# Clear-sky closure studies of lower tropospheric aerosol and water vapor during ACE‑2 using airborne sunphotometer, airborne in-situ, space-borne, and ground-based measurements

Beat Schmid^1\*^, John M. Livingston^2^, Philip B. Russell^3^, Philip A.
Durkee^4^, Haflidi H. Jonsson^4^,

Donald R. Collins^5^, Richard C. Flagan^5^, John H. Seinfeld^5^,
Santiago Gassó^6^, Dean A. Hegg^6^, Elisabeth Öström^7#^, Kevin J.
Noone^7^, Ellsworth J. Welton^8&^, Kenneth J. Voss^8^, Howard R.
Gordon^8^, Paola Formenti^9^, and Meinrat O. Andreae^9^

^1^Bay Area Environmental Research Institute, San Francisco, CA, U.S.A.

^2^SRI International, Menlo Park, CA, U.S.A.

^3^NASA Ames Research Center, Moffett Field, CA, U.S.A.

^4^Naval Postgraduate School, Monterey, CA, U.S.A.

^5^California Institute of Technology, Pasadena, CA, U.S.A.

^6^University of Washington, Seattle, WA, U.S.A.

^7^Meteorological Institute Stockholm University, Stockholm, Sweden

^\#^Now at Meteorological Office, Hadley Centre, Bracknell, UK

^8^University of Miami, Miami, FL, U.S.A.

^&^Now at Science Systems and Applications Inc., Greenbelt, MD, U.S.A.

^9^Max Planck Institute for Chemistry, Mainz, Germany

\*Corresponding author: Beat Schmid, MS-245, NASA Ames Research Center,
Moffett Field, CA 94035-1000, U.S.A, Phone: 650 604 5933, Fax: 650 604
3625, E-mail: bschmid@mail.arc.nasa.gov

Paper submitted to Tellus ACE-2 special issue December 22, 1998; First
revision submitted August 2, 1999; Second revision submitted August 24,
1999; Final version submitted October 1, 1999

This document was prepared with Microsoft Word 97 (PC)

## Abstract

We report on clear-sky column closure experiments (CLEARCOLUMN)
performed in the Canary Islands during the second Aerosol
Characterization Experiment (ACE-2) in June/July 1997. We present
CLEARCOLUMN results obtained by combining airborne sunphotometer and
in-situ (optical particle counter, nephelometer, and absorption
photometer) measurements taken aboard the Pelican aircraft, space-borne
NOAA/AVHRR data and ground-based lidar and sunphotometer measurements.
During both days discussed here, vertical profiles flown in cloud free
air masses revealed three distinctly different layers: a marine boundary
layer (MBL) with varying pollution levels, an elevated dust layer, and a
very clean layer between the MBL and the dust layer. A key result of
this study is the achievement of closure between extinction or layer
aerosol optical depth (AOD) computed from continuous in-situ aerosol
size-distributions and composition and those measured with the airborne
sunphotometer. In the dust, the agreement in layer AOD (λ=380-1060 nm)
is 3-8%. In the MBL there is a tendency for the in-situ results to be
slightly lower than the sunphotometer measurements (10-17% at λ=525 nm),
but these differences are within the combined error bars of the
measurements and computations.

## Introduction

The second Aerosol Characterization Experiment (ACE-2) of the
International Global Atmospheric Chemistry Project (IGAC) ran from 16
June to 25 July 1997. The results presented in this study are part of
the "Clear-sky column closure experiment" (CLEARCOLUMN) activity, one of
7 ACE-2 activities (Raes et al., 2000). Clear-sky column closure
experiments call for characterization of aerosol layers by simultaneous
measurements using different techniques that can be related using models
(Quinn et al., 1996). During ACE-2, several CLEARCOLUMN experiments
involving different platforms were carried out successfully (Russell and
Heintzenberg, 2000). In this paper, we report on CLEARCOLUMN results
obtained by combining airborne sunphotometer and in-situ measurements
taken aboard the Pelican aircraft, space-borne NOAA/AVHRR data and
ground-based lidar and sunphotometer measurements.

A wide range of aerosol types was encountered throughout the ACE-2 area,
including background Atlantic marine, European pollution-derived, and
African mineral dust. Of the 21 flights performed by the Pelican in
ACE-2, five were designed as CLEARCOLUMN missions. In this study we
report on Pelican flights tf15 and tf20 performed on the 8^th^ and
17^th^ of July 1997. On both days, vertical profiles flown in cloud free
air masses revealed three distinctly different layers: a marine boundary
layer (MBL) with varying pollution levels, an elevated dust layer and a
very clean layer between the MBL and the dust layer.

## 2. Measurements

### 2.1. Airborne Measurements

#### 2.1.1. The Pelican aircraft

The Pelican is operated by the Marina, California based Center for
Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) (Bluth et
al., 1996). The Pelican, a highly modified Cessna 337 Skymaster, was the
smallest of the six aircraft participating in ACE-2. For a complete list
of the Pelican ACE-2 payload, see Russell and Heintzenberg (2000). The
Pelican has the option to be piloted remotely; however, on all flights
performed during ACE-2 out of the north airport of Tenerife (28.48° N,
16.34° W, 632 m) it was flown by two on-board pilots. The typical
cruising speed was about 50 ms^-1^, and the maximum flight altitude was
close to 4000 m.

#### 2.1.2 The NASA Ames Airborne Tracking 14-channel Sunphotometer (AATS-14)

AATS-14 measures the transmission of the direct solar beam in 14
spectral channels (380 to 1558 nm). Azimuth and elevation motors
controlled by a differential sun sensor rotate a tracking head to lock
on to the solar beam and keep detectors normal to it. The tracking head
mounts outside the aircraft skin, to minimize blockage by aircraft
structures and to avoid data contamination by aircraft-window effects.
Each channel consists of a baffled entrance path, interference filter,
photodiode detector, and integral preamplifier. The
filter/detector/preamp sets are temperature controlled to avoid
thermally induced calibration changes. Data are digitized and recorded
by a self-contained data acquisition and control system. Radiometric
calibration is determined via Langley plots (Schmid and Wehrli, 1995),
either at high-mountain observatories or during specially designed
flights. The first science flights of AATS-14 were made on the Pelican
during the Tropospheric Aerosol Radiative Forcing Observational
Experiment (TARFOX) in July 1996 (Russell et al., 1999a and 1999b).
AATS-14 is an extended version of the 1985-built AATS-6 instrument
(Matsumoto et al., 1987). After having been operated on a variety of
aircraft or at land-based sites during all previous missions, AATS-6 was
operated on board a ship -- the R/V Vodyanitskiy - during ACE-2. These
results are presented by Livingston et al. (2000).

