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Secondary organic aerosol (SOA) is a very important component of fine particulate matter (PM

Nitro-phenolic compounds (NPs) have attracted increasing attention because of their health risks and impacts on visibility, climate, and atmospheric chemistry. Despite many measurements of particulate NPs, the knowledge of their gaseous abundances, sources, atmospheric fates, and impacts remains incomplete. Here, 18 gaseous NPs were continuously measured with a time-of-flight chemical ionization mass spectrometer at a background site in South China in autumn and winter. Abundant NPs were observed in the continental outflows from East Asia, with a total concentration up to 122.1 pptv. Secondary formation from the transported aromatics dominated the observed NPs, with mono-NPs exhibiting photochemical daytime peaks and nighttime enrichments of di-NPs and Cl-substituted NPs. The budget analysis indicates that besides the •OH oxidation of aromatics, the NO3• oxidation also contributed significantly to the daytime mono-NPs, while the further oxidation of mono-NPs by NO3• dominated the nocturnal formation of di-NPs. Photolysis was the main daytime sink of NPs and produced substantial HONO, which would influence atmospheric oxidation capacity in downwind and background regions. This study provides quantitative insights on the formation and impacts of gaseous NPs in the continental outflow and highlights the role of NO3• chemistry in the secondary nitro-aromatics production that may facilitate regional pollution.

The understanding at a molecular level of ambient secondary organic aerosol (SOA) formation is hampered by poorly constrained formation mechanisms and insufficient analytical methods. Especially in developing countries, SOA related haze is a great concern due to its significant effects on climate and human health. We present simultaneous measurements of gas-phase volatile organic compounds (VOCs), oxygenated organic molecules (OOMs), and particle-phase SOA in Beijing. We show that condensation of the measured OOMs explains 26-39% of the organic aerosol mass growth, with the contribution of OOMs to SOA enhanced during severe haze episodes. Our novel results provide a quantitative molecular connection from anthropogenic emissions to condensable organic oxidation product vapors, their concentration in particle-phase SOA, and ultimately to haze formation.

Semi-volatile and intermediate-volatility organic compounds (S–IVOCs) are considered critical precursors of secondary organic aerosol (SOA), which is an important component of fine particulate matter (PM2.5). However, knowledge of the contributions of S–IVOCs to SOA is still lacking in the Pearl River Delta (PRD) region, southern China. Therefore, in this study, an emission inventory of S–IVOCs in the PRD region was developed for the first time for the year 2010. The S–IVOC emissions were calculated based on a parameterization method involving the emission factors of POA (primary organic aerosol), emission ratios of S–IVOCs to POA, and domestic activity data. The total emissions of S–IVOCs were estimated to be 323.4 Gg, with major emissions from central cities in the PRD, i.e., Guangzhou, Foshan, and Shenzhen. On-road mobile sources and industries were the two major contributors of S–IVOC emissions, with contributions of ∼42 % and ∼35 %, respectively. Furthermore, uncertainties of the emission inventory were evaluated through Monte Carlo simulation. The uncertainties ranged from −79 % to 229 %, which could be mainly attributed to mass fractions of OC (organic carbon) to PM2.5 from on-road mobile emissions and emission ratios of IVOCs ∕ POA. The developed S–IVOC emission inventory was further incorporated into the Weather Research and Forecasting with Chemistry (WRF-Chem) model with a volatility basis-set (VBS) approach to improve the performance of SOA simulation and to evaluate the influence of S–IVOCs on SOA formation at a receptor site (Wan Qing Sha (WQS) site) in the PRD. The following results could be obtained. (1) The model could resolve about 34 % on average of observed SOA concentrations at WQS after considering the emissions of S–IVOCs, and 18 %–77 % with the uncertainties of the S–IVOC emission inventory considered. (2) The simulated SOA over the PRD region was increased by 161 % with the input of S–IVOC emissions, and it could be decreased to 126 % after the reaction coefficient of S–IVOCs with OH radical was improved. (3) Among all anthropogenic sources of S–IVOCs, industrial emission was the most significant contributor of S–IVOCs for SOA formation, followed by on-road mobile, dust, biomass burning, residential, and off-road mobile emissions. Overall, this study firstly quantified emissions of S–IVOCs and evaluated their roles in SOA formation over the PRD, which contributes towards significantly improving SOA simulation and better understanding of SOA formation mechanisms in the PRD and other regions in China.

