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Oxygenated organic molecules (OOMs) are critical intermediates linking volatile organic compound oxidation and secondary organic aerosol (SOA) formation. Yet, the understanding of OOM components, formation mechanism, and impacts are still limited, especially for urbanized regions with a cocktail of anthropogenic emissions. Herein, ambient measurements of OOMs were conducted at a regional background site in South China in 2018. The molecular characteristics of OOMs revealed dominant nitrogen-containing products, and the influences of different factors on OOM composition and oxidation state were elucidated. Positive matrix factorization analysis resolved the complex OOM species to factors featured with fingerprint species from different oxidation pathways. A new method was developed to identify the key functional groups of OOMs, which successfully classified the majority species into carbonyls (8%), hydroperoxides (7%), nitrates (17%), peroxyl nitrates (10%), dinitrates (13%), aromatic ring-retaining species (6%), and terpenes (7%). The volatility estimation of OOMs was improved based on their identified functional groups and was used to simulate the aerosol growth process contributed by the condensation of those low-volatile OOMs. The results demonstrate the predominant role of OOMs in contributing sub-100 nm particle growth and SOA formation and highlight the importance of dinitrates and anthropogenic products from multistep oxidation.

Multiphase oxidation of sulfur dioxide (SO 2 ) by ozone (O 3 ) in alkaline sea salt aerosols is an important source of sulfate aerosols in the marine atmosphere. However, a recently reported low pH of fresh supermicron sea spray aerosols (mainly sea salt) would argue against the importance of this mechanism. Here, we investigated the impact of ionic strength on the kinetics of multiphase oxidation of SO 2 by O 3 in proxies of aqueous acidified sea salt aerosols with buffered pH of ∼4.0 via well-controlled flow tube experiments. We find that the sulfate formation rate for the O 3 oxidation pathway proceeds 7.9 to 233 times faster under high ionic strength conditions of 2-14 mol kg -1 compared to the dilute bulk solutions. The ionic strength effect is likely to sustain the importance of multiphase oxidation of SO 2 by O 3 in sea salt aerosols in the marine atmosphere. Our results indicate that atmospheric models should consider the ionic strength effects on the multiphase oxidation of SO 2 by O 3 in sea salt aerosols to improve the predictions of the sulfate formation rate and the sulfate aerosol budget in the marine atmosphere.

Oxidation of volatile organic compounds (VOCs) forms oxygenated organic molecules (OOMs), which contribute to secondary pollution. Herein, we present measurement results of OOMs using chemical ionization mass spectrometry with nitrate as the reagent ion in Shanghai. Compared to those in forests and laboratory studies, OOMs detected at this urban site were of relatively lower degree of oxygenation. This was attributed to the high NO x concentrations (∼44 ppb), which overall showed a suppression on the propagation reactions. As another result, a large fraction of nitrogenous OOMs (75%) was observed, and this fraction further increased to 84% under a high NO/VOC ratio. By applying a novel framework on OOM categorization and supported by VOC measurements, 50 and 32% OOMs were attributed to aromatic and aliphatic precursors, respectively. Furthermore, aromatic OOMs are more oxygenated (effective oxygen number, n O eff = 4-6) than aliphatic ones ( n O eff = 3-4), which can be partly explained by the difference in initiation mechanisms and points to possible discrimination in termination reactions. This study highlights the roles of NO x in OOM formation in urban areas, as well as the formation of nitrogenous products that might show discrimination between aromatic and aliphatic VOCs.

