Air pollution levels vary across the seasons due to changing weather conditions and emission levels. The general trends can be visualised by average measurements taken at a rural background monitoring site:
Levels of particulate matter and nitrogen oxides (NOx) tend to peak in winter and early spring. The trends reflects emissions from winter heating and the relatively poor dispersion conditions.
NOx levels are an indicator of urban air quality since they closely relate to local traffic emissions. Discerning how individual variables impact air quality can be difficult, due to seasonal variability etc. Segmenting NOx data into monthly subsets helps reveal the impact that weather has:
The plots show that all occurrences of high NOx levels are in winter months and are associated with lower temperatures and wind speeds. This is likely attributable to poor dispersion conditions.