Observations and explicit modeling of isoprene chemical 1 processing in polluted air masses in rural areas of the Yangtze 2 River Delta region : radical cycling and formation of ozone and 3 formaldehyde 4 5

15 Ozone pollution has become one of the most severe environmental problems in China in 16 recent years. Our online observations showed that high level of O3 were observed in rural 17 areas of the Yangtze River Delta (YRD) region even there was no obvious ozone transport 18 from the urban regions. To better understand the formation mechanism of local O3 pollution 19 and investigate the potential role of isoprene chemistry in the budgets of ROx (OH+HO2+RO2) 20 radicals, synchronous observations of volatile organic compounds (VOCs), formaldehyde 21 (HCHO) and meteorological parameters were conducted at a rural site of the YRD region in 22 2018. Five episodes with elevated O3 concentrations under stagnant meteorological 23 conditions were first identified; an observation-based model (OBM) with the Master 24 Chemical Mechanism was applied to analyze the photochemical processes in these high-O3 25 https://doi.org/10.5194/acp-2020-728 Preprint. Discussion started: 21 August 2020 c © Author(s) 2020. CC BY 4.0 License.

one of the most active species, and also the most abundant biogenic VOCs (BVOCs) species 51 globally (Wennberg et al., 2018). Isoprene emissions from biogenic sources have been 52 extensively studied over past decades (Gong et al., 2018) and recent works have switched 53 from emissions to the degradation pathways and the impact of isoprene chemistry on regional 54 forest chemistry (Gong et al., 2018;Wolfe et al., 2016a). Previous studies showed that 55 isoprene could be quickly oxidized by atmospheric oxidants (e.g. OH, O3 or NO3) (Wolfe et 56 al., 2016a;Gong et al., 2018;Jenkin et al., 2015). Due to the rapid reaction between OH and 57 isoprene (100× 10 -12 cm 3 molecule -1 s -1 at 298 K), more than 90% of the total daytime 58 isoprene is removed via this reaction (Wennberg et al., 2018). The reaction between OH and 59 isoprene is initiated by the addition of OH and can generate isoprene hydroxyperoxy radicals 60 (ISOPO2) (Wennberg et al., 2018;D'Ambro et al., 2017;Liu et al., 2013;Jenkin et al., 2015).

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ISOPO2 isomers could then interconvert rapidly due to reversible O2 addition and are finally 62 removed via reactions with HO2 or NO (Jenkin et al., 2015;Wolfe et al., 2016a). Hence, the 63 degradation process of isoprene is tightly associated with ROx recycling. According to He et 64 al. (2019), isoprene chemistry could strongly influence the photochemical formation of O3, photochemical processes (Tan et al., 2019;Zhu et al., 2020b). Previous studies have pointed 76 out that high level of O3 at suburban areas of Shanghai could be attributed to the transport of 77 O3 or O3 precursors from urban areas (Lin et al., 2020;Zhang et al., 2019). However, high O3 78 concentration was frequently observed in suburban areas under stable meteorological 79 conditions. Given the high vegetation coverage in rural YRD and weak transport of air 80 masses, the importance of local isoprene chemistry to ozone formation remains unclear.

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In this study, we conducted a comprehensive set of in-situ observations of isoprene,  Before each simulation, the model will run 3 days as spin up to reach a steady state for 146 unmeasured species (e.g., OH and NO3 radicals). The comparison of simulated observed O3 147 concentrations can is shown in Figure S1 (Supplemental Information parallel scenarios (S0 and S1) were conducted with isoprene chemistry disabled in S1. In 153 both cases, identical chemical mechanism and meteorological conditions were used to drive 154 model simulations. Through a comparative analysis of the scenarios, the impact of isoprene 155 chemistry on AOC can be obtained.    suggesting that the nighttime chemistry in the suburban site was also very important. were not considered in this model.  (Xue et al., 2016b;Liu et al., 2012). In addition, RONO2 and RO2NO2 could in

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The daytime (6:00-18:00) recycling of ROx is shown in Figure 6,with primary sources 248 (in red) and sinks (in blue) of ROx. In the recycling of ROx, the production of OH was 249 dominated by the reaction of HO2+NO (9.43 ppbv h -1 ). As for RO2, it was produced by the 250 reaction of OH with OVOC (2.56 ppbv h -1 ), alkyl (RH) (1.51 ppbv h -1 ), and peroxides (0.14 251 ppbv h -1 ). The reaction of RO2+NO can result in strong production of RO (4.36 ppbv h -1 ).

