SO₂ over central China: Measurements, numerical simulations and the tropospheric sulfur budget

Abstract

SO 2 in central China was measured in situ from an aircraft and remotely using the Ozone Monitoring Instrument (OMI) from the Aura satellite; results were used to develop a numerical tool for evaluating the tropospheric sulfur budget ‐ sources, sinks, transformation and transport. In April 2008, measured ambient SO 2 concentrations decreased from ∼7 ppbv near the surface to ∼1 ppbv at 1800 m altitude (an effective scale height of ∼800 m), but distinct SO 2 plumes were observed between 1800 and 4500 m, the aircraft's ceiling. These free tropospheric plumes play a major role in the export of SO 2 and in the accuracy of OMI retrievals. The mean SO 2 column contents from aircraft measurements (0.73 DU, Dobson Units) and operational OMI SO 2 products (0.63 ± 0.26 DU) were close. The OMI retrievals were well correlated with in situ measurements (r = 0.84), but showed low bias (slope = 0.54). A new OMI retrieval algorithm was tested and showed improved agreement and bias (r = 0.87, slope = 0.86). The Community Multiscale Air Quality (CMAQ) model was used to simulate sulfur chemistry, exhibiting reasonable agreement (r = 0.62, slope = 1.33) with in situ SO 2 columns. The mean CMAQ SO 2 loading over central and eastern China was 54 kT, ∼30% more than the estimate from OMI SO 2 products, 42 kT. These numerical simulations, constrained by observations, indicate that ∼50% (35 to 61%) of the anthropogenic sulfur emissions were transported downwind, and the overall lifetime of tropospheric SO 2 was 38 ± 7 h.