Title: Investigation of aerosol and ozone interaction over Zanjan, Iran: Ozone formation and radiative forcing implications

Abstract

The variations in ozone concentration, aerosol properties and ozone-aerosol interactions can be investigated to better understand the atmospheric radiation budget and its effects on climate. The scattering and absorption of solar radiation due to the direct effect of aerosols can change solar radiation reaching the earth's surface. Consequently, aerosols change the photolysis rate and cause changes in ozone production. Moreover, aerosols affect the temperature structure of the atmosphere and thus modify ozone production. In this study, the effects of aerosol on ozone were analyzed over Zanjan, Iran during 2008-2018. The high values of Aerosol Optical Depth (AOD), Particulate Matter (PM10) and Absorption Aerosol Optical Depth (AAOD) were observed in the spring and summer, whereas low values in the fall and winter were determined. In the dusty days and months, there was a negative correlation between tropospheric and surface ozone and AOD, as aerosol has reduced the concentration of ozone. The small and negative values of Aerosol Index (AI) in the late spring, summer, and fall were due to the dominance of scattering aerosol like dust in the atmosphere of Zanjan. A comprehensive study of the effects of the types of aerosols on ozone showed that coarse mode aerosols such as dust reduced the ozone concentration. On the other hand, anthropogenic fine mode aerosols such as urban/industrial had a positive effect on ozone production. The relationship between AOD, Aerosols Radiative Forcing (ARF) and ozone, revealed that coarse mode aerosols such as dust with higher AOD, reduced the sunlight reaching the surface. As a result, ARF values were found to be more negative at the surface, thus the photolysis rate decreased considerably, and the concentration of surface ozone was reduced considerably.