Weather conditions underlie the high particulate matter (PM) episodes which can occur in the early spring months in the UK. Such episodes may arise from the build up of local emissions due to poor dispersion, or due to easterly airflows which imports air pollution from Europe.
Except for bonfire night, the highest daily PM measurements at the various monitoring sites tend to occur in March and April. This is shown by the mean monthly PM2.5 levels for a rural background site, where PM levels reflect regional pollution sources:
A typical spring PM episode occurred in March 2014. The event can be visualised in a time series plot of data from sites such as Leeds, which recorded its highest daily PM2.5 levels in recent years:
Wind rose plots indicate that these high PM2.5 levels were largely a result of weather conditions involving moderate easterly winds:
This suggests that imported air pollution from continental Europe can make a significant contribution to UK air pollution levels.