₂ and OH ( [Formula: see text]  × OH) were 0.93 in period Ⅰ, 0.95 in period Ⅱ. These excellent correlation coefficients suggested that the unknown sources released HONO highly related to the solar radiation and the variation of OH radicals.

Contribution of Vehicle Emission and NO2 Surface Conversion to Nitrous Acid (HONO) in Urban Environments: Implications from Tests in a Tunnel

Nitrous acid (HONO) is an important photochemical precursor to hydroxyl radicals particularly in an urban atmosphere, yet its primary emission and secondary production are often poorly constrained. Here, we measured HONO and nitrogen oxides (NO_x) at both the inlet and the outlet in a busy urban tunnel (>30 000 vehicles per day) in south China. Multiple linear regression revealed that 73.9% of the inlet-outlet incremental HONO concentration was explained by NO₂ surface conversion, while the rest was directly emitted from vehicles with an average HONO/NO_x ratio of 1.31 ± 0.87%, which was higher than that from previous tunnel studies. The uptake coefficient of NO₂, γ(NO₂), on the tunnel surfaces was calculated to be (7.01 ± 0.02) × 10^-5, much higher than that widely used in models. As tunnel surfaces are typical of urban surfaces in the wall and road materials, the dominance of HONO from surface reactions in the poorly lit urban tunnel demonstrated the importance of NO₂ conversion on urban surfaces, instead of NO₂ conversion on the aerosol surface, for both daytime and night-time HONO even in polluted ambient air. The higher γ(NO₂) on urban surfaces and the elevated HONO/NO_x ratio from this study can help explain the missing HONO sources in urban areas.

Comparative observation of atmospheric nitrous acid (HONO) in Xi'an and Xianyang located in the GuanZhong basin of western China

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 NO₂) 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 NO₂, 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 NO₂ upon aerosols surface was identified as an important source of HONO for two sites. The conversion of NO₂ 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 × 10⁷ and 1.3 × 10⁷ molecule/(cm³ s) for Xian and Xianyang, 5.2 and 3.5 times higher than from O₃ 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.

Simulation Verification of Barrierless HONO Formation from the Oxidation Reaction System of NO, Cl, and Water in the Atmosphere

Nitrous acid (HONO) is a major source of hydroxyl (OH) radicals, and identifying its source is crucial to atmospheric chemistry. Here, a new formation route of HONO from the reaction of NO with Cl radicals with the aid of one or two water molecules [(Cl) (NO) (H₂O)_n (n = 1-2)] as well as on the droplet surface was found by Born-Oppenheimer molecular dynamic simulation and metadynamic simulation. The (Cl) (NO) (H₂O)₁ (monohydrate) system exhibited a free-energy barrier of approximately 0.95 kcal mol^-1, whereas the (Cl) (NO) (H₂O)₂ (dihydrate) system was barrierless. For the dihydrate system and the reaction of NO with Cl radicals on the droplet surface, only one water molecule participated in the reaction and the other acted as the "solvent" molecule. The production rates of HONO suggested that the monohydrate system ([NO] = 8.56 × 10¹² molecule cm^-3, [Cl] = 8.00 × 10⁶ molecule cm^-3, [H₂O] = 5.18 × 10¹⁷ molecule cm^-3) could account for 40.3% of the unknown HONO production rate (P_unknown) at site 1 and 53.8% of P_unknown at site 2 in the East China Sea. This study identified the importance of the reaction system of NO, Cl, and water molecules in the formation of HONO in the marine boundary layer region.

Aerosol Promotes Peroxyacetyl Nitrate Formation During Winter in the North China Plain

