**Shipboard sunphotometer measurements of aerosol optical depth
spectra**

**and columnar water vapor during ACE-2 and comparison with**

**selected land, ship, aircraft, and satellite measurements**

John M. Livingston^1^, Vladimir N. Kapustin^2^, Beat Schmid^3^, Philip
B. Russell^4^,

Patricia K. Quinn^5^, Timothy S. Bates^5^, Philip A. Durkee^6^, Peter J.
Smith^6^, Volker Freudenthaler^7^,

Matthias Wiegner^7^, Dave S. Covert^8^, Santiago Gassó^8^, Dean Hegg^8^,
Donald R. Collins^9^,

Richard C. Flagan^9^, John H. Seinfeld^9^, Vito Vitale^10^, Claudio
Tomasi^10^

^1^SRI International, Menlo Park, CA

^2^ Department of Oceanography, University of Hawaii, Honolulu, HI

^3^Bay Area Environmental Research Institute, San Francisco, CA

^4^NASA Ames Research Center, Moffett Field, CA

^5^NOAA-Pacific Marine Environmental Laboratory, Seattle, WA

^6^Naval Postgraduate School, Monterey, CA

^7^University of Munich, Munich, Germany

^8^University of Washington, Seattle, WA

^9^California Institute of Technology, Pasadena, CA

^10^Istituto FISBAT-C.N.R., Bologna, Italy

# ABSTRACT

Analyses of aerosol optical depth (AOD) and columnar water vapor (CWV)
measurements acquired with NASA Ames Research Center's six-channel
Airborne Tracking Sunphotometer (AATS-6) operated aboard the R/V
Professor Vodyanitskiy during the second Aerosol Characterization
Experiment (ACE-2) are discussed. Data are compared with various in-situ
and remote measurements for selected cases. The focus is on 10 July,
when the Pelican airplane flew within 70 km of the ship near the time of
a NOAA-14/AVHRR satellite overpass and AOD measurements with the
14-channel Ames Airborne Tracking Sunphotometer (AATS-14) above the
marine boundary layer (MBL) permitted calculation of AOD within the MBL
from the AATS-6 measurements. A detailed column closure test is
performed for MBL AOD on 10 July by comparing the AATS-6 MBL AODs with
corresponding values calculated by combining shipboard particle size
distribution measurements with models of hygroscopic growth and
radiosonde humidity profiles (plus assumptions on the vertical profile
of the dry particle size distribution and composition). This closure
investigation is not successful, however, in large part because of its
great sensitivity to hygroscopic growth models, which vary considerably
and have not been validated over the necessary range of particle
size/composition distributions. The wavelength dependence of AATS-6 AODs
is compared with the corresponding dependence of aerosol extinction
calculated from shipboard measurements of aerosol size distribution and
of total scattering measured by a shipboard integrating nephelometer for
several days. Results are highly variable, illustrating further the
great difficulty of deriving column values from point measurements.
AATS-6 CWV values are shown to agree well with corresponding values
derived from radiosonde measurements during eight soundings on seven
days and also with values calculated from measurements taken on 10 July
with the AATS-14 and the University of Washington Passive Humidigraph
aboard the Pelican.

1\. INTRODUCTION

The second Aerosol Characterization Experiment (ACE-2) of the
International Global Atmospheric Chemistry Project (IGAC) was conducted
from 16 June through 25 July 1997. The goal of ACE-2 was to characterize
and understand the properties and controlling factors of the aerosol in
the anthropogenically modified atmosphere of the North Atlantic and
assess their relevance for radiative forcing (Raes et al., this issue).

During ACE-2, the six-channel NASA Ames Airborne Tracking Sunphotometer
(AATS-6) was operated aboard the Institute of Biology of Southern Seas
(Sebastopol, Ukrania) Research Vessel (R/V) Professor Vodyanitskiy in
the North Atlantic Ocean off the coast of Portugal and North Africa
during the period 22 June through 23 July 1997. Measurements were taken
in support of the CLEARCOLUMN (Heintzenberg and Russell, this issue) and
LAGRANGIAN (Johnson et al., this issue) components of ACE-2.

Since 1985, the AATS-6 has been operated successfully aboard a variety
of aircraft platforms and from the ground in several stratospheric and
tropospheric research studies (e.g.: Livingston and Russell, 1989;
Pueschel and Livingston, 1990; Russell et al., 1993; Russell et al.,
1999a), but the instrument had never been operated from aboard a ship
before ACE-2. Shipboard operation of the instrument during transit over
the open ocean presented a new set of challenges to the automatic
tracking and data acquisition capabilities of the AATS-6 due to platform
pitch and roll, radio frequency interference from the ship's radio early
in the cruise, intermittent obscuration of the solar disk by shipboard
structures and overhanging antenna and support wires, sea salt spray
coating of the optical windows that required frequent cleaning, and
general operation of the instrument electronics in the corrosive sea
salt environment. These problems were mitigated, to a large extent, by
conformally coating all electronics within the instrument's telescope
head prior to the cruise, by frequent operator monitoring of instrument
operation and intervention when necessary during data acquisition, and
by careful screening and analysis of the measurements. The result is an
archived data set that includes aerosol optical depth (AOD) spectra and
associated uncertainties for portions of thirteen days of the cruise and
measurements of columnar water vapor (CWV) on ten of these days.

During ACE-2, AATS-6 measurements were taken during a variety of lower
tropospheric aerosol loading conditions that included relatively clean
air masses that originated over the north Atlantic Ocean and moderately
polluted air masses transported from the European continent. In this
paper, we show selected AATS-6 AOD and CWV data only and compare these
results with corresponding in-situ and remote measurements for a small
subset of the available measurements. Our primary objective is to
investigate MBL AOD closure for the single measurement event that
occurred on 10 July 1997, when a variety of simultaneous and
near-simultaneous in-situ and remote sensing measurements was acquired
from ship, aircraft, and satellite. For this case, we compare AODs
derived from satellite radiometer (AVHRR) measurements of upscattered
radiance with temporally and spatially coincident AATS-6 column AODs,
and with AODs measured just prior to satellite overpass within 70 km of
the ship by the fourteen-channel Ames Airborne Tracking Sunphotometer
(AATS-14) that operated aboard the Pelican airplane. We calculate MBL
AODs at the ship location by subtracting the free tropospheric AODs
measured by AATS-14 from the column AODs measured by AATS-6, and these
values are compared with MBL AODs estimated from shipboard in-situ
aerosol size distribution measurements (Quinn et al., this issue; Bates
et al., this issue) and from a shipboard aerosol backscatter lidar. We
show that demonstration of MBL AOD closure using the shipboard sun
photometer and in-situ aerosol size measurements is a complex task that
requires a number of critical assumptions.

2\. INSTRUMENTATION

## 2.1. 6-Channel NASA Ames Airborne Tracking Sunphotometer (AATS-6)

The AATS-6 \[Matsumoto et al., 1987; Russell et al., 1993\] tracks the
sun and measures the direct beam solar transmission through the earth's
atmosphere in six spectral channels. Each channel consists of a doubly
baffled entrance tube, interference filter, photodiode detector, and
integral preamplifier. The entrance baffles define a detector field of
view (FOV) with measured half angle of 2.2°. The six
filter/detector/preamplifier sets are mounted in a common heat sink
maintained at 45° ± 1°C. During ACE-2, the filters were centered at
wavelengths 380.1, 450.7, 525.3, 863.9, 941.4, and 1020.7 nm, with
filter bandwidths (FWHM) of 5.0 - 5.8 nm. Solar tracking is achieved by
azimuth and elevation motors driven by error signals derived from a
differential-shadowing sun sensor. Photodiode voltages were digitized at
3 Hz, averaged over 3 s, and recorded every 12 s together with signal
standard deviations and various ancillary data.

Calibration of the AATS-6 was performed by taking sunrise measurements
at Mauna Loa Observatory on a series of days during late April/early May
prior to ACE-2 and again in August following ACE-2. In all channels
except 941.4 nm, the Langley plot technique (e. g., Russell et al.,
1993) was used to derive the exoatmospheric detector voltages that are
necessary for subsequent calculation of instantaneous optical depths. At
941.4 nm, because water vapor and aerosol are the primary attenuators
and attenuation by water vapor does not follow Beer's Law, a modified
Langley technique (Michalsky et al., 1995; Schmid et al., 1996) was
used. AODs and associated uncertainties were calculated according to the
methodology described in Russell et al. (1993), and CWV estimates and
uncertainties were derived using the methodology described in Schmid et
al. (1996). Columnar ozone amounts necessary for the calculation of AOD
from total optical depth were taken from the Total Ozone Monitor Sensor
(TOMS) on the Earth Probe satellite.

