Surface O3 photochemistry over the South China Sea: Application of a near-explicit chemical mechanism box model

Comprehensive measurement of carbonyls in Lhasa, Tibetan Plateau: Implications for strong atmospheric oxidation capacity

Carbonyls are ubiquitous in the troposphere and play a crucial role in atmospheric oxidation capacity (AOC), particularly in photochemistry-active regions such as the Tibetan Plateau (TP). However, the composition and evolution of carbonyls over the TP is still poorly understood due to a lack of comprehensive observations and modelling. Here, we conducted an intensive field measurement of 37 carbonyls and their precursors at a suburban site in Lhasa during summer 2022. Markedly higher levels of carbonyls (7.24 ± 3.83 ppbv) were found during ozone pollution episodes, with 36 % higher than those during non-episodes. Formaldehyde was the most abundant carbonyl (38 %), which primarily originating from photochemical secondary formations. Simulations using the Rapid adaptive Optimization Model for Atmospheric Chemistry (ROMAC) indicated strong AOC in Lhasa, with the daytime maximum of ·OH and ·HO2 of 9.8 × 106 and 4.2 × 108 molecules cm-3, respectively, which were even higher than that in most of the megacities in China. Notably, AOC significantly enhanced with the increasing carbonyls during the episodes, with the concentrations of ·OH and ·HO2 were boosted 21 % and 67 % than those during non-episodes, respectively. Budget analysis revealed that the ·HO2 + NO (88 %) and ·OH + VOC (74 %) pathways dominated the generation and loss of ·OH, respectively. And for ·HO2, they were ·RO2 + NO (67 %) and ·HO2 + NO (83 %). This study provides valuable insights into the strong AOC in the ecologically-fragile and climate-sensitive TP region, and highlighted the crucial role of anthropogenic-biogenic interactions in the active photochemistry of TP.

Biogenic volatile organic compounds dominated the near-surface ozone generation in Sichuan Basin, China, during fall and wintertime

The complex air pollution driven by both Ozone (O3) and fine particulate matter (PM2.5) significantly influences the air quality in the Sichuan Basin (SCB). Understanding the O3 formation during autumn and winter is necessary to understand the atmospheric oxidative capacity. Therefore, continuous in-site field observations were carried out during the late summer, early autumn and winter of 2020 in a rural area of Chongqing. The total volatile organic compounds (VOCs) concentration reported by a Proton-Transfer-Reaction Time-of-Flight Mass Spectrometry (PTR-ToF-MS) were 13.66 ± 9.75 ppb, 5.50 ± 2.64 ppb, and 9.41 ± 5.11 ppb in late summer, early autumn and winter, respectively. The anthropogenic VOCs (AVOCs) and biogenic VOCs (BVOCs) were 8.48 ± 7.92 ppb and 5.18 ± 2.99 ppb in late summer, 3.31 ± 1.89 ppb and 2.19 ± 0.93 ppb in autumn, and 6.22 ± 3.99 ppb and 3.20 ± 1.27 ppb in winter. A zero-dimensional atmospheric box model was employed to investigate the sensitivity of O3-precursors by relative incremental reactivity (RIR). The RIR values of AVOCs, BVOCs, carbon monoxide (CO), and nitrogen oxides (NOx) were 0.31, 0.71, 0.09, and -0.36 for late summer, 0.24, 0.59, 0.22, and -0.38 for early autumn, and 0.30, 0.64, 0.33 and -0.70 for winter, and the results showed that the O3 formation of sampling area was in the VOC-limited region, and O3 was most sensitive to BVOCs (with highest RIR values, > 0.6). This study can be helpful in understanding O3 formation and interpreting the secondary formation of aerosols in the winter.

Traceability and policy suggestions for ozone pollution in heavy industrial city in Northeast China

In the heavy industrial city of Northeast China, there has been a significant decrease in particulate matter pollution while experiencing a sharp increase in ozone (O3) pollution. However, the main influencing factors and source contributions to O3 remain unclear. Taking the case of Siping as an example, this study analyzed the spatiotemporal characteristics, assessed local source contributions to O3, and revealed regional transmission effects using numeric simulation and statistical methods. Temporally, higher O3 concentrations were observed in summer and the afternoon, with hourly peaks up to 254 µg/m3. Spatially, O3 pollution was mainly contributed by background concentrations (34.52%), external transport (34.50%), and local emissions (30.98%) during the case study period (June 11-18, 2021). Among the local emission sources, biological emissions, the industrial sector, and the traffic sector accounted for 35.30%, 32.09%, and 23.58% of the O3 concentration, respectively. For regional atmospheric transmission, high O3 pollution was accompanied by wind from the southwest directions, and the trajectory of air mass transport suggests that eastern Mongolia, the Korean Peninsula, and its neighboring regions contribute to O3 pollution. Furthermore, sensitivity analysis showed that O3 pollution in Siping is a co-controlled region by anthropogenic volatile organic compounds (AVOCs) and NOX, which implies control in an optimal ratio of VOCs and NOX emissions. Thus, our results highlight the importance of joint prevention and control of O3 pollution in the region, optimization of biogenic landscape ecology, and control of VOCs and NOx in both the industrial and transport sectors.

Reaction Kinetics of CH2OO and syn-CH3CHOO Criegee Intermediates with Acetaldehyde

Criegee intermediates exert a crucial influence on atmospheric chemistry, functioning as powerful oxidants that facilitate the degradation of pollutants, and understanding their reaction kinetics is essential for accurate atmospheric modeling. In this study, the kinetics of CH2OO and syn-CH3CHOO reactions with acetaldehyde (CH3CHO) were investigated using a flash photolysis reaction tube coupled with the OH laser-induced fluorescence (LIF) method. The experimental results indicate that the reaction of syn-CH3CHOO with CH3CHO is independent of pressure in the range of 5-50 Torr when using Ar as the bath gas. However, the rate coefficient for the reaction between CH2OO and CH3CHO at 5.5 Torr was found to be lower compared to the near-constant values observed between 10 and 100 Torr. Furthermore, the reaction of syn-CH3CHOO with CH3CHO demonstrated positive temperature dependence from 283 to 330 K, with a rate coefficient of (2.11 ± 0.45) × 10-13 cm3 molecule-1 s-1 at 298 K. The activation energy and pre-exponential factor derived from the Arrhenius plot for this reaction were determined to be 2.32 ± 0.49 kcal mol-1 and (1.66 ± 0.61) × 10-11 cm3 molecule-1 s-1, respectively. In comparison, the reaction of CH2OO with CH3CHO exhibited negative temperature dependence, with a rate coefficient of (2.16 ± 0.39) × 10-12 cm3 molecule-1 s-1 at 100 Torr and 298 K and an activation energy and a pre-exponential factor of -1.73 ± 0.31 kcal mol-1 and (1.15 ± 0.21) × 10-13 cm3 molecule-1 s-1, respectively, over the temperature range of 280-333 K.

Roles of photochemical consumption of VOCs on regional background O3 concentration and atmospheric reactivity over the pearl river estuary, Southern China

Understanding of the photochemical ozone (O3) pollution over the Pearl River Estuary (PRE) of southern China remains limited. We performed an in-depth analysis of volatile organic compounds (VOCs) data collected on an island (i.e., the Da Wan Shan Island, DWS) located at the downwind of Pearl River Delta (PRD) from 26 November to 15 December 2021. Abundances of O3 and its precursors were measured when the air masses originated from the inland PRD. We observed that the VOCs levels at the DWS site were lower, while the mixing ratio of O3 was higher, compared to those reported at inland PRD, indicating the occurrence of photochemical consumption of VOCs during the air masses transport, which was further confirmed by the composition and diurnal variations of VOCs, as well as ratios of specific VOCs. The simulation results from a photochemical box model showed that the O3 level in the outflow air masses of inland PRD (O3(out-flow)) was the dominant factor leading to the intensification of O3 pollution and the enhancement of atmospheric radical concentrations (ARC) over PRE, which was mainly contributed by the O3 production via photochemical consumption of VOCs during air masses transport. Overall, our findings provided direct quantitative evidence for the roles of outflow O3 and its precursors from inland PRD on O3 abundance and ARC over the PRE area, highlighting that alleviation of O3 pollution over PRE should focus on the impact of photochemical loss of VOCs in the outflow air masses from inland PRD.

