Relative Incremental Reactivity Research Articles

Carbonyl Compounds Observed at a Suburban Site during an Unusual Wintertime Ozone Pollution Event in Guangzhou

Carbonyl compounds are important oxygenated volatile organic compounds (VOCs) that play significant roles in the formation of ozone (O3) and atmospheric chemistry. This study presents comprehensive field observations of carbonyl compounds during an unusual wintertime ozone pollution event at a suburban site in Guangzhou, South China, from 19 to 28 December 2020. The aim was to investigate the characteristics and sources of carbonyls, as well as their contributions to O3 formation. Formaldehyde, acetone, and acetaldehyde were the most abundant carbonyls detected, with average concentrations of 7.11 ± 1.80, 5.21 ± 1.13, and 3.00 ± 0.94 ppbv, respectively, on pollution days, significantly higher than those of 2.57 ± 1.12, 2.73 ± 0.88, and 1.10 ± 0.48 ppbv, respectively, on nonpollution days. The Frame for 0-D Atmospheric Modeling (F0AM) box model simulations revealed that local production accounted for 62–88% of observed O3 concentrations during the pollution days. The calculated ozone formation potentials (OFPs) for various precursors (carbonyls and VOCs) indicated that carbonyl compounds contributed 32.87% of the total OFPs on nonpollution days and 36.71% on pollution days, respectively. Formaldehyde, acetaldehyde, and methylglyoxal were identified as the most reactive carbonyls, and formaldehyde ranked top in OFPs, and it alone contributed 15.92% of total OFPs on nonpollution days and 18.10% of total OFPs on pollution days, respectively. The calculation of relative incremental reactivity (RIR) indicates that ozone sensitivity was a VOC-limited regime, and carbonyls showed greater RIRs than other groups of VOCs. The model simulation showed that secondary formation has a significant impact on formaldehyde production, which is primarily controlled by alkenes and biogenic VOCs. The characteristic ratios and backward trajectory analysis also indicated the indispensable impacts of local primary sources (like industrial emissions and vehicle emissions) and regional sources (like biomass burning) through transportation. This study highlights the important roles of carbonyls, particularly formaldehyde, in forming ozone pollution in megacities like the Pearl River Delta region.

The chemical characteristics and sources of formaldehyde on O3 and non-O3 polluted days in Wuhan, central China

In August 2023, a detailed investigation of formaldehyde (HCHO) chemical characteristics on ozone (O3) polluted days and non-O3 polluted days in Wuhan was undertaken. The mean value of HCHO on O3 polluted days (3.02 ± 1.15 ppbv) was 122% higher than that on non-O3 polluted days (1.35 ± 0.41 ppbv). Utilizing Positive Matrix Factorization (PMF) model revealed secondary formation as the dominant HCHO source (58.3% on non-O3 polluted days and 66.2% on O3 polluted days). On O3 polluted days, the contribution of liquefied petroleum gas (LPG)/solvent usage and industrial emissions to HCHO were 13.7% and 8.2%, respectively, whereas on non-O3 polluted days, LPG/solvent use and diesel exhaust contributed 15.4% and 14.7%, respectively. The top ten species, with the highest Relative Incremental Reactivity (RIR) to HCHO, were mainly alkenes and aromatics, which remained consistent on both O3 and non-O3 polluted days. It is noteworthy that the RIR of isobutane to HCHO is significant in this study. Further reapportionment of secondary HCHO by a photochemical box model indicated that LPG/solvent usage (33.2%) contributed the most to HCHO on O3 polluted days, while diesel exhaust (31.9%) dominated on non-O3 polluted days. This research enhances understanding of HCHO in Wuhan, providing a theoretical basis for targeted pollution reduction and supporting efforts to improve air quality and public health.

Ozone response to precursors changes in the Chengdu-Chongqing economic circle, China, from satellite and ground-based observations