Our methods for data reduction, calibration, and error analysis have
been described previously (Russell et al., 1993, Schmid and Wehrli,
1995, Schmid et al., 1996, 1997 and 1998). A very brief summary is given
here: The AATS-14 channels are chosen to allow separation of aerosol,
water vapor, and ozone transmission. From these slant-path transmissions
we retrieve spectral aerosol optical depth (AOD) in 13 narrow wavelength
bands and the columnar amounts of water vapor (CWV) and ozone. For the
results shown here the ozone retrieval was turned off and the total
column ozone values were taken from the Total Ozone Monitor Sensor
(TOMS) on the Earth Probe satellite. In addition to the usual
corrections for Rayleigh scattering, O~3~ and NO~2~ absorption, some
channels required corrections for H~2~O and O~2~-O~2~ (Michalsky et al.,
1999) absorption.

During ACE-2, AATS-14 acquired data of good quality during 14 of 21
flights. Maximum (but constant) altitude flights during sunset allowed
us to perform three successful in-flight Langley-plot calibrations of
AATS-14. In addition, two months before ACE-2 a calibration consisting
of six Langley plots had been performed at Mauna Loa Observatory in
Hawaii. The averaged calibration constants obtained at Mauna Loa and
during ACE-2 differed by 0.2 to 1.6% depending on wavelength. The
uncertainty of the retrieved AOD due to uncertainties in calibration,
signal measurement, airmass computation, and corrections of molecular
scattering and absorption, is computed according to Russell et al.
(1993) and is smaller than 0.01 for the data presented here (see also
Schmid et al., 1999). The uncertainty in CWV is computed according to
Schmid et al. (1996) and is smaller than 0.2 g/cm^2^.

The Pelican was able to fly as low as 20 m above the ocean surface, thus
allowing measurement of the entire overlying atmospheric column. Having
the Pelican fly narrow up or down spirals allowed us to retrieve
vertical profiles of spectral AOD and CWV. Differentiation of those
profiles leads to spectral aerosol extinction and water vapor density
profiles. In the AATS-14 vertical profiles shown in this paper,
occasionally the AOD or the CWV decreased (increased) when the plane
descended (ascended). This is of course non-physical. However, this is a
natural consequence of the facts that (1) the sunphotometer can only
measure the transmittance of the sunphotometer-to-sun path, (2) that
path in general passes through a horizontally inhomogeneous,
time-varying atmosphere, and (3) the path and the atmosphere move with
respect to each other as the plane moves and the wind blows. Before the
sunphotometer AOD or the CWV profile is vertically differentiated to
obtain extinction or H~2~O density, it needs to be smoothed (in a
non-biased manner) to eliminate increases in AOD or CWV with height. In
this study, smoothed spline fits have been used for this purpose.
However, to avoid over-smoothing we occasionally allow the extinction or
the H~2~O density to become slightly negative as it can be seen in
Figure 2 and Figure 4. The need for smoothing of AOD profiles prior to
differentiation has been recognized by others and varying
implementations have been chosen (Clarke et al., 1996; Hartley et al.,
1999). It is noteworthy that the errors in AOD or CWV cancel out when
the profiles are differentiated. We therefore estimate the error in
aerosol extinction from the scatter in the AOD profile (7% in the MBL,
4% in the dust) plus an uncertainty caused by the splining procedure
(0.005 km^-1^).

Finally, we estimate aerosol size distributions by inverting AOD or
extinction spectra using the constrained linear inversion method of King
et al. (1978) \[see also King (1982); Gonzàlez Jorge and Ogren (1996)\].
In its present formulation, the inversion assumes an aerosol consisting
of homogeneous spherical particles which are nondispersive (i.e.
refractive index independent of wavelength or size) over the wavelength
range of the observations.

#### 2.1.3. In-situ aerosol size distributions

A differential mobility analyzer (DMA) and two optical particle counters
(OPC) on-board the Pelican provided continuous composite aerosol size
distributions for diameters between 0.005 and 8 µm. The DMA system
mounted in the nose of the Pelican was the Caltech Automated Classified
Aerosol Detector (ACAD) (see Collins et al., 2000). The OPCs mounted on
the wings of the Pelican were a Passive Cavity Aerosol Spectrometer
Probe (PCASP-100X) and a Forward Scattering Spectrometer Probe
(FSSP-100) manufactured by PMS, Boulder, CO. The size range covered by
each instrument was 0.005 to \~0.2 µm [^1] for the ACAD, 0.15 to \~3 µm
for the PCASP and 0.5 to \~8 µm for the FSSP.

Unfortunately, the Pelican FSSP was not operational during the two
flights discussed here. Therefore, we extrapolated the PCASP size
distribution by using the shape of the distribution expected based on
FSSP measurements under similar conditions. For the marine boundary
layer aerosol, the shape was taken from Pelican FSSP measurements during
flights tf1 to tf12. Fortunately, the shape changed only moderately
throughout these flights. For the dust aerosol, the shape was taken from
FSSP measurements taken aboard the Merlin aircraft on July 8, 1997
(Brenguier et al., 2000). For details of this extrapolation procedure,
see Collins et al. (2000).

