## 2 Data and Methods  
### Data  
* Satellite Imagery  
We used Maxar Technologies (formerly DigitalGlobe, USA) WorldView2 (WV2) images available over Greater London, Vancouver, LA, and NYC for the year 2010. WV2 was launched on 8 October 2009, as part of the Maxar satellite constellation. It is a Sunsynchronous satellite with a 10:30 a.m. descending node overpass, located 770 km from Earth. Images are produced in 8 spectral bands in the VIS-NIR range (400 nm-1040 nm) with a spatial resolution of 50 cm (for the panchromatic imagery) and 2.5 m (for the multi-spectral imagery) [27]. The latter have been used in this study. Access to the Maxar imagery archive was completed under the 'NextView' license framework. Over the study period, we obtained and pre-processed raw imagery over all study areas, including radiometric correction and cloud cleaning, resulting in a total of 717 non-overlapping 2.5 m spatial-resolution satellite images that cover all the studied cities for 2010. These images were used to construct a total of 612,248 100 m \(\times\) 100 m (40 pixels \(\times\) 40 pixels) image patches that were then used as input to the model. Table 1 shows the number of available images and constructed patches for each individual city (more details in the \"Data Preparation\" section). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**No. of** & **London** & **Vancouver** & **Los Angeles** & **NYC** \\ \hline Available images & & & & \\ (patches) & 61 (105,242) & 337 (117,924) & 315 (369,602) & 4 (19,480) \\ \hline Annual mean LUR & & & & \\ PM\({}_{2.5}\)/NO\({}_{2}\) (\(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\)) & 14.4/41.0 & 2.22/8.07 & 7.48/37.8 & 9.46/47.4 \\ \hline Co-located PM\({}_{2.5}\)/NO\({}_{2}\) ground monitoring sites * & 11/7 & 8/12 & 8/10 & 6/2 \\ \hline \multicolumn{5}{c}{**Ground monitoring sites—PM\({}_{2.5}\)/NO\({}_{2}\)**} \\ \hline Annual Mean (\(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\)) & 16.17/61.32 & 5.65/8.11 & 10.91/25.99 & 10.17/- \\ \hline Annual SD (\(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\)) & 2.43/27.85 & 2.49/4.63 & 1.85/8.71 & 1.64/- \\ \hline \hline \end{tabular}  
* Only sites that had co-located model-estimated values within a distance less than 500 m were included. For London, daily station data was used to calculate annual means of measured pollution over ground stations that had at least one measured value every month. For the other cities, annual means of measured data were used for the stations that had collected data for over 50% of the year.  
\end{table} 
Table 1: Summary details of data availability over the study areas (2010). spatial resolutions. These high spatial resolutions are key for detecting pollution sources and are a proxy for human activity and urban planning. We use information from 2.5 m spatial-resolution satellite imagery to understand the relationship between PM\({}_{2.5}\)/NO\({}_{2}\) and surface reflectance (in a broader look). The advantage of using very high-spatial-resolution satellite images is the ability to capture urban features that are known to influence pollution levels (e.g., roads, buildings, trees) that are often used as inputs to LUR models but not visible at lower resolutions. This approach helps our proposed model to account for more than a single pixel (2.5 \(\times\) 2.5 m\({}^{2}\) area) but understand the urban composition causing that pollution level. This study introduces an opportunity to use meter-scale images that are globally available to develop a more parsimonious model for estimating AQ. This approach can then be applicable in developed and developing urban environments that lack the data required for developing LUR models. * Ground monitoring data  
Ground monitoring data of PM\({}_{2.5}\) and NO\({}_{2}\) concentrations in ug/m\({}^{3}\) were obtained for 2010 at all study areas and annual concentrations were calculated. For London, daily ground monitoring data were used to calculate annual means of measured pollution over ground stations that had at least one measured value every month.