Ozone (O3) pollution has become a noticeable problem in the Chengdu-Chongqing Economic Circle in China. The April–September MDA8 O3 level increases significantly by 2.26 μg m−3 year−1 from 2015 to 2023, with meteorological factors occupying merely 18 % in line with multivariate linear regression. To reveal the impact of anthropogenic emissions on O3 increase, O3 production sensitivity is accurately diagnosed by deriving localized thresholds for satellite formaldehyde (HCHO) to NO2 ratio and validated by in-situ measurements and observation-based model. Tracking volatile organic compounds (VOCs) and NOx through satellite HCHO and NO2, the O3 responses to precursor changes are assessed for long-term and special cases, and appropriate precursor reduction ratios are inferred. The results present that the transition range of satellite HCHO/NO2 from VOC-limited to NOx-limited in the region ranges from 2.7 to 4.3. The VOC-limited regime is concentrated in the urban areas of Chongqing and Chengdu as well as the central of the neighboring cities such as Deyang, Mianyang, and Meishan. The relative incremental reactivity from in-situ observations and box model at three sites in August 2019 demonstrates that O3 is most sensitive to anthropogenic VOC at urban and suburban sites, consistent with satellite results. Satellite and surface NO2 decrease at an annual rate of −2.1 % and − 2.9 % between 2015 and 2023, with larger decreases in Chengdu and Chongqing. In contrast, the trend of satellite HCHO is insignificant, indicating effective reduction in NOx but no significant reduction in VOC. This inappropriate reduction results in an increase in urban O3. The three short-term cases further validate the need for synergistic NOx and VOC reductions. Based on the relationship between O3 and satellite NO2 and HCHO, the minimum and optimal reduction ratios of VOC to NOx are 0.4:1 and 2.4:1 for the entire region, with higher ratios in Chengdu and Chongqing.

Sensitivity Analysis of Atmospheric Ozone and Source Apportionment of Volatile Organic Compounds in Yinchuan City

Based on the observation data of O3 concentration in Yinchuan in 2022, the monthly variation characteristics of O3 concentrations were analyzed. Further, based on the observation data of meteorological elements, conventional pollutants, and volatile organic compounds （VOCs） concentrations at an urban site in Yinchuan from May to July, the difference in meteorological elements and precursor concentrations between the polluted days and the non-polluted days were compared. Then, the O3 sensitivity and the VOCs sources were discussed using the Framework for 0-D Atmospheric Modeling （F0AM） and positive matrix factorization （PMF） model, respectively. The results showed that： ① The O3 pollution occurred from May to July in 2022, and the concentrations of O3-8h-90per were 156 μg·m-3, 170 μg·m-3, and 174 μg·m-3, respectively, with exceeding standard rates of 9.7%, 26.7%, and 29.0%, respectively. ② Compared with those on the non-polluted days, the hourly mean values of temperature, total solar radiation, and concentrations of various precursors on the O3-polluted days increased, including the volume concentrations of propane, isobutane, ethane, n-butane, and dichloromethane, which increased significantly by 33.1%, 29.1%, 25.0%, 22.7%, and 21.3%, respectively. The results showed that the combined increase in pollutant emissions and adverse meteorological conditions contributed to the formation of O3. ③ From May to July 2022, the top five VOCs species in terms of ozone formation potential （OFP） value on whole, non-polluted, and polluted days were the same. They were acetaldehyde, m/p-xylene, ethylene, isoprene, and toluene, mainly from solvent use sources, natural sources, and chemical industry emissions. ④ The local O3 production was mostly controlled by VOCs, and the relative incremental reactivity （RIR） results revealed that O3 production showed strong positive sensitivity to alkene and aromatic hydrocarbon but showed negative sensitivity to NOx on both polluted and non-polluted days. The relative contributions of active species such as acetone, ethylene, and isobutane to O3 production were high, and the implementation of an emission reduction scheme with the ratio of VOCs to NOx emission reduction much greater than 1 could effectively reduce the local O3 concentration. ⑤ The main sources of atmospheric VOCs in Yinchuan were motor vehicle emission sources （32.3%）, process sources （20.7%）, combustion sources （19.2%）, solvent use sources （12.7%）, gasoline volatile sources （9.1%）, and natural sources （6%）, and the contribution rate of motor vehicle emission sources on polluted days increased by 4.6% compared with that on non-polluted days, indicating that the motor vehicle emission source was an important object of summer VOCs control in Yinchuan.

Ozone production sensitivity analysis for the Chengdu Plain Urban Agglomeration based on a multi-site and two-episode observation

