Comment on acp-2021-811

doi:10.5194/acp-2021-811-rc1

Abstract

Nitrate aerosol plays an increasingly important role in wintertime haze pollution in China. Despite intensive research on the 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 two-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, 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.

Highlights

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 nitrogen oxides (NOx) to HNO3 through both heterogeneous and gas-phase processes
When the O3 level was low (< 10 ppb), the heterogeneous process was relatively slow, even with NO2 concentration exceeding 60 ppb. These results suggest that the atmospheric oxidation capacity, which affected the production of N2O5, played a vital role in controlling the nitrate formation rate from the heterogeneous process
The chemical mechanisms and key controlling factors of wintertime nitrate formation in the eastern Yangtze River Delta (YRD) of China were investigated using a combination of online field observations and detailed model simulations

Summary

Introduction

Atmospheric fine particulate matter (PM2.5) has profound impacts on air quality, climate, and public health (Huang et al, 2014; Wang et al, 2014; Lelieveld et al, 2015; von Schneidemesser et al., 2015). Anthropogenic emissions of major air pollutants such as sulfur dioxide (SO2), nitrogen oxides (NOx), and primary PM have declined dramatically and the nationwide PM2.5 air quality have improved significantly (Shao et al, 2018; Zheng et al, 2018; Ding et al, 2019; Zhang et al, 2019). Sulfate aerosol concentration has decreased dramatically nationwide since 2013, but wintertime nitrate concentration has not decreased much (Ding et al., 2019; Li et al, 2019a; Xu et al, 2019; Fu et al, 2020; Wang et al, 2020b); nitrate has become an increasingly important component of PM2.5 in most regions of China during winter (Ye et al, 2017; Yun et al, 2018; Li et al, 2019a; Xu et al, 2019; Chen et al, 2020; Fu et al, 2020; Kong et al., 2020; Lin et al, 2020; Xie et al, 2020; Zhai et al, 2021; Zhang et al, 2021).

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Conclusion