Differences in the key volatile organic compound species between their emitted and ambient concentrations in ozone formation

The emissions and secondary transformations of volatile organic compounds (VOCs) play a significant role in ozone (O₃) formation. Previous studies have often relied on ambient VOC concentrations to identify key species. However, ambient concentrations represent the residual concentrations after the emitted VOCs have been consumed, which can introduce substantial uncertainties. To address this issue, this study proposes a novel method to identify the key VOC species in both anthropogenic and biogenic emissions. The emitted VOC concentrations are calculated during both nighttime and daytime in summer using the nitrate radical, O₃, and hydroxyl radical reaction rates and ambient concentrations of 99 VOCs measured at Deyang, Chengdu, and Meishan, China. The emitted concentrations of alkenes and aromatics are higher than the ambient concentrations. The largest differences between emitted and ambient concentrations are 1.04 ppbv for cis-2-butene at Deyang, 0.81 ppbv for isoprene at Chengdu, and 1.79 ppbv for isoprene at Meishan. In contrast, due to secondary production, the emitted concentrations of oxygenated VOCs are lower than the ambient concentrations. The largest differences are −0.54 ppbv for acetone at Deyang, −0.58 ppbv for acetaldehyde at Chengdu, and −0.5 ppbv for acetone at Meishan. Based on the emitted concentrations, isoprene is one of the top three species contributing to O₃ formation at all three sites, which may be overlooked in observed concentrations. Comprehensive calculation of the emitted VOC concentrations enables the key VOC species in O₃ formation to be accurately identified.

1 Introduction

Volatile organic compounds (VOCs), mainly including non-methane hydrocarbons (NMHCs) and oxygenated VOCs (OVOCs) (Mo et al., 2021; Gkatzelis et al., 2021; Xia et al., 2020; Huang et al., 2020), have an important role in ozone (O₃) formation (Fry et al., 2018; Seinfeld and Pandis, 2016; Finlayson-Pitts and Pitts, 2000; Haagen-Smit and Fox, 1956). VOCs primarily originate from anthropogenic sources, such as biomass burning, vehicle emissions, solvent use, and industrial activities, as well as biogenic sources (Wu and Xie, 2017). The VOCs emitted from these sources are referred to as emitted VOCs. Additionally, OVOCs could be formed through the oxidation of NMHCs (Calvert et al., 2015; Birdsall and Elrod, 2011; Finlayson-Pitts and Pitts, 2000). It is crucial to accurately identify the key emitted VOC species that influence O₃ formation to develop effective VOC emission reduction strategies.

The evaluation of the contribution of VOC species to O₃ formation typically involves methods such as the photochemical O₃ creation potentials (Derwent et al., 2007, 1998), the relative incremental reactivity method (Cardelino and Chameides, 1995), O₃ formation pathway tracking based on box models (Zhan et al., 2023), and the maximum incremental reactivity (MIR) method (Carter et al., 1995; Carter, 1994). The MIR method has been widely used since 1994 (Kong et al., 2023; Carter et al., 1995; Carter, 1994). The MIR values have previously been obtained by adding VOCs in the smog chamber experiments and performing numerical simulations. These values reflect the changes in O₃ concentrations resulting from adding a unit of VOCs (Carter et al., 1995; Carter, 1994). The importance of VOC species is ranked based on the product of the MIR and the ambient VOC concentrations at sampling sites, and the results are used to identify the key species contributing to O₃ formation potential (OFP) in Chengdu (Wang et al., 2023; Kong et al., 2023; Xiong et al., 2021; Tan et al., 2020a, b; Deng et al., 2019; Song et al., 2018) and Deyang (Chen et al., 2021). For example, the top three VOC species at Chengdu contributing to the OFP were ethylene, propylene, and m,p-xylene (Song et al., 2018). The VOCs added in smog chamber experiments should correspond to those emitted into the atmosphere from sources. Most studies have determined these by relying on the ambient VOC concentrations measured in the atmosphere. However, there are substantial uncertainties in accurately identifying the key VOC species.

Ambient VOC concentrations consist of the residual concentrations after the emitted VOCs have been consumed during the day and night, along with the concentrations of secondary OVOC formation during the day. Most studies have calculated emitted concentrations based on the daytime consumption of NMHCs by hydroxyl radicals (OH) (He et al., 2019; Shao et al., 2009). However, the consumption of alkenes at night by O₃ or nitrate radicals (NO₃) and the primary emissions and secondary formation of OVOCs are not considered. First, although OH mainly reacts with NMHCs during the day, nighttime alkene consumption is majorly driven by NO₃ or O₃ (Zhu et al., 2020; Brown and Stutz, 2012; Finlayson-Pitts and Pitts, 1997). The ratio of nighttime NO₃ and O₃ consumption to daytime OH consumption is 1.28±1.28 for isoprene, styrene, 1,3-butadiene, ethylene, propylene, 1-butene, cis-2-butene, and trans-2-butene (de Gouw et al., 2017). Therefore, if nighttime alkene consumption is not considered, the emitted concentrations of alkenes are likely underestimated.

Second, ambient concentrations of OVOCs include contributions from both primary emissions and secondary formation. The OFP of secondary OVOC concentrations is already included within their precursor NMHCs because the MIR is expressed as the maximum value. Thus, without excluding the secondary OVOC formation, their OFP would likely be overestimated. The differences in the reaction rates of VOCs and secondary formation of OVOCs cause discrepancies in the species rankings of emitted and ambient VOC concentrations, which in turn affects the OFP rankings of key VOC species. Due to the instantaneous emissions, mixing, and oxidation changes in the ambient atmosphere, directly obtaining the emitted VOC concentrations during the day and night is difficult. These cause uncertainties in accurately identifying the emitted VOC species that are key to O₃ formation.

To address these issues, this study calculates the emitted concentrations of alkenes at night and NMHCs and OVOCs during the day based on reaction rates and the measured ambient VOC concentrations. The differences in the OFP rankings of the VOC species are assessed based on the emitted and ambient VOC concentrations. These investigations are conducted through hourly measurements of concentrations of 99 VOCs in rural Deyang, suburban Chengdu, and forest Meishan in the Sichuan Basin, China, from August to September 2019. The main aim is to determine the emitted VOC concentrations and identify the key species affecting O₃ formation by constraining the consumption of VOCs both at night and during the day and the secondary formation of OVOCs during the day.

2 Materials and methods

2.1 Field measurements

Three monitoring sites are located in the Sichuan Basin, China (Fig. S1 in the Supplement): the village of Guihong (31°1^′ N, 104°12^′ E) in Deyang (expressed as rural Deyang); Huangjueshu community (30°24^′ N, 103°51^′ E) in Chengdu (suburban Chengdu); and a bamboo forest wetland park (29°48^′ N, 103°49^′ E) in Meishan (forest Meishan). In summer (July to September), the temperature at these sites ranges from 25–35 °C, and the relative humidity ranges from 40 % to 80 %. The Deyang–Chengdu–Meishan urban agglomeration is located between the eastern Tibetan Plateau and the western Longquan Mountains (Fig. S1b). Air masses in this agglomeration are not easily dispersed and tend to concentrate within the basin (Fig. S1c). Strong ultraviolet light, high temperatures, and low wind speeds are conducive to VOC emissions and consumption during the mid-latitude summer in the basin.