AATS-6 measurements acquired during ACE-2 have been filtered to
eliminate data affected by clouds by rejecting, for each channel, those
points for which the standard deviation of the recorded mean signal
voltage exceeded a specified fraction (typically 0.5%) of the mean
signal over each 3-s sampling period. Although this screening works
quite well in removing large amplitude data excursions that result from
solar obscuration (as might be caused by clouds) that lasts for a
significant fraction of the 3-s sampling period, it cannot and does not
remove shorter-lived or partial obscuration events. One of the major
problems associated with shipboard operation of the AATS-6 during ACE-2
was partial or intermittent obscuration of the sun by shipboard
structures such as masts and antenna wires. Inclusion of these
"contaminated" data in the analysis always results in an overestimate of
the calculated optical depth (from 10-100%), can cause an artificial
apparent periodic variation in the optical depth temporal traces, and,
in general, results in optical depths that are significantly noisier
than those calculated during periods when no such interference occurred.
The data processing filter described above was unable to remove points
like these from the data set, and no attempt has been made to do so,
because of the subjectivity required in any such attempt. In particular,
one approach to removing these data would be to assume that only the
relative minima in calculated optical depth are "uncontaminated", but
defining "relative minima" is itself subjective and prone to large
uncertainty. The frequency of occurrence of data affected by ship
structure obscuration is estimated as less than 10% of the total
archived AATS-6 data set. The inclusion of such data in certain figures
of this paper is so noted in the text.

## 2.2. 14-Channel NASA Ames Airborne Tracking Sunphotometer (AATS-14)

During ACE-2, the AATS-14 was operated on a single engine modified
Cessna Skymaster aircraft, the Pelican, which is operated by the Marina,
California Center for Interdisciplinary Remotely Piloted Aircraft
Studies (CIRPAS). The Pelican flew in and out of the North Airport
(28.48˚N, 16.34˚W, 632 m MSL) on Tenerife, Canary Islands, and included
a variety of instruments that measured aerosol chemical, physical, and
optical properties, plus radiative fluxes and meteorological parameters.
These are summarized by Heintzenberg and Russell (this issue). The
AATS-14 was designed and developed by NASA Ames, and its first science
flights were on the Pelican during the summer 1996 Tropospheric Aerosol
and Radiative Forcing Observational Experiment (TARFOX) (Livingston and
Russell, 1997; Russell et al., 1999a, 1999b). It measures the
transmission of the direct solar beam in fourteen spectral channels
between 380 and 1558 nm. During ACE-2, the Pelican flew in the vicinity
of the R/V Vodyanitskiy only during a single flight on 10 July to the
northeast of the Canary Islands. Only measurements taken with the
AATS-14 on this day are discussed in this study. Schmid et al. (this
issue) present in-depth column closure analyses of AATS-14 measurements
taken on 8 and 17 July, while Collins et al. (this issue) use AATS-14
AODs measured on 21 June, and 8, 10 and 17 July in their study that
focuses on the Pelican in-situ aerosol measurements. Calibration of the
AATS-14 was performed from the ground at Mauna Loa prior to ACE-2 and
from aboard the Pelican during several flights near sunset during ACE-2.

## 2.3. AVHRR

The Advanced Very High Resolution Radiometer (AVHRR) is a spaceborne
instrument that measures solar radiance backscattered by the
earth-atmosphere system in five wavelength passbands. In our study, we
compare AATS-6 AOD spectra with corresponding AOD spectra derived
(Durkee et al., this issue) from AVHRR radiances in Channels 1 and 2
(centered at 630 and 860 nm, respectively) during NOAA-14 satellite
overpasses on 10 July, 24 and 25 June. Durkee et al. (this issue)
present an in-depth validation of AODs derived from the AVHRR radiances
during ACE-2 by comparing the AVHRR-derived values with corresponding
AODs measured by various sunphotometers during satellite overpasses.

## 2.4. Shipboard In-Situ Particle Sampling Instruments

The NOAA Pacific Marine Environmental Laboratory (PMEL) operated a suite
of in-situ instruments aboard the R/V Vodyanitskiy to measure aerosol
size distribution and total and backscattered aerosol light scattering
coefficient (at 450, 550 and 700 nm). These instruments and the
associated data processing methodology are described in detail by Quinn
et al. (this issue) and Bates et al. (this issue). Aerosol particles
were sampled at 10 m above sea level through a heated mast. The
instruments included an ultrafine differential mobility particle sizer
(UDMPS), a differential mobility particle sizer (DMPS), TSI 3010 and TSI
3025 particle counters, a TSI 3310A aerodynamic particle sizer (APS),
and a TSI 3563 integrating nephelometer. The two DMPSs and the APS were
located within a humidity controlled box at the base of the mast. The
sample air flow was maintained at approximately 55% RH to match the
sampling RH of the impactors and the integrating nephelometer (Quinn and
Coffman, 1998). Particle number-size distributions were collected every
10 minutes. In this paper, we use the nephelometer total scattering
measurements, and the particle number distribution data averaged over 30
minute periods centered on the hour and half hour and archived at dry RH
of 10% in terms of geometric particle diameter. We adjust the size
distributions at 10% RH to ambient RH using either the Swietlicki et al.
(this issue) ACE-2 ship hygroscopic growth data or the model from Hoppel
et al. (1990).

## 2.5. Shipboard Aerosol Backscatter Lidar

The University of Munich Meteorological Institute (LMU) operated its
three-wavelength aerosol backscatter lidar (Wiegner et al., 1995) aboard
the R/V Vodyanitskiy. The lidar is equipped with a Nd:YAG laser that
simultaneously emits radiation at 1064 nm, 532 nm, and 355 nm with pulse
energies of 220 mJ, 100 mJ, and 155 mJ, respectively. The receiving
telescope with 30 cm diameter has a variable field of view between 0.5
and 8 mrad, and the line of sight of the lidar can be changed in azimuth
and zenith direction to observe atmospheric cross sections or even
volumes. During ACE-2, the lidar was operated in a zenith-pointing mode
with a field of view between 2 and 8 mrad. Vertical profiles of aerosol
backscatter were measured as means over 20 s (200 laser shots) about
every 30 min at a sampling rate of 30 x 10^6^ samples/s, which results
in a vertical resolution of 5 m at altitudes at least 150-200 m above
the lidar. Data below these altitudes are not used due to the incomplete
overlap of receiver and transmitter FOV in this region. Measurements
acquired with the lidar are used in this study to determine the height
of the MBL and to estimate the vertical distribution of aerosols within
and above the MBL. For 10 July and 22 July, aerosol extinction
coefficients have been derived from the lidar measurements by assuming
height-dependent extinction-to-backscatter ratios for different aerosol
models according to a parameterization \[Ackermann, 1998\] with respect
to the height-dependent humidity profiles from radiosoundings onboard
the vessel.

3\. RESULTS

## 3.1. OVERVIEW

As noted by Quinn et al. (this issue) and Bates et al. (this issue),
during ACE-2 the R/V Vodyanitskiy encountered air masses from five
source regions: the central north Atlantic, the Arctic north Atlantic,
the Iberian peninsula, the Mediterranean, and western Europe. Figure 1
shows representative AOD spectra that illustrate the range of AOD values
measured by the AATS-6 during flow from the central north Atlantic (22
June), the Mediterranean (6 July), the Iberian peninsula (18 July), and
Western Europe (10, 22 July). Mid-visible AODs ranged from a minimum of
0.03 on 22 June to a maximum of 0.35 on 18 July.

AATS-6 AOD and CWV data are available for comparison with measurements
by other sensors on several days. Table 1 lists those days for which the
highest quality (i.e., fewest number of obscuration events by clouds
and/or ship structures) AATS-6 data are available and includes a partial
list of available comparative data. In Sections 3.2 and 3.3, we present
detailed case studies of selected AATS-6 AOD measurements acquired on 10
and 22 July, respectively. We also compare AATS-6 CWV values with
corresponding values measured by the AATS-14 and by the UWPH on 10 July
and derived from radiosonde measurements on seven days. In Section 4.0
(Summary and Conclusions), we compare the spectral behavior of AATS-6
AODs for those days listed in Table 1 with analogous results calculated
from shipboard in-situ aerosol size distribution and scattering
measurements.