New particle formation (NPF) induces a sharp increase in ultrafine particle number concentrations and potentially acts as an important source of cloud condensation nuclei (CCN). As the densely populated area of China, the Yangtze River Delta (YRD) region shows a high frequency of observed NPF events at the ground level, especially in spring. Although recent observational studies suggested a possible connection between NPF at the higher altitudes and ground level, the role played by vertical mixing, particularly in the planetary boundary layer (PBL) is not fully understood. Here we integrate measurements in Nanjing on 15-20 April 2018, and the NPF-explicit Weather Research and Forecast coupled with chemistry (WRF-Chem) model simulations to better understand the governing mechanisms of the NPF and CCN. Our results indicate that newly formed particles at the boundary layer top could be transported downward by vertical mixing as the PBL develops. A numerical sensitivity simulation created by eliminating aerosol vertical mixing suppresses both the downward transport of particles formed at a higher altitude and the dilution of particles at the ground level. The resulting higher Fuchs surface area at the ground level, together with the lack of downward transport, yields a sharp weakening of NPF strength and delayed start of NPF therein. The aerosol vertical mixing, therefore, leads to a more than double increase of surface CN 10 - 40 and a one third decrease of boundary layer top CN 10 - 40 . Additionally, the continuous growth of nucleated ultrafine particles at the boundary layer top is strongly steered by the upward transport of condensable gases, with close to half increase of particle number concentrations in Aitken mode and CCN at a supersaturation rate of 0.75%. The findings may bridge the gap in understanding the complex interaction between PBL dynamics and NPF events, reducing the uncertainty in assessing the climate impact of aerosols., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Secondary organic aerosol, formed through atmospheric oxidation processes, plays an important role in affecting climate and human health. In this study, we conducted a comprehensive campaign in the megacity of Shanghai during the 2019 International Import Expo (EXPO), with the first deployment of a chemical ionization─Orbitrap mass spectrometer for ambient measurements. With the ultrahigh mass resolving power of the Orbitrap mass analyzer (up to 140,000 Th/Th) and capability in dealing with massive spectral data sets by positive matrix factorization, we were able to identify the major gas-phase oxidation processes leading to the formation of oxygenated organic molecules (OOM) in Shanghai. Nine main factors from three independent sub-range analysis were identified. More than 90% of OOM are of anthropogenic origin and >60% are nitrogen-containing molecules, mainly dominated by the RO 2 + NO and/or NO 3 chemistry. The emission control during the EXPO showed that even though the restriction was effectual in significantly lowering the primary pollutants (20-70% decrease), the secondary oxidation products responded less effectively (14% decrease), or even increased (50 to >200%) due to the enhancement of ozone and the lowered condensation sink, indicating the importance of a stricter multi-pollutant coordinated strategy in primary and secondary pollution mitigation.

Gas-phase oxygenated organic molecules (OOMs) can contribute substantially to the growth of newly formed particles. However, the characteristics of OOMs and their contributions to particle growth rate are not well understood in urban areas, which have complex anthropogenic emissions and atmospheric conditions. We performed long-term measurement of gas-phase OOMs in urban Beijing during 2018-2019 using nitrate-based chemical ionization mass spectrometry. OOM concentrations showed clear seasonal variations, with the highest in the summer and the lowest in the winter. Correspondingly, calculated particle growth rates due to OOM condensation were highest in summer, followed by spring, autumn, and winter. One prominent feature of OOMs in this urban environment was a high fraction (∼75%) of nitrogen-containing OOMs. These nitrogen-containing OOMs contributed only 50-60% of the total growth rate led by OOM condensation, owing to their slightly higher volatility than non-nitrate OOMs. By comparing the calculated condensation growth rates and the observed particle growth rates, we showed that sulfuric acid and its clusters are the main contributors to the growth of sub-3 nm particles, with OOMs significantly promoting the growth of 3-25 nm particles. In wintertime Beijing, however, there are missing contributors to the growth of particles above 3 nm, which remain to be further investigated.

Gaseous sulfuric acid (H 2 SO 4 ) is a crucial precursor for secondary aerosol formation, particularly for new particle formation (NPF) that plays an essential role in the global number budget of aerosol particles and cloud condensation nuclei. Due to technology challenges, global-wide and long-term measurements of gaseous H 2 SO 4 are currently very challenging. Empirical proxies for H 2 SO 4 have been derived mainly based on short-term intensive campaigns. In this work, we performed comprehensive measurements of H 2 SO 4 and related parameters in the polluted Yangtze River Delta in East China during four seasons and developed a physical proxy based on the budget analysis of gaseous H 2 SO 4 . Besides the photo-oxidation of SO 2 , we found that primary emissions can contribute considerably, particularly at night. Dry deposition has the potential to be a non-negligible sink, in addition to condensation onto particle surfaces. Compared with the empirical proxies, the newly developed physical proxy demonstrates extraordinary stability in all the seasons and has the potential to be widely used to improve the understanding of global NPF fundamentally.