Characteristics and sources of oxygenated VOCs in Hong Kong: Implications for ozone formation

To investigate the characteristics of oxygenated volatile organic compounds (OVOCs) and their potential contribution to ozone (O3) generation, we conducted 3-h high-resolution observations during the summertime of 2022 and the wintertime of 2021. This study focused on a total of 28 OVOCs in five different chemical classes, which were encompassed at two representative sites in Hong Kong, including a roadside and an urban area. During the summertime, the total concentrations of quantified OVOCs (∑OVOCs) were 45 ± 12 and 63 ± 20 μg m-3 at the roadside and urban sites, respectively, whereas the ∑OVOCs decreased by 31 ± 11 % and 38 ± 13 %, respectively, during the wintertime. Among the classes of OVOCs, carbonyls and alcohols were the two predominant at both sites, with relatively higher concentration levels of acetone, methanol, butanaldehyde, and acrolein. The sources of OVOCs have significant spatial and temporal characteristics. Spatially, OVOCs were predominately attributed to primary emission and background at the roadside site, whereas they were a combination of primary emission, secondary formation, and background at the urban site. Temporally, background sources dominated the summertime OVOCs, while the contribution of primary emissions increased for the wintertime OVOCs. The O3 formation potential (OFP) for the OVOCs was calculated. The OFPs were 67 ± 16 and 119 ± 31 μg m-3 at the roadside and urban sites during the summertime, whereas the winter OFPs declined 30 % at the roadside and 38 % at the urban site. The background sources of carbonyls and alcohols at the roadside and of carbonyls and acrylates in the urban area were the major contributors to the summer OFP. Controlling the OVOC sources from local non-combustion sources such as gasoline-fuel evaporation and volatile chemical-containing products could lead to a reduction of OVOCs in the background and subsequently mitigate the OFP. This is beneficial for local O3 reduction in Hong Kong and surrounding regions.

Atmospheric oxidation capacity and O3 formation in a coastal city of southeast China: Results from simulation based on four-season observation

The pollution of atmospheric ozone in China shows an obvious upward trend in the past decade. However, the studies on the atmospheric oxidation capacity and O3 formation in four seasons in the southeastern coastal region of China with the rapid urbanization remain limited. Here, a four-season field observation was carried out in a coastal city of southeast China, using an observation-based model combining with the Master Chemical Mechanism, to explore the atmospheric oxidation capacity (AOC), radical chemistry, O3 formation pathways and sensitivity. The results showed that the average net O3 production rate (14.55 ppbv/hr) in summer was the strongest, but the average O3 concentrations in autumn was higher. The AOC and ROx levels presented an obvious seasonal pattern with the maximum value in summer, while the OH reactivity in winter was the highest with an average value of 22.75 sec-1. The OH reactivity was dominated by oxygenated VOCs (OVOCs) (30.6%-42.8%), CO (23.2%-26.8%), NO2 (13.6%-22.0%), and alkenes (8.4%-12.5%) in different seasons. HONO photolysis dominated OH primary source on daytime in winter, while in other seasons, HONO photolysis in the morning and ozone photolysis in the afternoon contributed mostly. Sensitivity analysis indicated that O3 production was controlled by VOCs in spring, autumn and winter, but a VOC-limited and NOx-limited regime in summer, and alkene and aromatic species were the major controlling factors to O3 formation. Overall, the study characterized the atmospheric oxidation capacity and elucidated the controlling factors for O3 production in the coastal area with the rapid urbanization in China.

Comparison of the ozone formation mechanisms and VOCs apportionment in different ozone pollution episodes in urban Beijing in 2019 and 2020: Insights for ozone pollution control strategies

Ground-level ozone (O3) pollution has been a tough issue in urban areas of China in the past decade. Clarifying the formation mechanisms of O3 and the sources of its precursors is necessary for the effective prevention of O3 pollution. In this study, a comparative analysis of O3 formation mechanisms and VOCs apportionment for five O3 pollution episodes was carried out at two urban sites (CRAES and CGZ) in Beijing in 2019 and 2020 by applying an observation-based modeling approach in order to obtain insights into O3 pollution control strategies. Results indicated that O3 pollution levels were generally more severe in 2019 than in 2020 during the observation periods. O3 formation at the two sites was both VOCs-limited on O3 polluted days and non-O3 polluted days. Stronger atmospheric oxidation capacity and ROx radicals cycling processes were found on O3 polluted days which could accelerate the local production of O3, and local photochemical production dominated the observed O3 concentrations at the two sites even on non-O3 polluted days. Emission reduction of VOCs should be a priority for mitigating O3 pollution, and alkenes and biogenic VOCs was the priority species at the CRAES and CGZ sites, respectively. Additionally, the reduction of oxygenated VOCs should also be important for the ozone control. Gasoline exhaust at the CRAES site, and solvent utilization and fuel evaporation at the CGZ site were main anthropogenic sources of VOCs. Therefore, local control measures should be further strengthened and differentiated control strategies of VOCs in the aspects of area, time, sources and species should be adopted in urban Beijing in the future. Overall, the findings of this study could provide a scientific understanding of the causes of O3 pollution and significant guidelines for formulating O3 control strategies from the perspective of different ozone pollution episodes in urban Beijing.

Peroxy radical chemistry during ozone photochemical pollution season at a suburban site in the boundary of Jiangsu-Anhui-Shandong-Henan region, China

Ambient peroxy radical (RO2⁎ = HO2 + RO2) concentrations were measured at a suburban site in a major prefecture-level city (Huaibei) in the boundary of Jiangsu-Anhui-Shandong-Henan region, which is the connecting belt of air pollution in the Beijing-Tianjin-Hebei region and the Yangtze River Delta. Measurements were carried out during the period of September to October 2021 to elucidate the formation mechanism of O3 pollution. The observed maximum concentration of peroxy radicals was 73.8 pptv. A zero-dimensional box model (Framework for 0-Dimensional Atmospheric Modeling, F0AM) based on Master Chemical Mechanism (MCM3.3.1) was used to predict radical concentrations for comparison with observations. The model reproduced the daily variation of peroxy radicals well, but discrepancies still appear in the morning hours. As in previous field campaigns, systematic discrepancies between modelled and measured RO2⁎ concentrations are observed in the morning for NO mixing ratios higher than 1 ppbv. Between 6:00 and 9:00 am, the model significantly underpredicts RO2⁎ by a mean factor of 7.2. This underprediction can be explained by a missing RO2⁎ source of 1.2 ppbv h-1 which originated from the photochemical conversion of an alkene-like chemical species. From the model results it shows that the main sources of ROx (= OH + HO2 + RO2) are the photolysis of oxygenated volatile organic compounds (OVOCs, 33 %), O3 and HONO (25 %), and HCHO (24 %). And the major sinks of ROx transitioned from a predominant reaction of radicals with NOx in the morning to a predominant peroxy self- and cross-reaction in the late afternoon. The introduction of an alkene-like species increased RO2 radical concentration and resulted in 14 % increase in net daily integrated ozone production, indicating the possible significance of the mechanism of alkene-like species oxidation to peroxy radicals. This study provides important information for subsequent ozone pollution control policies in Jiangsu-Anhui-Shandong-Henan region.

Integrating ambient carbonyl compounds provides insight into the constrained ozone formation chemistry in Zibo city of the North China Plain

Quantifying the impact of carbonyl compounds (carbonyls) on ozone (O₃) photochemical formation is crucial to formulating targeted O₃ mitigation strategies. To investigate the emission source of ambient carbonyls and their integrated observational constraint on the impact of O₃ formation chemistry, a field campaign was conducted in an industrial city (Zibo) of the North China Plain from August to September 2020. The site-to-site variations of OH reactivity for carbonyls were in accordance with the sequence of Beijiao (BJ, urban, 4.4 s^-1) > Xindian (XD, suburban, 4.2 s^-1) > Tianzhen (TZ, suburban, 1.6 s^-1). A 0-D box model (MCMv3.3.1) was applied to assess the O₃-precursor relationship influenced by measured carbonyls. It was found that without carbonyls constraint, the O₃ photochemical production of the three sites was underestimated to varying degrees, and the biases of overestimating the VOC-limited degree were also identified through a sensitivity test to NO_x emission changes, which may be associated with the reactivity of carbonyls. In addition, the results of the positive matrix factorization (PMF) model indicated that the main source of aldehydes and ketones was secondary formation and background (81.6% for aldehydes, 76.8% for ketones), followed by traffic emission (11.0% for aldehydes, 14.0% for ketones). Incorporated with the box model, we found that biogenic emission contributed the most to the O₃ production at the three sites, followed by traffic emission as well as industry and solvent usage. Meanwhile, the relative incremental reactivity (RIR) values of O₃ precursor groups from diverse VOC emission sources featured consistencies and differences at the three sites, which further highlights the importance of the synergetic mitigation of target O₃ precursors at regional and local scales. This study will help to provide targeted policy-guiding O₃ control strategies for other regions.