Hourly VOC samples are collected, preprocessed, and analyzed by a custom-built online gas chromatography–mass spectrometry flame ionization detector (GC-MS/FID) system (TH-PKU 300B, Tianhong, China; GCMS-QP2010SE-Plus, Shimadzu, Japan) from 8 August to 14 September 2019. After removing particulate matter, moisture, and carbon dioxide, 300 mL of air is concentrated within an electronic refrigeration unit at −150 °C for each sample. The concentrated VOCs are quickly desorbed by heating at 100 °C and transferred to the GC column. C₂ to C₅ hydrocarbons are separated on a nonpolar capillary column (HP-PLOT Al₂O₃, Agilent, Santa Clara, CA, USA) and measured with the FID. Other compounds are separated on a semi-polar column (DB-624, Agilent) and detected by MS. Standard curves are established for six concentrations from 0.1 to 8 ppbv for 57 VOCs at photochemical assessment monitoring stations (PAMSs) and 55 customized OVOCs and halocarbons using four internal standard gases (Linde, Danbury, CT, USA). Data validation is performed almost every night using the 2 ppbv standard concentration at the PAMSs and for the customized gas samples. The coefficients of determination of the calibration curves are greater than 0.99, and the method detection limits range from 0.003 to 0.070 ppbv for each VOC. A total of 99 VOCs are detected at the three sites: 29 alkanes, 11 alkenes, 16 aromatics, 28 halocarbons, 13 OVOCs, acetonitrile, and acetylene. Given the absence of secondary formation, VOCs other than OVOCs are classified as NMHCs, which include alkanes, alkenes, acetylene, acetonitrile, halocarbons, and aromatics. Similarly, no secondary formation is found for methyl tert-butyl ether (MTBE). The hourly O₃ and NO₂ concentrations are monitored concurrently by the Sichuan Environmental Monitoring Center at their sites at Deyang (31°8^′ N, 104°26^′ E), Chengdu (30°37^′ N, 104°8^′ E), and Meishan (30°4^′ N, 103°52^′ E) in a similar surrounding environment near each of the three VOC sampling sites, respectively.

2.2 Calculations of emitted VOC concentrations

Source classification is crucial for calculating emitted VOC concentrations. NMHCs (except isoprene) and MTBE are generally emitted from anthropogenic activities. Isoprene is typically emitted from biogenic sources and oxidized into methyl vinyl ketone (MVK) and methacrolein (MACR). In addition to secondary production, 10 of the 13 OVOCs (except MTBE, MVK, and MACR) are also emitted from both anthropogenic and biogenic sources (Zou et al., 2024; Lyu et al., 2024; Li et al., 2023; Wu et al., 2020). The emitted VOC concentrations are classified into three categories: anthropogenic sources in Sect. 2.2.1, biogenic sources in Sect. 2.2.2, and a combination of both in Sect. 2.2.3.

First, the major atmospheric oxidants for the consumption of emitted VOCs are NO₃, O₃, and OH. Due to the absence of sunlight, OH concentrations and photolysis rates are very low from 20:00 to 06:00 LT (Fig. S2). Either NO₃ or O₃ is the primary oxidant for the consumption of emitted alkene and styrene during the nighttime. During the local nighttime, emitted alkene and styrene concentrations are estimated through the NO₃ or O₃ exposure methods based on the relative loss rates of reported species between NO₃ and O₃ in the Los Angeles Basin (de Gouw et al., 2017). Other VOCs are excluded from the analysis due to their slow reaction rates with NO₃ and O₃ during the nighttime. The NO₃ or O₃ exposure method indicates that the concentration ratios of a stable tracer species to a reactive tracer species would increase with both NO₃ or O₃ concentrations and reaction time after emissions. Emitted concentrations are calculated based on NO₃ or O₃ reaction rates with VOCs and their exposure. Unreported alkenes are classified through comparison with reported alkenes in reaction rates of both NO₃ (kNO₃) and O₃ (kO₃) (Fig. S3). For example, the nocturnal consumption of 1-butene is over 96 % through reaction with O₃ (de Gouw et al., 2017). The kO₃ for 1-pentene is higher than the kO₃ for 1-butene, but the kNO₃ for 1-pentene is lower than the kNO₃ for 1-butene. Therefore, the emitted 1-pentene concentrations are estimated using the O₃ exposure method. Briefly, styrene and 1,3-butadiene are determined using the NO₃ exposure method, while 8 of the 10 alkenes are determined using the O₃ exposure method.

During the daytime from 07:00 to 19:00 LT (Fig. S2), NO₃ is highly unstable and rapidly photolyzed. Therefore, VOC consumption by its oxidation is negligible. Alkenes and styrene can react with both OH and O₃. For alkenes and styrene, the ratio of the product of the OH reaction rates (Carter, 2010) and the ambient OH concentration in the Chengdu Plain ($\mathrm{6.14}\times {\mathrm{10}}^{\mathrm{6}}\mathrm{molecules}{\mathrm{cm}}^{-\mathrm{3}}$; Yang et al., 2021) to the product of the O₃ reaction rates (Carter, 2010; Atkinson and Arey, 2003) and the ambient O₃ concentration (45.71 ppbv) is 19.20. This indicates that OH predominantly consumes VOCs during the daytime. The emitted concentrations of NMHCs and MTBE are quantified during the daytime using the OH exposure method in Sect. 2.2.1. The OH exposure method is similar to the NO₃ and O₃ exposure methods.

Second, Brown et al. (2009b) calculated emitted isoprene concentrations during the nighttime based on the steady-state NO₃ production from the reaction of NO₂ with O₃ and its consumption by isoprene. The mean ratios of measured O₃ to NO₂ concentrations during the nighttime are 4.64 ppbv ppbv⁻¹ at Deyang, 1.42 ppbv ppbv⁻¹ at Chengdu, and 2.23 ppbv ppbv⁻¹ at Meishan. The reported method is not suitable for this study because O₃ concentrations must be much larger than the NO₂ concentrations (Brown et al., 2009a). Similar to styrene and 1,3-butadiene, emitted isoprene concentrations are determined using the NO₃ exposure method during the nighttime. During the day, emitted isoprene concentrations are calculated using the OH exposure method and the ambient concentrations of MVK and MACR. Emitted MVK and MACR concentrations are calculated based on their measured concentrations and isoprene consumption.

