Attributing improvement of Air Quality in Colombo to COVID19 curfews distracts focus on Sri Lanka’s major sources of air pollution

Abstract:

The increasing concern about deteriorating air quality in Sri Lanka and public access to the monitoring devices on the South-West over the last three years has led to recent attention to air quality. The improvement in Air Quality was attributed by some officials and media to COVID19 curfews.

Hourly particulate measurements have been obtained since March of 2017 by the US embassy within central Colombo with an EPA accredited BAM-1020 pm2.5 monitor. Starting in March 2018, we deployed lower costs sensors (IQAir Node & Purple Air II) five sites. These instruments had been assessed well by California’s Southern Coast Air Quality Management District and calibrated against the US embassy Instrument. Wind measurements were obtained with automatic weather stations (accessible on www.disaster.lk). During March to April the direction changes from North-Easterly to Westerly and wind roses shows extended durations of North-Westerlies. Data were also reported by three other agencies.

These air quality data time series shows that there was no improvement in Air Quality immediately after the imposition of the curfews except for suspicious outlier stations . The improvement in air quality only began one week later after the reversal in wind-seasonality as the north-east trade winds gave way to south-western trade winds. Such a reversal happens every year in early April. Even then the drop in Air Quality indexes was around 30% only. As the wind reverses, it brings in pristine air from the Southern Indian Ocean to Colombo rather than from Sri Lanka Northern land mass and the lands to the North. After April, the pollution from these sources shall be directed to the interior of Sri Lanka and beyond but not to Colombo. Regional pollution transport models implemented for this period using NOAA and EPA confirm the role of monsoonal reversal.

There was a drop in vehicular emissions in Colombo and a few other other industrialized and urbanized localities due to COVID19 curfews but this was not the primary cause of drops in AQI. The drop in air quality was largely due to seasonality. Thus the improvement during the curfews should not distract pollution mitigation focus from the coal power plant, the agricultural residue burning, forest and garbage fires, and the industries that continued to function.