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.

Transmission paths and source areas of near-surface ozone pollution in the Yangtze River delta region, China from 2015 to 2021

Near-ground ozone in the Yangtze River Delta (YRD) region has become one of the main air pollutants that threaten the health of residents. However, to date, the transport behavior and source areas of ozone in the YRD region have not been systematically analyzed. In this study, by combining the ozone observational record with a HYSPLIT (hybrid single-particle Lagrangian integrated trajectory) model, we tried to reveal the spatiotemporal regularity of the airflow transport trajectory of ozone. Spatially, high ozone concentrations mainly clustered in industrial cities and resource-based cities. Temporally, the center of the ozone pollution shifted westward of Nanjing from 2015 to 2021. With the passage of time, the influence of meteorological elements on the ozone concentration in the YRD region gradually weakened. Marine atmosphere had the most significant impact on the transmission path of ozone in Shanghai, of which the trajectory frequency in 2021 accounted for 64.21% of the total frequency. The transmission trajectory of ozone in summer was different from that in other seasons, and its transmission trajectory was mainly composed of four medium-distance transmission paths: North China-Bohai Sea, East China Sea-West Pacific Ocean, Philippine Sea, and South China Sea-South China. The contribution source areas mainly shifted to the southeast, and the emission of pollutants from the Shandong Peninsula, the Korean Peninsula-Japan, and the Philippine Sea-Taiwan area increased the impact of ozone pollution in the Shanghai area from 2019 to 2021. This study identified the regional transport path of ozone in the YRD region and provided a scientific reference for the joint prevention and control of ozone pollution in this area.

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 review of gas-phase chemical mechanisms commonly used in atmospheric chemistry modelling

The atmospheric chemical mechanism is an essential component of airshed models used for investigating the chemical behaviors and impacts of species. Since the first tropospheric chemical mechanism was proposed in the 1960s, various mechanisms including Master Chemical Mechanism (MCM), Carbon Bond Mechanism (CBM), Statewide Air Pollution Research Center (SAPRC) and Regional Atmospheric Chemistry Mechanism (RACM) have been developed for different research purposes. This work summarizes the development and applications of these mechanisms, introduces their compositions and lumping methods, and compares the ways the mechanisms treat radicals with box model simulations. CBM can reproduce urban pollution events with relatively low cost compared to SAPRC and RACM, whereas the chemical behaviors of radicals and the photochemical production of ozone are described in detail in RACM. The photolysis rates of some oxygenated compounds are low in SAPRC07, which may result in underestimation of radical levels. As an explicit chemical mechanism, MCM describes the chemical processes of primary pollutants and their oxidation products in detail. MCM can be used to investigate certain chemical processes; however, due to its large size, it is rarely used in regional model simulations. A box model case study showed that the chemical behavior of OH and HO₂ radicals and the production of ozone were well described by all mechanisms. CBM and SAPRC underestimated the radical levels for different chemical treatments, leading to low ozone production values in both cases. MCM and RACM are widely used in box model studies, while CBM and SAPRC are often selected in regional simulations.

The interplays among meteorology, source, and chemistry in high particulate matter pollution episodes in urban Shanghai, China

High particulate matter (PM) pollution episodes still occur occasionally in urban China, despite of improvements in recent years. Investigating the influencing factors of high-PM episodes is beneficial in the formulation of effective control measures. We herein present the effects of weather condition, emission source, and chemical conversion on the occurrence of high-PM episodes in urban Shanghai using multiple online measurements. Three high-PM episodes, i.e., locally-accumulated, regionally-transported, and dust-affected ones, as well as a clean period were selected. Stagnant air with temperature inversion was found in both locally-accumulated and regionally-transported high-PM episodes, but differences in PM evolution were observed. In the more complicated dust-affected episode, the weather condition interacted with the emission/transport sources and chemical conversion, resulting in consecutive stages with different PM characteristics. Specifically, there were (1) stronger local accumulation in the pre-dust period, (2) dust-laden air with aged organic aerosol (OA) upon dust arrival, (3) pollutants being swept into the ocean, and (4) back to the city with aged OA. Our results suggest that (a) local emissions could be rapidly oxidized in some episodes but not all, (b) aged OA from long-range transport (aged in space) had a similar degree of oxygenation compared to the prolonged local oxidation (aged in time), and (c) OA aged over land and over the ocean were similar in chemical characteristics. The findings help better understand the causes and evolution of high-PM episodes, which are manifested by the interplays among meteorology, source, and chemistry, providing a scientific basis for control measures.

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 (NO_x) 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 NO_x 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 NO_x.

Attributing Increases in Ozone to Accelerated Oxidation of Volatile Organic Compounds at Reduced Nitrogen Oxides Concentrations

Surface ozone (O₃) is an important secondary pollutant affecting climate change and air quality in the atmosphere. Observations during the COVID-19 lockdown in urban China show that the co-abatement of nitrogen oxides (NO_x) and volatile organic compounds (VOCs) caused winter ground-level O₃ increases, but the chemical mechanisms involved are unclear. Here we report field observations in the Shanghai lockdown that reveals increasing photochemical formation of O₃ from VOC oxidation with decreasing NO_x. Analyses of the VOC profiles and NO/NO₂ indicate that the O₃ increases by the NO_x reduction counteracted the O₃ decreases through the VOC emission reduction in the VOC-limited region, and this may have been the main mechanism for this net O₃ increase. The mechanism may have involved accelerated OH-HO₂-RO₂ radical cycling. The NO_x reductions for increasing O₃ production could explain why O₃ increased from 2014 to 2020 in response to NO_x emission reduction even as VOC emissions have essentially remained unchanged. Model simulations suggest that aggressive VOC abatement, particularly for alkenes and aromatics, should help reverse the long-term O₃ increase under current NO_x abatement conditions.

Evidence for Reducing Volatile Organic Compounds to Improve Air Quality from Concurrent Observations and In Situ Simulations at 10 Stations in Eastern China

Ground-level ozone (O₃) has been an emerging air pollution in China and interacts with fine particulate matters (PM_2.5). We synthesized observations of O₃ and its precursors in two summer months of 2020 at 10 sites in the Zhejiang province, East China and simulated the in situ photochemistry. O₃ pollution in the northeastern Zhejiang province was more serious than that in the southwest. The site-average daytime O₃ increment correlated well (R² = 0.73) with the total reactivity of volatile organic compounds (VOCs) and carbon monoxide toward the hydroxyl radical (OH) in urban areas. Model simulation revealed that the main function of nitrogen oxides (NO_x) at the rural sites where isoprene accounted for >85% of OH reactivity of VOCs was to facilitate the radical cycling. With NO_x reduction from 0 to 90%, the self-reactions between peroxy radicals (Self-Rxns), a proven pathway for secondary organic aerosol formation, were intensified by up to 23-fold in a NO_x-rich environment. In contrast, reducing VOCs could weaken the Self-Rxns while reducing O₃ production rate and atmospheric oxidation capacity. This study observes and simulates O₃ chemistry based on extensive measurements in typical Chinese cities, highlighting the necessity of reducing VOCs for co-benefit of O₃ and PM_2.5.

Does Ozone Pollution Share the Same Formation Mechanisms in the Bay Areas of China?

As important regions of transition between land and sea, the three bay areas of Bohai Bay (BHB), Hangzhou Bay (HZB), and Pearl River Estuary (PRE) in China often suffer from severe photochemical pollution despite scarce anthropogenic emissions. To understand the causes of high ozone (O₃) concentrations, the high O₃ episode days associated with special synoptic systems in the three bays were identified via observations and simulated by the weather research and forecasting coupled with community multiscale air quality (WRF-CMAQ) model. It was revealed that the interaction between synoptic winds and mesoscale breezes resulted in slow wind speeds over the HZB and PRE, where air pollutants transported from upwind cities gained a long residence time and subsequently participated in intensive photochemical reactions. The net O₃ production rates within the bay areas were even comparable to those in surrounding cities. This finding was also applicable to BHB but with lower net O₃ production rates, while high levels of background O₃ and the regional transport from farther upwind BHB partially elevated the O₃ concentrations. Hence, these three bay areas served as O₃ "pools" which caused the accumulation of air pollutants via atmospheric dynamics and subsequent intense photochemical reactions under certain meteorological conditions. The results may be applicable to other similar ecotones around the world.

