Peroxyacetyl Nitrate Concentrations Research Articles

Sensitivity of atmospheric peroxyacetyl nitrate (PAN) formation and its impact on ozone pollution in a coastal city

Peroxyacetyl nitrate (PAN) ranks among the troposphere's most prevalent organic peroxy radicals, and the photochemical production and loss pathways of PAN are mainly related to the generation and consumption of PA radicals. PA radicals can affect the atmospheric O3 generation by competing or providing precursors and affecting the atmospheric radical cycling. Field measurements took place at two sites in Shenzhen: the suburban site of Yangmeikeng Station (YMK) from September to October 2018, and the urban site of University town of Shenzhen (DXC) from September to October 2019. The average PAN levels of YMK and DXC were 0.57 and 0.87 ppb, which were similar to PAN concentrations in other southeastern coastal regions and lower than those in inland and northern regions in China. The model simulations showed that except PAN thermal decomposition, acetaldehyde (40.7% and 24.9%), other OVOCs (40.0% and 59.2%) and radical cycling (19.3% and 16.0%) were the major production pathways peroxyacetyl (PA) radicals at YMK and DXC. During the sampling period, the production of PAN resulted in a decreasing of local O3 generation at YMK (5.32 ± 3.2 ppb) and DXC (3.53 ± 12.21 ppb). The findings could make us more comprehensively understand the mechanism of atmospheric formation of PAN in urban and suburban sites in Shenzhen and its influence on the process of atmospheric photochemical reaction in Shenzhen.

Peroxyacetyl nitrate can be used as a comprehensive indicator of air pollution complex

With the acceleration of air cleaning activities in China, air pollution has entered a new stage characterized by seasonal interplay and predominance of fine particulate matter (PM2.5) and ozone (O3) pollutants. However, the differing peak seasons of these two pollution preclude the use of a unified indicator for air pollution complex. Given that peroxyacetyl nitrate (PAN) originates from secondary formation and persists under low-temperature conditions for extended periods, it is vital to determine whether its concentration can be used as an indicator to represent air pollution, not only in summer but also in winter. Here, PAN observational data from 2018 to 2022 for Beijing were analyzed. The results showed that during photochemical pollution events in summer, secondary formation of PAN was intense and highly correlated with O3 (R = 0.8), while during PM2.5 pollution events in winter, when the lifetime of PAN is extended due to the low temperature, the PAN concentration was highly consistent with the PM2.5 concentration (R = 0.9). As a result, the PAN concentration essentially exhibited consistency with both the seasonal trends in the exceedance of air pollution (R = 0.6) and the air quality index (R = 0.8). When the daily average concentration exceeds 0.5 and 0.9 ppb, the PAN concentration can be used as a complementary indicator of the occurrence of primary and secondary standard pollution, respectively. This study demonstrated the unique role of PAN as an indicator of air pollution complex, highlighting the comprehensive ability for air quality characterization and reducing the burden of atmospheric environment management.

Pollution characteristics of peroxyacetyl nitrate in karst areas in Southwest China

In recent years, the issue of photochemical pollution in the Guangxi region of China has escalated considerably. However, there remains a notable dearth of related research in this area. Peroxyacetyl nitrate (PAN), recognized as a reliable indicator of photochemical pollution, was the focus of our study. This study marks the inaugural observation of PAN levels in Guilin, a renowned world tourist destination situated in a typical karst region, during 1–31 October 2021. Throughout this observation period, the average volume concentration of PAN ranged from 0.087 to 2.559 ppb, which was 3.61 times higher than the South China background site of the Nanling. Combined with meteorological factors and potential source analysis, the causes of a typical high-value PAN process were explored during 24–29 October. The results showed that, during this high-value PAN event, pollution primarily originated from the horizontal transport of polluted air masses and the descent of high-altitude air masses from Hunan Province in the northeast direction. Additionally, the meteorological conditions, including high temperatures, intense radiation, and low humidity, fostered local PAN formation. Notably, traffic emissions emerged as the primary source of PAN's locally generated precursor volatile organic compounds. Furthermore, we estimated the background concentration of O3 to be approximately 20.347 ppb based on PAN monitoring data, constituting 44.4% of the total O3 levels in Guilin City. This study offers valuable insights for addressing and mitigating photochemical pollution in southern Chinese cities, while providing a theoretical foundation for regional pollution control efforts.摘要桂林是世界著名的地处喀斯特地区的旅游胜地, 其光化学污染问题日益严重.过氧乙酰硝酸酯(PAN)被认为是光化学污染的可靠指标, 也是本研究的重点. 本研究于2021年10月首次观测了桂林的PAN的浓度为0.087–2.559 ppb, 同时探讨了PAN典型高值过程的成因. 此次污染主要来源于东北方向污染气团的水平和高空输送, 同时, 高温, 强辐射和低湿度等气象条件也促进了本地PAN的形成. 本研究同时估算了桂林市的O3背景浓度为20.347 ppb.这项研究为城市的光化学污染控制工作提供了理论基础.