Peroxyacetyl nitrate (PAN) is an important indicator for photochemical pollution, formed similar to ozone in the photochemistry of certain volatile organic compounds (VOCs) in the presence of nitrogen oxides, and has displayed surprisingly high concentrations during wintertime that were better correlated to particulate rather than ozone concentrations, for which the reasons remained unknown. In this study, wintertime observations of PAN, VOCs, PM_2.5, HONO, and various trace gases were investigated to find the relationship between aerosols and wintertime PAN formation. Wintertime photochemical pollution was affirmed by the high PAN concentrations (average: 1.2 ± 1.1 ppb, maximum: 7.1 ppb), despite low ozone concentrations. PAN concentrations were determined by its oxygenated VOC (OVOC) precursor concentrations and the NO/NO₂ ratios and can be well parameterized based on the understanding of their chemical relationship. Data analysis and box modeling results suggest that PAN formation was mostly contributed by VOC aging processes involving OH oxidation or photolysis rather than ozonolysis pathways. Heterogeneous reactions on aerosols have supplied key photochemical oxidants such as HONO, which produced OH radicals upon photolysis, promoting OVOC formation and thereby enhancing PAN production, explaining the observed PM_2.5-OVOC-PAN intercorrelation. In turn, parts of these OVOCs might participate in the formation of secondary organic aerosol, further aggravating haze pollution as a feedback. Low wintertime temperatures enable the long-range transport of PAN to downwind regions, and how that will impact their oxidation capacity and photochemical pollution requires further assessment in future studies.

Characteristics of HONO and its impact on O3 formation in the Seoul Metropolitan Area during the Korea-US Air Quality Study

Photolysis of nitrous acid (HONO) is recognized as an early-morning source of OH radicals in the urban air. During the Korea-US air quality (KORUS-AQ) campaign, HONO was measured using quantum cascade - tunable infrared laser differential absorption spectrometer (QC-TILDAS) at Olympic Park in Seoul from 17 May, 2016 to 14 June, 2016. The HONO concentration was in the range of 0.07-3.46 ppbv, with an average of 0.93 ppbv. Moreover, it remained high from 00:00-05:00 LST. During this time, the mean concentration was higher during the high-O3 episodes (1.82 ppbv) than the non-episodes (1.20 ppbv). In the morning, the OH radicals that were produced from HONO photolysis were 50% higher (0.95 pptv) during the high-O3 episodes than the non-episodes. Diurnal variations in HOx and O3 concentrations were simulated by the F0AM model, which revealed a difference of ~20 ppbv in the daily maximum O3 concentrations between the high-O3 episodes and non-episodes. Furthermore, the HONO concentration increased with an increase in relative humidity (RH) up to 80%; the highest HONO was associated with the top 10% NO2 in each RH group, confirming that NO2 is one of the main precursors of HONO. At night, the conversion ratio of NO2 to HONO was estimated to be 0.88×10-2 h-1; this ratio was found to increase with an increase in RH. The Aitken mode particles (30-120 nm), which act as catalyst surfaces, exhibited a similar tendency with a conversion ratio that increased along with RH, indicating the coupling of surfaces with HONO conversion. Using an artificial neural network (ANN) model, HONO concentrations were successfully simulated with measured variables (r2 = 0.66 as an average of five models). Among these variables, NOx, aerosol surface area, and RH were found to be the main factors affecting the ambient HONO concentrations. The results reveal that RH facilitates the conversion of NO2 to HONO by constraining the availability of aerosol surfaces. This study demonstrates the coupling of HONO with the HOx-O3 cycle in the Seoul Metropolitan Area (SMA) and provides practical evidence of the heterogeneous formation of HONO by employing the ANN model.

Influence of Chinese New Year overlapping COVID-19 lockdown on HONO sources in Shijiazhuang

Nitrous acid (HONO) is an important precursor of hydroxyl radical (OH) in the atmosphere. It is also toxic to human health. In this work, HONO concentrations were measured in Shijiazhuang using a Monitor for AeRosols and Gases in ambient Air (MARGA) from December 15, 2019 to March 15, 2020, which covered the heavy air pollution season, the Chinese New Year (CNY) vocation and the Corona Virus Disease-19 (COVID-19) lockdown period. During & after CNY overlapping COVID-19 lockdown, the air quality was significantly improved because of both the emission reduction and the increase in diffusion ability of air masses. The mean HONO concentration was 2.43 ± 1.08 ppbv before CNY, while it decreased to 1.53 ± 1.16 ppbv during CNY and 0.97 ± 0.76 ppbv after CNY. The lockdown during & after CNY reduced ~31% of ambient HONO along with ~62% of NO and ~36% of NO2 compared with those before CNY after the improvement of diffusion ability had been taken into consideration. Heterogeneous reaction of NO2 on ground surface dominated the nocturnal HONO sources, followed by heterogeneous reaction on aerosol surface, vehicle emission, reaction between NO and OH and emission from soil on pollution days throughout the observation. Except for elevated soil emission, other nighttime HONO sources and sinks decreased significantly during & after CNY. The relative importance of heterogeneous reaction of NO2 on surfaces further increased because of both the decrease in vehicle emission and the increase in the heterogeneous conversion kinetics from NO2 to HONO during & after CNY.

