Nonlinear response of nitrate to NOx reduction in China during the COVID-19 pandemic

Abstract

In recent years, nitrate plays an increasingly important role in haze pollution and strict emission control seems ineffective in reducing nitrate pollution in China. In this study, observations of gaseous and particulate pollutants during the COVID-19 lockdown, as well as numerical modelling were integrated to explore the underlying causes of the nonlinear response of nitrate mitigation to nitric oxides (NO x ) reduction. We found that, due to less NO x titration effect and the transition of ozone (O 3 ) formation regime caused by NO x emissions reduction, a significant increase of O 3 (by ∼ 69%) was observed during the lockdown period, leading to higher atmospheric oxidizing capacity and facilitating the conversion from NO x to oxidation products like nitric acid (HNO 3 ). It is proven by the fact that 26–61% reduction of NO x emissions only lowered surface HNO 3 by 2–3% in Hebi and Nanjing, eastern China. In addition, ammonia concentration in Hebi and Nanjing increased by 10% and 40% during the lockdown, respectively. Model results suggested that the increasing ammonia can promote the gas-particle partition and thus enhance the nitrate formation by up to 20%. The enhanced atmospheric oxidizing capacity together with increasing ammonia availability jointly promotes the nitrate formation, thereby partly offsetting the drop of NO x . This work sheds more lights on the side effects of a sharp NO x reduction and highlights the importance of a coordinated control strategy.

Integrated process analysis retrieval of changes in ground-level ozone and fine particulate matter during the COVID-19 outbreak in the coastal city of Kannur, India

“…These discrepancies highly depended on the different study areas and the duration of the lockdown period. O 3 enhancement during the COVID-19 lockdown resulting from less NOx titration has been proposed previously over other regions such as cities in China ( Meng et al, 2021 ; Ren et al, 2021 ; Wu et al, 2021 ; Yin et al, 2021 ), Baghdad, Iraq ( Hashim et al, 2021 ), Milan, Italy ( Zoran et al, 2020 ) and the Auvergne-Rhône-Alpes region, France ( Sbai et al, 2021b ). During the lockdown in Europe, O 3 was found to increase over more urbanized regions such as Central Northern Europe and the Po Valley ( Cuesta et al, 2021 ; Grange et al, 2021 ; Matthias et al, 2021 ; Sbai et al, 2021b ; Zoran et al, 2020 ); O 3 photochemistry in these regions with high NOx emissions from industry and traffic was typically dominated by VOC-limited conditions and O 3 titration with NO was reduced when NOx emissions were reduced ( Cuesta et al, 2021 ; Matthias et al, 2021 ).…”

“…In particular, the phenomenon of elevated ozone levels during the lockdown periods has been extensively reported ( He et al, 2021 ; Meng et al, 2021 ; Sbai et al, 2021b ; Wu et al, 2021 ; Yin et al, 2021 ). Previous studies have analyzed the reasons for O 3 changes from the perspective of emission restrictions and meteorological factors including relative humidity, temperature, solar radiation, and ozone-forming mechanisms ( Meng et al, 2021 ; Mor et al, 2021 ; Ren et al, 2021 ; Saxena and Raj, 2021 ; Tibrewal and Venkataraman, 2022 ; Tobias et al, 2020 ). An increase in air temperature and a rise in solar radiation was responsible for the O 3 increase during the lockdown because higher temperatures could decrease the stability of the atmosphere and correspondingly increase the mixing height of pollutants, and stronger solar radiation could have enhanced the intensity of photochemical reactions in the atmosphere ( Dang and Liao, 2019 ; Mor et al, 2021 ; Ravindra et al, 2019 ; Saxena and Raj, 2021 ; Sbai et al, 2021b ).…”

Chemical characteristics and sources of PM2.5 in Hohhot, a semi-arid city in northern China: insight from the COVID-19 lockdown

“…4). During this period, the SNA contributed 55.3 % to the total PM 2.5 , slightly higher than those of the cities in northern China such as Xi'an (50.0 %; Tian et al, 2021) and Beijing (48.5 %; Ren et al, 2021) and lower than the cities in southern China such as Guangzhou (78.7 %; Wang et al, 2021), Nanjing (68.2 %; Ren et al, 2021) and Shanghai (75.4 %; Chen et al, 2020). Sulfate was the predominant component of SNA in Hohhot during this period, whereas nitrite was the main contributor to SNA in Guangzhou, Nanjing and Shanghai.…”

Effect of restricted emissions during COVID-19 on atmospheric aerosol chemistry in a Greater Cairo suburb: Characterization and enhancement of secondary inorganic aerosol production

“…An increase in the oxidation capacity (O 3 ) of the atmosphere led to the enhancement of sulfur oxidation ratio, and then SO 4 2− formation during the COVID lockdown ( Duan et al, 2021 ). Ren et al (2021) reported that a 26–61% reduction of NOx emissions only lowered surface HNO 3 by 2–3% and enhanced the NO 3 − formation in the presence of a high concentration of NH 3 reduction in China during the COVID-19 pandemic due to the enhanced atmospheric oxidizing capacity.…”