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

Characterization and source apportionment of volatile organic compounds in Hong Kong: A 5-year study for three different archetypical sites

Initial success has been achieved in Hong Kong in controlling primary air pollutants, but ambient ozone levels kept increasing during the past three decades. Volatile organic compounds (VOCs) are important for mitigating ozone pollution as its major precursors. This study analyzed VOC characteristics of roadside, suburban, and rural sites in Hong Kong to investigate their compositions, concentrations, and source contributions. Here we show that the TVOC concentrations were 23.05 ± 13.24, 12.68 ± 15.36, and 5.16 ± 5.48 ppbv for roadside, suburban, and rural sites between May 2015 to June 2019, respectively. By using Positive Matrix Factorization (PMF) model, six sources were identified at the roadside site over five years: Liquefied petroleum gas (LPG) usage (33%-46%), gasoline evaporation (8%-31%), aged air mass (11%-28%), gasoline exhaust (5%-16%), diesel exhaust (2%-16%) and fuel filling (75-9%). Similarly, six sources were distinguished at the suburban site, including LPG usage (30%-33%), solvent usage (20%-26%), diesel exhaust (14%-26%), gasoline evaporation (8%-16%), aged air mass (4%-11%), and biogenic emissions (2%-5%). At the rural site, four sources were identified, including aged air mass (33%-51%), solvent usage (25%-30%), vehicular emissions (11%-28%), and biogenic emissions (6%-12%). The analysis further revealed that fuel filling and LPG usage were the primary contributors to OFP and OH reactivity at the roadside site, while solvent usage and biogenic emissions accounted for almost half of OFP and OH reactivity at the suburban and rural sites, respectively. These findings highlight the importance of identifying and characterizing VOC sources at different sites to help policymakers develop targeted measures for pollution mitigation in specific areas.

Understanding the impact of elevated CO2 and O3 on growth and yield in Indian wheat cultivars: Implications for food security in a changing climate

The pressing issue of increasing tropospheric ozone (O3) levels necessitates the development of effective stress management strategies for plant protection. While considerable research has elucidated the adverse impacts of O3, understanding the combined effects of O3 and CO2 requires further investigation. This study focuses on assessing the response of stomatal O3 flux under various O3 and CO2 treatments, individually and in combination, and their repercussions on physiological, growth, and yield attributes in two Indian wheat cultivars, HUW-55 and PBW-550, which exhibit varying levels of sensitivities against elevated O3. Results indicated significant alterations in stomatal O3 flux in both O3-sensitive and tolerant wheat cultivars across different treatments, influencing the overall yield outcomes. Particularly, the ECO2+EO3 treatment demonstrated more positive yield protection in the O3-sensitive cultivar PBW-550, compared to HUW-55 indicating enhanced allocation of photosynthates towards reproductive development in PBW-550, compared to the tolerant cultivar HUW-55, as evidenced by higher harvest index (HI). Furthermore, the study revealed a stronger correlation between yield response and stomatal O3 flux in PBW-550 (R2 = 0.88) compared to HUW-55 (R2 = 0.79), as indicated by a steeper regression slope for PBW-550. The research also confirmed the role of elevated CO2 in reducing stomatal O3- flux in the tested cultivars, with discernible effects on their respective yield responses. Further experimentation is necessary to confirm these results across different cultivars exhibiting varying sensitivities to O3. These findings can potentially revolutionize agricultural productivity in regions affected by O3 stress. The criteria for recommending cultivars for agricultural practices should not be based only on their sensitivity/tolerance to O3. Still, they should also consider the effect of CO2 fertilization in the growing area. This experiment offers hope to sustain global food security, as the O3-sensitive wheat cultivar also showed promising results at elevated CO2. In essence, this research could pave the way for more resilient agricultural systems in the era of changing climate under elevated O3 and CO2 conditions.

Unexpected changes in source apportioned results derived from different ambient VOC metrics

Although most source apportionments of VOCs use mixing ratios, about 23 % of published studies use mass concentrations. Thus, systematically exploring the changes in VOC source apportioned results caused by metric differences is important to assess the differences in key precursor apportionment results given the observed increases in O3 pollution situation. Different monitoring instruments measured hourly VOC volumetric concentrations in three typical Chinese cities (i.e., Qingdao, Shijiazhuang, and Zhumadian). Converting volumetric to mass concentrations under standard and/or actual temperature-pressure conditions, VOC values with different metrics were obtained. The impacts of different metrics on the source apportionments were then investigated. Compared to the positive matrix factorization of the volumetric data (VC-PMF), the VOC species concentrations with low relative molecular mass (RMM) in the factor profiles substantially decreased in mass data analyses (MC-PMF). However, those species with high RMM substantially increased. There were no substantial differences in the apportioned source contributions based on standard and actual condition mass concentrations. However, the high-low rankings of percent contributions apportioned using the volumetric and mass data produced substantial differences. Compared with the VC-PMF results, the percent contributions of sources dominated by species with low RMM (e.g., natural gas usage and mixed sources containing natural gas usage) apportioned by MC-PMF decreased, while those of sources that emitted high RMM species (e.g., solvent usage and mixed sources containing solvent usage) increased. Source apportionments based on the volumetric concentration data had more practical significance compared to the mass concentration data results for control strategy development since the mass data analyses created issues.

