3.2.1 OH reactivity

The OH reactivity of trace gases was categorized into SO₂, CO, O₃, NO_x, CH₄ and total NMVOCs, which were grouped into alkanes, alkenes, aromatics and OVOCs (Table S1 lists the NMVOCs included in each group), as shown in Fig. 4a and b. The total OH reactivity was between 9.2 and 69.6 s⁻¹, with an average of 27.5±9.7 s⁻¹ (± standard deviation). Statistically, the average total OH reactivity was much higher than those determined in Beijing (16.4 s⁻¹ and 20±11 s⁻¹) (Tan et al., 2019; Yang et al., 2017), Shanghai (13.5 s⁻¹) (Tan et al., 2019), Chongqing (17.8 s⁻¹) (Tan et al., 2019), Jinan (19.4±2.1 s⁻¹) (Lyu et al., 2019), Wangdu (10–20 s⁻¹) (Fuchs et al., 2017), Houston (9–22 s⁻¹) (Mao et al., 2010), London (18.1 s⁻¹) (Whalley et al., 2016) and Nashville (11.3±4.8 s⁻¹) (Kovacs et al., 2003) but was comparable to or lower than those in Heshan (31±20 s⁻¹) (Yang et al., 2017), Backgarden (mean maximum value of 50 s⁻¹) (Lou et al., 2010) and New York (25 s⁻¹) (Ren et al., 2006b). The OH reactivity towards SO₂, CO and NO_x was higher than the values reported in various Chinese cities (Xu et al., 2011; Zhu et al., 2020; Liu et al., 2009) (Table 1). It should be noted that the OH reactivity in this study was calculated from the sum of the products of measured species and their rate coefficients for reactions with OH and does not involve species that were not measured, such as monoterpenes and alcohols. Previous studies have shown that there are some discrepancies between the actual measured values and the calculated values of OH reactivity, which may be attributed to missing OH reactivity that originates from VOC oxidation products of both biogenic and anthropogenic origin (Di Carlo et al., 2004; Dolgorouky et al., 2012; Yoshino et al., 2006; Zhu et al., 2020). Therefore, the OH reactivity calculated in this study is somewhat underestimated.

The total OH reactivity was mainly contributed by NO_x (12.0±7.1 s⁻¹, 43.7 %), followed by NMVOCs (7.9±4.8 s⁻¹, 28.5 %), CO (7.2±2.6 s⁻¹, 26.0 %) and CH₄ (0.3±0.1 s⁻¹, 1.3 %) and to a lesser extent by SO₂ and O₃ (0.2±0.1 s⁻¹, 0.6 %), indicating the strong influence of anthropogenic emissions in Xianghe. The majority of total OH reactivity values were below 30 s⁻¹, as seen in the frequency distribution, which was dominated by the sum of low-OH-reactivity contributions and less influenced by single compounds with high OH reactivity (Fig. S2a–f). Specifically, the cumulative frequency distribution (Fig. S3a) clearly showed that the OH reactivity at values >40 s⁻¹ was dominated entirely by OH reactivity towards NO_x, and the OH reactivity at values from 20–40 s⁻¹ was nearly completely dominated by OH reactivity towards NO_x and total NMVOCs. In general, the frequency distributions and cumulative frequency distributions of OH reactivity highlighted the necessity of considering a large number of species to obtain a better understanding of OH reactivity.