Third, emitted OVOC concentrations are determined using the photochemical age method during the daytime, due to their primary emissions, secondary production, and losses by either photolysis or photooxidation. Among meteorological factors, temperature is the primary driver of O₃ production (Jacob and Winner, 2009), with the OH reaction rate showing small variations between 25–35 °C. For example, the reaction rate ratio for isoprene between these two temperatures is 0.96 (Saunders et al., 2003). Consequently, all VOC reaction rate constants are adjusted for a temperature of 300 K.

2.2.1 NMHC concentrations emitted by anthropogenic activities

For nighttime NO₃ consumption, the emitted concentrations of styrene and 1,3-butadiene are estimated using the NO₃ exposure method (de Gouw et al., 2017):

$$\begin{array}{}\text{(1)}& {\displaystyle }\begin{array}{rl}\left[{\mathrm{NO}}_{\mathrm{3}}\right]\mathrm{\Delta }t=& {\displaystyle \frac{\mathrm{1}}{(k{{\mathrm{NO}}_{\mathrm{3}}}_{\text{benzene}}-k{{\mathrm{NO}}_{\mathrm{3}}}_{\text{styrene}})}}\\ & \times \left[\ln \left({\displaystyle \frac{\left[\text{e-benzene}\right]}{\left[\text{e-styrene}\right]}}\right)-\ln \left({\displaystyle \frac{\left[\text{a-benzene}\right]}{\left[\text{a-styrene}\right]}}\right)\right]\end{array}\text{(2)}& {\displaystyle }\left[{\text{e-alkene}}_j\right]=\left[{\text{a-alkene}}_j\right]\times \exp \left(k{{\mathrm{NO}}_{\mathrm{3}}}_{{\text{alkene}}_j}\right[{\mathrm{NO}}_{\mathrm{3}}\left]\mathrm{\Delta }t\right),\end{array}$$

where [NO₃] and Δt, together referred to as NO₃ exposure ([NO₃]Δt), are the NO₃ concentrations and nocturnal reaction time, respectively. $k{{\mathrm{NO}}_{\mathrm{3}}}_{\text{benzene}}$ and $k{{\mathrm{NO}}_{\mathrm{3}}}_{\text{styrene}}$ are the reaction rate constants of benzene ($\mathrm{3.0}\times {\mathrm{10}}^{-\mathrm{17}}{\mathrm{cm}}^{\mathrm{3}}{\mathrm{molecule}}^{-\mathrm{1}}{\mathrm{s}}^{-\mathrm{1}}$) and styrene ($\mathrm{1.5}\times {\mathrm{10}}^{-\mathrm{13}}{\mathrm{cm}}^{\mathrm{3}}{\mathrm{molecule}}^{-\mathrm{1}}{\mathrm{s}}^{-\mathrm{1}}$) with NO₃, respectively (Carter, 2010; Atkinson and Arey, 2003). [e-benzene]$/$[e-styrene] is the emission ratio between benzene and styrene. The estimated emission ratios are $\mathrm{1.0}\pm \mathrm{0.4}\mathrm{ppbv}{\mathrm{ppbv}}^{-\mathrm{1}}$ at Deyang, $\mathrm{1.1}\pm \mathrm{0.5}\mathrm{ppbv}{\mathrm{ppbv}}^{-\mathrm{1}}$ at Chengdu, and $\mathrm{2.7}\pm \mathrm{0.5}\mathrm{ppbv}{\mathrm{ppbv}}^{-\mathrm{1}}$ at Meishan based on the ambient concentrations with a low degree of nocturnal consumption, respectively (Fig. S4). [a-benzene]$/$[a-styrene] is the hourly ambient concentration ratio between benzene and isoprene. [a-alkenes_j] and $k{{\mathrm{NO}}_{\mathrm{3}}}_{{\text{alkene}}_j}$ refer to the ambient concentrations and NO₃ reaction rate constants (Fig. S3) of species j in styrene or 1,3-butadiene, respectively.

For nighttime O₃ consumption, the emitted concentrations of eight reactive alkenes are estimated using the O₃ exposure method (de Gouw et al., 2017):

$$\begin{array}{}\text{(3)}& {\displaystyle }\begin{array}{rl}\left[{\mathrm{O}}_{\mathrm{3}}\right]\mathrm{\Delta }t=& {\displaystyle \frac{\mathrm{1}}{(k{{\mathrm{O}}_{\mathrm{3}}}_{\text{benzene}}-k{{\mathrm{O}}_{\mathrm{3}}}_{\mathit{cis}\text{-2-butene}})}}\\ & \times \left[\ln \left({\displaystyle \frac{\left[\text{e-benzene}\right]}{\left[\text{e-}\mathit{cis}\text{-2-butene}\right]}}\right)\right.\\ & \left.-\ln \left({\displaystyle \frac{\left[\text{a-benzene}\right]}{\left[\text{a-}\mathit{cis}\text{-2-butene}\right]}}\right)\right]\end{array}\text{(4)}& {\displaystyle }\left[{\text{e-alkene}}_j\right]=\left[{\text{a-alkene}}_j\right]\times \exp \left(k{{\mathrm{O}}_{\mathrm{3}}}_{{\text{alkene}}_j}\right[{\mathrm{O}}_{\mathrm{3}}\left]\mathrm{\Delta }t\right),\end{array}$$

where [O₃] and Δt, together referred to as O₃ exposure ([O₃]Δt), are the O₃ concentrations and nocturnal reaction time. $k{{\mathrm{O}}_{\mathrm{3}}}_{\text{benzene}}$ and $k{{\mathrm{O}}_{\mathrm{3}}}_{\mathit{cis}\text{-2-butene}}$ are the reaction rate constants of benzene ($\mathrm{1.0}\times {\mathrm{10}}^{-\mathrm{20}}{\mathrm{cm}}^{\mathrm{3}}{\mathrm{molecule}}^{-\mathrm{1}}{\mathrm{s}}^{-\mathrm{1}}$) and cis-2-butene ($\mathrm{1.3}\times {\mathrm{10}}^{-\mathrm{16}}{\mathrm{cm}}^{\mathrm{3}}{\mathrm{molecule}}^{-\mathrm{1}}{\mathrm{s}}^{-\mathrm{1}}$) with O₃ (Carter, 2010; Atkinson and Arey, 2003). [e-benzene]$/$[e-cis-2-butene] is the emission ratio between benzene and cis-2-butene. The estimated emission ratios are $\mathrm{0.5}\pm \mathrm{0.3}\mathrm{ppbv}{\mathrm{ppbv}}^{-\mathrm{1}}$ at Deyang, $\mathrm{4.5}\pm \mathrm{1.0}\mathrm{ppbv}{\mathrm{ppbv}}^{-\mathrm{1}}$ at Chengdu, and $\mathrm{6.5}\pm \mathrm{1.0}\mathrm{ppbv}{\mathrm{ppbv}}^{-\mathrm{1}}$ at Meishan based on measured data with a low degree of nocturnal consumption, respectively (Fig. S5). After mixing from different sources, the emission ratios obtained at different sampling sites may vary. [a-benzene]$/$[a-cis-2-butene] is the hourly ambient concentration ratio between benzene and cis-2-butene. [a-alkenes_j] and $k{{\mathrm{O}}_{\mathrm{3}}}_{{\text{alkene}}_j}$ refer to the ambient concentrations and O₃ reaction rate constants (Fig. S3) of the species j in alkenes, respectively. cis-2-Butene is replaced by trans-2-butene at Chengdu due to the unavailability of cis-2-butene data.