Investigating the long-term trends of alkyl nitrates (RONO

The dicarbonyls, glyoxal (Gly) and methylglyoxal (Mgly) have been recognized as important precursors of secondary organic aerosols (SOAs) through the atmospheric heterogeneous process. In this study, field measurement was conducted at a receptor site in the Pearl River Delta (PRD) region in south China, and an observation based photochemical box model was subsequently applied to investigate the production and evolution of Gly and Mgly as well as their contributions to SOA formation. The model was coupled with a detailed gas-phase oxidation mechanism of volatile organic compounds (VOCs) (i.e., MCM v3.2), heterogeneous processes of Gly and Mgly (i.e., reversible partitioning in aqueous phase, irreversible volume reactions and irreversible surface uptake processes), and the gas-particle partitioning of oxidation products. The results suggested that without considering the heterogeneous processes of Gly and Mgly on aerosol surfaces would overpredict the mixing ratios of Gly and Mgly by factors of 3.3 and 3.5 compared to the observed levels. The agreement between observation and simulation improved significantly when the irreversible uptake and the reversible partitioning were incorporated into the model, which in total contributed ~ 72 and ~ 73 % to the destruction of Gly and Mgly, respectively. Further analysis on the photochemical budget of Gly and Mgly showed that the oxidation of aromatics by the OH radical was the major pathway producing Gly and Mgly, followed by degradation of alkynes and alkenes. Furthermore, based on the improved model mechanism, the contributions of VOCs oxidation to SOA formed from gas-particle partitioning (SOAgp) and from heterogeneous processes of Gly and Mgly (SOAhet) were also quantified. It was found that o-xylene was the most significant contributor to SOAgp formation (~ 29 %), while m/p-xylene and toluene made dominant contributions to SOAhet formation. Overall the heterogeneous processes of Gly and Mgly can explain ~ 21 % of SOA mass in the PRD region. The results of this study demonstrated the important roles of heterogeneous processes of Gly and Mgly in SOA formation, and highlighted the need for a better understanding of the evolution of intermediate oxidation products.

Historical haze episodes (2013–16) in Guangzhou were examined and classified according to synoptic weather systems. Four types of weather systems were found to be unfavorable, among which “foreside of a cold front” (FC) and “sea high pressure” (SP) were the most frequent (>75% of the total). Targeted case studies were conducted based on an FC-affected event and an SP-affected event with the aim of understanding the characteristics of the contributions of source regions to fine particulate matter (PM2.5) in Guangzhou. Four kinds of contributions—namely, emissions outside Guangdong Province (super-region), emissions from the Pearl River Delta region (PRD region), emissions from Guangzhou–Foshan–Shenzhen (GFS region), and emissions from Guangzhou (local)—were investigated using the Weather Research and Forecasting–Community Multiscale Air Quality model. The results showed that the source region contribution differed with different weather systems. SP was a stagnant weather condition, and the source region contribution ratio showed that the local region was a major contributor (37%), while the PRD region, GFS region and the super-region only contributed 8%, 2.8% and 7%, respectively, to PM2.5 concentrations. By contrast, FC favored regional transport. The super-region became noticeable, contributing 34.8%, while the local region decreased to 12%. A simple method was proposed to quantify the relative impact of meteorology and emissions. Meteorology had a 35% impact, compared with an impact of -18% for emissions, when comparing the FC-affected event with that of the SP. The results from this study can provide guidance to policymakers for the implementation of effective control strategies.

Alkyl nitrates (RONO2) are important reservoirs of atmospheric nitrogen, regulating nitrogen cycling and ozone (O3) formation. In this study, we found that propane and n-butane were significantly lower at the offshore site (WSI) in Hong Kong (p 0.05). Stronger oxidative capacity at WSI led to more efficient RONO2 formation. Relative incremental reactivity (RIR) was for the first time used to evaluate RONO2–precursor relationships. In contrast to a consistently volatile organic compounds (VOC)-limited regime at TC, RONO2 formation at WSI switched from VOC-limited regime during O3 episodes to VOC and nitrogen oxides (NOx) colimited regime during nonepisodes. Furthermore, unlike the predominant contributions of parent hydrocarbons to C4–C5 RONO2, the production of C1–C3 RONO2 was more sensitive to other VOCs like aromatics and carbonyls, which accounted for ∼40–90% of the productions of C1–C3 alkylperoxy (RO2) and alkoxy radicals (RO...