The OH reactivity towards total NMVOCs was 7.9±4.8 s⁻¹, which was much lower than those in Beijing (11.2 s⁻¹) and Heshan (18.3 s⁻¹) (Yang et al., 2017) due to the higher content of reactive hydrocarbons (e.g., alkenes and aromatics) in Beijing and Heshan and due to the unmeasured species (e.g., acetaldehyde) in this study. Alkenes (3.4±3.7 s⁻¹, 42.9 %) dominated over OVOCs (2.4±1.5 s⁻¹, 30.2 %), aromatics (1.5±1.7 s⁻¹, 18.6 %) and alkanes (0.7±0.5 s⁻¹, 8.3 %) in the OH reactivity towards total NMVOCs. The majority of the values of OH reactivity towards total NMVOCs were below 13 s⁻¹ (Fig. S4a–d). The cumulative frequency distribution showed that the OH reactivity towards total NMVOCs at values of >6 s⁻¹ was dominated by OH reactivity towards alkenes, aromatics and OVOCs and that the OH reactivity towards total NMVOCs at values of <6 s⁻¹ was dominated by OH reactivity towards alkanes (Fig. S5). Alkanes accounted for >50 % of the mixing ratio of NMVOCs but only 8.3 % of the OH reactivity towards total NMVOCs. In contrast, aromatics, alkenes and OVOCs accounted for 44.6 % of the mixing ratio of NMVOCs, providing 91.7 % of the OH reactivity towards total NMVOCs. Significantly, isoprene accounted for only 4 % of the mixing ratio of NMVOCs but provided 31.2 % of the OH reactivity towards total NMVOCs. This result was explained by (1) the relatively low concentration of aromatics, alkenes, and OVOCs measured during the campaign; (2) the relatively high concentration of isoprene; and (3) the generally large isoprene reaction rate coefficient with OH ($\mathrm{101}\times {\mathrm{10}}^{-\mathrm{12}}$ ${\mathrm{cm}}^{\mathrm{3}}{\mathrm{molecule}}^{-\mathrm{1}}{\mathrm{s}}^{-\mathrm{1}}$) (Atkinson et al., 2006). The top 10 species, in terms of OH reactivity towards total NMVOCs, consisted of isoprene, HCHO, m/p-xylene, ethylene, hexanal, o-xylene, propylene, styrene, MACR and cis-2-butene (Fig. 3b). These species contributed only 39.1 % to NMVOC emissions but accounted for 80.3 % of OH reactivity towards total NMVOCs. As shown in Table 1, the OH reactivity towards the speciated NMVOCs in this study was basically within the values reported in various Chinese cities (Tan et al., 2019; Xu et al., 2011; Yang et al., 2017; Zhu et al., 2020).

Table 1Comparison of speciated OH reactivity with former studies in China.

^a Xu et al. (2011). ^b Yang et al. (2017). ^c Tan et al. (2019). ^d Liu et al. (2009). ^e Zhu et al. (2020).

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Figure 5Mean diurnal variations in (a–f) OH reactivity, (g–k) NO₃ reactivity and (l–o) O₃ reactivity of trace gases during the field campaign at Xianghe from 6 July to 6 August 2018.

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Figure 6Mean diurnal variations in OH reactivity (a–d), NO₃ reactivity (e–h) and O₃ reactivity (i–l) of NMVOC groups during the field campaign at Xianghe from 6 July to 6 August 2018.

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The mean diurnal profiles of the OH reactivity of trace gases and NMVOCs are presented in Figs. 5a–f and 6a–d, respectively. In general, the total OH reactivity was the lowest in the afternoon and the highest during rush hours, reaching a maximum of 33.0 s⁻¹ during the morning rush hour and a nighttime peak of 30.5 s⁻¹ (Fig. 5a). Most campaigns have also reported slightly higher OH reactivity in the morning traffic rush hour, which can be explained by higher levels of reactive gases such as NO and NMVOCs due to heavy traffic, as well as slower reactions (Fuchs et al., 2017; Yang et al., 2016). A similar diurnal profile was also observed for contributions from NO_x, CO, alkane and aromatic species, which are typically connected to emissions from anthropogenic activities. The shape of the total OH reactivity diurnal pattern was slightly shifted in the direction of OH reactivity towards NO_x, strengthening the idea that the local pollution in Xianghe was possibly impacted by traffic emissions. However, a different diurnal behavior to that of the above species was observed for alkenes (Fig. 6b) and OVOCs (Fig. 6d), which are emitted by plants or produced photochemically. The OH reactivity from OVOCs increased by a factor of approximately 2 from nighttime to daytime, suggesting that during the daytime, dilution or chemical removal had a weaker influence on the observed OVOCs than fresh production by photochemistry. The opposite diurnal variation was reported in Wangdu, which showed a weak diurnal variation with a decrease by a factor of approximately 2 from the morning to the evening (Fuchs et al., 2017). The diurnal profile of OH reactivity towards isoprene appears to be the major driver for the diurnal profile of OH reactivity towards alkenes. Biogenic isoprene is dependent on temperature and light intensity (Chang et al., 2014), and anthropogenic isoprene is predominantly emitted by road traffic (Ye et al., 1997); hence, the OH reactivity from alkenes increased during the daytime, with a morning peak of 4.1 s⁻¹ at 09:00 h and a nighttime peak of 7.4 s⁻¹ at 18:00 h. Many rainforest campaigns have also reported a significant diurnal pattern with higher OH reactivity from alkenes and OVOCs at noontime or a maximum at the beginning of the night (Yang et al., 2016). Notably, the large amplitude of the standard deviation bars highlighted the large diel variability.