We restrict discussion of other AATS-6 measurements to the following
brief summary of results from comparative analyses of data acquired on
three other days. In particular, AVHRR AODs derived from measurements
acquired on 24 and 25 June were found to agree with corresponding AATS-6
AODs to within 0.01-0.02. These data are included in Durkee et al. (this
issue). AATS-6 AOD measurements taken near Sagres on 18 July have been
compared with simultaneous measurements taken with the Istituto FISBAT
IR-RAD multiwavelength sun-radiometer (Vitale et al., this issue) at the
Sagres 50-m site (8˚57'W, 36˚59'N). Rms differences for data acquired
during the period 1000-1700 UTC ranged from 0.015 to 0.021 in absolute
AOD. A similar comparison of CWV values derived from the AATS-6 and
Istituto FISBAT ASP-1 sun photometer measurements (Tomasi et al., this
issue) for the same time period on 18 July yielded rms differences of
0.41 g/cm^2^ for AATS-6 CWV values of 3.2-3.3 g/cm^2^.

The analyses presented in Section 3 focus on data collected on 10 July,
when nearby AATS-14 measurements aboard the Pelican provided a measure
of AOD above the MBL and permitted calculation of MBL AOD from the
shipboard AATS-6 measurements of total column AOD at the time of an
AVHRR overpass. For this day, aerosol extinction profiles are calculated
for a range of refractive indices from the shipboard particle size
distribution measurements by using radiosonde RH profiles as input to a
hygroscopic growth model. These profiles are then compared with
analogous profiles inferred from the LMU lidar backscatter measurements,
and they are integrated within the MBL to yield estimates of AOD that
are then compared with AATS-6 MBL AODs. Comparisons of AATS-6 and
AATS-14 AODs with corresponding values derived from the AVHRR radiance
measurements are also shown. For 22 July, AATS-6 AODs are compared with
corresponding AODs calculated from simultaneous lidar backscatter
measurements.

## 3.2. 10 JULY 1997

## 3.2.1. Air Mass Origin

Five-day backward trajectories calculated by the Royal Dutch
Meteorological Institute (KNMI; Scheele et al., 1996) for the ship
location (\~29.2N, 11.7W) at 1200 UTC on 10 July indicate that the MBL
was dominated by flow from western Europe. In particular, air parcels
arriving at 975 hPa at the ship location originated over Great Britain
at 814 hPa, followed a southward trajectory over France, west over the
northern coast of Spain, and finally south over the ocean just west of
the Iberian peninsula. Air parcels arriving at the ship location at 850
hPa and above originated five days earlier over the north central
Atlantic and followed a strictly marine trajectory. All trajectories
showed subsidence.

## 3.2.2. Comparison of AATS-6, AATS-14 and AVHRR Column AODs

AATS-6 measurements of AOD can be compared with corresponding AODs that
have been derived (Durkee et al., this issue) from AVHRR radiance
measurements acquired during a NOAA-14 satellite overpass at 1515 UTC.
Measurements of AOD above the Pelican were acquired with the AATS-14 at
altitudes 55-70 m MSL in a region approximately 70 km ESE of the ship
just prior to the AVHRR overflight. Figure 2 presents a color coded map
of AVHRR AOD calculated from measurements in the 630-nm AVHRR channel,
and shows the locations of the ship at 1515 UTC and the airplane during
its low altitude traverse between 1457 and 1500 UTC. Note that AVHRR
AODs are similar at the ship and Pelican locations**.** (In fact, as
shown in Figures 3b, 3c below, values at the two locations in AVHRR
Channel 2 were equal, and those in AVHRR Channel 1 were within 0.01.)
Figure 3a shows the temporal variation of AOD measured by AATS-6 during
the period 1451-1524 UTC. Relative maxima in AOD were measured from 1454
to 1457 UTC, or about twenty minutes prior to satellite overpass, with
mid-visible values near 0.20. Relative minima were measured from 1509 to
1513 UTC, with mid-visible values about 0.15. Values at 1515 UTC were
about 0.01 greater. The magnitude of the difference between mean AOD
values measured in the relative maxima and minima was about 0.05 at all
wavelengths, and this near wavelength-independent variation in AOD
strongly suggests that attenuation by large particles ­­-- perhaps thin
clouds -- may have been the source of the larger AOD values. However,
there is no evidence of cirrus or lower level clouds in the simultaneous
lidar backscatter profiles at 1064 nm, so the attenuating aerosol
particles must have been within the layer between the sea surface and
\~150 m MSL, where no useful LMU lidar backscatter data have been
retrieved.

Figure 3b compares a two-minute mean AOD spectrum measured by AATS-6
during the time of the AVHRR overflight with AODs calculated in the two
AVHRR channels for the ship location. Vertical bars on the AATS-6 AODs
represent mean measurement uncertainties (including calibration, gas
optical depth, etc. per Eq. (A22a) of Russell et al. \[1993\]); they are
slightly greater than the sample standard deviations during the
averaging period. Vertical bars on AVHRR AODs represent the uncertainty
based solely on the radiometric resolution of the satellite and do not
include any uncertainty associated with the retrieval algorithm (Durkee
et al., this issue). AVHRR AODs exceeded the AATS-6 values by about
0.04, which is more than the estimated uncertainties. The source of this
difference has not been identified. Figure 3c compares AOD measurements
acquired with the AATS-14 during the Pelican's low altitude traverse
fifteen minutes before the AVHRR overpass with AVHRR AODs calculated for
the Pelican location. AVHRR values also exceeded mean AATS-14 AODs by
about 0.04, but AATS-14 values do not include the AOD in the layer
between the sea surface and the altitude of the Pelican. Figure 4 shows
vertical profiles of AOD measured by the AATS-14 during aircraft ascent
from 70 m to 3.8 km during the period 1500-1605 UTC, and the
corresponding retrieved aerosol extinction profiles (see Schmid et al.,
this issue, for details of the retrieval algorithm). It is estimated by
summing extrapolated values of the retrieved aerosol extinction shown in
Figure 4 down to the sea surface that about 0.010-0.015 (depending on
wavelength) of the 0.04 difference between AVHRR and AATS-14 AOD can be
explained by the AOD in the layer between the sea surface and the
Pelican. The remainder of the difference is attributed to temporal
variation in AOD between the time of the AATS-14 low altitude
measurements and the AVHRR overpass time.

## 3.2.3. Columnar Water Vapor Measurements

Figure 5 compares values of CWV and water vapor density derived from
AATS-14 measurements taken in the 940-nm channel (see Schmid et al.,
this issue, for details of the retrieval algorithm) during aircraft
ascent with corresponding values derived from coincident measurements
taken aboard the Pelican with the University of Washington Passive
Humidigraph (UWPH) (Gassó et al., 1999) and with values calculated from
the 1657 UTC ship radiosonde measurements. The mean CWV value derived
from shipboard measurements taken in the AATS-6 941.4-nm channel during
the 1500-1730 UTC time period is also shown in Figure 5. Horizontal bars
on the AATS-14 and AATS-6 CWV values represent the uncertainty in the
measurements and are due primarily to the uncertainties in the
calibration voltages in the respective water vapor channels. Derivation
of these uncertainties is described in detail by Schmid et al. (1996).
The AATS-6 CWV mean value slightly underestimates corresponding values
derived from the AATS-14, the UWPH, or the radiosonde, but values agree
to within measurement uncertainties. Figure 6 compares CWV values
calculated from AATS-6 measurements with corresponding values derived
from eight radiosonde measurements for seven days during ACE-2. Values
agree well, with an rms difference of 0.10 g cm^-2^ for a range of 1.2
to 3.2 g cm ^-2^.

## 3.2.4. MBL Aerosol Extinction and Optical Depth

At the ship location, AODs have been calculated within the MBL by
subtracting from the AATS-6 total column AOD values the
free-tropospheric AODs measured by the AATS-14 in the altitude range
1.09-1.17 km (cf. Figure 4), which was just above the top of the MBL at
the Pelican location. As will be shown below, the top of the MBL at the
Pelican location was about 0.3 km higher than that measured at the ship
location by lidar at 1513 UTC. The free-tropospheric AODs ranged from
0.027 at 380.1 nm to 0.008 at 1020.7 nm. The resulting AATS-6 MBL AODs
will be compared with corresponding values calculated from coincident
shipboard mean in-situ measurements of aerosol size distribution
combined with an assumption on the vertical mixing behavior (e.g.,
uniform mixing) within the MBL.

