Abstract
Halogen (Cl, Br, and I) chemistry has been reported to influence the formation of secondary air pollutants. Previous studies mostly focused on the impact of chlorine species on air quality over large spatial scales. Very little attention has been paid to the effect of the combined halogen chemistry on air quality over Europe and its implications for control policy. In the present study, we apply a widely used regional model, the Community Multiscale Air Quality Modeling System (CMAQ), incorporated with the latest halogen sources and chemistry, to simulate the abundance of halogen species over Europe and to examine the role of halogens in the formation of secondary air pollution. The results suggest that the CMAQ model is able to reproduce the level of O3, NO2, and halogen species over Europe. Chlorine chemistry slightly increases the levels of OH, HO2, NO3, O3, and NO2 and substantially enhances the level of the Cl radical. Combined halogen chemistry induces complex effects on OH (ranging from –0.023 to 0.030 pptv) and HO2 (in the range of –3.7 to 0.73 pptv), significantly reduces the concentrations of NO3 (as much as 20 pptv) and O3 (as much as 10 ppbv), and decreases NO2 in highly polluted regions (as much as 1.7 ppbv); it increases NO2 (up to 0.20 ppbv) in other areas. The maximum effects of halogen chemistry occur over oceanic and coastal regions, but some noticeable impacts also occur over continental Europe. Halogen chemistry affects the number of days exceeding the European Union target threshold for the protection of human beings and vegetation from ambient O3. In light of the significant impact of halogen chemistry on air quality, we recommend that halogen chemistry be considered for inclusion in air quality policy assessments, particularly in coastal cities.
Highlights
Halogen (Cl, Br, and I) species and related processes have been known to deplete stratospheric ozone (O3) for several decades (Molina and Rowland, 1974; Farman et al, 1985)
The base model without halogen chemistry (BASE) simulation underpredicts O3 compared to observations at both coastal and continental stations (Table 1), possibly due to the uncertainty of the nonmethane volatile organic compounds (NMVOCs) emission inventory (Sherwen et al, 2017) and the underestimated NOx (Table 1)
Since a direct comparison is not possible, here we present a comparison of the simulated concentrations with observations from previous studies (Table 3) to provide an approximate assessment of the representation of halogen species in the HAL simulation of the Community Multiscale Air Quality Modeling System (CMAQ) model over Europe
Summary
Halogen (Cl, Br, and I) species and related processes have been known to deplete stratospheric ozone (O3) for several decades (Molina and Rowland, 1974; Farman et al, 1985). The chemistry of halogens has been described in detail in recent reviews and references therein (Saiz-Lopez and von Glasow, 2012; Simpson et al, 2015), so it is just briefly outlined here. Halogen species affect the concentration of air pollutants by, e.g., directly destroying O3 (Reaction R1), indirectly decreasing O3 production by reducing NO2 (Reactions R2 and R3), and influencing the NO/NO2 ratio (Reactions R2 and R4) and the HO2/OH ratio (Reactions R5 and R6). Q. Li et al.: Halogen impact on air quality in Europe
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