Volatile Organic Compounds Mixing Ratios Research Articles

Characterization and Sources of VOCs during PM2.5 Pollution Periods in a Typical City of the Yangtze River Delta

To investigate the characteristics and sources of volatile organic compounds (VOCs) as well as their impacts on secondary organic aerosols (SOAs) formation during high-incidence periods of PM2.5 pollution, a field measurement was conducted in December 2019 in Hefei, a typical city of the Yangtze River Delta (YRD). During the whole process, the mixing ratios of VOCs were averaged as 21.1 ± 15.9 ppb, with alkanes, alkenes, alkyne, and aromatics accounting for 59.9%, 15.3%, 15.0%, and 9.8% of the total VOCs, respectively. It is worth noting that the contributions of alkenes and alkyne increased significantly during PM2.5 pollution periods. Based on source apportionment via the positive matrix factorization (PMF) model, vehicle emissions, liquefied petroleum gas/natural gas (LPG/NG), and biomass/coal burning were the main sources of VOCs during the research in Hefei. During pollution periods, however, the contribution of biomass/coal burning to VOCs increased significantly, reaching as much as 47.6%. The calculated SOA formation potential (SOAFP) of VOCs was 0.38 ± 1.04 µg m−3 (range: 0.04–7.30 µg m−3), and aromatics were the dominant contributors, with a percentage of 96.8%. The source contributions showed that industrial emissions (49.1%) and vehicle emissions (28.3%) contributed the most to SOAFP during non-pollution periods, whereas the contribution of biomass/coal burning to SOA formation increased significantly (32.8%) during PM2.5 pollution periods. These findings suggest that reducing VOCs emissions from biomass/coal burning, vehicle, and industrial sources is a crucial approach for the effective control of SOA formation in Hefei, which provides a scientific basis for controlling PM2.5 pollution and improving air quality in the YRD region.

Indoor Emission, Oxidation, and New Particle Formation of Personal Care Product Related Volatile Organic Compounds.

Personal care products (PCPs) contain diverse volatile organic compounds (VOCs) and routine use of PCPs indoors has important implications for indoor air quality and human chemical exposures. This chamber study deployed aerosol instrumentation and two online mass spectrometers to quantify VOC emissions from the indoor use of five fragranced PCPs and examined the formation of gas-phase oxidation products and particles upon ozone-initiated oxidation of reactive VOCs. The tested PCPs include a perfume, a roll-on deodorant, a body spray, a hair spray, and a hand lotion. Indoor use of these PCPs emitted over 200 VOCs and resulted in indoor VOC mixing ratios of several parts per million. The VOC emission factors for the PCPs varied from 2 to 964 mg g-1. We identified strong emissions of terpenes and their derivatives, which are likely used as fragrant additives in the PCPs. When using the PCPs in the presence of indoor ozone, these reactive VOCs underwent oxidation reactions to form a variety of gas-phase oxidized vapors and led to rapid new particle formation (NPF) events with particle growth rates up to ten times higher than outdoor atmospheric NPF events. The resulting ultrafine particle concentrations reach ∼34000 to ∼200000 cm-3 during the NPF events.

Effect of Relative Humidity on the Rate of New Particle Formation for Different VOCs

Atmospheric new particle formation (NPF) is an important source of aerosol particles and cloud condensation nuclei, which affect both climate and human health. In pristine environments, oxidation of biogenic volatile organic compounds (VOCs) is a major contributor to NPF. However, the impact of relative humidity (RH) on NPF from these precursors remains poorly understood. Herein, we report on NPF, as inferred from measurements of total particle number density with a particle diameter (dp) > 7 nm, from three VOCs (sabinene, α-terpineol, and myrtenol) subjected to dark ozonolysis. From a series of comparative experiments under humid (60% RH) and dry (~0% RH) conditions and a variety of VOC mixing ratios (ξVOC, parts per billion by volume, ppbv), we show varied behavior in NPF at elevated RH depending on the VOC and ξVOC. In general, RH-dependent enhancement of NPF at an ξVOC between <1 ppbv and 20 ppbv was observed for select VOCs. Our results suggest that gaseous water at particle genesis enhances NPF by promoting the formation of low-volatility organic compound gas-phase products (LVOCs). This is supported by measurements of the rate of NPF for α-pinene-derived SOA, where RH had a greater influence on the initial rate of NPF than did ξVOC and ξO3.