Pollution characteristics and potential sources of nitrous acid (HONO) in early autumn 2018 of Beijing

Nitrous Acid (HONO) is an important precursor of hydroxyl radical (OH) and has significant impacts on the formation of Ozone (O₃) and Secondary Organic Aerosol (SOA). The atmospheric concentrations of HONO were measured during early autumn in downtown, Beijing (China). This study investigated HONO pollution characteristics and potential sources during day and night. The maximum hourly HONO levels reached 5.16 ppb, with 1.23 ppb on average. HONO concentration exhibited typical diurnal variation characteristics, with maximum at nighttime and minimum at daytime. The potential sources mainly included vehicle emission, heterogeneous reaction of NO₂ on aerosol surfaces (Photo-enhanced at the daytime) and photolysis of particulate nitrate (NO₃^-) in Beijing. Vehicle emission was an important HONO source, particular at the morning rush period and lower HONO concentration. The simulated results highlighted that the main contribution of HONO was NO₂ heterogeneous reaction on aerosol surfaces. The photolysis of particulate NO₃^- was also an important daytime HONO source, particularly in the pollution period. The main loss routine was the photolysis of HONO and dry deposition at day and night, respectively.

The observation of isotopic compositions of atmospheric nitrate in Shanghai China and its implication for reactive nitrogen chemistry

The occurrence of PM_2.5 pollution in China is usually associated with the formation of atmospheric nitrate, the oxidation product of nitrogen oxides (NO_X = NO + NO₂). The oxygen-17 excess of nitrate (Δ¹⁷O(NO₃^-)) can be used to reveal the relative importance of nitrate formation pathways and get more insight into reactive nitrogen chemistry. Here we present the observation of isotopic composition of atmospheric nitrate (Δ¹⁷O and δ¹⁵N) collected from January to June 2016 in Shanghai China. Concentrations of atmospheric nitrate ranged from 1.4 to 24.1 μg m^-3 with the mean values being (7.6 ± 4.4 (1SD)), (10.2 ± 5.8) and (4.1 ± 2.4) μg m^-3 in winter, spring and summer respectively. Δ¹⁷O(NO₃^-) varied from 20.5‰ to 31.9‰ with the mean value being (26.9 ± 2.8) ‰ in winter, followed by (26.6 ± 1.7) ‰ in spring and the lowest (23.2 ± 1.6) ‰ in summer. Δ¹⁷O(NO₃^-)-constrained estimates suggest that the conversion of NO_X to nitrate is dominated by NO₂ + OH and/or NO₂ + H₂O, with the mean possible contribution of 55-77% in total and even higher (84-92%) in summer. A diurnal variation of Δ¹⁷O(NO₃^-) featured by high values at daytime (28.6 ± 1.2‰) and low values (25.4 ± 2.8‰) at nighttime was observed during our diurnal sampling period. This trend is related to the atmospheric life of nitrate (τ) and calculations indicate τ is around 15 h during the diurnal sampling period. In terms of δ¹⁵N(NO₃^-), it changed largely in our observation, from -2.9‰ to 18.1‰ with a mean of (6.4 ± 4.4) ‰. Correlation analysis implies that the combined effect of NO_X emission sources and isotopic fractionation processes are responsible for δ¹⁵N(NO₃^-) variations. Our observations with the aid of model simulation in future study will further improve the understanding of reactive nitrogen chemistry in urban regions.