For daytime OH consumption, the emitted concentrations of each NMHC [e-NMHC], including MTBE, are estimated using the OH exposure method (Ma et al., 2022; Shao et al., 2011; de Gouw et al., 2005; Roberts et al., 1984):

$$\begin{array}{}\text{(5)}& {\displaystyle }\begin{array}{rl}\left[\mathrm{OH}\right]\mathrm{\Delta }t=& {\displaystyle \frac{\mathrm{1}}{(k{\mathrm{OH}}_{\text{ethylbenzene}}-k{\mathrm{OH}}_{m,p\text{-xylenes}})}}\\ & \times \left[\ln \left({\displaystyle \frac{\left[\text{e-ethylbenzene}\right]}{[\text{e-}m,p\text{-xylenes}]}}\right)\right.\\ & \left.-\ln \left({\displaystyle \frac{\left[\text{a-ethylbenzene}\right]}{[\text{a-}m,p\text{-xylenes}]}}\right)\right]\end{array}\text{(6)}& {\displaystyle }\left[{\text{e-NMHC}}_j\right]=\left[{\text{a-NMHC}}_j\right]\times \exp \left(k{\mathrm{OH}}_{{\text{NMHC}}_j}\right[\mathrm{OH}\left]\mathrm{\Delta }t\right),\end{array}$$

where [OH] and Δt, together referred to as OH exposure ([OH]Δt), are the OH concentrations and reaction time, respectively. [e-ethylbenzene]$/$[e-m,p-xylenes] is the emission ratio between ethylbenzene and m,p-xylenes (Fig. S6). The major source of ethylbenzene and m,p-xylenes in the Chengdu Plain is solvent use (Wu and Xie, 2017). There is a strong linear correlation between ethylbenzene and m,p-xylenes (R²=0.96). kOH_ethylbenzene and kOH_m,p-xylenes are the reaction rate constants of ethylbenzene ($\mathrm{7.0}\times {\mathrm{10}}^{-\mathrm{12}}{\mathrm{cm}}^{\mathrm{3}}{\mathrm{molecule}}^{-\mathrm{1}}{\mathrm{s}}^{-\mathrm{1}}$) and m,p-xylenes ($\mathrm{1.9}\times {\mathrm{10}}^{-\mathrm{11}}{\mathrm{cm}}^{\mathrm{3}}{\mathrm{molecule}}^{-\mathrm{1}}{\mathrm{s}}^{-\mathrm{1}}$) with OH, respectively (Carter, 2010). [a-NMHC_j] and $k{\mathrm{OH}}_{{\text{NMHC}}_j}$ denote the hourly ambient concentrations and OH reaction rate constants (Fig. S7) of the species j in NMHCs, respectively.

2.2.2 Isoprene concentrations emitted by biogenic sources

For nighttime NO₃ consumption, the emitted concentrations of isoprene are estimated using the NO₃ exposure method (de Gouw et al., 2017):

$$\begin{array}{}\text{(7)}& {\displaystyle }\begin{array}{rl}\left[{\mathrm{NO}}_{\mathrm{3}}\right]\mathrm{\Delta }t=& {\displaystyle \frac{\mathrm{1}}{(k{{\mathrm{NO}}_{\mathrm{3}}}_{\text{MVK}}-k{{\mathrm{NO}}_{\mathrm{3}}}_{\text{isoprene}})}}\\ & \times \left[\ln \left({\displaystyle \frac{\left[\text{e-MVK}\right]}{\left[\text{e-isoprene}\right]}}\right)-\ln \left({\displaystyle \frac{\left[\text{a-MVK}\right]}{\left[\text{a-isoprene}\right]}}\right)\right]\end{array}\text{(8)}& {\displaystyle }\begin{array}{rl}\left[\text{e-isoprene}\right]=& \left[\text{a-isoprene}\right]\\ & \times \exp \left(k{{\mathrm{NO}}_{\mathrm{3}}}_{\text{isoprene}}\right[{\mathrm{NO}}_{\mathrm{3}}\left]\mathrm{\Delta }t\right),\end{array}\end{array}$$

where [NO₃] and Δt, together referred to as NO₃ exposure ([NO₃]Δt), are the concentrations of NO₃ and nocturnal reaction time, respectively. $k{{\mathrm{NO}}_{\mathrm{3}}}_{\text{MVK}}$ and $k{{\mathrm{NO}}_{\mathrm{3}}}_{\text{isoprene}}$ are the reaction rate constants of MVK ($\mathrm{5.4}\times {\mathrm{10}}^{-\mathrm{18}}{\mathrm{cm}}^{\mathrm{3}}{\mathrm{molecule}}^{-\mathrm{1}}{\mathrm{s}}^{-\mathrm{1}}$ with O₃) and isoprene ($\mathrm{6.8}\times {\mathrm{10}}^{-\mathrm{13}}{\mathrm{cm}}^{\mathrm{3}}{\mathrm{molecule}}^{-\mathrm{1}}{\mathrm{s}}^{-\mathrm{1}}$) with NO₃, respectively (Carter, 2010; Atkinson and Arey, 2003). The $k{{\mathrm{NO}}_{\mathrm{3}}}_{\text{MVK}}$ value is very small. Due to the unavailability of the $k{{\mathrm{NO}}_{\mathrm{3}}}_{\text{MVK}}$ value, $k{{\mathrm{O}}_{\mathrm{3}}}_{\text{MVK}}$ is used as a substitute. [e-MVK]$/$[e-isoprene] is the emission ratio between MVK and isoprene. Although MVK and isoprene emissions are low at night, many field studies have demonstrated that they can accumulate in the early nighttime from 20:00 to 21:00 LT (Wennberg et al., 2018). Therefore, the measured MVK and isoprene concentrations in the early nighttime are the “emitted” concentrations for nighttime NO₃ consumption. The estimated emission ratios are $\mathrm{0.5}\pm \mathrm{0.2}\mathrm{ppbv}{\mathrm{ppbv}}^{-\mathrm{1}}$ at Deyang, $\mathrm{0.1}\pm \mathrm{0.1}\mathrm{ppbv}{\mathrm{ppbv}}^{-\mathrm{1}}$ at Chengdu, and $\mathrm{0.1}\pm \mathrm{0.1}\mathrm{ppbv}{\mathrm{ppbv}}^{-\mathrm{1}}$ at Meishan from measured data with a low degree of nocturnal consumption, respectively (Fig. S8). There are no significant differences in the estimated emission ratios between early and late nighttime. Therefore, nighttime low MVK and isoprene emissions may not influence this calculation method. The emission ratios are directly linked to emission sources. After mixing from different sources, the emission ratios obtained at different sampling sites may vary. Although MVK and isoprene may originate from different sources during the nighttime, both anthropogenic and biogenic activities in the Chengdu Plain are relatively stable at nighttime based on both our unpublished results and the reported findings of a study using positive matrix factorization (Zheng et al., 2023; Kong et al., 2023; Xiong et al., 2021). Furthermore, as surrogates for traffic flows, the traffic congestion indices during the nighttime in Chengdu remain relatively stable (https://jiaotong.baidu.com/congestion/city/urbanrealtime, last access: 8 February 2025). Therefore, their emission ratios may remain consistent. [a-MVK]$/$[a-isoprene] is the hourly ambient concentration ratio between MVK and isoprene. MACR is not used as a stable biogenic tracer due to its relatively high NO₃ reaction rate ($\mathrm{3.5}\times {\mathrm{10}}^{-\mathrm{15}}{\mathrm{cm}}^{\mathrm{3}}{\mathrm{molecule}}^{-\mathrm{1}}{\mathrm{s}}^{-\mathrm{1}}$) compared to MVK.

