2007 May 01: WI Madison: An Inter-comparison of two Black Carbon aerosol instruments and a semi-continuous elemental carbon instrument in the urban environment

 

2007 May 01: WI Madison: An Inter-comparison of two Black Carbon aerosol instruments and a semi-continuous elemental carbon instrument in the urban environment

 

Aerosol Science and Technology

, 41:463–474, 2007

Copyright

c American Association for Aerosol Research

ISSN: 0278-6826 print / 1521-7388 online

DOI: 10.1080/02786820701222819

An Inter-Comparison of Two Black Carbon Aerosol

Instruments and a Semi-Continuous Elemental Carbon

Instrument in the Urban Environment

David C. Snyder and James J. Schauer

Environmental Chemistry and Technology Program, University of Wisconsin—Madison, Madison,

Wisconsin, USA

Aerosol absorption coefficients were obtained using two versions

of the Magee Scientific Aethalometer and a Particle Soot Absorption

Photometer (PSAP) in Riverside, California during July and

August of 2005. These measurements were subsequently compared

to each other and to hourly elemental carbon (EC) mass concentrations

as determined by a Sunset Labs semi-continuous OCEC

analyzer. Measurements from all four instruments were shown to

be highly correlated (R

2 = 0.83 to 0.92). Differences between absorption

values measured by the PSAP and the Aethalometer were

found to be dominated by differences in the filter media used by the

respective instruments. Comparison of optical and thermal measurements

revealed that the specific attenuation cross section (σATN)

of light absorbing carbon (LAC) varied as a function of the time

of the day, most notably during weekdays. Minimum

σATN values

were observed during morning rush hour when EC concentrations

were at their greatest and maxima were seen in the late afternoon.

These variations correlated with changes in the OC/EC ratio and

the Angstrom exponent for absorption, which is consistent with

changes in the mixing state of elemental carbon associated with

secondary aerosol condensation on primary EC particles.

INTRODUCTION

The ability to determine the mass concentration of lightabsorbing

carbon (LAC) in ambient aerosols using filter-based

optical instruments depends on two important assumptions: first,

that the attenuation of light through a particle-laden filter is

Received 29 October 2006; accepted 16 January 2007.

Funding for this work was provided by the US Environmental Protection

Agency STAR grants # R831080 and RD832161010 and a grant

from the National Science Foundation (NSF grant ATM-0449815).

We gratefully acknowledge the assistance of Jose Jimenez and Ken

Docherty from Colorado University—Boulder and Paul Ziemann from

the University of California—Riverside for assistance with site preparation

and support.We would also like to thank the Prather Group from

the University of California—San Diego for supplying Aethalometer

data. Finally, many thanks to Brian Majestic for his assistance with the

filter comparison work at the Midwest Supersite.

Address correspondence to James J. Schauer, Environmental Chemistry

and Technology Program, University of Wisconsin—Madison,

660 N. Park Street, Madison, WI 53706, USA. E-mail: jjschauer@

wisc.edu

directly related to the mass of light absorbing material on the filter

and second, that this relationship, represented by the specific

attenuation cross section (σATN), remains constant over time at

a given receptor site. The relationship between filter attenuation

and absorption of light by particles is, unfortunately, not entirely

straight forward. While the attenuation of light through a filter is

a function of absorption, it is also, to some extent, a function of

scattering by both particles and filter fibers (Hitzenberger 1993;

Horvath 1993; Petzold et al. 1997; Bond et al. 1999). The scattering

of light by the filter material can be particularly complex

as multiple scattering by fibers creates a tortuous path for incident

light along which there exists an increased probability for

photons to encounter and be absorbed by particles. Attenuation

due to scattering is not expected to be strongly correlated with

the mass of absorbing material on the filter (Petzold et al. 1997)

and thus can potentially result in an overestimation of LAC as

the apparent attenuation cross section fluctuates in response to

scattering effects (Hitzenberger 1993; Horvath 1993; Petzold

et al. 1997; Bond et al. 1999).

Attenuation enhancement caused by filter scattering is dependant

on the type of filter used, both in terms of the amount

of scattering and the extent to which particles are embedded

within the filter matrix (Bond et al. 1999; Arnott et al. 2005).

These differences are thought to be contributing factors in the

differences between light absorption measurements reported by

two of the most commonly deployed filter-based optical instruments,

the Radiance Research Particle Soot Absorption Photometer

(PSAP) and the Magee Scientific Aethalometer (Hansen

et al. 1984;Weingartner et al. 2003). Whether filter enhancement

is the dominant factor in the differences between these two instruments

is unclear due in part to uncertainties regarding how

filter loading affects both absorption and scattering(Weingartner

et al. 2003; Bond and Bergstrom 2006). Numerous correction

factors for both instruments have been proposed but, with the

exception of those included in the PSAP algorithm, these corrections

are not universally applied making it all the more difficult

to compare PSAP and Aethalometer data.