Contribution of Ship Emission to Volatile Organic Compounds Based on One‐Year Monitoring at a Coastal Site in the Pearl River Delta Region

AbstractShip emission impacts ambient air quality, especially in coastal regions, by emitting air pollutants such as fine particles, nitrogen oxides (NOx), and sulfur dioxide (SO2), yet its contributions to volatile organic compounds (VOCs) and the formation of secondary organic aerosol (SOA) and ozone (O3) are much less constrained with the challenge in distinguish ship emission from land diesel emission. In this study, we conducted a 1‐year online measurement of VOCs with a 1‐hr resolution at a coastal site in south China's Pearl River Delta region, which holds three of the world's top 10 container ports. The results revealed that C10–C12 n‐alkanes, as typical diesel‐related emission tracers, were significantly enhanced and strongly related to oceanic air masses. Receptor modeling revealed two diesel‐related sources of land diesel emission and ship emission, which could be differentiated based on their source profiles, seasonal trends and air mass back trajectories. Ship emissions contributed 6.4%, 5.0%, and 13.6% of total VOC mixing ratios, ozone formation potentials (OFPs), and secondary organic aerosol formation potentials (SOAFPs), while these percentages were 3.4%, 14.7%, and 15.9% for land diesel emission, respectively. In particular, in July, ship emissions could contribute 21.7%, 14.6%, and 31.2% of VOCs, OFPs, and SOAFPs, respectively. Our results highlight the important contribution of diesel‐related emission VOCs in forming O3 and SOA in coastal regions, and ship emission is a non‐negligible source of VOCs, particularly after the strict control of land emission sources.

Analysis of Influencing Factors of Ozone Pollution Difference Between Chengdu and Chongqing in August 2022

In August 2022, Chengdu and Chongqing showed significant differences in ozone (O3) pollution. Chengdu had O3 pollution days for 20 days, whereas Chongqing had no O3 pollution days. In this study, we analyzed the influencing factors of this difference from the emission level of precursors and meteorological conditions. The results showed that:① the total mixing ratio of 52 VOCs (volatile organic compounds) (including 26 alkanes, 16 aromatics, and 10 alkenes) in Chengdu (18.8×10-9) was 2.8 times that of Chongqing (6.6×10-9), and the total O3 formation potential (OFP) (51.2×10-9) was 2.0 times that of Chongqing (25.0×10-9). The·OH radical loss rate (L·OH) (3.9 s-1) was 1.7 times that of Chongqing (2.3 s-1). The top three OFP in Chengdu were ethylene, m/p-xylene, and isoprene, and those in Chongqing were isoprene, ethylene, and propylene. The contribution rate of alkenes to O3 in Chongqing was 60.7%, whereas the OFP of alkenes and aromatics in Chengdu were 1.6 times and 2.9 times that in Chongqing. In conclusion, the total mixing ratio of VOCs, atmospheric photochemical activity, and O3 formation potential of Chengdu were higher than those of Chongqing. ② Isoprene was ranked first place in L·OH in both Chengdu and Chongqing, indicating that the contribution of biogenic sources to O3 pollution in August was significant. However, the biogenic source emission activity was in response to temperature. From August 14 to 24, the high temperature in Chongqing (38.3℃) decreased biogenic source emission activity, whereas the temperature in Chengdu (34.9℃) increased the biogenic sources emission activity. ③ The horizontal and vertical atmospheric diffusion conditions of Chongqing were better than those of Chengdu, and Chengdu was affected by regional pollution transmission.

Origin and transformation of volatile organic compounds at a regional background site in Hong Kong: Varied photochemical processes from different source regions

