Influence of semi-volatile aerosols on physical and optical properties of aerosols in the Kathmandu Valley

Abstract. A field study was conducted in the urban atmosphere of the Kathmandu Valley to study influence of the semi-volatile aerosol fraction on physical and optical properties of aerosols. The study was carried out during the pre-monsoon season of 2015. Our experimental setup consisted of a single ambient air inlet from which the flow was split into two sets of identical sampling instruments; the first set was connected directly with an ambient sample while the second set received the air sample through a thermodenuder (TDD). Four sets of experiments were conducted for our study to understand aerosol number, size distribution, absorption, and scattering properties using Condensation Particle Counter (CPC), Scanning Mobility Particle Sizer (SMPS), Aethalometer (AE33) and Nephelometer respectively. The influence of semi-volatile aerosols were calculated based on the difference of aerosol properties at room temperature, 50 °C, 100 °C, 150 °C, 200 °C, 250 °C and 300 °C through set TDD temperatures to ambient sample. Our results show that with increasing TDD temperature, the evaporated fraction of semi-volatile aerosols also increased. At room temperature the semi-volatile fraction of aerosol number was 12 %, while at 300 °C it was as high as 49 % of ambient aerosol. Aerosol size distribution analysis from SMPS shows that with an increase in temperature from 50 °C to 300 °C, the peak mobility diameter of particles shifted from around 60 nm to 40 nm. However, no distinct change in the effective diameter of the aerosol size distribution was observed with increase in set TDD temperature. The change in size of aerosols due to loss of semi-volatile component had a stronger influence (~ 70 %) at larger size bins when compared to (~ 20 %) at smaller bins of SMPS. At 300 °C, the semi-volatile aerosols amplified BC absorption by approximately 28 % while scattering by the semi-volatile aerosols contributed up to 71 % of total scattering. The Scattering Angstrom Exponent (SAE) of the semi-volatile aerosol fraction was found to be more sensitive at lower temperatures (< 100 °C) than at higher temperatures. However the Absorption Angstrom Exponent (AAE) of the semi-volatile aerosol fraction did not show any significant temperature dependence.