2.2. High-volume filter sampling and analysis methods

From 1 June to 16 July 2013, PM_2.5 samples were collected onto Tissuquartz™ Filters (8 × 10 in., Pall Life Sciences) using high-volume PM_2.5 samplers (Tisch Environmental) operated at 1 m³ min⁻¹ at an ambient temperature described in detail elsewhere (Budisulistiorini et al., 2015; Riva et al., 2016). All quartz filters were prebaked prior to collection. The procedure consisted of baking filters at 550 °C for 18 h followed by cooling to 25 °C over 12 h.

The sampling schedule is given in Table 1. Either two or four samples were collected per day. The regular schedule consisted of two samples per day - one during the day, the second at night - each collected for 11 h. On intensive sampling days, four samples were collected, with the single daytime sample being subdivided into three separate periods. The intensive sampling schedule was conducted on days when high levels of isoprene, SO42− and NO_x were forecast by the National Center for Atmospheric Research (NCAR) using the Flexible Particle dispersion model (FLEXPART) (Stohl et al., 2005) and Model for Ozone and Related Chemical Tracers (MOZART) (Emmons et al., 2010) simulations. Details of these simulations have been summarized in Bud-isulistiorini et al. (2015); however, these model data were only used qualitatively to determine the sampling schedule. The intensive collection frequency allowed enhanced time resolution for offline analysis to examine the effect of anthro-pogenic emissions on the evolution of isoprene SOA tracers throughout the day.

Sampling schedule during SOAS at the BHM ground site.

In total, 120 samples were collected throughout the field campaign with a field blank filter collected every 10 days to identify errors or contamination in sample collection and analysis. All filters were stored at −20 °C in the dark until extraction and analysis. In addition to filter sampling of PM_2.5, SEARCH provided a suite of additional instruments at the site that measured meteorological and chemical variables, including temperature, relative humidity (RH), solar radiation (SR), barometric pressure (BP), trace gases (i.e., CO, O₃, SO₂, NO_x, and NH₃), and continuous PM monitoring. The exact variables measured with their respective instrumentation are summarized in Table S1 of the Supplement.

SOA collected in the field on quartz filters was extracted and isoprene tracers quantified by GC/EI-MS with prior trimethylsilylation. A 37 mm diameter circular punch from each filter was extracted in a pre-cleaned scintillation vial with 20 mL of high-purity methanol (LC-MS CHROMA-SOLV grade, Sigma-Aldrich) by sonication for 45 min. The extracts were filtered through polytetrafluorethylene (PTFE) syringe filters (Pall Life Science, Acrodisc^®, 0.2-μm pore size) to remove insoluble particles and residual quartz fibers. The filtrate was then blown dry under a gentle stream of N₂ at room temperature. The dried residues were immediately trimethylsilylated by reaction with 100 μL of N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) + trimethylchlorosilane (TMCS) (99:1 v/v, Supelco) and 50 μL of pyridine (anhydrous, 99.8%, Sigma-Aldrich) at 70 °C for 1 h. Trimethylsilyl derivatives of carbonyl and hydroxyl functional groups were measurable by our GC/EI-MS method. Derivatized samples were analyzed within 24 h after trimethylsilylation using a Hewlett-Packard (HP) 5890 Series II gas chromatograph coupled to an HP 5971A mass selective detector. The gas chromatograph was equipped with an Econo-Cap®-EC®−5 Capillary Column (30 m x 0.25 mm i.d.; 0.25 pm film thickness) to separate trimethylsilyl derivatives before MS detection. 1 μL aliquots were injected onto the column. Operating conditions and procedures have been described elsewhere (Surratt et al., 2010).