Seasonal discrepancies and inter-relationship of peroxyacetyl nitrate (PAN), ozone, and other environmental factors in Hangzhou, East China

Peroxyacetyl nitrate (PAN, CH3C(O)OONO2) and ozone (O3) are both important photochemical products, both of which are detrimental to air quality, human health and ecosystem. In this study, we measured atmospheric PAN, O3 and other pollutants in Hangzhou during the whole 2021 year. Observed results revealed that the average concentrations of PAN and O3 during the whole observation period were 1.59 and 27.09 ppbv, respectively. PAN concentrations declined from the weekday to weekend, while O3 presented the reverse pattern. The abundance of PAN and O3 showed different variations in all seasons. Serious photochemical pollution in spring and autumn were featured by higher PAN levels (mean of 1.74 and 1.66 ppbv); whereas those in summer and spring were characterized by higher O3 levels (mean of 32.34 and 30.98 ppbv). With the extremely higher PAN/O3 ratio of 0.10, the results suggested the high-level photochemical pollution in Hangzhou. In all seasons, the diurnal peak of PAN appeared earliest in summer could be associated with rapid decomposition rate of PAN at noon with higher temperatures. The correlation between PAN and other factors varies across all seasons due to the environmental conditions are unique in each season. Through analysis of conditional probability function (CPF) and toluene/benzene ratio, traffic sources were identified as the main factor contributing to high PAN and O3 accumulation at a regional scale. Furthermore, backward trajectory analysis revealed that marine air masses affected the air of the monitoring site in both summer and autumn; whereas continental air masses impacted it in both spring and winter. This study provides novel insights into photochemical pollution process in a coastal city, eastern China.

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.

Simultaneous observations of peroxyacetyl nitrate and ozone in Central China during static management of COVID-19: Regional transport and thermal decomposition

Peroxyacetyl nitrate (PAN) and ozone (O3) are two typical indicators of photochemical pollution, which are harmful to human body and environment. In this study, PAN and O3 were continuously monitored during static management (SM) of the COVID-19 epidemic in Zhengzhou. Briefly, SM refers to control policies in that all except for essential business are closed; work at home; and no people going out and no gathering, etc. During the SM, the temperature suddenly dropped from 26 °C to 14 °C, which provided a good experimental condition for the study of source transformation, accumulation, and consumption of PAN in the atmosphere. The concentrations of PAN were 0.6, 0.8, and 0.5 ppbv in three periods: before, during, and after the SM. During the SM, the lifetime of PAN was increased due to low temperature, and the accumulation time of PAN was longer, resulting in the increase of PAN concentration in daytime faster than the other two periods. During SM, especially after the temperature drops, the average atmospheric lifetime of PAN was 23.8 h, which was higher than 17.6 h before SM and 13.8 h after SM. This indicated that low temperature was more conducive to the accumulation of PAN. The high potential PAN source (CBPF>0.8) mainly came from the northeast of Zhengzhou, and the wind speed range was between 2.5 and 3 m/s. The backward trajectory analysis showed that 83.3% of the air mass from the northeast and northwest had a greater impact on the PAN concentration in Zhengzhou.