Five C1-C4 alkyl nitrates (RONO2) were measured at a coastal site in Hong Kong in four selected months of 2011 and 2012. The total mixing ratios of C1-C4 RONO2 (Σ5RONO2) ranged from 15.4 to 143.7 pptv with an average of 65.9 ± 33.0 pptv. C3-C4 RONO2 (2-butyl nitrate and 2-propyl nitrate) were the most abundant RONO2 during the entire sampling period. The mixing ratios of C3-C4 RONO2 were higher in winter than those in summer, while the ones of methyl nitrate (MeONO2) were higher in summer than those in winter. Source analysis suggests that C2-C4 RONO2 were mainly derived from photochemical formation along with biomass burning (58.3-71.6%), while ocean was a major contributor to MeONO2 (53.8%) during the whole sampling period. The photochemical evolution of C2-C4 RONO2 was investigated, and found to be dominantly produced by the parent hydrocarbon oxidation. The notable enrichment of MeONO2 over C3-C4 RONO2 was observed in a summer episode when the air masses originating from the South China Sea (SCS) and MeONO2 was dominantly derived from oceanic emissions. In order to improve the accuracy of ozone (O3) prediction in coastal environment, the relative contribution of RONO2 from oceanic emissions versus photochemical formation and their coupling effects on O3 production should be taken into account in future studies.

A systematic field measurement was conducted at an island site (Wanshan Island, WSI) over the South China Sea (SCS) in autumn 2013. It was observed that mixing ratios of O3 and its precursors (such as volatile organic compounds (VOCs), nitrogen oxides (NOx = NO + NO2) and carbon monoxide (CO)) showed significant differences on non-episode days and episode days. Additional knowledge was gained when a photochemical box model incorporating the Master Chemical Mechanism (PBM-MCM) was applied to further investigate the differences/similarities of O3 photochemistry between non-episode and episode days, in terms of O3-precursor relationship, atmospheric photochemical reactivity and O3 production. The simulation results revealed that, from non-O3 episode days to episode days, 1) O3 production changed from both VOC and NOx-limited (transition regime) to VOC-limited; 2) OH radicals increased and photochemical reaction cycling processes accelerated; and 3) both O3 production and destruction rates increased significantly, resulting in an elevated net O3 production over the SCS. The findings indicate the complexity of O3 pollution over the SCS.

Data from an in situ monitoring network and five ozone sondes are analysed during August of 2012, and a high tropospheric ozone episode is observed around the 8th of AUG. The Community Multi-scale Air Quality (CMAQ) model and its process analysis tool were used to study factors and mechanisms for high ozone mixing ratio at different levels of ozone vertical profiles. A sensitive scenario without chemical initial and boundary conditions (ICBCs) from MOZART4-GEOS5 was applied to study the impact of stratosphere-troposphere exchange (STE) on vertical ozone. The simulation results indicated that the first high ozone peak near the tropopause was dominated by STE. Results from process analysis showed that: in the urban area, the second peak at approximately 2 km above ground height was mainly caused by local photochemical production. The third peak (near surface) was mainly caused by the upwind transportation from the suburban/rural areas; in the suburban/rural areas, local photochemical production of ozone dominated the high ozone mixing ratio from the surface to approximately 3 km height. Furthermore, the capability of indicators to distinguish O3-precursor sensitivity along the vertical O3 profiles was investigated. Two sensitive scenarios, which had cut 30% anthropogenic NOX or VOC emissions, showed that O3-precursor indicators, specifically the ratios of O3/NOy, H2O2/HNO3 or H2O2/NOZ, could partly distinguish the O3-precursor sensitivity between VOCs-sensitive and NOx-sensitive along the vertical profiles. In urban area, the O3-precursor relationship transferred from VOCs-sensitive within the boundary layer to NOx-sensitive at approximately 1–3 km above ground height, further confirming the dominant roles of transportation and photochemical production in high O3 peaks at the near-ground layer and 2 km above ground height, respectively.

Over the past 10 years (2005–2014), ground-level O3 in Hong Kong has consistently increased in all seasons except winter, despite the yearly reduction of its precursors, i.e. nitrogen oxides (NOx = NO + NO2), total volatile organic compounds (TVOCs), and carbon monoxide (CO). To explain the contradictory phenomena, an observation-based box model (OBM) coupled with CB05 mechanism was applied in order to understand the influence of both locally produced O3 and regional transport. The simulation of locally produced O3 showed an increasing trend in spring, a decreasing trend in autumn, and no changes in summer and winter. The O3 increase in spring was caused by the net effect of more rapid decrease in NO titration and unchanged TVOC reactivity despite decreased TVOC mixing ratios, while the decreased local O3 formation in autumn was mainly due to the reduction of aromatic VOC mixing ratios and the TVOC reactivity and much slower decrease in NO titration. However, the decreased in situ O3 formation in autumn was overridden by the regional contribution, resulting in elevated O3 observations. Furthermore, the OBM-derived relative incremental reactivity indicated that the O3 formation was VOC-limited in all seasons, and that the long-term O3 formation was more sensitive to VOCs and less to NOx and CO in the past 10 years. In addition, the OBM results found that the contributions of aromatics to O3 formation decreased in all seasons of these years, particularly in autumn, probably due to the effective control of solvent-related sources. In contrast, the contributions of alkenes increased, suggesting a continuing need to reduce traffic emissions. The findings provide updated information on photochemical pollution and its impact in Hong Kong.

