OH and HO<sub>2</sub> radical chemistry at a suburban site during the EXPLORE-YRD campaign in 2018

Abstract

Abstract. The first OH and HO2 radical observation in Yangtze River Delta, one of the four major urban agglomerations in China, was carried out at a suburban site (Taizhou) in summer 2018 from May to June, aiming to elucidate the atmospheric oxidation capacity in this region. The maximum diurnal averaged OH and HO2 concentrations were 1.0×107 and 1.1×109 cm−3, respectively, which were the second highest HOx (sum of OH and HO2) radical concentrations observed in China. HONO photolysis was the dominant radical primary source, accounting for 42 % of the total radical initiation rate. Other contributions were from carbonyl photolysis (including HCHO, 24 %), O3 photolysis (17 %), alkene ozonolysis (14 %), and NO3 oxidation (3 %). A chemical box model based on the RACM2-LIM1 mechanism could generally reproduce the observed HOx radicals, but systematic discrepancy remained in the afternoon for the OH radical, when the NO mixing ratio was less than 0.3 ppb. An additional recycling mechanism equivalent to 100 ppt NO was capable to fill the gap. The sum of monoterpenes was on average up to 0.4 ppb during daytime, which was all allocated to α-pinene in the base model. A sensitivity test without monoterpene input showed the modeled OH and HO2 concentrations would increase by 7 % and 4 %, respectively, but modeled RO2 concentration would significantly decrease by 23 %, indicating that monoterpene was an important precursor of RO2 radicals in this study. Consequently, the daily integrated net ozone production would reduce by 6.3 ppb without monoterpene input, proving the significant role of monoterpene in the photochemical O3 production in this study. In addition, the generally good agreement between observed and modeled HOx concentrations suggested no significant HO2 heterogeneous uptake process during this campaign. Incorporation of HO2 heterogeneous uptake process would worsen the agreement between HOx radical observation and simulation, and the discrepancy would be beyond the combined measurement–model uncertainties using an effective uptake coefficient of 0.2. Finally, the ozone production efficiency (OPE) was only 1.7 in this study, a few folds lower than other studies in (sub)urban environments. The low OPE indicated a slow radical propagation rate and short chain length. As a consequence, ozone formation was suppressed by the low NO concentration in this study.

Radical chemistry in the Pearl River Delta: observations and modeling of OH and HO2 radicals in Shenzhen in 2018

“…S1b-c. Additionally, the ROOOH values modeled in our another campaign (Taizhou, 2018) were comparable to or even slightly higher than the simulations in this study, and the chemical modulation tests in Taizhou confirmed the ROOOH is not a significant OH interference in our PKU-LIF system (Ma et al, 2022). Overall, the OH interference during this campaign was negligible according to the analysis of the behavior of the PKU-LIF system in previous campaigns, the comparison of environmental conditions between this campaign and Wangdu campaign, and the exploration of the impact of ROOOH on the discrepancy of OH observations and simulations.…”

“…2) dropped from 10:00 CST gradually, a systematic difference existed, with the observed OH concentration being about 1 × 10 6 cm −3 higher than the modeled OH concentration. The OH concentrations observed in the environments with low NO levels were underestimated by the state-of-theart models at the Backgarden (summer) and Heshan (autumn) sites in PRD as well, and the OH underestimation was identified to be universal at low-NO conditions in China (Lu et al, 2013(Lu et al, , 2012Ma et al, 2019a;Tan et al, 2017;Ma et al, 2022). The reason of OH underestimation was further discussed in Sect.…”

“…Additionally, prior studies reported that OH measurement might be affected by the potential interference, when the sampled air contains ozone, alkenes, and BVOCs (Mao et al, 2012;Fuchs et al, 2016;, indicating the environmental conditions are important to the production of interference. The pre-injector is usually used to test the potential OH interference and has been applied to our PKU-LIF system to quantify the possible interferences for several campaigns, including the campaigns conducted at the Wangdu, Heshan, Huairou, Taizhou, and Chengdu sites (Tan et al, 2017(Tan et al, , 2019(Tan et al, , 2018Yang et al, 2021;Ma et al, 2022). No significant internal interference was found in the prior studies, demonstrating that the accuracy of the PKU-LIF system has been determined for several times.…”

“…The LIM, which has been integrated into the current radical mechanism, is still insufficient to explain the OH missing sources. The X mechanism was identified several times, but the amount of the numerical species, X, varied in different environments, and the nature of X is still unknown (Hofzumahaus et al, 2009;Lu et al, 2013Lu et al, , 2012Tan et al, 2017Tan et al, , 2019Yang et al, 2021;Ma et al, 2022). Therefore, further exploration of radical regeneration sources is necessary.…”

“…Due to the strong photochemistry influenced by high temperatures and strong radiation, severe O 3 pollution appeared to occur in YRD and PRD, especially in PRD (Ma et al, 2019b;. Radicals, the dominant oxidant in the troposphere, have been measured during warm seasons in NCP (Yufa 2006, Wangdu 2014, and Beijing 2016, YRD (Taizhou 2018), SCB (Chengdu 2019), and PRD (Backgarden 2006 andHeshan 2014) in China (Lu et al, 2013(Lu et al, , 2012Tan et al, 2017Tan et al, , 2019Yang et al, 2021;Tan et al, 2021;Ma et al, 2022). The radical observations in PRD, where the cities are suffering from severe O 3 pollution, have not been conducted since 2014, and thus the oxidation capacity here has not been clear in recent years.…”