O3 and PAN in southern Tibetan Plateau determined by distinct physical and chemical processes

Abstract. Tropospheric ozone (O3) and peroxyacetyl nitrate (PAN) are both photochemical pollutants harmful to the ecological environment and human health. In this study, measurements of O3 and PAN as well as their precursors were conducted from May to July 2019 at Nam Co station (NMC), a highly pristine high-altitude site in the southern Tibetan Plateau (TP), to investigate how distinct transport processes and photochemistry contributed to their variations. Results revealed that, despite highly similar diurnal variations with steep morning rises and flat daytime plateaus that were caused by boundary layer development and downmixing of free-tropospheric air, day-to-day variations in O3 and PAN were in fact controlled by distinct physicochemical processes. During the dry spring season, air masses rich in O3 were associated with high-altitude westerly air masses that entered the TP from the west or the south, which frequently carried high loadings of stratospheric O3 to NMC. During the summer monsoon season, a northward shift of the subtropical jet stream shifted the stratospheric downward entrainment pathway also to the north, leading to direct stratospheric O3 entrainment into the troposphere of the northern TP, which traveled southwards to NMC within low altitudes via northerly winds in front of ridges or closed high pressures over the TP. Westerly and southerly air masses, however, revealed low O3 levels due to the overall less stratospheric O3 within the troposphere of low-latitude regions. PAN, however, was only rich in westerly or southerly air masses that crossed over polluted regions such as northern India, Nepal or Bangladesh before entering the TP and arriving at NMC from the south during both spring and summer. Overall, the O3 level at NMC was mostly determined by stratosphere–troposphere exchange (STE), which explained 77 % and 88 % of the observed O3 concentration in spring and summer, respectively. However, only 0.1 % of the springtime day-to-day O3 variability could be explained by STE processes, while 22 % was explained during summertime. Positive net photochemical formation was estimated for both O3 and PAN based on observation-constrained box modeling. Near-surface photochemical formation was unable to account for the high O3 level observed at NMC, nor was it the determining factor for the day-to-day variability of O3. However, it was able to capture events with elevated PAN concentrations and explain its day-to-day variations. O3 and PAN formation were both highly sensitive to NOx levels, with PAN being also quite sensitive to volatile organic compound (VOC) concentrations. The rapid development of transportation networks and urbanization within the TP may lead to increased emissions and loadings in NOx and VOCs, resulting in strongly enhanced O3 and PAN formation in downwind pristine regions, which should be given greater attention in future studies.

A WRF-CMAQ modeling of atmospheric peroxyacetyl nitrate and source apportionment in Central China

Atmospheric peroxyacetyl nitrate (PAN), as an essential constituent in the photochemical smog, is formed from photochemical reactions between volatile organic compounds (VOCs) and NOx. However, limited regional studies on distribution, formation and sources of PAN restrict the further understanding of the atmospheric behavior and environmental significance of PAN. In this study, the variation characteristics of PAN and the influencing factors to PAN concentrations were investigated using the WRF-CMAQ model simulation in the central China during July 2019. The results showed that the monthly mean concentration of PAN in the near-surface layer was 0.4 ppbv and increased with the height rising, accompanied by strong intra-day variation. The process analysis suggested that the removal was mainly controlled by dry deposition (57 %), followed by the gas-phase chemistry (43 %) which was mainly attributed to the thermal decomposition. Based on the sensitivity simulation, PAN concentrations decreased effectively in most of the simulated regions when precursors of VOCs and NOx emissions were reduced, and PAN concentrations were more sensitive to VOCs emissions than NOx emissions. The reduction of NOx and VOCs could lead to enhanced atmospheric oxidation in east-central region, which in turn hindered the decrease of PAN concentrations. During the simulation period, we found that emissions from industry and transportation sectors had the greatest impact on PAN concentrations in the central China, with contributions of 39 %–49 % and 33 %–41 %, respectively.

