Version 2 Ozone Monitoring Instrument SO 2 product (OMSO2 V2): new anthropogenic SO 2 vertical column density dataset

Abstract. The Ozone Monitoring Instrument (OMI) has been providing global observations of SO 2 pollution since 2004. Here we introduce the new anthropogenic SO 2 vertical column density (VCD) dataset in the version 2 OMI SO 2 product (OMSO2 V2). As with the previous version (OMSO2 V1.3), the new dataset is generated with an algorithm based on principal component analysis of OMI radiances but features several updates. The most important among those is the use of expanded lookup tables and model a priori profiles to estimate SO 2 Jacobians for individual OMI pixels, in order to better characterize pixel-to-pixel variations in SO 2 sensitivity including over snow and ice. Additionally, new data screening and spectral fitting schemes have been implemented to improve the quality of the spectral fit. As compared with the planetary boundary layer SO 2 dataset in OMSO2 V1.3, the new dataset has substantially better data quality, especially over areas that are relatively clean or affected by the South Atlantic Anomaly. The updated retrievals over snow/ice yield more realistic seasonal changes in SO 2 at high latitudes and offer enhanced sensitivity to sources during wintertime. An error analysis has been conducted to assess uncertainties in SO 2 VCDs from both the spectral fit and Jacobian calculations. The uncertainties from spectral fitting are reflected in SO 2 slant column densities (SCDs) and largely depend on the signal-to-noise ratio of the measured radiances, as implied by the generally smaller SCD uncertainties over clouds or for smaller solar zenith angles. The SCD uncertainties for individual pixels are estimated to be ∼  0.15–0.3 DU (Dobson units) between ∼  40 ∘  S and ∼  40 ∘  N and to be ∼  0.2–0.5 DU at higher latitudes. The uncertainties from the Jacobians are approximately ∼  50 %–100 % over polluted areas and are primarily attributed to errors in SO 2 a priori profiles and cloud pressures, as well as the lack of explicit treatment for aerosols. Finally, the daily mean and median SCDs over the presumably SO 2 -free equatorial east Pacific have increased by only ∼  0.0035 DU and ∼  0.003 DU respectively over the entire 15-year OMI record, while the standard deviation of SCDs has grown by only ∼  0.02 DU or ∼  10%. Such remarkable long-term stability makes the new dataset particularly suitable for detecting regional changes in SO 2 pollution.