Table 3: Impact of Covid Mobility Restrictions on Air Pollution Measures (Hourly Averages, 2020 & 2023)

||(1)<br>Regulatory|(2)<br>Satellite|(3)<br>Low-cost|(4)<br>PA Local Calibration|
|---|---|---|---|---|
|Policy Period (Mar 14-May 12)|-37.58<sup>_∗∗∗_</sup>|-29.41<sup>_∗∗∗_</sup>|-4.18<sup>_∗∗∗_</sup>|-6.32<sup>_∗∗∗_</sup>|
||(2.91)|(7.92)|(0.66)|(1.04)|
|2020|20.54<sup>_∗∗∗_</sup>|31.90<sup>_∗∗∗_</sup>|2.61<sup>_∗∗∗_</sup>|4.21<sup>_∗∗∗_</sup>|
||(1.46)|(2.95)|(0.62)|(0.98)|
|Policy Period (March 14-May 12)*2020|-24.98<sup>_∗∗∗_</sup><br>(1.71)|-30.51<sup>_∗∗∗_</sup><br>(3.67)|-8.50<sup>_∗∗∗_</sup><br>(0.50)|-13.61<sup>_∗∗∗_</sup><br>(0.78)|
|Average|100.07|129.38|23.08|58.72|
|% of Average|-24.97|-23.58|-36.85|-23.18|

**Note** : Controlling for temperature and humidity, with hourly, weekly, monthly, day-of-week and monitor fixed effects. Contains data from 22nd January to 12th May for 2020 and 2023. The ’Average’ is based off the pre-Covid average in 2020. Regulatory data comes from monitors operated by the CGQA. Satellite estimates are generated from the MERRA-2 assimilation. The satellite value is the MERRA-2 PM2.5 estimate closest to the Bel-air regulatory-grade monitor station. The Low-cost measurements in Column 3 are the unadjusted values from all Purple Air monitors active in 2020 or 2023. The Local calibration is based on calibrating the unadjusted Purple Air data using co-located regulatory data from two local monitors. Winsorizing outliers at 99th percentile.<sup>_∗_</sup> _p <_ 0 _._ 05,<sup>_∗∗_</sup> _p <_ 0 _._ 01,<sup>_∗∗∗_</sup> _p <_ 0 _._ 001.

the coefficient is larger (though still smaller than the coefficient in the regression using regulatory data), and the percent decrease becomes perfectly in line with the regulatory and satellite data.

These findings are encouraging because they demonstrate that it is possible to measure similar impacts of a policy on PM2.5 pollution at the city level using three different data sources. Thus, if there is data available from only one source, it is still possible to measure the impacts of a policy that affects pollution at a larger geographic level. Nevertheless, the ability to accurately measure the impact of a policy with any data source will depend on the type of policy and the spatial and temporal frequency of interest. For instance, if there is interest in evaluating the impact of an intervention that is spatially concentrated and is unlikely to affect air pollution globally in a city, the satellite data would not be adequate since it provides only one value for the city. Similarly, if policy makers are interested in the impact of a policy to reduce short-term peaks in air pollution, the satellite data would not be a good choice since it is not well aligned with data from regulatory-grade monitors at the hourly level. In these cases, it would be necessary to use data from air pollution monitors on the ground that collect data across space (whether low-cost or regulatory-grade), and the accuracy would depend on the density of the network.

Finally, the results up to this point suggest that the unadjusted PM2.5 measurements from the lowcost monitors give much lower magnitudes (and a lower coefficient) relative to the regulatory and satellite data, even as the effects of a policy measured in percent change is largely aligned across data sources. Therefore, we also evaluate the impact of the policy at the hourly level using the four different calibrations for the Purple Air Data: the two off-the-shelf calibrations from the literature (Barkjohn and Jaffe), and our two co-located calibrations based on PM2.5 measurements from regulatory-grade monitors and satellite estimates (Table 4). Both off-the-shelf calibrations estimate much smaller coefficients (treatment effect in levels), even as the treatment effect as a percent change stays around the same value for the Barkjohn calibration. The co-located calibrations result in a larger coefficient than the unadjusted data and are more in line with the coefficient estimated with the data from the regulatory-grade monitors.

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