Seasonal variation characteristics of atmospheric peroxyacetyl nitrate (PAN) and its source apportionment in a megacity in southern China

Owing to its biotoxicity and inductive effect on photochemical pollution, atmospheric peroxyacetyl nitrate (PAN), which is a typical product of atmospheric photochemical reactions, has attracted much research attention. However, to the best of our knowledge, few comprehensive studies have been conducted on the seasonal variation and key influencing factors of PAN concentrations in southern China. In this study, PAN, ozone (O3), precursor volatile organic compound (VOC), and other pollutant concentrations were measured online for 1 year (from October 2021 to September 2022) in Shenzhen, a megacity in the Greater Bay Area of China. The average concentrations of PAN and peroxypropionyl nitrate (PPN) were 0.54 and 0.08 parts per billion (ppb), and the maximum hourly concentrations reached 10.32 and 1.01 ppb, respectively. The results of the generalized additive model (GAM) showed that the atmospheric oxidation capacity and precursor concentration were the most important factors affecting the PAN concentration. According to the steady-state model, the average cumulative contribution to the peroxyacetyl (PA) radical formation rate by six major carbonyl compounds was calculated at 4.2 × 106 molecules cm−3 s−1, and acetaldehyde (63.0 %) and acetone (13.9 %) contributed the most. Furthermore, the photochemical-age-based parameterization method was used to analyze the source contributions of carbonyl compounds and PA radicals. The results showed that although the primary anthropogenic (40.2 %), biogenic (27.8 %), and secondary anthropogenic (16.4 %) sources were the most important contributors of PA radicals, the biogenic and secondary anthropogenic source contributions both increased considerably in summer, and the cumulative proportion of both sources reached ~70 % in July. In addition, a comparison of PAN pollution processes in different seasons revealed that in summer and winter, the PAN concentration was predominantly limited by precursors and meteorological parameters, such as light intensity, respectively.

Characteristics and sources of peroxyacetyl nitrate (PAN) in the rural North China Plain: Results from 1-year continuous observations

Peroxyacetyl nitrate (PAN) is an important photochemical pollutant in the troposphere, whereas long-term measurements are scarce in rural areas in North China Plain (NCP), resulting in unclear seasonal variations and sources of PAN in rural NCP. In this study, we conducted a 1-year observation of PAN during 2021-2022 at the rural NCP site. The average concentrations of PAN were 1.10, 0.75, 0.65, and 0.88 ppbv in spring, summer, autumn, and winter, respectively, with a 1-year average of 0.81 ± 0.60 ppbv. Calculations indicate that the loss of PAN through thermal decomposition in summer accounts for 43.2% of the total formed PAN, which is an important reason for the low concentration of PAN in summer. We speculate that since the correlation between PAN and O3 in winter is significantly lower than that in other seasons, the observed regional transport of PAN cannot be ignored in winter. Through budget analysis, regional transport accounted for 12.8% and 55.9% of the observed PAN on the spring and winter pollution days, respectively, which showed that regional transport played key roles during the photochemical pollution of the rural NCP in winter. The potential source contribution function revealed that the transported PAN mainly comes from southern Hebei in spring. In winter, the transported PAN was mainly from Langfang, Hengshui, and southern Beijing. Our findings may aid in understanding PAN variations in different seasons in rural areas and highlight the impact of regional transport on the PAN budget.

Heterogeneous Reaction of Peroxyacetyl Nitrate on Real-World PM2.5 Aerosols: Kinetics, Influencing Factors, and Atmospheric Implications.

The formation and decomposition of peroxyacetyl nitrate (PAN), an important atmospheric nitrogen oxide reservoir, can impact the level and cycling of free radicals and nitrogen compounds in the atmosphere. PAN sinks are poorly understood, highlighting the importance of elucidating the heterogeneous reaction of PAN on aerosol surfaces. Here, we report for the first time the uptake behavior, kinetics, and potential mechanism of PAN uptake on real-world aerosol PM2.5 using a flow tube system. The uptake coefficients (γ) of PAN increased non-linearly from (1.5 ± 0.7) × 10-5 at 0% relative humidity (RH) to (9.3 ± 2.0) × 10-5 at 80% RH. The γ decrease with increasing initial PAN concentration is consistent with the Langmuir-Hinshelwood mechanism. Organic components of aerosols may promote heterogeneous loss of PAN through redox reactions. Higher γ occurs with higher water content, lower pH, and lower ionic strength in the aqueous phase of aerosols. The present study suggests that heterogeneous reaction of PAN on ambient aerosols plays a non-negligible role in the atmospheric PAN budget and provides new insights into the role of PAN in promoting atmospheric oxidation capacity during hazy periods with cold and wet weather conditions.

