Air pollution levels are driven mainly by two factors. Firstly, the level of emissions from various source points. For example, the more fossil fuels are burned for transport or energy, the higher the levels of air pollution.
Secondly, the day-to-day variability in weather conditions determines if pollutants are dispersed or accumulate close to the sources. For much of South Asia, very different weather patterns during the two monsoons play a large role in determining air quality. Air pollution levels in Colombo are highest during the north-east monsoonal period (from November to February) and lowest during the south-west monsoonal period.
The main reason for the increase in air pollution during the north-east monsoon is two-fold: pollutants transported from the Indian subcontinent to Sri Lanka (known as transboundary pollution), and cooler conditions caused by stable air flowing off the Himalayas, which retard pollutant dispersion.