Interpreting C[O.sub.2] fluxes over a suburban lawn: the influence of traffic emissions

Turf-grass lawns are ubiquitous in the United States. However direct measurements of land-atmosphere fluxes using the eddy-covariance method above lawn ecosystems are challenging due to the typically small dimensions of lawns and the heterogeneity of land use in an urbanised landscape. Given their typically small patch sizes, there is the potential that C[O.sub.2] fluxes measured above turf-grass lawns may be influenced by nearby C[O.sub.2] sources such as passing traffic. In this study, we report on two years of eddy-covariance flux measurements above a 1.5 ha turf-grass lawn in which we assess the contribution of nearby traffic emissions to the measured C[O.sub.2] flux. We use winter data when the vegetation was dormant to develop an empirical estimate of the traffic effect on the measured C[O.sub.2] fluxes, based on a parametrised version of a three-dimensional Lagrangian footprint model and continuous traffic count data. The C[O.sub.2] budget of the ecosystem was adjusted by 135 g C [m.sup.-2] in 2007 and by 134 g C [m.sup.-2] in 2008 to determine the natural flux, even though the road crossed the footprint only at its far edge. We show that bottom-up flux estimates based on C[O.sub.2] emission factors of the passing vehicles, combined with the crosswind-integrated footprint at the distance of the road, agreed very well with the empirical estimate of the traffic contribution that we derived from the eddy-covariance measurements. The approach we developed may be useful for other sites where investigators plan to make eddy-covariance measurements on small patches within heterogeneous landscapes where there are significant contrasts in flux rates. However, we caution that the modelling approach is empirical and will need to be adapted individually to each site. Keywords Eddy covariance * Footprint model * Traffic emission factors * Turf-grass lawn * Urban micrometeorology