INDOEX aerosol: A comparison and summary of chemical,
microphysical, and optical properties observed from land, ship,
and aircraft
A. D. Clarke,
1
S. Howell,
1
P. K. Quinn,
2
T. S. Bates,
2
J. A. Ogren,
3
E. Andrews,
3
A. Jefferson,
3
A. Massling,
4
O. Mayol-Bracero,
5
H. Maring,
6
D. Savoie,
6
and G. Cass
7
Received 1 March 2001; revised 16 August 2001; accepted 30 August 2001; published 15 October 2002.
[1] The Indian Ocean Experiment (INDOEX) measurements on land, sea, and in the air
were designed to provide complementary assessment of chemical, physical, and optical
properties of the haze aerosol over the Indian Ocean. Differences in platform requirements
and objectives resulted in diverse techniques, measurements, and analyses being
employed. In order to best interpret the properties of the INDOEX aerosol, comparisons of
data by platform, air mass origin, and light scattering intensity were undertaken. These
revealed significant variability in platform averages of aerosol extensive properties (e.g.,
mass, light scattering, and absorption) but less variability in intensive properties (e.g.,
mass scattering efficiency, single scattering albedo, backscatter fraction, and A
˚
ngstro¨m
exponent) and the ratios of constituents. In general, ratios of chemical species were found
to show greater variability than properties of the size distributions or aerosol optical
properties. Even so, at higher haze concentrations with higher scattering values, various
determinations of the mass scattering efficiency (MSE) at 33% relative humidity
converged on values of about 3.8 ± 0.3 m
2
g
1
, providing a firm constraint upon the
description and modeling of haze optical properties. MSE values trended lower with more
dilute haze but became more variable in clean air or regions of low concentrations. This
cross-platform comparison resolved a number of measurement differences but also
revealed that regional characterization from different platforms results in differences
linked to variability in time and space. This emphasizes the need to combine such efforts
with coordinated satellite and modeling studies able to characterize large-scale regional
structure and variability. These comparisons also indicate that ‘‘closure’’ between
chemical, microphysical, and optical properties across platforms to better than about 20%
will require significant improvements in techniques, calibration procedures, and
comparison efforts.
INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and
particles (0345, 4801); 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305);
0394 Atmospheric Composition and Structure: Instruments and techniques; K
EYWORDS: INDOEX, data
comparison, optical properties, chemistry, microphysics, size distributions
Citation: Clarke, A. D., et al., INDOEX aerosol: A comparison and summary of chemical, microphysical, and optical properties
observed from land, ship, and aircraft, J. Geophys. Res., 107(D19), 8033, doi:10.1029/2001JD000572, 2002.
1. Introduction
[2] A major goal of the Indian Ocean Experiment
(INDOEX) was to identify the dominant aerosol constitu-
ents advected over the Indian Ocean and to establish links
between their properties and related radiative effects. Our
intent was to provide a database of both natural and
anthropogenic aerosol species and their contribution to
regional aerosol radiative properties and related climate
effects. One objective was to reduce uncertainties in aerosol
radiative forcing through ‘‘closure’’ experiments wherein
several alternate m easurements and approaches are
employed to establish a property. This ‘‘redundancy’’ pro-
vides a means of testing measurements in order to identify
the s ources and nature of the uncertainties involved.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. D19, 8033, doi:10.1029/2001JD000572, 2002
1
Department of Oceanography, University of Hawa ii, Honolulu,
Hawaii, USA.
2
Pacific Marine Environmental Laboratory, National Oceanic and
Atmospheric Administration, Seattle, Washington, USA.
3
Climate Monitoring and Diagnostics Laboratory, National Oceanic and
Atmospheric Administration, Boulder, Colorado, USA.
4
Institute for Tropospheric Research, Leipzig, Germany.
5
Department of Biogeochemistry, Max Planck Institute for Chemistry,
Mainz, Germany.
