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

 

4 Results and discussion

4.1 Humidification factors (f(RH))

20

The extent of agreement between measured and modeled f (RH) values is limited by

the number of simplifying assumptions required to perform the calculations. Characterizing

heterogeneous, non-spherical, chemically complex particles as homogeneous internally

mixed spheres of known composition, size, and water content obviously could

4241

a

ffect how well the calculated and measured values of f (RH) agree. The assumptions

and limitations are important to keep in mind when evaluating the comparisons

between modeled and measured values of

f (RH), and sensitivity to some of these

assumptions will be discussed in the next section.

5

We present comparisons of f (RH) values for smoke particles generated from burning

of four fuel types (Forest/Pine, Brush, southeast US/Tropical and Du

ff) and their

relation to other measured smoke properties such as composition and microstructure.

Comparisons of measured to modeled

f (RH) are difficult to quantify over the entire RH

range, therefore we compared

f (RH) over RH=85–90% to avoid deliquescence RH val10

ues, and we separated the comparisons between the two model estimates (metastable

and deliquescence). The average modeled

f (RH) was computed for the same RH values

over which the measurements were performed. Measured and modeled results for

all of the fuels are reported in Table 2.

The

f (RH) curve corresponding to particles generated from burns of ponderosa pine

15

is presented in Fig. 4. Ponderosa pine is representative of the other fuels in the Forest/

Pine fuel category, all of which produced either weak or non-hygroscopic particles,

with minimal growth observed above 85% (see Table 2). Values of

f (RH) for

smoke from this fuel type did not exceed 1.1

±0.08 for RH=85–90%. The modeled

f

(RH) values agreed within experimental uncertainty for fuels of this type, with model

20 results ranging between 1.03 and 1.05 for both deliquescence and metastable equilibrium.

The modeled curves were flat over most of the RH range and started to increase

around 95%. No deliquescence behavior was observed in either measured or modeled

results. Smoke particles generated from the burning of these fuels were dominated by

carbon, with combined POM and LAC mass fractions of 83% or more. Inorganic salt to

25

carbon (POM+LAC) ratios were small, typically less than 0.02. Figure 5 shows an SEM

image of particles generated in burns of ponderosa pine. Morphology of these particles

was very typical for those generated in burns of the Forest/Pine type of fuels. The

oily organic content of particles was largely electron-transparent and was seen in the

images as dark areas that commonly contain soot inclusions, which are seen as bright

4242

fragments. X-ray microanalysis of both dark areas and bright inclusions showed almost

no elements other than carbon and oxygen, which was consistent with the bulk analysis

data. Additionally, DMPS-derived shape factors for these particles were fairly small

(1.0–1.2), suggesting that airborne particles were nearly spherical (Table 2), which is

5

also consistent with substantial coating of particles with oily organic material. Finally,

densities ranged from 1.45–1.47 g cm

3, consistent with the dominance of POM and

its density (1.4 g cm

3) that was assumed in the calculation. Refractive indices also

reflected the dominance of POM with real values around 1.6 and imaginary parts less

than 0.07. The organic carbon multipliers were fairly low (1.5–1.6) for these young par

10

ticles, as opposed to estimates reported by Malm et al. (2005) for ambient aged smoke

(

1.8).

Smoke particles generated in the burns of Brush fuels were significantly hygroscopic,

with

f (RH) values ranging from 1.18 (juniper) to 2.15 (sage/rabbit brush). The f (RH)

curves shown in Fig. 6 for sage/rabbit brush fuel demonstrated flat growth (

f (RH)=1)

15

until around 55% RH after which the f (RH) smoothly increased. The measurements

suggest deliquescence around 75% RH while the modeled deliquescence was shifted

lower by about 5% (70%). Both model approaches overestimated the data until

75%

RH when the data fell between the deliquescence and metastable curves. Above



85% the measurements were in closer agreement with the deliquescence curve. The

20

modeled f (RH=85–90%) was 2.16 and 1.87 for the deliquescence and metastable

estimates, respectively. The continuous growth observed for smoke particles from

sage/rabbit brush burns was typical for fuels of this type (Lewis et al., 2009). Inorganic

constituents contributed substantially to the mass of particles, with carbon mass

fraction ranging from 22–80% and inorganic salt to carbon ratios from 0.12–0.55 (Ta

