Objectives: Siem Reap in Cambodia is home to the World Heritage Site of Angkor, and the number of tourists is increasing year by year. In recent years, there have been concerns about air and water pollution associated with the development of the tourism industry. However, there is no available data on air pollution except for PAHs for two decades. Squall-induced heavy rainfall (SHR) frequently occurs in the rainy season in Southeast Asia, but research on rainwater chemistry is also limited in Cambodia. The objectives of this study are to clarify the current status of air pollution in Siem Reap and the impact of air pollutants on SHR.
Methods: We conducted observational campaigns in Siem Reap, Cambodia for 1-4 weeks in September 2018, August 2019 (as rainy season), February 2019 (as dry season), and April 2019 (as exchange season), respectively. SPM samples collected onto a quartz filter with a high-volume air sampler in the day/night time. Forty-one trace metals, six organic aerosols as molecular markers for biomass, and water-soluble components such as sulfate and nitrate in the particulate matter were analyzed. VOCs, gaseous elemental mercury (GEM), and acidic gases such as HNO₃ and SO₂ were also determined. The pH and major ions in SHR were analyzed. PM_2.5 and BC mass concentration were monitored every one minute.
Results & Discussion: NH₃ concentration was very high in the dry season but the differences between cations and anions suggested the presence of sulfuric acid mist. BSOA, which is oxidized from BVOCs such as Isoprene and monoterpenes in the daytime, could coat the surface of sulfuric acid aerosols and interfere with neutralization by NH₃. On the other hand, NH₄⁺ and SO₄^2- were major ions in SHR. This indicated that the BSOA-coated sulfuric acid aerosols formed raindrop as cloud condensation nuclei (CCN) and then was quickly neutralized by NH₃ (Figure 1). In the presentation, we will discuss the mechanism in more detail.

Phul PHORSDA

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