Two models (the Positive Matrix Factorization (PMF) model and a photochemical box model with Master Chemical Mechanism (PBM-MCM)) were applied to analyze the formaldehyde (HCHO) data collected in July 2006 at an urban site (GPEMC) in the Pearl River Delta (PRD), southern China. Three major HCHO sources (secondary formation, vehicular exhaust, and solvent usage) were identified and they were found to contribute in average 53%, 31% and 16% respectively to the total HCHO loading at GPEMC. Alkenes was the most important group contributing to the secondary formation of HCHO, followed by aromatics and alkanes. Among them, trans-2-butene had the largest contribution to secondary HCHO formation, with the average percentage of 16 ± 4%, followed by i-butene, cis-2-butene, propene, isoprene and m,p-xylene. Secondary HCHO and HCHO emitted from vehicular emissions contributed comparably to ground-based measured O3 and HOx radical at GPEMC, higher than that from solvent usage (1.3 ± 0.1 ppbv and (4.1 ± 0.3) × 106 molecule/cm3 for O3 and HOx radical). Our results highlight the importance of secondary HCHO formation for both photochemical formation of ozone and the oxidative capacity of the atmosphere in this region. It is hence critical for policy makers to propose strategies for controlling VOCs from vehicular emissions in order to reduce secondary HCHO formation. Our results also have important implication for improving the understanding of the source apportionments of HCHO and their contributions to photochemical pollution in the PRD region in China.

From 2005 to 2013, volatile organic compounds (VOCs) and other trace gases were continuously measured at a suburban site in Hong Kong. The measurement data showed that the concentrations of most air pollutants decreased during these years. However, ozone (O3) and total non-methane hydrocarbon levels increased with the rate of 0.23 ± 0.03 and 0.34 ± 0.02 ppbv/year, respectively, pointing to the increasing severity of photochemical pollution in Hong Kong. The Hong Kong government has ongoing programs to improve air quality in Hong Kong, including a solvent program implemented during 2007-2011, and a diesel commercial vehicle (DCV) program since 2007. From before to after the solvent program, the sum of toluene, ethylbenzene and xylene isomers decreased continuously with an average rate of -99.1 ± 6.9 pptv/year, whereas the sum of ethene and propene increased by 48.2 ± 2.0 pptv/year from before to during the DCV program. Despite this, source apportionment results showed that VOCs emitted from diesel exhaust decreased at a rate of -304.5 ± 17.7 pptv/year, while solvent related VOCs decreased at a rate of -204.7 ± 39.7 pptv/year. The gasoline and liquefied petroleum gas vehicle emissions elevated by 1086 ± 34 pptv/year, and were responsible for the increases of ethene and propene. Overall, the simulated O3 rate of increase was lowered from 0.39 ± 0.03 to 0.16 ± 0.05 ppbv/year by the solvent and DCV programs, because O3 produced by solvent usage and diesel exhaust related VOCs decreased (p

Photochemical smog, characterized by high concentrations of ozone (O3) and fine particles (PM2.5) in the atmosphere, has become one of the top environmental concerns in China. Volatile organic compounds (VOCs), one of the key precursors of O3 and secondary organic aerosol (SOA) (an important component of PM2.5), have a critical influence on atmospheric chemistry and subsequently affect regional and global climate. Thus, VOCs have been extensively studied in many cities and regions in China, especially in the North China Plain, the Yangtze River Delta and the Pearl River Delta regions where photochemical smog pollution has become increasingly worse over recent decades. This paper reviews the main studies conducted in China on the characteristics and sources of VOCs, their relationship with O3 and SOA, and their removal technology. This paper also provides an integrated literature review on the formulation and implementation of effective control strategies of VOCs and photochemical smog, as well as suggestions for future directions of VOCs study 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.