Chengdu Plain Urban Agglomeration (CPUA) is one of the most serious areas suffering from ozone pollution in China. A comprehensive field observation focused on the ozone production rate and its sensitivity was conducted at CPUA in the summer of 2019. Six sampling sites were set and two ozone pollution episodes were recognized. The daily maximum 8-h average (MDA8) O3 concentration reached 137.9 ppbv in the urban sites during the ozone pollution episode. Peak concentration of O3 was closely related to intense solar radiation, high temperatures, and precursor emissions. The OH-HO2-RO2 radical chemistry and ozone production rate (P(O3)) were calculated using an observation-based model (OBM). The daily peak OH concentration varied in the range of 3-13 × 106 molecules cm−3, and peak HO2 and RO2 were in the range of 2–14 × 108 molecules cm−3 during ozone pollution episodes. During the ozone pollution episode, the average maximum of P(O3) in suburban sites (about 30 ppbv h-1.) was compared with urban sites, and the maximum of P(O3) was 18 ppbv h-1 in rural sites. The relative incremental reactivity (RIR) results demonstrate that it was a VOCs-limited regime in the central urban area of Chengdu, with NOx suppression effect in some regions. In the southern neighboring suburb of Chengdu, it was VOCs-limited as well. However, the northern suburban area was a transition region. In the remote rural areas of the southern CPUA, it was highly NOx-limited. Local ozone production driven by the photochemical process is crucial to the ozone pollution formation in CPUA. The geographically differentiated recognition of the ozone regime found by this study can help to tailor control strategies for local conditions and avoid the negative effects of a one-size-fits-all approach.

Analysis of Photochemical Characteristics and Sensitivity of Atmospheric Ozone in Nanjing in Summer

Tropospheric ozone (O3) is mainly produced through a series of photochemical reactions of nitrogen oxides (NOx) and volatile organic compounds (VOCs). The reaction process presents complex non-linear relationships. In this work, datasets of atmospheric ozone and volatile organic compounds (VOCs) observed during the summer of 2018 in Nanjing were used. Combining with the framework for 0-D atmospheric model-master chemical mechanism (F0AM-MCM), the characteristics of photochemical reactions for ozone (O3) formation in Nanjing during the O3 episode days and non-episode days were investigated. The results showed that φ(O3) and φ(TVOCs) in the O3 episode days were 47.8×10-9 and 49.0×10-9, respectively, exceeding those in the non-episode days by factors of 1.8 and 1.6. Furthermore, F0AM, the empirical kinetic modeling approach (EKMA), and relative incremental reactivity (RIR) were utilized for the calculation of ozone chemical sensitivity. It was found that O3 formation in Nanjing was attributed to both VOCs and NOx limitation. In addition, the modeled ·OH and HO2 concentrations in the O3 episode days were 1.3 and 1.8 times higher than those in the non-episode days. The higher formation and loss rates of ·OH and HO2 were also found during O3 episode days. These findings reflected that the enhancements of atmospheric oxidation capacity resulted in increased production rates of O3, providing an explanation for the enhancements of O3 concentrations in Nanjing during the O3 episode days. The findings also improved the understanding of the O3 photochemical characteristics over Nanjing in the summer during the O3 episode days.

Composition and Reactivity of Volatile Organic Compounds and the Implications for Ozone Formation in the North China Plain

Enhanced ozone (O3) pollution has emerged as a pressing environmental concern in China, particularly for densely populated megacities and major city clusters. However, volatile organic compounds (VOCs), the key precursors to O3 formation, have not been routinely measured. In this study, we characterize the spatial and temporal patterns of VOCs and examine the role of VOCs in O3 production in five cities (Dongying (DY), Rizhao (RZ), Yantai (YT), Weihai (WH), and Jinan (JN)) in the North China Plain (NCP) for two sampling periods (June and December) in 2021 through continuous field observations. Among various VOC categories, alkanes accounted for the largest proportion of VOCs in the cities. For VOCs, chemical reactivities, aromatic hydrocarbons, and alkenes were dominant contributors to O3 formation potential (OFP). Unlike inland regions, the contribution to OFP from OVOCs increased greatly at high O3 concentrations in coastal regions (especially YT). Model simulations during the O3 episode show that the net O3 production rates were 27.87, 10.24, and 10.37 ppbv/h in DY, RZ, and JN. The pathway of HO2 + NO contributed the most to O3 production in JN and RZ, while RO2 + NO was the largest contributor to O3 production in DY. The relative incremental reactivity (RIR) revealed that O3 formation in DY was the transitional regime, while it was markedly the VOC-limited regime in JN and RZ. The O3 production response is influenced by NOx concentration and has a clear daily variation pattern (the sensitivity is greater from 15:00 to 17:00). The most efficiencies in O3 reduction could be achieved by reducing NOx when the NOx concentration is low (less than 20 ppbv in this study). This study reveals the importance of ambient VOCs in O3 production over the NCP and demonstrates that a better grasp of VOC sources and profiles is critical for in-depth O3 regulation in the NCP.

