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

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.

Significant contribution of carbonyls to atmospheric oxidation capacity (AOC) during the winter haze pollution over North China Plain

Atmospheric carbonyl compounds play significant roles in the cycling of radicals and have exhibited surprisingly high levels in winter that were well correlated to particulate matter, for which the reason have not been clearly elucidated. Here we measured carbonyl compounds and other trace gasses together with PM2.5 over urban Jinan in North China Plain during the winter. Markedly higher carbonyl concentrations (average: 14.63 ± 4.21 ppbv) were found during wintertime haze pollution, about one to three-times relative to those on non-haze days, with slight difference in chemical composition except formaldehyde (HCHO). HCHO (3.68 ppbv), acetone (3.17 ppbv), and acetaldehyde (CH3CHO) (2.83 ppbv) were the three most abundant species, accounting for ∼75% of the total carbonylson both haze and non-haze days. Results from observational-based model (OBM) with atmospheric oxidation capacity (AOC) indicated that AOC significantly increased with the increasing carbonyls during the winter haze events. Carbonyl photolysis have supplied key oxidants such as RO2 and HO2, and thereby enhancing the formation of fine particles and secondary organic aerosols, elucidating the observed haze-carbonyls inter-correlation. Diurnal variation with carbonyls exhibiting peak values at early-noon and night highlighted the combined contribution of both secondary formation and primary diesel-fuel sources. 1-butene was further confirmed to be the major precursor for HCHO. This study confirms the great contribution of carbonyls to AOC, and also suggests that reducing the emissions of carbonyls would be an effective way to mitigate haze pollution in urban area of the NCP region.

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.

Severe photochemical pollution formation associated with strong HONO emissions from dew and guttation evaporation

The unknown daytime source of HONO has been extensively investigated due to unexplained atmospheric oxidation capacity and current modelling bias, especially during cold seasons. In this study, abrupt morning increases in atmospheric HONO at a rural site in the North China Plain (NCP) were observed almost on daily basis, which were closely linked to simultaneous rises in atmospheric water vapor content and NH3 concentrations. Dew and guttation water formation was frequently observed on wheat leaves, from which water samples were taken and chemically analyzed for the first time. Results confirmed that such natural processes likely governed the daily nighttime deposition and daytime release of HONO and NH3, which have not been considered in the numerous HONO budget studies investigating its large missing daytime source in the NCP. The dissolved HONO and NH3 in leaf surface water droplets reached 1.4 and 23 mg L-1 during the morning on average, resulting in averaged atmospheric HONO and NH3 increases of 0.89 ± 0.61 and 43.7 ± 29.3 ppb during morning hours, with relative increases of 186 ± 212 % and 233 ± 252 %, respectively. The high atmospheric oxidation capacity contained within HONO was stored in near surface liquid water (such as dew, guttation and soil surface water) during nighttime, which prevented its atmospheric dispersion after sunset and protected it from photodissociation during early morning hours. HONO was released in a blast during later hours with stronger solar radiation, which triggered and then accelerated daytime photochemistry through the rapid photolysis of HONO and subsequent OH production, especially under high RH conditions, forming severe secondary gaseous and particulate pollution. Results of this study demonstrate that global ecosystems might play significant roles in atmospheric photochemistry through nighttime dew formation and guttation processes.

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.

Strong relations of peroxyacetyl nitrate (PAN) formation to alkene and nitrous acid during various episodes

Peroxyacetyl nitrate (PAN) is one of the critical secondary pollutants in photochemical smog. This study investigated the relationship between PAN and PAN precursors with the Regional Atmospheric Chemical Mechanism version 2 model in six episodes recorded in Zhengzhou. In all episodes, peroxyacetyl radical (PA) was primarily produced by acetaldehyde oxidation, with more than 70% contributions. In photochemical episodes and photochemical-haze co-occurring episodes (combined episodes), methylglyoxal secondarily contributes 8.1%-10.6% to PA while in haze pollution, the propagation of other radicals to PA is the second most important source (12.0%-19.1%). Among anthropogenic non-methane hydrocarbons, alkene restricted PAN formation as first-generation precursors, with the relative incremental reactivity of PAN (RIR_PAN) more than 0.6 during three-type episodes. Nitrous acid (HONO) also played important role in PAN formation. Especially during photochemical episodes, RIR_PAN(HONO) reached 0.79, which was comparable to the RIR_PAN value of alkene. Through sensitivity analysis of the relative formation of PAN to O₃ (the amount of PAN generated when 100 ppb O₃ formed), HONO was identified as the key precursor of PAN in haze pollution by promoting the oxidation of NMHC, while alkene predominated the relative formation of PAN to O₃ in photochemical and combined pollution through producing acetaldehyde. The sensitivity of PAN to HONO is obviously enhanced with higher NOx/VOC ratios during photochemical and combined pollution.

