Wildfire Smoke Along Colorado's Front Range in 2020: An Evaluation of PM 2.5 Monitor Spacing and Smoke Forecasts

Abstract The 2020 wildfires caused widespread elevated fine particulate matter (PM 2.5 ) concentrations across the western U.S. The Colorado Front Range experienced multiple smoke episodes from nearby fires and fires hundreds of kilometers away. Multiple days (88 total) with elevated PM 2.5 along the Front Range occurred in 2020 between July and October. Smoke that originated solely from regional fires, defined here as the wildfires in Colorado, southern Wyoming, and eastern Utah, was present on 25 days, 23 days were impacted by solely long‐range transported smoke, and 40 days were impacted by both types of smoke. We leverage these events to (a) quantify the required spacing between PM 2.5 monitors to resolve concentration gradients in smoke plumes, and (b) evaluate the ability of a forecast system to warn the public of regional versus transported smoke. We use regulatory and community low‐cost PurpleAir monitoring networks and the High‐Resolution Rapid Refresh Smoke (HRRR‐Smoke) forecast. During smoke events, the PM 2.5 concentration can deviate significantly from the nearest regulatory monitor, suggesting that the regulatory monitor network is insufficient to quantify exposure during smoke events. We find a steady decrease in correlation with distance during periods with smoke from regional fires when monitors are further than ∼8 km apart. While overall, the HRRR‐Smoke predicts >84% of the elevated hourly concentrations to within one U.S. Air Quality Index category, it performs better when long‐range smoke is present. Characterizing exposure from regional fires continues to challenge both regulatory monitoring networks and forecasting systems.