Investigation on the budget of peroxyacetyl nitrate (PAN) in the Yangtze River Delta: Unravelling local photochemistry and regional impact.

Peroxyacetyl nitrate (PAN) is a significant indicator of atmospheric photochemical pollution, which can influence the regional distribution of ozone (O3) and hydroxyl radical (OH) through long-range transport. However, investigations of PAN incorporating comprehensive measurement and explicit modeling analysis are limited, hindering complete understandings of its temporal behavior, sources, and impacts on photochemistry. Here we conducted a 1-year continuous observation of PAN and relative atmospheric species in Nanjing located in Yangtze River Delta (YRD). The annual mean concentration of PAN was 0.62 ± 0.49 ppbv and showed a bimodal monthly variation, peaking in April-June and November-January, respectively. This pattern is different from the typical pattern of photochemistry, suggesting important contributions of other non-photochemical processes. We further analyzed the PAN budget using an observation-based model, by which, PAN from local photochemical production and regional source could be decoupled. Our results revealed that local photochemical production of PAN is the sole contributor to PAN in summer, whereas about half of the total PAN concentration is attributed to regional source in winter. Although the formation of PAN can suppress the atmospheric oxidation capacity by consuming the peroxyacetyl radical and nitrogen dioxide (NO2), our analyses suggested this effect is minor at our station (-3.2 ± 1.1 % in summer and - 7.2 ± 2.8 % in winter for O3 formation). However, it has the potential to enhance O3 and OH formation by 14.16 % and 5.93 %, if transported to cleaner environments with air pollutants halved. Overall, our study highlights the importance of both local photochemistry and regional process in PAN budget and provides a useful evaluation on the impact of PAN on atmospheric oxidation capacity.

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.

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.

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.

Characteristics of peroxyacetyl nitrate (PAN) in a coastal city of southeastern China: Photochemical mechanism and pollution process.

Peroxyacetyl nitrate (PAN) can effectively indicate photochemical pollution, and also plays a vital role in regional oxidant balance. One-year continuous monitoring of PAN in a coastal city of southeastern China was investigated. The mean concentration of PAN in winter (0.64 ppb) was close to that in autumn (0.73 ppb), indicating that photochemical pollution was still non-negligible in the cold season. The peak occurrence time between O3 and PAN had a delay of 1-2 h in four seasons, due to the rapid decomposition rate of PAN in midday. Emission sources of the precursors are located to the south of the monitoring site, so high concentrations of PAN and O3 are frequently observed under southerly wind conditions. The air mass with low concentration of PAN (0.22-0.34 ppb) and O3 (18.17-23.67 ppb) originated from the ocean with less anthropogenic air pollutants. Continental air mass with high PAN concentration might be related to the contribution of heterogeneous reactions of PM2.5 to the promotion of PAN formation. In the polluted case, PAN concentration was often higher than 1.0 ppb and reached the peak of 4.2 ppb, suggesting the influence of photochemical reactions and local accumulation. High concentrations of HONO and sufficient ultraviolet radiation might be the main factors for rapid photochemical production of PAN. Besides, the lifetime of PAN in winter under the condition of high PM2.5 concentration (≥35 µg·m-3) was up to 3.246 days. This study provided insights into photochemical mechanism and pollution process in a coastal city of southeastern China.

[Analysis of Peroxyacetyl Nitrate and Ozone During a Typical Photochemical Pollution Process at the Panyu Atmospheric Composition Station].

The typical photochemical pollution process in Guangzhou was analyzed from 2010 to 2016 by observing photochemical pollutants (ozone (O3), peroxyacetyl nitrate (PAN), volatile organic compounds (VOCs), nitrogen dioxide (NO2), and nitric oxide (NO)) and meteorological factors at the Guangzhou Panyu Atmospheric Composition Station (GPACS). The results showed that during this photochemical pollution process, the average O3 and PAN concentrations were relatively high:O3,max 1 h concentration was 140.6×10-9 and PANmax 1 h concentration was 4.7×10-9. The average concentration of NO was low, which had little effect on O3 titration and PAN elimination. The high NO2 concentration, strong radiation, and low wind speed all led to the formulation and accumulation of O3 and PAN. The linear relationship between O3 and PAN (R2=0.55) was affected by the fact that their precursor VOCs were not similar. Ethene, propane, isoprene, and toluene had a large influence in PAN formation, while isoprene, 1,3,5-trimethylbenzene, propene, m,p-xylene, and toluene had a large influence on O3 formation potential. When evaluating PA concentration, we found that its daily average concentration ranged from 0.11×10-12 to 0.16×10-12, which were higher than many places around the world, indicating that this photochemical process was relatively intensive.

[Transport Characteristics of PAN and O3 in the Lower Atmosphere of the Boundary Layer in Tianjin in Summer].

The volume concentration of peroxyacetyl nitrate (PAN) and O3 in the atmosphere were measured at the Tianjin Meteorological Tower in summer 2017 by using the online instrument with meteorological parameters and back trajectory analysis to analyze the delivery characteristics of PAN and O3. The average volume concentrations of PAN and O3 during the observational period are (0.73±0.56)×10-9 and (53±25)×10-9, respectively. The hourly maximum concentrations of PAN and O3 are 3.49×10-9 and 137×10-9. The volume concentrations of PAN and O3 show pronounced diurnal profiles, which are both characterized by much higher values at daytime than at nighttime. In addition, the correlation coefficient between PAN and O3 at daytime (R2=0.52) is notably higher than that at nighttime (R2=0.21). The air masses originating from the south show the highest volume concentration of PAN and O3, with the lowest volume concentration originating from the east. The wind rose plot and cluster analysis of the back trajectories show that the highest concentration of pollutants mainly originates in the southwest. The air massess originating from the east and circulating through the Bohai Sea and coastal areas of the Hebei and Liaoning provinces show the lowest volume concentrations of PAN and O3. The transportation within the boundary layer plays an important role in the concentration distribution of PAN and O3.

Characteristics of peroxyacetyl nitrate pollution during a 2015 winter haze episode in Beijing.

Peroxyacetyl nitrate (PAN) are effective indicators of photochemical pollution, and also play an important role in regional oxidant balance. Surprisingly, in recent years, PAN have also been detected under conditions that do not favor the photochemical processes. To obtain a better understanding of the mechanisms of formation of atmospheric compound pollution, this study examined the relationships between concentrations of PAN and other pollutants (e.g., ozone [O3] and PM2.5) during a winter haze episode. The observation periods were from December 31, 2015, to February 2, 2016, and from February 19, 2016, to March 4, 2016. The maximum daily concentration of PAN during haze episodes was 4-10 times higher than that during non-haze episodes. The continuous cumulative increase in PAN concentrations was the result of a combination of photochemical production during the daytime and production based on free radical chemical reactions during the nighttime. During the haze episode, the correlation between concentrations of PAN and O3 was weak, while a significant correlation was observed between PAN and PM2.5 concentrations (R2 = 0.82). This may have been due to higher concentrations of particulate matter impairing illumination, which can then inhibit the photochemical reactions that produce PAN and O3. OH radicals can replace the role of light in PAN formation, which can cause concentrations of PAN and O3 to vary independently. During the haze episode, the ratio of PAN/O3 was around 0.3, which was much higher than that during the clean period.