3.2.2 NO₃ reactivity

The NO₃ reactivity of trace gases was categorized into SO₂, NO_x, CH₄ and NMVOCs, as shown in Fig. 4c and d. The campaign-averaged values of total NO₃ reactivity were 2.2±2.6 s⁻¹, ranging from 0.7 to 27.5 s⁻¹. The average total NO₃ reactivity was much higher than those determined during the IBAIRN campaign (Liebmann et al., 2018a) and at a rural mountain site (988 m a.s.l.) in southern Germany in 2017 (Liebmann et al., 2018b) due to higher contributions from NO_x. We noted that NO_x was by far the main contributor to the total NO₃ reactivity, representing 99 % of the total NO₃ reactivity on average. NO exhibited the most prominent contribution to the total NO₃ reactivity and represented an average of 78.0 % of the total NO₃ reactivity. In contrast to NO, NO₂ had a maximum contribution during the nighttime and represented, on average, 27 % of the total NO₃ reactivity. The NO₃ reactivity towards CH₄, NMVOCs and SO₂ was very minor, accounting for no more than 1 % of the total NO₃ reactivity over the whole campaign. The majority of the total NO₃ reactivity values were below 3 s⁻¹, but values below $\mathrm{5.5}\times {\mathrm{10}}^{-\mathrm{5}}$, 0.1, 3 and $\mathrm{1}\times {\mathrm{10}}^{-\mathrm{8}}$ s⁻¹ were observed for NO₃ reactivity towards CH₄, total NMVOCs, NO_x and SO₂, respectively, as seen in the frequency distribution (Fig. S2g–k). The cumulative frequency distribution clearly showed that the total NO₃ reactivity at low and high values was entirely dominated by NO₃ reactivity towards SO₂ and NO_x, respectively (Fig. S3b). In total, the frequency distributions and cumulative frequency distributions of NO₃ reactivity highlighted the necessity of considering a large number of species to obtain a complete picture of NO₃ reactivity.

The NO₃ reactivity towards total NMVOCs was $\mathrm{2.4}\pm \mathrm{3.0}\times {\mathrm{10}}^{-\mathrm{2}}$ s⁻¹ on average, with a minimum of $\mathrm{1.1}\times {\mathrm{10}}^{-\mathrm{3}}$ s⁻¹ and a maximum of 0.3 s⁻¹. The largest fraction of attributed NO₃ reactivity towards total NMVOCs was provided by alkenes (77.8 %), followed by aromatics (20.7 %) and OVOCs (1.3 %). The measured alkanes played virtually no role in NO₃ reactivity towards total NMVOCs, although they accounted for more than 50 % of the mixing ratio of NMVOCs. This result can be largely explained by the fact that the reaction rate coefficients of alkenes, aromatics and OVOCs with NO₃ are 1–5 orders of magnitude higher than the alkane reaction rate coefficients with NO₃ (Atkinson and Arey, 2003; Atkinson et al., 2006). The majority of the NO₃ reactivity values towards alkanes, alkenes, aromatics and OVOCs were below $\mathrm{5.0}\times {\mathrm{10}}^{-\mathrm{5}}$, 0.1, $\mathrm{1.0}\times {\mathrm{10}}^{-\mathrm{2}}$ and $\mathrm{1.0}\times {\mathrm{10}}^{-\mathrm{3}}$ s⁻¹, respectively (Fig. S4e–f). The cumulative frequency distribution showed that the NO₃ reactivity towards total NMVOCs at values of >0.1 s⁻¹ was entirely dominated by NO₃ reactivity towards alkenes, the NO₃ reactivity towards total NMVOCs at values from 0.01 to 0.1 s⁻¹ was dominated by NO₃ reactivity towards alkenes and aromatics, and the NO₃ reactivity towards total NMVOCs at values of $<\mathrm{1.0}\times {\mathrm{10}}^{-\mathrm{5}}$ s⁻¹ was entirely dominated by NO₃ reactivity towards alkanes (Fig. S6). The top 10 species in terms of NO₃ reactivity towards total NMVOCs consisted of isoprene, styrene, cis-2-butene, trans-2-butene, cis-2-pentene, hexanal, HCHO, propylene, 1,3-butadiene and trans-2-pentene (Fig. 3c). These species contributed only 27.7 % to NMVOC emissions but accounted for 99.2 % of the NO₃ reactivity towards total NMVOCs.