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.

Spatiotemporal variations, sources, and atmospheric transformation potential of volatile organic compounds in an industrial zone based on high-resolution measurements in three plants

Industrial emissions are significant sources of volatile organic compounds (VOCs). This study conducted a field campaign at high temporal and spatial resolution to monitor VOCs within three plants in an industrial park in southern China. VOC concentrations showed significant spatial variability in this industrial zone, with median concentrations of 75.22, 40.53, and 29.41 μg/m3 for the total VOCs in the three plants, respectively, with oxygenated VOCs (OVOCs) or aromatics being the major VOCs. Spatial variability within each plant was also significant but VOC-dependent. Seasonal variations in the VOC levels were governed by their industrial emissions, meteorological conditions, and photochemical losses, and they were different for the four groups of VOCs. The temporal and spatial variations in the VOC compositions suggest similar sources of each class of VOCs during different periods of the year in each plant. The diurnal patterns of VOCs (unimodal or bimodal) clearly differed from those at most industrial/urban locations previously, reflecting a dependence on industrial activities. The secondary transformation potential of VOCs also varied temporally and spatially, and aromatics generally made the predominant contributions in this industrial park. The loss rate of OH radicals and ozone formation potential were highly correlated, but the linear relationship substantially changed in summer and autumn due to the intensive emissions of an OVOC species. The lifetime cancer and non-cancer risks via occupational inhalation of the VOCs in the plants were acceptable but merit attention. Taking the secondary transformation potential and health risks into consideration, styrene, xylene, toluene, trichloroethylene, and benzene were proposed to be the priority VOCs regulated in the plants.

Characteristics and source apportionment of ambient volatile organic compounds and ozone generation sensitivity in urban Jiaozuo, China

In recent years, many cities have taken measures to reduce volatile organic compounds (VOCs), an important precursor of ozone (O3), to alleviate O3 pollution in China. 116 VOC species were measured by online and offline methods in the urban area of Jiaozuo from May to October in 2021 to analyze the compositional characteristics. VOC sources were analyzed by a positive matrix factorization (PMF) model, and the sensitivity of ozone generation was determined by ozone isopleth plotting research (OZIPR) simulation. The results showed that the average volume concentration of total VOCs was 30.54 ppbv and showed a bimodal feature due to the rush-hour traffic in the morning and at nightfall. The most dominant VOC groups were oxygenated VOCs (OVOCs, 29.3%) and alkanes (26.7%), and the most abundant VOC species were acetone and acetylene. However, based on the maximum incremental reactivity (MIR) method, the major VOC groups in terms of ozone formation potential (OFP) contribution were OVOCs (68.09 µg/m3, 31.5%), aromatics (62.90 µg/m3, 29.1%) and alkene/alkynes (54.90 µg/m3, 25.4%). This indicates that the control of OVOCs, aromatics and alkene/alkynes should take priority. Five sources of VOCs were quantified by PMF, including fixed sources of fossil fuel combustion (27.8%), industrial processes (25.9%), vehicle exhaust (19.7%), natural and secondary formation (13.9%) and solvent usage (12.7%). The empirical kinetic modeling approach (EKMA) curve obtained by OZIPR on O3 exceedance days indicated that the O3 sensitivity varied in different months. The results provide theoretical support for O3 pollution prevention and control in Jiaozuo.

Diurnal variation characteristics and meteorological causes of autumn ozone in the Pearl River Delta, China

