Single particle mass spectral signatures from on-road and non-road vehicle exhaust particles and their application in refined source apportionment using deep learning.

With advances in vehicle emission control technology, updating source profiles to meet the current requirements of source apportionment has become increasingly crucial. In this study, on-road and non-road vehicle particles were collected, and then the chemical compositions of individual particles were analyzed using single particle aerosol mass spectrometry. The data were grouped using an adaptive resonance theory neural network to identify signatures and establish a mass spectral database of mobile sources. In addition, a deep learning-based model (DeepAerosolClassifier) for classifying aerosol particles was established. The objective of this model was to accomplish source apportionment. During the training process, the model achieved an accuracy of 98.49 % for the validation set and an accuracy of 93.36 % for the testing set. Regarding the model interpretation, ideal spectra were generated using the model, verifying its accurate recognition of the characteristic patterns in the mass spectra. In a practical application, the model performed hourly source apportionment at three specific field monitoring sites. The effectiveness of the model in field measurement was validated by combining traffic flow and spatial information with the model results. Compared with other machine learning methods, our model achieved highly automated source apportionment while eliminating the need for feature selection, and it enables end-to-end operation. Thus, in the future, it can be applied in refined and online source apportionment of particulate matter.

Evaluation of laboratory and environmental exposure systems for protein modification upon gas pollutants and environmental factors.

Chemical modifications of proteins induced by ambient ozone (O3) and nitrogen oxides (NOx) are of public health concerns due to their potential to trigger respiratory diseases. The laboratory and environmental exposure systems have been widely used to investigate their relevant mechanism in the atmosphere. Using bovine serum albumin (BSA) as a model protein, we evaluated the two systems and aimed to reduce the uncertainties of both the reactants and products in the corresponding kinetic study. In the laboratory simulation system, the generated gaseous pollutants showed negligible losses. Ten layers of BSA were coated on the flow tube with protein extraction recovery of 87.4%. For environmental exposure experiment, quartz fiber filter was selected as the upper filter with low gaseous O3 (8.0%) and NO2 (1.7%) losses, and cellulose acetate filter was appropriate for the lower filter with protein extraction efficiency of 95.2%. The protein degradation process was observed without the exposure to atmospheric oxidants and contributed to the loss of protein monomer mass fractions, while environmental factors (e.g., molecular oxygen and ultraviolet) may cause greater protein monomer losses. Based on the evaluation, the study exemplarily applied the two systems to protein modification and both showed that O3 promotes the protein oligomerization and nitration, while increased temperature can accelerate the oligomerization and increased relative humidity can inhibit the nitration in the environmental exposure samples. The developed laboratory and environmental systems are suitable for studying protein modifications formed under different atmospheric conditions. A combination of the two will further reveal the actual mechanism of protein modifications.

Multiphase reactions of proteins in the air: Oligomerization, nitration and degradation of bovine serum albumin upon ambient exposure.

Proteins in atmospheric aerosol can react with atmospheric pollutants such as ozone (O3) and nitrogen dioxide (NO2) in the atmosphere via the reactions of oxidation, nitration, and cross-linking etc. Currently, the reactions have been more thoroughly studied in the laboratory but rarely investigated in the ambient environment. In this study, we used bovine serum albumin (BSA) as the model protein to conduct the exposure experiment in the ambient environment in southern China, an area with increasing oxidative capacity, to investigate the reactions of proteins in the atmosphere. We observed the occurrence of oligomerization, nitration and degradation of BSA upon exposure. The mass fraction of BSA monomer decreased by 5.86 ± 1.61% after exposure and those of dimers, trimers and higher oligomers increased by 1.04 ± 0.49%, 1.37 ± 0.74% and 3.40 ± 1.06%, respectively. Simultaneously, the nitration degrees of monomers, dimers, trimers and higher oligomers increased by 0.42 ± 0.15%, 0.53 ± 0.15%, 0.55 ± 0.28% and 2.15 ± 1.01%, respectively. The results show that oligomerization was significantly affected by O3 and temperature and nitration was jointly affected by O3, temperature and relative humidity, indicating the important role of atmospheric oxidants in the atmospheric reactions of protein. Atmospheric degradation of BSA was observed with the release of free amino acids (FAAs) such as glycine, alanine, serine and methionine. Glycine was the dominant FAA with a molar yield ranging from ∼8% to 33% for BSA. The estimated stoichiometric coefficient (α) of glycine is 10-7-10-6 for the degradation of BSA upon O3. Our observation suggests the occurrence of protein reactions in the oxidative ambient environment, leading to the production of nitrated products, oligomers and low molecular weight products such as peptides and FAAs. This study may deepen the current understanding of the atmospheric reaction mechanisms and reveal the influence of environmental factors in the atmosphere.

