NIST Measures Biogenic Volatile Organic Compounds that Contribute to Summertime Smog in the Los Angeles Basin

Critical to the success of regional and national programs designed to improve air quality and comply with the National Ambient Air Quality Standards, is knowing how much man-made (controllable) pollution exists in a region relative to the amount of natural (uncontrollable) pollution.  NIST is working with the U.S. EPA to apply previously developed carbon isotope methods to summertime emissions of ozone forming volatile organic compounds (VOC).  Through accurate radiocarbon (¹⁴C) measurements of VOC separated from whole air, relative contributions of fossil fuel and biogenic sources of these pollutants are determined. 

An objective of the 1997 Southern California Ozone Study (SCOS97) was to provide an up-to-date assessment of the importance of biogenic emissions for tropospheric ozone production in the South Coast Air Basin.  Through an interagency agreement between the US EPA and NIST, ambient air samples were collected during September 1997 at the Azusa air monitoring station by the US EPA, processed at NIST to isolate the VOC fractions, and measured by NIST for the radiocarbon (¹⁴C) content.  The ¹⁴C/¹³C ratio is proportional to the fraction of a sample’s VOC-carbon that is biogenic, and calibrated using local samples of vegetation, gasoline and ambient CO₂ collected during the same period.  The median fraction of biogenic VOC observed from 0600 h to 0900 h was 7 % with a range of –8 % to 24 %, from 1300 h to 1600 h it was 27 % with a range of 11 % to 39%, and from 1700 h to 2000 h it was 34 % for a single sample. Based on calculated 24-h air mass back-trajectories, the dominant source region associated with periods of high biogenic VOC-carbon levels was a sector extending from the north to the east.  Over all time and space that the samples represent, the median fraction of biogenic VOC was 18 %.   Expressed as an atmospheric mixing ratio, the overall (median and 95 % confidence interval) biogenic VOC-carbon contribution was 80 ± 50 nmol mol^-1 that is representative of the natural VOC-carbon background for the Los Angeles air basin.

NIST radiocarbon results of VOC from the SCOS97 Study will provide to the California Air Resources Board an independent measure of source strengths that can be compared to their modeled estimates based on emissions inventory.  The method is now being applied to samples collected during the Texas 2000 Air Quality Study (TexAQS2000) to investigate sources of VOC in the Houston-Galveston area, a region faced with air quality issues comparable to those in the Los Angeles area.