6
RSMAS/MAC, University of Miami, Miami, Florida, USA.
7
School of Earth and Atmospheric Sciences, Georgia Tech, Atlanta,
Georgia, USA.
Copyright 2002 by the American Geophysical Union.
0148-0227/02/2001JD000572
INX2 32 - 1
INDOEX employed several platforms and diverse instru-
mentation coordinated to meet these objectives over
extended spatial and temporal scales [Ramanathan et al.,
2001]. Because radiative effects depend upon relationships
between aerosol size, composition and optical properties it
was important that different measurements be interpreted
consistently on a given platform and between platforms. In
spite of broad agreement among many of the observations
and data sets from the INDOEX intensive field phase (11
February 1999–25 March 1999), some measurement differ-
ences and uncertainties were evident. The intent of this
paper is to identify uncertainties and to provide a consensus
on the INDOEX aerosol chemistry, optical properties and
aerosol size distributions such that other researchers and
modelers have a common reference for fundamental
INDOEX in-situ observations.
2. Approach
2.1. Strategy
[
3] A logical approach to ensure that instrument per-
formance and measurement techniques were comparable
during INDOEX was to provide side-by-side comparison
of various platforms during the experiment. These plat-
forms included the long-term site at Kashidhoo Climate
Observatory (KCO), the R/V Sagar Kanya (SK) [Jayara-
man et al., 2001], the R/V Ronald H. Brown (RB) and the
NCAR C-130 aircraft (C-130) [Rama nathan et al., 2001].
Actual opportunities for side-by-side comparisons include
one C-130-R B flyby and several KCO flybys with the C-
130 as well as some periods when the RB sampled near
KCO. These were less frequent than hoped due to logistical
difficulties and because both instrument operation and
environmental conditions were l ess consistent than
expected. Also, in order to compare aircraft, shi p and
ground-based data, only the low altitude legs from the C-
130 flights are used. Even so, for a 10-min surface leg,
comparison of C-130 data collected at speeds of 110 ms
1
to surface-based data requires data to be averaged over the
same air volume sampled by the C-130. For example, if
surface winds are about 6 m s
1
then ship or ground data
corresponding to the 10 min of C-130 data is about 3 h (10
min 110/6). Even though the C-130 flew along the wind
axis parallel to the surface platform for a 10 min sample leg
the passage of air past the surface platform for the next 3 h
seldom maintains the same speed or direction. Hence, in
the presence of aerosol gradients, the C-130 measurements
and surface measurements can be intrinsically different
even for a 10 min sample, the shortest practical C-130
sample leg. Furthermore, even under ideal circumstances
the C-130 near-surface legs flown at 35 m altitudes may
not reflect surface values when a near surface gradient
exists.
[
4] In vie w of the problematic nature of side-by-side
measurements between platforms, a strategy for more
extended comparison is employed here. A rapid and sensitive
measurement of aerosol changes common to all platforms is
the aerosol scattering coefficient, (s
sp
). Light scattering
measurements (550 nm) at a constrained RH near 55% wer e
used to establish Low (s
sp
<25 Mm
1
), Medium (25 Mm
1
<
s
sp
<55 Mm
1
) and High (s
sp
>55Mm
1
) aerosol regimes.
Data were partitioned into characteristic values/properties
observed for each of these L, M, and H ranges and stratified
into identifiable source regions when possible. This approach
allowed comparison of similar data for similar plume proper-
ties even when platforms were not colocated and increased
opportunities for cross platform comparisons of INDOEX
aerosol characteristics.
[
5 ] Only near-surface flight legs of the C- 130 are
included. For the size distributions, this was defined as legs
below 100m, while chemical composition was compared for
legs up to 600m to increase the number o f available
samples. In the optics section the level legs below 1000 m
were used to compare C-130 measurements with surface
platforms. A Student t test showed that differences in
average submicrometer optical properties were insignificant
at the 95% confidence level whether the altitude was 1000
m, 600 m or 35 m.