Influence of ozone pollution on the mixing state and formation of oxygenated organics containing single particles.

The formation and aging processes of oxygenated organic molecules (OOMs) are important for understanding the formation mechanisms of secondary organic aerosols (SOAs) in the field. In this study, we investigated the mixing states of OOM particles by identifying several oxygenated species along with the distributions of secondary organic carbon (SOC) during both clean and ozone (O3)-polluted periods in July and September of 2022 in Guangzhou, China. OOM-containing particles accounted for 57 % and 49 % of the total detected single particles in July and September, respectively. Most of the OOM particles were internally mixed with sulfate and nitrate, while elemental carbon and hydrocarbon species were absent. Despite the higher SOC/OC ratio in September (81 %) than it in July (72 %), comparative investigations of the mass spectra, diurnal patterns, and distributions of OOM particles revealed the same composition and aging states of OOMs in two O3 pollution periods. As the O3 concentration increased from the clean to the polluted periods, the ratio of SOC to OC increased along with the relative abundance of secondary OOM particles among total OOM particles. In contrast, the relative abundance of OC-type OOM particles gradually decreased, indicating the conversion of hydrocarbon species into OOMs as the SOC/OC ratio increased. Both the bulk analysis of SOC from filter measurement and the mixing states of OOM particles suggested that OOM production and degree of oxidation were higher in the O3-polluted periods than in the clean periods. These results elucidate the effects of O3 pollution on the OOM formation process and offer new perspectives for the joint investigation of SOA production based on filter sampling and single-particle measurements.

The influence of lightning activity on NOx and O3 in the Pearl River Delta region.

Extremely high-temperature lightning generates NOx by electrolyzing nitrogen and oxygen molecules, regulating ozone concentration. The Pearl River Delta (PRD) is located in the world's high-value area of lightning density, and lightning-generated NOx (LNOx) cannot be ignored. Using the flash data from Guangdong-Hong Kong-Macao Lightning Location System and multi-site atmospheric composition data, we estimate the NOx variations in lightning activity and its impact on O3 across the PRD region. The cloud-to-groud (CG) frequency from 2013 to 2021 shows a decreasing trend driven by urban regions. We observe that the lightning density is steadily decreasing from the south-central part of Guangzhou City to the surrounding area. A comparison of the different sites with lightning days and non-lightning days shows that a significant amount (13. 84-20. 47 %) of ground-level NOx concentration at urban stations can be attributed to lightning NOx emissions. A lower lightning frequency and low background concentration observed at suburban sites indicated a limited contribution of LNOx. The average decrease in O3 concentration at urban stations (15.92-25.06 %) was significantly higher than that at suburban stations (5.34-8.95 %) due to the influence of titration and lower actinic radiation. There was a greater fluctuation in NOx and O3 concentrations during the cases, and the surface NOx concentration displayed the most significant responsiveness to LNOx under direct lightning striking in the tall tower. This phenomenon has not been reported, however, it is consistent with the laboratory-based observations suggesting the amount of LNO increases with peak current. LNOx significantly impacts air quality in the PRD during the high convective season. Further in situ and vertical distribution observations are necessary to explore the ground-level impact of LNOx.

Seasonal variations of mass absorption efficiency of elemental carbon in PM2.5 in urban Guangzhou of South China.