## 3.2.4.1. Hygroscopic Growth

First, the in-situ aerosol size distribution must be adjusted for
hygroscopic growth. Two separate hygroscopic growth models were used
together with the RH profile measured by ship radiosonde at 1700 UTC.
The first model is that of Swietlicki et al. (this issue), in which

D~p~(RH) = D~p10~ 10^c^ (1 - RH/100)^γ^; (1)

where D~p~(RH) is the particle diameter at RH; D~p10~ is the particle
diameter at 10% RH; and c and γ are parameters derived from H-TDMA
measurements taken aboard the R/V Vodyanitskiy during ACE-2 European
outbreak air masses for a range of particle sizes (50-250 nm dry
particle diameter) within the Aitken and accumulation modes. Although
the actual measurements do not extend beyond 250 nm diameter, we have
applied the Swietlicki et al. model to all particles with dry (10% RH)
diameters ≤ 1.2 μm. This represents an extrapolation of the actual
measurements. For particles with dry diameters \> 1.2 μm, we have used
an analogous growth curve based on the Swietlicki et al. measurements
for sea spray particles at Punta del Hidalgo, Tenerife during incidents
of local pollution. We shall refer to this composite model as the S
model. The second model is that of Hoppel et al. (1990), in which
separate growth curves are presented for particles with dry diameters \<
1 μm and for sea spray aerosol particles with diameters \> 3 μm. The
Hoppel et al. sea spray curve is taken from Hänel (1976). Following the
procedure used by Hoppel, we interpolated between the two Hoppel et al.
model curves for intermediate particle sizes. We shall refer to this
model as the H model. Figure 7a shows the 1500 UTC aerosol size
distribution dried to 10% RH and resultant humidified size distributions
after application of the S hygroscopic growth model for relative
humidities of 80% and 90%. Figure 7b shows the analogous results after
application of the H model.

Figures 7c,d present the corresponding particulate extinction spectra
calculated from the in-situ aerosol size distribution measurements for
three refractive indices: 1.33-0i (similar to pure water), 1.40-.0035i
(tropospheric aerosol model: Tanré et al., 1997), and 1.48-.01i
(polluted continental aerosol: von Hoyningen-Huene et al., this issue).
Calculations using the S and H particle growth models give similar
results, with the expected strong dependence of extinction on RH due to
change in particle size, and a weaker dependence on refractive index.
Model-to-model extinction differences (S-H) vary from +6% (380 nm) to
-11% (1020 nm) for RH 80%, and +12% (380 nm) to +1% (1020 nm) for RH
90%.

Table 2 presents percent mass fractions calculated from the shipboard
seven-stage Berner impactor sample collected during the period 0650 UTC
on 10 July to 0600 UTC on 11 July. For stage 4, for which the dry
particle geometric cutoff diameter is about 0.67 μm (which corresponds
to a 55% RH particle diameter of about 0.81 μm), 18.6% of the mass was
sea salt and 62.4% was non-seasalt sulfate. For stage 5, for which the
dry particle geometric diameter is about 1.3 μm, 75% of the mass was sea
salt, 9% was non-seasalt sulfate, and 11% was ammonium. Using chemical
composition measurements made on Tenerife, Collins et al. (this issue)
have constructed a model of the size-segregated chemical composition of
the boundary layer aerosol for this day. For a particle diameter of 1.0
μm, their results indicate that about 75% of the mass was sea salt.
Hence, our assumption that all particles \> 1.2 μm were sea salt is not
entirely correct. This probably leads to a small overestimate of aerosol
extinction, because the 90% RH Swietlicki et al. growth factor for sea
salt exceeds the corresponding non-sea salt growth factor by about 23%.
We note further that our extinction calculations make the unrealistic
assumption that refractive index is independent of particle size and
wavelength. Clearly, the variation in chemical composition and
associated variation in refractive index should be taken into account,
but Tang (1996) concluded that the chemical effect on light scattering
was outweighed by the size effect for the range of aerosol systems that
he considered, which included various ammonium, sodium, sulfate, and
nitrate species.

## 3.2.4.2. Aerosol Extinction Profile Comparison

A spectral aerosol extinction profile can be inferred from the measured
shipboard particle size distribution by combining the measured
radiosonde RH profile with an assumed vertical distribution of particle
number concentration. Using the 1657 UTC radiosonde sounding in an
approach similar to that of Hoppel et al. (1990), we have assumed that
particle number concentration remained constant up to the height of the
maximum in RH (which Hoppel defined as the top of the mixed layer), and
then decreased linearly with altitude to a value equal to 50% of the
surface value up to the top of the MBL. There is some question as to the
heights of the mixed layer and the MBL. NOAA/PMEL analysis (Johnson et
al., 1993) of the radiosonde data defines the top of the mixed layer as
the altitude where the relative humidity stops increasing with altitude,
and the top of the boundary layer as the altitude where the temperature
inversion starts and the RH begins to decrease with increasing altitude.
This definition is similar, but not identical, to the Johnson et al.
(this issue) definition of the surface mixed layer (SML). The SML is
defined as the layer closest to the surface and is identified by
constant equivalent potential temperature with a top defined by the
lowest height where the RH stops increasing with height. Johnson et al.
(this issue) note that RH also increases again in the cloud layer (CL),
which is the layer (above the SML) in which clouds form and is also
characterized by a nearly equal but smaller equivalent potential
temperature. The temperature and RH profiles measured by the ship
radiosonde at about 1100 and 1700 UTC and the RH profile measured by the
Pelican during its ascent 1500-1515 UTC are shown in Figure 8a. The NOAA
analysis gives a mixed layer height of 570 m and a boundary layer height
of 620 m for the 1657 UTC sounding, which measured a near surface RH of
83%, a peak of 91% between 570 and 610 m, and a decrease to 30% at 730
m. A subsidence temperature inversion extended from 610 to 810 m.

Background range-corrected lidar backscatter profiles measured at 532 nm
by the LMU shipboard lidar are overplotted in Figure 8b for seven times
from 1414 UTC to 1726 UTC. The lidar relative backscatter profile
measured at 1513 UTC indicates an aerosol layer with a peak backscatter
return at about 670 m, and a rapid signal decrease over the next 100 m
in altitude. As can be seen in Figure 8c, the peak of this maximum lidar
backscatter signal generally decreased in altitude with time. At 1700
UTC, although the maximum backscatter signal was measured at 470 m, the
peak was broad with no significant decrease in backscatter signals up to
620 m, which is consistent with the radiosonde measured height of the
maximum RH. The 499-nm aerosol extinction profile (Figure 4) derived
from the AATS-14 optical depth measurements during the period 1500-1515
UTC indicated a broad peak in extinction of 0.21 km^-1^ up to about 350
m, a linear decrease to 0.16 km^-1^ at about 640 m, and a less steep
decrease to about 0.05 km^-1^ at 0.9 km, with free tropospheric values
(\<0.01 km^-1^) above 1.2 km. In the discussion that follows, we use a
value of 610 m for the height of the well-mixed layer and a value of 780
m for the height of the MBL.

Figure 9 shows plots of aerosol extinction profiles calculated for a
refractive index of 1.40-.0035i from the 30-min average shipboard
aerosol size distribution measurements at a wavelength of 525 nm for
1500 UTC (Figure 9a) and at all five AATS-6 wavelengths for 1700 UTC
(Figure 9b). The 1657 UTC radiosonde RH values are listed at each
height. Also plotted in Figure 9a is the corresponding extinction
profile calculated when the Pelican RH profile is used with the ship
aerosol measurements. Figure 9c compares the 525-nm extinction profile
shown in Figure 9a with a profile derived from the AATS-14 airborne
optical depth measurements in the 499.4-nm channel (data in the 524.7-nm
channel were not usable during this flight due to dirt on the channel
aperture window), a profile calculated at 525 nm by Collins et al. (this
issue) from airborne (Pelican) in-situ size distribution and RH
measurements, and two profiles derived at 532 nm from the shipboard
lidar backscatter measurements. The lidar extinction profile was derived
from the lidar backscatter measurements using the Ackermann (1998)
formulation of an extinction to backscatter model representative of a
continental aerosol. This extinction to backscatter model is height
dependent in that it varies with RH. The range of RH values (83%-91%)
measured in the mixing layer by the 1657 UTC sounding results in model
extinction to backscatter ratios of 65-72 sr. The shape of the aerosol
extinction profile calculated from the shipboard in-situ size
distribution data is tied directly to the shape of the 1700 UTC
radiosonde RH profile through the hygroscopic growth model. It differs
markedly from the shape of the extinction profile derived from the
AATS-14 measurements, and its extinction magnitudes significantly exceed
the corresponding AATS-14 values. The shapes of the lidar extinction
profiles at 1513 and 1524 UTC are similar to that of the shipboard
in-situ extinction profile. However, the corresponding peaks in
extinction occur at greater altitudes, and indicate that the height of
the mixed layer was, in fact, greater at 1500 UTC than at the time of
the sounding at 1700 UTC. This is confirmed in Figure 9d, where lidar
profiles measured near 1700 UTC peak at altitudes near the RH peak
measured by the radiosonde. Also, the radiosonde RH maximum decreased
with time, as shown in Figure 8. Lidar extinction values at altitudes
between 300 and 800 m significantly exceed the AATS-14 values. The shape
of the Collins et al. profile (labeled CalTech), which is derived from
in-situ measurements in the Pelican profile, is similar to that of the
AATS-14 profile, and the values slightly underestimate the AATS-14
extinction (cf. Collins et al., this issue).