In addition to LAC mass concentration, attenuation data

from filter-based instruments is also utilized to determine the

463

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464 D. C. SNYDER AND J. J. SCHAUER

TABLE 1

Published values for the specific attenuation cross-section of ambient aerosols

σ

ATN (m2g1) Optical method λ (nm) Thermal method Authors

9.8 to 25.4 Aethalometer/ PSAP 550

a Cachier (Liousse et al. 1993)

5 to 25 PSAP 550

a NIOSH (Quinn et al. 2004)

5.6 to 10.8 PSAP 550a EGA (Mayol-Bracero et al. 2002)

6.6 to 7.0 Aethalometer 880 Cachier (Kuhlbusch 1995)

3 to 9 PSAP 550

a NIOSH (Huebert et al. 2003)

6.4 to 20.1 Aethalometer/ PSAP 550

a Various (Sharma et al. 2002)

9.3 Aethalometer 880 Cachier (Lavanchy et al. 1999)

5.9 to 54.8 Aethalometer 880 Sunset (Jeong et al. 2004)

a

Wavelength adjusted from 565 nm assuming inverse wavelength dependence.

mass absorption efficiency or specific absorption cross section

(

σ) of ambient aerosols. To determine σ, attenuation is

typically divided by the mass concentration of elemental carbon

(EC) as determined by a thermal-optical method. Determination

of

σ by such methods also relies on an important

assumption, namely that absorbance of light by particles collected

on a filter is analogous to the absorbance of light by ambient

aerosols. Given the difficulties in relating filter attenuation

with absorbance outlined previously, it is difficult to rely on

this assumption, and as a result, it is perhaps more appropriate

to refer to the quotient of attenuation and EC as the specific

attenuation cross section (

σATN) of filter-bound particles. It

should also be noted that the specific attenuation cross section

is dependant on how EC is defined by a given thermal-optical

method as the EC to OC split can vary significantly between

methods utilizing different temperature profiles (Schauer et al.

2003b).

As shown in Table 1, published values of

σATN span a fairly

wide range, which can be explained by several factors, including

differences in the optical instruments and thermal methods

used to measure attenuation and EC. Additionally, the absorptive

properties of ambient aerosols are a function of sources,

aerosol aging, and mixing state, and as a result, the attenuation

cross section of LAC can vary from location to location (Liousse

et al. 1993; Petzold and Niessner 1995). From studies at different

locations, Liousse et al. (1993) concluded that regional values

of

σATN are determined by the dominant source(s) of aerosols

within that region and as such, are not expected to vary significantly.

Subsequent studies, however, indicate that the absorptive

properties of LAC can also exhibit seasonal variations and can

undergo rapid changes when a site is impacted by sulfate haze

events and bio-mass burning (Sharma et al. 2002; Jeong et al.

2004). This suggests that the ability to predict changes in

σATN

is dependant on an understanding of the factors that influence

the sources and mixing state of LAC.

The motivation behind the present study is to provide a

framework for the inter-comparison of ambient measurements

of LAC by filter-based light absorption methods and the intercomparison

ofLACmeasurements with EC mass concentrations

reported by a common thermal-optical method. Filter based light

absorption instruments, in particular the PSAP and Aethalometer,

provide investigators with a method for measuring LAC that

is relatively inexpensive, robust, and simple to operate when

compared to thermal methods. These factors have combined to

make such instruments extremely popular and ideal for a variety

of uses by a variety of users. At last count, there were more than

one thousand Aethalometers in use across the globe (Hansen,

personal communication, 2006) making it imperative to have a

firm understanding of how measurements reported by this instrument

compare to the PSAP and to commonly used thermal

methods.

In order to accomplish this objective, this study uses atmospheric

data to compare attenuation values generated by a

PSAP and two versions of the Aethalometer in order to determine

the level of reproducibility in such measurements and

to ascertain what factors dominate the differences in attenuation

values reported by the PSAP and Aethalometer. Additionally,

hourly attenuation, elemental carbon, and organic carbon

(OC) measurements are utilized to demonstrate the range of

the specific attenuation cross section of LAC in the urban environment

and to suggest some of the factors influencing this

range.

EXPERIMENTAL SECTION

Aerosol Measurements

Aerosol measurements were obtained at the Air Pollution

Research Center on the campus of the University of California—

Riverside during July and August of 2005. The sampling site

was located approximately 5.0 km from downtown Riverside

and 1.0 km from a major highway (US 215) in an area where

EC concentrations have historically been dominated by mobile

sources (Schauer et al. 1996; Gray and Cass 1998).