Extraction efficiency was assessed and taken into account for the quantification of all SOA tracers. Efficiency was determined by analyzing four prebaked filters spiked with 50 ppmv of 2-methyltetrols, 2-methylglyceric acid, levoglucosan, and cis- and trans-3 —MeTHF-3,4-diols. Extraction efficiency was above 90 % and was used to correct the quantification of samples. Extracted ion chromatograms (EICs) of m/z 262, 219, 231, and 335 were used to quantify the cis-/trans-3-MeTHF-3,4-diols, 2-methyltetrols and 2-methylglyceric acid, C₅-alkene triols, and IEPOX-dimers, respectively (Surratt et al., 2006).

2-Methyltetrols were quantified using an authentic reference standard that consisted of a mixture of racemic dias-teroisomers. Similarly, 3-MeTHF-3,4-diol isomers were also quantified using authentic standards; however, 3-MeTHF-3,4-diol isomers were detected in few field samples. 2-Methylglyceric acid was also quantified using an authentic standard. Procedures for synthesis of the 2-methyltetrols, 3-MeTHF-3,4-diol isomers, and 2-methylglyceric acid have been described elsewhere (Zhang et al., 2012; Budisulistiorini et al., 2015). C₅-alkene triols and IEPOX dimers were quantified using the average response factor of the 2-methyltetrols.

To investigate the effect of IEPOX-derived OS hydrolysis or decomposition during GC/EI-MS analysis, known concentrations (i.e., 1, 5, 10, and 25 ppmv) of the authentic IEPOX-derived OS standard (Budisulistiorini et al., 2015) were directly injected into the GC/EI-MS following trimethylsilylation. Ratios of detected 2-methyltetrols to the IEPOX-derived OS were applied to estimate the total IEPOX-derived SOA tracers in order to avoid double counting when combining the GC/EI-MS and UPLC/ESI-HR-QTOFMS SOA tracer results.

A 37 mm diameter circular punch from each quartz filter was extracted following the same procedure as described in Sect. 2.2.2 for the GC/EI-MS analysis. However, after drying, the dried residues were reconstituted with 150 μL of a 50: 50 (v/v) solvent mixture of methanol (LC-MS CHROMASOVL grade, Sigma-Aldrich) and high-purity water (Milli-Q, 18.2 ΜΩ). The extracts were immediately analyzed by the UPLC/ESI-HR-QTOFMS (6520 Series, Agilent) operated in the negative ion mode. Detailed operating conditions have been described elsewhere (Riva et al., 2016). Mass spectra were acquired at a mass resolution of 7000–8000.

Extraction efficiency was determined by analyzing three prebaked filters spiked with propyl sulfate and octyl sulfate (electronic grade, City Chemical LLC). Extraction efficiencies were in the range of 86–95%. EICs of m/z 215, 333, and 199 were used to quantify the IEPOX-derived OS, IEPOX-derived dimer OS, and the MAE-derived OS, respectively (Surratt et al., 2007a). EICs were generated with a ±5 ppm tolerance. Accurate masses for all measured organosulfates (OSs) were within ±5 ppm. For simplicity, only the nominal masses are reported in the text when describing these products. IEPOX-derived OS and IEPOX-derived dimer OS were quantified by the IEPOX-derived standard synthesized in-house (Budisulistiorini et al., 2015). The MAE-derived OS was quantified using an authentic MAE-derived OS standard synthesized in-house by a procedure to be described in a forthcoming publication (¹H nuclear magnetic resonance (NMR) trace, Fig. S2). Although the MAE-derived OS (Gomez-Gonzalez et al., 2008), which is more formally called 3-sulfooxy-2-hydroxy-2-methyl propanoic acid, has been chemically verified from the reactive uptake of MAE on wet acidic sulfate aerosol (Lin et al., 2013a), the term MAE/HMML-derived OS will be used hereafter to denote the two potential precursors (MAE and HMML) contributing to this OS derivative as recently discussed by Nguyen et al. (2015). It should be noted that Nguyen et al. (2015) provided indirect evidence for the possible existence of HMML. As a result, further work is needed to synthesize this compound to confirm its structure and likely role in SOA formation from isoprene oxidation.