Photochemical Mechanism and Control Strategy Optimization for Summertime Ozone Pollution in Yining City

To explore the formation mechanism of the ozone （O3） and emission reduction strategy in a northwestern city， an extensive field campaign was conducted in summertime in 2021 in Yining City， in which the 0-D box model incorporating the latest explicit chemical mechanism （MCMv3.3.1） was applied for the first time to quantify the O3-NOx-VOCs sensitivity and formulate a precise O3 control strategy in this city. The results showed that： ① the three indicators ［i.e.， O3 formation potential （OFP）， ·OH reaction rate （k·OH）， and relative incremental reactivity （RIR）］ jointly indicated that alkenes， oxygenated volatile organic compounds （OVOCs）， and aromatics were the highest contributors among anthropogenic volatile organic compounds （AVOC） to O3 formation， and the contribution of biogenic volatile organic compounds （BVOC） also could not be ignored. Additionally， the results based on RIR calculation implied that that the acetaldehyde， ethylene， and propylene were the most sensitive individual VOCs species in Yining City. ② The in-situ photochemical O3 variations were primarily influenced by the local photochemical production and export process horizontally to downwind areas or vertically to the upper layer， and the reaction pathways of HO2·+ NO and ·OH + NO2 contributed the most to the gross Ox photochemical production （60%） and photochemical destruction production （53%）， respectively. Hence， the reduction in local emissions for O3 precursors was more essential to alleviate O3 pollution in this city. ③ The outcome based on RIR（NOx） / RIR（AVOC） and EKMA jointly suggested that the photochemical regime in this city can be considered a transitional regime that was also nearly a VOCs-limited regime. Detailed mechanism modeling based on multiple scenarios further suggested that the NOx and VOCs synergic emission reduction strategies was helpful to alleviate O3 pollution. These results are useful to provide policy-related guidance for other cities facing similar O3 pollution in northwest China.

Comparative analysis for the impacts of VOC subgroups and atmospheric oxidation capacity on O3 based on different observation-based methods at a suburban site in the North China Plain

The persistent O3 pollution in the Beijing-Tianjin-Hebei (BTH) region remains unresolved, largely due to limited comprehension of O3-precursor relationship and photochemistry drivers. In this work, intraday O3 sensitivity evolution from VOC-limited (volatile organic compound) regime in the forenoon to transition regime in the late afternoon was inferred by relative incremental reactivity (RIR) in summer 2019 at Xianghe, a suburban site in BTH region, suggesting that VOC-focused control policy could combine with stringent afternoon NOx control. Then detailed impacts of VOC subgroups on O3 formation were further comprehensively quantified by parametric OH reactivity (KOH), O3 formation potential (OFP), as well as RIR weighted value and O3 formation path tracing (OFPT) approach based on photochemical box model. O3 episode days corresponded to stronger O3 formation, depicted by higher KOH (10.4 s−1), OFP (331.7 μg m−3), RIR weighted value (1.2), and F(O3)-OFPT (15.5 ppbv h−1). High proportions of isoprene and OVOCs (oxygenated VOCs) to the total KOH and the OFPT method were demonstrated whereas results of OFP and RIR-weighted presented extra great impacts of aromatics on O3 formation. The OFPT approach captured the process that has already happened and included final O3 response to the original VOC, thus reliable for replicating VOC impacts. The comparison results of the four methods showed similarities when utilizing KOH and OFPT methods, which reveals that the potential applicability of simple KOH for contingency VOC control and more complex OFPT method for detailed VOC- and source-oriented control during policy-making. To investigate propulsion of VOC-involved O3 photochemistry, atmospheric oxidation capacity (AOC) was quantified by two atmospheric oxidation indexes (AOI). Both AOIp_G (7.0 × 107 molec cm−3 s−1, potential AOC calculated by oxidation reaction rates) and AOIe_G (8.5 μmol m−3, estimated AOC given redox electron transfer for oxidation products) were stronger on O3 episode days, indicating that AOC promoted the radical cycling initiated from VOC oxidation and subsequent O3 production. Result-oriented AOIe_G reasonably characterized actual AOC inferred by good linear correlation between AOIe_G and O3 concentrations compared to process-oriented AOIp_G. Therefore, with continuous NOx abatement, AOIe_G should be considered to represent actual AOC, also O3-inducing ability.

