Between 1997 and 2015, VOC emissions decreased by 65% which was mainly caused by the decline in emissions from non-combustion and road transport sectors. Non-combustion and road transport sectors were the top two sources of VOC emissions, accounting for 58% and 18% of total VOC emissions in 2015, respectively.
During the same period, the ozone (O3) levels measured at the EPD's general air quality monitoring stations and the estimated VOC emission trend went in nearly opposite directions. Ozone is a secondary pollutant, formed by the photochemical reaction of VOC and NOx. Despite the reduction of local emissions of VOC and NOx over the past years, the ambient O3 level was still rising which indicates the contribution of regional influence.