2010: CO & WA: Part 1 of 4: Scientific Paper: Measured and modeled humidification factors from biomass burning: role of inorganic constituents

Atmos. Chem. Phys. Discuss., 10, 4225–4269, 2010

http://www.atmos-chem-phys-discuss.net/10/4225/2010/

© Author(s) 2010. This work is distributed under

the Creative Commons Attribution 3.0 License.

Atmospheric

Chemistry

and Physics

Discussions

This discussion paper is/has been under review for the journal Atmospheric Chemistry

and Physics (ACP). Please refer to the corresponding final paper in ACP if available.

Measured and modeled humidification

factors of fresh smoke particles from

biomass burning: role of inorganic

constituents

J. L. Hand

1, D. E. Day1, G. M. McMeeking2,*, E. J. T. Levin2, C. M. Carrico2,

S. M. Kreidenweis

2, W. C. Malm3, A. Laskin4, and Y. Desyaterik4,2

1

Cooperative Institute for Research in the Atmosphere, Colorado State University, Fort Collins,

CO, USA

2

Department of Atmospheric Science, Colorado State University, Fort Collins, CO, USA

4225

3

National Park Service, Cooperative Institute for Research in the Atmosphere, Colorado State

University, Fort Collins, CO, USA

4

William R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National

Laboratory, Richland, WA, USA

now at: Center for Atmospheric Science, University of Manchester, Manchester, UK

Received: 27 January 2010 – Accepted: 5 February 2010 – Published: 12 February 2010

Correspondence to: J. L. Hand (hand@cira.colostate.edu)

Published by Copernicus Publications on behalf of the European Geosciences Union.

4226

Abstract

During the 2006 FLAME study (

Fire Laboratory at Missoula Experiment), laboratory

burns of biomass fuels were performed to investigate the physico-chemical, optical,

and hygroscopic properties of fresh biomass smoke. As part of the experiment,

5

two nephelometers simultaneously measured dry and humidified light scattering coefficients

(

bsp(dry) and bsp(RH), respectively) in order to explore the role of relative

humidity (RH) on the optical properties of biomass smoke aerosols. Results from

burns of several biomass fuels showed large variability in the humidification factor

(

f (RH)=bsp(RH)/bsp(dry)). Values of f (RH) at RH=85–90% ranged from 1.02 to 2.15

10

depending on fuel type. We incorporated measured chemical composition and size

distribution data to model the smoke hygroscopic growth to investigate the role of inorganic

and organic compounds on water uptake for these aerosols. By assuming only

inorganic constituents were hygroscopic, we were able to model the water uptake within

experimental uncertainty, suggesting that inorganic species were responsible for most

15

of the hygroscopic growth. In addition, humidification factors at 85–90% RH increased

for smoke with increasing inorganic salt to carbon ratios. Particle morphology as observed

from scanning electron microscopy revealed that samples of hygroscopic particles

contained soot chains either internally or externally mixed with inorganic potassium

salts, while samples of weak to non-hygroscopic particles were dominated by soot and

20

organic constituents. This study provides further understanding of the compounds responsible

for water uptake by young biomass smoke, and is important for accurately

assessing the role of smoke in climate change studies and visibility regulatory efforts.

1 Introduction

The significant contribution of biomass burning emissions to the global and regional

25

aerosol burden has been documented by several studies (e.g. Park et al., 2003, 2007;

Spracklen et al., 2007). Quantifying the role of biomass burning aerosols in climate

4227

forcing and visibility degradation requires characterizing their optical, physical, chemical

and hygroscopic properties. Many studies have been performed in the ambient

atmosphere to measure radiative properties of smoke particles from wild fire and prescribed

burning (see the review by Reid et al., 2005a, b). However, interpreting these

5

results is complicated due to the variety of conditions under which the smoke was

generated in the ambient atmosphere. Typically, ambient smoke is generated by a

combination of biomass fuels of varying moisture, under a variety of flame conditions

and is also a

ffected by atmospheric aging. Previous studies have provided a wealth of

data for ambient biomass smoke properties and are necessary to understand smoke

10

behavior in the atmosphere. However, measurements performed in combustion facilities

also provide opportunities to further investigate specific biomass smoke properties

under known conditions, such as for a given fuel type and flame condition.

The Fire Laboratory at Missoula Experiment (FLAME) was designed to investigate

many important questions regarding smoke properties from biomass burning. These

15

experiments were conducted at the US Forest Service’s Fire Science Laboratory in

Missoula MT in 2006 and 2007. Results from several subsets of these experiments

include particle and gas emissions (McMeeking et al., 2009; Chakrabarty et al., 2006;

Chen et al., 2006, 2007; Engling et al, 2006; Smith et al., 2009; Laskin et al., 2009),

smoke marker properties (Sullivan et al., 2008), particle physical and optical properties

20

(Hopkins et al., 2007; Lewis et al., 2009; Levin et al., 2010), aerosol hygroscopicity,

cloud condensation and ice nucleation ability (Day et al., 2006; Carrico et al., 2008,

2010; Petters et al., 2009; Demott et al., 2009). Burns were performed on fuels that

represented biomass found in locations often involved in wild and prescribed fires, such

as in the western and southeastern United States (McMeeking et al., 2009).