This study investigates seasonal variations of mass absorption efficiency of elemental carbon (MAEEC) and possible influencing factors in urban Guangzhou of South China. Mass concentrations of elemental carbon (EC) and organic carbon (OC) in PM2.5 and aerosol absorption coefficient (bap) at multi-wavelengths were simultaneously measured in four seasons of 2018-2019 at hourly resolution by a semi-continuous carbon analyzer and an aethalometer. Seasonal average mass concentrations of EC were in the range of 1.36-1.70 µgC/m3 with a lower value in summer than in the other seasons, while those of OC were in the range of 4.70-6.49 µgC/m3 with the lowest value in summer and the highest in autumn. Vehicle exhaust from local traffic was identified to be the predominant source of carbonaceous aerosols. The average aerosol absorption Ångström exponents (AAE) were lower than 1.2 in four seasons, indicating EC and bap were closely related with vehicle exhaust. Seasonal MAEEC at 550 nm was 11.0, 8.5, 10.4 and 11.3 m2/g in spring, summer, autumn, and winter, respectively. High MAEEC was related with the high mass ratio of non-carbonaceous aerosols to EC and high ambient relative humidity.

Vertical distribution and transport of microplastics in the urban atmosphere: New insights from field observations.

The distribution and transport of atmospheric microplastics (AMPs) have raised concerns regarding their potential effects on the environment and human health. Although previous studies have reported the presence of AMPs at ground level, there is a lack of comprehensive understanding of their vertical distribution in urban environments. To gain insight into the vertical profile of AMPs, field observations were conducted at four different heights (ground level, 118 m, 168 m and 488 m) of the Canton Tower in Guangzhou, China. Results showed that the profiles of AMPs and other air pollutants had similar layer distribution patterns, although their concentrations differed. The majority of AMPs were composed of polyethylene terephthalate and rayon fibers ranging from 30 to 50 µm. As a result of atmospheric thermodynamics, AMPs generated at ground level were only partially transported upward, leading to a decrease in their abundance with increasing altitude. The study found that the stable atmospheric stability and lower wind speed between 118 m and 168 m resulted in the formation of a fine layer where AMPs tended to accumulate instead of being transported upward. This study for the first time delineated the vertical profile of AMPs within the atmospheric boundary layer, providing valuable data for understanding the environmental fate of AMPs.

Characterizing atmospheric biological aerosols at a suburban site in Guangzhou, southern China by airborne microbes, proteins and saccharides.

Bioaerosols in ambient environment can be evaluated using various techniques. However, the results of bioaerosols obtained using different methods are rarely compared. The relationships between different bioaerosol indicators and their behaviors under the influence of environment factors are seldom investigated. Here we used airborne microbial numbers, proteins and saccharides concentrations as the indicators to characterize bioaerosols in two seasons with different source contribution, air pollution situation and meteorological conditions. The observation was conducted at a suburban site in Guangzhou, southern China, during the winter and spring periods of 2021. Airborne microbes were observed with an average of (1.82 ± 1.33) × 106 cells/m3, converted to the mass concentration level of 0.42 ± 0.30 µg/m3, comparable but lower than that of proteins (0.81 ± 0.48 µg/m3). Both of them were much higher than the average concentration of saccharides (19.93 ± 11.53 ng/m3). During the winter period, significant and good correlations were observed between the three components. In spring, a biological outbreak was observed in late March with a strong elevation of airborne microbes followed by elevations of proteins and saccharides. The retardation of proteins and saccharides could be the result of the enhanced release from microorganisms under the influence of atmospheric oxidation processes. Saccharides in PM2.5 were studied to reveal the contribution of specific sources of bioaerosols (e.g. fungi, pollen, plants and soil). Our results show that primary emissions and secondary processes should play their roles in the variations of these biological components. By comparing the results of the three methods, this study provides an insight into the applicability and variability of bioaerosol characterization in the ambient environment with respect to various influences of sources, atmospheric processes and environmental conditions.

The contributions of non-methane hydrocarbon emissions by different fuel type on-road vehicles based on tests in a heavily trafficked urban tunnel.

