Comparison of methods for the determination of NO-O3-NO2 fluxes and chemical interactions over a bare soil

Abstract

Tropospheric ozone (O₃ ) is a known greenhouse gas responsible for impacts on human and animal health and ecosystem functioning. In addition, O₃ plays an important role in tropospheric chemistry, together with nitrogen oxides. The determination of surface-atmosphere exchange fluxes of these trace gases is a prerequisite to establish their atmospheric budget and evaluate their impact onto the biosphere. In this study, O₃ , nitric oxide (NO) and nitrogen dioxide (NO₂ ) fluxes were measured using the aerodynamic gradient method over a bare soil in an agricultural field. Ozone and NO fluxes were also measured using eddy-covariance and automatic chambers, respectively. The aerodynamic gradient measurement system, composed of fast response sensors, was capable to measure significant differences in NO and O₃ mixing ratios between heights. However, due to local advection, NO₂ mixing ratios were highly non-stationary and NO₂ fluxes were, therefore, not significantly different from zero. The chemical reactions between O₃ , NO and NO₂ led to little ozone flux divergence between the surface and the measurement height (less than 1% of the flux on average), whereas the NO flux divergence was about 10% on average. The use of fast response sensors allowed reducing the flux uncertainty. The aerodynamic gradient and the eddy-covariance methods gave comparable O₃ fluxes. The chamber NO fluxes were down to 70% lower than the aerodynamic gradient fluxes, probably because of either the spatial heterogeneity of the soil NO emissions or the perturbation due to the chamber itself.

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