Assessment of 24-hour moving average PM2.5 concentrations in Bangkok, Thailand against WHO guidelines

Abstract Currently, the particulate matter with diameter less than 2.5 micron (PM2.5) pollutant has gained more concerned as can be seen from the WHO revised the air quality guideline value. The 24-hour average concentration has been strengthened from 25 µg m-3 to 15 µg m-3. However, the continuous PM2.5 monitoring system provides data on an hourly basis, which can be averaged at a 24-hour value compare with the WHO air quality guidelines. The value given by the moving average technique can be stored at the leftmost, center or rightmost hour. Three moving average PM2.5 time series would differ from the hourly observed PM2.5 data. Similarity testing by cross-correlation and Euclidean distance was performed to present a suitable 24-hour moving average time series for hourly data. The 24-hour moving average time series recorded at center is more suitable than the leftmost and rightmost 24-hour moving average time series in terms of shape and distance. It has less time lag and distance to the hourly PM2.5 time series. Comparing the 24-hour moving average time series to the WHO interim targets and the guideline value reveals PM2.5 concentration level lower than the guideline value (15 µg m-3) about 40% during the nighttime, whereas the proportion during daytime is around 28%. Also, the NAAQS of Thailand for 24-hour PM2.5 was changed from 50 µg m-3 to 37.5 µg m-3 corresponding to the interim targets 3 and 2, respectively. From this study, concentrations higher than the NAAQs level will increase from 10 to about 22%. The increase in the number of exceedances based on the same data means the state of air quality is similar. Therefore, residents may misunderstand and know the air quality becomes more severe. The government should spend more effort to reduce emissions and ambient air concentrations than earlier endeavors.