There are a number of possible explanations for the large differences in
magnitude and in shape between the aerosol extinction profiles
calculated from the shipboard measurements and the corresponding
profiles derived from the airborne measurements. First, although nearly
coincident in time, the shipboard and airborne measurements were
separated by 70 km. Thus, it is likely that the instruments did, in
fact, sample air masses with slightly different vertical distributions
of aerosol. Second, because the above analyses (except Collins et al.,
this issue) assume that particle number concentration is constant within
the well-mixed layer, our methodology for calculating extinction from
the ship in-situ measurements is critically dependent on the RH profile.
This means that any overestimate in the true RH profile due to a lack of
available coincident data, such as occurred at 1500 UTC, or due to
inherent uncertainty in the RH measurement can lead to large
overestimates in extinction. The RH profile measured at the Pelican
location during aircraft ascent 1500-1515 UTC differed significantly
from that measured by the 1700 UTC ship sounding (c.f. Figure 8a). Use
of these Pelican RH measurements to adjust the ship in-situ measurements
of aerosol size distribution for hygroscopic growth yields a profile
with values that are significantly closer in magnitude to those of the
aircraft sensor profiles. Third, we assume that all particles regardless
of size are equally mixed within the mixing layer, and this can lead to
an overestimate of extinction and AOD because it does not account for
sedimentation of large particles. Fourth, because the hygroscopic growth
model for sea salt particles with dry diameters \> 1.2 μm yields growth
factors that exceed those for the smaller particles in the Aitken and
accumulation size modes, our application of the sea salt growth curve to
all particles \> 1.2 μm likely leads to a small overestimate in
extinction, since all particles \> 1.2 μm were not sea salt. However,
this effect is not enough to account for the differences in magnitude
between the extinction profile calculated from the ship in-situ aerosol
size distribution measurements and that derived from the aircraft
measurements. Finally, we acknowledge that our assumption of a particle
size- and wavelength-invariant refractive index (1.40-.0035i) is an
oversimplification. Nevertheless, the extinction values in Figure 7c,d,
calculated for a range of refractive indices (1.33-0i, 1.40-.0035i,
1.48-.01i), suggest that reasonable variations in refractive index
(caused, e.g., by water uptake) would change extinction by less than the
size effects explored here.

## 3.2.4.3. Aerosol Optical Depth Comparison

Figure 10 compares temporally averaged AATS-6 MBL AOD spectra and
corresponding AOD spectra derived by integrating to 780 m the extinction
profiles calculated (at the three values of refractive index) from the
30-min average shipboard in-situ aerosol size distribution measurements
centered at 1500 and 1700 UTC. For the range of refractive indices
assumed, when non sea-salt growth factors are applied to all particles ≤
1.2 μm and sea salt growth factors are applied to all particles \> 1.2
μm prior to calculating extinction, the in-situ values significantly
overestimate the sunphotometer optical depths for both times. These
results are shown by the solid red, green, and blue lines. For a
refractive index of 1.40-.0035i, the calculated mid-visible AOD exceeds
the AATS-6 measurement by about 0.03 (15%) at 1500 UTC, and by about
0.10 (40%) at 1700 UTC. However, use of only the non-sea salt Aitken and
accumulation mode growth curves for particles of all sizes (red, green,
and blue dashed lines) leads to agreement, within the AOD measurement
uncertainties, between measured and calculated AOD for this refractive
index at 1500 UTC, but does not completely account for the disagreement
at 1700 UTC. Lowering the maximum altitude limit of integration (not
shown) to 610 m for the 1700 UTC case (consistent with both lidar and
radiosonde) further reduces AODs by about 0.05, which is still 0.03
greater than the mean AATS-6 mid-visible AOD but only about 0.015
greater that the upper limit measurement uncertainty. It should be noted
that the AATS-6 mean for 1500 UTC is only a 20-min average, because data
before 1450 UTC were affected by ship structure obscuration of the sun.
Vertical bars on the AATS-6 values represent the larger of the standard
deviation over the time period and the measurement uncertainty. As
discussed above and shown in Figure 3, total column AODs decreased by
about 0.05 at all wavelengths during the 1450-1510 UTC time period.

The time history of AATS-6 AOD and CWV is shown in Figure 11a for the
time period 1400-1830 UTC. Figure 11b shows corresponding values
computed by integrating to 780 m the particulate extinction profiles
calculated from the shipboard aerosol size distribution measurements for
a refractive index of 1.40-.0035i after applying the S hygroscopic
growth model as described above, using the 1657 UTC RH profile. In
general, mid-visible AODs calculated from the shipboard in-situ particle
size measurements overestimate coincident AATS-6 AODs by 0.02 to 0.10.

## 3.2.4.4. Spectral Dependence of AOD, Extinction, Scattering

The wavelength dependence of aerosol optical depth, extinction (E), or
scattering (s) is often approximated by a power law,

![](media/image1.pct){width="116.0in" height="48.0in"} (2)

where the Mie scattering parameter p is AOD, E, or s; λ is wavelength; α
represents Ångström exponents, α ~D~, α ~E~, or α ~s~, that can be
derived from linear least squares fits to the AOD(λ), E(λ), or s(λ)
data, respectively. We calculate α~D~ from AATS-6 AOD spectra, α~E~ from
E(λ) derived from the ship in-situ particle size measurements at AATS-6
wavelengths, and α~s~ from the ship nephelometer spectral measurements
of total scattering. As noted by Quinn et al. (1998), the actual limits
of integration over zenith angle are truncated within the nephelometer
because of geometrical limitations. This angular truncation is most
pronounced for supermicron particles, which preferentially scatter light
in the forward direction. We note that no adjustment has been made to
the nephelometer data to correct for this instrument angular truncation
error, and this can result in calculated Ångström exponents that are too
large, especially when scattering is dominated by large sea salt
particles.

In previous studies (e.g., von Hoyningen-Huene and Raabe, 1987; Reddy et
al., 1990; Hoppel et al., 1990; Smirnov et al., 1994; Villevalde et al.,
1994; Smirnov et al., 1995; Quinn et al., 1998), α has been used as an
index to compare air masses with different aerosol signatures or merely
to characterize the dependence of scattering on wavelength and hence on
particle size. These studies led us to question whether α might be used
to assess agreement between the shipboard in-situ and sunphotometer
measurements. We would expect agreement only for those cases where the
MBL is well-mixed and the total atmospheric column AOD is dominated by
the MBL aerosol. Like Reddy et al. (1990), we recognize that there is
inherent error in the use of α because the shapes of the MBL/total
columnar aerosol size distribution and the associated spectral
extinction, scattering, or AOD curves are not necessarily
well-represented by a power law distribution.