[
6] Although average concentrations and their variation
are presented here for each platform for these classifica-
tions, the intent is not to focus on these extensive aerosol
properties, since they are expected to differ markedly with
the sampling time each platform spent exposed to each
regime. Rather, the intent is to group such data i nto these
three optically stratified classifications to see if differences
exist in intensive properties such as the mass scattering
efficiency (MSE), single-scattering albedo (v
o
), Black
Carbon (BC) to Total Carbon (TC) ratio, etc. This analysis
depends on the assumptions that 1) each platform accumu-
lated a representative range of values for each regime and 2)
fundamental aerosol properties and characteristics were
similar on all platforms within each of the L, M or H
categories. Hence, differences in ratios evident between
platforms or measurement stratifications point to differences
in techniques or sampling or to invalid assumptions. Con-
sistent trends in ratios among L, M, or H regimes for
different platforms or measurements could also indicate a
change in intensive aerosol properties with increasing
pollution concentrations.
2.2. Relative Humidity
[
7] One issue important to all platforms and this assess-
ment is the role of relative humidity (RH) on water uptake
by the aerosol [Tang and Munkelwitz, 1977]. Water uptake
depends on whether or not some of the particle mass is
insoluble or partially soluble which, in turn, depends on
which chemical species are present. Condensed water is a
major aerosol constituent with a pronounced impact on
particle size, density, refractive index and scattering extinc-
tion [Kochenruther et al., 1999]. It is not only important to
the goal of linking aerosol radiative forcing to the various
aerosol species but also because most aerosol measurement
techniques (e.g., size, mass, optical properties) require
understanding and accounting for RH effects.
[
8] Hence, both interpretation of measurements at instru-
ment RH and the extrapolation of measured properties to
ambient RH conditions introduce uncertainties that must be
accounted for. For example, various impactors used to size
segregate aerosol prior to measurement (e.g., nephelometer,
filter, or gravimetric mass) will have a size cut that will
fluctuate with RH due to changes in particle size and
density. Efforts to control impactor RH on the ground were
often successful but were problematic or not practical for
the varied conditions aboard aircraft. Also, aerosol sizing
INX2 32 - 2 CLARKE ET AL.: INDOEX AEROSOL—A COMPARISON AND SUMMARY
instruments determine size classe s by various methods
including diffusion, aerodynamic properties, and optical
properties and these often involve assumptions about par-
ticle shape, density, and refractive index all of which can
change with RH and often in very different ways. Even
gravimetric mass will depend on the RH during weighing.
Indications of some of these competing effects on INDOEX
measurements are illustrated in Table 1. Here we indicate
the impact on a measurement as a result of an actual RH
value being higher than that assumed to be correct for a
measurement. INDOEX planning called for conditioning
RH to target values of 55% when possible (for impactors,
nephelometry, etc.) and filter mass measurements at 33%.
However, for the variable conditions aboard the C-130
(pressure, ambient RH etc.) this was not always possible
and such adjustments are often necessary.
[
9] Two fundamental aerosol measurements made during
INDOEX provide valuable constraints on how these RH
adjustments are implemented for optical and sizing meas-
urements. The first is the change in scattering coefficient
with humidity, typically called f(RH). This is the ratio of
scattering at a given RH to that at some reference humidity
(chosen here as 40%RH). The f(RH) for INDOEX was
measured at KCO by maintaining one nephelometer at 40%
RH while gradually scanning the humidity within a second
nephelometer. For the near surface data compared here we
assume the dominant dependency found at KCO applies
(Figure 1) although variability of about ±15% around this
line is present in the full data set (J. A. Ogren, personal
communication, 2001). The average relationship shown in
Figure 1 has been used to adjust light scattering measured at
one RH to light scattering at a different RH when necessary.