Species profiles, in-situ photochemistry and health risk of volatile organic compounds in the gasoline service station in China

Accompanying with increases in vehicle population and gasoline consumption, gasoline evaporation accounted for an enlarged portion of total volatile organic compound (VOC) emissions in China, raising increasing environmental concerns especially in megacities. In this study, an intensive sampling campaign was performed in a gasoline service station, to reveal emission characteristics, environmental and health impacts of VOCs. It was strikingly found that 24 % of air samples exceeded the national standard of 4 mg/m³ for non-methane hydrocarbons (NMHCs) on the boundary of the station, with the equipment of Stage I and II controls. VOC groups and species profiles showed that alkanes dominated total VOCs. As typical markers of evaporative loss of gasoline, C4-5 species (i-pentane, n-pentane and n-butane) as well as methyl tert-butyl ether (MTBE) accounted for 49.6 % of VOCs. Species profile and diagnostic ratios indicated the prominent contribution of gasoline evaporative losses from refueling or breathing processes, as well as the interference of vehicle exhaust in the ambient air at the site. Intensive O₃ production was reproduced by the photochemical box model, demonstrating that O₃ formation was co-limited by both VOCs (especially trans-2-butene) and NO_x. Inhalation health risk assessment proved that exposure to hazardous VOCs caused non-cancer risk (HQ = 3.08) and definitely posed cancer risks at a probability of 1.3 × 10^-4 to workers. Remarkable health risks were mainly imposed by halocarbons, aromatics and alkenes, in which 1,2-dichloropropane caused the highest non-cancer risk (HQ = 1.3) and acted as the primary carcinogen (ICR = 5.1 × 10^-5). This study elucidated the high unqualified rate in gasoline service stations after the implementation of latest standards in China, where new regulations targeted halocarbons and updates in existing vapor recovery systems were suggested for VOC mitigation.

Detection of excited triplet species from photolysis of carbonyls: Direct evidence for single oxygen formation in atmospheric environment

Excited triplet species play an important role in the photolytic formation of ¹O₂ from carbonyls, but the related mechanism is still uncertain, due to lack of direct evidence. In this study, steady-state and transient photolysis of eleven carbonyls to produce ¹O₂ was investigated. Dicarbonyl displayed greater ¹O₂ production ability than monocarbonyl, while dicarbonyl containing both ketone and carboxyl groups connected by CC bond (i.e., pyruvic acid (PA)) showed the highest ¹O₂ steady-state concentration ([¹O₂]_SS). For the first time, the production of ³PA* from PA with narrow energy gap was confirmed by laser flash photolysis technique and the second-order decay rate constant of ³PA* was 2.78 × 10⁷ M^-1 s^-1. Quenching results verified the dominant contribution of ³PA* to ¹O₂ production from PA. Addition of inorganic salt or increase in solution pH showed negligible effect on ³PA*, but significantly decreased the [¹O₂]_SS of PA by up to two orders of magnitude, due to reduction of hydrate content. Photolysis of methylglyoxal and dimethylamine mixture led to higher content of excited triplet species at pH ≈ 11 and remarkably enhanced [¹O₂]_SS, which was 2.3 times of that from PA and dimethylamine mixture. These findings provide direct evidence for the contribution of transient species from carbonyls or their product to ¹O₂ formation in atmospheric environment.

A Theoretical Perspective on the Actinic Photochemistry of 2-Hydroperoxypropanal

The photochemical reactions triggered by the sunlight absorption of transient volatile organic compounds in the troposphere are notoriously difficult to characterize experimentally due to the unstable and short-lived nature of these organic molecules. Some members of this family of compounds are likely to exhibit a rich photochemistry given the diversity of functional groups they can bear. Even more interesting is the photochemical fate of volatile organic compounds bearing more than one functional group that can absorb light—this is the case, for example, of α-hydroperoxycarbonyls, which are formed during the oxidation of isoprene. Experimental observables characterizing the photochemistry of these molecules like photoabsorption cross-sections or photolysis quantum yields are currently missing, and we propose here to leverage a recently developed computational protocol to predict in silico the photochemical fate of 2-hydroperoxypropanal (2-HPP) in the actinic region. We combine different levels of electronic structure methods—SCS-ADC(2) and XMS-CASPT2—with the nuclear ensemble approach and trajectory surface hopping to understand the mechanistic details of the possible nonradiative processes of 2-HPP. In particular, we predict the photoabsorption cross-section and the wavelength-dependent quantum yields for the observed photolytic pathways and combine them to determine in silico photolysis rate constants. The limitations of our protocol and possible future improvements are discussed.

Remarkable spring increase overwhelmed hard-earned autumn decrease in ozone pollution from 2005 to 2017 at a suburban site in Hong Kong, South China

Ozone (O₃) pollution has been a persistent problem in Hong Kong, particularly in autumn when severe O₃ pollution events are often observed. In this study, linear regression analyses of long-term O₃ data in suburban Hong Kong revealed that the variation of autumn O₃ obviously leveled off during 2005-2017, mainly due to the significant decrease of autumn O₃ in 2013-2017 (period II), despite the increase in 2005-2012 (period I). In addition, the rise of O₃ in summer and winter also ceased since 2013. In contrary, O₃ continuously increased throughout the spring of 2005-2017, especially in period II. Consequently, an incessant increase of overall O₃ was observed during 2005-2017. A statistical model combining Kolmogorov-Zurbenko filter with multiple linear regressions, and a photochemical box model incorporating CB05 mechanism were applied to probe the causes of the above trends. In general, O₃ production was controlled by VOC-limited regime throughout 13 years. The meteorological variability and regional transport facilitated the O₃ growth in period Ι. In contrast, the unchanged O₃ level in period II was attributable to the negative impact of meteorological variability and reduction of regional transport effect on O₃ formation and accumulation, as well as the negligible change in locally-produced O₃. In autumn of period II, the inhibitory meteorological variability, reduced regional transport, and alleviated local production were the driving force for the hard-earned decrease of O₃. However, the remarkable rise of spring O₃ was caused by the reduction of NO_x, especially in the spring of period II. The findings of the long-term and seasonal variations of O₃ pollution in Hong Kong are helpful for future O₃ mitigation.

Assessment of summertime O3 formation and the O3-NOX-VOC sensitivity in Zhengzhou, China using an observation-based model

Zhengzhou, the provincial capital of Henan province in Central China and a major hub of the country's transportation network, has been suffering from severe summertime ozone (O₃) pollution. Simultaneous field measurements of O₃ and its precursors, including NO_x, CO, HONO, and 106 volatile organic compounds (VOCs), were conducted at an urban site (the municipal environmental monitoring station, MEM) in Zhengzhou in July 2019. The Community Multiscale Air Quality (CMAQ) model, which incorporates the Master Chemical Mechanism (MCMv3.3.1), was modified to work as a 0-D observation-based photochemical box model to assess the sources and sinks of HO_x radicals and O₃, and the OH reactivity (K_OH) and ozone formation potential (OFP) of major VOC groups. In addition, the O₃-NO_x-VOC sensitivity was evaluated using the relative incremental reactivity (RIR) and O₃ formation isopleth techniques. The OH radicals were mainly generated from the propagation reaction of HO₂ + NO (91-95%). The daily average mixing ratios of OH and HO₂ radicals were significantly higher during high O₃ days, reaching as high as 4.8 × 10⁶ and 7.7 × 10⁸ molecules cm^-3, respectively. Photochemical O₃ formation was mostly due to the conversion of NO to NO₂ by HO₂ radicals (52-54%), while the NO₂ + OH reaction was the main contributor to O₃ destruction (70- 76%). Alkenes and aromatics were the main anthropogenic VOC contributors to K_OH and OFP. Contributions of biogenic VOCs became much more important on high O₃ days, correlating with the increase in temperature and solar radiation. RIR analysis showed that the O₃ formation was under the VOC-limited on low O₃ days but was in the transition regime during the O₃ pollution buildup and persisting days. These results are generally consistent with those based on the O₃ formation isopleth. This paper provides important corroborative scientific evidence urgently needed by local governments to formulate O₃ pollution control strategies.