For daytime OH consumption, the emitted isoprene concentrations are estimated using the OH exposure method in Eqs. (9–12) (Paulot et al., 2009; Stroud et al., 2001).

$$\begin{array}{}\text{(9)}& {\displaystyle }\begin{array}{rl}\left[\mathrm{OH}\right]\mathrm{\Delta }{t}_{\text{MVK}}=& {\displaystyle \frac{\mathrm{1}}{(k{\mathrm{OH}}_{\text{isoprene}}-k{\mathrm{OH}}_{\text{MVK}})}}\\ & \times \ln \left(\mathrm{1}-{\displaystyle \frac{\left[\text{a-MVK}\right]}{\left[\text{a-isoprene}\right]}}\right.\\ & \left.\times {\displaystyle \frac{k{\mathrm{OH}}_{\text{MVK}}-k{\mathrm{OH}}_{\text{isoprene}}}{\mathrm{0.32}\times k{\mathrm{OH}}_{\text{isoprene}}}}\right)\end{array}\text{(10)}& {\displaystyle }\begin{array}{rl}\left[\mathrm{OH}\right]\mathrm{\Delta }{t}_{\text{MACR}}=& {\displaystyle \frac{\mathrm{1}}{(k{\mathrm{OH}}_{\text{isoprene}}-k{\mathrm{OH}}_{\text{MACR}})}}\\ & \times \ln \left(\mathrm{1}-{\displaystyle \frac{\left[\text{a-MACR}\right]}{\left[\text{a-isoprene}\right]}}\right.\\ & \left.\times {\displaystyle \frac{k{\mathrm{OH}}_{\text{MACR}}-k{\mathrm{OH}}_{\text{isoprene}}}{\mathrm{0.23}\times k{\mathrm{OH}}_{\text{isoprene}}}}\right)\end{array}\text{(11)}& {\displaystyle }\left[\mathrm{OH}\right]\mathrm{\Delta }{t}_{\text{isoprene}}={\displaystyle \frac{\left[\mathrm{OH}\right]\mathrm{\Delta }{t}_{\text{MVK}}+\left[\mathrm{OH}\right]\mathrm{\Delta }{t}_{\text{MACR}}}{\mathrm{2}}}\text{(12)}& {\displaystyle }\begin{array}{rl}\left[\text{e-isoprene}\right]=& \left[\text{a-isoprene}\right]\\ & \times \exp \left(k{\mathrm{OH}}_{\text{isoprene}}\right[\mathrm{OH}\left]\mathrm{\Delta }{t}_{\text{isoprene}}\right),\end{array}\end{array}$$

where kOH_isoprene, kOH_MVK, and kOH_MACR are the reaction rate constants of isoprene ($\mathrm{1}\times {\mathrm{10}}^{-\mathrm{10}}{\mathrm{cm}}^{\mathrm{3}}{\mathrm{molecule}}^{-\mathrm{1}}{\mathrm{s}}^{-\mathrm{1}}$), MVK ($\mathrm{2.0}\times {\mathrm{10}}^{-\mathrm{11}}{\mathrm{cm}}^{\mathrm{3}}{\mathrm{molecule}}^{-\mathrm{1}}{\mathrm{s}}^{-\mathrm{1}}$), and MACR ($\mathrm{2.8}\times {\mathrm{10}}^{-\mathrm{11}}{\mathrm{cm}}^{\mathrm{3}}{\mathrm{molecule}}^{-\mathrm{1}}{\mathrm{s}}^{-\mathrm{1}}$) with OH (Carter, 2010; Atkinson and Arey, 2003). [a-isoprene], [a-MVK], and [a-MACR] refer to the ambient concentrations of isoprene, MVK, and MACR, respectively.

For photolysis and photooxidation loss, the emitted concentrations of MVK and MACR ([e-OVOC]) are estimated based on isoprene consumption:

$$\begin{array}{}\text{(13)}& {\displaystyle }\left[\text{c-isoprene}\right]=\left[\text{e-isoprene}\right]-\left[\text{a-isoprene}\right]\text{(14)}& {\displaystyle }\left[{\text{s-OVOC}}_j\right]=p\times \left[\text{c-isoprene}\right]\text{(15)}& {\displaystyle }\begin{array}{rl}[\text{e-}& {\text{OVOC}}_j]=[{\text{a-OVOC}}_j]-[{\text{s-OVOC}}_j]+[\text{c-isoprene}]\\ & \times {\displaystyle \frac{\left(\right[{\text{a-OVOC}}_j]-[{\text{s-OVOC}}_j\left]\right)\times k{\mathrm{OH}}_{{\text{OVOC}}_j}^{\ast }}{\left[\text{a-isoprene}\right]\times k{\mathrm{OH}}_{\text{isoprene}}}},\end{array}\end{array}$$

