WHAT PIDJ^ MEAS13BE? The Radiocarbon Method • Naturally-occuning radioactive carbon (14C) is present at trace levels in all living material, resulting from the photosynthesis of atmospheric carbon dioxide that contains 14C. • 14C is not present in fossil fuels (coal, petroleum, natural gas) due to its radioactive decay. • percent Modern Carbon (pMC) = percentage of 14C in a sample of unknown origin relative to that in a sample of living material. • pMC ~ percentage of carbon in a sample that originated from non- fossil (biogenic) sources. • pMC has been increased above natural levels by nuclear weapons testing during the 1950's and 1960's (a correction can be applied). (see details below) 14C Basics 14C Decay: 14C -»14N + e (half-life = 5730 yr) Percent Modem Carbon (sample x): 14c/13cx Tree harvested in 1999 (equal-width tree rings) pMC Atmospheric C02 vs. Year 1990 2000 Cars Or Trees: Which Contribute More to Particulate Matter Air Pollution? Charles W. Lewis1, George A„ Klouda2, and William D„ Ellenson3 lU.S. EPA, national Exposure Research Laboratory, Research Triangle Park, MC; 2Mational Institute of Standards and Technology, Gaithersburg, MD 3ManTech Environmental Technology, Inc., Research Triangle Park, MC Mo ving Science into Actioji * MP WE MEASURE?^ ..,haT PIP we FIWD? Field Sampling in Mashville TN and Houston TX Samples of air particles were collected at a rural site (Figure 3) near Nashville TN under the following conditions: • Cornelia Fort Airport, 8 km east of Nashville urban center • June 21 - July 14, 1999 • 11.5-hr sample durations, starting 7 am & 7 pm • PM2.5 sampling system: 300 L min1, 90-mm dia. quartz filter During the following summer, August 9-30, 2000, similar sampling was performed at three suburban/rural sites - Aldine, Conroe, & Galveston - near Houston TX (Figure 4). Cornelia Fort Airport site (Mashville) Houston-Galveston-Conroe PM2 5 Sampling Sites Galveston - Background Site, Aldine - Suburban Site, Gulf of Mexico N.Houston mm ¦ p Xn Conroe - Forested Site, N. of Houston •- r-sSH7 pMC Results Definitions The table below summarizes the pMC results from radiocarbon measurements performed on the samples from the two field studies. The values are surprisingly high, implying a correspondingly high biogenic contribution to the total carbon contained in the samples. Nashville Houston pMC range m 56-80 27-77 pMC mean 09 69 55 pMC (Total Carbon) vs OC/EC (Nashville, 1999) OC/EC Ratio Figure 5 Secondary Organic Aerosol (SOA) Interpretation The "OC/EC ratio" is commonly regarded as being an SOA indicator.* A ratio greater than (OC/EC)0 — 2-4 indicates the onset of SOA, and the larger the ratio, the larger the relative contribution of SOA: SOA/OC = 1 - (OC/EC)0 / (OC/EC) In Figures 5 and 6 the individual Nashville and Houston pMC results are given, plotted against the OC/EC ratio for each sample. For both field studies most of the samples have an OC/EC ratio that is well above the onset of SOA. Furthermore, the greater the amount of inferred SOA, the greater the biogenic percentage (~pMC) of the sample's carbon content. * Atmospheric Environment 25A, 207 (1991) Secondary Organic Aerosol (SOA): carbon-containing particles formed in the atmosphere from volatile organic compounds (VOC), rather than emitted from sources directly as particles (primary aerosol). Elemental Carbon (EO : refractory, graphitic, light-abs orbing component of total carbon (TC). Organic Carbon (OC): all remaining carbon (OC = TC - EC). pMC (Total Carbon) vs OC/EC (Houston, 2000) Aldine Conroe Galveston 20 30 OC/EC Ratio J. Geophys. Res. 107(D22), 4613 (2002) Figure 6 BOTTOM UNE Results from two regions show large biogenic contributions - in the form of Secondary Organic Aerosol - to the carbon component of PM 2 5 Regulatory-based attempts to control PM 2 5 only affect man-made sources and will not control biogenically-produced PM 25 Partnering to Protect Human ------- |