Radiocarbon measurement of the biogenic contribution to summertime PM-2.5 ambient aerosol in Nashville, TN
Introduction
A radiocarbon (14C) measurement performed on an ambient air sample provides a means of quantitatively distinguishing the separate contributions to carbon in the sample from fossil-fuel- and non-fossil-fuel-related sources. The method depends on the fact that 14C is present at a small but measurable, approximately constant, level in living materials, but absent in fossil fuels. The two source categories are loosely referred to as anthropogenic and biogenic, although circumstances can blur the sharpness of this classification (e.g., biomass burning for residential heating, or using ethanol as a gasoline additive). Regarded as a tracer for “biogenic” sources 14C is extremely robust, retaining its identity throughout any atmospheric chemical changes, in contrast to a molecular organic tracer. However a molecular tracer has the potential for distinguishing more specific source categories whereas 14C alone cannot.
Since the early 1980s a primary use of 14C measurements has been to estimate the impact of wintertime residential wood burning on ambient aerosol levels (Lewis et al., 1988). More recently, interest has turned to performing such measurements during the warm-weather seasons, as a way of investigating forest fire and biogenic secondary organic aerosol (SOA) influences on PM-2.5 (Lemire et al., 2002, Bench and Herckes, 2004) and PM-10 (Tanner et al., 2004, Szidat et al., 2004). The measurements reported here are the second in a series that is intended to survey non-wintertime 14C levels in PM-2.5 at different sites in the US. The present measurements were a component of the Southern Oxidant Study conducted in the Nashville, TN area during the summer of 1999. Highlights of the present work include (1) formulation and application of a new biologically-based model to correct present-day 14C measurements for the lingering effects of atmospheric nuclear weapons testing during the mid-20th century; (2) comparison of 14C results with and without solvent extraction of the samples; and (3) assessment of the importance of biogenic precursors to SOA.
Section snippets
Sample collection
Ambient air sampling was performed at the Cornelia Fort Airport site, located about 8 km east of the Nashville urban center. Samplers were situated on the “aerosol platform,” a 1.5-m high metal scaffold constructed for the field study at the site. Sampling occurred from June 21 to July 14, 1999. Samples for 14C analysis were collected on quartz-fiber filters, mostly for 11.5-h durations beginning at 7 a.m. and 7 p.m., although a smaller subset of samples was collected that separated the daytime
Converting pMC results to biogenic percentages
“Percent modern carbon” is a precise well-defined measurement quantity, but in environmental research the related quantity “percent biogenic carbon” has a more practical usefulness. This is because the pMC result for a biological system whose growth has included any period after 1950 will be inflated by the abrupt increase in the 14C content of atmospheric —the source of 14C in the earth's biosphere—due to atmospheric nuclear weapons testing that occurred in the 1950s and early 1960s. The
Leaf and gasoline samples
The average pMC found from the radiocarbon measurements performed on the ten leaf samples was (sd). Within its uncertainty this agrees with recent pMC measurements for background atmospheric (Levin and Hesshaimer, 2000), extrapolated to the year 1999, and with Figs. 1a and b.
A summary of the organic species composition of the gasoline samples has been published previously (Harley et al., 2001). Ethanol is the only gasoline additive used in the US that is biogenically-derived, and
Conclusions
The radiocarbon results of this study show a large contribution of non-fossil-fuel sources to PM-2.5 carbon during summer in Nashville, TN. Concurrent measurements of OC/EC ratios are consistent with biogenic SOA being a significant non-fossil-fuel contributor. The latter is also consistent with recent work, not specific to Nashville but to the Southeastern US, based on Chemical Mass Balance modeling (Zheng et al., 2002) and biogenic SOA tracer species (Edney et al., 2003).
These radiocarbon
Acknowledgements
We are grateful to EPA staff Leonard Stockburger for technical assistance throughout this project, Chris Geron for informing us of the Chapman–Richards tree growth model, Bill Lonneman for gasoline sample analysis and Shelly Eberly for statistical analysis assistance; ManTech Environmental Technology, Inc. staff David Stiles for sampling preparation, Chris Fortune for field assistance and Robert Kellogg for X-ray fluorescence analysis; David Smith (Sunset Laboratory East) for OC/EC
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2022, Environment InternationalCitation Excerpt :F14Cnf is slightly larger than 1 due to the excess 14C from nuclear bomb tests in the 1960s. F14Cnf was estimated as 1.09 ± 0.05 for OC fractions and 1.10 ± 0.05 for EC (see details in Ni et al., 2019b) from a tree growth model and the contemporary atmospheric 14CO2 over the past years (Lewis et al., 2004; Mohn et al., 2008; Levin et al., 2010), with the assumption that biomass burning and biogenic emissions contribute to 85% and 15% of total OC, respectively. Knowing the fraction of non-fossil carbon, carbon concentrations were apportioned into carbon from non-fossil sources (ECbb, OCnf, WIOCnf, WSOCnf) and fossil sources (ECfossil, OCfossil, WIOCfossil, WSOCfossil) (Eqs. S8–S15 in Table S3).
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