Abstract
Abstract. SO2 from volcanic eruptions is now operationally monitored from space in both the ultraviolet (UV) and thermal infrared (TIR) spectral range, but anthropogenic SO2 has almost solely been measured from UV sounders. Indeed, TIR instruments are well known to have a poor sensitivity to the planetary boundary layer (PBL), due to generally low thermal contrast (TC) between the ground and the air above it. Recent studies have demonstrated the capability of the Infrared Atmospheric Sounding Interferometer (IASI) to measure near-surface SO2 locally, for specific atmospheric conditions. In this work, we develop a retrieval method allowing the inference of SO2 near-surface concentrations from IASI measurements at a global scale. This method consists of two steps. Both are based on the computation of radiance indexes representing the strength of the SO2 ν3 band in IASI spectra. The first step allows the peak altitude of SO2 to be retrieved and near-surface SO2 to be selected. In the second step, 0–4 km columns of SO2 are inferred using a look-up table (LUT) approach. Using this new retrieval method, we obtain the first global distribution of near-surface SO2 from IASI-A, and identify the dominant anthropogenic hotspot sources and volcanic degassing. The 7-year daily time evolution of SO2 columns above two industrial source areas (Beijing in China and Sar Cheshmeh in Iran) is investigated and correlated to the seasonal variations of the parameters that drive the IASI sensitivity to the PBL composition. Apart from TC, we show that humidity is the most important parameter which determines IR sensitivity to near-surface SO2 in the ν3 band. As IASI provides global measurements twice daily, the differences between the retrieved columns for the morning and evening orbits are investigated. This paper finally presents a first intercomparison of the measured 0–4 km columns with an independent iterative retrieval method and with observations of the Ozone Monitoring Instrument (OMI).
Highlights
Sulfur dioxide (SO2) is an atmospheric trace gas with both natural and anthropogenic sources
It is important to point out that we found that a number of Infrared Atmospheric Sounding Interferometer (IASI) measurements, mainly associated with negative thermal contrast, were not covered by the look-up table (LUT)
We have presented a method for retrieving SO2 in the low troposphere from IASI at a global scale
Summary
Sulfur dioxide (SO2) is an atmospheric trace gas with both natural and anthropogenic sources. Volcanic emissions are the largest natural contributors to tropospheric and stratospheric SO2, and account for 7.5–10.5 Tg of S per year on average (Andres and Kasgnoc, 1998; Halmer et al, 2002). Anthropogenic sources emit on average 60–100 Tg of S per year (Stevenson et al, 2003), with the major contribution coming from combustion of sulfur-rich fuels, such as coal and oil, and smelting of heavy metals (Smith et al, 2011). The sinks of SO2 are dry (Wesely, 2007) and wet (Ali-Khodja and Kebabi, 1998) deposition, and oxidation by the OH radical in the gas phase or by O3 and H2O2 in the aqueous phase (Eisinger and Burrows, 1998; Stevenson et al, 2003). Significant amounts of SO2 are in this case mainly injected in the high troposphere or stratosphere and cover large ar-
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