Apportioned contributions of PM2.5 fine aerosol particles over the Maldives (northern Indian Ocean) from local sources vs long-range transport.

Urban-like plumes of gases and particulate matter originating from the South Asian region are frequently observed over the Indian Ocean, especially during the dry winter period. However, in addition to the strong sources on mainland South Asia, there are also local Maldivian emissions. The local contributions to the load of fine particulate matter (PM2.5) in the Maldivian capital Malé was assessed using the well-established Maldives Climate Observatory at Hanimaadhoo (MCOH) to represent local background, recording the long-range transported component for a full-year synoptic campaign at both sites in 2013. The year-round levels in both Malé and MCOH are strongly influenced by the seasonality of the monsoon cycle, including precipitation patterns and air-mass transport pathways, with lower levels during the wet summer season. The annual-average PM2.5 levels in Malé are higher (avg. 19 μg/m3) than at MCOH (avg. 13 μg/m3) with the difference being the largest during the summer, when local emissions play a larger role. The 24-h World Health Organization (WHO) PM2.5 health guideline was surpassed for the weeklong collections in 71% of the cases in Malé and in 74% of the cases for Hanimaadhoo. This study shows that in the dry/winter season 90±11% of PM2.5 levels in Malé could be from long-range transport with only 8±11% from local emissions while in the wet/monsoon season the relative contributions are about equal. The concentrations of organic carbon (OC) and elemental carbon (EC) showed similar seasonal patterns as bulk mass PM2.5. The relative contribution of total carbonaceous matter to bulk mass PM2.5 was 17% in Malé and 13% at MCOH, suggesting larger contributions from incomplete combustion practices in the Malé local region.