Table S1. Sample collection dates for aerosol WSOC measurements, measurements performed on each sample during this study, and supporting measurements presented previously (Wozniak et al., in press). All TOC, fWSOC, and WSOC kinetics measurements were performed in triplicate.

Millbrook / 5/23-24/06 / 5/25-26/06 / 5/25-26/06 / 8/18-20/06 / 12/1-2/06 / 12/3-4/06 / 12/5-6/06 / 3/5-6/07 / 3/6-7/07 / 3/8-9/07 / 3/9-10/07 / 5/13-15/07 / 5/15-16/07
TSP / x / x / x / x / x / x / x / x / x / x / x / x / x
TOC / x / x / x / x / x / x / x / x / x / x / x / x / x
TOC Δ14C, δ13C / x / x / x / y / x / x / y / x
WSOC Δ14C, δ13C / x / y / x / x / y / x / x / x / y
WSOC kinetics / x / x / x / x
Harcum / 6/8-9/06 / 6/20-21/06 / 7/6-7/06 / 7/18-19/06 / 8/1-2/06 / 8/30-31/06 / 9/26-27/06 / 11/15-16/06 / 1/4-5/07 / 1/24-26/07 / 2/1-2/07 / 2/14-15/07 / 3/15-16/07 / 3/22-23/07 / 4/5-6/07 / 5/17-18/07 / 5/31-6/1/07
TSP / x / x / x / x / x / x / x / x / x / x / x / x / x / x / x / x / x
TOC / x / x / x / x / x / x / x / x / x / x / x / x / x / x / x / x / x
TOC Δ14C, δ13C / x / x / y / x / x / y / x / x
WSOC Δ14C, δ13C / x / x / x / x / y / x / x / x / x / x
WSOC kinetics / x / x / x / x

‘x’ denotes a sample that was measured in triplicate on one occasion. ‘y’ denotes samples that were measured in triplicate on two separate occasions.

For isotopic analyses (Δ14C and δ13C of TOC and WSOC), ‘x’ denotes samples for which a single measurement was performed and ‘y’ denotes samples for which duplicate measurements were performed.

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Table S2. TOC, BC, and WSOC concentrations (μg C m-3) for aerosol particulate samples from Millbrook, NY and Harcum, VA in 2006-2007 that were also analyzed for δ13C and Δ14C (see Table 2).

Sampling Site/Date / TOCa / BCa / WSOC
Millbrook
5/23-24/06 / 3.12 / bdlb / 1.02
8/18-20/06 / 3.40 / nmc / 1.83
12/1-2/06 / 1.49 / 0.02 / 0.61
12/3-4/06 / 2.04 / bdlb / 0.89
12/5-6/06 / 2.75 / bdlb / 1.10
3/5-6/07 / 1.50 / 0.10 / 0.29
3/9-10/07 / 2.59 / bdlb / 0.82
5/13-15/07 / 6.31 / bdlb / 2.09
5/15-16/07 / 20.43 / 0.01 / 6.39
Site Meand / 4.85 ± 2.01 / nce / 1.67 ± 0.62
Harcum
6/8-9/06 / 9.61 / nmc / 5.63
6/20-21/06 / 5.21 / nmc / 3.75
7/6-7/06 / 12.37 / 0.04 / 3.22
8/30-31/06 / 3.32 / bdlb / 1.01
9/26-27/06 / 4.56 / 0.13 / 1.18
11/15-16/06 / 4.31 / 0.05 / 1.37
1/4-5/07 / 3.28 / bdlb / 0.77
2/1-2/07 / 3.69 / 0.05 / 1.26
3/22-23/07 / 6.45 / bdlb / 2.12
5/31-6/1/07 / 3.82 / bdlb / 1.95
Site Meand / 5.67 ± 0.96 / nce / 2.22 ± 0.49

aTOC and BC data is taken from Wozniak et al., 2011.

bbdl refers to samples that measured BC values below the detection limit of our instrumental protocol. See Wozniak et al. (2011) for details.

c’nm’ refers to samples for which BC was not measured.

dSite means represent parameter mean values for all samples at each site. Errors represent standard errors of the mean. For Millbrook, n=9, and for Harcum WSOC parameters, n=10.

eBC site means are recorded as ‘nc’, not calculated. Site means were not calculated due to the number of samples that measured below the detection limit.

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Figure S1. Map showing locations of the Millbrook, NY and Harcum, VA sites sampled during this study.

Appendix A.Field Sampling.

Twenty four-hour integrated high-volume total suspended particulate aerosol samples (~0.8 m3min-1, ~1,150 m3, Model GS2310 TSP sampler, ThermoAndersen, Smyrna, GA) were collected at both sites during 2006-2007 using total suspended particulate (TSP) air samplers (Model GS2310, ThermoAndersen, Smyrna, GA) by drawing air through pre-ashed (3 hrs, 525°C) and pre-weighed high-purity quartz microfibre filters (20.3 cm x 25.4 cm, nominal pore size 0.6 μm; Whatman QM-A grade). Particulate matter samples were stored in pre-ashed aluminum foil pouches in the dark in carefully cleaned air-tight polycarbonate desiccators (≤ 10% relative humidity) until analysis. Pre-ashed filter blank samples were transported to the field, briefly removed from then returned to aluminum foil pouches, and stored in a manner identical to aerosol samples.