This study investigates the diurnal variation of ozone (O3) in the Pearl River Delta (PRD) during autumn from 2016 to 2021, focusing on the main O3 modes and their relationship with meteorological conditions. Utilizing K-means clustering, four patterns of O3 variation were identified: Cluster 1 (extremely low O3), Cluster 2 (close to autumn average), Cluster 3 (abnormally high O3 at night), and Cluster 4 (extremely high O3). In Cluster 1, the PRD was situated on the northwest side of the western Pacific subtropical high (WPSH), resulting in increased cloud cover, weakened radiation, and the lowest O3 growth rate during the day, with weak nighttime changes. Cluster 2 presents O3 changes under normal autumn conditions, closely resembling the autumn average. In Cluster 3, the PRD was located between continental high pressure and the low-pressure system over the South China Sea. The enhanced horizontal pressure gradient led to an increase in the horizontal wind speed, promoting the formation of a low-level jet (LLJ). The LLJ caused decoupling between the residual layer and stable boundary layer at night, leading to increased surface O3 concentration and a higher background O3 concentration before sunrise the next day. In Cluster 4, favorable meteorological conditions for O3 generation and accumulation were created by the influence of the WPSH and peripheral tropical cyclones. O3 rapidly increased during the day, reaching extremely high values in the afternoon, with an exceedance rate of 80 %. Comparing the four diurnal patterns and their meteorological conditions highlights the significance of meteorological processes in O3 variations.

Passive air sampling of VOCs, O3, NO2, and SO2 in the large industrial city of Ulsan, South Korea: spatial-temporal variations, source identification, and ozone formation potential

Concerns about volatile organic compounds (VOCs) have increased due to their toxicity and secondary reaction with nitrogen oxides (NOX) to form ozone (O3). In this study, passive air sampling of VOCs, O3, NO2, and SO2 was conducted in summer, fall, winter, and spring from 2019 to 2020 at six industrial and ten urban sites in Ulsan, the largest industrial city in South Korea. Over the entire sampling period, the concentration of toluene (mean: 8.75 μg/m3) was the highest of the 50 target VOCs, followed by m,p-xylenes (4.52 μg/m3), ethylbenzene (4.48 μg/m3), 3-methylpentane (4.40 μg/m3), and n-octane (4.26 μg/m3). Total (Σ50) VOC levels did not statistically differ between seasons, indicating that large amounts of VOCs are emitted into the atmosphere throughout the year. On the other hand, O3, NO2, and SO2 exhibited strong seasonal variation depending on the meteorological conditions and emission sources. The spatial distribution of Σ50 VOCs, NO2, and SO2 indicated that industrial complexes were major sources in Ulsan, while O3 had the opposite spatial distribution. Using a positive matrix factorization model, five major sources were identified, with industrial effects dominant. Aromatic compounds, such as m,p,o-xylenes, toluene, and 1,2,4-trimethylbenzene, significantly contributed to O3 formation. The VOC/NO2 ratio and O3 concentrations suggested that reducing VOC emissions is more effective than reducing NO2 emissions in terms of preventing the secondary formation of O3. The findings of this study allow for a better understanding of the relationship between VOCs, O3, NO2, and SO2 in industrial cities.

Summertime tropospheric ozone source apportionment study in the Madrid region (Spain)

The design of emission abatement measures to effectively reduce high ground-level ozone (O3) concentrations in urban areas is very complex. In addition to the strongly non-linear chemistry of this secondary pollutant, precursors can be released by a variety of sources in different regions, and locally produced O3 is mixed with that transported from the regional or continental scales. All of these processes depend also on the specific meteorological conditions and topography of the study area. Consequently, high-resolution comprehensive modeling tools are needed to understand the drivers of photochemical pollution and to assess the potential of local strategies to reduce adverse impacts from high tropospheric O3 levels. In this study, we apply the Integrated Source Apportionment Method (ISAM) implemented in the Community Multiscale Air Quality (CMAQ v5.3.2) model to investigate the origin of summertime O3 in the Madrid region (Spain). Consistent with previous studies, our results confirm that O3 levels are dominated by non-local contributions, representing around 70 % of mean values across the region. Nonetheless, precursors emitted by local sources, mainly road traffic, play a more important role during O3 peaks, with contributions as high as 25 ppb. The potential impact of local measures is higher under unfavorable meteorological conditions associated with regional accumulation patterns. These findings suggest that this modeling system may be used in the future to simulate the potential outcomes of specific emission abatement measures to prevent high-O3 episodes in the Madrid metropolitan area.

A comprehensive investigation on source apportionment and multi-directional regional transport of volatile organic compounds and ozone in urban Zhengzhou

