Validation of SO₂ retrievals from the Ozone Monitoring Instrument over NE China

Abstract

[1] The Dutch-Finnish Ozone Monitoring Instrument (OMI) launched on the NASA Aura satellite in July 2004 offers unprecedented spatial resolution, coupled with contiguous daily global coverage, for space-based UV measurements of sulfur dioxide (SO 2 ). We present a first validation of the OMI SO 2 data with in situ aircraft measurements in NE China in April 2005. The study demonstrates that OMI can distinguish between background SO 2 conditions and heavy pollution on a daily basis. The noise (expressed as the standard deviation, s) is $1.5 DU (Dobson units; 1 DU = 2.69 Á 10 16 molecules/cm 2 ) for instantaneous field of view boundary layer (PBL) SO 2 data. Temporal and spatial averaging can reduce the noise to s $ 0.3 DU over a remote region of the South Pacific; the long-term average over this remote location was within 0.1 DU of zero. Under polluted conditions collection 2 OMI data are higher than aircraft measurements by a factor of two. Improved calibrations of the radiance and irradiance data (collection 3) result in better agreement with aircraft measurements on polluted days. The air mass-corrected collection 3 data still show positive bias and sensitivity to UV absorbing aerosols. The difference between the in situ data and the OMI SO 2 measurements within 30 km of the aircraft profiles was about 1 DU, equivalent to $5 ppb from 0 to 3000 m altitude. Quantifying the SO 2 and aerosol profiles and spectral dependence of aerosol absorption between 310 and 330 nm are critical for an accurate estimate of SO 2 from satellite UV measurements.

Recent large reduction in sulfur dioxide emissions from Chinese power plants observed by the Ozone Monitoring Instrument

“…It is derived by assuming cloud-and aerosol-free conditions, solar zenith angle (SZA) of 30°, surface albedo of 0.05, surface pressure of 1013.25 hPa, summer mid-latitude ozone profile of 325 DU, and SO 2 mostly distributed in the PBL, centered at ∼900 hPa. The noise is ∼1.5 DU for the instantaneous field of view PBL SO 2 data, but can be greatly reduced through averaging over time and space [Krotkov et al, 2008]. Error may also arise due to the constant AMF employed in the operational product.…”

“…The AMF is a function of SO 2 vertical distribution, satellite viewing geometry, total column ozone, aerosols, and clouds [Krotkov et al, 2008]. A constant AMF of 0.36 is used in the current operational PBL SO 2 product.…”

“…[5] The OMI sensor retrieves planetary boundary layer (PBL) SO 2 column amounts from measurements of backscattered solar UV (BUV) radiation in the wavelength range of 311-315 nm using a Band Residual Difference (BRD) algorithm [Krotkov et al, 2006[Krotkov et al, , 2008. The retrieved SO 2 slant column density (SCD) (i.e., the effective total column along the mean path of BUV photons) is converted to the total SO 2 vertical column density (VCD) in Dobson Units (1 DU = 2.69 × 10 16 molecules/cm 2 ) using an air mass factor (AMF),…”

“…Launched in 2004, the DutchFinnish built Ozone Monitoring Instrument (OMI) aboard NASA's EOS Aura satellite provides daily global coverage and spatial resolution of 13 km × 24 km at nadir [Levelt et al, 2006]. OMI-retrieved tropospheric NO 2 and SO 2 column amounts have been used to study regional emissions [e.g., Witte et al, 2009], synoptic pollution events [e.g., Krotkov et al, 2008], and large point emission sources [e.g., Carn et al, 2007].…”

Sulfur dioxide retrievals from TROPOMI onboard Sentinel-5 Precursor: algorithm theoretical basis

“…5.1). Equally important are also limb-DOAS or in situ instruments to provide information on the vertical distribution of SO 2 which is crucial for satellite validation (e.g., Krotkov et al, 2008).…”