Volatile organic compounds (VOCs) are important gaseous constituents in the troposphere, impacting local and regional air quality, human health, and climate. Oxidation of VOCs, with the participation of nitrogen oxides (NOx), leads to the formation of tropospheric ozone (O3). Accurately apportioning the emission sources and transformation processes of ambient VOCs, and effectively estimation of OH reactivity and ozone formation potential (OFP) will play an important role in reducing O3 pollution in the atmosphere and improving public health. In this study, field measurements were conducted at a regional background site (Hok Tsui; HT) in Hong Kong from October to November 2020 with proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS). VOC data coupled with air mass back trajectory cluster analysis and receptor modelling were applied to reveal the pollution pattern, emission sources and transformation of ambient VOCs at HT in autumn 2020. Seven sources were identified by positive matrix factorization (PMF) analysis, namely vehicular + industrial, solvent usage, primary oxygenated VOCs (OVOCs), secondary OVOCs 1, secondary OVOCs 2 (aged), biogenic emissions, and background + biomass burning. Secondary formation and vehicular + industrial emissions are the vital sources of ambient VOCs at HT supersite, contributing to 20.8 % and 46.7 % of total VOC mixing ratios, respectively. Integrated with backward trajectory analysis and correlations of VOCs with their oxidation products, short-range transport of air masses from inland regions of southeast China brought high levels of total VOCs but longer-range transport of air masses brought more secondary OVOCs in aged air masses. Photolysis of OVOCs was the most important contributor to OH reactivity and OFP, among which aldehyde was the dominant contributor. The results of this study highlight the photochemical processing of VOCs from different source regions which should be considered in strategy making for pollution reduction.

Identification of volatile organic compounds and their sources driving ozone and secondary organic aerosol formation in NE Spain

Volatile organic compounds (VOCs) play a crucial role in the formation of ozone (O3) and secondary organic aerosol (SOA). We conducted measurements of VOC ambient mixing ratios during both summer and winter at two stations: a Barcelona urban background station (BCN) and the Montseny rural background station (MSY). Subsequently, we employed positive matrix factorization (PMF) to analyze the VOC mixing ratios and identify their sources. Our analysis revealed five common sources: anthropogenic I (traffic & industries); anthropogenic II (traffic & biomass burning); isoprene oxidation; monoterpenes; long-lifetime VOCs. To assess the impact of these VOCs on the formation of secondary pollutants, we calculated the ozone formation potential (OFP) and secondary organic aerosol formation potential (SOAP) associated with each VOC. In conclusion, our study provides insights into the sources of VOCs and their contributions to the formation of ozone and SOA in NE Spain. The OFP was primarily influenced by anthropogenic aromatic compounds from the traffic & industries source at BCN (38–49 %) and during winter at MSY (34 %). In contrast, the summer OFP at MSY was primarily driven by biogenic contributions from monoterpenes and isoprene oxidation products (45 %). Acetaldehyde (10–35 %) and methanol (13–14 %) also made significant OFP contributions at both stations. Anthropogenic aromatic compounds originating from traffic, industries, and biomass burning played a dominant role (88–93 %) in SOA formation at both stations during both seasons. The only exception was during the summer at MSY, where monoterpenes became the primary driver of SOA formation (41 %). These findings emphasize the importance of considering both anthropogenic and biogenic VOCs in air quality management strategies.

Simulations of winter ozone in the Upper Green River basin, Wyoming, using WRF-Chem

Abstract. In the Upper Green River basin (UGRB) of Wyoming and the Uintah Basin of Utah, strong wintertime ozone (O3) formation episodes leading to O3 mixing ratios occasionally exceeding 70 parts per billion (ppb) have been observed over the last 2 decades. Wintertime O3 events in the UGRB were first observed in 2005 and since then have continued to be observed intermittently when meteorological conditions are favorable, despite significant efforts to reduce emissions from oil and natural gas extraction and production. While O3 formation has been successfully simulated using observed volatile organic compound (VOC) and nitrogen oxide (NOx) mixing ratios, successful simulation of these wintertime episodes using emission inventories in a 3-D photochemical model has remained elusive. An accurate 3-D photochemical model driven by an emission inventory is critical to understanding the spatial extent of high-O3 events and which emission sources have the most impact on O3 formation. In the winter of 2016/17 (December 2016–March 2017) several high-O3 events were observed with 1 h mixing ratios exceeding 70 ppb. This study uses the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) to simulate one of the high-O3 events observed in the UGRB during March 2017. The WRF-Chem simulations were carried out using the 2014 edition of the Environmental Protection Agency National Emissions Inventory (EPA NEI2014v2), which, unlike previous versions, includes estimates of emissions from non-point oil and gas production sources. Simulations were carried out with two different chemical mechanisms: the Model for Ozone and Related Chemical Tracers (MOZART) and the Regional Atmospheric Chemistry Mechanism (RACM), and the results were compared with data from seven weather and air quality monitoring stations in the UGRB operated by the Wyoming Department of Environmental Quality (WYDEQ). The simulated meteorology compared favorably to observations with regard to temperature inversions, surface temperature, and wind speeds. Notably, because of snow cover present in the basin, the photolysis surface albedo had to be modified to predict O3 in excess of 70 ppb, although the models were relatively insensitive to the exact photolysis albedo if it was over 0.65. O3 precursors, i.e., NOx and VOCs, are predicted similarly in simulations with both chemical mechanisms, but simulated VOC mixing ratios are a factor of 6 or more lower than the observations, while NOx is also underpredicted but to a lesser degree. Sensitivity simulations revealed that increasing NOx and VOC emissions to match observations produced slightly more O3 compared to baseline simulations, but an additional sensitivity simulation with doubled NOx emissions resulted in a considerable increase in O3 formation. These results suggest that O3 formation in the basin is most sensitive to NOx emissions.

