Effects of Meteorological Factors on the Vertical Distribution of Peroxyacetyl Nitrate in Autumn in Tianjin

With the decrease of PM2.5 concentration, atmospheric photochemical pollution in North China Plain has attracted more and more attention. Peroxyacetyl nitrate (PAN) is an important product and reliable indicator of atmospheric photochemical reaction. In this study, the measured concentrations of PAN near the surface and at 220 m were obtained from the Tianjin Meteorological Tower in October 2018. The results showed that the average concentrations of PAN at ground and 220 m platform were 1.65 +/- 1.34 ppb and 1.76 +/- 1.41 ppb respectively during the day, and the mixture was relatively uniform. The average concentrations of PAN were 0.99 +/- 0.94 ppb and 1.42 +/- 1.28 ppb during the nighttime, respectively, indicating that the vertical difference was significantly increased. The standard deviation of PAN concentration was close to the average value, indicating that the concentration fluctuated greatly. The correlation coefficient between PAN and PM2.5 was higher than that of O3, indicating that stable weather was an important factor affecting PAN concentration. Based on the analysis of the vertical wind speed by the wind profile radar and ozone concentration vertical distribution data, we believe that there is a downdraft in the upper atmosphere of the boundary layer. The higher concentration of PAN at 220 m at night came from the high-altitude pollution air mass. The analysis of 40 m and 200 m turbulence data showed that the weak turbulence hindered the vertical exchange between the surface and the upper atmosphere, resulting in the high concentration of PAN unable to reach the surface. The boundary layer structure is one of the important reasons for the vertical difference of PAN concentration at night in autumn in NCP.