Because total column AOD on 10 July appears to have been dominated by
well-mixed aerosol within the MBL, we examine α values for this day in
detail. Figure 12a compares α values from the various instruments for
the period 1200-1900 UTC.  Values of α~D~ are shown for calculations
performed for AATS-6 MBL AOD and for AATS-6 total column AOD spectra.
Three separate time series of α~E~ are shown to investigate the effect
of choice of wavelengths on the α~E~ calculation. These include one each
for E(λ) calculated at the five AATS-6 AOD wavelengths and at the three
nephelometer wavelengths for RH 80% (approximate ambient), and one for
E(λ) calculated at the three nephelometer wavelengths for RH 55% (the
nephelometer sampling RH). A refractive index of 1.40-.0035i was assumed
in the E(λ) calculations. The time series for α~s~ was calculated as
described above. The magnitudes differ significantly, and the particle
size distribution values do show some dependence on RH and on the range
of specific wavelengths chosen for analysis. In general, however, the
correlation appears very good for the 1500-1800 UTC time period. When
total column α~D~ values are averaged over 30-min time periods, the
(α~D~ - α~s~) rms difference is 0.20 with a bias of -0.19±0.04. The
analogous (α~D~ - α~E~) rms difference (for RH 80%) is 0.10, with a
(α~D~ - α~E~) bias of 0.04±0.09. Coefficients of determination, r^2^,
are 0.98 for α~s~ vs α~D~ and 0.60 for α~E~ vs α~D~. The spectral
dependence of the AATS-6 MBL AODs is slightly flatter than that of the
AATS-6 total column AODs, as evidenced by α~D~ values that are 0.05-0.10
less than those derived from the total column AODs. Figure 12b compares
values of 550-nm aerosol scattering coefficient measured by the
nephelometer with corresponding extinction values calculated at 550 nm
for a refractive index of 1.40-0.0035i from the ship height particle
size measurements adjusted to the nephelometer sampling RH of 55%.
Results are shown for the entire day. Differences between the calculated
extinction and the measured scattering range from -.01 to +.019 km^-1^
(-12 to +33%), with a mean difference of .005 km^-1^ (+7%). Results are
essentially unchanged (to a precision of .001 km^-1^) if a non-absorbing
refractive index of 1.40-0i is assumed.

## 3.3. 22 JULY 1997

On 22 July, the R/V Vodyanitskiy sailed in a region located about 1.5
deg longitude off the Portugal coast and about 3 deg latitude and
longitude northwest of Sagres. A large region of high pressure extended
from the Azores to the North Sea (Raes et al, this issue) and resulted
in outflow from western Europe on this day. Five-day backward
trajectories indicate that air parcels below 850 hPa at the ship
location were transported from the northern European continent west,
then south through France, west over the ocean north of Spain, and then
south to the ship. Ship radiosonde soundings at about 1100 and 1700 UTC
indicated a weak subsidence inversion at 410-470 m and 320-400 m,
respectively, with an estimated boundary layer top at 300 m at 1100 UTC
and no definable layer at 1700 UTC.

Figure 13a presents the time series of AATS-6 AOD and CWV measurements
for this day. Due to ship heading and sun position relative to the ship,
data prior to 1330 UTC were subject to frequent obscuration by shipboard
wires/structures. However, the general temporal trend is still
discernible. Peak values of AOD (about 0.25 at 525 nm) were measured
just before noon, with a gradual decrease during the early afternoon,
followed by a relative maximum (0.20 at 525 nm) at about 1500 UTC and
lower relatively constant values (0.17 at 525 nm) from 1600 to 1800 UTC.
No shipboard particle size data were available for the time period
during which AATS-6 measurements were taken. Figure 13b overplots α~D~
and α~s~, with good agreement after 1300 UTC.

Lidar aerosol backscatter measurements (λ=532 nm) during the period
1429-1739 UTC can be compared with the AATS-6 measurements. Lidar
background- and range-corrected relative backscatter profiles are shown
in Figure 14a for five times. The data show significant aerosol
backscatter returns up to 1.8 km, but the peak signals occur in the MBL
below 350 m, which is consistent with the 1700 radiosonde data. Figure
14b shows the corresponding extinction profiles derived from the lidar
backscatter profiles by using the best estimate lidar system calibration
constant and assuming height-dependent extinction-to-backscatter ratios
calculated from the Ackermann (1998) continental aerosol model and the
radiosonde RH profile. These profiles indicate a peak in the MBL aerosol
extinction at 1518 UTC, with significant decreases after that time.
Above the boundary layer, the aerosol backscatter and extinction
structures are more complicated. Time-dependent lidar-derived AODs have
been estimated by integrating each extinction profile after assuming a
constant value of extinction coefficient within the overlap region below
120 m equal to the maximum derived value between 100 and 250 m. This
procedure leads to optical depth errors on the order of ±10%; additional
errors of ±10-20% can result from uncertainties associated with required
profile splicing and overlap correction. Figure 14c and Table 3 compare
AODs calculated from the lidar-inferred extinction profiles for the
Ackermann continental and marine aerosol extinction-to-backscatter
models with coincident mean AATS-6 total column AODs calculated for
three-minute time periods. The error bars on the lidar AODs are those
due to calibration only; error bars on the AATS-6 AODs represent upper
limit combined measurement and temporal variability uncertainties. In
general, similar temporal behavior is observed, with a peak AATS-6 AOD
at 525 nm of 0.187 at 1518 UTC, followed by a subsequent decrease to
0.160 at 1738 UTC. Consistent with the European source of the aerosol,
lidar AOD results derived using the Ackermann continental aerosol
extinction-to-backscatter model yield significantly better agreement
with the AATS-6 values than do those derived using the marine aerosol
model.

# 4. SUMMARY AND CONCLUSIONS

Shipboard sunphotometer (AATS-6) measurements of AOD and CWV have been
presented for a subset of the data collected during ACE-2. AATS-6 values
of CWV have been shown to agree well with corresponding radiosonde
measurements on several days and with airborne AATS-14 and UWPH
measurements on 10 July. Detailed analyses of AATS-6 AOD measurements
acquired on 10 and 22 July have been presented. Spatially and temporally
coincident measurements of AOD by AATS-6 and AVHRR on 10 July have been
compared, and AVHRR AODs were found to have exceeded corresponding mean
AATS-6 AODs by about 0.04, which is greater than the estimated
measurement uncertainties, although the uncertainty in the AVHRR AOD
retrieval algorithm (Durkee et al., this issue) has yet to be
quantified. AATS-14 AODs have also been presented for measurements taken
from the Pelican at an altitude of about 60 m MSL at a location about 70
km from the ship fifteen minutes prior to the NOAA-14 satellite
overpass. Comparison of AATS-14 AODs with the corresponding AVHRR AODs
calculated for the aircraft location indicates AVHRR values also
exceeded the AATS-14 AODs by about 0.04. It is estimated that
approximately 0.010-0.015 of this difference can be explained by AOD in
the layer between the sea surface and the aircraft altitude. The
remainder is attributed to temporal variations in the overlying aerosol
between the times of the AATS-14 and AVHRR measurements.

An in-depth attempt to demonstrate column closure between measured
AATS-6 MBL AODs and corresponding values calculated from shipboard
in-situ aerosol size distribution measurements on 10 July was not
successful, with significant overestimates of AATS-6 measured AOD by the
calculated AOD. In large part, we attribute this lack of success to the
high sensitivity of the closure methodology to models of hygroscopic
growth, which have not been validated over the necessary range of
particle size/composition distributions. Our assumption that particle
refractive index (hence, composition) is constant with particle size,
RH, and wavelength is not valid. Although our sensitivity studies with
three refractive indices indicate that this chemical composition effect
is less important than the particle size effect due to hygroscopic
growth, the measurement-based size-segregated chemical composition
modeling methodology of Collins et al. (this issue) represents a
significantly more rigorous and realistic approach that should be
adopted in future studies.

Accurate characterization of the vertical distribution of aerosol is
critical to the calculation of AOD from a single size distribution
measurement, because extinction is directly proportional to the particle
number density. Our assumption of constant particle number density up to
the height of the peak in RH (i.e., equal mixing of all particles
regardless of size) and then a linear decrease above that height may be
incorrect and, in any event, is certainly not an ideal approach.
Multiwavelength aerosol lidar backscatter measurements probably offer
the best means for quantifying the vertical distribution of aerosol
within the marine boundary layer because only lidar can provide an
instantaneous, quantitative range-resolved view of the overlying
atmosphere. However, calculation of aerosol extinction and optical depth
from measurements taken by a zenith-pointing aerosol backscatter lidar
requires that the lidar be well-calibrated, that pulse-to-pulse
variations in transmitted energy variations be known and accounted for,
and that signal degradation due to incomplete overlap between the
transmitted and received laser beams at ranges near the lidar can be
accurately characterized and corrected. Moreover, the
extinction-to-backscatter ratio, which depends on both particle size and
composition (hence RH and aerosol type) must be known throughout the
aerosol layer. Achieving all these proved difficult aboard the ship;
additional analysis of the lidar data set may lead to a better
quantification of errors and permit additional comparisons.