Total NO₃ reactivity displayed a weak diel variation, with a campaign-averaged morning peak value of 4.0 s⁻¹ at 06:00–07:00 h (Fig. 5g). The diurnal profile of NO₃ reactivity towards NO_x (Fig. 5i) appears to be the major driver for the diurnal profile of total NO₃ reactivity. The morning peak value of total NO₃ reactivity could be explained by the accumulation of NO_x due to traffic emissions that are released into the shallow nocturnal boundary layer during the morning rush hours. In contrast, the average diurnal profile of NO₃ reactivity towards total NMVOCs (Fig. 5k) had a maximum at 18:00 h, which was slightly shifted in the direction of NO₃ reactivity towards alkenes (Fig. 6j). The evening peak value of NO₃ reactivity towards total NMVOCs could be accounted for by the accumulation of alkenes due to vegetation emissions and traffic emissions that are released into the shallow nocturnal boundary layer. NO₃ reactivity towards alkanes (Fig. 6e), alkenes (Fig. 6f), aromatics (Fig. 6g), OVOCs (Fig. 6h) and SO₂ (Fig. 5h) played virtually no role in the diurnal variations in total NO₃ reactivity and NO₃ reactivity towards total NMVOCs but exhibited a more distinct diurnal profile.

3.2.3 O₃ reactivity

The O₃ reactivity of trace gases was categorized into NO_x, CH₄ and total NMVOCs, as shown in Fig. 4e and f. The total O₃ reactivity at the site varied between a minimum of $\mathrm{3.3}\times {\mathrm{10}}^{-\mathrm{4}}$ s⁻¹ and a maximum of $\mathrm{1.8}\times {\mathrm{10}}^{-\mathrm{2}}$ s⁻¹ and was $\mathrm{1.2}\pm \mathrm{1.7}\times {\mathrm{10}}^{-\mathrm{3}}$ s⁻¹ on average. NO exhibited the most prominent contribution to the total O₃ reactivity and represented >99 % of the total O₃ reactivity on average, whereas nearly all other contributions were <1 %. This result can be largely accounted for by the generally large NO reaction rate coefficients with O₃ ($\mathrm{1.8}\times {\mathrm{10}}^{-\mathrm{14}}$ ${\mathrm{cm}}^{\mathrm{3}}{\mathrm{molecule}}^{-\mathrm{1}}{\mathrm{s}}^{-\mathrm{1}}$) (Atkinson et al., 2006), which are several orders of magnitude higher than the reaction rate coefficients of NO₂, alkanes, alkenes, aromatics and OVOCs with NO₃ (Atkinson et al., 2006; Atkinson and Arey, 2003; Yuan et al., 2013; Ferracci et al., 2018; Jenkin et al., 2015). The majority of the total O₃ reactivity values were below $\mathrm{2}\times {\mathrm{10}}^{-\mathrm{3}}$ s⁻¹, but values below $\mathrm{5.5}\times {\mathrm{10}}^{-\mathrm{10}}$, $\mathrm{2}\times {\mathrm{10}}^{-\mathrm{6}}$ and $\mathrm{2}\times {\mathrm{10}}^{-\mathrm{3}}$ s⁻¹ were observed for the O₃ reactivity towards CH₄, total NMVOCs and NO_x, respectively, as seen in the frequency distribution (Fig. S2l–o). The cumulative frequency distribution clearly showed that the total O₃ reactivity at low and high values was entirely dominated by O₃ reactivity towards CH₄ and NO_x, respectively (Fig. S3c). Generally, the frequency distributions and cumulative frequency distributions of O₃ reactivity highlight the necessity of considering a large number of species to obtain a complete picture of O₃ reactivity.