Atmospheric oxidation capacity and secondary pollutant formation potentials based on photochemical loss of VOCs in a megacity of the Sichuan Basin, China

Volatile organic compounds (VOCs) are significant precursors to photochemical pollution. However, reactive VOC species are easily oxidized during transportation, resulting in a systematic underestimate of the measured concentrations. To address this, we applied an improved calculation method to correct the measured VOC concentrations into photochemical initial concentrations (PICs) in Chengdu, a megacity in the Sichuan Basin, China, which is highly vulnerable to complex pollution. In this study, 56 VOC species on the Photochemical Assessment Monitor Station (PAMS) target list were quantitatively monitored throughout all four seasons. Comparing to directly measured values, photochemically initialized total mixing ratios of VOCs increased by 18.6 % in general. The photochemical loss percentages of alkenes and aromatics were prominent in summer (68.6 %, 28.7 %) and spring (65.9 %, 24.7 %), respectively. Furthermore, we examined contributions of VOCs to atmospheric oxidation capacity (AOC) depending on PICs and found that maximum daily total AOC showed a surge in spring and summer. Besides hydroxyl radicals, daytime O3 in spring and late-afternoon nitrate radicals in summer were essential for AOC with PICs. As expected, alkenes and aromatics dominated PIC-based ozone formation potentials (OFPs). Furthermore, contribution of alkenes to secondary organic aerosol formation potentials reached 15.5 % and 7.6 % in spring and summer, respectively. Using positive matrix factorization model, we identified five VOC sources including vehicular exhaust, industrial emissions, solvent usage, biogenic sources, and liquefied petroleum gas/natural gas use. Based on PICs, biogenic sources were significantly underestimated in spring and summer. Meanwhile, m,p-xylene from solvent usage and isoprene from biogenic sources were the primary contributors to OFPs. Consequently, these results emphasize the significance of photochemically oxidized VOC concentrations, especially for reactive species in typical seasons.

Methodological advances to improve repeatability of SOA generation in environmental chambers

Most laboratory atmospheric chamber studies probing the chemical and physical properties of secondary organic aerosol (SOA) perform such experiments with mixing ratios of volatile organic compounds (VOCs) above atmospheric relevance (≳50 ppbv). When performing ozonolysis of biogenic VOCs at mixing ratios of atmospheric relevance (≲10 ppbv), a major hinderance on the repeatability of replicate experiments is due to the limitations of conventional VOC injection techniques. To overcome these limitations, two novel components (stop/flow and split valves) were embedded in a conventional VOC injection setup, thereby permitting the use of higher VOC volumes of injection to attain low VOC mixing ratios, and the delivery of the VOC to the environmental chamber as a short, discrete pulse for subsequent reaction. Implementation of these novel VOC injection components has resulted in improvements in variability between replicate chamber experiments of up to a factor of 7 with respect to particle number, mass, and size distributions at both high and low VOC mixing ratios (50 and 10 ppbv, respectively). These improvements permit extension of quantitative measurements of SOA formation to VOC mixing ratios at or near atmospheric levels, where new particle formation (NPF) and SOA mass loading are typically within experimental variability.

