RAL IMS retrieval of SO2 and sulphates (January to April 2022)

<p><strong>RAL IMS retrieval of SO2 and sulphates</strong></p> <p><strong>Description of the product</strong></p> <p>The RAL (Rutherford Appleton Laboratory) Infrared/Microwave Sounder (IMS) retrieval core scheme (Siddans, 2019) uses an optimal estimation spectral fitting procedure to retrieve atmospheric and surface parameters jointly from co-located measurements by IASI (Infrared Atmospheric Sounding Interferometer), AMSU (Advanced Microwave Sounding Unit) and MHS(Microwave Humidity Sounder) on MetOp-B spacecraft, using RTTOV 12 (Radiative Transfer for TOVS)(Saunders et al., 2017) as the forward radiative transfer model. The use of RTTOV 12 enables the quantitative retrieval of volcanic-specific aerosols (sulphate aerosol) and trace gases (SO2). The present dataset includes IMS SO2 and sulphate aerosols retrievals from its near-real time implementation. The IMS scheme &nbsp;&nbsp;retrieves the SO2 in the sensitive region around 1100-1200 cm^&minus;1, in ppbv assuming a uniform vertical mixing ratio. It retrieves sulphate-specific AOD (Aerosol Optical Depth) at 1170 cm^&minus;1 (i.e. the peak of the mid-infrared extinction cross section (Sellitto and Legras, 2016)), assuming a Gaussian extinction coefficient profile shape peaking at 20 km altitude, with 2 km full-width half-maximum. The bulk of the spectroscopic information on SO2 and sulphate aerosols, in the IMS scheme, thus comes from the IASI Fourier transform spectrometer (Clerbaux et al., 2009).</p> <p>We refer to the two retrieved products as IMS SO2 and IMS SA OD.</p> <p><strong>References</strong></p> <p>Clerbaux, C., Boynard, A., Clarisse, L., George, M., Hadji-Lazaro, J., Herbin, H., Hurtmans, D., Pommier, M., Razavi, A., Turquety, S., Wespes, C., and Coheur, P.-F.: Monitoring of atmospheric composition using the thermal infrared IASI/MetOp sounder, Atmospheric Chemistry and Physics, 9, 6041&ndash;6054, https://doi.org/10.5194/acp-9-6041-2009, 2009.</p> <p>Saunders, R., Hocking, J., Rundle, D., Rayer, P., Hayemann, S., Matricardi, A., Lupu, C., Brunel, P., and Vidot, J.: RTTOV-12 SCIENCE AND VALIDATION REPORT; Version : 1.0, Doc ID : NWPSAF-MO-TV-41, https://nwp-saf.eumetsat.int/site/download/documentation/rtm/docs_rttov12/rttov12_svr.pdf, 2017.</p> <p>Sellitto, P. and Legras, B.: Sensitivity of thermal infrared nadir instruments to the chemical and microphysical properties of UTLS secondary sulfate aerosols, Atmospheric Measurement Techniques, 9, 115&ndash;132, https://doi.org/10.5194/amt-9-115-2016, 2016.</p> <p>Siddans, R.: Water Vapour Climate Change Initiative (WV-CCI) - Phase One, Deliverable 2.2; Version 1.0, https://climate.esa.int/documents/1337/Water_Vapour_CCI_D2.2_ATBD_Part2-IMS_L2_product_v1.0.pdf, 2019.</p> <p><strong>Description of the data</strong></p> <p>The archive IMS-2022.tgz contains level 3 daily gridded files for the two retrieved products IMS SO2 and IMS SA OD in the period 13 January to 30 April 2022. A few days are missing between 9 March and 13 March. The first 8 letters of the name of each file contain the date. There are 4 files per day as the two products are in separate files and there is a file collecting day-time orbits and another one for night-time orbits every day.</p> <p>For the 28 April 2022, the four files are</p> <p>20220428_ims_metopb_tir_qnrt_aot0_day_global_g0.5_qc0.nc &nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; SA OD day-time orbits</p> <p>20220428_ims_metopb_tir_qnrt_aot0_night_global_g0.5_qc0.nc &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; SA OD night-time orbits</p> <p>20220428_ims_metopb_tir_qnrt_so2_day_global_g0.5_qc0.nc&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; SO2 day-time orbits</p> <p>20220428_ims_metopb_tir_qnrt_so2_night_global_g0.5_qc0.nc&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; SO2 night-time orbits</p> <p>The names for other dates can be derived by changing the first 8 letters.</p> <p>The format is netdcf4 that is readable with many programming languages and graphics packages.</p> <p>The data are on a [-90,90] x [-180,180] lat x lon grid with resolution 0.25&deg;, that is a 720 x 1440 array of centered values.</p> <p>For both SO2 and SA OD, the values are in the &lsquo;data&rsquo; variable. The variable &lsquo;qa_value&rsquo; is a quality control value used to screen values for plotting; 0 means do not plot; -1 means mask is not defined so the mask is not used (data will be plotted).</p> <p>SO2 units are ppbv (assuming a uniform mixing ratio vertical profile ). SA OD is an optical depth with no unit.</p> <p><strong>Reading software</strong></p> <p>A python package to read and process the data is available at https://github.com/bernard-legras/ASTuS/tree/master/IMS and in the IMS-reader.tgz archive</p>