SO₂ data and alert service

Introduction

Slant column density vs. vertical column density

Geographic regions

Data presentation and delivery

Solar Zenith Angle

What is the Dobson Unit?

Slant column retrieval

Background correction

Reference spectrum

Cloud cover fraction

Near-real time service

Criteria for exceptional SO2 concentrations

Air-mass factor using look-up tables

Air-mass factor using a chemistry transport model

SO2 column from OMI

Time period of available data

Data format specification

Data and Service version history

Validation of the data products

South Atlantic Anomaly

Downloading data & image files

Documentation

References

Acronyms

Acknowledgments

SO2 total column based on OMI data

The SO2 data product based on OMI data is derived quite differently -- namely with a "band residual method" using the residuals of the DOAS-based ozone retrieval -- and it does not give a comparable slant column density as intermediate step. The method used for OMI should in principle lead to the same SO2 total column as those derived with the other methods.

The OMI data product file contains three estimates of the total SO2 column in Dobson Units. These correspond to three a-priori vertical profiles for the SO2 vertical distribution used in the retrieval algorithm. The three vertical profiles were selected to represent typical SO2 vertical distributions for three SO2 source regimes: SO2 in the Planetary Boundary Layer (PBL, below 2 km) from anthropogenic sources, SO2 distributed between 5 and 10 km emitted by passive volcanic degassing in the free troposphere, and SO2 distributed between 15 and 20 km representing injection from explosive volcanic eruptions. The images shown on the website represent the second of these three regimes.

SO2 total column based on OMI measurements on 7 April 2007. The SO2 plume is related to the eruption of the Piton de la Fournaise volcano on Reunion Island, which started a few days earlier and continued for several days. [Image courtesy Simon Carn, OMI/NASA.]

In the OMSO2 product, all PBL data are processed with the Band Residual Difference (BRD) method, while all 5 km and 15 km data are processed with the Linear Fit (LF) algorithm. The algorithms use different subsets of calibrated residuals produced by the NASA operational ozone algorithm. This set contains residuals centered at the Earth Probe (EP) TOMS wavelengths, four residuals at extrema of the SO2 absorption cross-section from 310.8-314.4 nm, and two non-absorbing wavelengths at 345 and 370 nm. Both OMSO2 algorithms use the temperature-dependent SO2 cross-sections data, residual correction for background regions, and different parameterizations of the AMF.

The BRD algorithm uses differential residuals at 3 SO2 sensitive OMI UV2 wavelength pairs, and the pair average is used to produce all PBL data. A constant AMF of 0.36 is used to estimate total SO2 (vertical column). For strong volcanic degassing and eruptions, SO2 loading can be very large and the BRD algorithm may fail. The LF algorithm has been developed to optimally select residuals from the set of available OMTO3 bands to retrieve SO2 under these conditions. The LF algorithm minimizes a subset of the residuals by simultaneously adjusting total SO2, total column ozone, reflectivity at 331 nm, and polynomial coefficients (linear and quadratic) to account for the wavelength dependent effect of surface albedo and aerosol on the effective reflectivity.

More information, references and links regarding the OMI SO2 data product can be found at the website of the Aura OMI Sulphur Dioxide Data Product See also the website of the OMI Sulphur Dioxide Group

OMI SO2 data presented on this web site

The image here shows the SO2 distribution (in this case there is virtually no SO2) over Dem.Rep. Congo. The times of the measurement are plotted at the middle of each orbit, once every 50 swaths. The image also shows bands of no data: pixels 53 and 54 of the swath -- these have an instrumental problem and since that can lead to a spurious SO2 signal, the pixels are omitted from the OMI plots.

Note that in the NRT service only images are shown; data files are not supplied. The archive service provides images and links to data files.