Journal of Environmental Science and Health Part A (2009) 44, 1011–1018 ISSN: 1093-4529 (Print); 1532-4117 (Online) DOI: 10.1080/10934520902996963 Measurement of greenhouse gas emissions from agricultural sites using open-path optical remote sensing method KYOUNG S. RO 1 , MELVIN H. JOHNSON 1 , RAVI M. VARMA 2 , RAM A. HASHMONAY 3 and PATRICK HUNT 1 1 Coastal Plains Soil, Water & Plant Research Center, USDA-ARS, Florence, South Carolina, USA 2 Department of Physics, National University of Ireland, Cork, Ireland 3 ARCADIS Inc., Research Triangle Park, North Carolina, USA Improved characterization of distributed emission sources of greenhouse gases such as methane from concentrated animal feeding operations require more accurate methods. One promising method is recently used by the USEPA. It employs a vertical radial plume mapping (VRPM) algorithm using optical remote sensing techniques. We evaluated this method to estimate emission rates from simulated distributed methane sources. A scanning open-path tunable diode laser was used to collect path-integrated concentrations (PICs) along different optical paths on a vertical plane downwind of controlled methane releases. Each cycle consists of 3 ground-level PICs and 2 above ground PICs. Three- to 10-cycle moving averages were used to reconstruct mass equivalent concentration plum maps on the vertical plane. The VRPM algorithm estimated emission rates of methane along with meteorological and PIC data collected concomitantly under different atmospheric stability conditions. The derived emission rates compared well with actual releasedrates irrespective of atmospheric stability conditions. The maximum error was 22 percent when 3-cycle moving average PICs were used; however, it decreased to 11% when 10-cycle moving average PICs were used. Our validation results suggest that this new VRPM method may be used for improved estimations of greenhouse gas emission from a variety of agricultural sources. Keywords: Open-path tunable diode laser (OP-TDLAS), vertical radial plume mapping (VRPM), agricultural sites, greenhouse gas, methane. Introduction Agricultural sites such as rice paddies, feedlots and treat- ment lagoons are significant emission sources of the green- house gas methane. [1−3] Although accurate assessment of gas emission from agricultural sites is very important, the accuracy depends strongly on the methods employed and the surrounding environments. [4] Greenhouse gas emissions from a point source such as an animal house with mechan- ical ventilation could be adequately estimated by multiply- ing ventilation rates and gas concentrations at the fan out- let. However, estimation of greenhouse gas emission from distributed sources such as treatment lagoons, treatment wetlands, land spread of manure, and feedlots requires more complicated methods such as chamber and various micrometeorological methods. Furthermore, these more sophisticated methods do not provide higher reproducibil- ity of measurements. For example, different measurement Address correspondence to Kyoung S. Ro Coastal Plains Soil, Water, and Plant Research Center, 2611 West Lucas Street, Flo- rence, SC 29501-1241. E-mail: Kyoung.Ro@ars.usda.gov Received February 13, 2009. methods taken in over-lapping days for a treatment lagoon produced widely different volatilization rates. [5] Harper [6] reported more than an order of magnitude difference in ammonia, methane and nitrous oxide emissions from swine lagoons using these methods. Clearly, new reliable meth- ods to measure emission of these trace gases are urgently needed. Among various micrometeorological methods, Wilson et al. [7] concluded that the integrated horizontal flux method (IHF) proved to be the most satisfactory, followed by the backward Lagrangian stochastic method (BLS). Laubach and Kelliher [8] also preferred the IHF technique because it does not rely on the similarity assumptions. Al- though the IHF technique estimates the emission rate by simply determining the difference of the integrated mass fluxes from up- and down-wind sides, successful applica- tion of the IHF technique requires accurate concentration profile information. Unfortunately, these values continu- ously changes in both time and height. Venkatram [9] sug- gested the use of a dispersion model to better estimate the concentration profile for the IHF. Instead of using the dispersion model, the new USEPA’s path-integrated optical remote sensing (PI-ORS) method