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The reaction of RO and O2 was the major contributor to HO2 production, followed by the  suggesting that O3 could efficiently accumulate during daytime. The net production of O3 283 (P(O3)-L(O3)) is also shown in Figure 7. The maximum O3 concentration was found at

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As aforementioned, high levels of HCHO was observed at DSL, a rural site over the 291 Yangtze River Delta region. Figure 8 (A) shows the production and loss pathways of HCHO OVOC+OH only contributed minor amount of the total production rate during whole day.

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As for HCHO depletion, the photolysis of HCHO and the reaction of HCHO+OH was 314 the two dominate pathway, accounting for ~50% and ~50% of the total depletion rate, 315 respectively. The net HCHO (equals to P(HCHO) + L(HCHO)) production rate was also 316 shown in Figure 8. After sunrise, the net production rate of HCHO raised gradually until 8:00, The above analysis indicates that the photolysis of OVOC, HCHO, O3 and HONO was 326 the primary source of ROx, which offers high oxidizing environment for the degradation of  As aforementioned, the degradation of isoprene is tightly related to the cycling of ROx.

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To roughly explain the impact of isoprene chemistry on ROx budget, we carried out a parallel 337 simulation (S1) where isoprene chemistry is disabled (see in Figure 9). The diurnal variation 338 of OH, HO2, RO2 and NO3 in S1 is also shown in Figure 4 (B). Most of the reaction rates 339 show a decrease trend in S1, suggesting that the absence of isoprene slows down the ROx ROx. However, without isoprene, the photolysis rate of OVOC decreased by 0.71 ppbv h -1 .

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The total production and depletion rate of OH dropped to 6.22 and 5.53 ppbv h -1 , respectively.

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Although the absence of isoprene could reduce the consumption of OH, the OH concentration 344 would be reduced by ~16% compared to S0, suggesting that the amount of OH produced via 345 isoprene chemistry is large enough to compensate for the shift from OH to peroxy radicals in 346 the ROx family. As for RO2, the daytime production and destruction rate falls to 1.87 and 347 3.01 ppbv h -1 , respectively. This means the concentration of RO2 would be in a stage of 348 gradual decline. In addition, the absence of isoprene could also reduce RO2 concentration by 349 ~20%, suggesting that isoprene was an important source of RO2 at DSL site. As for HO2, 350 drastic decrease of ~53% was found in S1. The above-mentioned decrease in ROx obviously Isoprene is an important precursor of O3. To investigate the detailed impact of isoprene 366 on O3 formation, the production and loss pathways of O3 in S1 was also calculated (see 367 Figure 7 (B)). The simulated daily average level of O3 dropped to 39.31 ppbv in S1, which is 368 ~36% lower than that in S0. In addition, the maximum O3 concentration falls to 80.06 ppbv 369 in S1, which is ~45.95 ppbv lower than that in S0. Comparisons of S1 and S0 show that the  production, the major reactions that dominate the formation and depletion of HCHO in S1 384 were also analyzed by OBM model (see Figure 8 (B)). Comparison of S0 and S1 shows that from isoprene oxidation play an important role in the photochemical production of HCHO.

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To investigate the role of isoprene in ROx recycle and the formation of secondary pollutant, a 412 sensitivity scenario without isoprene (S1) input was simulated by OBM model. By comparing 413 S1 to the standard simulation, S0, we find that isoprene chemistry is important to local ROx