We have combined the measurements of ACAD, PCASP and FSSP into a
consistent data set of time resolved size distributions of the *ambient*
aerosol. The details of this procedure are described by Collins et al.
(2000). An important step is correcting the PCASP sizing for the
difference of the complex refractive index *m* of the polystyrene latex
spheres (*m*=1.56 --0*i*) used for calibration versus that of the
actually measured aerosol. (Note that the PCASP inlet is designed to
minimize inertial particle losses, thereby enabling penetration and
detection of particles exceeding 3μm). The other important issue is the
potential growth or shrinkage of a hygroscopic particle if measured
under non-ambient conditions (i.e. RH). The FSSP measures the aerosol
particles at ambient conditions, and therefore no correction is
necessary here. The sample stream reaching the ACAD was in general
several degrees °C above ambient, resulting in reduced RH, and
consequently partial evaporation of some particles. Also for the PCASP
(even when the de-icing heaters remained off during ACE-2) combined ram
and sheath air heating caused a heating of ∼3.5°C (equivalent to an RH
decrease of 15-20%). Adjusting for the differences in refractive index
and RH requires knowledge of the chemical make-up of the individual
particle, information that was not available at the same temporal and
size resolution as the size measurements. Filter measurements were made
on board the Pelican (Schmeling et al., 2000). However, because these
samples were not size-resolved and often combined contributions from the
free troposphere (FT) and the MBL, they only partly provided the
necessary information. Therefore for each flight, size-resolved aerosol
composition for the MBL and the FT was assumed using ground measurements
(Putaud et al., 2000) made at two sites in Tenerife: Punte del Hidalgo
(located at the northern end of the island at an elevation of 30 m) and
Izaña (in the central region of the island at 2367 m).

Given the aerosol composition the response of the size distribution to
changes in RH and *m* can be deduced. The assumed compositions consist
of sulfates (H~2~SO~4~, NH~4~HSO~4~ and (NH~4~)~2~SO~4~), sea salt,
organic carbon (OC), elemental carbon (EC) and dust. Hygroscopic growth
and *m* of the salts was calculated using published data on solution
thermodynamics (Tang and Munkelwitz, 1994; Tang 1996; Tang et al.,
1997). The remaining components were assumed to be non-hygroscopic. We
assumed *m*=1.96-0.66*i* (Seinfeld and Pandis, 1998) for elemental
carbon, *m*=1.55-0.0*i* (Larson et al., 1988) for organic carbon and
*m*=1.56‑(0.17×10^‑0.0025λ(nm)^)*i* for dust (from Patterson et al.
1977). The complex refractive index of each of the aerosol constituents
is assumed independent of wavelength. The only exception is the
imaginary part of dust, where an equation fitted through the data (300
to 700 nm) measured by Patterson et al. (1977) on Tenerife had been
used. An external mixture of the salts and dust was assumed with most of
the EC and OC assumed to be internally mixed (homogeneously) with the
salt particles. Mie code was then used to calculate extinction (σ~e~),
scattering (σ~s~), and absorption (σ~a~), coefficients. A detailed error
analysis of the resulting ambient aerosol size distributions and of the
derived quantities is given by Collins et al. (2000). In this paper we
will refer to these results as Caltech computations.

#### 2.1.4. Nephelometers

We use aerosol particle scattering coefficients, σ~s~, measured by three
separate integrating nephelometers aboard the Pelican. One of the 3
nephelometers was a TSI 3563 three-wavelength (450, 550 and 700 nm)
integrating nephelometer (Öström and Noone, 2000) operated by the
Meteorological Institute Stockholm University, Sweden (MISU). The sample
air used in this instrument was heated to 30°-40° C. The resulting
sample RH was 35-45% in the MBL and \<10% in the FT. The resulting σ~s~
were smoothed by using a 30 second running average on the 1 Hz data. The
other two nephelometers (Radiance Research, model M903, λ=530 nm) are
part of the University of Washington Passive Humidifier (UW-PH) system
(Gassó et al., 2000). One of them measured σ~s~ at typically 5° C above
ambient conditions (sample RH=50-60% in the MBL, 10-20% in the FT) and
the other had a humidity control that allowed measuring σ~s~ at an RH
higher than ambient (RH=85-95% in the MBL, 50-95% in the FT). The data
were averaged over 6 seconds.

The fact that the three nephelometers measure σ~s~ (at the green common
wavelength) at three different RH allows us to estimate σ~s~ at ambient
RH from a statistical fit of the equation (Gassó et al., 2000).

$\sigma_{s}\left( \text{RH} \right) = \sigma_{s,\text{RH} = 0}\left\lbrack 1 - \frac{\text{RH}}{\text{100}} \right\rbrack^{- \gamma}$
(1)

Beforehand the UW-PH 6-sec σ~s~ data were smoothed to exhibit a similar
smoothness as the MISU 30-sec averages. The resulting γ values were then
used to estimate σ~s,amb~ at the two other wavelengths of the MISU
nephelometer. This assumes γ being wavelength independent. Before
σ~s,amb~ was computed we applied two corrections to the nephelometer
data:

Anderson et al. (1996) investigated the non-idealities in the light
source as well as the truncation of near forward and near backward
scattered light for the TSI 3563. Because no similar data are available
for the Radiance Research nephelometers, their characteristics were
assumed equivalent to the TSI instrument. Mie computations with the
continuous size-distribution results described above and the resulting
nephelometer angular sensitivity led to time resolved correction
factors. The correction of the nephelometer non-idealities increases
σ~s~ by a factor of 1.13-1.2 in the MBL and 1.3-1.5 in the dust.

All three nephelometers sampled aerosol from a common isokinetic inlet
mounted in front of the Pelican's nose. To obtain an upper size cut, the
particles were pulled through a set of cyclone impactors with a nominal
50% cut-off at an aerodynamic diameter *D*~a~ = 2.5µm. According to
Stern (1977) the geometric particle diameter, *D*~g~, is related to
*D*~a~ by

$D_{g} = \frac{D_{a}}{\sqrt{\frac{\rho}{\rho_{0}}}}$ (2)

where ρ is the particle density and  ρ~0~ =1 g/cm^3^. Based on Tang and
Munkelwitz (1994), Tang (1996) and Tang et al. (1997), we estimate
ρ = 1.3 g/cm^3^ for the MBL aerosol at RH = 80%. For the dust we use ρ =
2.65 g/cm^3^ (Tegen and Fung, 1994). Hence, the geometric 50% cut-off
diameter *D*~g~ is approximately 2.2 μm in the MBL and 1.5 μm in the
dust. Mie computations with the continuous Caltech size-distribution
results and an S-curve shaped cyclone penetration efficiency (centered
at the appropriate 50% cut-off *D*~g~) led to time resolved correction
factors. The correction of the nephelometer inlet cut-off effects
increases σ~s~ (λ = 450--700 nm) by a factor of 1.17-1.47 in the MBL and
2.8-3.2 in the dust.