25

The focus of this paper is the hygroscopic properties of biomass smoke measured

during FLAME 2006. Quantifying the behavior of aerosols with respect to changing relative

humidity (RH) is necessary for accurately estimating their role in climate forcing

(Hansen et al, 2005) visibility degradation (Malm et al., 2005), cloud nucleating ability

(Vestin et al., 2007; Petters et al., 2009), and health e

ffects (Naeher et al., 2007).

4228

One description of the hygroscopic response of a particle is the diameter growth factor,

GF, defined as the ratio of particle diameter at a specific relative humidity to the

dry particle diameter (GF=Dp(RH)/Dp(dry)). However, estimating the effects of RH on

aerosol radiative properties requires additional information, such as changes in par-

5

ticle mass, composition, size, shape and morphology as a function of RH, as these

properties also a

ffect the amount of light scattered by a particle. The humidification

factor,

f (RH), describes changes in particle light scattering as a function of RH and is

defined as the ratio of light scattering coe

fficients (bsp) of humidified to dry aerosols

(

f (RH)=bsp(RH)/bsp(dry)). A computed f (RH) value for a pure, spherical inorganic salt

10

particle composed of ammonium sulfate with a mass mean diameter of 0.4 μm and geometric

standard deviation of 1.9 is 3.7 at 85–90% RH, implying that these humidified

particles scatter almost four times as much light as compared to dry conditions. An

experimental humidification factor is typically determined by parallel measurements of

light scattering coe

fficients under dry and humidified conditions (e.g. Day et al., 2000;

15

Malm et al., 2003).

Previous results of

f (RH) for biomass smoke particles demonstrate a wide range of

estimates. Comparisons of

f (RH) from various studies is complicated by the fact that

the definitions of “dry” and “humidified” vary per study. Typically, humidified RH values

range from 80–90%, and although dry RH often range between 10–30%, particles may

20

still absorb or retain water at RH values considered “dry”. Day et al. (2006) summarized

f

(RH) values for a variety of locations and platforms, including aircraft, ground-based

and laboratory studies. Observations of

f (RH) ranged from 1.01 to 2.1 (RH=80–90%)

for ambient smoke; the same range of

f (RH) was observed for measurements in combustion

facilities (Day et al., 2006; Lewis et al., 2009). The range of results undoubtedly

25

was due to the range of RH values, the variety of fuel types and conditions, chemical

composition of the smoke and the degree of aging and processing in the atmosphere.

For example, smoke sampled in association with regional haze could be significantly

mixed with inorganic species in the atmosphere that could alter hygroscopic properties

of the particles (Li et al., 2003; P´ osfai et al., 2003; Semeniuk et al., 2007; Zhang et al.,

4229

2008; Khalizov et al., 2009). In fact, Malm et al. (2005) found that

f (RH) for days-old

smoke in Yosemite National Park CA increased from 1.2 to

2.0 as the ratio of organic

carbon mass to ammonium sulfate mass decreased. Laboratory measurements

of

f (RH) for young (minutes- to hours-old) smoke reported by Day et al. (2006) were

5

1.01–1.76 (RH=80–90%), demonstrating a significant range in the hygroscopic properties

of unprocessed young smoke. Comprehensive measurements performed during

FLAME 2006 study allowed us to investigate the range of

f (RH) for young biomass

smoke in the laboratory, as well as model

f (RH) as a function of RH.

Modeling of aerosol humidification factors requires predicting changes in aerosol

10

physical and optical properties as a function of RH and aerosol water content. Thermodynamic

equilibrium models such as the Extended Aerosol Inorganic Model (E-AIM)

(Clegg et al., 1998) allow for the prediction of aerosol water content as a function of RH.

Aerosol water content for an aerosol mixture can be computed using the Zdanovskii-

Stokes-Robinson (ZSR) approach (Stokes and Robinson, 1966) wherein the amount of

15

water associated with a mixture of compounds is the sum of the water content from the

individual species. Computing changes in aerosol size (GF), physical and optical properties

as a function of RH is straightforward once the aerosol water content is known.

Mie theory is used to compute changes in light scattering coe

fficients for spherical particles

as a function of changing particle size and optical properties. We modeled

f (RH)

20

by applying thermodynamic models and Mie theory to aerosol chemical composition

and size distributions measured during the FLAME 2006 study.

This manuscript presents measured and modeled

f (RH) estimates for young

biomass smoke generated in a controlled laboratory burns of di

fferent biomass fuels.

These results are noteworthy because

f (RH) was measured in conjunction with particle

25

size, bulk PM2.5 composition and morphology measurements. Combining all of these

data sets in a modeling framework provided for the interpretation of aerosol hygroscopicity

for very young, unprocessed (by the atmosphere) smoke. The manuscript is organized

in the following way. Following the introduction, Sect. 2 describes the experiments

conducted during FLAME 2006, including light scattering coe

fficient measurements

4230

(Sect. 2.1), chemical composition (Sect. 2.2), size distributions (Sect. 2.3), scanning

electron microscopy (Sect. 2.4) and particle diameter growth factors (Sect. 2.5). Section

3 provides a description of the modeling technique for computing

f (RH). Results

are discussed in Sect. 4 and include reports of uncertainties and sensitivity studies. A

5

summary is provided in Sect. 5.

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