Semi-volatile and intermediate volatility organic compounds (S/IVOCs) are considered as critical precursors of secondary organic aerosol (SOA), which is an important component of fine particulate matter (PM2.5). However, the knowledge on the contributions of S/IVOCs to SOA is still poorly understood in the Pearl River Delta (PRD) region, southern China. Therefore, in this study, an emission inventory of S/IVOCs in the Pearl River Delta (PRD) region was developed for the first time for the year 2010. The S/IVOCs emission was calculated based on a parameterization method involving the emissions factors of POA (primary organic aerosol), emission ratios of S/IVOCs to POA, and domestic activity data. The total emission of S/IVOCs was estimated to be 323.4 Gg, with major emissions from central cities in PRD, i.e., Guangzhou, Foshan, and Shenzhen. On-road mobile sources and industries were the two major contributors of S/IVOC emissions, with contributions of ~ 42 % and ~ 35 %, respectively. Furthermore, uncertainties of the emission inventory were evaluated through Monte Carlo simulation. The uncertainties ranged from −79 % to 229 %, which could be mainly attributed to mass fractions of OC (organic compound) to PM2.5 from on-road mobile emissions and emission ratios of IVOCs/POA. The developed S/IVOC emission inventory was further incorporated into the Weather Research and Forecasting with Chemistry (WRF-Chem) model with a volatility basis-set (VBS) approach to improve the performance of SOA simulation and to evaluate the influence of S/IVOCs on SOA formation at a receptor site (Wan Qing Sha (WQS) site) of PRD. The following results could be obtained: (1) The model could resolve about 34 % on average of observed SOA concentrations at WQS after considering the emissions of S/IVOCs, and 18 %–77 % with the uncertainties of the S/IVOC emission inventory considered. (2) The simulated SOA over the PRD region was increased by 161 % with the input of S/IVOC emissions, and it could be decreased to 126 % after the reaction coefficient of S/IVOCs with OH radical was improved. (3) Among all anthropogenic sources of S/IVOCs, industrial emission was the most significant contributor of S/IVOCs for SOA formation, followed by on-road mobile, dust, biomass burning, residential, and off-road mobile emissions. Overall, this study firstly quantified emissions of S/IVOCs and evaluated their roles in SOA formation over PRD, which contribute towards significantly improving SOA simulation and better understanding of SOA formation mechanisms in PRD and other regions in China.

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. NOx 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 NOx 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 NOx at TC consumed more radicals, whereas the concentration of radicals increased from TC to WSI because of NOx 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.

An emission inventory of precursors is a prerequisite for the simulation of secondary organic aerosol (SOA), which could provide valuable information on the evolution of precursors, formation of SOA, and its influence on fine particle (PM2.5) abundance, oxidative capacity, and climate change. However, an emission inventory of semi-volatile and intermediate volatility organic compounds (S/IVOCs), the key precursor of SOA, particularly the gridded inventory that is appropriate for input into regional air quality models, remains limited in China, leading to an incomplete understanding of S/IVOCs sources and roles in SOA formation and the atmospheric environment. Therefore, a gridded emission inventory of S/IVOCs in China for 2016 was developed based on ample source-specific measured data on emission ratios of S/IVOCs to primary organic aerosols (POA) from literatures. The total emission of S/IVOCs was estimated to be 9.6 Tg, and industry and residential sectors were major sources of S/IVOCs, with contributions of 48.0% and 30.2%, respectively. The spatial variations suggested that S/IVOC emissions were mainly distributed in the highly industrialized and urbanized regions in China, such as Beijing-Tianjin-Hebei (BTH), the Yangtze River Delta (YRD), the Pearl River Delta (PRD), and the Sichuan-Chongqing (SC) regions, though the contributions and temporal patterns varied between different regions. Furthermore, uncertainty of the emission inventory was estimated to be within the range of -66%-153%, which was mainly attributed to emission ratios of IVOCs/POA for industry, transportation, and power plants. The gridded emission inventory developed in this study can be used to estimate the emissions of S/IVOCs in different regions, and can be applied to different models for a better understanding of the environmental effects of S/IVOCs.

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

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 NOx emissions. However, the O3 formation at QA was co-limited by VOCs and NOx 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