The random error in the UW-PH and the MISU nephelometers was estimated
0.0025 km^-1^ by adjusting the value specified by Gassó et al. (2000)
for 6 sec averages (0.005 km^-1^) to the 30 sec averages, following the
dependency indicated by Anderson et al. (1996). The combined uncertainty
of the angular sensitivity and inlet-cut off correction is assumed to be
30%. The uncertainty in interpolating σ~s,amb~ using equation (1) is
estimated to be \<15% in the MBL (based on Gassó et al., 2000) and
assumed negligible in the dust.

#### 2.1.5. Particle soot / absorption photometer

The aerosol light absorption coefficient, σ~a,dry~, at 565 nm was
measured with a Radiance Research Particle Soot/Absorption Photometer
(PSAP) operated by MISU (Öström and Noone, 2000). The instrument works
on the basic principle of Beer's law in which the change in optical
transmission of a filter caused by particle deposition is related to the
optical absorption coefficient and a calibration transfer coefficient.
The instrument can be operated continuously to get a time series of the
aerosol absorption coefficient. The PSAP σ~a,dry~ data have been
corrected for scattering contributions by subtracting 3% of σ~s,dry~
measured by the MISU nephelometer at the green wavelength (Öström and
Noone, 2000). The particles sampled by the PSAP were pulled through the
same set of cyclones and underwent the same heating as those measured by
the MISU nephelometer. Mie computations with the continuous
size-distribution results led to time resolved correction factors to
account for the inlet cut-off. These corrections increase σ~a,dry~ by a
factor of 1.04-1.19 in the MBL and 3.9-4.9 in the dust. No attempt has
been made to correct the PSAP σ~a,dry~ to ambient RH.

The PSAP data obtained during aircraft ascent or descent are unreliable,
possibly due to the pressure variation causing bending of the filter
membrane. Therefore, we only use PSAP data averaged over horizontal runs
flown at several altitudes immediately before or after the vertical
profiles discussed here. Adding σ~a,dry~ (from PSAP) and σ~s,amb~ (from
the nephelometers) will allow us to compare aerosol extinction σ~e~ with
the values measured by AATS-14 or with the Caltech computational
results. To extrapolate σ~a,dry~ (λ=565 nm) to the nephelometer
wavelengths we assumed σ~a~ ∝ λ^−1^ (see Hartley et al., 1999 and
references therein).

The instrumental uncertainty in σ~a,dry~ is assumed to 25% (Hartley et
al., 1999). The uncertainty of the cut-off correction is assumed to 20%
and errors due to the spatial interpolation are estimated to be \<30%.

#### 2.1.6. Auxiliary data

Temperature, pressure and dewpoint were measured by sensors outside the
Pelican. After a careful end-to-end recalibration, the pressure values
should be correct within 2 hPa. Unfortunately, the dewpoint sensor was
not functional during ACE-2. However, knowledge of the dewpoint, or,
equivalently, ambient RH, is crucial for adjusting in-situ aerosol size
distributions and nephelometer measurements to ambient conditions.
Therefore, the ambient RH is computed as a function of ambient pressure
and temperature, and, pressure, temperature and RH measured at the inlet
of the UW-PH system assuming a constant water vapor mixing ratio. For
comparison with the AATS-14 water vapor retrievals, we then computed the
water vapor density as a function of ambient RH, temperature and
pressure using an expression given by Bögel (1977).

The aircraft position (i.e., geographical latitude and longitude and
altitude above sea level) was determined using two onboard Global
Positioning System (GPS) receivers. After the experiment, better
positioning accuracy was achieved by differentially correcting the
onboard GPS positions by relying on error corrections generated by a GPS
receiver placed at a known station, the north airport of Tenerife. In
order to have accurate time stamps for the data stream generated by each
instrument the clock offsets between the Pelican Payload Data Management
System (PDMS) and the various instrument clocks were recorded during
each flight. The offset between the PDMS clock and UTC was determined
using the onboard GPS receivers.

### 2.2. Space-borne Measurements

We use data from the Advanced Very High Resolution Radiometer (AVHRR)
aboard the polar orbiting NOAA-14 spacecraft. Radiance data of the first
two bands (Channel 1 - 0.63 µm, Channel 2 - 0.86 µm center wavelengths)
of the five channel AVHRR instrument are used in an automated process
based on work developed by Brown (1997) to compute aerosol optical
depth. The procedure includes a check for sun glint, a cloud screening
algorithm, and a linearized single-scatter radiative transfer estimate
of optical depth. Accurate estimates of optical depth from measurements
of backscattered solar radiance require assumptions about the scattering
phase function and single scatter albedo. In addition, estimates of the
surface reflectance, water vapor and ozone absorption and molecular
scattering are required. Seven model aerosol size distributions (based
on Brown, 1997) were created to represent a range of variations of
aerosol in the marine environment. The scattering phase functions and
extinctions for these models were calculated using Mie theory. As
proposed by Durkee et al. (1991) the measured aerosol radiance
differences in each AVHRR pixel (channel 1 and 2) is used to chose the
'best' of the seven phase functions. For this study a non-absorbing
aerosol was assumed. A complete description of AVHRR retrieved AOD
during ACE-2 is given by Durkee et al. (2000).

### 2.3 Ground-based Measurements

We use a vertical AOD profile obtained from a Micro Pulse Lidar (MPL) in
conjunction with a Cimel sunphotometer (Smirnov et al., 1998), both
operated at the Izaña observatory (28.30° N, 16.50° W, 2367 m) on
Tenerife. The MPL operates at a wavelength of 524 nm. The inversion of
the lidar data was constrained so that the integrated extinction profile
returns the AOD (interpolated to 524 nm) observed simultaneously by the
Cimel sunphotometer. This assumes an altitude independent
backscatter-to-extinction ratio. Details of the MPL system, its
calibration, and data processing are discussed by Welton et al. (2000).

In addition, we use spectral AOD measurements at 415, 501, 615, 675 and
868 nm, performed with a Multi-Filter Rotating Shadow-Band Radiometer
(MFR-7, Yankee Environmental Systems, Inc., Turners Falls, MA) near the
summit of "El Pico del Teide" (28.27° N, 16.60° W, 3570 m) (Formenti et
al., 2000).

