Pollutant vertical mixing in the nocturnal boundary layer enhanced by density currents and low-level jets: two representative case studies

Abstract

Turbulent and intermittent characteristics of the nocturnal boundary layer play an important role in determining the transport, diffusion, and storage of momentum, heat, and atmospheric pollutants. Here we use micrometeorological observations and numerical simulations to analyze two coherent turbulent structures—a density current and a low-level jet (LLJ)—that enhance mixing and intermittent turbulence in the nocturnal boundary layer. We analyze in detail their impact on the distribution of simulated pollutant concentrations. The Weather Research and Forecast (WRF) model is coupled with the Community Multiscale Air Quality photochemical model to simulate the dispersion of pollutants for the density-current and LLJ cases. The terms of the pollutant-concentration budget are then analyzed to quantify the contribution of different atmospheric processes on the net $$\hbox {NO}_2$$ and $$\hbox {O}_3$$ concentrations. The results show that the WRF model reproduces successfully the essential characteristics observed in the density-current and LLJ cases. The density-current simulation shows turbulent-mixing enhancement in the frontal zone of the density current, leading to significant vertical pollutant diffusion. Results also indicate that the LLJ is able to transport pollutants from the residual layer to the surface layer due to vertical diffusion. In both cases, the main mechanism responsible for pollutant transport is the horizontal advection.