This approach of scanning a range of RH used at KCO was
too slow for f(RH) measurements on the C-130 and a
simpler method was used where one nephelometer operated
at aircraft temperature while the second was controlled to a
target humidity near 85%. This provided a two-point
characterization of the plot shown in Figure 1.
[
10] The ratio of wet to dry aerosol diameters or the so-
called growth factor, D/Do, places another empirical con-
straint on the effect of soluble/insoluble constituents on
particle growth behavior. The associated measurement of
D/D
ref
(where ‘‘ref’’ here is about 55%RH for this data) was
measured on the RB using a Tandem Differential Mobility
Analyzer (TDMA) technique (A. Massling et al., Hygro-
scopic properties and solubility of different aerosol types
over the Indian Ocean, submitted to Journal of Geophysical
Reasearch, 2001) and provides an empirical approach for
scaling sizes measured at one RH to sizes at some other RH
(Figure 2). Here we show the RB measured data for the
largest ‘‘dry’’ particle sizes characterized (250 nm geometric
diameter at less than 10%RH) by the TDMA. Since most
aerosol optical properties during INDOEX were dominated
by accumulation mode aerosol (see below), this is also the
most appropriate TDMA size range to use for corrections for
particle growth related to their optical properties.
[
11] A second plot of D/D
ref
included in Figure 2 is taken
from the ACE2 measurements of Swietliki et al. [2000].
Their data has been ‘‘normalized’’ to agreement with Mas-
sling data at 55%RH. This was done because Swietliki
growth was originally referenced to ‘‘dry’’ diameters at a
low RH of about 13% while Massling data was referenced
to about 5%RH for ‘‘dry’’ diameters. Normalizing data sets
to 55%RH avoids apparent differences caused by uncertain
Table 1. Expected Influence of an Uncertainty in RH on Various Measurements
a
Property Measurement Sensitive To Effect of RH
Increase
Impact on Measurement
Gravimetric mass Mass at 30% RH Weighing RH Increase mass Overestimate mass
Analytical mass
less then 1 mmDp
(impactor)
Ionic Mass Size cut Decrease cutsize Underestimate mass
OC Size cut ? ?
TC Size cut ?
FSSP size dist.
on wing (C-130)
Ambient forward
scatter
Ref. Index Decrease R.I. Size Decrease
PCASP size. dist. Integrated scatter Size Increase Dp Size Increase
On wing (C-130) Ref. Index Decrease Dp Size Decrease
DMA size dist. Aerosol mobility Measured RH Decrease Size Increase
APS size dist. Aerodynamic Drag
relative to reference
Density Decrease Size Decrease
Size Increase Size Increase
OPC size dist. Optical Scatter
relative to reference
Ref. Index Decrease Size Decrease
Size Increase Size Increase
Light scattering Fixed RH, 1 mm cut Size cut Decrease Scatter Decrease
Measured RH Increase Scatter Increase
Light absorption Filter transmission Size cut Decrease Absorption Decrease?
Measured RH ?
a
RH increase assumed.
Figure 1. Equation of curve fit to average f(RH) a
function of relative humidity measured by CMDL at KCO
for Dp <10 mm. Scatter in original data is about ±15%
around this line (not shown).
CLARKE ET AL.: INDOEX AEROSOL—A COMPARISON AND SUMMARY INX2 32 - 3
growth beh avior b elow 10% RH inclu ding th e greater
uncertainty associated with RH measurements at low RH.
Also shown in Figure 2 is the calculated normalized growth
for a limited period of size-resolved chemical data taken at
KCO that shows very similar inferred growth behavior for
RH values above about 35% RH. The growth equation
obtained by fitting these data provides a consistent way to
adjust sizes at one RH (above 35% RH) to sizes expected at
another RH.