Insight into decreased ozone formation across the Chinese National Day Holidays at a regional background site in the Pearl River Delta

Investigating the formation mechanism of ozone (O3) during the Chinese National Day Holidays (CNDH, Oct. 1st to 7th) in the background area of megacity clusters provided a valuable opportunity for surveying the relative influence of anthropogenic and biogenic sources on O3 pollution. Here we conducted an intensive observation campaign during Sep. 24 to Oct. 14, 2021 at a regional background site in the Pearl River Delta region, southern China. Results showed that O3 concentrations exhibited a downtrend across the CNDH, with a decreasing rate of 14.8% from prior-CNDH to CNDH and 28.9% from CNDH to post-CNDH. Simulation of O3 photochemical process by using a photochemical box model with the master chemical mechanism (PBM-MCM) indicated that the decreased atmospheric oxidation capacity and decelerated free radical cycling led to a decrease in the net O3 production rate (PO3) across the CNDH. O3 isopleth diagrams and relative incremental reactivity (RIR) analysis revealed that O3 formation (FO3) in prior-CNDH and CNDH was jointly limited by VOCs and NOx, while was VOC-limited in post-CNDH. VOCs was still shown to be the largest contributor to FO3, suggesting VOCs reduction was the most effective way to reduce O3 pollution. Further simulation by combining the PBM-MCM and positive matrix factorization (PMF) model showed that anthropogenic sources have a smaller contribution to the PO3 in CNDH compared to the other periods, showing a significant holiday effect of “decreasing during holidays and rebounding after holidays”. However, due to the more biogenic emission during CNDH, the net PO3 was higher than that post-CNDH. This study improved our understanding of the enhanced biogenic contributions to FO3 in future scenarios of anthropogenic emissions reduction.

Characteristics, Sources, and Contributions to Ozone Formation of Ambient Volatile Organic Compounds in Huanggang, China

Based on the offline sampling data of volatile organic compounds (VOCs) and the simultaneous online measurements of conventional gaseous air pollutants and meteorological parameters in urban Huanggang, the volume fractions and component characteristics of VOCs were analyzed. The sources and ozone (O3) formation sensitivity of VOCs during severe ozone pollution episodes were analyzed using the positive matrix factorization (PMF) model and the photochemical box model coupled with master chemical mechanism (PBM-MCM), respectively. The results revealed that the average volume fractions of total volatile organic compounds were (21.57±3.13)×10-9, with higher volume fractions in winter and spring compared to those in summer and autumn. Among these, alkanes (49.9%) and alkenes (16.4%) accounted for the highest proportion. The PMF analysis results showed that fuel combustion (27.8%), vehicle emission (19.9%), solvent use (15.7%), industrial halogenated hydrocarbon emission (12.1%), chemical enterprise emission (10.5%), natural sources (7.8%), and diesel vehicle emission (6.2%) were the main sources of VOC emissions. Anthropogenic VOCs emitted by solvent use, fuel combustion, and chemical enterprises contributed significantly (60.9% in total) to generating O3, which indicates that these three types of anthropogenic sources should be controlled first when it comes to preventing and controlling ozone pollution. Further, the relative incremental reactivity (RIR) and empirical kinetic method approach (EKMA) revealed that O3 formation was in a VOCs-limited regime during the observation period in Huanggang, China. Furthermore, O3 formation was more sensitive to m-xylene, p-xylene, ethylene, 1-butene, and toluene; therefore, reducing these VOCs should be prioritized.

Elucidating the mechanisms of rapid O3 increase in North China Plain during COVID-19 lockdown period

Ozone (O3) levels in North China Plain (NCP) suffered from rapid increases during the COVID-19 period. Many previous studies have confirmed more rapid NOx reduction compared with VOCs might be responsible for the O3 increase during this period, while the comprehensive impacts of each VOC species and NOx on ambient O3 and their interactions with meteorology were not revealed clearly. To clarify the detailed reasons for the O3 increase, a continuous campaign was performed in a typical industrial city of NCP. Meanwhile, the machine-learning technique and the box model were employed to reveal the mechanisms of O3 increase from the perspective of meteorology and photochemical process, respectively. The result suggested that the ambient O3 level in Tangshan increased from 18.7 ± 4.63 to 45.6 ± 8.52 μg/m3 (143%) during COVID-19 lockdown, and the emission reduction and meteorology contributed to 54 % and 46 % of this increment, respectively. The lower wind speed (WS) coupled with regional transport played a significant role on O3 increase (30.8 kg/s). The O3 sensitivity verified that O3 production was highly volatile organic compounds (VOC)-sensitive (Relative incremental reactivity (RIR): 0.75), while the NOx showed the negative impact on O3 production in Tangshan (RIR: −0.59). It suggested that the control of VOCs rather than NOx might be more effective in reducing O3 level in Tangshan because it was located on the VOC-limited regime. Besides, both of ozone formation potential (OFP) analysis and observation-based model (OBM) demonstrated that the alkenes (36.3 ppb) and anthropogenic oxygenated volatile organic compounds (OVOCs) (15.2 ppb) showed the higher OFP compared with other species, and their reactions released a large number of HO2 and RO2 radicals. Moreover, the concentrations of these species did not experience marked decreases during COVID-19 lockdown, which were major contributors to O3 increase during this period. This study also underlined the necessity of priority controlling alkenes and OVOCs across the NCP.

