BIOGENIC VOLATILE ORGANIC COMPOUNDS (BVOCS) EMISSIONS BY SELECTED STREET TREES IN SINGAPORE

Abstract

Ecosystem services provided by trees have been widely documented. It is important to note that while trees provide ecosystem services, they also emit biogenic volatile organic compounds (BVOCs) like isoprene and monoterpenes into the atmosphere. These BVOCs, on the contrary, can result in the formation of ground-level ozone which is an air pollutant and can potentially cause harm to human health. However, there is a lack of studies on the BVOCs emissions in the Singapore. Through this study, I look into the ecosystem services and disservices provided by trees and focus particularly on the BVOCs emissions from 39 species of selected street trees in Singapore. An estimation of the BVOCs emissions is provided with the use of biogenic emission model. At the same time, impacts of temperature and Photosynthetically Active Radiation (PAR) on BVOCs emissions will be explored. Finally, a comparison of BVOCs emissions in Singapore is done against emissions in other countries to study the significance of the emissions in Singapore.

Findings from this study show that the annual isoprene and monoterpenes emission in Singapore amount to about 4.2 Tg yr-1 and 1.1 Tg yr-1 respectively. Of the 39 species of street trees selected, 77% of the species belong to low emitter species which account for only 1% of the isoprene emission. The remaining 23% of the selected species belong to the high emitter species, accounting for 99% of the isoprene emission estimated in Singapore. Similarly, for monoterpenes emission, 48% of the selected species belong to low and moderate emitter species that collectively account for only 4% of the monoterpenes emissions estimated. The remaining 52% are high emitters contributing to 96% of the monoterpenes emissions. Therefore, it is important for planners to be cautious when selecting the species of trees to be planted in the extended efforts to continue to green Singapore.

Both isoprene and monoterpenes are highly dependent on temperature, where an increase in temperature would result in increasing emissions. This has further implications on BVOCs emissions in the future where temperature in Singapore is likely to increase by an average of 2.8oC due to climate change. It is estimated that the amount of BVOCs emitted would increase by at least 20% with climate change. Further, the impact of PAR on isoprene emission is strong but less so for monoterpenes as they continue to be emitted in the absence of light. Comparison of the levels of BVOCs emissions with other areas conclude that the isoprene emissions in Singapore is higher than those observed in Hong Kong and China, while monoterpenes emissions are significantly lower. However, when these estimations are normalised, the emissions in Singapore are significantly lower.

It is recommended that low emitter species should be planted to keep BVOCs emissions within a safe level, preventing further formation of ground-level ozone. At the same time, selecting the right species of trees would also ensure that the community continues to enjoy the ecosystem services provided by the trees.