Impacts on Urban VOCs and PM2.5 during a Wildfire Episode

Abstract

This study focuses on the impact of wildfire smoke emissions on regional, urban air quality during a wildfire event. We measured volatile organic compounds (VOCs) and fine particulate matter (PM2.5) in the San Francisco Bay Area to assess air quality during a wildfire event and compared them to those in a later non-wildfire period. VOCs were collected using thermal desorption tubes and quantified using thermal desorption-gas chromatography/mass spectrometry (TD-GC/MS). Elevated concentrations of VOCs such as 1,3,5-trimethylbenzene (0.33 ± 0.01 µg/m3), benzene (1.03 ± 0.02 µg/m3), toluene (2.15 ± 0.04 µg/m3), ethylbenzene (0.60 ± 0.02 µg/m3), and m, p-xylene (0.77 ± 0.07 µg/m3) were observed in the wildfire event. Compared with that in the non-wildfire season, the toluene concentration during the wildfire period was more than three times the non-wildfire concentration. Similarly, the benzene concentration during the wildfire was almost four times higher, and that of p, m-xylene was three times higher. The higher wildfire levels were statistically significant for sec-butylbenzene, 1,2,4-trimethylbenzene, n-propylbenzene, o-xylene, styrene, 1,3,5-trimethylbenzene, benzene, toluene, ethylbenzene, and p,m-xylene (p < 0.00001). These higher VOC levels compared with those for the non-fire period may potentially pose a public health concern. Open face passive sampler (OFPS)-collected PM was analyzed using scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS) and revealed organic carbon tar balls. The highest daily average beta-attenuation-monitored PM2.5 during the fire period was 200 µg/m3 and the highest hourly average was 270 µg/m3. Monitoring gas phase species in addition to PM concentrations is useful during wildfire season to inform public health guidance.

A Preliminary Case Study on the Compounding Effects of Local Emissions and Upstream Wildfires on Urban Air Pollution

“…More studies like Rice et al 2023 [15] are needed across a range of cities and smoke sources (grassland versus timber or other fires) to better understand the complexity of WUIs [49,50]. As discussed by Wang et al [14], we need "Monitoring gas phase species in addition to PM during wildfire season to inform public health guidance". Satellite data, in concert with expanded spatial long-term monitoring of wildfire pollutants encroaching on cities, will be key to elucidating more fully the impact of wildfire pollutants on urban air quality.…”

“…Interactions between urban and wildfire pollution emissions are active areas of research, and numerous aircraft field campaigns and satellite analyses of wildfire pollution have been conducted in recent years [9][10][11]. A number of studies have found that elevated ozone and particulate pollution levels are both generally associated with wildfire smoke in urban areas [12,13], as well as elevated black carbon [14]. In addition to particulate pollution, a wide range of hazardous air pollutants (HAPS) have been measured during wildfire smoke episodes over urban regions, including acetaldehyde, formaldehyde, and tetrachloroethylene [15].…”

“…In addition to particulate pollution, a wide range of hazardous air pollutants (HAPS) have been measured during wildfire smoke episodes over urban regions, including acetaldehyde, formaldehyde, and tetrachloroethylene [15]. As discussed by Wang et al 2024 [14], the quantities of volatile organic compounds (VOCs) such as benzene and toluene are also found to be very elevated in urban air that is influenced by wildfire smoke. McClure and Jaffe [16] found enhancements of 32% in maximum 8 h tropospheric ozone levels during wildfire smoke events at a study site in Idaho.…”