Mechanisms for pollutant transport between the boundary layer and the free troposphere

Pollutants are longer-lived in the free troposphere than the boundary layer, hence the transport of pollutants from the boundary layer to the free troposphere has significant implications for long-range transport and global warming. It is important to quantify the transport of air between the boundary layer and the free troposphere and to understand the role different meteorological mechanisms play, Idealised passive tracer experiments, with tracer initially only in the boundary layer, are performed in a numerical model for three case study days with different synoptic conditions. After 24 hours, more than 50% of the tracer resides in the free troposphere for the two frontal cases, and 40% resides there for the high-pressure case. The tracer was transported to maximum heights of 8 km, To elucidate the role of different mechanisms for each case, the tracer amount transported by advection only, advection and turbulent mixing, and advection and convection was calculated. Advection is found to be the most important mechanism in transporting the tracer to the free troposphere; however, the addition of upright convection and turbulent mixing increases the amount by up to 24% with convection transporting the tracer to heights of 5 km. The inclusion of convection and turbulent mixing to the advection are not linearly additive processes. This study shows the possibility of a large proportion of the pollutant emitted in the boundary layer being transported to the free troposphere in a short time and the importance of representing all the meteorological processes.