Aerosol absorption data were obtained from two single

wavelength PSAPs (Radiance Research, Seattle,WA), operated

in succession during the campaign, and two 7-wavelength

Downloaded At: 21:03 16 February 2010

AN URBAN COMPARISON OF BC AND EC INSTRUMENTS

465

model AE-31 Aethalometers (Magee Scientific, Berkeley, CA)

operated concurrently. Aethalometer #1 was fitted with a standard

“high sensitivity” sampling head featuring a 0.5 cm

2 circular

collection spot and operated at a flowrate of 5.9Lmin1 without

benefit of a size-selective inlet. Aethalometer #2 was fitted

with the optional “extended range” sampling head, which contains

a 1.67 cm

2 oval shaped collection spot, and operated a flow

rate of 5.0 L min1 from an inlet which included a PM2.5 sharpcut

cyclone (BGI, Waltham, MA). Both Aethalometers were

programmed to provide data over a five minute averaging period.

PSAP #1 was operated throughout the majority of the campaign

and was replaced by PSAP #2 during the last two days of

sampling. PSAP #1 was operated at a flow rate of 1.0 L min

1

initially, but in order to facilitate less frequent filter changes, the

flow rate was changed to 0.5 L min

1 during the first week of the

campaign. Sampling was accomplished through the same inlet

used by Aethalometer #1, which did not include a cyclone. The

averaging period for PSAP data throughout the campaign was

one minute.

Thermal measurements of elemental carbon and organic carbon

were obtained by two methods. Hourly EC and OC concentrations

were measured using a Sunset Labs semi-continuous

OCEC analyzer (Sunset Labs, Tigard, OR) programmed to collect

aerosol for 47 minutes beginning at the top of each hour with

the analysis cycle executed during the remainder of the hour. The

analysis of carbonaceous aerosols by this instrument is based on

the NIOSH 5040 method and a description of its operation and

validation can be found in Bae et al. (2004). Sample collection

was accomplished at flow rate of 24.0 L min

1 through an inlet

equipped with a sharp-cut PM

2.5 cyclone and a carbon impregnated

parallel plate organics denuder (Sunset Labs, Tigard, OR)

designed to remove gas-phase organic compounds upstream of

the collection filter. Calibration of the Sunset Labs field analyzer

was accomplished by an internal calibration using a 5%

methane in helium mixture in a fixed volume loop automatically

repeated at the conclusion of each analysis cycle, and an external

calibration using sucrose spikes on clean, pre-baked filters.

As a part of the quality assurance procedure for the semicontinuous

analyzer, aerosol samples were also collected on

pre-baked 90 mm quartz-fiber filters (Pall Gellman, Ann Arbor,

MI) by four URG-3000B (URG, Chapel Hill, NC) particulate

samplers. Filters were stored in sealed Petri dishes lined with

baked aluminum foil and kept in a freezer at or below 20

F before

and after sampling. The URG samplers were operated at

92 L min

1 and were each fitted with a PM2.5 cyclone. Three

five hour samples and one nine hour sample were collected consecutively

each day, along with 20% field blanks, and samples

were subsequently analyzed for OC and EC content using the

ACE-Asia method. (Schauer et al. 2003a).

Filter Comparison Experiment

Two PSAPs were operated concurrently at the Midwest

Supersite in East St. Louis during January of 2006, and hourly

absorption coefficients were obtained from each instrument

over 48 hours in two phases of 24 hours each. In the initial

phase, both instruments collected aerosol on the tissue-glass

filters used by both PSAPs in Riverside (Pallflex E70-2075W).

In the second phase, one of the tissue-glass filters was replaced

by a length of the filter tape used by the Aethalometer (Pallflex

quartz 2500 Q205). The sampling regime, including flow rate

and inlet construction, used in this comparison was identical to

that used in Riverside.

Data Treatment

The PSAP provides a measurement of the absorption coefficient

(bap) of LAC by dividing the change in light attenuation,

ATN (λ, t), by the optical pathlength of the sample, V/A (sample

air volume divided by the collection area of the filter), such

that bap is defined as the attenuation of light by the sample in

the air per unit pathlength (Equation [1]). As the Aethalometer

provides a direct reading of LAC mass concentration, average

hourly b

ap values were calculated for each Aethalometer from

attenuation data wherever possible using Equation (1) in order

to provide consistency between the treatment of PSAP and

Aethalometer data. During hours when the filter tape advanced,

causing an interruption in attenuation readings, direct concentration

readings taken from the Aethalometer were utilized instead

(Equation [2]). A comparison of the two calculation methods

revealed that there are no significant differences between them.

b

ap (Mm1) = ATN(λ, t)/(V/A) [1]

b

ap (Mm1) = σ(1)[LAC] (g m3) [2]

Attenuation as determined by the Aethalometer is defined in

Equation (3) with the factor of 100 added by the manufacturer

for numerical convenience. This factor was removed when calculating

b

ap values. In Equation (2), σ (1/λ), which has units of

m

2 g1, is the instrumental specific attenuation cross section of

LAC provided by the manufacturer. This constant is wavelength

dependant and is used to provide a measurement of LAC mass

concentration by relating light attenuation through the filter to

the mass of light absorbing material on the filter via Equation (4).