Pollution characteristics, sources, and photochemical roles of ambient carbonyl compounds in summer of Beijing, China

Ambient carbonyls are important precursors of radicals and ground-level ozone (O3). In this study, sources, precursors, and impacts on radicals and O3 of carbonyls were investigated based on online observations of volatile organic compounds (VOCs) at an urban site in Beijing during June 2021. Carbonyls accounted for 36% and 42% of mixing ratios and OH reactivity for total measured VOCs, respectively. Formaldehyde was the most abundant carbonyl, with the mean level of 4.13 ± 2.28 ppb. Source apportionment results based on the multi linear regression (MLR) method suggested that secondary production contributed 41%, 25%, 36%, and 30% of formaldehyde, acetaldehyde, propanal, and acetone, respectively. Key precursors of carbonyls were then identified based on the calculation of their production rates. It was found that alkenes contributed 59%–80% of aldehydes production. Impacts of carbonyls on HOx radicals (OH and HO2) and O3 production were explored using a box model based on observations (OBM). Photolysis of HONO, formaldehyde, and O3 were the dominant primary sources of HOx radicals during daytime of O3 pollution days, with average relative contributions of 52%, 28%, and 19% to the total primary production rate of HOx, respectively. Aldehydes accounted for 32% (20% from formaldehyde) of average HOx removal rates. The relative incremental reactivity (RIR) values of NOx determined by the OBM were negative, suggesting that the O3–VOCs-NOx sensitivity was in the VOCs-limited regime. Using the observed concentrations of carbonyls as constraints of OBM, the absolute values of RIR for NOx tended to increase but those for anthropogenic VOCs tended to decrease. Formaldehyde showed the largest RIR value for anthropogenic VOCs during O3 pollution days. These findings indicated the important impacts of carbonyls on O3 production and O3–VOCs-NOx sensitivity.

Analysis of O3 Pollution Characteristics, Formation Sensitivity, and Transport Impact in Southern Nanjing

As one of the most important city clusters in China, surface ozone (O3) pollution in the Yangtze River Delta (YRD) Region has become a prominent air quality problem in recent years. Online observations of ambient O3, nitrogen oxides (NOx), and volatile organic compounds (VOCs) were conducted in southern Nanjing from July-September 2020 and April-May 2021. On this basis, the pollution characteristics of O3 were analyzed. The O3-VOCs-NOx sensitivity and the transport influence of its precursors were further discussed using models. The frequency statistics of the daily maximum 8-hour moving average (DMA-8h O3) and hourly concentration (O3-1h) showed that O3 pollution in southern Nanjing was more serious than that in urban areas. Three typical O3 pollution episodes were selected during the whole observation period, which included August 16th-27th, 2020; September 3rd-11th, 2020; and May 17th-25th, 2021. The O3-VOCs-NOx sensitivities in these three pollution episodes were analyzed using the observation-based model (OBM). The results of the relative incremental reactivity (RIR) and empirical kinetics modeling approach (EKMA) showed that the sensitivities of O3 formation in the three pollution episodes were in the NOx-limited regime, the transition regime, and the VOCs-limited regime, respectively. This difference in O3-VOCs-NOx sensitivity reflects that the site may have been influenced by transport. Therefore, the potential source contribution function (PSCF) and the concentration weighted trajectory (CWT) method were further used to analyze the potential source areas of NOx, VOCs, and O3 in these three pollution episodes. The results showed that there were obvious regional transport effects of NOx, VOCs, and O3 in these three pollution episodes. The location of potential sources differed in these three pollution episodes, which were affected by the eastern cities of the Lishui site; the urban area of Nanjing and eastern area of Anhui Province; and the urban area of Nanjing and the middle of the YRD Region, respectively. The impact of transport on O3 and its precursors also indicated the necessity of regional joint prevention and control of O3 pollution in the YRD Region.