EICs of m/z 155, 169, and 139 were used to quantify the glyoxal-derived OS, methylglyoxal-derived OS, and the hydroxyacetone-derived OS, respectively (Surratt et al., 2007a). In addition, EICs of m/z 211, 260, and 305 were used to quantify other known isoprene-derived OSs (Surratt et al., 2007a). Glycolic acid sulfate synthesized in-house was used as a standard to quantify the glyoxal-derived OS (Galloway et al., 2009), and propyl sulfate was used as a surrogate standard to quantify the remaining isoprene-derived OSs.

A 1.5 cm² square punch from each quartz filter was analyzed for total organic carbon (OC) and elemental carbon (EC) by the thermal-optical method (Birch and Cary, 1996) on a Sunset Laboratory OC-EC instrument (Tigard, OR) at the National Exposure Research Laboratory (NERL) at the US Environmental Protection Agency, Research Triangle Park, NC. The details of the instrument and analytical method have been described elsewhere (Birch and Cary, 1996). In addition to the internal calibration using methane gas, four different mass concentrations of sucrose solution were used to verify the accuracy of the instrument during the analysis.

Water-soluble organic carbon (WSOC) was measured in aqueous extracts of quartz fiber filter samples using a total organic carbon (TOC) analyzer (Sievers 5310C, GE Water & Power) equipped with an inorganic carbon remover (Sievers 900). To maintain low background carbon levels, all glass-ware used was washed with water, soaked in 10 % nitric acid, and baked at 500 °C for 5 h and 30 min prior to use. Samples were extracted in batches that consisted of 12–21 PM_2.5 samples and field blanks, one laboratory blank, and one spiked solution. A 17.3 cm² filter portion was extracted with 15 mL of purified water (> 18Ω Barnstead Easypure II, Thermo Scientific) by ultra-sonication (Branson 5510). Extracts were then passed through a 0.45 μm PTFE filter to remove insoluble particles. The TOC analyzer was calibrated using potassium hydrogen phthalate (KHP, Sigma-Aldrich) and was verified daily with sucrose (Sigma-Aldrich). Samples and standards were analyzed in triplicate; the reported values correspond to the average of the second and third trials. Spiked solutions yielded recoveries that averaged (±1 standard deviation) 96 ± 5 % (n = 9). All ambient concentrations were field blank subtracted.

Aerosol pH was estimated using a thermodynamic model, ISORROPIA-II (Nenes et al., 1998). SO42−, nitrate (NO3−), and ammonium (NH4+) ion concentrations measured in PM_2.5 collected from BHM, as well as RH, temperature, and gas-phase ammonia (NH3) were used as inputs into the model. These variables were obtained from the SEARCH network at BHM, which collected the data during the period covered by the SOAS campaign. The ISORROPIA-II model estimates particle hydronium ion concentration per unit volume of air (H⁺, μg m⁻³), aerosol liquid water content (LWC, μgm⁻³), and aqueous aerosol mass concentration (μgm⁻³). The model-estimated parameters were used in the following formula to calculate the aerosol pH:

where a_H⁺ is H⁺ activity in the aqueous phase (molL⁻¹), LMASS is total liquid-phase aerosol mass (μgm⁻³), and ρ_er is aerosol density. Details of the ISORROPIA-II model and its ability to predict pH, LWC, and gas-to-particle partitioning are not the focus of this study and are discussed elsewhere (Fountoukis et al., 2009).

Nitrate radical (NO₃) production (P[NO₃]) was calculated using the following equation:

where [NO₂] and [O₃] correspond to the measured ambient NO₂ and O₃ concentrations (mol cm⁻³), respectively, and k is the temperature-dependent rate constant (Herron and Huie, 1974; Graham and Johnston, 1978). Since no direct measure of NO₃ radical was made at this site during SOAS, P[NO₃] was used as a proxy for NO₃ radicals present in the atmo-sphere to examine whether there is any association of it with isoprene-derived SOA tracers.