Influence of thermal decomposition and regional transport on atmospheric peroxyacetyl nitrate (PAN) observed in a megacity in southern China

Peroxyacetyl nitrate (PAN) is a typical secondary product of photochemical smog and is widely considered an indicator when evaluating the severity of photochemical pollution, but there have been few quantitative studies on the thermal decomposition of ambient PAN in southern China. In this study, online sampling of PAN and ozone (O3) was conducted from August to November 2020, which is the most severe photochemical pollution period of the year, in Shenzhen, a megacity in southern China. During the sampling period, two photochemical pollution periods occurred in August (EP1) and October (EP2). The mean O3 concentrations during EP1 and EP2 were very close, 36.3 and 36.9 ppb, respectively, while PAN concentrations were very different (0.66 and 1.06 ppb, respectively). The diurnal variation showed that the PAN formation rate in the initial stage of photochemical reactions (9:00–11:00) during EP1 was significantly faster than that during EP2, while there was significant consumption at noon and almost complete decomposition at night in EP1. Calculating the thermal decomposition of PAN (TPAN), we found that the average concentration of TPAN in EP1 was 8.47 ppb, approximately 9 times that in EP2 (0.94 ppb), which significantly reduced the concentration of PAN in EP1. This indicates that when using PAN as an indicator of photochemical reaction activity, the consumption of PAN due to thermal decomposition must be considered. In addition, we found that regional transport may significantly contribute to the formation of PAN by increasing the concentration of precursor VOCs, although the lifetime of PAN was only a few hours due to a large amount of thermal decomposition.

Effects of Meteorological Factors on the Vertical Distribution of Peroxyacetyl Nitrate in Autumn in Tianjin

With the decrease of PM2.5 concentration, atmospheric photochemical pollution in North China Plain has attracted more and more attention. Peroxyacetyl nitrate (PAN) is an important product and reliable indicator of atmospheric photochemical reaction. In this study, the measured concentrations of PAN near the surface and at 220 m were obtained from the Tianjin Meteorological Tower in October 2018. The results showed that the average concentrations of PAN at ground and 220m platform were 1.65 ± 1.34 ppb and 1.76 ± 1.41 ppb respectively during the day, and the mixture was relatively uniform. The average concentrations of PAN were 0.99 ± 0.94 ppb and 1.42 ± 1.28 ppb during the nighttime, respectively, indicating that the vertical difference was significantly increased. The standard deviation of PAN concentration was close to the average value, indicating that the concentration fluctuated greatly. The correlation coefficient between PAN and PM2.5 was higher than that of O3, indicating that stable weather was an important factor affecting PAN concentration. Based on the analysis of the vertical wind speed by the wind profile radar and ozone concentration vertical distribution data, we believe that there is a downdraft in the upper atmosphere of the boundary layer. The higher concentration of PAN at 220 m at night came from the high-altitude pollution air mass. The analysis of 40 m and 200 m turbulence data showed that the weak turbulence hindered the vertical exchange between the surface and the upper atmosphere, resulting in the high concentration of PAN unable to reach the surface. The boundary layer structure is one of the important reasons for the vertical difference of PAN concentration at night in autumn in NCP.

Measurement report: Fast photochemical production of peroxyacetyl nitrate (PAN) over the rural North China Plain during haze events in autumn