## Case Studies

### Flight tf15 - 8 July 1997

#### 3.1.1. Situation based on satellite data, AATS-14 measurements, and back trajectories

The satellite images from NOAA-12 for 08:18 UT (Figure 1), METEOSAT for
12:00 UT and NOAA-14 for 15:34 UT show an extensive sheet of
stratocumulus clouds to the north of the Canary Islands. The flight
pattern for Pelican flight tf15, chosen to avoid cloudiness, was
therefore covering the mostly cloud free region southwest of Tenerife.
Here we will focus on a downward spiral flown between 10:19 and 11:03 UT
close to the west coast of Gran Canaria.

The AOD and aerosol extinction profile obtained from the AATS-14
measurements is shown in Figure 2. Below 700 m, the direct solar beam
was occasionally blocked by clouds, leading to some missing AOD values.
A three layer structure can be observed: an MBL within which the
mid-visible AOD rapidly decreases from 0.3 to 0.2, a clean layer where
the total column AOD remains virtually constant up to 2.7 km, and an
implied elevated layer above 2.7 km where the AOD starts to decrease
again. Mid-visible aerosol extinction values derived by differentiating
the AOD profile are close to 0.15 km^‑1^ near the sea surface and 0.065
km^-1^ in the elevated layer. A similar, although less pronounced,
three-layer structure can be seen in the CWV and H~2~O density profiles
shown in Figure 3. The MBL contains around 2/3 of the total CWV. The
clean layer is rather dry, and water vapor density in the elevated layer
exceeds that in the clean layer. The elevated layer extends above the
Pelican's maximum flight altitude. The radio sounding taken at the north
shore of Tenerife at 6:00 UT shows a temperature inversion around 5.7 km
suggesting that the top of the elevated layer was at that altitude.

The five-day back trajectories for this day ending at 12:00 UT on
Tenerife suggest that the air in the lowest part of the MBL originated
over the British Isles and Spain where it had been exposed to pollution
sources. The clean air between the MBL and the elevated layer had its
origin over the Atlantic to the west of Tenerife and had subsided in the
free troposphere for several days. The air at the level of the elevated
layer originated over Africa, where it had resided for several days
before arriving in Tenerife. It is considered to consist of Saharan
dust. This assumption is further supported by the results of the filter
analysis aboard the Pelican aircraft (Schmeling et al., 2000).

#### 3.1.2 Water vapor comparison

In Figure 3 the H~2~O density profile obtained from the differentiated
AATS-14 CWV profile is compared to the in-situ measured H~2~O density
profile. Integrating the in-situ measured densities and adding the CWV
value at the top of the profile (from AATS-14) allows comparison of the
two CWV profiles. With the exception of the lowest altitudes, the CWV
profiles agree within the error bars of the AATS-14 retrieval alone. The
CVW and H~2~O density profiles agree within 0.07 g/cm^2^ and 1.3 g/m^3^
(rms), respectively. The largest differences occur at the bottom of the
profile.

#### 3.1.3 Aerosol extinction and layer AOD closure

The right panel of Figure 4 shows the aerosol extinction profiles at the
green wavelength obtained from the differentiated AATS-14 AOD profile,
from the in-situ measured σ~e,amb~ profile (combining the σ~s~
measurements of the 3 nephelometers and the σ~a~ measurement of the PSAP
instrument as described earlier) and from the Caltech computations.
Integrating the in-situ measured σ~e~ values and adding the AOD value at
the top of the profile (from AATS-14) allows comparison of the AOD
profiles (left panel of Figure 4). At most altitudes, the three AOD
profiles agree within the error bars of the AATS-14 AOD alone. The
extinction profiles agree very closely in the dust layer with somewhat
larger discrepancies in the MBL. The agreement can be described more
quantitatively by considering the absolute and relative differences
between in-situ and AATS-14 σ~e~ profiles along with the combined (rms)
error bars shown in Figure 5. The Caltech computations agree with the
AATS-14 results within the error bars of both techniques, with the
exception of three points near the top of the MBL. In the dust layer,
the Caltech and AATS-14 σ~e~ values agree within 0.005 km^-1^ (rms). In
the MBL the agreement is 0.017 km^-1^. The results from
nephelometer/PSAP and AATS-14 agree within error bars in the dust layer,
but in the MBL the differences are larger than the error bars. In the
dust layer the nephelometer/PSAP and AATS-14 σ~e~ values agree within
0.006 km^-1^ (rms). In the MBL the rms difference is 0.028 km^-1^.

Next, we focus on layer AODs caused by the dust and the MBL aerosols,
but this time we consider the full spectral information available. For
AATS-14, layer AODs are obtained by subtracting the AOD at the top of
the considered layer from the AOD at the bottom of the layer. The
in-situ results are integrated vertically through the same layer. For
the MBL (Figure 6), the largest layer AODs are obtained from the AATS-14
measurements. The Caltech layer spectra is steeper: the AODs are lower
by 0.002 (2%) at 380 nm, 0.008 (10%) at 525 nm, and 0.02 (40%) at 1558
nm. The nephelometer/PSAP spectrum is even steeper than the Caltech
spectrum. The AODs differ from the AATS-14 results by 0.018 (20%) at 450
nm, 0.022 (28%) at 530 nm and 0.028 (44%) at 700 nm. These differences
are larger than the combined error bars. For the dust (Figure 7), the
layer AODs of Caltech and AATS-14 agree almost perfectly at all
wavelengths (3-8%) except at 1558 nm (23%). The nephelometer/PSAP layer
AOD spectrum is again steeper but at all wavelengths, the differences
(3-19%) are within error bars.

### 3.2. Flight tf20 - 17 July 1997

#### 3.2.1. Situation based on satellite data, AATS-14 measurements, and back trajectories

For this day also, the satellite image (Figure 8) from NOAA-14 (15:36
UT) shows an extensive sheet of stratocumulus clouds to the north of the
Canary Islands. The sky was mostly clear to the south of the islands
with some island-induced wake clouds. The flight pattern for Pelican
flight tf20, again chosen to avoid cloudiness, is shown in Figure 9. The
entire flight lasted from 14 to 20 UT. Here we will focus on the down
spiral and the horizontal runs south of Tenerife, and on the down and up
spirals above the Izaña observatory on Tenerife.

