Optical closure experiments for biomass smoke aerosols

L A Mack,D Obrist,C E Wold,W P Arnott,W C Malm,A P Sullivan,H Moosmüller,J L Collett,S M Kreidenweis,G R Mcmeeking,W.-M Hao,E J T Levin,K A Lewis

doi:10.5194/acp-10-9017-2010

Abstract

Abstract. A series of laboratory experiments at the Fire Laboratory at Missoula (FLAME) investigated chemical, physical, and optical properties of fresh smoke samples from combustion of wildland fuels that are burned annually in the western and southeastern US The burns were conducted in the combustion chamber of the US Forest Service Fire Sciences Laboratory in Missoula, Montana. Here we discuss retrieval of optical properties for a variety of fuels burned in FLAME 2, using nephelometer-measured scattering coefficients, photoacoustically-measured aerosol absorption coefficients, and size distribution measurements. Uncertainties are estimated from various instrument characteristics and instrument calibration studies. Our estimates of single scattering albedo for different dry smoke samples varied from 0.428 to 0.990, indicative of observed wide variations in smoke aerosol chemical composition. In selected case studies, we retrieved the complex refractive index from measurements but show that these are highly sensitive to uncertainties in measured size distributions.

Highlights

Absorbing aerosols represent large contributions to aerosol optical depth (AOD) attributed to “atmospheric brown clouds” (e.g., Ramanathan et al, 2007), which have been shown to have widespread effects on climate due to the surface dimming and atmospheric solar heating with which they are associated. Ramanathan and Feng (2009) discuss a variety of impacts attributable to atmospheric brown clouds: for example, absorption of light and the resulting heating affects atmospheric dynamics locally by stabilizing atmospheric temperature profiles and, on larger scales, by affecting monsoon circulations, and deposition of absorbing aerosols onto snow and ice can accelerate melting
We focus on contributing to the database of knowledge of optical properties of biomass burning particles that have not been processed in the atmosphere, via laboratory measurements made on dry particles within a few hours of emission from open burning
Particle number and mass concentrations and measured values of bscat and babs decreased during the course of each experiment, calculated values of ωmeas were relatively constant with time, with the mean standard deviation during an experiment ranging from ±0.002 to ±0.028

Summary

Introduction

Calculated climate and visibility impacts of biomass burning aerosols are sensitive to the relative amounts of scattering and absorption, which in turn depend on the size distribution of particles and on composition (Chylek and Wong, 1995). We focus on contributing to the database of knowledge of optical properties of biomass burning particles that have not been processed in the atmosphere, via laboratory measurements made on dry particles within a few hours of emission from open burning. We further relate these optical properties to measured bulk particle composition to show. 0.910±0.009 tree the extent to which simple assumptions can be used to model scattering and absorption coefficients

Experimental

Measured single scattering albedos

Retrieval of refractive indices

Summary and conclusions