A national crowdsourced network of low-cost fine particulate matter and aerosol optical depth monitors: results from the 2021 wildfire season in the United States

Fine particulate matter (PM2.5) is a leading contributor to premature death, disease, and environmental degradation globally. Wildfire smoke is a primary source of air pollution in the United States. However, reference-grade ground monitors are cost prohibitive to deploy at the spatial scales needed to assess the variability of wildfire smoke. In many regions lacking adequate monitoring, health impact assessments and epidemiological studies on smoke may rely on satellite-based instruments, which estimate PM2.5 based on Aerosol Optical Depth (AOD). However, AOD measurements may not capture surface-level impacts; thus, measurements in under-monitored regions are necessary to interpret satellite observations. While low-cost PM2.5 sensors have been deployed at large scales, these sensors typically lack AOD measurement capability. In prior work, we designed the Aerosol Mass and Optical Depth (AMODv2) sampler, which can simultaneously measure PM2.5 (optical and gravimetric filter-based) and AOD. In this work, we distributed AMODv2s to volunteers at 31 locations in the contiguous United States, forming a nationwide network for the summer of 2021. A majority of our network was successful, with 86.6% of sampling periods resulting in a valid sample. We found our AOD sensors agreed closely with nearby (<25 km) reference monitor measurements (taken <180 seconds apart), with mean absolute error results of 0.03, 0.02, 0.02, and 0.03 at 440, 500, 675, and 870 nm, respectively. In a regional analysis of wildfire smoke, we observed elevated PM2.5 and AOD on smoky days at study sites in most regions, which led to similar PM2.5:AOD ratios regardless of smoke. However, at the California study sites, median PM2.5 remained similar on smoky days relative to non-smoky days, while AOD increased, implying that the smoke may have been lofted above the surface during the study period. At the California study sites, the median PM2.5:AOD ratio was 67.2 mu g m(-3) on non-smoky days, compared with 30.2 mu g m(-3) on smoky days. At study sites in other regions, the average ratio was 24.7 to 33.5 mu g m(-3) on non-smoky days and 20.3 to 29.4 mu g m(-3) on smoky days. We show that paired PM2.5 and AOD measurements collected by a crowdsourced network can highlight anomalies in air quality during smoke events and provide insights into the relationship between satellite-based and ground-based air quality observations. Environmental significance Ambient fine-particulate air pollution (PM2.5) is a leading risk factor for premature disease and death, globally, and a contributor to Earth's energy balance. Both satellite remote sensing and ground-based monitoring have a role to play in quantifying surface PM2.5 concentrations, but networks that monitor both aerosol optical depth (AOD; the satellite product) and PM2.5 mass (the ground-based monitor product) are sparse. Here, we demonstrate that a low-cost monitor for PM2.5 and AOD, when deployed in a national crowdsourced monitoring network, can provide valuable insights into air quality.