Insights on In-Situ Photochemistry Associated with Ozone Reduction in Guangzhou during the COVID-19 Lockdown

Increases in ground-level ozone (O3) have been observed during the COVID-19 lockdown in many places around the world, primarily due to the uncoordinated emission reductions of O3 precursors. In Guangzhou, the capital of Guangdong province in South China, O3 distinctively decreased during the lockdown. Such a phenomenon was attributed to meteorological variations and weakening of local O3 formation, as indicated by chemical transport models. However, the emission-based modellings were not fully validated by observations, especially for volatile organic compounds (VOCs). In this study, we analyzed the changes of O3 and its precursors, including VOCs, from the pre-lockdown (Pre-LD) to lockdown period (LD) spanning 1 week in Guangzhou. An observation-based box model was applied to understand the evolution of in-situ photochemistry. Indeed, the ambient concentrations of O3 precursors decreased significantly in the LD. A reduction of 20.7% was identified for the total mixing ratios of VOCs, and the transportation-related species experienced the biggest declines. However, the reduction of O3 precursors would not lead to a decrease of in-situ O3 production if the meteorology did not change between the Pre-LD and LD periods. Sensitivity tests indicated that O3 formation was limited by VOCs in both periods. The lower temperature and photolysis frequencies in the LD reversed the increase of O3 that would be caused by the emission reductions otherwise. This study reiterates the fact that O3 abatement requires coordinated control strategies, even if the emissions of O3 precursors can be significantly reduced in the short term.

Accelerated toluene degradation over forests around megacities in southern China

Toluene is a typical anthropogenic pollutant that has profound impacts on air quality, climate change, and human health, but its sources and sinks over forests surrounding megacities remain unclear. The Nanling Mountains (NM) is a large subtropical forest and is adjacent to the Pearl River Delta (PRD) region, a well-known hotspot for toluene emissions in southern China. However, unexpectedly low toluene concentrations (0.16 ± 0.20 ppbv) were observed at a mountaintop site in NM during a typical photochemical period. A backward trajectory analysis categorized air masses received at the site into three groups, namely, air masses from the PRD, those from central China, and from clean areas. The results revealed more abundant toluene and its key oxidation products, for example, benzaldehyde in air masses mixed with urban plumes from the PRD. Furthermore, a more than three times faster degradation rate of toluene was found in this category of air masses, indicating more photochemical consumption in NM under PRD outflow disturbance. Compared to the categorized clean and central China plumes, the simulated OH peak level in the PRD plumes (15.8 ± 2.2 × 106 molecule cm-3) increased by approximately 30% and 55%, respectively, and was significantly higher than the reported values at other background sites worldwide. The degradation of toluene in the PRD plumes was most likely accelerated by increased atmospheric oxidative capacity, which was supported by isoprene ozonolysis reactions. Our results indicate that receptor forests around megacities are not only highly polluted by urban plumes, but also play key roles in environmental safety by accelerating the degradation rate of anthropogenic pollutants.

Ambient volatile organic compounds at a receptor site in the Pearl River Delta region: Variations, source apportionment and effects on ozone formation

We present the continuously measurements of volatile organic compounds (VOCs) at a receptor site (Wan Qing Sha, WQS) in the Pearl River Delta (PRD) region from September to November of 2017. The average mixing ratios of total VOCs (TVOCs) was 36.3 ± 27.9 ppbv with the dominant contribution from alkanes (55.5%), followed by aromatics (33.3%). The diurnal variation of TVOCs showed a strong photochemical consumption during daytime, resulting in the formation of ozone (O₃). Five VOC sources were resolved by the positive matrix factorization (PMF) model, including solvent usage (28.6%), liquid petroleum gas (LPG) usage (24.4%), vehicle exhaust (21.0%), industrial emissions (13.2%) and gasoline evaporation (12.9%). The regional transport air masses from the upwind cities of south China can result in the elevated concentrations of TVOCs. Low ratios of TVOCs/NO_x (1.53 ± 0.88) suggested that the O₃ formation regime at WQS site was VOC-limited, which also confirmed by a photochemical box model with the master chemical mechanism (PBM-MCM). Furthermore, the observation on high-O₃ episode days revealed that frequent O₃ outbreaks at WQS were mainly caused by the regional transport of anthropogenic VOCs especially for aromatics and the subsequent photochemical reactions. This study provides valuable information for policymakers to propose the effective control strategies on photochemical pollution in a regional perspective.

Photochemical ozone pollution in five Chinese megacities in summer 2018

To investigate photochemical ozone (O₃) pollution in urban areas in China, O₃ and its precursors and meteorological parameters were simultaneously measured in five megacities in China in summer 2018. Moderate wind speeds, strong solar radiation and high temperature were observed in all cities, indicating favorable meteorological conditions for local O₃ formation. However, the unusually frequent precipitation caused by typhoons reaching the eastern coastline resulted in the least severe air pollution in Shanghai. The highest O₃ level was found in Beijing, followed by Lanzhou and Wuhan, while relatively lower O₃ value was recorded in Chengdu and Shanghai. Photochemical box model simulations revealed that net O₃ production rate in Lanzhou was the largest, followed by Beijing, Wuhan and Chengdu, while it was the lowest in Shanghai. Besides, the O₃ formation was mainly controlled by volatile organic compounds (VOCs) in most cities, but co-limited by VOCs and nitrogen oxides in Lanzhou. Moreover, the dominant VOC groups contributing to O₃ formation were oxygenated VOCs (OVOCs) in Beijing and Wuhan, alkenes in Lanzhou, and aromatics and OVOCs in Shanghai and Chengdu. Source apportionment analysis identified six sources of O₃ precursors in these cities, including liquefied petroleum gas usage, diesel exhaust, gasoline exhaust, industrial emissions, solvent usage, and biogenic emissions. Gasoline exhaust dominated the O₃ formation in Beijing, and LPG usage and industrial emissions made comparable contributions in Lanzhou, while LPG usage and solvent usage played a leading role in Wuhan and Chengdu, respectively. The findings are helpful to mitigate O₃ pollution in China.

Exploration of O3-precursor relationship and observation-oriented O3 control strategies in a non-provincial capital city, southwestern China

To investigate photochemical ozone (O₃) formation and provide localized control strategies, an intensive sampling of O₃ and its precursors (i.e. volatile organic compounds (VOCs) and nitrogen oxides (NO_x = NO + NO₂)) were conducted at an urban site in Leshan, Sichuan province during 4-13 August 2019. The mixing ratios of O₃, total VOCs (TVOCs) and NO_x were 40.0 ± 5.3, 22.5 ± 2.6 and 14.6 ± 3.8 ppbv, respectively (±95% confidence intervals). O₃ and its precursors existed a well negative correlation, indicating intensive local O₃ formation. To further explore the O₃-precursors relationship and observation-oriented O₃ control strategies, a photochemical box model coupled with master chemical mechanism (PBM-MCM) was adapted. The relative incremental reactivity (RIR) calculated by model results showed that Leshan was in the VOCs-limited and O₃ production was most sensitive to alkenes. Moreover, O₃ isopleth diagram was drawn using the PBM-MCM simulation results and seven reduction scenarios were evaluated in Leshan. The reduction ratio of VOCs/NO_x on 3:1 was proposed to be the best solution, which can be achieved effective reduction on local O₃ formation. At last, since VOCs were the key precursors of O₃ in Leshan, VOC sources and their potential contributions to O₃ formation were investigated by using the positive matrix factorization (PMF) model. Seven sources were identified, and traffic related emissions (including vehicle exhaust and gasoline evaporation, 29.9%) and fixed combustion (27.7%) had the large contribution to ambient VOCs. Among anthropogenic sources, fixed combustion and solvent usage in painting were the large contributors to O₃ formation, accounting for 30.9% and 18.3%, respectively, which should have high priorities on source reduction. This study provides scientific advices for future O₃ pollution control strategies in Leshan, which can be extended to other cities.

