Preliminary comparison of OMI PBL SO2 data to in-situ measurements in Beijing

Abstract

Planet Boundary Layer sulfur dioxide (PBL-SO₂) derived from Ozone Monitoring Instrument (OMI) are compared with in-situ measurements from Differential Optical Absorption Spectroscopy (DOAS) and gas analyzer observations at three sites in Beijing (Jan-Dec, 2007) and Hebei province (Jan-May, 2007). We use an Air Mass Factor (AMF) lookup table, which was calculated via Linearized Discrete Ordinate Radiative Transfer (LIDORT) model, to convert OMI PBL-SO₂ slant column density to vertical column density. Co-locate Lidar (UV) aerosol extinction profiles are used to correct the effect of aerosol. Results show that, AMF decreases less than 3% with the increasing solar zenith angle from 0° to 45°, AMF is more sensitive to surface albedo and the viewing zenith angle. AMF reduces by 6% with the increasing Ozone density from 275DU to 325DU. Normally, absorption aerosol reduces AMF and scattering aerosol increases AMF, aerosol profiles are critical to AMF estimation. Under very clear conditions, from winter to later spring, OMI observed SO₂ values are underestimated by 3.6ppbv to 20ppbv, but in reasonable agreement with in-situ measurements. Because of the effects of Sub-pixel cloud contamination, long slant path (higher solar zenith angles or viewing zenith angles), differences in aerosol types and large Aerosol Optical Depth (AOD), direct comparisons between the OMI retrieval and the in situ measurements show that the correlation is low and the differences vary with months, while averaging over half a month can significantly reduces the bias.