Abstract. Photochemical pollution over the North China Plain (NCP) is attracting much concern. We usually view peroxyacetyl nitrate (PAN) as the second most important photochemical pollutant featuring high mixing ratios during warm seasons. Our observations at a background site in the NCP identified high PAN concentrations, even during haze events in autumn. The substantial increasing ratios of PAN, by 244 % and 178 %, over the morning hours (08:00–12:00 local time) on 20 and 25 October 2020 were 10.6 and 7.7 times larger than those on clean days. Polluted days are characterized by higher temperature, higher humidity, and anomalous southerly winds compared with clean days. Enhanced local photochemistry has been identified as being the dominant factor that controls the PAN increase in the morning at the rural site, as the time when prevailing wind turns to a southerly wind is too late to promote direct transport of PAN from the polluted urban region. By removing the effect of direct transport of PAN, we provide a quantitative assessment of net PAN chemical production rate of 0.45 ppb h−1 for the mornings of polluted days, also demonstrating the strong local photochemistry. Using observations and calculated photolysis rates, we find that acetaldehyde oxidation by hydroxyl radical (OH) is the primary pathway of peroxyacetyl radical formation at the rural site. Acetaldehyde concentrations and production rates of HOx (HOx= OH + HO2) on polluted days are 2.8 and 2 times as large as those on clean days, leading to a remarkable increase in PAN in the morning. Formaldehyde (HCHO) photolysis dominates the daytime HOx production, thus contributing to fast photochemistry of PAN. Our observational results suggest the cause of a rapid increase in PAN during haze events in autumn at a rural site of the NCP and provide evidence of important role of HCHO photolysis in secondary pollutants at lower nitrogen oxide emissions. This highlights the urgency of carrying out strict volatile organic compound controls over the NCP during the cold season and not just in summer.

Spatial distribution and source apportionment of peroxyacetyl nitrate (PAN) in a coastal region in southern China

Peroxyacetyl nitrate (PAN) is a typical secondary product of photochemical pollution and has attracted considerable attention because of its important role in atmospheric photochemical reactions and biotoxicity. Although existing studies have determined the dominant species of volatile organic compounds (VOCs) that can generate PAN, there are few studies including quantitative source analysis of PAN and precursor VOCs. In this study, PAN and precursor VOCs were measured at three sites in Shenzhen, a megacity on the southeast coast of China. The mean daytime (8:00–18:00) concentrations of PAN at the three sites during the sampling period were 0.70 ppbv (YMK, rural), 1.31 ppbv (DXC, residential) and 1.52 ppbv (FC, industrial), which had a different spatial distribution pattern compared to those of O3 and better reflect the degree of local anthropogenic emissions. Based on the steady-state model, the calculated PA radical formation rates cumulative contribution by six major carbonyl compounds at YMK and DXC were 4.4 × 106 molecules*cm−3*s−1 and 6.8 × 106 molecules*cm−3*s−1, respectively, and acetaldehyde contributed the highest percentage (44.4–54.3%) followed by acetone (15.1–25.6%), diacetyl (10.0–10.9%) and methylglyoxal (12.2–13.4%). Using an improved photochemical age-based parameterization method to analyze the daytime sources of PA radicals, the results show that biogenic sources were the most important source of PA radicals, accounting for 27.1% and 29.8% at YMK and DXC, respectively, and the contribution of anthropogenic secondary sources at DXC (25.8%) was significantly higher than at YMK (17.0%). The contribution of biomass burning at YMK reached 24.2%, emphasizing a critical role of biomass burning in atmospheric photochemical pollution in rural areas.

Formation of peroxyacetyl nitrate (PAN) and its impact on ozone production in the coastal atmosphere of Qingdao, North China.

Peroxyacetyl nitrate (PAN), acting as a relatively long-lived reservoir for both NOx and radicals, plays a crucial role in ozone (O3) formation in the troposphere. However, its quantitative impacts on radical concentrations and O3 production were rarely studied in the coastal atmosphere. In this study, ambient concentrations of PAN, O3, and related species were simultaneously measured from October 5 to November 10, 2018 (autumn), and July 14 to August 24, 2019 (summer) at a rural coastal site in Qingdao, North China. The formation mechanism of PAN and its impact on in-situ O3 production were explored with an observation-based chemical box model. Photochemical formation of PAN and O3 was controlled by both NOx and VOCs, and acetaldehyde and methylglyoxal were the main contributors to PAN formation. However, the sensitivities of PAN to precursors were larger than that of O3 in autumn while smaller in summer, which was mainly caused by the rapid decomposition of PAN at high temperatures. Zero-out sensitivity simulation showed that PAN could either promote or inhibit the in-situ O3 formation by affecting the radical chemistry. It tended to suppress O3 production by competing with precursors and terminating radical chain reactions under low-NOx and low-ROx circumstances but enhanced O3 production by supplying RO2 radicals under conditions with sufficient NOx. This study provides some new complementary insights into the formation mechanism of PAN and its impacts on O3 production, and has implications for the formulation of control policy to mitigate regional photochemical pollution in northern China.