[
12] Specification of RH is also essential to interpreting
aerosol intensive parameters discussed toward the end of this
paper, such as the ratio of aerosol scattering to aerosol mass
(the mass scattering efficiency or MSE). We will show this
important intensive parameter at RH = 33%. This was the
lowest RH at which measurements were routi nely made
(gravimetric mass on the RB) and is selected here because
gravimetric mass change in response to RH was not meas-
ured. In order to facilitate linking these data (e.g., MSE) to
measurements at other conditions the functional depend-
encies used to describe f(RH) and D/D
ref
have also been
included.
[
13] We note that while dry MSE might be more useful for
incorporating these data into chemical transport models or
other applications that use dry mass as a variable, it would
require extrapolation of both mass and scattering to RH
conditions well below any measurements we made. Figure 2,
which shows discrepancies between measured D/D
ref
and
chemically estimated values at low RH, suggests that such
extrapolations are risky. However, if we had pursued such
extrapolation, the lower RH Massling TDMA data indicate
that D(33)/D(0) is 1.14, which translates into 1.48 times the
dry volume or 1.22 times the mass for spherical particles
with dry density of 2.2 g cm
3
. Similarly, the f(RH) curve fit
(Figure 1) suggests that scattering at 33%RH exceeds dry
scattering by 16%, but no data are available to confirm that
extrapolation. Both cases indicate significant water remain-
ing at 33% compared to the dry state.
2.3. Comparison of Indoex Data Products
[
14] Measurements can reflect differences in sample plat-
forms, instrument, instrument operation/configuration, sam-
pling inle ts, environmen tal conditions, sample pe riods,
locations etc. In preparation for INDOEX, effor t s were
made to make various measurements as comparable as
possible within the constraints common to most field
studies. Summaries of the sampling approach for each of
the platforms can be found in Appendix A. Additional
detailed discussions of the chemical, physical, and optical
properties measured and specific issues related to sampling
and instrumentation are also presented in Appendix B. We
encourage the reader who is concerned with these measure-
ment issues to read these appendices as a reference for the
discussions that follow.
2.4. Chemical Properties
[
15] The aerosol chemical species considered here and
measured on the RB, the C-130, and at KCO are those
important to aerosol radiative forcing. These include non
sea-salt (nss) SO
4
2
, black carbon (BC), organic carbon (OC),
total carbon (TC) which is the sum of BC and OC, and aerosol
mass. Sea-salt is not considered because it was a minor
component in the submicrometer s ize range in the NH
samples and because submicromete r aerosol dominated
INDOEX optical properties (see size discussion below).
Absolute concentrations of these species are compared as
are ratios of the mass concentration of various species,
scattering to mass, and absorption to BC. Only submicrom-
eter aerosol (those with an aerodynamic diameter less than or
equal to one micrometer) are included in these comparisons.
[
16] No side-by-side comparisons between platforms or
between KCO-UMiami and KCO-Caltech were possible due
to a lack of data on at least one platform during those time
periods. Instead, comparisons of the chemical species (con-
centrations and ratios) were made based on similarities in
sampling conditions for L, M, and H scattering categories
(Table 2), trajectories to the platf orms (from the Bay of
Bengal (BoB) or the Arabian Sea (AS)) (Table 3), and, for
the RB and C-130, geographical regions (Northern Hemi-
sphere, ITCZ, and Southern Hemisphere) (Table 4). Compar-
isons for the low scattering category do not include cases
where the RB or C-130 experienced trajectories from the
northern or southern Indian Oceans in order to make those
data more comparable to the KCO data. Figures 3a – 3c
summarizes the typical values for major components and
properties on each platform and under each L, M or H
condition.
2.4.1. Nss SO
4
2
[17] Mean nonsea-salt sulfate (nss SO
4
2
) concentrations
from KCO-Caltech and the RB agreed within 26% for the
low and medium scattering regimes (Table 2; Figure 3a).
KCO-UMiami and C-130 mean values were considerably
lower and did not agree with each other or with the other
platforms within 1 standard deviation. For the high scatter-
ing category (Table 2; Figure 3a), the mean RB concen-
tration was higher than the KCO-UMiami and C-130 values
by 40% and higher than the KCO-Caltech values by 20%.
