International Centre for Integrated Mountain Development, GPO Box 3226, Kathmandu, Nepal, Tel +977 1 5003222, Email info@icimod.org, www.icimod.org

Influence of volatility on aerosol single scattering albedo

IntroductionField experiments were conducted at the Integrated Centre for International Mountain Development (ICIMOD) headquarter in Lalitpur, Nepal to quantify semi-volatile aerosol influence on aerosol physical and optical properties over Kathmandu valley.

Objectives and experimental setup•To study number of ambient particle reduction due to semi-volatile fraction of aerosol that

evaporates out of ambient aerosol was measured using Condensation Particle Counter (CPC) and thermodenuder (TDD) coupled with CPC setup.

•To examine the relationship between volatility and aerosol size distribution, experiments were conducted using identical Scanning Mobility Particle Sizer (SMPS) and TDD coupled with SMPS setup.

•Contribution to the aerosol absorption by semi-volatile fraction of aerosols were measured using identical aethalometer and TDD coupled with aethalometer setup.

•Contribution of aerosol scattering by semi-volatile fraction of aerosols were measured using identical nephelometer and TDD coupled with nephelometer setup.

Influence of volatility on number concentration

Influence of volatility on aerosol size distribution Influence of volatility on aerosol scattering

Influence of volatility on aerosol absorption

Particle loss increased from 16% to 49% with the increase in TDD set temperature from 50°C to 300°C. Strong correlation between wet and dry particle concentration was observed, except during very high ambient particle number concentration.

Highly volatile aerosol contribution was almost similar throughout the day, while the moderately volatile fraction changed significantly during peak events.

With the increase in set temperature, peak mobility diameter of dry aerosol shifted towards lower size.

Semi-volatile aerosol absorption contribution ranged from 20% to 28% while increasing set temperature from 50°C to 300°C at 880nm wavelength.

Semi-volatile aerosol AAE was observed in between 1.10 to 1.43. Highly and moderately volatile aerosol AAE was observed around 1.1 and 1.1-1.4 respectively. The highly volatile aerosol absorption was mainly from black carbon mixing state. This mixing state AAE was around 1.

Semi-volatile aerosol scattering contribution was observed to be 8% - 71% of wet aerosol scattering while increasing set temperature from 50°C to 300°C at different wavelengths.

Scattering contribution of semi-volatile fraction was almost eight times higher at 450nm wavelength compared to 700nm wavelength.

For other set temperatures, semi-volatile aerosol fraction contribution to scattering was around three times higher at 450nm wavelength compared to 700nm wavelength.

Conclusions•Semi-volatile aerosol number fraction ranged from 12 to 49% at TDD set temperatures from

room to 300°C respectively.

•Highly volatile aerosols does not exhibit diurnal variability while moderately volatile aerosols contribution increases during peak concentration events.

•The reduction of aerosol size was high for larger diameter aerosols compared to smaller diameter aerosols due to removal of semi-volatile aerosol fraction

•Semi-volatile aerosol mixing state contributed around 20% to total aerosol absorption.

•Calculated brown carbon contribution to aerosol absorption ranged from 0 to 9%.

•Semi-volatile aerosol scattering contribution was found to be two times higher than its absorption, implying removal of semi-volatile aerosols will lead to more absorbing atmosphere.

AcknowledgementThis project was funded by ICIMOD contributed by the government of Afghanistan, Australia, Austria, Bangladesh, Bhutan, China, India, Myanmar, Nepal, Norway, Pakistan, Sweden, Switzerland, and the United Kingdom.

Special acknowledgements to Dr. Prakash Bhave, Shreta Ghimire, Alpha Thapa and Pradeep Dangol.

TDD set temp. in °C

Semi-volatile aerosol fraction Single Scattering Albedo (SSA) at assumed wet aerosol fraction 0.9 and 0.95

SSA0.9 SSA0.95 SSA0.9 SSA0.95 SSA0.9 SSA0.95

Wavelength = 450nm Wavelength = 550nm Wavelength = 700nm

Room temp. 0.91 0.95 0.90 0.95 0.85 0.92

50 0.86 0.93 0.83 0.91 0.78 0.88

100 0.93 0.97 0.93 0.97 0.92 0.96

150 0.94 0.97 0.94 0.97 0.94 0.97

200 0.94 0.97 0.94 0.97 0.94 0.97

250 0.95 0.96 0.95 0.97 0.95 0.97

300 0.95 0.98 0.95 0.98 0.96 0.98

Schematic of instrument setup

Wet instrument

Dry instrument

Inlet

Results and Discussion

Influence of Semi-Volatile Aerosol on Physical and Optical Properties of Aerosol in Kathmandu ValleySujan Shrestha1,2, Siva Praveen Puppala1, Bhupesh Adhikary1, Kundan Lal Shrestha2, Arnico K. Panday1

1 International Centre for Integrated Mountain Development (ICIMOD), Khumaltar, Lalitpur, Nepal2 Kathmandu University, Department of Environmental Science and Engineering, Dhulikhel, Kavre, Nepal

The reduction of aerosol size was high for larger diameter aerosols compared to smaller diameter aerosols.

The semi-volatile aerosol fraction SSA was observed to be minimum for 50°C at all wavelengths. If this process is applicable in atmospheric condition, mid-day temperature rise may make atmosphere more absorbing in nature.