Characteristics, chemical transformation and source apportionment of volatile organic compounds (VOCs) during wintertime at a suburban site in a provincial capital city, east China

Based on the continuous online measurements of volatile organic compounds (VOCs) at a suburban site in Jinan, Shandong province in the wintertime of 2021, characteristics, chemical transformations and sources of VOCs were analyzed. The total mixing ratio of VOCs was 30.35 ± 19.9 ppbv, with alkanes being the most abundant species. Alkenes and aromatics were the major contributors to the hydroxyl radical loss rate (LOH) and the ozone formation potential (OFP), accounting for 44.9% and 32.7% to LOH, and 43.2% and 33.4% to OFP, respectively. Aromatics had the primary contribution (98.1%) to the secondary organic aerosol potential (SOAP), and the high correlation between SOAP and PM2.5 concentrations (R = 0.70) suggested that VOCs were critical precursors of PM2.5. Coal/biomass burning and motor vehicle emissions were the main sources of VOCs (24.6% and 23.8%, respectively), followed by petrochemical and fuel evaporation (19.9%), industrial sources (16.9%), and liquefied petroleum gas/natural gas use (14.8%). Backward trajectories and the potential source contribution function (PSCF) analysis results suggested that short-range transport from the southeast was the primary potential source of VOC concentrations in suburban Jinan. The results can provide theoretical support for local governments to develop VOC emission control strategies in suburban areas.

Variations of Wintertime Ambient Volatile Organic Compounds in Beijing, China, from 2015 to 2019

Air quality in China significantly improved after strict regulations were put in place. To assess the efficacy of volatile organic compound (VOC) control strategies, online measurements of wintertime ambient VOCs at a Beijing urban location were taken in December and January from 2014 to 2019. We assessed the changes in mixing ratios of 81 VOC species and cancer risks of 10 hazard species. The impact of meteorology on VOC mixing ratios was decoupled using a machine-learning approach. The highest and lowest VOC mixing ratios were observed in wintertime 2017 (December 2016 and January 2017) and 2019 (December 2018 and January 2019), respectively, with a decrease in 63.5%. Both emission control policies and meteorological conditions helped reduce VOC mixing ratios in the winter in Beijing. Alkenes and alkynes decreased significantly from 2015 to 2019. In contrast, the contribution of aromatics increased considerably from 2015 to 2017, and many aromatics had higher mixing ratios in 2019 than in 2015, indicating that aromatics may be a key group for future air quality improvement. The cancer risks due to VOC exposure in the wintertime in Beijing in 2019 were lower than in other years but were still much higher than the safety level.

Characterization of odorous industrial plumes by coupling fast and slow mass spectrometry techniques for volatile organic compounds.

This study aimed to develop a technique to chemically characterize odor issues in neighborhoods of designated industrial zones with pronounced emissions of volatile organic compounds (VOCs). Due to the elusive nature of odor plumes, speedy detection with sufficient sensitivity is required to capture the plumes. In this demonstration, proton-transfer-reaction mass spectrometry (PTR-MS) was used as the front-line detection tool in an industrial zone to guide sampling canisters for in-laboratory analysis of 106 VOCs by gas chromatography-mass spectrometry/flame ionization detector (GC-MS/FID). The fast but less accurate PTR-MS coupled with the slow but accurate GC-MS/FID method effectively eliminates the drawbacks of each instrument and fortifies the strength of both when combined. A 10-day PTR-MS field screening period was conducted to determine suitable trigger VOC species with exceedingly high mixing ratios that were likely the culprits of foul odors. Twenty canister samples were then collected, triggered by m/z 43, 61 (ethyl acetate, fragments, EA), m/z 73 (methyl ethyl ketone, MEK), or m/z 88 (morpholine) in all cases. Internal consistency was confirmed by the high correlation of critical species in the PTR-MS and trigger samples. Several long-lived halocarbons were exploited as the intrinsic internal reference for quality assurance. Oxygenated VOCs (OVOCs) accounted for 15%-75% of the total VOC mixing ratios in the triggered samples. However, EA and MEK, the most prominent OVOC species, did not appear to have common sources with morpholine, which presented with PTR-MS peaks incoherent with the other OVOCs. Nevertheless, these distinctive OVOC plumes were consistent with the multiple types of odor reported by the local residents. In contrast with the triggered sampling, random samples in the same industrial zone and roadside samples in a major metropolitan area were collected. The pronounced OVOC content in the triggered samples highlighted the advantage over random grab sampling to address odor issues.