However, due to the large variability in concentrations in
this category, all differences between platforms were within
1 standard deviation of the means.
[
18] Separating the data accord ing to trajectories reveals
that the large variability is due in part to air mass flow patterns
to the sampling platforms. For trajectories from the AS, the
RB mean nss SO
4
2
concentration is lower than the KCO-
Figure 2. D/D
ref
a function of RH measured by DMPS on
the RB for both INDOEX (solid line) and ACE-2 (dashed
line) e xperiments and e stimated values d erived from
Caltech KCO chemistry data (see text).
INX2 32 - 4 CLARKE ET AL.: INDOEX AEROSOL—A COMPARISON AND SUMMARY
Table 2. Mean Concentrations and Mass Ratios Obtained for KCO, RB, and C-130 Measurements for Submicrometer Chemical Species
for Harmony Categories ‘‘Low,’’ ‘‘Medium, and ‘‘High’’ Scattering
a
Species KCO-UMiami KCO-Caltech
b
C-130 RB
‘‘Low’’ Scattering (s
sp
25 Mm
1
)
Concentrations mgm
3
mgm
3
mgm
3
Nss SO
4
2
N/A 2.3 ± 0.04 (1) 0.51 ± 0.30 (3) 1.7 ± 0.40 (8)
BC N/A 0.55 ± 0.03 (1) cnd - 0.5 (3) 0.17 ± 0.21 (2)
OC N/A 0.61 ± 0.10 (1) cnd (3) 0.27 ± 0.20 (2)
TC 1.2 ± 0.11 (1) cnd (3) 0.44 ± 0.01 (2)
Total mass N/A 7.9 ± 0.50 (1) cnd (3) 3.9 ± 0.66 (4)
Mass ratios
BC/TC N/A 0.47 ± 0.05 (1) cnd (3) 0.39 ± 0.47 (2)
BC/OC N/A 0.90 ± 0.14 (1) cnd (3) 1.4 ± 1.8 (2)
BC/nss SO
4
2
N/A 0.24 ± 0.01 (1) cnd (3) 0.07 ± 0.08 (2)
OC/nss SO
4
2
N/A 0.26 ± 0.04 (1) cnd (3) 0.17 ± 0.17 (2)
Nss K
+
/BC N/A N/A cnd (3) 0.56 ± 0.24 (2)
Nss SO
4
2
/mass N/A 0.29 ± 0.02 (1) cnd (3) 0.39 ± 0.08 (3)
Scattering to mass ratios m
2
g
1
m
2
g
1
m
2
g
1
m
2
g
1
s
sp
(33%)/mass (33%)
c
N/A 3.7 (1) 4.1 ± 1.2 (7)
s
sp
(33%)/mass (chem anal)
d
N/A cnd 5.2 ± 2.5 (7)
s
ap
/BC N/A 20 (1) 12 (1) 12 (1)
‘‘Medium’’ Scattering (25 < s
sp
55 Mm
1
)
Concentrations mgm
3
mgm
3
mgm
3
nss SO
4
2
3.2 ± 0.58 (4) 4.5 ± 0.04 (2) 2.2 ± 0.68 (4) 4.5 ± 0.78 (7)
BC 1.4 ± 0.04 (2) 0.8 ± 0.7 (4) 0.43 ± 0.18 (6)
OC 1.0 ± 0.11 (2) 1.3 ± 1.2 (4) 0.42 ± 0.06 (6)
TC 2.4 ± 0.12 (2) 2.1 ± 1.6 (4) 0.85 ± 0.22 (6)