25

ble 2). Smoke from these fuels had large shape factors, with the largest (

=1.8) corresponding

to particles from juniper burns. The large shape factors were consistent with

observations from the SEM images that revealed the presence of highly fractal soot

particles in the samples. The top two panels of Fig. 7 show two images of particles

in the same field of view obtained using secondary electron (SE) and backscattered

4243

electron (BSE) imaging modes, respectively. Soot and organic particles are clearly

seen and labeled in the SE image. Carbonaceous materials have very low backscattering

e

fficiency. Therefore, both fractal soot and organic particle are nearly invisible in

the BSE image. In contrast, higher atomic number inorganic particles and inclusions

5

are visible in the BSE image. The bottom panel of Fig. 7 shows characteristic X-ray

spectrum of these inorganic particles indicating their mixed KCl/K

2SO4 composition.

Similar observations were reported for particles sampled from chamise burns (Lewis

et al., 2009) and sagebrush (Chakrabarty et al., 2006). Similarly, internally mixed soot

and organic particles with inorganic species have been observed previously for am

10

bient biomass smoke samples (e.g. Li et al., 2003; Po´sfai et al., 2003; Hand et al.,

2005; Semeniuk et al., 2007). OC multipliers tended to be higher for smoke particles

from burns of these fuels, as did refractive indices (real and imaginary) and densities,

reflecting the higher inorganic and LAC content of these fuels (see Table 2). With the

exception of ceanothus, all of the fuels of this type had LAC mass fractions of

30% or

15

greater, consistent with the dominance of soot in the SEM images.

Smoke particles from the southeastern US/Tropical fuels tended to be less hygroscopic

than those from the Brush type, with the values of

f (RH) at 85–90% RH of

1.11 and 1.39 for smoke from Puerto Rico fern and wax myrtle fuels, respectively. The

data did not show any visible deliquescence and indicated continuous growth at higher

20

RH. For both fuels, measured f (RH) values were lower than the model estimates at

RH

=85–90%. In the case of Puerto Rico fern, the model estimates were 1.41 and 1.31

for the deliquescence and metastable cases, respectively. The models significantly

overestimated

f (RH) for particles from wax myrtle burns, with f (RH)=2.03 and 1.71

for the deliquescence and metastable estimates, respectively. The carbon content for

25

particles from fuels of this type ranged from 58% (wax myrtle) to 82% (Puerto Rico

fern). Inorganic salt to carbon ratios were

<0.3, ranging from 0.10 to 0.26 (Puerto Rico

fern and wax myrtle, respectively). The shape factors, organic multipliers, densities

and refractive indices for particles from Puerto Rico fern burns were similar to particles

from burns of the Forest/Pine fuels (see Table 2), but these properties were somewhat

4244

higher for wax myrtle. SEM images showed heterogeneity in particle microstructure

for these fuels. Round particles rich in carbon and oxygen and having inorganic inclusions

dominated the Puerto Rico fern sample. In comparison, the wax myrtle sample

is abundant with NaCl and KCl particles internally and externally mixed with carbon but

5

without the oily/liquid particles seen in the ponderosa pine sample.

The final fuel type, Du

ff, included ponderosa pine duff and Alaskan duff. Smoke

particles from these burns were weak to non-hygroscopic with

f (RH) values of 1.05 and

1.09, respectively. The weak hygroscopic behavior observed for these particles was

similar to those from the first fuel type and was indicated by very flat, smooth curves that

10

exhibit little to no growth at high RH. Within experimental uncertainty, model estimates

(both deliquescence and metastable) agreed well, with values of

f (RH) ranging from

1.03–1.05. Smoke particles from both of these fuels had high carbon mass fractions

(

>90%) and inorganic salt to carbon ratios of 0.02. The SEM images and EDX analysis

of particles from burns of ponderosa pine du

ff showed round and irregular particles

15

dominated by carbon and oxygen. Some particles appeared to have been flattened

upon impaction onto the sample substrate; however, they do not contain oily organic

constituents characteristic of particles from the Forest/Pine fuels. Particles sampled

from Alaskan du

ff burns showed fractal soot particles both internally and externally

mixed with round and irregular shaped particles dominated by carbon and oxygen. The

20

internally mixed particles may have been a result of coagulation. Only trace amounts of

inorganic species were measured by X-ray microanalysis of particles from both fuels.

Particle densities and refractive indices were similar to those of the Forest/Pine type,

consistent with their high POM content.