The O₃ reactivity towards total NMVOCs was $\mathrm{1.1}\pm \mathrm{0.8}\times {\mathrm{10}}^{-\mathrm{6}}$ s⁻¹ on average, ranging from a minimum of $\mathrm{2.5}\times {\mathrm{10}}^{-\mathrm{7}}$ s⁻¹ to a maximum of $\mathrm{1.0}\times {\mathrm{10}}^{-\mathrm{5}}$ s⁻¹. Alkenes clearly dominated the O₃ reactivity towards total NMVOCs, with a campaign-averaged contribution of 94.0 %. Aromatics were the second largest contributor, comprising an average of 5.2 % of the O₃ reactivity towards total NMVOCs. In comparison, OVOCs accounted for only 0.8 % of the O₃ reactivity towards total NMVOCs. In contrast, the measured alkanes played nearly no role in the O₃ reactivity towards total NMVOCs due to their small reaction rate coefficients with O₃ ($<\mathrm{1.0}\times {\mathrm{10}}^{-\mathrm{23}}$ ${\mathrm{cm}}^{\mathrm{3}}{\mathrm{molecule}}^{-\mathrm{1}}{\mathrm{s}}^{-\mathrm{1}}$) (Atkinson and Arey, 2003; Atkinson et al., 2006). The majority of the O₃ reactivity values towards alkanes, alkenes, aromatics and OVOCs were below $\mathrm{5.0}\times {\mathrm{10}}^{-\mathrm{12}}$, $\mathrm{3.0}\times {\mathrm{10}}^{-\mathrm{6}}$, $\mathrm{2.0}\times {\mathrm{10}}^{-\mathrm{7}}$ and $\mathrm{2.0}\times {\mathrm{10}}^{-\mathrm{8}}$ s⁻¹, respectively (Fig. S4i–l). The cumulative frequency distribution (Fig. S7) clearly showed that the O₃ reactivity towards total NMVOCs at $>\mathrm{1.0}\times {\mathrm{10}}^{-\mathrm{7}}$ s⁻¹ was dominated by O₃ reactivity towards alkenes and aromatics; the O₃ reactivity towards total NMVOCs between $\mathrm{1.0}\times {\mathrm{10}}^{-\mathrm{9}}$ and $\mathrm{1.0}\times {\mathrm{10}}^{-\mathrm{7}}$ s⁻¹ was dominated by O₃ reactivity towards alkenes, aromatics and OVOCs, and the O₃ reactivity towards NMVOCs $<\mathrm{1.0}\times {\mathrm{10}}^{-\mathrm{11}}$ s⁻¹ was entirely dominated by O₃ reactivity towards alkanes. In terms of individual species, isoprene, cis-2-butene, trans-2-butene, cis-2-pentene, propylene, styrene, ethylene, 1-butene, trans-2-pentene and 1-pentene were the top 10 species (Fig. 3d), accounting for 28 %, 25 %, 20 %, 8 %, 7 %, 5 %, 5 %, 3 %, 2 % and 1 %, respectively, of the O₃ reactivity towards total NMVOCs and 3.1 %, 0.3 %, 0.1 %, 0.1 %, 1 %, 0.4 %, 4.1 %, 0.4 %, 0.1 % and 0.1 %, respectively, of the total NMVOC emissions.

Compared with the OH and NO₃ reactivities, O₃ reactivity displayed a much weaker diel variation, especially the O₃ reactivity towards alkenes and aromatics, as shown in Figs. 5 and 6. This weakness can be explained by the following reasons. First, for a given species, the reaction rate coefficients with O₃ were much smaller than the corresponding reaction rate coefficients with OH and NO₃. For example, the ethylene reaction rate coefficients with OH ($\mathrm{8.52}\times {\mathrm{10}}^{-\mathrm{12}}$ ${\mathrm{cm}}^{\mathrm{3}}{\mathrm{molecule}}^{-\mathrm{1}}{\mathrm{s}}^{-\mathrm{1}}$) and NO₃ ($\mathrm{2.05}\times {\mathrm{10}}^{-\mathrm{16}}$ ${\mathrm{cm}}^{\mathrm{3}}{\mathrm{molecule}}^{-\mathrm{1}}{\mathrm{s}}^{-\mathrm{1}}$) are 6 and 2 orders of magnitude higher, respectively, than the ethylene reaction rate coefficient with O₃ ($\mathrm{1.59}\times {\mathrm{10}}^{-\mathrm{18}}$ ${\mathrm{cm}}^{\mathrm{3}}{\mathrm{molecule}}^{-\mathrm{1}}{\mathrm{s}}^{-\mathrm{1}}$) (Atkinson and Arey, 2003; Atkinson et al., 2006). Second, the high-emission species reaction rate coefficients with O₃ are smaller than the low-emission species reaction rate coefficients with O₃. For instance, the m/p-xylene (one of the top five species in terms of emissions) reaction rate coefficient with O₃ ($<\mathrm{1.0}\times {\mathrm{10}}^{-\mathrm{20}}$ ${\mathrm{cm}}^{\mathrm{3}}{\mathrm{molecule}}^{-\mathrm{1}}{\mathrm{s}}^{-\mathrm{1}}$) are much smaller than the 1-hexene (one of the bottom five emissions species) reaction rate coefficients with O₃ ($\mathrm{1.13}\times {\mathrm{10}}^{-\mathrm{17}}$ ${\mathrm{cm}}^{\mathrm{3}}{\mathrm{molecule}}^{-\mathrm{1}}{\mathrm{s}}^{-\mathrm{1}}$) (Atkinson and Arey, 2003; Atkinson et al., 2006). The above two factors largely weaken the diurnal variation in O₃ reactivity.