Total mass 11 ± 1.7 (4) 14 ± 0.68 (2) 7.1 ± 1.3 (3) 8.9 ± 2.6 (8)
Mass ratios
BC/TC 0.58 ± 0.03 (2) 0.40 ± 0.20 (4) 0.49 ± 0.10 (6)
BC/OC 1.4 ± 0.16 (2) 0.90 ± 0.60 (4) 1.0 ± 0.39 (6)
BC/nss SO
4
2
0.31 ± 0.01 (2) 0.36 ± 0.28 (3) 0.10 ± 0.03 (6)
OC/nss SO
4
2
0.22 ± 0.02 (2) 0.90 ± 0.80 (3) 0.10 ± 0.02 (6)
nss K
+
/BC 0.30 ± 0.30 (3) 0.74 ± 0.20 (6)
nss SO
4
2
/mass 0.30 ± 0.05 (4) 0.32 ± 0.015 (2) 0.35 ± 0.13 (3) 0.45 ± 0.04 (6)
Scattering to mass ratios m
2
g
1
m
2
g
1
m
2
g
1
m
2
g
1
s
sp
(33%)/mass (33%)
c
3.4 ± 0.09 (2) 3.9 ± 0.51 (6)
s
sp
(33%)/mass (chem anal)
d
2.2 ± 0.57 4.7 ± 1.0 4.7 ± 0.36 (6)
s
ap
/BC 13 (1) 13 ± 8.9 (4) 17 ± 4.6 (6)
‘‘High’’ Scattering (55 Mm
1
< s
sp
)
Concentrations mgm
3
mgm
3
mgm
3
nss SO
4
2
6.4 ± 1.1 (12) 7.4 ± 0.09 (3) 6.2 ± 2.7 (12) 8.9 ± 3.9 (14)
BC 2.5 ± 0.07 (3) 2.5 ± 1.8 (12) 1.4 ± 0.48 (8)
OC 2.1 ± 0.12 (3) 3.1 ± 2.2 (11) 0.95 ± 0.44 (8)
TC 4.6 ± 0.14 (3) 5.5 ± 3.3 (11) 2.3 ± 0.66 (8)
Total mass 19 ± 4.3 (11) 21 ± 0.49 (3) 14 ± 6.2 (9) 17 ± 3.5 (3)
Mass ratios
BC/TC 0.55 ± 0.02 (3) 0.50 ± 0.10 (11) 0.59 ± 0.12 (8)
BC/OC 1.2 ± 0.07 (3) 1.1 ± 0.60 (11) 1.6 ± 0.64 (8)
BC/nss SO
4
2
0.34 ± 0.01 (3) 0.46 ± 0.18 (13) 0.16 ± 0.04 (8)
OC/nss SO
4
2
0.28 ± 0.02 (3) 0.50 ± 0.30 (9) 0.13 ± 0.11 (8)
nss K
+
/BC 0.11 ± 0.03 (9) 0.37 ± 0.11 (8)
nss SO
4
2
/mass 0.30 ± 0.02 (12) 0.35 ± 0.03 (3) 0.41 ± 0.09 (10) 0.52 ± 0.11 (7)
Scattering to mass ratios m
2
g
1
m
2
g
1
m
2
g
1
m
2
g
1
s
sp
(33%)/mass (33%)
c
3.5 ± 0.16 (3) 4.1 ± 0.23 (3)
s
sp
(33%)/mass (chem anal)
d
2.9 ± 0.8 (11) 5.8 ± 2.0 (9) 4.4 ± 0.08 (3)
s
ap
/BC 14 (1) 9.2 ± 2.8 (12) 10 ± 2 (8)
a
N/A, no sample available; cnd, could not determine. Also shown are ±1 standard deviations. Number of samples collected are shown in parentheses.
b
Mean and confidence interval.
c
Based on scattering coefficients adjusted to 33% RH and mass measured gravimetrically at 33% RH (RB) or 39% RH (KCO-Caltech).
d
Based on scattering coefficients adjusted to 33% RH and the sum of the chemically analyzed mass.
CLARKE ET AL.: INDOEX AEROSOL—A COMPARISON AND SUMMARY INX2 32 - 5
