Go to https://earthdata.nasa.gov/ and type "sulphur dioxide" or "nitrogen" (or anything else you are looking for in the search window. You should get a number of hits. All data are freely available. If you need help or have questions, you can click on the "Feedback" bubble on the top right corner. You can type any detailed questions or comments and they will be passed on to the appropriate experts.
(Looks like Jochen and I were typing at the same time! Links given by us are slightly different, but lead to the the same data.)