Automobile exhaust is a major source of volatile organic compounds (VOCs) in metropolitan areas, yet it is difficult to accurately determine the contributions of different types of on-road vehicles. Tunnel tests are an effective way to measure real-world vehicle emissions, and the data collected are also suitable for receptor modeling to analyze the contributions of non-methane hydrocarbons (NMHCs) from different types of vehicles, as the closed environment ensures good mixing and minimal aging. In this study, tunnel tests were conducted inside a heavily trafficked city tunnel in Guangzhou in south China, and the positive matrix factorization (PMF) model was applied to the inlet-outlet incremental NMHC data. The results revealed that gasoline vehicles (GVs), Liquefied Petroleum Gas vehicles (LPGVs), and diesel vehicles (DVs) were responsible for 39 %, 45 % and 16 % of NMHCs, and 52 %, 23 %, and 24 % of the ozone formation potentials, respectively. LPGVs were the largest contributor of (56 %) alkanes, and GVs were the largest contributor of aromatics (61 %) and C2-C4 alkenes (55 %). With the video-recorded traffic counts the emissions of different fuel types are further compared on a per-vehicle-per-kilometer basis, and the results reveal that LPGVs and GVs were comparable in the OFPs of NMHCs emitted per kilometer, while on average a DV emitted 2.0 times more NMHCs than a GV with 2.4 times more OFPs. This study highlights substantial contribution of reactive alkenes and aromatics by DVs and the benefits of strengthening diesel exhaust control in terms of preventing ozone pollution.

Atmospheric microplastics at a southern China metropolis: Occurrence, deposition flux, exposure risk and washout effect of rainfall.

Atmospheric microplastics (AMPs) have raised much concern for public health due to their potential for exposure. In this study, temporal distribution, characteristics and exposure risk of AMPs were studied in the urban area of Guangzhou, a metropolis in Southern China, and the washout effect of rainfall on AMPs was investigated. It was found that AMP abundances in Guangzhou were in a range of 0.01-0.44 items/m3, with higher abundance in the wet season (0.19 ± 0.01 items/m3) than in the dry season (0.15 ± 0.02 items/m3). The distribution of AMPs did not correspond to that of common air pollutants (e.g., PM2.5 and PM10), implying that their pollution sources might be distinct. In Guangzhou, a total of 1.26 × 1011 items AMPs are in the air every year, and annual inhalation exposure of adults was estimated to be in the range of 79.65-3.50 × 103 items. The annual deposition flux of AMPs is 65.94 ± 7.53 items/m2/d, and the deposition flux in the wet season (84.00 ± 6.95 items/m2/d) was much greater than that in the dry season (47.88 ± 8.35 items/m2/d). Furthermore, rainfall has an effective mechanism for removing AMPs from the atmosphere, with an average washout ratio of (19.39 ± 6.48) × 104 for rainfall washing AMPs out. Compared to moderate rain (2.5-10 mm/h) and heavy rain (10-50 mm/h), light rain (rainfall intensity <2.5 mm/h) had a better washout effect. This study contributes to the evaluation of AMP exposure risk and understanding of AMP environmental behavior and fate by providing long-term monitoring data on AMPs and quantifying the washout effect of rainfall on AMPs for the first time.

Vertical measurements of stable nitrogen and oxygen isotope composition of fine particulate nitrate aerosol in Guangzhou city: Source apportionment and oxidation pathway.

Nowadays, the emission source and formation mechanism of fine particulate nitrate (pNO3-) in China are mired in controversy. In this study, the stable nitrogen isotope (δ15N-NO3-) and triple oxygen isotope (Δ17O-NO3-) were determined for the pNO3- samples collected at three heights under different atmospheric oxidation capacity (AOC) (Ox = O3 + NO2: 107 ± 29 µg m-3 at ground, 102 ± 28 µg m-3 at 118 m, 122 ± 23 µg m-3 at 488 m) conditions during the sampling period based on the Canton Tower, Guangzhou, China. The Bayesian mixing model showed that coal combustion was the largest contributor to pNO3- in this city, followed by biomass burning, vehicle exhaust, and soil emission. Interestingly, we found that vertical NOx and pNO3- concentrations displayed an opposite pattern owing to the different formation mechanisms among heights. The average contributions of oxidation pathways for (NO2 + OH, P1), (NO3 + DMS/HC, P2), and (N2O5 + H2O, P3) were 61 %, 12 %, and 27 % at the ground, respectively, and these values would vary greatly among heights. These results implied that both AOC and NOx loading played an important role in pNO3- production. The pNO3- displayed a positive correlation with NOx (r = 0.95) with an enhanced contribution of the P1 pathway under the relatively high AOC condition. However, pNO3- has a negative correlation with NOx (r = -0.99) with a rise of heterogeneous reaction (P2 and P3) under the relatively low AOC condition. Therefore, the current emission control strategy for air pollution in China needs to consider the AOC conditions among regions to effectively mitigate particulate air pollution.

