Air quality in Singapore during the 2013 smoke-haze episode over the Strait of Malacca: Lessons learned

The widespread wildfires that raged across Indonesia's island of Sumatra in June 2013 triggered the worst air pollution episode ever recorded in Singapore. The highest 24-h moving average PM2.5 concentration reported by the authorities reached a whopping 310 μg m−3 on June 20, 2013. However, a top 3-h moving average Pollutant Standard Index (PSI) of 401 (index ceiling is 500) based only in PM10 concentration data occurred the following day, and a maximum 24-h moving average PSI of 246 with an associated 24-h PM10 concentration of 382 μg m−3 was reported on June 22, 2013. The decoupling between these air quality health advisories evidences the flaw in informing to the public on the hour when the air quality was at its worst. Up until now, despite the positive changes on the public dissemination of air quality information, the real hourly pollution levels experienced in Singapore during the 2013 smoke-haze episode are still unknown. To unveil the hourly PM2.5 concentrations this work applies a set of statistical models to the available air quality data during the episode and the lately 1-h PM2.5 concentration records published since April 2014 on a day-by-day bases. Results suggest maximum likelihood hourly PM2.5 concentrations over 600 μg m−3, twice the maximum 24-h moving average reported by the authorities.