Daytime atmospheric oxidation capacity in four Chinese megacities during the photochemically polluted season: a case study based on box model simulation

Abstract

Abstract. Atmospheric oxidation capacity is the basis for converting freshly emitted substances into secondary products and is dominated by reactions involving hydroxyl radicals (OH) during daytime. In this study, we present in situ measurements of ROx radical (hydroxy OH, hydroperoxy HO2, and organic peroxy RO2) precursors and products; the measurements are carried out in four Chinese megacities (Beijing, Shanghai, Guangzhou, and Chongqing) during photochemically polluted seasons. The atmospheric oxidation capacity is evaluated using an observation-based model and radical chemistry precursor measurements as input. The radical budget analysis illustrates the importance of HONO and HCHO photolysis, which account for ∼50 % of the total primary radical sources. The radical propagation is efficient due to abundant NO in urban environments. Hence, the production rate of secondary pollutants, that is, ozone (and fine-particle precursors (H2SO4, HNO3, and extremely low volatility organic compounds, ELVOCs) is rapid, resulting in secondary air pollution. The ozone budget demonstrates its high production in urban areas; also, its rapid transport to downwind areas results in rapid increase in local ozone concentrations. The O3–NOx–VOC (volatile organic compound) sensitivity tests show that ozone production is VOC-limited and that alkenes and aromatics should be mitigated first for ozone pollution control in the four studied megacities. In contrast, NOx emission control (that is, a decrease in NOx) leads to more severe ozone pollution. With respect to fine-particle pollution, the role of the HNO3–NO3 partitioning system is investigated using a thermal dynamic model (ISORROPIA 2). Under high relative humidity (RH) and ammonia-rich conditions, nitric acid converts into nitrates. This study highlights the efficient radical chemistry that maintains the atmospheric oxidation capacity in Chinese megacities and results in secondary pollution characterized by ozone and fine particles.

Estimation of Secondary Organic Aerosol Formation During a Photochemical Smog Episode in Shanghai, China

“…[OH] is the average OH radical concentration (1.7 × 10 6 molecule cm −3 ) estimated from the empirical equation (Ehhalt & Rohrer, 2000). As shown in Figure S6, the average of the daily maximum concentration was about 6 × 10 6 molecule cm −3 , comparable with the only published result (7 × 10 6 molecule cm −3 ) in the summer of 2016 in Shanghai by Tan et al (2019) from the observation‐based model. Accordingly, the first item in equation represents the primary emission of OC and its removal (denoted by POC).…”

Observation Constrained Aromatic Emissions in Shanghai, China

“…Moreover, maximum OH radical concentrations at noon increased from 0.6 × 10 6 molecule per cubic centimeter in December to 8.1 × 10 6 molecule per cubic centimeter in August ( Figure S9), which was consistent with previous findings in Beijing Yuan et al, 2013). Tan et al (2019) studied the OH radical concentrations in summer of Shanghai by observation-based model, and their results were very close to our simulated OH radical concentrations. The abovementioned to some extent proved that our simulated results were reasonable.…”

Observationally constrained modelling of atmospheric oxidation capacity and photochemical reactivity in Shanghai, China

“…The OH reactivity of Case 3 in the clean environment was significantly lower than that of Case 2 and Case 3, which is consistent with previous studies (Mao et al, 2010;Li et al, 2018). In general, the OH reactivity assessed in Shanghai was in the range of 4.6-28.0 s -1 under different air quality conditions, which was at a relatively low level compared to that calculated for other big cities in China such as Guangzhou (20-30 s -1 ), Chongqing (15-25 s -1 ) and Beijing (15-25 s -1 ) (Tan et al, 2019), reflecting that the abundance of pollutants in Shanghai is relatively lower compared to other metropolitan areas in Total OH reactivity has been measured in many urban areas over the past two decades. Compared to the studies in other regions, the estimated average OH reactivity in Shanghai was much lower than that in Paris (Dolgorouky et al, 2012), New York (Ren et al, 2003;Ren et al, 2006), and Tokyo (Yoshino et al, 2006), and was equivalent to Nashville (Kovacs et al, 2003) and…”

“…Since the calculation of OH reactivity did not include unmonitored alcohols, the results may underestimate the contribution of OVOCs to OH reactivity. In a previous study, the average contribution of OVOCs to OH reactivity was about 2.97 s -1 when the maximum ozone was 80 ppbv (similar to Case 2, kOH = 1.43 s -1 ) in Shanghai in August (Tan et al, 2019). This also confirms the underestimation of atmospheric photochemical effects of OVOCs due to some missing OVOC measurements in this study.…”