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

Sujan Shrestha,Arnico K Panday,Kundan Lal Shrestha,Bhupesh Adhikary,Siva Praveen Puppala

doi:10.4209/aaqr.2017.11.0492

Abstract

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.

Highlights

Aerosols are suspended particles in the air that are solid, liquid, or a mixture of both states, ranging in size from a few nanometers to several micrometers (Ehhalt et al, 1999; Warneck, 1999)
The TDD operated at room temperature by not providing power to the TDD thermal desorption section, as TDD works with only set as opposed to ambient temperatures
The present study provides the first information on quantifying the semi-volatile aerosol fraction influence on aerosol physical and optical properties over the Kathmandu Valley, a polluted site in the Hindu Kush Himalayan region of Nepal

Summary

Introduction

Aerosols are suspended particles in the air that are solid, liquid, or a mixture of both states, ranging in size from a few nanometers to several micrometers (Ehhalt et al, 1999; Warneck, 1999). Aerosols are classified as primary or secondary depending upon their origin. Secondary aerosols are formed due to condensation, oxidation and chemical transformation (Seinfeld and Pandis, 2006). Secondary aerosols tend to be semi-volatile in nature (Hennigan et al, 2008). The aerosol components that do not condense under normal atmospheric conditions are considered as volatile aerosols, while those aerosols that remain in a condensed phase under certain atmospheric conditions are classified as semi-volatile. The non-volatile aerosols have negligible vapor pressure and remain in a condensed phase under normal atmospheric conditions (Fuzzi et al, 2006), semi-volatile aerosols are believed to contribute most significantly to the toxicity of particles (Stevanovic et al, 2015). The volatility of an aerosol provides an indication about its emission sources, history, and chemical composition

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