At Millbrook, samples were collected over 5-day periods in May, August, and December 2006, and March 2007 and a three day period in May 2007 (Table S1). At Harcum, samples were collected approximately twice each month from June 2006 until June 2007. Two samples (Millbrook, NY, 8/18-20/2006 and Harcum, VA, 1/24-26/2007) were collected over 2-day periods at a higher volumetric flow rate (~1.7 m3 min-1;>4,000 m3) to attain higher aerosol loadings.

Appendix B.

With respect to the assumption that fossil fuel and contemporary biomass are the dominant aerosol OC sources, soil OC represents a potential source to aerosols that has a variable and potentially confounding Δ14C source signature. For example, soil OC from agricultural (Rethemeyer et al., 2005), grassland (Wang et al., 1996; Rethemeyer et al., 2005), and forest soils (Trumbore, 1993; Wang et al., 1996; Richter et al., 1999) is composed primarily, but not entirely, of contemporary biogenic material, while desert soils (Wang et al., 1996) can be highly aged ( ~20 kyr BP) and depleted in 14C (Δ14C = ~ -900), though not as depleted as fossil fuels (i.e., Δ14C = -1,000‰). Malm et al. (2007) estimated that only ~5-10% of fine and ~30-40% of coarse particulate mass in the eastern United States is derived from soils. Furthermore, the majority of soil types have far lower OC fractional contributions (fOC = OC/TSP, fOC<0.05) (Trumbore, 1993; Li et al., 1994; Wang et al., 1996; Stevenson and Cole, 1999; Rethemeyer et al., 2005) than aerosols (fTOC ~0.17 for the present study; Table S3), with highly aged desert soils showing considerably lowerOC contents than prairie or forest soils (Wang et al., 1996). Assuming approximately equal contributions from fine and particulate mass (Malm et al., 2007) and using upper values for east coast soil dust contributions (fine mass = 10% soil; coarse mass = 40% soil; fOC = 0.05 for both) to aerosol TSP, soil TOC would account for a maximum of ~7% of aerosol OC, suggesting that the assumption of a two-source model consisting of contemporary biogenic and fossil fuel-derived OC is valid. On this basis, we assume that contributions from soil OC may be ignored as potential sources to both aerosol OC and aerosol-derived WSOC on the east coast of North America, and that radiocarbon signatures can be used to determine relative contributions solely from fossil fuel and contemporary biogenic aerosol OC sources.

Δ14C values can be converted to fraction modern (Fm) notation where “modern” is defined as 95% of the radiocarbon concentration contained in the internationally recognized Oxalic I standard in the year 1950 (Olsson 1970). The Fmconvention normalizes radiocarbon signatures to the amount of 14C present in the atmosphere in the year 1950 and must be adjusted to calculate the percent contribution from fossil and contemporary biogenic sources. Lewis et al. (2004) assumed that contemporary aerosol OC was likely to be derived from carbon fixed from present-day atmospheric CO2 rather than, for example,from biomass fixed over the potentially decades-long lifespan of a tree. These workers therefore divided Fm by a factor of 1.08, a value corresponding to the Fm of contemporary atmospheric CO2. Here, this same conversion is applied to calculate the % contemporary contribution to aerosol WSOC following the assumption that biogenic sources to aerosol WSOC will come primarily from newly formed biomass as opposed to biomass formed over the lifetime of long-lived vegetation. The percentage of fossil contributions can then be calculated using the assumption that aerosol total OC is the sum of fossil and contemporary OC contributions (i.e. % fossil OC = 100% - % contemporary OC). AMS measurement errors ranged between 3‰ and 11‰ corresponding to differences of ~30-120 years in terms of the date the OC was fixed. Thus, the AMS measurement error exceeds any difference between the Δ14C values of newly produced biomass and biomass produced over the lifespan of a given tree thereby further validating this approach.

Hsueh et al. (2007) measured Δ14C in corn leaves and found Δ14Csignatures as low as 55.2‰ (or Fm = 1.06) in areas (Ohio-Maryland region) influenced more heavily by fossil fuel-derived CO2. This finding suggests that the contemporary atmospheric CO2 and biomass end-member for determining contributions of contemporary vs. fossil OC to aerosol OC (including WSOC) may be as low as Fm=1.06. Using this value to calculate contributions from fossil and contemporary OC resulted in contemporary OC contributions that were 0 to 2% higher than those calculated using Fm=1.08 as the contemporary end-member. As a result, the inherent variability in the fractional contributions estimated here must also include the uncertainty in this contemporary end-member.

References

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Li, C., Frolking, S., Harriss, R., 1994. Modeling carbon biogeochemistry in agricultural soils, Global Biogeochemical Cycles, 8, 237-254.

Malm, W. C., Pitchford, M. L., McDade, C., Ashbaugh, L. L., 2007. Coarse particle speciation at selected locations in the rural continental United States, Atmospheric Environment, 41, 2225-2239.

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Richter, D. D., Markewitz, D.,Trumbore, S. E.,Wells, C. G.,1999. Rapid accumulation and turnover of soil carbon in a re-establishing forest, Nature, 400, 56-58.

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