Anthropogenic Pollutants Induce Changes in Peroxyacetyl Nitrate Formation Intensity and Pathways in a Mountainous Background Atmosphere in Southern China

Mountainous background areas are typically considered to have a clean atmosphere where peroxyacetyl nitrate (PAN) can be decomposed. This study demonstrated that PAN was photochemically formed with a simulated production rate of 0.28 ± 0.06 ppbv h^-1 in the Nanling mountains (1690 m a.s.l.) of South China and that net PAN formation was dependent on both volatile organic compounds (VOCs) and NO_x precursors (transition regime). In contrast to dominated acetaldehyde oxidation in previous urban and rural research, PAN at Nanling was primarily formed by methylglyoxal (38%), acetaldehyde (28%), radicals (20%), and other oxygenated volatile organic compounds (OVOCs) (13%). Moreover, when polluted air masses invaded the Nanling mountains, the PAN production rate was altered, primarily because anthropogenic aromatics intensified PAN formation via the oxidized pathways of methylglyoxal, other OVOCs, and radicals. Finally, net PAN formation at Nanling reduced the hydroxyl radical level by consuming NO_x, impaired local radical cycling, and thereby suppressed local O₃ production. This suppressing effect was exacerbated on polluted days. The findings of this study deepen our understanding of PAN photochemistry and the impact of anthropogenic intrusions on the background atmosphere of mountainous regions.

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.

A sensitive simultaneous detection approach for the determination of 30 atmospheric carbonyls by 2,4-dinitrophenylhydrazine derivatization with HPLC-MS technique and its preliminary application

Atmospheric carbonyls are important precursors of PM_2.5 and ground-level ozone, and some carbonyls are toxic and harmful; thus, it is crucial to obtain accurate information on the ambient levels of carbonyls. However, the detection of carbonyls is difficult due to their relatively higher reactivities and chemical instabilities; therefore, accurate determination of atmospheric carbonyls is important. In this study, an analytical method for atmospheric carbonyls with high concentration or reactivity was developed, the precursor ion of each carbonyl compound was selected, and the declustering potential (DP) and entrance potential (EP) for each precursor ion were optimized. A 2,4-dinitrophenylhydrazine cartridge derivatization-high performance liquid chromatography/atmospheric pressure chemical ionization-mass spectrometry (DNPH-HPLC/APCI-MS) method for the determination of 30 carbonyls was established. The results showed that the linear range of 24 carbonyls was 1.2-600 ng/mL, while other 6 carbonyls was 1.2-300 ng/mL, and the detection limits of 30 carbonyls ranged from 0.092 to 0.947 ng/mL (0.005-0.049 μg/m³ with an ambient air sampling volume of 96 L). The intra-day and inter-day repeatability ranges were 0.55-4.20% and 1.40-12.48%, respectively. A preliminary application of the method was carried out in the urban area of Beijing in spring and summer of 2021, and it was found that the mean total mass concentration of 30 carbonyls was 35.894 μg/m³. This study provided additional concentration information for 14 atmospheric carbonyls, including mono-, di-, oxygen-containing and heterocyclic carbonyls, which accounted for 38% and 35% of the total mass concentration and OH radical reactivities of 30 carbonyls, respectively. This is the first investigation of simultaneous quantitative analysis of multiple atmospheric carbonyls based on commercial standard derivatives. The optimized method could provide more comprehensive information for atmospheric carbonyls and further support research concerning the role of chemical reaction processes and health effects than traditional measuring techniques.

Nonlinear response of nitrate to NOx reduction in China during the COVID-19 pandemic

In recent years, nitrate plays an increasingly important role in haze pollution and strict emission control seems ineffective in reducing nitrate pollution in China. In this study, observations of gaseous and particulate pollutants during the COVID-19 lockdown, as well as numerical modelling were integrated to explore the underlying causes of the nonlinear response of nitrate mitigation to nitric oxides (NO_x) reduction. We found that, due to less NO_x titration effect and the transition of ozone (O₃) formation regime caused by NO_x emissions reduction, a significant increase of O₃ (by ∼ 69%) was observed during the lockdown period, leading to higher atmospheric oxidizing capacity and facilitating the conversion from NO_x to oxidation products like nitric acid (HNO₃). It is proven by the fact that 26-61% reduction of NO_x emissions only lowered surface HNO₃ by 2-3% in Hebi and Nanjing, eastern China. In addition, ammonia concentration in Hebi and Nanjing increased by 10% and 40% during the lockdown, respectively. Model results suggested that the increasing ammonia can promote the gas-particle partition and thus enhance the nitrate formation by up to 20%. The enhanced atmospheric oxidizing capacity together with increasing ammonia availability jointly promotes the nitrate formation, thereby partly offsetting the drop of NO_x. This work sheds more lights on the side effects of a sharp NO_x reduction and highlights the importance of a coordinated control strategy.