Using ambient concentrations as proxy for methane flux measurements from rice fields

Ambient concentrations of trace gases near their sources carry precise information about the emission rates. In this paper we use ambient measurements of methane near rice fields to estimate the emissions and compare them to actual flux measurements. The data are from a seven-year study of rice fields in China when both emissions and ambient concentrations were measured. The calculations are based on an empirical model of the dilution factor that relates emissions from the rice fields to the ambient concentrations caused by these emissions. The relationship is derived from the first year of the experiment (base year — 1988) and then applied to subsequent years (1989–1994) using only ambient measurements near the rice fields, to estimate methane emissions. The ambient data are about an order of magnitude fewer than the data for the direct flux measurements. Yet, the results of the ambient proxy calculations of flux are in agreement with the seasonal patterns and whole season emissions measured from the direct flux measurements. The seasonally averaged fluxes for each year agree to within ± 25%. There is a close agreement between the actual flux measurements and the proxy estimates of the seasonal pattern of emissions in the composite of all years of data (± 10 %). The seasonally averaged emissions over all seven years are nearly the same between the proxy] and actual measurements at about 30 mg/m=/hr (within 2%).These results are an independent validation of the direct flux measurements taken at this site.