where [c-isoprene] indicates consumed concentrations of isoprene, which are equivalent to its emitted concentrations ([e-isoprene]) calculated from Eq. (12) minus the ambient concentrations ([a-isoprene]). [s-OVOC_j] represents the secondary concentrations of species j in MVK or MACR produced from isoprene oxidation. The p values in Eq. (14) represent the molecular production from one molecular unit of isoprene consumption, with values of 0.32 for MVK and 0.23 for MACR, respectively (Paulot et al., 2009). Because the kOH_MVK and kOH_MACR values are 3.5 and 5 times lower than those of kOH_isoprene (Fig. S7), respectively, the ambient OVOC concentrations ([a-OVOC_j]) are assumed to be instantaneous total concentrations in Eq. (15). Therefore, [e-OVOC_j] is approximately equal to [a-OVOC_j] minus [s-OVOC_j] plus the corresponding photochemical consumption, which is calculated using [c-isoprene] and the reaction rates. The photochemical consumption of OVOCs includes both photolysis and reaction with OH. Therefore, the total OVOC_j loss rates ($k{\mathrm{OH}}_{{\text{OVOC}}_j}^{\ast }$) are estimated based on the photolysis rate ($J_{{\text{OVOC}}_j}$) and the loss rate with OH ([OH]$k{\mathrm{OH}}_{{\text{OVOC}}_j}$). The ratio of the $J_{{\mathrm{NO}}_{\mathrm{2}}}$ to OH concentrations in the Sichuan Basin is similar to that reported in the Los Angeles Basin (Yang et al., 2021; de Gouw et al., 2018); therefore the $J_{{\text{OVOC}}_j}$ and $\left[\mathrm{OH}\right]k{\mathrm{OH}}_{{\text{OVOC}}_j}$ may be comparable (Tan et al., 2018; de Gouw et al., 2018). Accordingly, we assume that the ratios (0.6 for MVK and MACR) of $J_{{\text{OVOC}}_j}$ to $\left[\mathrm{OH}\right]k{\mathrm{OH}}_{{\text{OVOC}}_j}$ established in the Los Angeles Basin (Fig. S9; de Gouw et al., 2018) are applicable for estimating $k{\mathrm{OH}}_{{\text{OVOC}}_j}^{\ast }=(\mathrm{1}+\mathrm{0.6})\times k{\mathrm{OH}}_{{\text{OVOC}}_j}$ in the Sichuan Basin. kOH_isoprene is the OH reaction rate constant of isoprene.

2.2.3 OVOC concentrations emitted by both anthropogenic and biogenic sources

To differentiate secondary production and consumption, we estimate the emitted concentrations of 10 of the 13 OVOCs ([e-OVOC]) during the daytime using the photochemical age method in Eqs. (16–17) (Wu et al., 2020; de Gouw et al., 2018; de Gouw et al., 2005). MTBE is excluded, with details provided in Sect. 2.2.1. The estimation methods for MVK and MACR are described in Sect. 2.2.2.

$$\begin{array}{}\text{(16)}& {\displaystyle }\begin{array}{rl}[\text{a-}& {\text{OVOC}}_j]={\text{ER}}_{{\text{OVOC}}_j}\times [\text{a-benzene}]\\ & \times \exp (-(k{\mathrm{OH}}_{{\text{OVOC}}_j}^{\ast }-k{\mathrm{OH}}_{\text{benzene}}\left)\right[\mathrm{OH}\left]\mathrm{\Delta }t\right)\\ & +{\text{ER}}_{\text{HC}}\times \left[\text{a-benzene}\right]\times {\displaystyle \frac{k{\mathrm{OH}}_{\text{HC}}}{k{\mathrm{OH}}_{{\text{OVOC}}_j}^{\ast }-k{\mathrm{OH}}_{\text{HC}}}}\\ & \times {\displaystyle \frac{\exp (-k{\mathrm{OH}}_{\text{HC}}[\mathrm{OH}\left]\mathrm{\Delta }t\right)-\exp (-k{\mathrm{OH}}_{{\text{OVOC}}_j}^{\ast }[\mathrm{OH}\left]\mathrm{\Delta }t\right)}{\exp (-k{\mathrm{OH}}_{\text{benzene}}[\mathrm{OH}\left]\mathrm{\Delta }t\right)}}\\ & +{\text{ER}}_{\text{biogenic}}\times \left[\text{e-isoprene}\right]\end{array}\text{(17)}& {\displaystyle }\begin{array}{rl}\left[{\text{e-OVOC}}_j\right]=& {\text{ER}}_{{\text{OVOC}}_j}\times \left[\text{a-benzene}\right]\\ & +{\text{ER}}_{\text{biogenic}}\times \left[\text{e-isoprene}\right],\end{array}\end{array}$$

where the measured concentrations of species j in OVOCs ([a-OVOC_j]) are equivalent to the sum of primary anthropogenic contributions, secondary anthropogenic contributions, and biogenic contributions, as represented sequentially in Eq. (16). Benzene is selected as the tracer of anthropogenic primary sources due to the dominance of combustion and industrial VOC emissions in the Sichuan Basin (Wu and Xie, 2017) and its relatively low OH reaction rate. ${\text{ER}}_{{\text{OVOC}}_j}$ and ER_HC are the emission ratios of species j in OVOCs and hydrocarbons to benzene, respectively. We assume that the ratios (R) for $J_{{\text{OVOC}}_j}$ to $\left[\mathrm{OH}\right]k{\mathrm{OH}}_{{\text{OVOC}}_j}$ established in the Los Angeles Basin (Fig. S9; de Gouw et al., 2018) are applicable for estimating $k{\mathrm{OH}}_{{\text{OVOC}}_j}^{\ast }=(\mathrm{1}+R)\times k{\mathrm{OH}}_{{\text{OVOC}}_j}$ in the Sichuan Basin. OH exposure is calculated using Eq. (5). ER_biogenic is the emission ratio between OVOCs and isoprene from biogenic sources. [e-isoprene] is estimated by Eqs. (9–12). The ${\text{ER}}_{{\text{OVOC}}_j}$, ER_HC, kOH_HC, and ER_biogenic values are determined using the nonlinear least-squares fit.

2.3 O₃ formation potential

The OFP is calculated using Eqs. (18) or (19):

$$\begin{array}{}\text{(18)}& {\displaystyle }\left[\text{OFP}\right]={\text{MIR}}_j\times \left[{\text{a-VOC}}_j\right]\text{(19)}& {\displaystyle }\left[\text{OFP}\right]={\text{MIR}}_j\times \left[{\text{e-VOC}}_j\right],\end{array}$$

where MIR_j, [a-VOC_j], and [e-VOC_j] indicate the MIR (Carter, 2010), the ambient concentration of species j in VOCs, and the emitted concentrations of species j in VOCs, respectively.

3 Results and discussion

Figure 1 shows the hourly ambient total VOC (TVOC) concentrations at rural Deyang, suburban Chengdu, and forest Meishan from August to September 2019. The average TVOC concentration is 37.63±15.34 ppbv (parts per billion by volume) at Deyang, which is higher than 33.42±13.96 ppbv at Chengdu and 25.17±8.46 ppbv at Meishan (Fig. 1).

Figure 1 Hourly concentrations of ambient VOCs at Deyang (a), Chengdu (b), and Meishan (c) from August to September 2019, respectively. The lack of data at rural Deyang from 30 August to 4 September 2019 is due to a power failure.