Grid field measurements of volatile organic compounds (VOCs) covering the entire territory of Hong Kong were simultaneously carried out twice daily on 27 September 2013 and 24 September 2014, respectively, to advance our understanding on the spatiotemporal variations of VOCs and ozone (O3) formation, the factors controlling O3 formation and the efficacy of a control measure in Hong Kong. From before to after the control measure on liquefied petroleum gas (LPG) fueled vehicles, the VOCs originated from LPG vehicle exhaust deceased from 41.3±1.2μg/m(3) (49.7±1.5%) to 32.8±1.4μg/m(3) (38.8±1.7%) (p0.05). In contrast, the contribution to VOCs made by gasoline and diesel vehicle exhaust and solvent usage increased (p0.05). VOCs and nitric oxide (NO) in LPG source experienced the highest reductions at the roadside sites, while the variations were not significant at the urban and new town sites (p0.05). For O3 production, LPG vehicle exhaust generally made a negative contribution (-0.17±0.06 ppbv) at the roadside sites, however it turned to a slightly positive contribution (0.004±0.038 ppbv) after the control measure. At the urban sites, although the reductions of VOCs and NO were minor (p0.05), O3 produced by LPG vehicle significantly reduced from 4.19±1.92 ppbv to 0.95±0.38 ppbv (p0.05). Meanwhile, O3 produced by LPG at the new town sites remained stable. The analysis of O3-precursor relationships revealed that alkenes and aromatics were the main species limiting roadside O3 formation, while aromatics were the most predominant controlling factor at urban and new town sites. In contrast, isoprene and sometimes NOx limited the O3 formation in rural environment.

C1–C4 alkyl nitrates (RONO2) were measured concurrently at a mountain site, Tai Mo Shan (TMS), and an urban site, Tsuen Wan (TW), at the base of the same mountain in Hong Kong from September to November 2010. Although the levels of parent hydrocarbons were much lower at TMS (p

A one-year field experiment was conducted in 2013 at an urban and suburban site in Guangzhou, Southern China to study the chemical compositions of PM_(2.5) and reconstruct the IMPROVE Algorithm to investigate the impact of aerosol components on visibility. Annual average PM_(2.5) mass concentration was 61.3 ± 27.6 and 54.2 ± 29.7 μg m^(-3) at the urban and suburban site, with organic matter (OM), sulfate (SO_4^(2-)), and nitrate (NO_3^-) among the dominant components, accounting for 40.3%, 16.3%, and 8.0% of the PM_(2.5) mass concentration respectively. Based on the modified IMPROVE Algorithm, the localized Mass Scattering Efficiencies (MSE) for sulfate and nitrate was obtained, with values of 2.16 ± 0.34 m^2 g^(-1) and 2.63 ± 0.66 m^2 g^(-1) at urban site and 2.22 ± 0.31 m^2 g^(-1) and 2.76 ± 0.84 m^2 g^(-1) at suburban site. Hygroscopic growth factors (Gf) for OM and EC were also taken into consideration with average values of 1.28 ± 0.13 m^2 g^(-1) and 1.15 ± 0.11 m^2 g^(-1) at the urban site and 1.18 ± 0.09 m^2 g^(-1) and 1.10 ± 0.09 m^2 g^(-1) at the suburban site. The estimated total light extinction coefficient was 294.7 ± 106.9 and 255.8 ± 119.0 Mm^(-1) at the urban and suburban site, with OM, SO_4^(2-) and NO_3^- contributing 41.7%, 16.5% and 11.5% at the urban site, and 42.1%, 21.3% and 9.1% at the suburban site. Secondary water-soluble ions and OM as a whole was becoming increasingly vital under polluted conditions, with nitrate in particular being more important under heavily polluted conditions in Guangzhou.

Intensive field measurements of formaldehyde (HCHO) and acetaldehyde (CH_3CHO) were concurrently conducted at a mountain site (TMS) and an urban site (TW) at the foot of the same mountain in Hong Kong from September to November 2010. The spatiotemporal variations of HCHO and CH_3CHO, the correlation between HCHO and CH_3CHO and the ratios of HCHO/CH3CHO indicated different impacts of primary emissions and secondary formation at the two sites. The source apportionments of HCHO and CH_3CHO at both sites were investigated using the Positive Matrix Factorization (PMF) model, while the in-situ formation of HCHO and CH_3CHO was estimated using a Photochemical Box Model coupled with Master Chemical Mechanism (PBM-MCM). At TMS, the in-situ formation was the most significant contributor to ambient HCHO and CH_3CHO, accounting for 51 ± 5 and 32 ± 2%, respectively, followed by the transport of photochemically-formed aldehydes, vehicular emissions, biogenic emissions, biomass burning and solvent usage. On the other hand, at TW, the in-situ formation and vehicular emissions explained 55 ± 7% and 18 ± 1% of ambient HCHO, respectively, while vehicular emissions and in-situ formation made comparable contributions to CH_3CHO (~35%). The findings are helpful for the formulation and implementation of appropriate control strategies for aldehydes and their precursors in Hong Kong.