Variations and sources of volatile organic compounds (VOCs) in urban region: insights from measurements on a tall tower

Abstract. Volatile organic compounds (VOCs) are key precursors of ozone and particulate matter, which are the two dominant air pollutants in urban environments. However, compositions and sources of VOCs in urban air aloft have rarely been reported so far. To address this matter, highly time-resolved measurements of VOCs were made by a proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) at a 450 m platform on the Canton Tower in Guangzhou, China. A combination of in situ measurements and modeling techniques was used to characterize variations in and sources of VOCs. Five sources were identified from positive matrix factorization (PMF) analysis, namely daytime mixed (e.g., biogenic emissions and secondary formation), visitor-related (e.g., human breath, cooking, and volatilization of ethanol-containing products), vehicular–industrial, regional transport, and volatile chemical product (VCP) (i.e., volatilization of personal care products), contributing on average to 21 %, 30 %, 28 %, 10 %, and 11 % of total VOC (TVOC) mixing ratios, respectively. We observe that contributions of the visitor-related source, mainly composed of ethanol, followed with the variation in visitor numbers on the tower well. The VCP-dominated source only had an average contribution of ∼5.7 ppb during the campaign, accounting for a small fraction (11 %) of TVOC mixing ratios but a large fraction (22 %) of the total OH reactivity. However, large fractions of reactive VOC species, e.g., monoterpenes (49 %), were attributed to the VCP-dominated source, indicating important contributions of VCPs to ambient concentrations of these species in urban environments. Vertical profiles of air pollutants (namely NOx, ozone, Ox, and PM2.5), measured at 5, 118, 168, and 488 m, exhibited more evident gradients at night than in the daytime owing to the stronger stability of the nocturnal boundary layer. Mixing ratios of VOC species during the nighttime generally decreased with time when the 450 m platform was located in the nocturnal residual layer and markedly increased when impacted by emissions at ground level. The results in this study demonstrated composition characteristics and sources of VOCs in urban air aloft, which could provide valuable implications in making strategies for control of VOCs and secondary air pollutants.

Accurate identification of key VOCs sources contributing to O3 formation along the Liaodong Bay based on emission inventories and ambient observations

In order to achieve the precise control of the volatile organic compounds (VOCs) species with high ozone (O3) formation contribution from key sources in Panjin and Yingkou, two coastal industrial cities with severe O3 pollution along the Liaodong Bay, northeast China, the ambient concentrations of 99 VOCs species were measured online at urban-petrochemical (XLT), suburban-industrial (PP), and rural (XRD) sites in July 2019, contemporary monthly anthropogenic VOCs emission inventories were developed. The source contribution of ambient VOCs resolved by positive matrix factorization (PMF) model was comparable with emission inventories, and the location of VOCs sources were speculated by potential source contribution function (PSCF). 17.5 Gg anthropogenic VOCs was emitted in Panjin and Yingkou in July 2019 with potential to form 54.7 Gg-O3 estimated by emission inventories. The average VOC mixing ratios of 47.1, 26.7, and 16.5 ppbv was observed at XLT, PP, and XRD sites, respectively. Petroleum industry (22 %), organic chemical industry (21 %), and mobile vehicle emission (19 %) were identified to be the main sources contributing to O3 formation at XLT site by PMF, while it is organic chemical industry (33 %) and solvent utilization (28 %) contributed the most at PP site. Taking the subdivided source contributions of emission inventories and source locations speculated by PSCF into full consideration, organic raw chemicals manufacturing, structural steel coating, petroleum refining process, petroleum products storage and transport, off-shore vessels, and passenger cars were identified as the key anthropogenic sources. High O3-formation contribution sources, organic chemical industry and solvent utilization were located in the industrial parks at the junction of the two cities and the southeast of Panjin, and petroleum industry distributed in the whole Panjin and offshore areas. These results identify the key VOCs species and sources and speculate the potential geographical location of sources for precisely controlling ground-level O3 along the Liaodong Bay.

Characteristics of VOC Composition at Urban and Suburban Sites of New Delhi, India in Winter