Understanding the atmospheric new particle formation (NPF) process within the global range is important for revealing the budget of atmospheric aerosols and their impacts. We investigated the seasonal characteristics of NPF in the urban environment of Beijing. Aerosol size distributions down to ∼1 nm and H 2 SO 4 concentration were measured during 2018-2019. The observed formation rate of 1.5 nm particles ( J 1.5 ) is significantly higher than those in the clean environment, e.g., Hyytiälä, whereas the growth rate is not significantly different. Both J 1.5 and NPF frequency in urban Beijing show a clear seasonal variation with maxima in winter and minima in summer, while the observed growth rates are generally within the same range around the year. We show that ambient temperature is a governing factor driving the seasonal variation of J 1.5 . In contrast, the condensation sink and the daily maximum H 2 SO 4 concentration show no significant seasonal variation during the NPF periods. In all four seasons, condensation of H 2 SO 4 and (H 2 SO 4 ) n (amine) n clusters contributes significantly to the growth rates in the sub-3 nm size range, whereas it is less important for the observed growth rates of particles above 3 nm. Therefore, other species are always needed for the growth of larger particles.

Peroxy acetyl nitrate (PAN) is a key component of photochemical smog and plays an important role in atmospheric chemistry. Though it has been known that PAN is produced via reactions of nitrogen oxides (NOx) with some volatile organic compounds (VOCs), it is difficult to quantify the contributions of individual precursor species. Here we use an explicit photochemical model--Master Chemical Mechanism (MCM) model--to dissect PAN formation and identify principal precursors, by analyzing measurements made in Beijing in summer 2008. PAN production was sensitive to both NOx and VOCs. Isoprene was the predominant VOC precursor at suburb with biogenic impact, whilst anthropogenic hydrocarbons dominated at downtown. PAN production was attributable to a relatively small class of compounds including NOx, xylenes, trimethylbenzenes, trans/cis-2-butenes, toluene, and propene. MCM can advance understanding of PAN photochemistry to a species level, and provide more relevant recommendations for mitigating photochemical pollution in large cities., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Measurements for size distribution and chemical components (including water-soluble ions, OC/EC and trace elements) of particles were taken in Jinan, China, during the 2008 Chinese New Year (CNY) to assess the impacts of firecracker burning on aerosol chemical characteristics and human health risk levels. On the eve of the CNY, the widespread burning of firecrackers had a clear contribution to the number concentration of small accumulation mode particles (100-500 nm) and PM2.5 mass concentration, with a maximum PM2.5 concentration of 464.02 μg/m(3). The firecracker activities altered the number size distribution of particles, but had no influence on the mass size distribution of major water-soluble ions. The concentrations of aerosol and most ions peaked in the rush hour of firecracker burning, whereas the peaks of NO3(-) and NH4(+) presented on the day following the burning of firecrackers. K(+), SO4(2-) and Cl(-) composed approximately 62% of the PM2.5 mass, and they existed as KCl and K2SO4 during the firecracker period. However, during the non-firecracker period, organic matter (OM), SO4(2-), NO3(-) and NH4(+) were the major chemical components of the PM2.5, and major ions were primarily observed as (NH4)2SO4 and NH4NO3. Estimates of non-carcinogenic risk levels to human health showed that the elemental risk levels during the firecracker period were substantially higher than those observed during the non-firecracker period. The total elemental risk levels in Jinan for the three groups (aged 2-6 years, 6-12 years and ≥70 years) were higher than 2 during the firecracker period, indicating that increased pollutant levels emitted from the burning of firecrackers over short periods of time may cause non-carcinogenic human health risks., (Copyright © 2014 Elsevier B.V. All rights reserved.)

Aerosol size distributions, trace gas, and PM(2.5) concentrations have been measured in urban Jinan, China, over 6 months in 2007 and 2008, covering spring, summer, fall, and winter time periods. Number concentrations of particles (10-2,500 nm) were 16,200, 13,900, 11,200, and 21,600 cm( -3) in spring, summer, fall, and winter, respectively. Compared with other urban studies, Jinan has higher number concentrations of accumulation-mode particles (100-500 nm) and particles (10-2,500 nm), but lower concentrations of ultrafine particles (10-100 nm). The number, surface and volume concentrations, and size distributions of particles showed obvious seasonal variation and are also influenced by traffic emissions. Through correlation analysis, traffic emissions are proposed to be a more important contributor to Atkien-mode and accumulation-mode particles than coal firing. Around midday, the presence of nanoparticles and new particle formation is limited to pre-existing particles from traffic emissions and the mass transport of particles from suburban and rural areas. Compared with other studies in urban areas of Europe and the USA, the variation of particle number concentration and related gas concentration in Jinan between weekdays and weekends is smaller and the reasons has been deduced.