Formation features of nitrous acid in the offshore area of the East China Sea

Nitrous acid (HONO) is an important precursor of hydroxyl radical (OH), which plays a key role in atmospheric chemistry. In this study, a shipboard-based measurement of HONO and related species in the offshore area of the East China Sea (ECS) was performed during June 2017. The HONO concentration ranged from 35 ppt to 1.95 ppb, with an average value of 0.44 ± 0.25 ppb during the entire campaign. HONO displayed a relatively higher level (0.48 ± 0.21 ppb) in the area within 30 km from the coastline (S1), whereas a lower level (0.40 ± 0.18 ppb) in the area between 30 km and 100 km from the coastline (S2). Five distinct hotspots of HONO were identified, including Ningbo Port, Yangshan Port, the Yangtze River estuary, northwest of the Zhoushan city, and the area adjacent to Jinshan Chemical Industry Park, suggesting the impact of local vessel emissions and land industrial emissions on HONO formation. During the nighttime, the direct vessel emissions contributed on average 18% of the HONO concentration. The averaged conversion frequency of NO₂-to-HONO (k_het) estimated from six nighttime cases was 1.18 × 10^-2 h^-1. Daytime budget analysis showed that the unknown HONO production rate (P_unknown) in S1 and S2 was 1.52 ppb h^-1 and 1.14 ppb h^-1, respectively. P_unknown was related to a light-induced HONO source from NO₂ on the sea surface and particulate nitrate. During the cruise campaign, the averaged daytime OH production rate from HONO photolysis was 1.35 ± 0.69 ppb h^-1, about 1.6 times higher than that from the O₃ photolysis (0.87 ± 0.55 ppb h^-1), which suggested an important role of HONO in the atmospheric chemistry of the offshore area of ECS.

Variations and sources of nitrous acid (HONO) during a severe pollution episode in Beijing in winter 2016

HONO is an important precursor of OH radical and plays a key role in atmospheric chemistry, but its source and formation mechanism remain uncertain, especially during complex atmospheric pollution processes. In this study, HONO mixing ratios were measured by a custom-made instrument during a severe pollution event from 16 to 23 December 2016, at an urban area of Beijing. The measurement was divided into three periods: I (haze), II (severe haze) and III (clean), according to the levels of PM_2.5. This pollution episode was characterized by high levels of NO (75 ± 39 and 94 ± 40 ppbV during periods I and II, respectively) and HONO (up to 10.7 ppbV). During the nighttime, the average heterogeneous conversion frequency during the two haze periods were estimated to be 0.0058 and 0.0146 h^-1, and it was not the important way to form HONO. Vehicle emissions contributed 52% (±16)% and 40% (±18)% to ambient HONO at nighttime during periods I and II. The contribution of homogeneous reaction of NO with OH should be reconsidered under high-NO_x conditions and could be noticeable to HONO sources during this pollution event. Furthermore, HONO was positively correlated with PM_2.5 during periods I and II, suggesting a potential chemical link between HONO and haze particles.

An observational study of nitrous acid (HONO) in Shanghai, China: The aerosol impact on HONO formation during the haze episodes

Continuous HONO measurement was conducted to study the formation features of HONO during the haze episodes at Shanghai, China. The HONO concentration ranged from 0.26 to 5.84ppb and averaged at 2.31ppb during the measurement period. The HONO concentration during the haze episode (P1), the haze-fog episode (P2) and the clean period (P3) were 2.80, 2.35 and 1.78ppb, respectively. Heterogeneous conversion of NO₂ was the dominate pathway for nocturnal HONO formation, and the heterogeneous conversion efficiency of NO₂ to HONO was closely associated with the PM_2.5 concentration. The averaged heterogeneous conversion rate of NO₂-to-HONO (C_HONO) during the pollution periods (P1+P2) was 1.58×10^-2h^-1, higher than that during the clean period (P3) (0.93×10^-2h^-1), suggesting the higher conversion potential of NO₂ to HONO during the pollution episodes. The daytime unknown HONO production rate (P_unknown) in the pollution period was 2.98ppb/h, higher than 1.78ppb/h in the clean period. Further, aerosol played a role in P_unknown during the transformation of the clean period to the pollution period. At a single particle scale, transmission electron microscopy (TEM) images revealed that most of the particles during P1 and P2 were agglomerated, whereas the particles collected from P3 were uniformly distributed and showed simple morphologies. The number percentage of the S/N-bearing particles during P1 (34%) and P2 (27%) were higher than that during P3 (20%). In addition, particles contained more internally mixed nitrates during P1 and P2 than those during P3, suggesting more intense heterogeneous conversion of NO₂ to HONO on particle surfaces during the pollution episodes. In the present study, the averaged HONO/NO_x ratio (5.60%), especially during P1 (7.80%) and P2 (7.50%) was much higher than that assumed global averaged value of 2.0%, suggesting a potentially important role for the HONO chemistry in Shanghai. This study provides new insights into the HONO formation mechanism in the atmosphere characterized by high fine particle level.