The AOD and aerosol extinction profiles obtained from the AATS-14
measurements during the down spiral are shown in Figure 10. Again, a
three layer structure can be observed: an MBL within which the total
column (mid-visible) AOD decreases from 0.37 to 0.32, a clean layer
between 1 and 2 km altitude where the total column AOD remains virtually
constant, and an elevated layer where the AOD decreases from 0.3 to
0.06. The elevated layer must have extended somewhat above the Pelican's
maximum flight altitude, because during ACE-2 the AOD at maximum Pelican
flight altitude was well below 0.01 on clean days. Indeed the lidar
operated at Izaña showed the top of the dust layer to be near 5 km
throughout the whole day. The mid-visible aerosol extinction values
derived by differentiating the AATS-14 AOD profile are close to 0.1
km^-1^ near the sea surface and 0.17 km^-1^ in the elevated layer. The
CWV and H~2~O density profiles are shown in Figure 11. The MBL contains
around 60% of the total CWV. Again, the clean layer is rather dry,
whereas the elevated layer shows enhanced water vapor densities.

The five-day back trajectories ending at 12:00 UT on Tenerife indicate a
marine source for the air in the lowest part of the MBL. The trajectory
suggests that the air remained in the MBL during all five days prior to
arriving at Tenerife. The clean air between the MBL and the elevated
layer originated over the Iberian Peninsula. The air at the level of the
elevated layer had its origin over Africa south of Tenerife. It is
considered to consist of Saharan dust, which is in accord with the
filter measurements of Schmeling et al. (2000).

Neglecting the free-tropospheric plus stratospheric background
(AOD\<0.01, see above), we observe that on July 17 the dust layer has an
AOD of 0.3 (λ=500 nm) versus 0.2 on July 8, and the dust extinction is
larger by a factor of 2.6. However, the extinction in the MBL is smaller
on July 17 than on July 8 probably because of the different origin of
these airmasses. Nevertheless, the AOD values obtained on July 17 at sea
level (the bottom of the spiral shown in Figure 10) are the largest
values measured by AATS-14 throughout the entire ACE-2 campaign.

#### 3.2.2 Water vapor comparison

In Figure 11 the H~2~O density profile obtained from the differentiated
AATS-14 CWV profile is compared to the in-situ measured H~2~O density
profile. The agreement is 1.7 g/cm^3^ (rms), with some larger
differences occurring in the 250-600 m altitude range. With the
exception of the lowest altitudes, the two CWV profiles agree within the
error bars of the AATS-14 retrieval alone. The rms difference is 0.09
g/cm^2^.

#### 3.2.3 Aerosol extinction and layer AOD closure

Figure 12 shows the aerosol extinction and AOD profiles obtained using
the three different methods (nephelometers/PSAP, Caltech computations
and AATS-14). For this case, the disagreement is larger than for the
July 8 case discussed before. The extinction profiles from Caltech and
AATS-14 are still fairly close. The nephelometer/PSAP results are
considerably lower in the dust. The absolute and relative differences
along with the combined error bars are shown in Figure 13. With the
exception of a few points, the Caltech computations agree with the
AATS-14 results throughout the entire profile within the error bars of
both techniques. The agreement between nephelometer/PSAP and AATS-14 is
outside the error bars in the dust layer, but within error bars for most
of the MBL. The rms differences in the MBL are 0.017 km^-1^ (Caltech)
and 0.016 km^-1^ (nephelometer/PSAP). In the dust, the rms differences
between in-situ and AATS-14 σ~e~ values are considerably larger than for
the July 8 case: \~0.04 km^-1^ (rms). This is caused by the especially
large difference at the top of the profile. The largest relative
differences between in-situ and AATS-14 σ~e~ occur at the boundaries of
the three layers, and at the top of the profile. This is probably a
result of the smoothing of the AATS-14 AOD profiles prior to
differentiation, and shows the limitations of retrieving extinction
profiles from airborne sunphotometer measurements.

The layer AOD comparisons shown in Figure 14 and Figure 15 do not
require such smoothing. In the MBL (Figure 14), the layer AODs obtained
with the three techniques agree within the combined error bars. This
time the slopes of Caltech and the AATS-14 spectra are similar, whereas
the nephelometer/PSAP spectrum is again slightly steeper. Compared to
AATS-14, the Caltech AODs are lower by 15% at 380 nm, 17% at 525 nm and
27% at 1558 nm. The nephelometer/PSAP AODs are lower by 13% at 450 nm,
16% at 530 nm and 23% at 700 nm. In the dust (Figure 15), the layer AODs
of Caltech and AATS-14 agree almost perfectly at all wavelengths (2-5%)
except at 1558 nm (18%). The nephelometer/PSAP AODs are lower by 20% at
450 nm, 26% at 530 nm and 38% at 700 nm. Only the result at the shortest
nephelometer wavelength agrees within the error bars. The
nephelometer/PSAP layer AOD spectrum is again steeper.

Part of the observed difference in spectral slope between
nephelometer/PSAP and AATS-14 results might stem from our use of a
wavelength independent value of γ in (2). In theory γ should be
increasing with increasing wavelength, because particle growth will
cause σ~s~(λ) to increase more at longer wavelengths. Experimental
evidence for such an increase was found by Kotchenruther et al. (1999)
and Carrico et al. (2000). Also our assumption of σ~a~ ∝ λ^−1^ can lead
to an error in the spectral slope.

The consistent discrepancy between the AATS-14 and the Caltech dust
spectra at 1558 nm (Figure 7 and Figure 15) might be caused by assuming
that the real part of the refractive index, *m* = *n\' - n''i*, of the
dust aerosol is *n\'* = 1.56 independent of wavelength. Indeed the
Patterson et al. (1977) measurements taken on "El Pico del Teide" on
Tenrife indicate that *n\'* = 1.575 at 440 nm and *n\'* = 1.55 at 650
nm. For a Sahara dust sample taken at Barbados, Volz (1973) obtained
*n\'* = 1.5 at 500 nm and *n\'* =1.45 at 2500 nm. Furthermore, the
spectral behavior of Sahara dust extinction as computed by d'Almeida
(1987) from measured size distributions using wavelength-dependent
values of *n\'* (by combining the *n\'* values from Patterson et al.
(1977) and Volz (1973)) is very similar to the spectral behavior of the
AATS-14 layer AODs shown in Figure 7 and Figure 15.