Table 2The top 10 NMVOC species in terms of concentration (ppb, first column), OH reactivity (s⁻¹, second column), NO₃ reactivity (s⁻¹, third column), and O₃ reactivity (s⁻¹, fourth column) and their corresponding contributions to concentration, OH, NO₃, and O₃ reactivity towards NMVOCs (%). Bold values denote the top 10 NMVOC species in terms of concentration, OH reactivity, NO₃ reactivity and O₃ reactivity.

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3.4.1 Modeling OVOCs, OH, HO₂, RO₂ and NO₃ by SOSAA

With the appropriate setup of the condensation sinks for the 10 calculated OVOCs (ACR, C₂H₅CHO, MACR, C₃H₇CHO, MVK, MEK, MPRK, C₄H₉CHO, DIEK and C₅H₁₁CHO), the modeled diurnal mean pattern generally followed the measured pattern within 1 standard deviation of the measurement data, although the model underestimated measurements, predicting values of less than 1 ppb from 19:00 to 24:00 h (Fig. S8a). With the inclusion of input MTBE (methyl tert-butyl ether) and CH₃COCH₃ (acetone), which constituted more than 50 % of the total OVOCs, the modeled total OVOC concentration agreed better with the measurements than expected (Fig. S8b). The modeled diurnal median number concentrations of OH, HO₂ and RO₂ showed an apparent diurnal pattern with peaks during midday and values approaching zero during night, which resulted from the dependence of their chemical production reactions on incoming solar radiation (Fig. S9a, b and c). The midday time (12:00–16:00 h) median number concentrations of OH, HO₂ and RO₂ were 1.2×10⁷, 5.9×10⁸ and 3.7×10⁸ molecules cm⁻³, respectively, which were comparable to previous studies (Tan et al., 2017). The diurnal variation of in the hourly median NO₃ concentration showed two peaks which were consistent with the high chemical production from NO₂+O₃ (Fig. S9d). Figure S10 shows the relationship between the modeled OH number concentration and the measured JO¹D. The coefficient of determination (R²) was 0.86, and the linear regression fit showed that the slope was 6.1×10¹¹ ${\mathrm{cm}}^{-\mathrm{3}}{\mathrm{s}}^{-\mathrm{1}}$ and the intercept was 0.9×10⁶ cm⁻³. These values were comparable to Tan et al. (2017), except that the slope here was approximately 36 % higher than the observed fit in Tan et al. (2017).