AbstractSimultaneous measurements of volatile organic compounds (VOCs) using two PTR‐TOF‐MS instruments were conducted at urban and suburban sites of New Delhi during the winter of 2018. The time series of VOC mixing ratios show substantial variations mainly influenced by local emissions and meteorological conditions. Mixing ratios of methanol (∼28 ppbv), acetaldehyde (7.7 ppbv), acetone (10.6 ppbv), isoprene (2.8 ppbv) and monoterpenes (0.84 ppbv) at the suburban site were higher than those at the urban site, while levels of aromatic VOCs were almost similar. The strong nighttime correlations of isoprene and monoterpenes with CO and benzene at the urban site indicate their predominant anthropogenic origin. Higher emission ratios of ∆VOCs/∆CO and ∆VOCs/∆benzene than those reported for vehicular exhaust suggest the contributions of other sources. In addition to vehicular emissions, episodes of biomass burning, industrial plumes and aged air strongly influenced the levels of VOCs at the suburban site. Despite the predominant primary anthropogenic emissions, the higher daytime enhancements of OVOCs/CO ratios indicate additional contributions of OVOCs from secondary/biogenic sources. The secondary formation of OVOCs in moderately aged air masses was noticeable at the suburban site. Using the source‐tracer‐ratio method, the estimated biogenic contributions of isoprene (71%) and acetone (65%) during daytime at the suburban site were significantly higher than those for the urban site. The photochemical box model simulations suggest that daytime ozone formation was under the VOC‐limited regime. The present study highlights the impact of different emission sources, photochemical processes and meteorological conditions on the composition and concentration of VOCs in the Delhi region.

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

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

An analysis of the temporal variability in volatile organic compounds (VOCs) within megacity Seoul and an identification of their sources

In this study, the volume mixing ratio of 55 volatile organic compound (VOC) species was measured every hour using the online TD-GC-FID system during the period from January 2018 to December 2019 in Seoul. The most abundant species were ethane (10.0 ppbv) and toluene (4.1 ppbv), with alkanes and aromatics accounting for 57.5% and 31.5% of the total VOCs (TVOC), respectively. The mixing ratios of the TVOC were markedly higher in July and November, mainly owing to the presence of aromatics and alkanes, respectively, but were lower in other months, without any significant differences in other months. Of the aromatics, the maximum mixing ratios of xylene, benzene, and toluene were observed in summer, winter, and both seasons, respectively. Principal component analysis showed that emissions from traffic and solvent use were the main VOC sources, which were further categorized into liquefied natural gas/liquefied petroleum gas/gasoline, diesel fuel, mixed combustion, industrial solvents, and paint solvents. The two solvent sources were well represented by toluene and xylene, respectively. In addition, the distinct VOC composition of the air masses clustered by the back trajectory reveals the origin, seasonality, and transport characteristic of the five source categories. This study is the first report on multiyear variability of VOC mixing ratios in Seoul that highlights the contribution from evaporation sources and fugitive emissions and the potential impact of intercity transport from industrial areas near Seoul.

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.

Tropospheric ozone changes and ozone sensitivity from the present day to the future under shared socio-economic pathways

Abstract. Tropospheric ozone is important to future air quality and climate. We investigate ozone changes and ozone sensitivity to changing emissions in the context of climate change from the present day (2004–2014) to the future (2045–2055) under a range of shared socio-economic pathways (SSPs). We apply the United Kingdom Earth System Model, UKESM1, with an extended chemistry scheme including more reactive volatile organic compounds (VOCs) to quantify ozone burdens as well as ozone sensitivities globally and regionally based on nitrogen oxide (NOx) and VOC mixing ratios. We show that the tropospheric ozone burden increases by 4 % under a development pathway with higher NOx and VOC emissions (SSP3-7.0) but decreases by 7 % under the same pathway if NOx and VOC emissions are reduced (SSP3-7.0-lowNTCF) and by 5 % if atmospheric methane (CH4) mixing ratios are reduced (SSP3-7.0-lowCH4). Global mean surface ozone mixing ratios are reduced by 3–5 ppb under SSP3-7.0-lowNTCF and by 2–3 ppb under SSP3-7.0-lowCH4. However, surface ozone changes vary substantially by season in high-emission regions under future pathways, with decreased ozone mixing ratios in summer and increased ozone mixing ratios in winter when NOx emissions are reduced. VOC-limited areas are more extensive in winter (7 %) than in summer (3 %) across the globe. North America, Europe, and East Asia are the dominant VOC-limited regions in the present day, but North America and Europe become more NOx-limited in the future mainly due to reductions in NOx emissions. The impacts of VOC emissions on ozone sensitivity are limited in North America and Europe because reduced anthropogenic VOC emissions are partly offset by higher biogenic VOC emissions. Ozone sensitivity is not greatly influenced by changing CH4 mixing ratios. South Asia becomes the dominant VOC-limited region under future pathways. We highlight that reductions in NOx emissions are required to transform ozone production from VOC to NOx limitation, but that these lead to increased ozone mixing ratios in high-emission regions, and hence emission controls on VOC and CH4 are also necessary.