We investigated variations of ozone (O3), its precursors, and their influence on photochemical reactivity by applying a photochemical box model to field data from three cities (Nantong, Xuzhou, and Yancheng) in the Yangtze River Delta. The photochemical box model was coupled with the near-explicit mechanism (Master Chemical Mechanism, MCM version 3.2) for the degradation of volatile organic compounds (VOCs), which provided detailed descriptions of different pathways for photochemical O3 formation and related atmospheric photochemical reactivity based on observed data. Alkanes were the most important VOC group overall, while the contributions of aromatics and alkenes varied by city. The highest levels of O3 and VOCs (indicating more severe photochemical pollution) were found in Nantong. This induced a higher contribution of O3 photolysis to HOx production and promoted OH-HO2 propagation, resulting in maximum HOx (OH + HO2) and higher O3 formation rates from RO2 + NO. In Yangchen, the highest occurrence of alkenes resulted in the maximum contribution of ozonolysis to HOx production. In Nantong, the highest levels of NO2 and HO2 led to the greatest contributions of OH + NO2 and HO2 + HO2 to HOx destruction. However, the longest OH chain length was found in Xuzhou, suggesting that O3 production was most efficient there. Although vehicular emissions were the dominant sources of VOCs overall, industrial emissions in Nantong, solvent usage in Xuzhou and vehicular emissions in Yancheng were the most important VOC sources for O3 formation. These results show that appropriate photochemical pollution control measures should consider both the emission sources and photochemical reactivities of VOCs.

Methacrolein (MACR) and methyl vinyl ketone (MVK) are two major intermediate products from the photochemical oxidation of isoprene, the most important biogenic volatile organic compound. In addition, MACR and MVK have primary emissions. Investigating the sources and evolution of MACR and MVK could provide helpful information for the oxidative capacity of the atmosphere. In this study, hourly measurements of isoprene, MACR, and MVK were conducted at a receptor site in the Pearl River Delta region (PRD), i.e., the Heshan site (HS), from 22 October to 20 November, 2014. The average mixing ratios of isoprene, MACR and MVK were 151 ± 17, 91 ± 6 and 79 ± 6 pptv, respectively. The daily variations and the ratios of MVK/MACR during daytime and nighttime suggested that other sources besides isoprene photooxidation influenced the MACR and MVK abundances at the HS. Positive matrix factorization was utilized to resolve the sources of MACR and MVK. Five sources were identified and quantified, including biogenic emissions, biomass burning, secondary formation, diesel, and gasoline vehicular emissions. Among them, secondary formation made the greatest contribution to observed MACR and MVK with average contributions of ~ 45% and ~ 70%, respectively. Through the yields of secondary products from the oxidation of MACR and MVK by the OH radical and the concentrations of MACR and MVK, it was found that methylglyoxal and formaldehyde were the main oxidation products of MACR and MVK at the HS site. Overall, this study evaluated the roles of primary emissions on ambient levels of MACR and MVK and advanced the understanding of photochemical oxidation of MACR and MVK in the PRD.

Peroxy acetyl nitrate (PAN) is an important photochemical product formed from the reactions between volatile organic compounds (VOCs) and nitrogen oxides (NOx) under sunlight. In this study, a field measurement was conducted at a rural site (the backgarden site, or BGS) of the Pearl River Delta (PRD) region in 2006, with the 10 min maximum PAN mixing ratios of 3.9 ppbv observed. The factors influencing the abundance of PAN at the BGS site was evaluated by the process analysis through the Weather Research and Forecasting-Community Multiscale Air Quality (WRF-CMAQ) model. The results suggested that the increase of PAN abundance at the BGS site was mainly controlled by the gas-phase chemistry, followed by vertical transport, while its loss was modulated mainly by dry deposition and horizontal transport. As the dominant important role of gas-phase chemistry, to provide detailed information on the photochemical formation of PAN, a photochemical box model with near-explicit chemical mechanism (i.e., the master chemical mechanism, MCM) was used to explore the relationship of photochemical PAN formation with its precursors based on the measured data at the BGS site. It was found that PAN formation was VOC-limited at the BGS site, with the oxidation of acetaldehyde the most important pathway for photochemical PAN production, followed by the oxidation and photolysis of methylglyoxal (MGLY). Among all the primary VOC precursors, isoprene and xylenes were the main contributors to PAN formation. Overall, our study provides new insights into the PAN photochemical formation and its controlling factors, and highlighted the importance of gas chemistry on the PAN abundance in the PRD region.