The comparisons of the average

f (RH) values at 85–90% RH for smoke from all

25

burns are shown as scatter plots in Fig. 8a, b for the deliquescence and metastable

curves, respectively. The symbols for the average

f (RH) values for smoke sampled for

a specific burn is denoted by a number (see Table 2) and a color (fuel type). The experimental

uncertainties (

±0.08) are shown as dotted lines parallel to the solid perfect

agreement line. The

f (RH) values for smoke particles with weak hygroscopic growth

4245

(Forest/Pine and Du

ff fuel types) agreed within experimental uncertainty for the two

model estimates. Greater disparity was observed for more hygroscopic particles depending

on the model approach. Overall better agreement was accomplished when

using the deliquescence curves, at least over the RH range considered here. The

5

exception was for smoke particles from ceanothus (#9) which agreed well with the

metastable curve. Humidification factors for particles generated in burns of two southeastern

US/Tropical fuels were overestimated by the models.

Particles with high carbon mass fractions corresponded to low

f (RH) values. Figure

9 presents a summary of measured

f (RH) values for RH=85–90% as a function of

10

inorganic salt to carbon ratios. The lowest f (RH) values (typically less than 1.1) corresponded

to the lowest inorganic salt to carbon ratios (

<0.05) observed for particles from

burns of Forest/Pine and Du

ff fuels. The most hygroscopic smoke particles from the

Brush fuel type had the highest inorganic salt to carbon ratios. Smoke from the southeast

US/Tropical type fell within this group as well. From Fig. 9 it is clear that as the

15

carbon content of particles increased relative to the inorganic content, their hygroscopicity

decreased. Orthogonal distance regression was applied to the data assuming uncertainties

in the inorganic/carbon ratio of

±0.02 based on measurement uncertainties

in mass concentrations. The regression resulted in a slope of 1.97

±0.15, an intercept

of 0.97

±0.03, and a correlation coefficient of r=0.97 (significant at 99% confidence

20

level). The intercept suggests an f (RH) value near one for carbon particles containing

no inorganic constituents. Previous results reported for other measurements of hygroscopicity

during FLAME 2006 and 2007 (Carrico et al., 2010) as well measurements of

ambient aged smoke particles measured in Yosemite National Park (Malm et al., 2005;

Carrico et al., 2005) suggested similar behavior. Semeniuk et al. (2007) reported on

25

the enhancement of hygroscopicity of carbonaceous biomass burning particles mixed

internally and externally with inorganic species during the SAFARI-2000 study in southern

Africa. Carbonaceous particles (soot and organic particles) processed with sulfate

species have also been shown to undergo enhancements in particle hygroscopicity

(e.g. Baynard et al., 2006; Garland et al., 2007; Zhang et al., 2008; Khalizov et al.,

4246

2009). Our results suggest that, for some fuel types, inorganic species formed through

the combustion of plant matter were present at the time of emission to produce significantly

hygroscopic particles without the influence of atmospheric processing. Similar

results were reported by Carrico et al. (2010) and Petters et al. (2009) using di

fferent

5

measurements of hygroscopicity during FLAME 2006 and 2007.

4.2 Sensitivity of modeled

f(RH)

As mentioned in the previous section, there were several assumptions and uncertainties

associated with the data and models used to predict

f (RH) values. Two of the

major assumptions included the value of the molecular weight per carbon weight mul

10

tiplier used to calculate POM, and the shape factor applied to the DMPS size distributions.

These assumptions were important because chemical composition influenced

the predicted aerosol water content, and the size distributions a

ffected the derived

optical properties. To test the impact on

f (RH) values from these assumptions, we

performed sensitivity analyses to explore the role of uncertainties in size distributions

15

and chemical composition.

The major source of discrepancy associated with the particle size distributions was

the di

fference in integrated mass from the DMPS and IMPROVE gravimetric fine mass

measurements because of the non-sphericity of the aerosols (see Sect. 2.3). The

derived shape factors forced the agreement between mass concentrations but also

20

significantly shifted the size distributions, in most cases to smaller size. We computed

f

(RH) with and without the shape factors applied to the size distributions. The differences

in

f (RH) were less than ±0.02 (at RH=85–90%) for all fuels and the two model

approaches, and were not systematically biased either high or low, suggesting

f (RH)

was fairly insensitive to change in size distributions. The estimates were well within

25

the ±0.08 experimental uncertainty. Averaging size distributions over an entire burn

may also have contributed some uncertainty, but changes in particle size distributions

during the sampling period were much smaller than any observed from applying shape

factors (up to 80% shift in diameter, see Table 2). The e

ffects from variables that affect

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