Observations and explicit modeling of summer and autumn ozone formation in urban Beijing: Identification of key precursor species and sources

Due to the complexity and nonlinearity of the ozone (O3) production, it is vital to understand the detailed knowledge of ozone–precursor relationship for the formulation of effective mitigation of surface ozone pollution. In this study, continuous measurements of O3, volatile organic compounds (VOCs), and related parameters were conducted at an urban site of Beijing in the summer and autumn of 2019. The photochemical box model contained the Master Chemical Mechanism (MCMv3.3.1) was used to reveal the detailed information on the photochemical O3 formation. Results show that the average concentration of total VOCs (TVOC) was 31.2 ± 10.7 ppb and 37.8 ± 18.9 ppb during the summer and autumn campaign, respectively, with the largest contribution from alkanes in both seasons, which mainly originated from liquefied petroleum gas (LPG) usage (20.2%–22.6%), gasoline vehicular exhaust (21.0%–21.6%), diesel vehicular exhaust (18.1%–18.3%), solvent usage (20.4%–24.1%), and fuel evaporation (16.4%–17.4%) that identified by positive matrix factorization (PMF) model. Heavy ozone pollution episodes were observed and the net local O3 production rate during summer (EP1) and autumn (EP2) pollution episode was 9.87 ± 0.62 ppbv h−1, and 9.44 ± 1.18 ppbv h−1, respectively, which was both dominated by the reaction between HO2 with NO. The analysis of relative incremental reactivity (RIR) and O3-NOx-VOC isopleth shows that ozone formation was VOC-limited in urban Beijing, with alkenes and aromatics contributed the most to O3 formation in summer and autumn, respectively. In addition, modellings for various emission reduction scenarios were carried out, indicating the reduction of gasoline vehicles emissions in summer and both reduction of gasoline vehicles and solvent usage emissions in autumn is the best measures to efficiently reduce O3 pollution. These results improved our understanding of O3 precursors and their impact on ozone production and provided a more targeted reference for controlling ozone pollution in Beijing.

Effects of long-distance transport on O3 and secondary inorganic aerosols formation in Qingdao, China

This work presents a three-year continuous observation of ozone (O3) and fine particle (PM2.5), as well as their precursors in Qingdao, China, from September 2018 to August 2021. The annual concentrations of O3 and PM2.5 were measured as 35.9–44.5 ppbv and 31.6–34.2 μg m−3, respectively. Analysis of the interannual variations of O3 and PM2.5 concentration indicated the effective control measures for PM2.5 and O3 pollution in Qingdao in recent years. Nevertheless, we still observed 85 O3 episodes and 80 PM2.5 episodes during the whole observation. And it was found that the fraction of secondary inorganic aerosols (SIA) in PM2.5 mass significantly increased during PM2.5 episodes. By employing the Lagrangian photochemical trajectory model (LPTM), we investigated the roles of local production and regional transport (i.e., short-distance transport and long-distance transport) that play in the O3 and SIA formation in these episodes. The contribution of local production, short-distance transport, and long-distance transport to O3 concentration was calculated as 36%, 25%, and 39%, respectively. Long-distance transport seemed to play a more significant role in the SIA formation, accounting for 56% of SIA concentration. Moreover, the calculation results of the relative incremental reactivity (RIR) showed that both O3 and SIA formation were mainly affected by NOx and VOCs emissions during the process of long-distance transport. The observed unexpectedly high contribution of long-distance transport to O3 and SIA formation suggests that more extensive regional joint prevention and control policies on NOx and VOCs emissions are warranted to mitigate secondary air pollution.

Ozone Pollution Characteristics and Sensitivity During the Ozone Pollution Days in Summer 2021 of Xinxiang City

This study was based on the observation of volatile organic compounds (VOCs), conventional gaseous air pollutants, and meteorological parameters observed at the Xinxiang Municipal Party School site from June to August 2021. The ozone (O3) characteristics and sensitivity of O3 pollution days and the control strategy of its precursors were studied using an observation-based model (OBM). It was found that the meteorological conditions were characterized by high temperature, low humidity, and low pressure in O3-pollution days. The concentrations of O3 and its precursors all increased in the O3 pollution days. Oxygenated volatile organic compounds (OVOCs) and alkanes were the highest-concentration components of VOCs on O3 pollution days in Xinxiang, and OVOCs had the highest ozone formation potential (OFP) and hydroxyl (·OH) reactivity. According to the relative incremental reactivity (RIR) analysis, during the O3 pollution days in Xinxiang, O3sensitivity was in the VOCs-limited regime in June and in the transitional regime in July and August. Ozone production was more sensitive to alkenes and OVOCs. The RIR values of the precursors in June changed throughout the day, but O3 sensitivity remained the VOCs-limited regime. In July and August, O3 sensitivity was the VOCs-limited regime in the morning, transitional regime at noon, transitional and NOx-limited regime, respectively in the afternoon. By simulating different precursor-reduction scenarios, the results showed that the reduction of VOCs was always beneficial to the control of O3, whereas the reduction of NOx had little effect on the control of O3 and a risk of increasing O3.