Exploring the nitrous acid (HONO) formation mechanism in winter Beijing: direct emissions and heterogeneous production in urban and suburban areas

Continuous measurements of nitrous acid (HONO) were performed from December 12 to December 22, 2015 in both urban and suburban areas of Beijing to study the formation mechanism of HONO. The measurement campaign in both sites included a clean-haze-clean transformation process. HONO concentrations showed similar variations in the two sites, while they were always higher in the urban area. Moreover, correlations of HONO with NOx, NO2, NO, PM2.5 and relative humidity (RH) were studied to explore possible HONO formation pathways, and the contributions of direct emissions, heterogeneous reactions, and homogeneous reactions were also calculated. This showed that HONO in urban and suburban areas underwent totally different formation procedures, which were affected by meteorological conditions, PM2.5 concentrations, direct emissions, homogeneous reactions and heterogeneous reactions. PM2.5 concentrations and RH would influence the NO2 conversion efficiency. Heterogeneous reactions of NO2 were more efficient in suburban areas and in clean periods while direct emissions and homogeneous reactions contributed more in urban areas and in polluted periods when the concentrations of NOx and NO were at a high level.

Concentration and sources of atmospheric nitrous acid (HONO) at an urban site in Western China

Highly time-resolved measurements of nitrous acid (HONO) were carried out with a highly sensitive long path absorption photometer (LOPAP) at an urban site of Xi'an in Western China from 24 July to 6 August 2015 to investigate the atmospheric variations, sources, and formation pathways of HONO. The concentrations of HONO vary from 0.02 to 4.3ppbv with an average of 1.12ppbv for the entire measurement period. The variation trends of HONO and NO₂ are very similar and positively correlated which, together with the similar diurnal profiles of HONO/NO₂ ratio and HONO, suggest the importance of heterogeneous conversion of HONO from NO₂. The nocturnal HONO level is governed by heterogeneous formation from NO₂, followed by homogeneous formation of NO with OH and then by direct emissions. Further, it is found that the heterogeneous formation of HONO is largely affected by relative humidity and aerosol surface. Daytime HONO budget analysis indicates that an additional unknown source with HONO production rate of 0.75ppbvh^-1 is required to explain the observed HONO concentration, which contributes 60.8% of the observed daytime HONO.

Measurements of nitrous acid (HONO) in urban area of Shanghai, China

Nitrous acid (HONO), as a precursor of the hydroxyl radical (OH), plays an important role in the photochemistry of the troposphere, especially in the polluted urban atmosphere. A field campaign was conducted to measure atmospheric HONO concentration and that of other pollutants (such as NO2 and particle mass concentration) in the autumn of 2009 at Shanghai urban areas. HONO mixing ratios were simultaneously measured by three different techniques: long path absorption photometer (LOPAP), differential optical absorption spectroscopy (DOAS) and chemical ionization mass spectrometer (CIMS). The measurements showed that the mixing ratios of HONO were highly variable and depended strongly on meteorological parameters. The HONO levels ranged from 0.5 to 7 ppb with maximum values during early morning and minimum levels during late afternoon. The three instruments reproduced consistent diurnal pattern of HONO concentrations with higher concentration during the night compared to the daylight hours. Comparison of HONOLOPAP/HONOCIMS ratios during daytime and nighttime periods exhibited a non-systematic disagreement of 0.93 and 1.16, respectively. This would indicate different chemical compositions of sampled air for the LOPAP and the CIMS instruments during daytime and nighttime periods, which have possibly affected measurements. Mean HONO concentration reported by LOPAP was 33 % higher than by DOAS on the whole period with no significant difference between daytime and nighttime periods. This revealed a systematic deviation from both instruments. The present data provides complementary information of HONO ambient levels in the atmosphere of Shanghai urban areas.