#### 3.2.4. Aerosol size-distribution closure

Instead of computing σ~e~ from the in-situ size-distribution data and
comparing this with the extinction or AOD obtained from AATS-14, we may
compare size distributions by inverting the AATS-14 extinction spectra.
For that purpose, the Caltech and the AATS-14 extinction values are
averaged over identical 8-minute intervals. In Figure 16 and Figure 17,
we show the area size distributions retrieved in the MBL and the dust.
Table 1 compares the integrated quantities such as number, *N*, surface
area, *S*, volume, *V*, and effective radius, *R*~eff~.

In the MBL (Figure 16) a wavelength-independent refractive index of
*m*=1.4-0.0035*i* (Tanré et al., 1997) has been used in the inversion of
the AATS-14 data. The agreement is within error bars for one third of
the size bins. As shown in Table 1, *S*, *V*, and *R*~eff~ agree almost
perfectly. The AATS-14 size distributions cover only the size range
where the aerosol particles are optically active. Therefore, we cannot
expect the particle number concentration, *N*, to agree. It should be
mentioned that the unusually large value of *N* = 6565 cm^-3^ (Table 1)
measured by Caltech in the MBL is caused by an island aerosol plume.

In the dust layer (Figure 17) the area size distributions look very
similar in shape and magnitude, but the distributions peak at different
diameters. The differences between the distributions exceed the error
bars except where the curves cross. Of the integrated quantities
(Table 1) only *S* agrees within error bars. We attribute the lack of
agreement to the assumption of a nondispersive aerosol (*m =* 1.55 -
0.005*i*) in the inversion of the AATS-14 extinction spectrum (see
discussion in section 3.2.3).

#### 3.2.5 AOD column closure

Figure 18 shows the AOD (λ=0.63 µm) retrieved from AVHRR radiances. In
order to identify areas where the AOD retrieval is contaminated by
clouds Figure 18 can be compared with Figure 8. Enhanced AODs up to 0.5
can be observed along the African coast extending into the Canary
Islands. A strong gradient from areas with dust to areas without dust
can be observed north of the islands. During Pelican flight tf20, an
oval-shaped horizontal run was flown close to the ocean surface within
the box depicted in Figure 18 (see also Figure 9). The low flight
altitude (20-35 m) allows a comparison of the total column AOD as
retrieved by AATS-14 and AVHRR. The results shown in Figure 19 represent
the most common AVHRR AODs within the box (15 by 15 pixels) and the mean
AATS-14 AODs along the Pelican run. The narrow error bars represent the
retrieval errors; the wide error bars are maximum and minimum AODs
within the box or along the Pelican run. At the shorter AVHRR
wavelength, the AODs agree within 0.03, that is within the wide error
bars of both methods. At the longer wavelength the difference is as
large as 0.08, larger than the error bars. This is in accord with the
results of Durkee et al. (2000), where the generally good agreement
between AVHRR and sunphotometer retrieved AOD (see also Livingston et
al., 2000) is reduced by the presence of dust, especially at the longer
AVHRR wavelength. This might be caused by the fact that the AVHRR
retrieval is currently assuming a non-absorbing aerosol, with phase
functions for marine aerosols.

#### 3.2.6. AOD intercomparisons

During flight tf20, the Pelican flew a down and an up spiral along the
vertical path of the Micro Pulse Lidar (MPL) operated simultaneously at
the Izaña observatory on Tenerife. The AOD profile obtained by
vertically integrating the lidar extinction (λ=524 nm) and the AODs
(λ=524.8 nm) obtained by AATS-14 are shown in Figure 20. Also plotted in
Figure 20 are the AOD value of the Cimel sunphotometer at the location
of the MPL and the AOD value measured by the MFR at nearby Teide at 3570
m a.s.l. (for both instruments the AODs are interpolated to λ=524 nm).
The agreement among all instruments is well within 0.02, and for most of
the profile the MPL and the AATS-14 agree within 0.01 or better.

The full AATS-14 AOD spectrum can be compared to the simultaneous
spectral AOD from the MFR on Teide, when the Pelican reaches the
corresponding altitude. The two AOD spectra (Figure 21) agree within
0.005. This level of agreement is remarkable, considering that the AOD
results are obtained by two independent systems, using a different
measurement approach and independent data reduction algorithms.

## Summary and Conclusions

Based on CLEARCOLUMN studies performed on July 8 and July 17, 1997 the
following conclusions can be drawn:

On both days columnar water vapor and water vapor density profiles from
0 to 4 km derived from in-situ RH measurements and solar transmission
measurements in the 0.94 μm band agree within 10% and 30% (rms),
respectively.

On both days closure was achieved for σ~e~ (λ=525 nm) profiles (0-4 km)
computed from in-situ size-distributions and AATS-14 measurements within
the 0.006-0.05 km^-1^ error bars of the measurements and the models.

Layer AOD (λ=380--1060nm) closure based on in-situ size-distributions
and AATS-14 measurements was achieved on both days in the MBL and the
elevated dust layer. In the dust the agreement is almost perfect 3-8%
(July 8) and 2--5% (July 17). On both days the Caltech computations
overpredict the measured dust layer AOD at λ=1558 nm, by \~20%. We
attribute this to the fact that the real part of the refractive index,
*n\'*, was held constant in the Caltech computations, whereas in reality
it might decreases with increasing wavelength (Patterson et al., 1977;
Volz, 1973). However, we are not aware of any measurements of *n\'* that
cover the entire spectral range (380 to 1558 nm) used here. In the MBL,
there is a tendency for the Caltech computations to be slightly lower
than the AATS-14 measurements. The best agreement is achieved at the
shortest wavelength (380 nm) 2% on July 8 and 15 % on July 17 whereas at
1558 nm the Caltech results are lower by 40% (July 8) and 27% (July 17).
These spectral differences might again be caused by assuming a
wavelength independent refractive index in the Caltech computations.
However, at the wavelengths where an error analysis of the Caltech σ~e~
was performed, the differences are within the error bars (40%) of the
Caltech computations alone.

Aerosol size-distribution closure based on in-situ size distributions
and inverted AATS-14 extinction spectra has been achieved in the MBL.
*S*, *V*, and R~eff~ agree within 0.2, 7 and 7%, respectively. In the
dust layer, size-distribution closure could not be achieved. We believe
that this is due to our use of a wavelength-independent refractive index
in the inversion of the AATS-14 spectra. If both the discrepancy at 1558
nm and the lack of closure can be resolved by using a
wavelength-dependent dust refractive index remains to be investigated.

