## 2 Materials and Methods  
### Calibration of PurpleAir Monitors 1 and 2  
Prior to the start of the present study, two of the PurpleAir monitors used were calibrated against three research-grade optical particle counters (TSI Model 510 Sidepaks equipped with PM\({}_{2.5}\) cutpoint inlets) [27]. The particle source was aerosol produced from a single puff of marijuana smoke from a vaping pen. The Sidepaks were part of a group of six Sidepaks, which were in turn calibrated against gravimetric samples sampling from the same indoor source [28]. Over an eight-month period, 47 experiments, each lasting 6-10 h, were carried out in a dedicated 30 m\({}^{3}\) room of a home in Santa Rosa, CA, USA. The experiments plotted the decay of the particle concentration. Following initial mixing, the decay becomes linear (on a logarithmic scale) for a period of well-mixed concentrations. The slope is given by the sum of the air change rate \(a\) and the deposition rate \(k\). The air exchange rate was measured by releasing a puff of carbon monoxide and tracking its decay using a Langan CO monitor T15. The regression line fitted to the decay curve can then be followed back to the beginning of the experiment to estimate the total mass released (in mg/puff), using a method developed in [29]. From the gravimetric experiments, the Sidepaks were determined to have a calibration factor (CF) of 0.44 (SE 0.03) [28]. The two PurpleAir monitors (numbers 1 and 2 with four independent Plantower sensors 1a, 1b, 2a, and 2b) were determined to have a calibration factor of 3.24, midway between the calibration factors of 3.0 and 3.4 found in various studies of outdoor air [4; 19; 20; 28]. A linear least-squares regression of the PurpleAir monitors against the SidePaks resulted in a slope of 1.00 and an R\({}^{2}\) of 98.6% (Figure 3).  
Two sites provided data using both the Plantower CF_1 and pm2.5 alt algorithms. At the Santa Rosa site where monitors 1 and 2 were calibrated, data were collected for one year (2021) using four monitors (eight Plantower sensors). Three PA-II monitors were collocated indoors in the same 30 m\({}^{3}\) room where monitors 1 and 2 were calibrated. They were placed 1.5 m high with the intake unobstructed. One monitor was outside at a height of 2 m and at 1 m distance from the house. The monitors collected data every two minutes, and the data were averaged over 10 min periods. In 2022, two new PurpleAir Flex monitors with two Plantower PMS 6003 sensors each were added to the four existing monitors with the Plantower PMS 5003 sensors. Therefore, a second data collection period was selected for the Santa Rosa site running from 1 January 2023 to 13 July 2023.  
At the Redwood City site, the data were collected by two collocated PA-II monitors at 1 m height in a 43 m\({}^{3}\) room. One PA-II monitor was located outdoors. Two data periods were selected from this site. The first runs from 29 April 2021 to 31 October 2022. The second runs from 22 September 2022 to 23 June 2023.  
In summary, four datasets from two locations were analyzed for this study (Table 4). Ultimately, 18 independent Plantower sensors were included.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Site** & **Year** & **Start Date** & **End Date** & **N obs** \\ \hline Santa Rosa & 2021 & 1 January 2021 & 31 December 2021 & 52,400 \\ \hline Santa Rosa & 2023 & 1 January 2023 & 13 July 2023 & 24,700 \\ \hline Redwood City & 2021–2022 & 29 April 2021 & 31 October 2022 & 78,900 \\ \hline Redwood City & 2022–2023 & 22 September 2022 & 23 June 2023 & 39,900 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Datasets analyzed in this study.  
Figure 3: PurpleAir (CF 3.24) estimates of particle source strength (mg/puff) compared to SidePak (CF 0.44). PurpleAir monitor IDs 1 and 2 (sensors 1a, 1b, 2a, 2b) were averaged for this comparison. SidePak values are means of triplicate SidePaks.