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about/index.html

@@ -128,9 +128,10 @@ <h3>About the data</h3>
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               <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 (2000-2021) 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 (V5 0.1 degree resolution)</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>Because so few historical observations of PM2.5 exist, so it is challenging to evaluate how good this approximation is, but in our approach the historical trend is taken from the computer model and the values are informed by the satellite.</p>
               <p>The historical concentrations for the UKESM model are calculated using the emissions inventory developed through the Community Emissions Data System (CEDS) by <a href="https://https://gmd.copernicus.org/articles/11/369/2018/" target="_new"> Hoesly et al, 2018</a>
+              <p>Modelling global concentrations of pollutants is very challenging, and models are continuously evaluated and improved. Previous research has shown that the CMIP6 multi-model simulations tend to underestimate PM2.5 concentrations when compared to global observations <a href="https://acp.copernicus.org/articles/20/14547/2020/" target="_new"(Turnock et al., 2020)</a>. To address this issue and to ensure a smooth time series between the model and satellite data, we take the following steps:</p> 
+              <p>For each city, we first calculate a three-year (2000-2002) mean of the satellite data for that city. Next, we calculate the three-year (2000-2002) mean of model concentrations for the same city. The ratio between these values represents the model's bias compared to observations. We then adjust (or "weight") the model values using this ratio. 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>Because so few historical observations of PM2.5 exist, so it is challenging to evaluate how good this approximation is, but in our approach the historical trend is taken from the computer model and the values are informed by the satellite.</p>
               <p>These images will be updated in the future as improved model simulations and observations become available.</p>
               <p>Both the computer model and the satellite use gridboxes that cover many kilometres, 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>
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