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Field Measurements for Quantifying Semi-Volatile Aerosol Influence on Physical and Optical Properties of Ambient Aerosols in the Kathmandu Valley, Nepal

Category: Urban Air Quality

Article In Press
DOI: 10.4209/aaqr.2017.11.0492
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Sujan Shrestha 1,2, Siva Praveen Puppala 1, Bhupesh Adhikary1, Kundan Lal Shrestha2, Arnico K. Panday1

  • 1 International Centre for Integrated Mountain Development (ICIMOD), Khumaltar, Lalitpur, Nepal
  • 2 Department of Environmental Science and Engineering, Kathmandu University, Dhulikhel, Kavre, Nepal

Highlights

Semi-volatile aerosol contributes up to 49% of total aerosol number concentration.
Peak mobility diameter changes due to loss of the semi-volatile aerosol.
The semi-volatile aerosol contributes up to 28% of the total aerosol absorption.
Brown carbon contribute up to 9% of the total aerosol absorption.
The semi-volatile aerosol contributes up to 70% of the total aerosol scattering.


Abstract

An intensive field campaign was conducted during the pre-monsoon season of 2015 in the urban atmosphere of the Kathmandu Valley to study the influence of the semi-volatile aerosol fraction on physical and optical properties of aerosols. Ambient air was siphoned through a specific ambient air inlet and then split into two parts. The first part connected directly with an ambient air sample while the second received the air sample through a thermodenuder (TDD). The aerosol properties, such as the aerosol number, size distribution, absorption, and scattering, were studied using Condensation Particle Counters (CPCs), Scanning Mobility Particle Sizers (SMPSs), Aethalometers (AE33) and Nephelometers, respectively. The differences in the properties of the aerosol fraction at room temperature and other TDD set temperatures (50°C, 100°C, 150°C, 200°C, 250°C, and 300°C) were calculated to study the influence of the semi-volatile aerosol fraction on ambient aerosols. The evaporated fraction of the semi-volatile aerosols increased with the TDD set temperature. The semi-volatile fraction of the aerosol number increased from 16% to 49% of ambient aerosol, while the peak mobility diameter of particles shifted from around 60 nm to 40 nm as the temperature increased from 50°C to 300°C. However, increasing the TDD set temperature had no influence on the effective diameter of the aerosol size distribution. Larger aerosol size bins of the SMPS experienced a significantly stronger influence (~70%) from temperature increments compared to smaller size bins (~20%). The semi-volatile aerosol fraction amplified BC absorption by up to 28%, while scattering by the semi-volatile aerosol fraction contributed up to 71% of the total.

Keywords

Aerosol number concentration Absorption Scattering Black carbon


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