The mass concentrations and major chemical components of PM(2.5) in Jinan, Shandong Province, China from Dec. 2004 to Oct. 2008 were analyzed using backward trajectory cluster analysis in conjunction with the potential source contribution function (PSCF) model. The aim of this work was to study the inter-annual variations of mass concentrations and major chemical components of PM(2.5), evaluate the air mass flow patterns and identify the potential local and regional source areas that contributed to secondary sulfate and nitrate in PM(2.5) in Jinan. The annual mean concentrations of PM(2.5), sulfate and nitrate in 2004-2008 were almost the highest in the world. The most significant air parcels contributing to the highest mean concentrations of mass and secondary ions in PM(2.5) originated from the industrialized areas of Shandong Province. Clusters with a lower ratio of NO(3)(-)/SO(4)(2-) in PM(2.5) originated from the Yellow Sea, while a higher ratio was observed in the clusters passing through Beijing and Tianjin. PSCF modeling indicated that the provinces of Shandong, Henan, Jiangsu, Anhui and the Yellow Sea were the major potential source regions for sulfate, in agreement with the cluster analysis results. Regional and long-range transport of NH(4)NO(3) played an important role in the nitrate concentration of Jinan. By comparing the distributions of secondary sulfate and nitrate over three years, enhanced emission control management before and during the 29(th) Olympic Games led to a discernible decrease in source contributions from Beijing and its environs in 2007-2008.

Atmospheric aerosols not only cause severe haze pollution, but also affect climate through changes in cloud properties. However, during the haze pollution, aerosol‐cloud interactions are not well understood due to a lack of in situ observations. In this study, we conducted simultaneous observations of cloud droplet and particle number size distribution, together with supporting atmospheric parameters, from ground to cloud base in East China using a high‐payload tethered airship. We found that high concentrations of aerosols and cloud condensation nuclei were constrained below cloud, leading to the pronounced "Twomey effect" near the cloud base. The cloud inhibited the pollutants dispersion by reducing surface heat flux and thus deteriorated the near‐surface haze pollution. Satellite retrievals matched well with the in situ observations for low stratus clouds, while were insufficient to quantify aerosol‐cloud interactions for other cases. Our results highlight the importance to combine in situ vertical and satellite observations to quantify the aerosol‐cloud interactions. Plain Language Summary: Atmospheric aerosols, one of the major pollutants contributing to air pollution, also play an important role in climate through their interactions with clouds. The impact of aerosols on cloud properties remains the largest uncertainty in climate projections, partly due to a lack of in situ observations. Here, we conducted simultaneous observations on atmospheric aerosols and clouds from ground to 1,200 m above ground level in East China using a high‐payload tethered airship. We found aerosols number concentration was high below the clouds, which increased the cloud droplet concentration and decreased the cloud droplet diameter near cloud base. The clouds deteriorated the near‐surface air pollution, thus increasing exposure to hazardous levels. For low stratiform clouds, the satellite retrievals matched well with the observations, suggesting the satellite observation is a powerful tool to investigate clouds. However, the aerosol‐cloud interactions can still be underestimated by satellite measurements as the satellites record cloud properties near cloud top. We emphasize the need for direct in situ observations from the ground to high altitudes to quantify the effects of aerosols on cloud properties. Key Points: The pronounced Twomey effect near the cloud base was directly observed during the haze pollution by the tethered airship measurementThe observed Twomey effect at the cloud base in East China contradicts the satellite‐detected anti‐Twomey effect at the top of cloudsSatellite retrieved cloud effective radius is comparable to observation near cloud base of low stratus clouds, while is biased for others [ABSTRACT FROM AUTHOR]

Peroxy acetyl nitrate (PAN) is a key component of photochemical smog and plays an important role in atmospheric chemistry. Though it has been known that PAN is produced via reactions of nitrogen oxides (NOx) with some volatile organic compounds (VOCs), it is difficult to quantify the contributions of individual precursor species. Here we use an explicit photochemical model--Master Chemical Mechanism (MCM) model--to dissect PAN formation and identify principal precursors, by analyzing measurements made in Beijing in summer 2008. PAN production was sensitive to both NOx and VOCs. Isoprene was the predominant VOC precursor at suburb with biogenic impact, whilst anthropogenic hydrocarbons dominated at downtown. PAN production was attributable to a relatively small class of compounds including NOx, xylenes, trimethylbenzenes, trans/cis-2-butenes, toluene, and propene. MCM can advance understanding of PAN photochemistry to a species level, and provide more relevant recommendations for mitigating photochemical pollution in large cities.

