The use of low-cost sensors (LCSs) for the mobile monitoring of oil and gas emissions is an understudied application of low-cost air quality monitoring devices. To assess the efficacy of low-cost sensors as a screening tool for the mobile monitoring of fugitive methane emissions stemming from well sites in eastern Colorado, we colocated an array of low-cost sensors (XPOD) with a reference grade methane monitor (Aeris Ultra) on a mobile monitoring vehicle from 15 August through 27 September 2023. Fitting our low-cost sensor data with a bootstrap and aggregated random forest model, we found a high correlation between the reference and XPOD CH4 concentrations (r = 0.719) and a low experimental error (RMSD = 0.3673 ppm). Other calibration models, including multilinear regression and artificial neural networks (ANN), were either unable to distinguish individual methane spikes above baseline or had a significantly elevated error (RMSDANN = 0.4669 ppm) when compared to the random forest model. Using out-of-bag predictor permutations, we found that sensors that showed the highest correlation with methane displayed the greatest significance in our random forest model. As we reduced the percentage of colocation data employed in the random forest model, errors did not significantly increase until a specific threshold (50 percent of total calibration data). Using a peakfinding algorithm, we found that our model was able to predict 80 percent of methane spikes above 2.5 ppm throughout the duration of our field campaign, with a false response rate of 35 percent.

Wildfires at the wildland-urban interface (WUI) have been increasing in frequency over recent decades due to increased human development and shifting climatic patterns. The work presented here focuses on the impacts of a WUI fire on indoor air using field measurements of volatile organic compounds (VOCs) by Proton-Transfer-Reaction Time-of-Flight Mass Spectrometry (PTR-TOF-MS). We found a slow decrease in VOC mixing ratios over the course of roughly 5 weeks starting 10 days after the fire, and those levels decreased to ∼20% of the initial indoor value on average. The VOC composition could be described by a combination of biomass burning emissions and indoor air composition. Comparisons were made between polycyclic aromatic hydrocarbon (PAH) distributions in the gas phase and ash, with differences observed in their distribution between each other and when compared to fresh fuel inventory measurements. Mitigation tests were conducted running air cleaners with activated carbon and opening windows to promote indoor-outdoor air exchange, with both methods showing a decrease greater than 50% for average VOC levels indoors while active. We compare our results with simulated smoke impact experiments that show the slow decline in VOCs must be understood in the context of indoor reservoirs, beyond just on surfaces, leading to the slow release of VOCs to indoor air.

Persistent cold air pools (PCAP), also referred to colloquially as inversions, are responsible for some of the greatest enhancements in air pollution in Utah's Wasatch Front. PCAPs, which can last fora period of days or weeks, trap warm air beneath a layer of colder air, which results in the accumulation of particulates during the inversion. Fine particulate matter (PM2.5) sampling occurred in seven field sites across Salt Lake County (SLCo) during Wintertime (November-April). Concentrations of the organic mass of PM 2.5 increased during inversion events ( = 4.0 mu g/m3) when compared to the wintertime baseline ( = 3.5 mu g/m3). However, organic mass enhancements during PCAPs were most pronounced at lowest-altitude field sites situated near potential PM 2.5 sources. Four sources of organic carbon were identified, comprised of industrial, abrasive, wood burning, and onroad sources. During PCAP events, PM 2.5 species profiles exhibited greater spatial heterogeneity, due to lower wind speeds and caps on vertical mixing (Coefficient of Determination = 0.51, Coefficient of Determination = 0.43). These results indicate both elevation and local source emissions maybe of increased importance in understanding PM 2.5 concentrations during PCAP events.

Quantifying changes in the properties of smoke aerosols under varying conditions is important for understanding the health and environmental impacts of exposure to smoke. Smoke composition, aerosol liquid water content, effective density (ρeff), and other properties can change significantly as smoke travels through areas under different ambient conditions and over time. During this study, we measured changes in smoke composition and physical properties due to oxidative aging and exposure to humidity. We found that smoke aging led to SOA formation and increases in ratios of organic carbon to elemental carbon. Aerosol liquid water content increased with increasing relative humidity (RH), and aged smoke took up more water than fresh smoke at all humidity levels, likely due to a combination of changes in aerosol surface polarity at low and medium RH and increases in surface area with aging at high RH. Growth factors ranged from 1.06 ± 0.08 for fresh smoke at low RH to 1.32 ± 0.08 for aged smoke at high RH. Oxidative aging and exposure to humidity led to increases in ρeff. For 100 nm particles, ρeff ranged from ∼1.2 for fresh smoke at low RH to ∼1.6 for aged smoke at high RH. Results from these experiments suggest that exposure to humidity leads to smoke restructuring and compaction and/or changes in surface chemistry.

The impacts of wildfires along the wildland urban interface (WUI) on atmospheric particulate concentrations and composition are an understudied source of air pollution exposure. To assess the residual impacts of the 2021 Marshall Fire (Colorado), a wildfire that predominantly burned homes and other human-made materials, on homes within the fire perimeter that escaped the fire, we performed a combination of fine particulate matter (PM 2.5 ) filter sampling and chemical analysis, indoor dust collection and chemical analysis, community scale PurpleAir PM 2.5 analysis, and indoor particle number concentration measurements. Following the fire, the chemical speciation of dust collected in smoke-affected homes in the burned zone showed elevated concentrations of the biomass burning marker levoglucosan (median levo = 4147 ng g −1 ), EPA priority toxic polycyclic aromatic hydrocarbons (median Σ 16 PAH = 1859.3 ng g −1 ), and metals (median Σ 20 Metals = 34.6 mg g −1 ) when compared to samples collected in homes outside of the burn zone 6 months after the fire. As indoor dust particles are often resuspended and can become airborne, the enhanced concentration of hazardous metals and organics within dust samples may pose a threat to human health. Indoor airborne particulate organic carbon (median = 1.91 μg m −3 ), particulate elemental carbon (median = .02 μg m −3 ), and quantified semi-volatile organic species in PM 2.5 were found in concentrations comparable to ambient air in urban areas across the USA. Particle number and size distribution analysis at a heavily instrumented supersite home located immediately next to the burned area showed indoor particulates in low concentrations (below 10 μg m −3 ) across various sizes of PM (12 nm–20 μm), but were elevated by resuspension from human activity, including cleaning. Graphical Abstract