Because the Ångström wavelength exponent, α, has been used often in
previous studies both as a measure of particle size and to characterize
the origin of air masses, we have examined the spectral dependence of
AATS-6 AOD measurements by calculating time-dependent values of α~D~
from the instantaneous total column AODs. Comparisons have been made of
AATS-6 α~D~ values with corresponding values, α~E~, derived for the ship
height from particulate extinction calculated for a refractive index of
1.40-.0035i from shipboard in-situ measurements of aerosol size
distribution adjusted for hygroscopic particle growth, and with
values, α~s~, calculated from nephelometer total particle scattering
measurements at 55% RH. Figure 15 summarizes the results of our
calculations for portions of nine days by showing scatter plots of α~E~
versus α~D~ (Figure 15a) and α~s~ versus α~D~ (Figure 15b). Data have
been segregated by air mass source region according to the
classification scheme of Quinn et al. (this issue). The results are
highly variable among the cases. For the composite data set, the rms
difference for (α~s~ - α~D~) is 0.32, with a bias of 0.10, and the rms
difference for (α~E~ - α~D~) is 0.50, with a bias of -0.10. Several
factors might have contributed to the high degree of variability in the
results. These include incorrect optical modeling of the aerosol size
distribution through use of a size- and wavelength-independent
refractive index, poorly known/modeled hygroscopic growth of supermicron
particles, use of different wavelength regions sampled by the
instruments to calculate α, underestimates of large particle
contributions to total scattering that result from truncation of angular
integration limits within the nephelometer, presence of particle
absorption not accounted for in the nephelometer total scattering
measurements, variations in the vertical distribution and optical
properties of a mixed clean marine and polluted continental aerosol
within the boundary layer, and significant contributions to AOD from
aerosol above the well-mixed layer (although none of the cases we
analyzed included a Saharan dust layer). These results illustrate
further the great difficulty of deriving column values from point
measurements.

*Acknowledgments*. This research was conducted as part of the second
Aerosol Characterization Experiment (ACE-2), which is a contribution to
the International Global Atmospheric Chemistry (IGAC) core project of
the International Geosphere-Biosphere Programme (IGBP). Financial
support for the measurements and analyses was provided by the U. S.
National Oceanic and Atmospheric Administration, Office of Naval
Research, National Aeronautics and Space Administration, and the
European Commission DG XII (Environment and Climate). We thank the Ozone
Processing Team at NASA Goddard Space Flight Center for making available
TOMS Earth Probe ozone data.

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Table 1 Summary of best AATS-6 measurements and partial list of
available spatially and temporally near-coincident comparative data

<table style="width:100%;">
<colgroup>
<col style="width: 7%" />
<col style="width: 10%" />
<col style="width: 9%" />
<col style="width: 8%" />
<col style="width: 7%" />
<col style="width: 8%" />
<col style="width: 9%" />
<col style="width: 14%" />
<col style="width: 13%" />
<col style="width: 12%" />
</colgroup>
<tbody>
<tr class="odd">
<td>Date</td>
<td>Air Mass Classification</td>
<td><p>AATS-6</p>
<p>(RVV)<a href="#fn1" class="footnote-ref" id="fnref1"
role="doc-noteref"><sup>1</sup></a></p></td>
<td>AATS-14 (Pelican)</td>
<td>In-situ (Pelican)</td>
<td><p>AVHRR</p>
<p>(NOAA-14)</p></td>
<td><p>In-situ</p>
<p>(RVV)</p></td>
<td><p>LMU Lidar</p>
<p>(RVV)</p></td>
<td>Radiosonde (RVV) Launch Time (UTC)</td>
<td>Sagres (50-m site) Sunphotometers</td>
</tr>
<tr class="even">
<td>22 June</td>
<td>Atlantic</td>
<td>AOD,CWV</td>
<td>­–</td>
<td>­–</td>
<td>­–</td>
<td>asd<a href="#fn2" class="footnote-ref" id="fnref2"
role="doc-noteref"><sup>2</sup></a> , tot scat<a href="#fn3"
class="footnote-ref" id="fnref3"
role="doc-noteref"><sup>3</sup></a></td>
<td>­–</td>
<td>1146, 2244</td>
<td>FISBAT Bologna, Univ. of Leipzig</td>
</tr>
<tr class="odd">
<td>24 June</td>
<td>Iberian</td>
<td>AOD,CWV</td>
<td>–</td>
<td>–</td>
<td>1445 UTC</td>
<td>asd, tot scat</td>
<td>range-corrected signals</td>
<td>1146, 2246</td>
<td>–</td>
</tr>
<tr class="even">
<td>25 June</td>
<td>Iberian</td>
<td>AOD,CWV</td>
<td>–</td>
<td>–</td>
<td>1434 UTC</td>
<td>asd, tot scat</td>
<td>range-corrected signals</td>
<td>1052, 2305</td>
<td>–</td>
</tr>
<tr class="odd">
<td>27 June</td>
<td>Atlantic/polar</td>
<td>AOD</td>
<td>–</td>
<td>–</td>
<td>–</td>
<td>asd, tot scat</td>
<td>range-corrected signals</td>
<td>1118</td>
<td>–</td>
</tr>
<tr class="even">
<td>30 June</td>
<td>polar</td>
<td>AOD</td>
<td>–</td>
<td>–</td>
<td>–</td>
<td>asd, tot scat</td>
<td>range-corrected signals</td>
<td>1047, 2248</td>
<td>–</td>
</tr>
<tr class="odd">
<td>6 July</td>
<td>Mediterranean</td>
<td>AOD,CWV</td>
<td>–</td>
<td>–</td>
<td>–</td>
<td>asd, tot scat</td>
<td>range-corrected signals</td>
<td>1058, 2302</td>
<td>–</td>
</tr>
<tr class="even">
<td>10 July</td>
<td>W. Europe</td>
<td>AOD,CWV</td>
<td>AOD,CWV</td>
<td>Various</td>
<td>1515 UTC</td>
<td>asd, tot scat</td>
<td>relative bks<a href="#fn4" class="footnote-ref" id="fnref4"
role="doc-noteref"><sup>4</sup></a> ; ext<a href="#fn5"
class="footnote-ref" id="fnref5"
role="doc-noteref"><sup>5</sup></a></td>
<td>1657, 2250</td>
<td>–</td>
</tr>
<tr class="odd">
<td>18 July</td>
<td>Iberian</td>
<td>AOD,CWV</td>
<td>–</td>
<td>–</td>
<td>1524 UTC</td>
<td>asd, tot scat</td>
<td>range-corrected signals</td>
<td>1058, 1650</td>
<td>FISBAT Bologna, Univ. of Leipzig</td>
</tr>
<tr class="even">
<td>22 July</td>
<td>W. Europe</td>
<td>AOD,CWV</td>
<td>–</td>
<td>–</td>
<td>­–</td>
<td>tot scat</td>
<td>relative bks; ext</td>
<td>1046, 1654, 2246</td>
<td>–</td>
</tr>
</tbody>
</table>
<aside id="footnotes" class="footnotes footnotes-end-of-document"
role="doc-endnotes">
<hr />
<ol>
<li id="fn1"><p><sup>1</sup> RVV: R/V Vodyanitskiy<a href="#fnref1"
class="footnote-back" role="doc-backlink">↩︎</a></p></li>
<li id="fn2"><p><sup>2</sup> asd: aerosol size distribution measured by
particle samplers/sizers<a href="#fnref2" class="footnote-back"
role="doc-backlink">↩︎</a></p></li>
<li id="fn3"><p><sup>3</sup> tot scat: aerosol total scattering
coefficient measured by integrating nephelometer<a href="#fnref3"
class="footnote-back" role="doc-backlink">↩︎</a></p></li>
<li id="fn4"><p><sup>4</sup> bks: aerosol backscatter<a href="#fnref4"
class="footnote-back" role="doc-backlink">↩︎</a></p></li>
<li id="fn5"><p><sup>5</sup> ext: aerosol extinction<a href="#fnref5"
class="footnote-back" role="doc-backlink">↩︎</a></p></li>
</ol>
</aside>

Table 2 Percent mass fractions calculated from measurements taken aboard
the R/V Vodyanitskiy with the NOAA/PMEL seven-stage Berner impactor
during the period 0650 UTC 10 July to 0600 UTC 11 July 1997. Data are
shown for each stage of the impactor.