3.4.2 Overall characteristics of AOC

The loss rates of NMVOCs, CH₄ and CO via reactions with OH, O₃ and NO₃ were calculated. The calculated AOC was up to 4.5×10⁸ $\mathrm{molecules}{\mathrm{cm}}^{-\mathrm{3}}{\mathrm{s}}^{-\mathrm{1}}$, with a campaign-averaged value of 7.8×10⁷ $\mathrm{molecules}{\mathrm{cm}}^{-\mathrm{3}}{\mathrm{s}}^{-\mathrm{1}}$, daytime average (06:00–18:00 h) of 1.4×10⁸ $\mathrm{molecules}{\mathrm{cm}}^{-\mathrm{3}}{\mathrm{s}}^{-\mathrm{1}}$ and nighttime average of 6.7×10⁶ $\mathrm{molecules}{\mathrm{cm}}^{-\mathrm{3}}{\mathrm{s}}^{-\mathrm{1}}$. As such, the total number of NMVOC, CH₄ and CO molecules depleted during the daytime and nighttime were, respectively, 6.0×10¹² and 2.9×10¹¹ per cm⁻³ of air. These AOC levels were higher than those determined at the Tung Chung air quality monitoring station (Xue et al., 2016), a polluted area in Santiago, Chile (Elshorbany et al., 2009), and Hong Kong Polytechnic University's air monitoring station at Hok Tsui (Li et al., 2018).

Figure 7Comparisons of calculated AOC by (a) modeled OH, (b) measured O₃ and (c) modeled NO₃ along with corresponding oxidation concentrations. Panels (a)–(c) show the time series, and (d)–(f) show scatterplots of calculated AOC and corresponding oxidation concentrations. Note: r and p are the correlation coefficient and the significance level, respectively.

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Figure 8Comparison of the relative contributions of OH, NO₃ and O₃ to the 24 h, daytime and nighttime-averaged loss rates. Data are calculated for the loss rates of (a, d, g) NMVOCs, CH₄ and CO; (b, e, h) NMVOCs only; and (c, f, i) unsaturated NMVOCs only.

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Comparisons of the AOC values calculated from OH, O₃, and NO₃ and the corresponding oxidation concentrations are shown in Fig. 7. The OH and NO₃ radical concentrations were simulated by the SOSAA box model. The AOC calculated from OH, O₃ and NO₃ correlated well with the corresponding oxidation concentrations, with correlation coefficients (r) of 0.91, 0.83 and 0.57, respectively, suggesting that the parameterized AOC here was consistent with that obtained using radical concentration to indicate AOC. Specifically, the average oxidation capacities of OH, O₃ and NO₃ radicals throughout the entire campaign were 7.7×10⁷, 1.2×10⁶ and 1.8×10⁵ $\mathrm{molecules}{\mathrm{cm}}^{-\mathrm{3}}{\mathrm{s}}^{-\mathrm{1}}$, representing 98.2 %, 1.5 % and 0.3 % of the total oxidation capacity, respectively. The total number of depleted molecules per day due to oxidation by OH, O₃ and NO₃ was 6.6×10¹², 1.0×10¹¹ and 1.5×10¹⁰ molecules cm⁻³, respectively; these values were slightly higher than those assessed in a polluted area in Santiago, Chile (Elshorbany et al., 2009). Accordingly, OH radicals are the driving force of AOC in Xianghe, especially during the daytime. Figure 8 shows a comparison of the oxidation capacities of OH, O₃ and NO₃. On average, the relative contribution of O₃ and NO₃ oxidation capacities when integrated over 24 h was less than 4 % (Fig. 8a–c). OH is the only oxidant of CO in the troposphere. As expected, OH was responsible for 99 % of the oxidation capacity regarding NMVOCs, CH₄ and CO during the daytime (Fig. 8d). The relative contribution of OH to oxidation capacity decreased to 98 % when restricting the calculation to NMVOC families alone (Fig. 8e). Focusing on the oxidation of unsaturated NMVOCs, OH was the dominant oxidant with a relative proportion of approximately 97 % (Fig. 8f). Note that the influence of NO₃ and O₃ on the oxidation of CO and VOCs can be neglected during the daytime. However, elevated relative contributions of O₃ and NO₃ to oxidation capacity can be observed during the nighttime. As expected, O₃ and NO₃ accounted for 10 % and 2 %, respectively, of the oxidation capacity with respect to NMVOCs, CH₄ and CO (Fig. 9g), but 19 % and 3 % of NMVOC families alone (Fig. 8h) occurred at night. Focusing on the oxidation of unsaturated NMVOCs, O₃ and NO₃ accounted for 20 % and 4 %, respectively, of the oxidation capacity (Fig. 8i). This quantitative intercomparison of the oxidation capacities of OH, O₃ and NO₃ confirms the important role of OH in the degradation of NMVOCs, CH₄ and CO. Compared with OH and O₃, NO₃ had a lower contribution during both the daytime and nighttime, which was mainly caused by high NO concentrations (Liebmann et al., 2018b).