Due to the influences of sunlight, the consumption of emitted VOCs is classified into nighttime and daytime periods. During the night, emitted concentrations of alkenes and styrene are consumed through oxidation by NO₃ radicals or O₃ (de Gouw et al., 2017; Calvert et al., 2015; Finlayson-Pitts and Pitts, 2000). Because the reaction rates of NO₃ and O₃ are very low, mostly below $\mathrm{10}\times {\mathrm{10}}^{-\mathrm{21}}{\mathrm{cm}}^{\mathrm{3}}{\mathrm{molecule}}^{-\mathrm{1}}{\mathrm{s}}^{-\mathrm{1}}$ (Carter, 2010; Atkinson and Arey, 2003; Atkinson, 2000), the nighttime consumption of aromatics except for styrene, alkanes, OVOCs, halocarbons, acetonitrile, and acetylene is not considered. During the daytime, emitted NMHC concentrations are primarily consumed by OH. Emitted OVOC concentrations are consumed by both OH and photolysis during the daytime, after excluding secondary formation.

The largest difference between the emitted and ambient average concentrations of TVOCs is found at Deyang, with a difference of 2.92 ppbv (Table 1). This is primarily attributable to the high reactivities of alkenes and aromatics (Carter, 2010). The average emitted concentrations of alkenes are 6.19 ppbv at Deyang, 5.16 ppbv at Chengdu, and 5.81 ppbv at Meishan, all exceeding the respective ambient concentrations of 3.32, 2.96 and 3.89 ppbv (Table 1). The average emitted concentrations of aromatics are 3.96 ppbv at Deyang, 5.00 ppbv at Chengdu, and 2.15 ppbv at Meishan, again exceeding the respective ambient concentrations of 2.99, 4.04, and 1.92 ppbv (Table 1). Among the chemical groups and the three sites, the largest difference is 2.87 ppbv for alkenes at Deyang (Table 1). This is attributed to the higher emitted alkene concentrations around Deyang (Table 1) and, consequently, higher consumption of alkenes. Specifically, the largest differences are 1.04 ppbv for cis-2-butene at Deyang, 0.81 ppbv for isoprene at Chengdu, and 1.79 ppbv for isoprene at Meishan, respectively (Fig. 2, Tables S1 and S2). The reaction rates of isoprene with NO₃ and OH are higher than those of cis-2-butene, but the largest difference is observed for cis-2-butene at Deyang. This is possibly due to the higher emitted concentrations of cis-2-butene compared to those of isoprene (Fig. 2, Tables S1 and S2), which leads to greater consumption and thus larger differences for cis-2-butene at Deyang.

Table 1 The ambient and emitted concentrations of VOC groups from August to September 2019.

Figure 2 The ambient and emitted VOC concentrations at Deyang (a), Chengdu (b), and Meishan (c) from August to September 2019, respectively. Square dots and whiskers show the means and standard errors (SEs), respectively. The details are given in Table S1.

The differences between the emitted and ambient concentrations of aromatics are 0.97 ppbv at Deyang, 0.96 ppbv at Chengdu, and 0.23 ppbv at Meishan (Table 1). Overall, the emitted concentrations of aromatics are higher at Chengdu than at Deyang, which likely leads to higher aromatics consumption. However, the larger consumption of styrene, a reactive aromatic, at Deyang results in similar consumption of aromatics: 0.97 ppbv at Deyang and 0.96 ppbv at Chengdu (Table 1). Styrene exhibited the largest difference between emitted and ambient concentrations among all aromatics, with differences of 0.70 ppbv at Deyang, 0.38 ppbv at Chengdu, and 0.11 ppbv at Meishan, respectively (Fig. 2, Tables S1 and S2). This is due to both its high OH and NO₃ reaction rates. For aromatics, the second largest difference is 0.28 ppbv for m,p-xylene at Chengdu (Fig. 2, Tables S1 and S2). This is possibly due to the higher emissions of solvent use around Chengdu compared to the other two sites, which likely causes higher consumption and thus higher differences. Due to secondary formation, the average emitted OVOC concentrations are lower than the ambient concentrations by 1.32 ppbv at Deyang, 1.29 ppbv at Chengdu, and 1.31 ppbv at Meishan, respectively (Table 1). Specifically, the largest differences between emitted and ambient concentrations are observed for acetone, with differences of −0.54 ppbv at Deyang and −0.50 ppbv at Meishan, respectively (Fig. 2, Tables S1 and S2). Acetaldehyde displays the largest difference of −0.58 ppbv at Chengdu (Fig. 2, Tables S1 and S2). This might be due to the higher emissions of isoalkanes at Deyang. They could be converted into acetone (Jacob et al., 2002), which thus causes a larger negative difference. Due to the low reaction rates of OH, NO₃, and O₃ (Carter, 2010), the emitted and ambient concentrations of alkanes, acetylene, acetonitrile, and halocarbons are almost the same (Table 1). Overall, major differences between emitted and ambient concentrations are found for alkenes, aromatics, and OVOCs because of their high reactivities and secondary OVOC formation.

Figure 3 Ozone formation potential based on ambient and emitted VOC concentrations at Deyang, Chengdu, and Meishan, from August to September 2019, respectively. Values are the means ± standard errors.

Ambient alkane concentrations are the largest among different VOC chemical groups (Fig. 1). However, due to their relatively low reaction activities, both lower OFP values and smaller OFP differences between emitted and ambient concentrations are found (Fig. 3). There are significant differences in the OFP between emitted and ambient alkene concentrations. The top three chemical groups with the largest differences are alkenes, aromatics, and OVOCs at all three sites. However, the OFP rankings of the chemical groups differ between emitted and ambient concentrations. The top three VOC chemical groups contributing to the OFP at Deyang are OVOCs, aromatics, and alkenes when based on ambient concentrations, and alkenes, aromatics, and OVOCs based on emitted concentrations. The top three VOC chemical groups at Chengdu are aromatics, OVOCs, and alkenes when based on ambient concentrations and aromatics, alkenes, and OVOCs when based on emitted concentrations. The top three VOC chemical groups at Meishan are alkenes, OVOCs, and aromatics when based on ambient concentrations and alkenes, aromatics, and OVOCs when based on emitted concentrations. The OFP of alkenes based on emitted concentrations is the highest at Deyang, reaching 135.52 µg m⁻³. The OFP of alkenes based on emitted concentrations is higher than that of aromatics at both Deyang and Meishan but lower at Chengdu. This might be due to the high emissions of solvents around Chengdu, leading to high emitted concentrations of aromatics (Table 1) and thus the higher OFP compared to that of alkenes.