Emission factors and source profiles of volatile organic compounds from typical industrial sources in Guangzhou, China.

Volatile organic compounds (VOCs) are important precursors of ozone (O3) and fine particulate matter (PM2.5). An accurate depiction of the emission characteristics of VOCs is the key to formulating VOC control strategies. In this study, the VOC emission factors and source profiles in five industrial sectors were developed using large-scale field measurements conducted in Guangzhou, China (100 samples for the emission factors and 434 samples for the source profile measurements). The emission factors based on the actual measurement method and the material balance method were 1.6-152.4 kg of VOCs per ton of raw materials (kg/t) and 3.1-242.2 kg/t, respectively. The similarities between the emission factors obtained using these two methods were examined, which showed a coefficient of divergence (CD) of 0.34-0.72. Among the 33 subdivided VOC source profiles developed in this study, sources including light guide plate (LGP), photoresist mask, and plastic products were the first time developed in China. Due to regional diversities in terms of production technologies, materials, and products, the emission characteristics of the VOCs varied, even in the same sector, thereby demonstrating the importance of developing localized source profiles of VOCs. The ozone formation potential (OFP) of the shipbuilding and repair sector from fugitive emissions was the highest value among all the industrial sectors. Controlling the emissions of aromatics and OVOCs was critical to reducing the O3 growth momentum in industrial sectors. In addition, 1,2-dibromoethane showed high carcinogenic risk potentials (CRPs) during most of the industrial sectors and should be prioritized for controlling.

Detection of polycyclic aromatic hydrocarbons using a high performance-single particle aerosol mass spectrometer.

The real-time detection of the mixing states of polycyclic aromatic hydrocarbons (PAHs) and nitro-PAHs in ambient particles is of great significance for analyzing the source, aging process, and health effects of PAHs and nitro-PAHs; yet there is still few effective technology to achieve this type of detection. In this study, 11 types of PAH and nitro-PAH standard samples were analyzed using a high performance-single particle aerosol mass spectrometer (HP-SPAMS) in lab studies. The identification principles 'parent ions' and 'mass-to-charge (m/z) = 77' of each compound were obtained in this study. It was found that different laser energies did not affect the identification of the parent ions. The comparative experiments of ambient atmospheric particles, cooking and biomass burning emitted particles with and without the addition of PAHs were conducted and ruled out the interferences from primary and secondary organics on the identification of PAHs. Besides, the reliability of the characteristic ions extraction method was evaluated through the comparative study of similarity algorithm and deep learning algorithm. In addition, the real PAH-containing particles from vehicle exhaust emissions and ambient particles were also analyzed. This study improves the ability of single particle mass spectrometry technology to detect PAHs and nitro-PAHs, and HP-SPAMS was superior to SPAMS for detecting single particles containing PAHs and nitro-PAHs. This study provides support for subsequent ambient observations to identify the characteristic spectrum of single particles containing PAHs and nitro-PAHs.

[Analysis of a Typical Ozone Pollution Process in Guangzhou in Winter].

