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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

Accepted Manuscripts
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 pre-monsoon season 2015 in the urban atmosphere of the Kathmandu Valley for studying 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 set received the air sample through a thermodenuder (TDD). The aerosol properties such as aerosol number, size distribution, absorption, and scattering properties were studied using Condensation Particle Counters (CPCs), Scanning Mobility Particle Sizers (SMPSs), Aethalometers (AE33) and Nephelometers, respectively. The difference in the fraction of aerosol properties between 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 semi-volatile aerosols fraction on ambient aerosols. The evaporated fraction of the semi-volatile aerosols increased with the increase in TDD set temperatures. The semi-volatile fraction of 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 with the increase in temperature from 50°C to 300°C. However, increase in set TDD temperature had no influence on the effective diameter of the aerosol size distribution. Larger aerosol size bins of SMPS had significantly stronger influence (~70%) of temperature increment 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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