Aerosol nitrate has become the most abundant compound during aerosol pollution in eastern China. The Chinese government implemented a stringent policy during 2013–2017 to tackle aerosol pollution. However, the response of nitrate to nitrogen oxides (NOx) reduction is unclear owing to the limitation of long‐term measurement. Here, we performed a 9‐year continuous measurement of aerosol compositions in Shanghai and confirmed a decrease in most species except nitrate. The contribution of nitrate to fine particulate matter (PM2.5) increased significantly, reaching up to 35% in pollution episodes after 2017. This is in contrast to the evident reduction in NOx emissions. We found that the elevated dinitrogen pentoxide (N2O5) hydrolysis is responsible for the observed nitrate trend. Increased ozone and decreased nitrogen dioxide (NO) facilitated the formation of N2O5, and increased nitrate proportion promoted the uptake of N2O5 and eventually enhanced the conversion efficiency of NO2 to nitrate. Our results highlight the importance of synergic control of aerosol and ozone pollution. Plain Language Summary: To tackle severe particulate matter (PM) pollution, the Chinese government tried to reduce the primary emission of pollutants, especially a stringent policy during 2013 and 2017, named Air Pollution Prevention and Control Action Plan. A particular need is to understand the respond of the chemical composition of PM2.5 to primary emission reductions, which supports the PM2.5 pollution control more effectively and scientifically. In this work, we performed a 9‐year online measurement of PM2.5 and its chemical compositions in the Yangtze River Delta from 2011 to 2019. We confirmed a significant decreasing trend for most species except nitrate, which remained unchanged in contrast to the evident reduction of nitrogen oxides. After ruling out the potential influences of meteorological parameters, we demonstrated the elevated dinitrogen pentoxide (N2O5) hydrolysis is responsible for the observed nitrate trend. Increased ozone and decreased nitrogen dioxide (NO) promoted the formation of N2O5, and increased nitrate proportion promoted the uptake of N2O5 and eventually promoted the conversion efficiency of NO2 to nitrate. Overall, the primary emission reduction strategies feedback to the elevated formation of nitrate mainly via the increased O3 production, highlighting the need for the synergic control of aerosol and atmospheric oxidation capacity in mitigating PM2.5 pollutions. Key Points: A 9‐year observation of fine particulate matter (PM2.5) chemical compositions in Shanghai from 2011 to 2019 confirmed a decrease in most species except nitrateThe conversion rate of nitrogen dioxide (NO2) to nitrate was promoted by dinitrogen pentoxide hydrolysis, resulting in a nonlinear response of nitrate to NO2 reductionIncreased ozone concentration, decreased NO concentration, and increased nitrate proportion played a key role in enhancing nitrate formation [ABSTRACT FROM AUTHOR]

Highly oxygenated multifunctional compounds (HOMs) play a key role in new particle formation (NPF), but their quantitative roles in different environments of the globe have not been well studied yet. Frequent NPF events were observed at two "flagship" stations under different environmental conditions, i.e. a remote boreal forest site (SMEAR II) in Finland and a suburban site (SORPES) in polluted eastern China. The averaged formation rate of 6 nm particles and the growth rate of 6-30 nm particles were 0.3 cm-3 s-1 and 4.5 nm h-1 at SMEAR II compared to 2.3 cm-3 s-1 and 8.7 nm h-1 at SORPES, respectively. To explore the differences of NPF at the two stations, the HOM concentrations and NPF events at two sites were simulated with the MALTE-BOX model, and their roles in NPF and particle growth in the two distinctly different environments are discussed. The model provides an acceptable agreement between the simulated and measured concentrations of sulfuric acid and HOMs at SMEAR II. The sulfuric acid and HOM organonitrate concentrations are significantly higher but other HOM monomers and dimers from monoterpene oxidation are lower at SORPES compared to SMEAR II. The model simulates the NPF events at SMEAR II with a good agreement but underestimates the growth of new particles at SORPES, indicating a dominant role of anthropogenic processes in the polluted environment. HOMs from monoterpene oxidation dominate the growth of ultrafine particles at SMEAR II while sulfuric acid and HOMs from aromatics oxidation play a more important role in particle growth. This study highlights the distinct roles of sulfuric acid and HOMs in NPF and particle growth in different environmental conditions and suggests the need for molecular-scale measurements in improving the understanding of NPF mechanisms in polluted areas like eastern China. [ABSTRACT FROM AUTHOR]