This study focused on an ozone pollution event occurring in winter (January) in Guangzhou. Various influencing factors were analyzed, including various atmospheric trace gases, meteorological conditions during the whole pollution process, as well as the characteristics of the main O3 precursor volatile organic compounds (VOCs). The main sources of VOCs and the O3 formation regime were analyzed using an array of tools:the ozone potential formation (OFP), positive matrix factorization (PMF) model, and empirical kinetic modeling approach (EKMA) curve. Feasible strategies for O3 control were suggested. The results showed that O3 and NO2 exceeded the corresponding standards in this winter pollution event, when the concentrations of PM10 and PM2.5 were also high, differing from the air pollution characteristics in summer and autumn. Low boundary layer height (<75 m) and high atmospheric stability at night exacerbated the accumulation of ozone precursors and fine particles. Meteorological conditions such as the increased daytime temperature (5℃), stronger solar radiation (10%), and low horizontal wind speed (<1 m·s-1) favored photochemical reactions and promoted the formation of ozone and fine particles. VOCs were mainly composed of alkanes, and the proportions of alkanes and alkynes in winter were higher than those in the other seasons. Aromatics (xylenes and toluene) and propylene were the key VOCs species leading to O3 formation. The main VOCs sources were vehicle exhaust (22.4%), solvent usage (20.5%), and industrial emissions (17.9%); however, the source with highest OFP was identified as solvent usage. O3 formation in this event was in the VOCs-limited regime, and reducing O3 precursors in the VOCs/NOx ratio of 3:1 was effective and feasible for O3 control. This study explored the causes of an O3 pollution event in winter, which will serve as reference for the synergistic control of O3 and PM2.5 in heavy pollution seasons.

Worsening ozone air pollution with reduced NOx and VOCs in the Pearl River Delta region in autumn 2019: Implications for national control policy in China.

Ozone (O3) pollution has emerged as a major air quality issue in China. Here we emphasize the great challenges in controlling O3 pollution by analyzing the recent experience of the Pearl River Delta (PRD) in southern China in reducing the autumn O3 peaks. Despite significant reductions in the concentration of O3 precursors, i.e., nitrogen oxides (NOx) and volatile organic compounds (VOCs), regional O3 pollution in the PRD was largely worse in autumn 2019 than in autumn 2018. We found that the supra-regional and regional background concentrations of O3 increased significantly in the PRD in autumn 2019 due to increased concentrations of O3 in the vast surrounding areas. We also observed slight increases in the concentrations of PRD-regionally and Guangzhou-locally produced O3. A chemical box-model analysis confirmed a slight increase in the in-situ production of O3 and revealed that increased biogenic VOCs (BVOCs) and decreased NOx levels negated the effect of significant decrease in the anthropogenic VOCs. Taken together, these aspects exacerbated O3 pollution in the PRD region in autumn 2019 relative to autumn 2018. The findings from this study highlight the strong interactions of O3 pollution over multiple regions and the need for collaborative inter-regional efforts to control O3 pollution. The experience of PRD also underlines the key role of BVOCs and the importance of science-based strategies to decrease VOCs and NOx.

Emissions and light absorption of PM2.5-bound nitrated aromatic compounds from on-road vehicle fleets.

Vehicle emissions are an important source of nitrated aromatic compounds (NACs) in particulate size smaller 2.5 µm (PM2.5), which adversely affect human health and biodiversity, especially in urban areas. In this study, filter-based PM2.5 samples were collected during October 14-19, 2019, in a busy urban tunnel (approximately 35,000 vehicles per day) in south China to identify PM2.5-bound NACs. Among them, 2,8-dinitrodibenzothiophene, 3-nitrodibenzofuran and 2-nitrodibenzothiophene were the most abundant nitrated polycyclic aromatic hydrocarbons (NPAHs), while 2-methyl-4-nitrophenol, 2,4-dinitrophenol, 3-methyl-4-nitrophenol and 4-nitrophenol were the most abundant nitrophenols (NPs). The observed mean fleet emission factors (EFs) of NPAHs and NPs were 2.2 ± 2.1 and 7.7 ± 4.1 µg km-1, and were 2.9 ± 2.7 and 10.2 ± 5.4 µg km-1 if excluding electric and liquefied petroleum gas vehicles, respectively. Regression analysis revealed that diesel vehicles (DVs) had NPAH-EFs (55.3 ± 5.3 µg km-1) approximately 180 times higher than gasoline vehicles (GVs) (0.3 ± 0.2 µg km-1), and NP-EFs (120.6 ± 25.8 µg km-1) approximately 30 times higher than GVs (4.1 ± 0.2 µg km-1), and thus 89% NPAH emissions and 56% NP emissions from the onroad fleets were contributed by DVs although DVs only accounted for 3.3% in the fleets. Methanol solution-based light absorption measurements demonstrated that the mean incremental light absorption for methanol-soluble brown carbon at 365 nm was 6.8 ± 2.2 Mm-1, of which the 44 detected NACs only contributed about 1%. The mean EF of the 7 toxic NACs was approximately 3% that of the 16 priority PAHs; However, their benzo(a)pyrene toxic equivalence quotients (TEQBaP) could reach over 25% that of the PAHs. Moreover, 6-nitrochrysene mainly from DVs contributed 93% of the total TEQBaP of the NACs. This study demonstrated that enhancing DV emission control in urban areas could benefit the reduction of exposure to air toxins such as 6-nitrochrysene.

