Atmospheric lifetime as a probe of radical chemistry in the boundary layer

Determination of the atmospheric lifetime of radical species provides a potentially powerful method to resolve discrepancies between their modelled and measured concentrations. Theoretical perturbation analysis is employed to investigate the relationship between atmospheric lifetime (τ) and decay time (λ−1) following concentration perturbation for OH and HO2 using a model system with constrained NOx. It is shown that feedback from HO2 to OH can lead to a non-equivalence of (τ) and (λ−1), depending on the size of the feedback term and the difference in the atmospheric lifetimes of the coupled species. The ideas are applied and extended in a discussion of atmospheric field data from the EASE96 campaign at a remote coastal site. Dynamic information about the relaxation modes and species couplings, available from the eigenvector elements, is investigated for the systems studied. In general, the results suggest that the assignment of a specific decay time to OH is feasible for background boundary layer tropospheric conditions. However, complex couplings between peroxy species lead to non-separable time scales for these species. The relevance of results for the development of a field instrument to measure OH lifetime is discussed.