High atmospheric oxidation capacity drives wintertime nitrate pollution in the eastern Yangtze River Delta of China

Abstract

Abstract. Nitrate aerosol plays an increasingly important role in wintertime haze pollution in China. Despite intensive research on wintertime nitrate chemistry in recent years, quantitative constraints on the formation mechanisms of nitrate aerosol in the Yangtze River Delta (YRD), one of the most developed and densely populated regions in eastern China, remain inadequate. In this study, we identify the major nitrate formation pathways and their key controlling factors during the winter haze pollution period in the eastern YRD using 2-year (2018–2019) field observations and detailed observation-constrained model simulations. We find that the high atmospheric oxidation capacity, coupled with high aerosol liquid water content (ALWC), made both the heterogeneous hydrolysis of dinitrogen pentoxide (N2O5) and the gas-phase OH oxidation of nitrogen dioxide (NO2) important pathways for wintertime nitrate formation in this region, with contribution percentages of 69 % and 29 % in urban areas and 63 % and 35 % in suburban areas during the haze pollution episodes, respectively. We further find that the gas-to-particle partitioning of nitric acid (HNO3) was very efficient so that the rate-determining step in the overall formation process of nitrate aerosol was the oxidation of NOx to HNO3 through both heterogeneous and gas-phase processes. The atmospheric oxidation capacity (i.e., the availability of O3 and OH radicals) was the key factor controlling the production rate of HNO3 from both processes. During the COVID-19 lockdown (January–February 2020), the enhanced atmospheric oxidation capacity greatly promoted the oxidation of NOx to nitrate and hence weakened the response of nitrate aerosol to the emission reductions in urban areas. Our study sheds light on the detailed formation mechanisms of wintertime nitrate aerosol in the eastern YRD and highlights the demand for the synergetic regulation of atmospheric oxidation capacity and NOx emissions to mitigate wintertime nitrate and haze pollution in eastern China.

The Aggravation of Summertime Nocturnal Ozone Pollution in China and Its Potential Impact on the Trend of Nitrate Aerosols

“…These analysis over different time scales based on the multi‐year observations in urban Beijing suggest that the nocturnal O 3 plays a vital role in the nocturnal particulate nitrate formation through N 2 O 5 heterogeneous hydrolysis. Although the gaseous HNO 3 during daytime will also be converted into $${{\mathrm{N}\mathrm{O}}_{3}}^{-}$$ at night, some literature demonstrated a very limited contribution of this pathway to nighttime $${{\mathrm{N}\mathrm{O}}_{3}}^{-}$$ (Guo et al., 2018; Zang et al., 2022). Compared with gas‐phase OH + NO 2 conversion, the production efficiency of N 2 O 5 heterogeneous hydrolysis was more efficient and was the main source of nocturnal $${{\mathrm{N}\mathrm{O}}_{3}}^{-}$$ pollution (Atkinson, 2000; H. Li et al., 2018; Y. Lin et al., 2020; P. Sun et al., 2018; Wen et al, 2015).…”

Reactive Chlorine Species Advancing the Atmospheric Oxidation Capacities of Inland Urban Environments

“…Thus, the AOC would be underestimated in the winter if the contributions of Cl radicals to the photochemistry were ignored. In other words, the formation of secondary air pollutants, such as by oxidation of NO x to nitrate, SO 2 to sulfate, and VOCs to SOA, would be facilitated by the enhanced AOC in winter due to the participation of Cl radicals. On the other hand, Ma et al and Wang et al found that the VOCs consumed by OH radicals were crucial to O 3 formation.…”

“…At present, the traditional gas-phase chemistry in the Master Chemical Mechanism cannot effectively explain the rapid formation rates of the secondary aerosols observed during wintertime haze events in China, – which suggests that an unknown mechanism for the AOC affects the formation of the secondary aerosols. On the other hand, model simulations usually underestimate the measured AOC at high NO x concentrations . This also indicates that the sources of atmospheric oxidants or the mechanism related to AOC might have been missed.…”

“…On the other hand, model simulations usually underestimate the measured AOC at high NO x concentrations. 15 This also indicates that the sources of atmospheric oxidants or the mechanism related to AOC might have been missed. Thus, a huge gap still exists in our understanding of high AOCs and their roles in the complex air pollution of the urban atmosphere.…”

Transport Pathways of Nitrate Formed from Nocturnal N2O5 Hydrolysis Aloft to the Ground Level in Winter North China Plain

“…Because aerosol water content (AWC) is the precondition for triggering the nocturnal reactions (–), these reactions can play significant roles in NO 3 – formation on severe haze days under high relative humidity (RH) . However, the ground O 3 concentrations on severe haze days usually approach zero in the nighttime due to the fast titration of O 3 by NO, which can effectively suppress the in situ formation of NO 3 radicals and N 2 O 5 near the ground (–). Considering the relatively high concentrations of NO 3 radicals and N 2 O 5 aloft, the vertical mixing of the air aloft with the ground air in the early morning due to the breaking of the boundary layer was proposed to explain the relatively high contribution of the nocturnal reactions to ground NO 3 – .…”