ABSTRACT
PM2.5 samples were collected at a near road air monitoring station during the first and harsh lockdown in Bangkok. The sampling campaign was set to gain the insight information on chemical speciation of PM2.5 during this rare event when most activities in Bangkok cease, and to establish a low episode of PM2.5 and its chemical component. The sampling was carried out during April and May 2020 with a Federal Reference Method (FRM) air sampler. sampler. Twenty-one metals were characterized by an inductively coupled plasma mass spectrometry (ICP-MS). Nine soluble ions were analyzed by an ion chromatography (IC), and a thermal-optical analysis for organic carbon (OC), and elemental carbon (EC). Daily PM2.5 concentrations at the sampling station ranged from 20.3 to 44.4 μg/m3 with the median of 26.3 ± 7.4 μg/m3. Secondary aerosol species accounted about 65% of PM2.5 mass and 14% for the primary aerosols, and unaccounted component was about 21%. OC was the major component of the secondary aerosol (59%), followed by sulfate ion (33%), ammonium ion (5%), and nitrate ion (3%). Sulfur dioxide as a precursor gas was well correlated with sulfate ion (r = 0.8, p < 0.05). EC accounted about 47% of the primary aerosol, followed by other water-soluble ions (27%), and elemental component (26%). Daily concentration of PM2.5 at the near road location in Bangkok during the low episode remained higher than the new guideline suggested by WHO (15 µg/m3). Lower the contribution of the secondary species of PM2.5 is crucial for the reduction of PM2.5 mass in the area and it is very challenging to achieve the target recommended by the guideline. © 2022, Thai Society of Higher Eduation Institutes on Environment. All rights reserved.