Although the OFP is primarily associated with daytime O₃ production, the nighttime OFP is also calculated to compare the potential differences in the contributions of VOCs and emitted VOCs to the OFP (Figs. S10 and S11). The temporal variation trends in the OFP values are generally consistent at the three sites based on emitted and ambient TVOC concentrations (Fig. S10a and b). The OFP based on emitted and ambient TVOC concentrations ranges from 65.60 to 1476.28 µg m⁻³ and from 55.86 to 827.30 µg m⁻³, respectively, at Deyang. Among VOC chemical groups, alkenes exhibit the greatest variations in the OFP, with ranges of 15.12 to 1081.41 µg m⁻³ and 7.87 to 282.78 µg m⁻³, respectively. The highest OFP based on the emitted TVOC concentrations was found at 16:00 LT on 11 August 2019, primarily due to the 1081.41 µg m⁻³ contribution from alkenes. At this time, the OFP based on the ambient TVOC concentrations is only 327.41 µg m⁻³. The ratio of the OFP between the emitted and ambient TVOC concentrations exceeds 4.5. This discrepancy is mainly because of the large consumption of alkenes at that time. If the consumption of alkenes is not considered, their contributions to the OFP would be greatly underestimated.

Table 2 Differences of top 10 VOC species contributing to the OFP based on ambient and emitted concentrations.

The OFP based on emitted and ambient TVOC concentrations ranges from 47.47 to 1143.74 µg m⁻³ and from 39.21 to 819.61 µg m⁻³, respectively, at Chengdu (Fig. S10c and d). Alkenes exhibit the greatest variations in the OFP among VOC chemical groups, with ranges from 12.34 to 780.85 µg m⁻³ and from 4.94 to 336.58 µg m⁻³, respectively. The highest OFP based on the emitted TVOC concentrations was observed at 12:00 LT on 30 August 2019, which is primarily due to the contributions of 500.64 µg m⁻³ from alkenes and 494.89 µg m⁻³ from aromatics. At this time, the OFP based on the ambient TVOC concentrations is 819.61 µg m⁻³. The OFP based on the emitted and ambient TVOC concentrations are similar, indicating that photochemical consumption is relatively low at Chengdu compared to Deyang. Furthermore, the OFP of the ambient OVOC concentrations is 171.94 µg m⁻³, which is higher than the OFP of emitted concentrations at 88.40 µg m⁻³. Compared to the other two sites, emissions from solvent use are higher around Chengdu. This leads to higher emissions of aromatics, which significantly contributes to the OFP.

The OFP based on emitted and ambient TVOC concentrations ranges from 59.30 to 1351.58 µg m⁻³ and from 48.43 to 1077.27 µg m⁻³, respectively, at Meishan (Fig. S10e and f). Alkenes display the greatest variations in the OFP, ranging from 13.98 to 1133.53 µg m⁻³ and from 7.62 to 864.52 µg m⁻³, respectively. The highest OFP of the emitted TVOC concentrations was found at 18:00 LT on 17 August 2019, due to the major contribution of 1133.53 µg m⁻³ from alkenes. Because of low photochemical consumption and secondary OVOC formation, the difference in the OFP between emitted and ambient TVOC concentrations was relatively low, at about 25 % at this time. Compared to the other two sites, there are more biogenic isoprene emissions around Meishan, which contributes to the OFP of alkenes.

The highest hourly OFP values of emitted TVOC concentrations are 391.07 µg m⁻³ at 12:00 LT at Deyang, 432.12 µg m⁻³ at 12:00 LT at Chengdu, and 403.80 µg m⁻³ at 18:00 LT at Meishan, respectively (Fig. S11). After considering the nighttime alkene consumption, the emitted alkene concentrations at night are close to those during the day at Deyang. Moreover, the MIR values of alkenes are generally higher across these VOC chemical groups (Carter, 2010). These may explain why the total OFP at night is similar to that during the day at Deyang. The diurnal variations in the OFP at Chengdu are consistent with those of sunlight intensity. The OFP at Meishan is primarily driven by isoprene. The surrounding bamboo forest acts as a source of biogenic emissions. The accumulation of isoprene causes the highest concentrations, and thus the highest OFP, occurring at 18:00 LT.

As shown in Table 2, 9 species rank in the top 10 VOCs contributing to the OFP among both ambient and emitted VOCs at Deyang, Chengdu, and Meishan. Although the number of species in common is high, their rankings differ. The importance of isoprene to the OFP increases notably from ambient to emitted VOC concentrations, rising from ninth to second at Deyang, rising from sixth to third at Chengdu, and remaining first with more than a 2-fold increase at Meishan. The species with the highest OFP based on emitted VOC concentrations is cis-2-butene, with the OFP value of 39.49 µg m⁻³ at Deyang, whereas this species only ranks tenth based on ambient concentrations. This indicates that not considering emitted concentrations could underestimate the importance of cis-2-butene for O₃ formation. At Chengdu, 5 of the top 10 OFP species based on emitted concentrations are aromatics, suggesting the need to control solvent usage. Although isoprene ranks first in its contribution to the OFP at Meishan based on both ambient and emitted VOC concentrations, its proportion increases due to relatively minor changes in the OFP contributions from other species. The top three species contributing to the OFP among emitted VOCs are cis-2-butene, isoprene, and m,p-xylene at Deyang; m,p-xylene, acetaldehyde, and isoprene at Chengdu; and isoprene, ethylene, and acetaldehyde at Meishan, respectively (Table 2). These results emphasize the importance of isoprene in O₃ formation. They differ from those based on ambient VOC concentrations (Table 2) and those reported at Chengdu and Deyang from 2016 to 2019, which often include three of m,p-xylene, ethylene, toluene, and acetaldehyde (Table S3; Wang et al., 2023; Kong et al., 2023; Chen et al., 2021; Xiong et al., 2021; Tan et al., 2020a, b; Deng et al., 2019; Song et al., 2018).

4 Conclusions

Using NO₃, O₃, and OH reaction rates along with hourly ambient concentrations of 99 VOCs measured at Deyang, Chengdu, and Meishan in southwest China, we calculate the emitted VOC concentrations during both nighttime and daytime in summer. These emitted VOC concentrations are compared with the ambient concentrations in terms of their OFP. Because the emitted VOC concentrations are directly linked to MIR values, this novel method substantially enhances the accurate identification of the key VOC species in O₃ formation. The emitted concentrations of alkenes and aromatics are significantly higher than the ambient concentrations. In contrast, because of the secondary production, the emitted OVOC concentrations are lower than the ambient ones. Based on the emitted VOC concentrations, the top three species contributing to the OFP are cis-2-butene, isoprene, and m,p-xylene at Deyang; m,p-xylene, acetaldehyde, and isoprene at Chengdu; and isoprene, ethylene, and acetaldehyde at Meishan. These results emphasize the importance of isoprene in O₃ formation and differ from those based on ambient concentrations. While many current environmental policies focus on reducing emissions of non-isoprene alkenes and aromatics, our study shows that isoprene emissions are also important. Therefore, the control of isoprene emissions should be considered in the mitigation of O₃ pollution. Our study provides new insights into improving the scientific understanding of the VOC emissions and their secondary transformations and serves as a reference for managing key VOC species in future control strategies.