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, PM2.5, 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 PM2.5-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.

CORRELACIÓN PAN-O3 EN EL SUROESTE DE LA CIUDAD DE MÉXICO

Peroxyacetyl nitrate (PAN) and NO 2 (true) were measured during two short campaigns in June and October, 2017 with a gas chromatography-luminol detection method in southwest Mexico City. The maximum and total hourly mixing ratio measured averages of PAN were 5.2 and 1.3 ppb v in June and 3.3 and 0.6 ppb v in October, respectively, being these values slightly lower than those reported in 2003 suggesting that PAN precursor levels have only partially decreased during the last 15 years. PAN hourly averages of the two campaigns were correlated with O 3 , NO 2 (by chemiluminescence), solar radiation, temperature, and wind speed measured in the same monitoring site in order to obtain a combined empirical model (CEM) for the diagnosis of PAN concentrations for other periods of the year. The resulting CEM was integrated with several multiple linear correlations which are applied depending on the period of the day and the ambient temperature. The correlation between both, the simulated and measured PAN resulted in a coefficient R 2 = 0.87. The CEM was applied to records of the above mentioned variables measured at the Centro de Ciencias de la Atmosfera monitoring station at Ciudad Universitaria for the 2017 ozone season. A positive temperature dependent correlation was obtained between PAN and ozone, but the correlation was weaker as ambient temperature exceeded the 20 oC.

Effect of potential HONO sources on peroxyacetyl nitrate (PAN) formation in eastern China in winter

As an important secondary photochemical pollutant, peroxyacetyl nitrate (PAN) has been studied over decades, yet its simulations usually underestimate the corresponding observations, especially in polluted areas. Recent observations in north China found unusually high concentrations of PAN during wintertime heavy haze events, but the current model still cannot reproduce the observations, and researchers speculated that nitrous acid (HONO) played a key role in PAN formation. For the first time we systematically assessed the impact of potential HONO sources on PAN formation mechanisms in eastern China using the Weather Research and Forecasting/Chemistry (WRF-Chem) model in February of 2017. The results showed that the potential HONO sources significantly improved the PAN simulations, remarkably accelerated the ROx (sum of hydroxyl, hydroperoxyl, and organic peroxy radicals) cycles, and resulted in 80%–150% enhancements of PAN near the ground in the coastal areas of eastern China and 10%–50% enhancements in the areas around 35–40°N within 3 km during a heavy haze period. The direct precursors of PAN were aldehyde and methylglyoxal, and the primary precursors of PAN were alkenes with C > 3, xylenes, propene and toluene. The above results suggest that the potential HONO sources should be considered in regional and global chemical transport models when conducting PAN studies.

A study of peroxyacetyl nitrate at a rural site in Beijing based on continuous observations from 2015 to 2019 and the WRF-Chem model

Peroxyacetyl nitrate (PAN) is one of the most important photochemical pollutants and has aroused much concern in China in recent decades. However, few studies described the long-term variations in PAN in China. In this study, we continuously monitored the PAN, O3 and NOx concentrations at a regional background site near Beijing from August 2015 to February 2019. Based on the observed concentrations and climate data, we analyzed the seasonal PAN variations. The results revealed that the monthly mean PAN concentration ranged from 0.33–2.41 ppb, with an average value of 0.94 ppb. The PAN concentration exhibited a distinct seasonal variation, with high values in spring and low values in winter. After analyzing the corresponding meteorological data, we found that stronger ultraviolet (UV) radiation, a relatively longer lifetime and a higher background PAN concentration contributed to the high PAN concentrations in spring. In addition, with the utilization of the WRF-Chem (Weather Research and Forecasting with Chemistry) model, the cause of the extremely high PAN concentration in spring 2018 was determined. The model results demonstrated that an anomalously low pressure and the southwesterly winds in northern China might be the main causes of the increased PAN concentration in Beijing and its surrounding area in spring 2018.