Decoding the Link Between Air Oxidation and Pollution

What is Air Oxidation and Pollution?

Air pollution is a serious environmental problem that affects human health and well-being. One of the key factors that influence air pollution is the atmospheric oxidation capacity, which refers to the ability of the atmosphere to oxidize pollutants emitted by human activities.

Atmospheric oxidation can transform primary pollutants, such as volatile organic compounds (VOCs), sulfur dioxide (SO2) and nitrogen oxides (NOx), into secondary pollutants, such as ozone (O3), sulfate (SO4) and nitrate (NO3). These secondary pollutants can form fine particulate matter (PM2.5) and affect the visibility, climate and air quality.

However, the relationship between air oxidation and air pollution is not straightforward. It depends on various factors, such as the concentration and composition of pollutants, the meteorological conditions, the regional transport and the chemical reactions.

In this article, we will review a recent study that reveals how air oxidation affects the formation of secondary pollutants and air quality in Lanzhou, a city in northwest China that suffers from frequent haze episodes. We will also discuss the implications and recommendations for air pollution control and management.

How to Study Air Oxidation and Pollution?

The study was conducted by researchers from the Northwest Institute of Eco-Environment and Resources of the Chinese Academy of Sciences. It was published in Science of the Total Environment on Dec. 30, 2023 .

The researchers used empirical equations to estimate the emission of primary pollutant particles and the formation of secondary pollutant particles in the atmosphere, based on the measured concentrations of PM2.5, PM10, SO2, NO2, O3 and VOCs in Lanzhou from January 2018 to December 2019.

They also analyzed the change characteristics of major atmospheric oxidants, such as O3, NO2 and Ox (Ox=O3+NO2), in Lanzhou during different seasons and weather conditions, using hourly data from 12 monitoring stations.

They further explored the possible causes of the change in pollutant source and concentration in heavily polluted weather, using backward trajectory analysis, potential source contribution function (PSCF) analysis and concentration weighted trajectory (CWT) analysis.

What are the Effects of Air Oxidation

The results show that atmospheric oxidation has a threshold effect on the secondary formation of atmospheric particles. When the concentration of atmospheric oxidant is low (MDA8 O3 ≤100μg/m3), the oxidation process of VOCs is easier to achieve, thus generating more secondary aerosols and promoting the formation of secondary particles.

When the concentration of atmospheric oxidant is higher than 160μg/m3, the proportion of secondary particulate matter is lower than that of primary particulate matter. The total oxidant agent Ox is negatively correlated with the sulfur oxidation rate and nitrogen oxidation rate, indicating that the formation rate of SO2 and NO2 is inhibited by the high concentration of Ox.

The researchers also found that a high-pressure ridge over Lanzhou was maintained for a long time during heavily polluted weather, which reduced the surface wind speed and weakened the transport and diffusion of pollutants near the surface.

The PM2.5 in Lanzhou’s atmosphere is mainly from the Hexi Corridor area, which is located to the west of Lanzhou and has intensive industrial activities and coal combustion. The PM10 is mainly from the southeast of Lanzhou, which is influenced by dust sources from desert areas.

Discussion

This study deepens the understanding of atmospheric oxidation affecting the formation mechanism of air pollution, and provides a reference for the prevention and control of air pollution and the improvement of air quality in Lanzhou.

The study suggests that reducing VOCs emissions is an effective way to decrease secondary aerosol formation and improve air quality when atmospheric oxidant concentration is low. However, when atmospheric oxidant concentration is high, reducing NOx emissions may be more beneficial to reduce O3 formation and inhibit SO2 and NO2 oxidation.

The study also implies that enhancing regional cooperation and coordination is necessary to mitigate transboundary pollution transport and improve regional air quality. Moreover, improving meteorological forecasting and early warning systems is important to cope with unfavorable weather conditions that aggravate air pollution.

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