Non-agricultural source dominates the ammonium aerosol in the largest city of South China based on the vertical δ15N measurements.

Ammonia (NH3) is the most prevalent alkaline gas in the atmosphere and plays a critical role in air pollution and public health. However, scientific debate remains over whether agricultural emissions (e.g., livestock and fertilizer application) dominate NH3 in urban atmosphere in China, which is one of the largest NH3 emitters in the world. In this study, we first simultaneously collected the fine atmospheric particles (PM2.5) at two heights (ground and 488 m) using the atmospheric observatories in Canton Tower, Guangzhou city, China for the measurements of stable nitrogen isotope composition in ammonium (δ15N-NH4+). Our results showed that the average δ15N-NH4+ value at the ground and the 488 m observatory were 16.9 ‰ and 3.8 ‰, respectively, implying that NH4+ aerosols between the two heights probably have different sources. Moreover, we found that the δ15N-NH4+ value would sharply decrease to -16.7 ‰ when the air masses came from western Guangzhou, where the urbanization is limited compared to other surrounding areas. The Bayesian mixing model indicated that NH4+ aerosol at the ground observatory was mainly derived from non-agricultural activities (76 %, e.g., vehicular exhaust), with the rest from agricultural sources (24 %). As for the 488 m observatory, the contribution of non-agricultural sources was 53 %, which is lower than the ground observatory. This is expected as the lower air receives more impacts from the local urban emission. However, the current "bottom-up" emission inventory illustrates that only ~20 % NH3 in Guangzhou is associated with non-agricultural emissions, which is significantly lower than our δ15N-based results. Overall, our findings strongly imply that non-agricultural sources dominate the urban NH3 in Guangzhou or maybe in adjacent cities of the Pearl River Delta region as well, suggesting that the emission inventory of NH3 in this region probably is urgently needed to be revisited in future studies.

Quantifying the relative contributions of aqueous phase and photochemical processes to water-soluble organic carbon formation in winter in a megacity of South China.

To identify potential formation mechanisms of water-soluble organic carbon (WSOC) and quantify their contributions to WSOC in urban Guangzhou of south China, a comprehensive campaign was carried out in winter of 2019-2020. During the campaign, WSOC, total carbon (TC), black carbon (BC), water-soluble inorganic ions (WSIIs) and fourteen elements in PM2.5 were collected using inline instruments. Bulk PM2.5 and size-segregated particle samples were also synchronously collected using offline instruments for analyzing the dominant chemical components including WSOC, organic carbon (OC), elemental carbon (EC) and WSIIs. In addition, gaseous pollutants (e.g., NH3, SO2, HNO3, NO2, O3) and meteorological parameters were also measured during the same period. PM2.5 pollution episodes during the campaign period were mainly driven by increased nitrate concentrations. The mass concentration of WSOC increased from 3.9 ± 1.1 µg m-3 on non-episode days to 6.8 ± 0.6 µg m-3 on episode days, although the mass ratio of WSOC to OC in PM2.5 changed little (<4%). Photochemical processes dominated WSOC formation in the afternoon and aqueous phase chemical processes played the dominant role in the night, from which newly formed WSOC distributed in the condensation mode and the droplet mode, respectively. Source apportionment analysis using positive matrix factorization (PMF) model suggested that on average 35% and 65% of WSOC mass in PM2.5 were related with the photochemical processes and aqueous phase chemical processes, respectively. Aqueous phase chemical processes were highly affected by nitrate pollution, which was closely related with O3 pollution.

