Update index.html

Added link to satellite data

about/index.html

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             <h3>About the data</h3>
             <div>
               <p>This website shows the concentration of particulate matter air pollution (PM2.5) in cities around the world.  Very few historical observations of PM2.5 exist before the year 2000 so instead we use data produced from a mix of computer model simulations and satellite observations.</p>
-              <p>For the most recent years (1998-2021) we use a dataset that combines ground-level and satellite observations of PM2.5 concentrations (from <a href="https://pubs.acs.org/doi/10.1021/acs.est.1c05309" target="_new">Van Donkelaar et at (2021)</a>, satellite dataset can be found here).  Satellite observations of PM2.5 aren’t available for the years before 1998, so instead we take the historical trend in air pollution from computer models (<a href="https://doi.org/10.5194/acp-20-14547-2020" target="_new">Turnock 2020</a>); publicly available model data was taken from the <a href="https://wcrp-cmip.org/" target="_new">Coupled Model Intercomparison Project (CMIP6)</a>, these are the climate models used for the IPCC assessment report.  We used data from the <a href="https://esgf-index1.ceda.ac.uk/projects/cmip6-ceda/" target-"_new">UKESM submission to CMIP</a> (data is <a href="http://cera-www.dkrz.de/WDCC/meta/CMIP6/CMIP6.CMIP.MOHC.UKESM1-0-LL" target="_new">here</a>).</p>
+              <p>For the most recent years (1998-2021) we use a dataset that combines ground-level and satellite observations of PM2.5 concentrations (from <a href="https://pubs.acs.org/doi/10.1021/acs.est.1c05309" target="_new">Van Donkelaar et at (2021)</a>, satellite dataset can be found <a href="https://sites.wustl.edu/acag/datasets/surface-pm2-5/#V5.GL.04" target="_blank">here</a>
+).  Satellite observations of PM2.5 aren’t available for the years before 1998, so instead we take the historical trend in air pollution from computer models (<a href="https://doi.org/10.5194/acp-20-14547-2020" target="_new">Turnock 2020</a>); publicly available model data was taken from the <a href="https://wcrp-cmip.org/" target="_new">Coupled Model Intercomparison Project (CMIP6)</a>, these are the climate models used for the IPCC assessment report.  We used data from the <a href="https://esgf-index1.ceda.ac.uk/projects/cmip6-ceda/" target-"_new">UKESM submission to CMIP</a> (data is <a href="http://cera-www.dkrz.de/WDCC/meta/CMIP6/CMIP6.CMIP.MOHC.UKESM1-0-LL" target="_new">here</a>).</p>
               <p>To ensure a smooth time series, we don’t use the model concentrations directly, instead we use them to calculate the historical trend for each city and apply this trend to a three-year average of the observations from 2000-2002. This is a similar approach to that taken by <a href="https://doi.org/10.1029/2023GH000812" target="_new">Turnock et al. (2023)</a> and <a href="https://doi.org/10.1029/2023EF003697" target="_new">Reddington et al. (2023)</a>.</p>
               <p>Very few historical observations of PM2.5 exist, so it is tricky to evaluate how good this approximation is, but in this approach the historical trend is taken from the computer model and the values are informed by the satellite.</p>
               <p>Both the computer model and the satellite use gridboxes that cover many kilometers, so concentrations from cities will likely be underestimated as the very concentrations in a city center may be mixed with lower concenrations within the same gridbox.   </p>