Assessing the role of mineral particles in the atmospheric photooxidation of typical carbonyl compound

Mineral particles are ubiquitous in the atmosphere and exhibit an important effect on the photooxidation of volatile organic compounds (VOCs). However, the role of mineral particles in the photochemical oxidation mechanism of VOCs remains unclear. Hence, the photooxidation reactions of acrolein (ARL) with OH radical (OH) in the presence and absence of SiO₂ were investigated by theoretical approach. The gas-phase reaction without SiO₂ has two distinct pathways (H-abstraction and OH-addition pathways), and carbonyl-H-abstraction is the dominant pathway. In the presence of SiO₂, the reaction mechanism is changed, i.e., the dominant pathway from carbonyl-H-abstraction to OH-addition to carbonyl C-atom. The energy barrier of OH-addition to carbonyl C-atom deceases 21.33 kcal/mol when SiO₂ is added. Carbonyl H-atom of ARL is occupied by SiO₂ via hydrogen bond, and carbonyl C-atom is activated by SiO₂. Hence, the main product changes from H-abstraction product to OH-adduct in the presence of SiO₂. The OH-adduct exhibits a thermodynamic feasibility to yield HO₂ radical and carboxylic acid via the subsequent reactions with O₂, with implications for O₃ formation and surface acidity of mineral particles.

Development of urban air monitoring with high spatial resolution using mobile vehicle sensors

Traditionally, the equipment used to measure air pollution is expensive and placed around cities or in mobile laboratories. It might only represent a certain area and not the entire city due to the locations and limited number of monitoring stations. Nowadays, a mobile sensing is becoming an alternative option to monitor air quality in urban environment due to its ease of use, high flexibility, and low price. This paper develops a vehicular-based mobile monitoring system for real time air quality sensing and visualization across large cities with high spatial resolution. The commercially available low-cost CO, NO[Formula: see text], NH[Formula: see text] O[Formula: see text], CH[Formula: see text], SO[Formula: see text], PM[Formula: see text], temperature and humidity sensors along with the microcontroller and GPS were integrated in a sensing device installed on the roof of taxi and sport utility vehicle (SUV). The developed device was calibrated through a reference monitoring station and validated through field measurement. We first split the entire city with a uniform grid discretization. We then propose a data processing methodology based on machine learning algorithms for generating 250 representative data set from 286 million data which is collected using the vehicular based mobile sensors. Next we present the representativeness of the data set by comparison of stationary data and mobile data. We also describe the analytical results and spatial distribution with high spatial resolution throughout the city. In addition, the collected mobile sensor data is also used to show that the significant differences and spatial variability in mean levels per street. Finally, we conclude that the proposed mobile monitoring system using high spatial resolution can effectively map the air quality in metropolitan environment and provide detail about the spatial variability that cannot be done with stationary monitoring systems.

Photochemistry of ozone pollution in autumn in Pearl River Estuary, South China

To explore the photochemical O₃ pollution over the Pearl River Estuary (PRE), intensive measurements of O₃ and its precursors, including trace gases and volatile organic compounds (VOCs), were simultaneously conducted at a suburban site on the east bank of PRE (Tung Chung, TC) in Hong Kong and a rural site on the west bank (Qi'ao, QA) in Zhuhai, Guangdong in autumn 2016. Throughout the sampling period, 3 days with high O₃ levels (maximum hourly O₃ > 100 ppbv) were captured at both sites (pattern 1) and 13 days with O₃ episodes occurred only at QA (pattern 2). It was found that O₃ formation at TC was VOC-limited in both patterns because of the large local NO_x emissions. However, the O₃ formation at QA was co-limited by VOCs and NO_x in pattern 1, but VOC-limited in pattern 2. In both patterns, isoprene, formaldehyde, xylenes and trimethylbenzenes were the top 4 VOCs that modulated local O₃ formation at QA, while they were isoprene, formaldehyde, xylenes and toluene at TC. In pattern 1, the net O₃ production rate at QA (13.1 ± 1.6 ppbv h^-1) was high, and comparable (p = 0.40) to that at TC (12.1 ± 1.5 ppbv h^-1), so was the hydroxyl radical (i.e., OH), implying high atmospheric oxidative capacity over PRE. In contrast, the net O₃ production rate was significantly higher (p < 0.05) at QA (16.3 ± 0.4 ppbv h^-1) than that at TC (4.7 ± 0.2 ppbv h^-1) in pattern 2, and the OH concentration and cycling rate were also higher, indicating much stronger photochemical reactions at QA. These findings enhanced our understanding of O₃ photochemistry in the Pearl River estuary, which could be extended to other estuaries.

Long-term variations of C1-C5 alkyl nitrates and their sources in Hong Kong

Investigating the long-term trends of alkyl nitrates (RONO₂) is of great importance for evaluating the variations of photochemical pollution. Mixing ratios of C₁-C₅ RONO₂ were measured in autumn Hong Kong from 2002 to 2016, and the average level of 2-butyl nitrate (2-BuONO₂) always ranked first. The C₁-C₄ RONO₂ all showed increasing trends (p < 0.05), and 2-BuONO₂ had the largest increase rate. The enhancement in C₃ RONO₂ was partially related to elevated propane, and dramatic decreases (p < 0.05) in both nitrogen monoxide (NO) and nitrogen dioxide (NO₂) also led to the increased RONO₂ formation. In addition, an increase of hydroxyl (OH) and hydroperoxyl (HO₂) radicals (p < 0.05) suggested enhanced atmospheric oxidative capacity, further resulting in the increases of RONO₂. Source apportionment of C₁-C₄ RONO₂ specified three typical sources of RONO₂, including biomass burning emission, oceanic emission, and secondary formation, of which secondary formation was the largest contributor to ambient RONO₂ levels. Mixing ratios of total RONO₂ from each source were quantified and their temporal variations were investigated. Elevated RONO₂ from secondary formation and biomass burning emission were two likely causes of increased ambient RONO₂. By looking into the spatial distributions of C₁-C₅ RONO₂, regional transport from the Pearl River Delta (PRD) was inferred to build up RONO₂ levels in Hong Kong, especially in the northwestern part. In addition, more serious RONO₂ pollution was found in western PRD region. This study helps build a comprehensive understanding of RONO₂ pollution in Hong Kong and even the entire PRD.

In Situ Measurements of Molecular Markers Facilitate Understanding of Dynamic Sources of Atmospheric Organic Aerosols

Reducing the amount of organic aerosol (OA) is crucial to mitigation of particulate pollution in China. We present time and air-origin dependent variations of OA markers and source contributions at a regionally urban background site in South China. The continental air contained primary OA markers indicative of source categories, such as levoglucosan, fatty acids, and oleic acid. Secondary OA (SOA) markers derived from isoprene and monoterpenes also exhibited higher concentrations in continental air, due to more emissions of their precursors from terrestrial ecosystems and facilitation of anthropogenic sulfate for monoterpenes SOA. The marine air and continental-marine mixed air had more abundant hydroxyl dicarboxylic acids (OHDCA), with anthropogenic unsaturated organics as potential precursors. However, OHDCA formation in continental air was likely attributable to both biogenic and anthropogenic precursors. The production efficiency of OHDCA was highest in marine air, related to the presence of sulfur dioxide and/or organic precursors in ship emissions. Regional biomass burning (BB) was identified as the largest contributor of OA in continental air, with contributions fluctuating from 8% to 74%. In contrast, anthropogenic SOA accounted for the highest fraction of OA in marine (37 ± 4%) and mixed air (31 ± 3%), overriding the contributions from BB. This study demonstrates the utility of molecular markers for discerning OA pollution sources in the offshore marine atmosphere, where continental and marine air pollutants interact and atmospheric oxidative capacity may be enhanced.

Kinetic and mechanism studies of the ozonolysis of three unsaturated ketones

Reaction rate constants and products of 1-octen-3-one, 3-octen-2-one and 4-hexen-3-one with ozone were studied in a 100-L fluorinated ethylene propylene (FEP) Teflon film bag using absolute rate method at 298 ± 1 K and atmospheric pressure. The rate constants were (1.09 ± 0.12) × 10^-17, (3.48 ± 0.36) × 10^-17 and (5.70 ± 0.60) × 10^-17 cm³/(molecule⋅sec), respectively. According to the obtained rate constants, the effects of carbonyl were discussed. The carbonyl group in β position has a net withdrawing effect with respect to an olefinic bond, then causing the decline of rate constants. The quantum chemical calculation was used to explain the results of rate constants. The products of ozonolysis were mainly aldehydes, which have significant influence on the formation of SOA, and hence play an important role in the atmosphere. In this work, we detected the main products of reaction and proposed the reaction mechanism by combining the results of quantum chemical calculations. Atmospheric lifetime for three unsaturated ketones reacted with ozone was 36.4, 11.4 and 6.9 hr for 1-octen-3-one, 3-octen-2-one and 4-hexen-3-one, respectively.