Decrease in ambient volatile organic compounds during the COVID-19 lockdown period in the Pearl River Delta region, south China.

During the COVID-19 lockdown, ambient ozone levels are widely reported to show much smaller decreases or even dramatical increases under substantially reduced precursor NOx levels, yet changes in ambient precursor volatile organic compounds (VOCs) have been scarcely reported during the COVID-19 lockdown, which is an opportunity to examine the impacts of dramatically changing anthropogenic emissions on ambient VOC levels in megacities where ozone formation is largely VOC-limited. In this study, ambient VOCs were monitored online at an urban site in Guangzhou in the Pearl River Delta region before, during, and after the COVID-19 lockdown. The average total mixing ratios of VOCs became 19.1% lower during the lockdown than before, and those of alkanes, alkenes and aromatics decreased by 19.0%, 24.8% and 38.2%, respectively. The levels of light alkanes (C < 6) decreased by only 13.0%, while those of higher alkanes (C ≥ 6) decreased by 67.8% during the lockdown. Disappeared peak VOC levels in morning rush hours and the drop in toluene to benzene ratios during the lockdown suggested significant reductions in vehicle exhaust and industrial solvent emissions. Source apportioning by positive matrix factorization model revealed that reductions in industrial emissions, diesel exhaust (on-road diesel vehicles and off-road diesel engines) and gasoline-related emissions could account for 48.9%, 42.2% and 8.8%, respectively, of the decreased VOC levels during the lockdown. Moreover, the reduction in industrial emissions could explain 56.0% and 70.0% of the reductions in ambient levels of reactive alkenes and aromatics, respectively. An average increase in O3-1 h by 17% and a decrease in the daily maximum 8-h average ozone by 11% under an average decrease in NOx by 57.0% and a decrease in VOCs by 19.1% during the lockdown demonstrated that controlling emissions of precursors VOCs and NOx to prevent ambient O3 pollution in megacities such as Guangzhou remains a highly challenging task.

Tower-based measurements of NMHCs and OVOCs in the Pearl River Delta: Vertical distribution, source analysis and chemical reactivity.

Measurements of vertical distribution of volatile organic compounds (VOCs) have attracted wide attentions, which could help to understand atmospheric oxidation mechanism and provide implications for VOC control. This study measured the non-methane hydrocarbons (NMHCs) and oxygenated VOCs (OVOCs) simultaneously for the first time at three different heights, namely ground, 118 m and 488 m, in the Canton Tower located in the urban core of the Pearl River Delta (PRD). The results show that NMHCs decreased while some OVOC species such as formaldehyde and acetaldehyde increased with increasing height. It was mainly attributed to the dilution and chemical loss of NMHCs but secondary production of OVOCs during vertical transport. Ratio analysis and receptor modeling indicate that vehicle exhausts (47%) and fuel evaporation (39%) were major sources of the total NMHCs. Interestingly, industry contributed much more at 118 m, probably affected by organic gas discharge from the high chimney of industrial factories. The chemical reactivities in terms of OH radical loss rate (LOH), ozone formation potential (OFP) and secondary organic aerosol potential (SOAP) were lowest at 118 m, smaller than those influenced by high fresh NMHC emissions at ground and strong formation of secondary species (e.g. OVOCs) at 488 m. OH exposure estimated by isoprene and m,p-xylene/ethylbenzene was different depending on their time scale of vertical turbulent mixing and chemical loss. OVOC species measured at different heights were positively correlated with Ox (R = 0.48-0.87), indicating that OVOCs were largely contributed by secondary formation in photochemical process. The tower measurements of NMHCs and OVOCs provided a unique opportunity to investigate the VOC distribution and chemical behaviors, which could give important information for understanding O3 and PM2.5 pollution mechanism in the PRD region with fast developing urban setting and substantially changing air quality.