Characteristic length scales for reactants in a convective boundary layer

Sources of and sinks for chemically active species, such as clouds or emissions and depositions in the canopy, lead to the generation of concentration fluctuations. In this study, the variance spectra of these fluctuations were determined in a dry convective boundary layer by means of large-eddy simulations. The numerical data were obtained from a domain defined by a large aspect ratio, in order to take all the relevant scales(from mesoscale to microscale) into account. The spectra were used to calculate the characteristic length scale of the concentration field variabilility.

For a simple first-order decay, the variance spectra maxima demonstrated a clear dependence on the reaction rate. The increase in the reaction rate produces a shift in the spectra maxima from the low to the high wave-numbers, and in consequence the characteristic reactant length scale decreases.

For a chemical cycle in a state of equilibrium, the spectra show that the fluctuation in the concentration are driven predominantly by scales larger than the boundary layer height. A spectral analysis of the variance budgets revealed that the contribution of the chemical term vanishes in chemical equilibrium. Under this condition, a relationship for the correlation coefficients for the chemical cycle species was derived from the chemical terms in the (co-)variance equation. Boundary layer processes, such as a passing cloud, may perturb this chemical balance, resulting in deviations from this relationship and have a direct impact on the variance spectra of all species involved. For non-abundant species, departure from a condition of equilibrium results in damping the mesoscale fluctuations.