Alkyl nitrates (RONO

The photochemical formation of alkyl nitrates (RONO2) and their impact on ozone (O3) formation were investigated using a Photochemical Box Model incorporating the Master Chemical Mechanism (PBM-MCM). The model was constrained with field measurement data collected on selected O3 episode days at Tai O, a rural-coastal site in southwestern Hong Kong, from August 2001–December 2002. The in-situ observations showed that the sum of C1–C5 RONO2 varied from 30.7 ± 14.8 pptv in spring to 120.7 ± 10.4 pptv in autumn, of which 2-butyl nitrate dominated with the highest average concentration of 30.8 ± 2.6 pptv. Model simulations indicated that the pathway of CH3O reacting with NO2, proposed in our previous study, made minor contributions (11.3 ± 0.7%) to methyl nitrate formation. Indeed, 51.8 ± 3.1% and 36.5 ± 6.3% of the methyl nitrate was attributed to the reaction of CH3O2+NO and to oceanic emissions/biomass burning, respectively. For the C2–C5 alkyl nitrates, the contribution of photochemical formation increased with increasing carbon number, ranging from 64.4 ± 4.0% for ethyl nitrate (EtONO2) to 72.6 ± 4.2% for 2-pentyl nitrate (2-PenONO2), while the contribution of oceanic emissions/biomass burning decreased from 35.1 ± 6.5% for EtONO2 to 26.8 ± 6.8% for 2-PenONO2. Model simulations of photochemical O3 levels influenced by RONO2 chemistry showed that the formation of methyl-, ethyl-, i-propyl-, n-propyl-, 2-butyl-, 2-pentyl-, and 3-pentyl-nitrates led to O3 reduction of 0.05 ± 0.03, 0.05 ± 0.03, 0.06 ± 0.02, 0.02 ± 0.02, 0.18 ± 0.04, 0.09 ± 0.02 and 0.06 ± 0.02 ppbv, respectively, with an average reduction rate of 11.0 ± 3.2 ppbv O3 per 1 ppbv RONO2 formation. The C1–C5 RONO2 constituted 18.6 ± 1.9% of the entire RONO2, and had a nitrogen reserve of 4.1 ± 0.2%, implying their potential influence on O3 production in downwind areas.

Recent studies have shown that fine particulate matter (PM2.5) is associated with multiple adverse health outcomes and PM2.5-induced oxidative stress is now commonly known as a proposed mechanism of PM2.5-mediated toxicity. However, the association between allergic symptoms in children and exposure to PM2.5 has not been fully elucidated, particularly the role of PM2.5 on the indoor environment involved in allergy or non-allergy is unknown. The aim of the present study was to explore whether indoor PM2.5 from the homes of children with allergic symptoms had more increased risks of allergy than that of healthy ones and then compare the toxicity and inflammatory response of them. In this study, indoor PM2.5 was collected from the homes of schoolchildren with allergic symptoms and those of healthy ones respectively, and components of PM2.5 were analyzed. PM2.5-mediated oxidative damage and inflammatory response were further evaluated in mouse peritoneal macrophages based on its effects on the levels of reactive oxygen species accumulation, lipid peroxidation, DNA damage or cytokine production. It seems that oxidative stress may contribute to PM2.5-induced toxicity, and PM2.5 from the allergic indoor environment produced more serious toxic effects and an inflammatory response on mouse peritoneal macrophages than that from a non-allergic indoor environment.

Field measurements were simultaneously conducted at a mountain (Mt.) site (Tai Mao Shan, TMS) and an urban site (Tsuen Wan, TW) at the foot of the Mt. TMS in Hong Kong. An interesting event with consecutive high-ozone (O₃) days from 08:00 on 28 Oct. to 23:00 on 03 Nov., 2010 was observed at Mt. TMS, while no such polluted event was found at the foot of the mountain. The Weather Research and Forecasting (WRF)-Community Multiscale Air Quality (CMAQ) models were used to understand this event. Model performance evaluation showed that the simulated meteorological parameters and air pollutants were well in agreement with the observations. The index of agreement (IOA) of temperature, relative humidity, wind direction and wind speed were 0.93, 0.83, 0.46 and 0.60, respectively. The multi-day high O₃ episode at Mt. TMS was also reasonably reproduced (IOA=0.68). Horizontally, the photochemical processes determined the O₃ levels in southwestern Pearl River Delta (PRD) and the Pearl River Estuary (PRE), while in eastern and northern PRD, the O₃ destruction was over the production during the event. Vertically, higher O₃ values at higher levels were found at both Mt. TMS and TW, indicating a vertical O₃ gradient over Hong Kong. With the aid of the process analysis module, we found positive contribution of vertical transport including advection and diffusion to O₃ mixing ratios at the two sites, suggesting that O₃ values at lower locations could be affected by O₃ at higher locations via vertical advection and diffusion over Hong Kong.