2.2 Particulate matter collection

A miniature single-stage electrostatic precipitator (ESP) collected PM directly onto the PDMS layer, see S1 Fig in S1 File. The device did not employ a charging stage; instead, we relied on PM native charge from the combustion process. Woodsmoke was generated by burning 3.8 cm by 1.9 cm Douglas fir sticks in a side-feed natural-draft cookstove [34]. Cigarette smoke was generated by lighting cigarettes in a sealed well-mixed aerosol test chamber [35]. Size distribution was measured using a scanning mobility particle sizer (SMPS, TSI Nanoscan 3910); peak particle concentration for woodsmoke and cigarette smoke was recorded for particle diameters of dp~60 nm, see Fig 1. The collector did not employ a charging stage; instead, we relied on PM native charge from the combustion process. Two coated collection substrates were placed in a Teflon holder, 5 mm apart, forming a rectangular flow channel, with PDMS layers facing the flow. Particle-laden flow was aspirated between the collection substrates at the flow rates of 1.8 slpm. Two copper electrodes positioned on either side were connected to a high voltage power supply (Glassman High Voltage Inc. EH Series, High Bridge, NJ, USA). An electric potential φ = 3 kV was applied between the electrodes resulting in electric field strength E = 0.42 kV/mm. The collection efficiency (CE%) was determined by comparison with a parallel reference channel plumbed to SMPS [36,37], and for these conditions, CE% ~ 40–60%, see Fig 1 (right). Higher efficiency for smaller (d_p < 50 nm) combustion-generated was observed compared to ambient aerosol. It is likely due to the greater residual charge on the combustion particle compared to Boltzman charge distribution associated with ambient aerosol aging.

Left—Particle size distribution for Cigarette and Cookstove smokes; Right–Collection efficiency of parallel plate collector for combustion-generated aerosols and ambient particles. Collection efficiency of parallel plate collector for ambient PM as a function of particle size at 0.75 slpm. Each data point corresponds to three or more particle count measurements by a TSI SMPS NanoScan particle counter with error bars representing one standard deviation.

The particles were collected onto PDMS coated slide, the tests were run until visible particle deposition was observed (90 min for woodsmoke and 10min for cigarette smoke). The substrates were removed from the collector and isolated for ~ 24 hours to allow adequate time to extract organic compounds into PDMS.

The reference sample was collected for LP-EEM analysis [32,33] on PTFE membrane filters (Pall Zefluor®, Cat.# P5PJ037, Pall Corp., Port Washington, NY, USA) using Harvard impactor (Cat. # HP2518, BGI, Butler, NJ, USA). After the gravimetric analysis, the samples were extracted in cyclohexane (Cat. #1.02822.2500, Uvasol®, Millipore Sigma, Burlington, MA, USA). The extracts were filtered with 0.2 μm PTFE syringe filters (Cat. #28145–491, VWR, Edison, NJ, USA) and diluted to achieve ~3.5–5 μg/mL concentration. The filtered extracts were then analyzed by EEM using a spectrofluorometer (Aqualog-880-C, HORIBA Inc, Edison, NJ, USA). The spectra from the liquid extracts were recorded in the range of excitation wavelength λ_ex = 200–600 nm with a 2 nm resolution. For each excitation wavelength, the instrument records emissions using a CCD array in the range of λ_em = 246–826 nm with a 0.58 nm resolution.

The extracts were analyzed using an Agilent 7000 GC/MS Triple Quad Mass Spectrometer using two 15 m columns (Part #: HP-5MS UI, Agilent, Santa Clara, CA, USA) equipped with a backflush. 24 PAHs (EPA 16 PAHs and eight additional compounds with MW up to 302 g/mol) were included in the analysis. Calibration curves for PAH species with seven concentration levels in the range of 1–1000 ng/mL were obtained. The calibration standards for the 24 PAHs were a mixture of 23 compounds (Wellington Laboratories Cat. # PAH-STK-A, Guelph, ON, Canada) and one additional compound, coronene, a standard PAH used in several studies on mechanisms of soot formation (AccuStandard Cat. # H-116, New Haven, CT, USA). Each calibrant included 16 deuterium-labeled PAH internal standards (Wellington Laboratories Cat. # PAH-LCS-A, Guelph, ON, Canada). PAH internal standards were at a concentration of 100 ng/mL in the calibrants. An equivalent concentration of the same internal standard was spiked into each sample for use in quantification. The instrument was operated in pseudo multiple reaction monitoring (PMRM) mode. Mahamuni et al. reported the wood smoke samples’ chemical composition, calculating that the total mass of 16 EPA PAHs was ~20–25 ng/μg of soot, see S6 Fig in S1 File. About 75% (by weight) of PAHs in the sample were high species with MW>202 g/mol (i.e., larger than Pyrene) [13]. TOC analysis was not performed here but typically reported TOC values for cookstoves are greater than 50% depending on the type of the cookstove and operational conditions [38–40]. The smoldering combustion during cigarette smoking does create any elemental carbon, and generated particles remain in the organic phase as the carbonization threshold is not reached [11,41].