  ------------------ ------- ------- ------- ------- ------- ------- -------
  Impactor stage:    1       2       3       4       5       6       7

  D, aero \[μm\]     0.184   0.308   0.549   1.063   2.024   4.13    10.3

  D~RH:55%~, geom    0.14    0.24    0.42    0.81    1.55    3.1     7.9
  \[μm\]                                                             

  D~RH:80%~, geom            0.35    0.61    1.18    2.26    4.53    11.54
  \[μm\]                                                             

  nssSO4             45.93   69.30   70.24   62.45   9.29    2.53    2.08

  Sea salt           5.00    2.03    3.18    18.59   75.36   86.08   81.92

  NO3                0.00    0.00    0.00    0.00    11.10   9.67    11.24

  MSA                0.00    0.48    0.77    0.62    0.13    0.00    0.00

  NH4                6.56    13.65   21.47   13.22   0.00    0.00    0.00

  Total carbon       42.51   14.54   4.33    5.11    4.12    1.71    4.77
  ------------------ ------- ------- ------- ------- ------- ------- -------

Table 3 Aerosol optical depths measured by AATS-6 and derived from
University of Munich lidar backscatter measurements taken from aboard
the R/V Vodyanitskiy on 22 July

  -------- ------- --------------- ---------- ------- ------- ---------- --------------- ----------
           LMU                                                AATS-6                     
           Lidar                                                                         
           AOD                                                                           
           (532                                                                          
           nm)                                                                           

  UTC              Continental     Marine             AOD                UTC             
                                                      (525                               
                                                      nm)                                

  142945           0.199 ± 0.025   0.105 ±            0.183 ±            142820-143108   
                                   0.017              0.010                              

  151810           0.206 ± 0.026   0.105 ±            0.187 ±            151647-151935   
                                   0.017              0.010                              

  160747           0.160 ± 0.046   0.082 ±            0.165 ±            160622-160910   
                                   0.030              0.010                              

  163248           0.179 ± 0.025   0.096 ±            0.161 ±            163121-163410   
                                   0.016              0.010                              

  173837           0.183 ± 0.025   0.100 ±            0.160 ±            173711-173946   
                                   0.016              0.010                              
  -------- ------- --------------- ---------- ------- ------- ---------- --------------- ----------

FIGURE CAPTIONS

Figure 1 Representative aerosol optical depth spectra measured by AATS-6
aboard the R/V Vodyanitskiy during ACE-2.

Figure 2 Aerosol optical depth (λ = 630 nm) retrieved from AVHRR
radiances (Durkee et al., this issue) during 1515 UTC NOAA-14 overpass
of the ACE-2 sampling region on 10 July 97; locations of R/V
Vodyanitskiy during the period 1451-1524 UTC and Pelican during the
period 1454-1500 UTC.

Figure 3 For 10 July 97: (a) Temporal variation of total column aerosol
optical depth measured by AATS-6 aboard the R/V Vodyanitskiy, (b)
comparison of AATS-6 aerosol optical depths and AVHRR aerosol optical
depths calculated for the ship location, (c) comparison of AATS-14
aerosol optical depths acquired from the Pelican at an altitude of 60 m
ASL and AVHRR aerosol optical depths calculated for the plane location.

Figure 4 Aerosol optical depth and extinction profiles retrieved from
AATS-14 measurements made between 1500 and 1515 UTC during Pelican
flight near R/V Vodyanitskiy on 10 July 97.

Figure 5 For 10 July 97: water vapor density and columnar water vapor
profiles calculated from AATS-14 measurements and from UWPH measurements
(Gassó et al., this issue) during Pelican ascent 1500-1605 UTC, and from
ship radiosonde 1656-1715 UTC; mean columnar water vapor retrieved from
AATS-6 measurements 1500-1730 UTC.

Figure 6 Scatter diagram of columnar water vapor calculated from
radiosonde measurements and from AATS-6 measurements during ACE-2. Each
AATS-6 data point represents the mean calculated over the time period
spanned by the radiosonde measurements; horizontal bars show the
corresponding standard deviation (wide ticks) and the range (narrow
ticks) for that time period.

Figure 7 NOAA/PMEL aerosol size distribution measured aboard R/V
Vodyanitskiy at 1500 UTC on 10 July 97 (a,b); corresponding calculated
extinction spectra (c,d). Dried (RH=10%: x symbols) size distribution
has been humidified to 80% (circles) and 90% (diamonds) RH using
hygroscopic growth models of (a) Swietlicki et al. (this issue), and (b)
Hoppel et al. (1990). Corresponding particulate extinction spectra
calculated at AATS-6 wavelengths for three refractive indices: m=1.33-0i
(smallest values for each set of three), m=1.40-.0035i (intermediate
values for each set of three), and m=1.48-.01i (largest values for each
set of three).

Figure 8 For 10 July 97: (a) ship radiosonde temperature and RH
profiles, Pelican RH profile; (b) shipboard lidar (University of Munich)
relative backscatter profiles at 532 nm between 1414 and 1726 UTC; and
(c) time variation of the height of aerosol lidar backscatter signal
peak and AATS-6 marine boundary layer aerosol optical depth at 525 nm.

Figure 9 For 10 July 97: (a) profiles of aerosol extinction calculated
at 525 nm for a refractive index of 1.40-.0035i by combining shipboard
mean aerosol size distribution measurements during the period 1445-1515
UTC with the radiosonde RH profile measured 1656-1715 UTC above ship
location and with the RH profile measured by Pelican UWPH (Gassó et al.,
this issue) during aircraft ascent 1500-1605 UTC; RH values are listed
beside the data points. (b) Aerosol extinction profiles calculated at
five AATS-6 wavelengths by combining shipboard 30-min average particle
size distribution measurements centered at 1700 UTC and 1700 UTC
radiosonde RH profile. (c) Comparison of mid-visible aerosol extinction
profiles calculated from ship (1445-1515 UTC) and from aircraft
(1500-1515 UTC; Collins et al., this issue) in-situ particle size
distribution measurements, from AATS-14 measurements (1500-1515 UTC),
and from shipboard lidar backscatter measurements for times near 1500
UTC. (d) Comparison of mid-visible aerosol extinction profiles
calculated from shipboard in-situ aerosol size distribution measurements
and from shipboard lidar backscatter measurements for times near 1700
UTC.

Figure 10 For 10 July 97: marine boundary layer aerosol optical depth
spectra derived from AATS-6 measurements (circles with error bars) and
calculated from shipboard in-situ aerosol size distribution measurements
for three values of refractive index for (a) 1500 UTC and (b) 1700 UTC.
For in-situ results connected by dashed red, green, and blue lines,
growth factors based only on the Swietlicki et al (this issue) shipboard
measurements for non-sea salt Aitken and accumulation modes were used.
For the results connected by solid red, green, and blue lines, the
non-sea salt growth factors were used for particles with dry diameter \<
1.2 μm, and growth factors based on the Swietlicki et al. (this issue)
sea salt measurements were applied to particles with dry diameter ≥ 1.2
μm.

Figure 11 (a) Marine boundary layer aerosol optical depth and columnar
water vapor measured by AATS-6, columnar water vapor measured by
radiosonde (triangle); (b) boundary layer aerosol optical depth
calculated from shipboard in-situ aerosol size distribution measurements
for a refractive index of 1.40-.0035i for 10 July 97.

Figure 12 For 10 July 97: (a) comparison of Ångström wavelength
exponent, α, calculated from AATS-6 MBL AODs (cyan) and from AATS-6
total column AODs (red), from extinction derived from shipboard aerosol
size distribution measurements (PSD) at AATS-6 wavelengths for RH 55%
(black triangles) and RH 80% (blue circles) and at nephelometer
wavelengths for RH 55% (magenta diamonds), and from shipboard
nephelometer measurements of particle scattering coefficient (green x
symbols); and (b) comparison of aerosol extinction coefficients
calculated at 550 nm from ship particle size data at RH 55% for a
refractive index 1.40-.0035i and from aerosol scattering coefficients
measured by shipboard nephelometer at a sampling RH 55%

Figure 13 For 22 July 97: (a) Columnar aerosol optical depth and water
vapor measured by AATS-6, columnar water vapor measured by radiosonde
(triangle); (b) comparison of Ångström wavelength exponent, α,
calculated from AATS-6 column AODs and from shipboard nephelometer
measurements of particle scattering coefficient at RH 55%.

Figure 14 For 22 July 97: (a) shipboard lidar relative backscatter
profiles at 532 nm, (b) corresponding aerosol extinction profiles
derived from the relative backscatter profiles, (c) AATS-6 AOD at 525 nm
and lidar-inferred AOD at 532 nm.

Figure 15 Scatter diagrams of Ångström wavelength exponents calculated
from (a) aerosol extinction spectra derived from shipboard particle size
measurements and (b) shipboard nephelometer scattering measurements
versus Ångström exponents calculated from AATS-6 total column AOD
measurements acquired during portions of nine days during ACE-2. Data
have been segregated by air mass source region according to the
classification scheme of Quinn et al. (this issue). The solid lines
represent the regression fits, and the dotted lines one-to-one
correspondence.