A case study on the characterization of non-methane hydrocarbons over the South China Sea: Implication of land-sea air exchange

We present the characteristics of non-methane hydrocarbons (NMHCs) over the northern South China Sea (SCS) during a cruise campaign from September to October 2013. The mixing ratios of the total NMHCs ranged from 1.45 to 7.13 ppbv with an average of 3.54 ± 1.81 ppbv. Among the measured NMHCs, alkanes and aromatics were the major groups, accounting for 45.8 ± 8.7% and 28.7 ± 12.3% of the total NMHCs, respectively. Correlations of NMHCs with typical source tracers suggest that light alkanes and benzene were largely contributed by vehicular exhaust via long-range transport, while the other aromatics might be related to industrial sources and marine ship emissions. The spatial variations of NMHCs were observed with higher mixing ratios of NMHCs in the samples collected in the offshore areas than those in the coastal areas. Air mass back-trajectory analysis and diagnostic ratios of NMHCs show that the elevations of the total NMHCs were caused by the regional pollution transport from the southeast coast of China and/or southern China. The ozone formation potentials (OFPs) of NMHCs were calculated and the results show that the aromatics associated with marine ship emissions were the important contributors to the total OFP. This study provides useful information on the interaction between continental outflow and marine atmosphere.

A study on the relationship between air pollution and pulmonary tuberculosis based on the general additive model in Wulumuqi, China

OBJECTIVE

This study aimed to explore the impact of atmospheric pollutants on the incidence of tuberculosis (TB), and provide new ideas for the prevention and control of TB in the future.

METHODS

It explored the relationship between air pollutants and meteorological factors, as well as between air pollutants and heating through Spearman correlation analysis and rank sum test. Additionally, it analyzed the relationship between air pollutants and TB incidence using the general additive model. Statistical analysis results at the p<0.05 level were considered significant.

RESULTS

Three months after exposure to air pollutants (PM_2.5, SO₂, NO₂, and CO) TB incidence increased. However, TB incidence increased 9 months after exposure to PM₁₀. The single pollutant model showed when concentrations of PM_2.5, PM₁₀, SO₂, NO₂, CO, and O₃ increased by 1μg/m³ (or 1mg/m³), the number of TB cases increased by 0.09%, 0.08%, 0.58%, 0.42%, 6.9%, and 0.57%, respectively. The optimal multi-pollutant model was a two-factor model (PM₁₀+NO₂).

CONCLUSION

Air pollutants including PM_2.5, PM₁₀, SO₂, NO₂, CO, and O₃ increased the risk of TB. Few studies have been conducted in this area of research, especially regarding the mechanism. The results of this study should contribute to the understanding of TB incidence and prompt additional research.

O3 Sensitivity and Contributions of Different NMHC Sources in O3 Formation at Urban and Suburban Sites in Shanghai

Ground-level ozone (O3) pollution is still one of the priorities and challenges for air pollution control in the Yangtze River Delta (YRD) region of China. Understanding the relationship of O3 with its precursors and contributions of different sources in O3 formation is essential for the development of an O3 control strategy. This study analyzed O3 sensitivity to its precursors using a box model based on online observations of O3, non-methane hydrocarbons (NMHCs), nitrogen oxides (NOx), and carbon monoxide (CO) at an urban site and a suburban site in Shanghai in July 2017. Anthropogenic sources of NMHCs were identified using the positive matrix factorization (PMF) receptor model, and then contributions of different sources in O3 formation were estimated by the observation-based model (OBM). The relative incremental reactivity (RIR) values calculated by the OBM suggest that O3 formation at the urban site was in the NMHC-limited regime, while O3 formation at the suburban site tended between the transition regime and the NMHC-limited regime. Vehicular emission and liquefied petrochemical gas (LPG) use or aged air mass were found to be the two largest contributors at the urban and suburban sites in July, followed by paint and solvent use, and the petrochemical industry. However, from the perspective of O3 formation, vehicular emission and paint and solvent use were the largest two contributors at two sites due to the higher RIR values for paint and solvent use. In addition, the influence of transport on O3 sensitivity was identified by comparing O3 sensitivity at the suburban site across two days with different air mass paths. The result revealed that O3 formation in Shanghai is not only related to local emissions but also influenced by emissions from neighboring provinces. These findings on O3–NMHC–NOX sensitivity, contributions of different sources in O3 formation, and influence of transport could be useful for O3 pollution control in the YRD region. Nevertheless, more quantitative analyses on transport and further evaluation of the uncertainty of the OBM are still needed in future.

Ambient air pollution in gastrointestinal endoscopy unit

BACKGROUND

The gastrointestinal endoscopy unit is frequently exposed to gastrointestinal gas expelled from patients and electrocoagulated tissue through carbonation. This can be potentially harmful to the health of not only the healthcare personnel but also patients who undergo endoscopy. This study aimed to measure the air quality in the endoscopy unit.

METHODS

We measured indoor air quality indices (CO₂, total volatile organic compounds (VOCs), PM_2.5, NO₂, CO, and ozone) using portable passive air quality monitoring sensors in the procedural area, recovery area, and cleansing-of-equipment area, at 1-min intervals for 1 week, and the type and number of endoscopic procedures were recorded.

RESULTS

CO₂, PM_2.5, NO₂, and ozone levels were the highest in the cleansing area, followed by the procedural and recovery areas, and VOC level was highest in the procedural area. The proportion of poor-quality level of CO₂ and VOCs was highest in the procedural area and that of NO₂ was highest in the cleansing area. The proportion of tolerable to poor-quality (exceeding acceptable level) level of CO₂ and total VOCs in the procedural area was 26% and 19.2% in all measurement times, respectively. The proportion of tolerable to poor-quality level of NO₂ in the cleansing area of the endoscopy unit was 32.1% in all measurement times. Multivariate analyses revealed that tolerable to poor-quality (exceeding acceptable level) level of VOCs was associated with the number of endoscopic procedures (odds ratio, 1.79; 95% confidence interval, 1.42-2.27) and PM_2.5 level (1.27, 1.12-1.44). Moreover, tolerable to poor-quality level of CO₂ was associated with the number of colonoscopy (5.35, 1.19-24.02), especially with electrocoagulation procedures (24.31, 1.31-452.44) in the procedural area.

CONCLUSIONS

Healthcare personnel and patients who undergo endoscopy are frequently exposed to ambient air pollution. Health-related protective strategies for ambient air pollution in the endoscopy unit are warranted. CLINICALTRIALS.

GOV REGISTRATION NUMBER

NCT03724565.

Aggravating O3 pollution due to NOx emission control in eastern China

During the past five years, China has witnessed a rapid drop of nitrogen oxides (NO_x) owing to the wildly-applied rigorous emission control strategies across the country. However, ozone (O₃) pollution was found to steadily deteriorate in most part of eastern China, especially in developed regions such as Jing-Jin-Ji (JJJ), Yangtze River Delta region (YRD) and Pearl River Delta region (PRD). To shed more light on current O₃ pollution and its responses to precursor emissions, we integrate satellite retrievals, ground-based measurements together with regional numerical simulation in this study. It is indicated by multiple sets of observational data that NO_x in eastern China has declined more than 25% from 2012 to 2016. Based on chemical transport modeling, we find that O₃ formation in eastern China has changed from volatile organic compounds (VOCs) sensitive regime to the mixed sensitive regime due to NO_x reductions, substantially contributing to the recent increasing trend in urban O₃. In addition, such transitions tend to bring about an ~1-1.5 h earlier peak of net O₃ formation rate. We further studied the O₃ precursors relationships by conducting tens of sensitivity simulations to explore potential ways for effective O₃ mitigation. It is suggested that the past control measures that only focused on NO_x may not work or even aggravate O₃ pollution in the city clusters. In practice, O₃ pollution in the three regions is expected to be effectively mitigated only when the reduction ratio of VOCs/NO_x is greater than 2:1, indicating VOCs-targeted control is a more practical and feasible way.