ATN

= −100ln(I0/I) [3]

ATN(

λ) = σ(1)LAC(g m2) [4]

During the treatment of the Aethalometer data, no correction

factors were utilized with the exception of correcting for differences

in reported versus actual flow rate and a correction for the

extended range sampling head that was applied when utilizing

Equation (1). The second correction was necessary because, according

to the manufacturer, for instruments which use the larger

spot size, sample deposition is not uniform across the entire collection

spot, and as a result, concentrations may be over-reported

as mass tends to accumulate preferentially in the center of the

spot directly above the photo-diode detector (Hansen, personal

communication, 2005). This has prompted Magee Scientific to

Downloaded At: 21:03 16 February 2010

466

D. C. SNYDER AND J. J. SCHAUER

include what it calls a “mean correction” factor to the algorithm

which calculates LAC concentration.

PSAP data was treated in much the same fashion as

Aethalometer data in order to determine absorption coefficients.

Although, as stated previously, the PSAP does provide a direct

reading of b

ap, it also applies what the manufacturer calls a

“transfer function” to this data in order to account for the effect

of increasing particle load on the filter.As the Aethalometer does

not incorporate any such correction and since the focus of this

study is to compare attenuation reported by both instruments, the

transfer function was not utilized in determining b

ap from the

PSAP. Instead the attenuation was calculated using the signals

from the reference and signal beams using Equation (5), which

is similar to the algorithm used by the Aethalometer. As with

the Aethalometer, the only corrections made were for collection

spot size and the sample flowrate. PSAP attenuation values were

then plugged into Equation (1) in order to generate b

ap values.

ATN

= −ln((sb0/sb)/(rb0/rb)) [5]

In Equation (5), sb and sb

0 refer to the signals of the sensing

beam at the beginning and the end of the averaging period respectively,

and similarly, rb and rb

0 refer to the reference beam

signals at the beginning and the end of the averaging period.

Slopes, intercepts, r-squared values, and uncertainties for all

regression analyses were calculated using SPSS for Windows

v.13.0.

CONCLUSIONS

Accurate measurement of LAC by filter-based optical instruments

requires a thorough understanding of the relationship between

the attenuation of light through a particle-laden filter and

the absorbance of light by ambient aerosols. Toward this objective,

we offer the following conclusions. First, we find that the

ratio of attenuation to EC mass concentration was not constant

during this study but instead varied in a fairly consistent pattern

during the weekdays. Secondly, variation in the specific attenuation

cross-section, which ranges from 25.58 to 35.31 m

2 g1 for

the Aethalometer and from 16.29 to 25.99 m

2 g1 for the PSAP,

appears to be related to changes in the wavelength dependence

of light attenuation and to changes in the bulk composition of

carbonaceous aerosols. Lastly, these relationships suggest that,

in urban environments, changes in the mixing state of LAC can

occur quite rapidly due to the condensation of semi-volatile organic

compounds on primary EC particles. Such changes need

to be taken into account when using filter-based light absorption

instruments such as the PSAP and the Aethalometer to measure

LAC in the urban environment.

With regards to comparisons between filter-based optical instruments,

we find that the largest single factor which accounts

for differences in attenuation reported by such instruments is

the type of filter used. Although there is evidence to suggest that

there may be some enhancement of attenuation due to filter loading

and light scattering by filter-bound particles, such artifacts

appear relatively minor when compared to the effects of filter

enhancement. On the whole, we find that attenuation reported

by the PSAP and the Aethalometer, regardless of the sampling

head, are highly reproducible.

Further study is clearly needed to investigate how changes

in

σATN are affected by seasonal conditions and by different environments

and how these may affect the inter-comparison of

light-absorption instruments. It should be noted, however, that

the accurate measurement ofLACconcentrations and a complete

understanding of its optical properties is dependant on the development

of a standardized method for the measurement of EC, as

this is the primary method for calibrating light-absorption instruments.

Clearly, until such a method is made available, an accurate

comparison of LAC measurements made in different environments

or during different seasons cannot be accomplished.

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