To understand the characteristics, source apportionment, and regional transport of volatile organic compounds (VOCs) and ozone (O₃) in a typical city with severe air pollution in central China, we observed and analyzed 115 VOC species at an urban site in Zhengzhou from 29 July to 26 September 2021. During this period, observation- and emission-based approaches revealed that Zhengzhou was in a VOC-limited regime. The average concentration of total VOCs (TVOCs) was 162.25 ± 71.42 μg/m³, dominated by oxygenated VOCs (OVOCs, 34.49%), alkanes (24.29%), and aromatics (19.49%). Six VOC sources were identified using positive matrix factorization (PMF) model, including paint solvent usage (25.32%), secondary production (24.11%), industrial production (19.22%), vehicle exhaust (16.18%), biogenic emission (8.87%), and combustion (6.30%). To assess the regional contribution and source apportionment of VOCs and O₃, Comprehensive Air Quality Model with Extensions (CAMx) with the Ozone Source Apportionment Technology (OSAT) was used for simulation. Results showed that the VOCs were significantly affected by local emissions (about 70%), while O₃ was mainly attributed to regional and super-regional transport. Regarding multi-directional regional transport of VOCs and O₃, dominant contributions were from the northeast and east-northeast directions, and O₃ contributions were also predominantly from the east and east-southeast directions. In terms of source apportionment, the transportation and industrial sectors (including solvent usage) were the major contributors to O₃ and VOCs. To alleviate VOCs and O₃ pollution, transportation and industrial emission reduction should be strengthened, and regional coordination, especially from the northeast to east-southeast directions, should be emphasized in addition to local management.

Chemical Characteristics and Source-Specific Health Risks of the Volatile Organic Compounds in Urban Nanjing, China

This work comprehensively investigated the constituents, sources, and associated health risks of ambient volatile organic compounds (VOCs) sampled during the autumn of 2020 in urban Nanjing, a megacity in the densely populated Yangtze River Delta region in China. The total VOC (TVOC, sum of 108 species) concentration was determined to be 29.04 ± 14.89 ppb, and it was consisted of alkanes (36.9%), oxygenated VOCs (19.9%), halogens (19.1%), aromatics (9.9%), alkenes (8.9%), alkynes (4.9%), and others (0.4%). The mean TVOC/NO_x (ppbC/ppbv) ratio was only 3.32, indicating the ozone control is overall VOC-limited. In terms of the ozone formation potential (OFP), however, the largest contributor became aromatics (41.9%), followed by alkenes (27.6%), and alkanes (16.9%); aromatics were also the dominant species in secondary organic aerosol (SOA) formation, indicative of the critical importance of aromatics reduction to the coordinated control of ozone and fine particulate matter (PM_2.5). Mass ratios of ethylbenzene/xylene (E/X), isopentane/n--pentane (I/N), and toluene/benzene (T/B) ratios all pointed to the significant influence of traffic on VOCs. Positive matrix factorization (PMF) revealed five sources showing that traffic was the largest contributor (29.2%), particularly in the morning. A biogenic source, however, became the most important source in the afternoon (31.3%). The calculated noncarcinogenic risk (NCR) and lifetime carcinogenic risk (LCR) of the VOCs were low, but four species, acrolein, benzene, 1,2-dichloroethane, and 1,2-dibromoethane, were found to possess risks exceeding the thresholds. Furthermore, we conducted a multilinear regression to apportion the health risks to the PMF-resolved sources. Results show that the biogenic source instead of traffic became the most prominent contributor to the TVOC NCR and its contribution in the afternoon even outpaced the sum of all other sources. In summary, our analysis reveals the priority of controls of aromatics and traffic/industrial emissions to the efficient coreduction of O₃ and PM_2.5; our analysis also underscores that biogenic emissions should be paid special attention if considering the direct health risks of VOCs.

Effects of coal chemical industry on atmospheric volatile organic compounds emission and ozone formation in a northwestern Chinese city

Coal is well known as the primary energy consumption in China, and the coal chemical industry (CCI) can serve as an important source of volatile organic compounds (VOCs) emissions. However, the characteristics of VOCs emitted from CCI along with their environmental consequences are still poorly understood. To pin down this, an intensive field campaign was carried out at a typical CCI city in northwestern China (Yulin) from February 26 to March 7, 2021. Results showed that VOC compositions in Yulin were distinct from those in the megacities of China as well as in the typical oilfields over the world. The concentration of naphthalene (1.6 ± 1.1 ppbv), an important byproduct of CCI, was significantly higher than that in other cities (<0.2 ppbv). Positive matrix factorization (PMF) model analysis revealed that the direct contribution of the CCI source for VOC emissions is 8.8 ± 1.8%. More importantly, these VOCs emitted from the CCI can account for 17.9 ± 6.8% of ozone (O₃) formation potential and 16.9 ± 7.4% of OH reactivity of VOCs, suggesting the significant impacts of the CCI on the air quality and atmospheric oxidizing capacity. During the observation, a rapid increase in O₃ concentration after a snowfall was encountered. The changing rate of O₃ concentration in the daytime was significantly higher than in its peripheral cities. The increased O₃ formation was partially attributed to the CCI, and this enhancement can be further magnified by snow cover due to the increment of surface albedo. These findings deepen the understanding of the characteristics and air quality impact of VOCs related to the CCI and provide valuable insights for the development of air quality control measures in the region influenced by intensive coal chemical production.