Photochemical evolution of continental air masses and their influence on ozone formation over the South China Sea

To investigate photochemical ozone (O₃) pollution over the South China Sea (SCS), an intensive sampling campaign was conducted from August to November simultaneously at a continental site (Tung Chung, TC) and a marine site (Wan Shan Island, WSI). It was found that when continental air masses intruded the SCS, O₃ episodes often occurred subsequently. To discover the causes, a photochemical trajectory model (PTM) coupled with the near-explicit Master Chemical Mechanism (MCM) was adopted, and the photochemical processes of air masses during the transport from TC to WSI were investigated. The simulated O₃ and its precursors (i.e. NO_x and VOCs) showed a reasonably good agreement with the observations at both TC and WSI, indicating that the PTM was capable of simulating O₃ formation for air masses traveling from TC to WSI. The modeling results revealed that during the transport of air masses from TC to WSI, both VOC and NO_x decreased in the morning while O₃ increased significantly, mainly due to rapid chemical reactions with elevated radicals over the SCS. The elevated radicals over the SCS were attributable to the fact that higher NO_x at TC consumed more radicals, whereas the concentration of radicals increased from TC to WSI because of NO_x dilution and destruction. Subsequently, the photochemical cycling of radicals accelerated, leading to high O₃ mixing ratios over the SCS. Furthermore, based on the source profiles of the emission inventory used, the contributions of six sources, i.e. gasoline vehicle exhaust, diesel vehicle exhaust, gasoline evaporation and LPG usage, solvent usage, biomass and coal burning, and biogenic emissions, to maritime O₃ formation were evaluated. The results suggested that gasoline vehicles exhaust and solvent usage largely contributed the O₃ formation over the SCS (about 5.2 and 3.8 ppbv, respectively). This is the first time that the contribution of continental VOC sources to the maritime O₃ formation was quantified.

Overview on the spatial-temporal characteristics of the ozone formation regime in China

Ozone (O3), a main component in photochemical smog, is a secondary pollutant formed through complex photochemical reactions involving nitrogen oxides (NOx) and volatile organic compounds (VOCs). In the past few decades, with the rapid economic development, industrialization and urbanization, the mixing ratio of O3 has increased substantially in China. O3 non-attainment days have been frequently observed. Despite great efforts made in the past few years, it is still difficult to alleviate O3 pollution in China, due to its non-linear relationship with the precursors. In view of the severe situation in China, this study presents a comprehensive review on the spatial-temporal variations of the relationship between O3 and its precursors (i.e. O3 formation regime), built upon the previous reviews of the spatial-temporal variations of O3 and its precursor levels. Valuable findings from previous studies are laid out for a better understanding of O3 pollution, followed by implications for the control of O3 pollution. This literature review indicates that O3 formation in most areas of the North China Plain (NCP), Yangtze River Delta (YRD) and Pearl River Delta (PRD) regions is in a VOC-limited regime during the high-O3 seasons due to dramatic emissions from human activities in cities. Outside these metropolitan areas, a NOx-limited regime dominates rural/remote areas. From summer to winter, the O3 formation regime over China shows a tendency to shift to a VOC-limited regime. Furthermore, O3 formation in China shifted toward increasing sensitivity to VOC emissions before the 12th Five-Year-Plan. However, after the 12th Five-Year-Plan, successful reduction of NOx slowed down this trend. Further effective control of VOCs is expected to achieve sustained O3 attainment in the future. To timely solve the current O3 pollution problem, precise control of O3 precursors is proposed, together with the joint prevention and control of regional air pollution.

The influence of terrestrial transport on visibility and aerosol properties over the coastal East China Sea

Air pollutants from East Asia continent can affect the physio-chemical and optical properties of marine aerosols under seasonal winds. We investigated the change of visibility and haze frequency from 1974 to 2017 over the coastal East China Sea (ECS), and reconstructed the light extinction coefficients according to the chemical compositions of PM_2.5 samples collected at Huaniao Island in the ECS. The annual average visibility significantly decreased from over 25 km in the early 1970s to <18 km in recent 4 years. The occurrence of daily maximum haze frequency was approximately 3-h later with respect to land sites, which could be explained by the diffusion of air pollutants from nearby cities (haze peak around rush hour) to the coastal ECS as well as the formation of secondary aerosols enhanced by photochemical reactions around noon at the condition of affluent gaseous precursors. Meanwhile, anthropogenic chloride transported from the land could increase the concentration of Cl^- in marine aerosol, which may weaken the Cl^- depletion phenomenon over coastal ECS and even induced considerable Cl^- enrichment during the severe haze event in Jan. 2013. The largest contributor to the light extinction was (NH₄)₂SO₄ followed by NH₄NO₃ and OM in almost all seasons. Especially in winter and spring, (NH₄)₂SO₄ accounted for 45% and 52% of total light extinction, respectively. The estimated b_ext was lower than the monitored values, suggesting that the contribution of some aerosol components (e.g. NH₄Cl and large mode components) might be underestimated. Further study on the combination of observation and estimation of specific aerosol contribution to the visibility impairment are needed.

Continuous effectiveness of replacing catalytic converters on liquified petroleum gas-fueled vehicles in Hong Kong

To mitigate the concentrations of air pollutants in the atmosphere, an intervention program of replacing the converters of liquefied petroleum gas (LPG) fueled vehicles was implemented by the Hong Kong government between October 2013 and April 2014. Data of ambient volatile organic compounds (VOCs) and other trace gases continuously monitored from September 2012 to April 2017 at a roadside site were used to evaluate the continuous effectiveness of the replaced catalytic converters on the reduction of air pollutants. The measurement data showed that LPG-related VOCs (propane and n/i-butanes) and several trace gases (CO, NO and NO₂) decreased significantly from before to after the program (p < 0.01). To further assess the efficiency of the program, five periods covering before the program, during the program, 1st year after the program, 2nd year after the program and 3rd year after the program were categorized. The values of propane and n/i-butanes decreased from Period-1 (before the program) to Period-2 (during the program), and from Period-2 to Periods 3-5 (after the program) (p < 0.01). In addition, the reduction rates of propane and n/i-butanes remained high and constant in Periods 3-5, suggesting that either had the vehicle owners themselves routinely replaced the converters at suitable interval afterwards, or were their vehicles caught by a remote sensing program checking excessive emissions. Source apportionment analysis indicated that LPG-fueled vehicular emissions were the top contributor to ambient VOCs in the roadside environment while the VOCs emitted from LPG-fueled vehicles indeed decreased at a rate of 4.21 ± 2.38 ppbv/year (average ± 95% confidence interval) from Period-1 to Period-5 (p < 0.01). Furthermore, the photochemical box model simulations revealed that the net negative contribution of VOCs and NO_x emitted from LPG-fueled vehicles to O₃ production strengthened at a rate of 1.9 × 10^-2 pptv/day from Period-1 to Period-5 (p < 0.01). The findings proved the continuous effectiveness of the intervention program, and are of help to future control strategies in Hong Kong.

Causes of ozone pollution in summer in Wuhan, Central China

In August 2016, continuous measurements of volatile organic compounds (VOCs) and trace gases were conducted at an urban site in Wuhan. Four high-ozone (O₃) days and twenty-seven non-high-O₃ days were identified according to the China's National Standard Level II (∼100 ppbv). The occurrence of high-O₃ days was accompanied by tropical cyclones. Much higher concentrations of VOCs and carbon monoxide (CO) were observed on the high-O₃ days (p < 0.01). Model simulations revealed that vehicle exhausts were the dominant sources of VOCs, contributing 45.4 ± 5.2% and 37.3 ± 2.9% during high-O₃ and non-high-O₃ days, respectively. Both vehicle exhausts and stationary combustion made significantly larger contributions to O₃ production on high-O₃ days (p < 0.01). Analysis using a chemical transport model found that local photochemical formation accounted for 74.7 ± 5.8% of the daytime O₃, around twice the regional transport (32.2 ± 5.4%), while the nighttime O₃ was mainly attributable to regional transport (59.1 ± 9.9%). The local O₃ formation was generally limited by VOCs in urban Wuhan. To effectively control O₃ pollution, the reduction ratio of VOCs to NO_x concentrations should not be lower than 0.73, and the most efficient O₃ abatement could be achieved by reducing VOCs from vehicle exhausts. This study contributes to the worldwide database of O₃-VOC-NO_x sensitivity research. Its findings will be helpful in formulating and implementing emission control strategies for dealing with O₃ pollution in Wuhan.