Coastal influences on pollution transport

Abstract

The ventilation of pollutants from the boundary layer into the free troposphere is an important process in controlling regional air quality. Coastal outflow is the horizontal ventilation of pollutants across a coastline from a layer within the continental boundary layer to above the marine boundary layer. It has been shown using the Met Office Unified Model that the ventilation by coastal outflow occurs with a similar order of magnitude to ventilation by convection, and that it possesses a diurnal cycle induced by the boundary layer height cycle over land. Pollutants with short lifetimes (typically several hours) exhibit the greatest diurnal variability in export by coastal outflow. Ventilation by coastal outflow by pollutants with longer lifetimes are less dependent on the boundary layer height over land and more dependent on the large scale cross-coastal wind strength. A simple model developed to simulate coastal outflow shows that increasing the pollutant lifetime, wind speed or convective boundary layer height increases export of tracer by coastal outflow. Above a threshold windspeed, coastal outflow is reduced due to tracer being exported across a coastline before being able to be mixed to a height greater than the marine boundary layer. Convection slightly decreases export by coastal outflow, although this effect is small.