Ozone and fine particle currently are the two air pollutants with great concerns in China. Continuous measurement of the both pollutants and relevant precursors are very important for improving the understanding of the impact of anthropogenic activities on air quality, atmospheric chemistry and climate change. Since 2011, a comprehensive "flagship" measurement station, the Station for Observing Regional Processes of the Earth System (SORPES), has been established in Nanjing in the Yangtze River Delta (YRD) in eastern China to continuously measure ozone, its precursors, PM2.5 mass concentration and the speciated chemical components (Ding et al., 2013; Ding et al., 2016). The suburban environment in upwind area of the city and downwind the most polluted northern and eastern China Plains as well as the city clusters in the YRD makes this site an ideal location to record the impact from intensive anthropogenic activities as well as strong control measures at regional scale (Ding et al., 2016). In this study, we report the results of continuous ozone and PM2.5 during the period of 2011-2018. An increasing trend of ozone associated with a decreasing trend of PM2.5 are observed at this site. The main factors and processes that influence the trend, interannual variability, and the interactions between ozone and PM2.5 as well as their precursors are discussed. Reference:Ding,A., C. Fu, et al., Ozone and fine particle in the western Yangtze River Delta: an overview of 1 yr data at the SORPES station, Atmos. Chem. Phys., 13,11, 5813-5830,2013.Ding, A., W. Nie, X. Huang, X. Chi, J. Sun, V.-M. Kerminen, Z. Xu, W. Guo, T. Petaja, X. Yang, M. Kulmala, and C. Fu: Long-term observation of air pollution-weather/climate interactions at the SORPES station: a review and outlook, Front. Environ. Sci. Eng.,10 (5): 15, di:10.1007/s11783-016-0877-3, 2016. [ABSTRACT FROM AUTHOR]

A correction is presented to the article "Polluted dust promotes new particle formation and growth."

Ammonia (NH 3) is an important air pollutant with crucial impacts on air quality, ecosystems and climate change. However, NH 3 is not included in routine air quality monitoring and the availability of long-term NH 3 measurements is still limited, resulting in large uncertainties in our knowledge of the spatial distribution and sources of NH 3. Here we performed 1 year (March 2021–February 2022) of atmospheric NH 3 measurements at a regional background station, the Station for Observing Regional Processes of the Earth System (SORPES) in the western Yangtze River Delta of China. We found that the annual mean NH 3 concentration was 12.2 ± 4.6 ppb and exhibited apparent seasonal variations, with a maximum in June and a minimum in February, influenced by agricultural activities, air temperature, gas-particle partitioning and precipitation. Moreover, air temperature and absolute humidity correlated well with NH 3 , indicating that they are important factors in influencing NH 3 levels. The diurnal variation of NH 3 showed a single peak in the morning and higher concentrations during the day. In spring and autumn, the NH 3 morning peak can be attributed to dew evaporation. The air mass backward trajectory, local wind direction and velocity analysis suggested that NH 3 was influenced by both local emissions and regional transport from nearby cities. During the COVID-19 lockdown, a strong reduction of NO x (−77%) and a weak reduction of NH 3 (−14%) were observed compared to the pre-lockdown, highlighting that traffic emissions have a minor impact on NH 3 at the SORPES station. Our results provide more insights into the characteristics and sources of atmospheric NH 3 in background regions influenced by mixed air pollution plumes. [Display omitted] • Long-term online measurements of NH 3 are conducted in Nanjing, eastern China. • Air temperature and absolute humidity have important impacts on NH 3 variation, and dew evaporation is a significant source of NH 3 morning peaks. • NH 3 and NO x decreased by 14% and 77% during the COVID-19 lockdown and traffic emissions have minor effects on NH 3 levels. [ABSTRACT FROM AUTHOR]