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

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

Changes in O3-VOCs-NOx Sensitivity and VOCs Sources at an Urban Site of Nanjing Between 2020 and 2021

The synergistic control of PM2.5 and ozone (O3) are the focus of air quality improvement during the 14th Five-Year Plan in China. The production of O3 shows a highly nonlinear relationship with its precursors volatile organic compounds (VOCs) and nitrogen oxides (NOx). In this study, we conducted online observations of O3, VOCs, and NOx at an urban site in downtown Nanjing from April to September of 2020 and 2021. The average concentrations of O3 and its precursors between these two years were compared, and then the O3-VOCs-NOx sensitivity and the VOCs sources were analyzed using the observation-based box model (OBM) and positive matrix factorization (PMF), respectively. The results showed that the mean daily maximum O3 concentrations, VOCs, and NOx concentrations decreased by 7% (P=0.031), 17.6% (P<0.001), and 14.0% (P=0.004) from April to September of 2021 compared with those from the same period in 2020, respectively. The average relative incremental reactivity (RIR) values of NOx and anthropogenic VOCs during the O3 non-attainment days in 2020 and 2021 were 0.17 and 0.14 and 0.21 and 0.14, respectively. The positive RIR values of NOx and VOCs indicated that O3 production was controlled by both VOCs and NOx. The O3 production potential contours (EKMA curves) based on the 50×50 scenario simulations also supported this conclusion. The PMF results showed that industrial and traffic-related emissions were the main sources of VOCs. The five PMF-resolved factors were identified as industrial emissions, including industrial liquefied petroleum gas (LPG) use, the benzene-related industry, petrochemistry, toluene-related industry, and solvent and paint use, which contributed 55%-57% of the average mass concentration of total VOCs. The summed relative contributions of vehicular exhaust and gasoline evaporation were 43%-45%. Petrochemistry and solvent and paint use showed the two highest RIR values, suggesting that VOCs from these two sources should be reduced with priority to control O3. With the implementation of VOCs and NOx control measures, the O3-VOCs-NOx sensitivity and VOCs sources have changed, and therefore we still need to follow their variations in the future to timely adjust O3 control strategies during the 14th Five-Year Plan.

VOCs sources and roles in O3 formation in the central Yangtze River Delta region of China

Surface ozone (O3) pollution has become a prominent air quality problem in the Yangtze River Delta (YRD) region of China in recent years. Since O3 is non-linearly related to its precursors (volatile organic compounds (VOCs), nitrogen oxides (NOx)), identifying the characteristics of VOCs is significant for the control of O3 pollution. In this study, online observation of ambient VOCs from August to October 2018 was conducted at 2 sites in Changzhou, an industrial city located in the central YRD region, to investigate the O3 pollution characteristics, sources of VOCs, and their roles in O3 formation. The average concentration of VOCs in Changzhou during the observational period was 39.52 ± 23.14 ppb. Alkanes, oxygenated VOCs (OVOCs), and halocarbons were the main contributors to the total VOCs concentration, with the average relative contributions of 39.4%, 23.1%, and 16.1%, respectively. An observation-based model (OBM) was used to investigate the sensitivity of O3 formation to VOCs and NOx. Results of relative incremental reactivity (RIR) of individual VOC and empirical kinetics modeling approach (EKMA) showed that the O3 formation in Changzhou was within the VOCs-limited regime. Anthropogenic VOCs with the largest RIR values were xylenes and propene, respectively. The positive matrix factorization (PMF) model was applied to quantitatively identify the sources of VOCs. Six factors were resolved, including vehicle exhaust, gasoline evaporation, paint and solvent usage, electronic manufacturing, petrochemical industry, and biogenic source & secondary formation. The average relative contribution of traffic-related emissions to the total VOCs mass concentration was about 49.5%, followed by electronic manufacturing (22.9%), paint and solvent usage (14.7%), and petrochemical industry (7.1%). The average relative contributions of individual sources for xylenes and propene were further compared. Xylenes were mainly emitted from paint and solvent usage (60%), electronic manufacturing (17%), and vehicle exhaust (16%). Petrochemical industry (67%) and vehicle exhaust (30%) were the major sources of propene. Therefore, VOCs from industrial emissions and traffic-related emissions should be given priority for the control of O3 pollution in the central YRD region.