Molecular Characterization and Photoreactivity of Organic Aerosols Formed from Pyrolysis of Urban Materials during Fires at the Wildland-Urban Interface

Abstract

Fires at the wildland-urban interface (WUI) are increasing in magnitude and frequency, emitting organic aerosol (OA) with unknown composition and atmospheric impacts. In this study, we investigated the chemical composition of OA produced through the 600 °C pyrolysis of ten urban materials in nitrogen, which were subsequently aged under UV light for 2 h. The analysis utilized ultrahigh-performance liquid chromatography (UHPLC) separation, coupled with a photodiode array (PDA) detector and a high-resolution mass spectrometer (HRMS) for molecular characterization. Hierarchical clustering analysis demonstrated that lumber-derived OA was the most diverse and distinct in composition. Unaged and aged OA (for each urban material) did not significantly differ in chemical identities. Potential aromatic brown carbon (BrC) chromophores (based on their degree of unsaturation) constituted 13-42% of all assigned compounds. PDA chromatograms revealed multiple BrC chromophoric species that were either enhanced or degraded as a result of UV aging, providing insights into specific BrC chromophores responsible for photobleaching and photoenhancement of the overall absorption coefficient. Thirty-six BrC chromophores were identified across the ten OA types, and their structures were confirmed using reference standards. Components of plasticizers and resins, such as phthalic and terephthalic acids, were structurally confirmed in the samples. We present potential species for WUI fires as components of resins, epoxies, dyes, and adhesives commonly used in manufacturing urban materials. Photolysis did not significantly impact the chemical composition of OA emitted from the burning of specific WUI materials.

Figure 1

UHPLC-ESI total ion chromatograms collected in the positive (red) and negative (blue) ionization modes for the ten unaged OA samples (panels a–j). Repeated peaks are indicated by dashed lines and labeled with the major ion’s assigned neutral molecular formula.

Figure 2

Hierarchical clustering of ESI(+) mass spectral species deconvoluted and identified using Compound Discover 3.3.200 SP3 software (Thermo Fisher Scientific). Columns represent unaged (blue) and aged (red) OA samples. Rows denote individual compounds (assigned with CH, CHO, and CHON molecular formulas) and are scaled using unit variance. The total number of rows (or compounds) detected across all 20 samples was 6509. The distance between clusters (top and left) is based on the correlation distance and average linkage for both rows and columns. The color gradient, ranging from white to purple, represents the abundance of each compound. Standardized matrix values, as determined by the Compound Discover program, were used to scale the data to ensure that all ions have the same range and importance in the color scale.

Figure 3

BrC chromophores present in the carpet and fiberboard OA samples before (a and c) and after UV aging (b and d), respectively. Benzoic acid, p-coumaric acid, catechol, and terephthalic acid were structurally identified with standards (Table S1). Note that the maximum absorbance value in the carpet and fiberboard OA panels are different. As classified in Hopstock et al., carpet OA exhibited photobleaching, whereas fiberboard OA exhibited photoenhancement after UV aging.

Figure 4

BrC chromophores present in the vinyl flooring OA sample before (a) and after UV aging (b). Terephthalic acid was verified with an internal standard (Table S1). As classified in Hopstock et al., vinyl flooring OA exhibited no net change in the overall absorbance after aging.

Figure 5

Plot of double bond equivalent (DBE) versus the number of carbon and nitrogen (C+N) atoms for assigned OA species in unaged and aged carpet (a and b), vinyl flooring (c and d), and drywall (e and f) OA. The size of each symbol is arbitrarily scaled to the cubic root of the corresponding MS peak intensity in order to display both stronger and weaker peaks. Reference lines indicate DBE/(C+N) values corresponding to fullerene-like hydrocarbons (0.9 × C limit) and linear polyenes (0.5 × C limit).^, Yellow circles represent the percentage of data points (by number) that fall within the dashed lines and orange shaded region, thus predicted to be potential BrC chromophores. The total number of points is provided in each panel. The presented data come from both the positive and negative ionization modes. For all formulas, the number of nitrogen atoms (N ≤ 6) is much smaller than the number of the carbon atoms.