A study of precipitation scavenging of semivolatile organic compounds in a tropical area

Rain samples were collected concurrently with gas phase and particulate samples from June 2007 to May 2008 in Singapore and processed to investigate precipitation scavenging of polycyclic aromatic hydrocarbons (PAHs) and organochlorine pesticides (OCPs). A comprehensive atmospheric scavenging model has been developed with inclusion of major atmospheric deposition processes such as particle scavenging, dissolution (Henry's law), and surface adsorption affecting the total scavenging ratio of SVOCs. This model was subsequently used in this study to calculate precipitation ratios. Total scavenging ratios ranged from (8.9 ± 4.3) × 10⁴ to (1.2 ± 0.4) × 10⁶ for PAHs and from (8.3 ± 5.1) × 10⁴ to (4.9 ± 1.5) × 10⁵ for OCPs. Particle scavenging rather than gas scavenging was the dominant removal mechanism, accounting for 86-99% for PAHs and 98-99% for OCPs in terms of the particle contribution to the total scavenging. The variation of both total and particle scavenging ratios over the study period is smaller compared to those reported in the literature, which might be attributed to uniform ambient temperature prevailing throughout the year in this tropical area. The effects of particle fraction, supercooled vapor pressure, and rainfall intensity on particle scavenging of SVOCs were assessed. The relationship between gas scavenging ratio and supercooled vapor pressure implied that the domination of gas scavenging might switch from dissolution to adsorption at supercooled vapor pressures around 10^-3.5-10 ^-4 Pa, especially for PAHs with five or more aromatic rings. The equations used to estimate total scavenging (W_T) based on the particle fraction (Φ) can be summarized as log W_T = 0.94 log Φ + 6.09 (R² = 0.74) for PAHs and log W_T = 0.82 log Φ + 6.34 (R² = 0.62) for OCPs.