Mechanisms for pollutant transport between the boundary layer and the freetroposphere

Pollutants are longer-lived in the free troposphere than the boundary layer , hence the transport of pollutants from the boundary layer to the free tro posphere has significant implications for long-range transport and global w arming. It is important to quantify the transport of air between the bounda ry layer and the free troposphere and to understand the role different mete orological mechanisms play, Idealised passive tracer experiments, with trac er initially only in the boundary layer, are performed in a numerical model for three case study days with different synoptic conditions. After 24 hou rs, 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 trace r was transported to maximum heights of 8 km, To elucidate the role of diff erent mechanisms for each case, the tracer amount transported by advection only, advection and turbulent mixing, and advection and convection was calc ulated. Advection is found to be the most important mechanism in transporti ng the tracer to the free troposphere; however, the addition of upright con vection and turbulent mixing increases the amount by up to 24% with convect ion 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 em itted in the boundary layer being transported to the free troposphere in a short time and the importance of representing all the meteorological proces ses.