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

Abstract

Abstract. The ambient radical concentrations were measured continuously by laser-induced fluorescence during the STORM (STudy of the Ozone foRmation Mechanism) campaign at the Shenzhen site, located in the Pearl River Delta in China, in the autumn of 2018. The diurnal maxima were 4.5×106 cm−3 for OH radicals and 4.2×108 cm−3 for HO2 radicals (including an estimated interference of 23 %–28 % from RO2 radicals during the daytime), respectively. The state-of-the-art chemical mechanism underestimated the observed OH concentration, similar to the other warm-season campaigns in China. The OH underestimation was attributable to the missing OH sources, which can be explained by the X mechanism. Good agreement between the observed and modeled OH concentrations was achieved when an additional numerical X equivalent to 0.1 ppb NO concentrations was added into the base model. The isomerization mechanism of RO2 derived from isoprene contributed approximately 7 % to the missing OH production rate, and the oxidation of isoprene oxidation products (MACR and MVK) had no significant impact on the missing OH sources, demonstrating further exploration of unknown OH sources is necessary. A significant HO2 heterogeneous uptake was found in this study, with an effective uptake coefficient of 0.3. The model with the HO2 heterogeneous uptake can simultaneously reproduce the OH and HO2 concentrations when the amount of X changed from 0.1 to 0.25 ppb. The ROx primary production rate was dominated by photolysis reactions, in which the HONO, O3, HCHO, and carbonyls photolysis accounted for 29 %, 16 %, 16 %, and 11 % during the daytime, respectively. The ROx termination rate was dominated by the reaction of OH+NO2 in the morning, and thereafter the radical self-combination gradually became the major sink of ROx in the afternoon. As the sum of the respective oxidation rates of the pollutants via reactions with oxidants, the atmospheric oxidation capacity was evaluated, with a peak of 11.8 ppb h−1 around noontime. The ratio of P(O3)net to AOCVOCs, which indicates the yield of net ozone production from VOC oxidation, trended to increase and then decrease as the NO concentration increased. The median ratios ranged within 1.0–4.5, with the maximum existing when the NO concentration was approximately 1 ppb. The nonlinear relationship between the yield of net ozone production from VOC oxidation and NO concentrations demonstrated that optimizing the NOx and VOC control strategies is critical to controlling ozone pollution effectively in the future.

An Observational Constraint of VOC Emissions for Air Quality Modeling Study in the Pearl River Delta Region

“…Given that SZ has the highest VOC emission rates among the PRD cities in both the priori emission inventory and observation‐constrained emissions, the uncertainty analysis for SZ was selected as a representative of the observation‐constrained emissions in the PRD region. The daily maximum OH concentrations observed in urban SZ, ranging from 2–9 × 10 6 molecules · cm − 3 (Yang et al., 2022), were employed in the Monte Carlo runs. The noon‐time PBLH was set to the range of 600–1200 m, according to the measured PBLH in Guangdong province during summer (J. Guo et al., 2016).…”

Intensive photochemical oxidation in the marine atmosphere: evidence from direct radical measurements

“…The suitable temperature (20-30 • C) and humidity (40 %-80 %) conditions promoted the stable oxidation of the diurnal photochemistry. The peak j (O 1 D) value was approximately 2.0 × 10 −5 s −1 , exhibiting the typical characteristics of intense light radiation in fall in the Pearl River Delta region (Yang et al, 2022;Tan et al, 2022).…”

“…As typical marine air components, the concentrations of NO x , CO, PM 2.5 , and other pollutants were lower than those detected in other observation campaigns in both urban and suburban areas in the Pearl River Delta region (Tan et al, 2019b;Lu et al, 2012;Yang et al, 2022). Serval observation campaigns have discovered the relationship between wind direction and radical chemistry (Lu et al, 2012;Niu et al, 2022).…”

“…Zero air measurements were taken every 8 h for a duration of 20 min to correct for instrument baseline fluctuations. This method has been extensively tested for its suitability in detecting HONO in complex atmospheric conditions, as demonstrated in previous studies by Yang et al (2022Yang et al ( , 2021b and Wang et al (2023). Eight measured photolysis rates (j (NO 2 ), j (H 2 O 2 ), j (HCHO), j (HONO), j (NO 2 ), j (NO 3 ), and j (O 1 D)) were used as model constraints.…”

Measurement report: Changes in light absorption and molecular composition of water-soluble humic-like substances during a winter haze bloom-decay process in Guangzhou, China

“…Although a remarkable decline in atmospheric particulate matter (PM 2.5 ) pollution has been observed in recent years owing to strict regulatory controls, O 3 and VOCs still remain at higher levels and severe haze pollution caused by fine particulate matter frequently occurs in winter (An et al, 2019;K. Li et al, 2019;Yang et al, 2022). Several studies have investigated the optical, chemical, and molecular properties of HULIS in the PRD region (Lin et al, 2010(Lin et al, , 2012Fan et al, 2016;Liu et al, 2018;Jiang et al, 2020Jiang et al, , 2021.…”