A new method to determine the aerosol optical properties from multiple wavelength O 4 absorptions by MAX-DOAS observation

Abstract. Ground based Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) observation was carried out from November 2016 to February 2017 in Beijing, China to measure the O 4 absorptions in UV and visible bands and further to illustrate its relationship with aerosol optical properties (AOPs) under different the weather types. According to relative humidity, visibility and PM 2.5 , we classified the observation periods into clear, non-haze, haze, heavy-haze, fog and rainy five different weather conditions. There are obvious differences for measured AOPs under different weather conditions, especially scattering coefficient (σ sca ) and absorption coefficient (σ abs ). It was also found that both the O 4 Differential Slant Column Densities (DSCDs) at UV and visible bands varied in the order of clear days > non-haze days > haze days > heavy-haze days > fog days. The correlation coefficients (R 2 ) between O 4 DSCDs at 360.8 and 477.1 nm mainly varied in the order of clear days > non-haze days > haze days > heavy-haze days. Based on the statistics of O 4 DSCDs at elevation angle 1o with the corresponding linear regression between UV and visible bands of segmental periods, the relationships between O 4 DSCDs and AOPs were established. It mainly should be clear or non-haze days when the correlation slope is greater than 1.0, correlation coefficient (R 2 ) greater than 0.9 and O4 DSCDs mainly greater than 2.5 × 10 43  molec cm −2 . Meanwhile, σ sca and σ abs are less than 45 and 12 Mm −1 , respectively. For haze or heavy-haze days, the correlation slope is less than 0.6, R 2 less than 0.8 and O 4 DSCDs mainly less than 1.3 × 10 43  molec cm −2 , under which σ sca and σ abs are mainly located at 200–900 and 20–60 Mm −1 . Additionally, the determination method was well validated based on another MAX-DOAS measurement at Gucheng from 19 to 27 November 2016. For more precise and accurate inversion of AOPs, more detailed look-up tables for O 4 multiple wavelength absorptions need to developed. Furthermore, the vertical spatial-resolved aerosol scattering and absorption information is worthy of being expected by using DSCDs at different elevation angles.