Extinction closure (λ=525 nm) between nephelometer/PSAP and AATS-14 was
achieved on July 8 for altitudes above 1.1 km within the 0.007-0.03
km^-1^ error bars of the measurements and the models. On July 17,
closure could not be achieved for most of the profile.

Layer AOD (λ=450--700nm) closure between nephelometer/PSAP and AATS-14
in the dust layer was achieved on July 8 (∆AOD: +15 to --19%) but not on
July 17 (∆AOD: --19 to --38%). In the MBL closure was achieved on July
17 (∆AOD: -13 to --23%) but not on July 8 (∆AOD: -20 to --44%). The fact
that the nephelometers and the PSAP sampled the aerosol through a
cyclone makes those measurements less useful for the closure study
carried out here. Large corrections (especially in the dust) had to be
applied. Therefore, it is not surprising that closure with AATS-14 was
not always achieved.

The agreement of the sea level AOD as retrieved from AVHRR radiances and
measured by AATS‑14 suffered from the presence of the elevated dust
layer due to the assumptions (i.e. non-absorbing aerosol, phase
functions) made in the AVHRR retrieval algorithm (Durkee et al., 2000).
In the absence of such dust layers, AVHRR and AATS-14 values typically
agree within error bars (Durkee et al., 2000; Livingston et al., 2000).

Comparing vertical AOD profiles obtained by AATS-14 and a ground-based
lidar leads to agreement well within 0.02 at all altitudes (better than
0.01 at most altitudes). Comparing a ground-based sunphotometer (MFR)
AOD spectrum taken on Teide (3570m) with an AATS-14 airborne AOD
spectrum at the same altitude leads to agreement within 0.005.

The aerosol extinction and layer AOD closure achieved in the dust
between the Caltech computations (based on continuous size-distribution
measurements) and AATS-14 (based on continuous airborne AOD measurement)
is noteworthy because several previous attempts to do this were not
successful (Carlson and Caverly, 1977; Fouquart et al., 1987; Clarke et
al., 1996). The extinction and layer AOD closure in the MBL accomplished
in this study has been achieved previously by Clarke et al. (1996)
during the 1992 Atlantic Stratocumulus Transition Experiment (ASTEX).

Various layer AOD closure studies based on airborne nephelometer/PSAP
and airborne or ground-based sunphotometer measurements have been
performed in the past (e.g. Remer et al., 1997; Hegg et al., 1999,
Hartley et al., 1999, and Kato et al., A comparison of the aerosol
optical thickness derived from ground-based and airborne measurements,
submitted to *Journal of Geophysical Research,* 1999). Closure (in-situ
results lower by 12 ± 5%, at 450 nm) was achieved by Hegg et al. (1999),
and Hartley et al. (1999) based on the same data set covering 12 MBL
cases obtained during TARFOX.

It is noteworthy, that extinction or layer AOD closure between in-situ
and sunphotometer measurements has been achieved only in those studies
(Clarke et al., 1996; Hegg et al., 1999; Hartley et al. 1999; Collins et
al., 2000, and this study) where both measurements were taken from the
same airplane. Also it is remarkable that in all the closure studies
mentioned above the in-situ measurements yield σ~e~ or layer AODs that
are almost always lower than those obtained with sunphotometer
measurements. Possible reasons for these differences are at this point
speculative, but there are some phenomena reported in the literature
that might provide at least partial explanations. These phenomena
include (1) possible loss of semi-volatile aerosol material during
direct sampling (e.g., Eatough et al., 1996), and (2) gas absorption
that may be present but is not accounted for in sunphotometer analyses
(e.g., Halthore et al., 1998).

## Acknowledgements

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 US National
Science Foundation, Office of Naval Research, National Oceanic and
Atmospheric Administration, and National Aeronautics and Space
Administration, the Max Planck Society, and the European Commission DG
XII (Environment and Climate). Acknowledgement is given to the AERONET
program for making available CIMEL sunphotometer aerosol optical depth
measurements and to the Ozone Processing Team at NASA Goddard Space
Flight Center for making available TOMS EP data. We also thank Q. Wang
at the Naval Postgraduate School in Monterey, CA, for providing us with
differentially corrected Global Positioning System data for the ACE-2
Pelican flights.

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Table 1: Quantities from integrated size distributions during Pelican
flight tf20 on July 17, 1997.

  ---------------- ------- ------- ---------- ------ ------ ----------- ------------- ------------- --------- ---------- ----------
  Instrument       Time            Altitude          Type   Number      Area          Volume        Density   Mass       R~eff~

                   UT              m                        1/cm^3^     µm^2^/cm^3^   µm^3^/cm^3^   g/cm^3^   µg/m^3^    µm

                                                                                                                         

  Caltech          15:31   15:39   3436       2603   Dust   291         267           159           2.65      422        1.79

  AATS-14          15:31   15:39   3436       2603   Dust   139         234           60            2.65      159        0.77

                                                                                                                         

  **Difference**                                            **110%**    **14%**       **166%**                **166%**   **133%**

  Unc. Caltech                                              23%         25%           27%                     27%        5%

  Unc. AATS-14                                              32%         33%           51%                     51%        18%

                                                                                                                         

  Caltech          15:55   16:03   882        203    MBL    6565        166           32.8          1.30      42.7       0.60

  AATS-14          15:55   16:03   882        203    MBL    450         165           35.4          1.30      46.0       0.64

                                                                                                                         

  **Difference**                                            **1360%**   **0.2%**      **-7%**                 **-7%**    **-7%**

  Unc. Caltech                                              15%         18%           21%                     21%        5%

  Unc. AATS-14                                              31%         33%           50%                     50%        19%
  ---------------- ------- ------- ---------- ------ ------ ----------- ------------- ------------- --------- ---------- ----------

##  List of Illustrations

Figure 1: NOAA-12 AVHRR Channel 1 (0.63µm center wavelength) image of
the Canary Islands, July 8, 1997. The islands are (from left to right):
Hierro, La Palma, Gomera, Tenerife, Gran Canaria, Fuerteventura, and
Lanzarote. The African coastline can be seen in the lower right corner
of the picture.

Figure 2: Aerosol optical depth and extinction profile retrieved from
AATS-14 measurements during Pelican flight tf15 on July 8, 1997. Only 4
of 13 aerosol channels are shown.

Figure 3: Columnar water vapor and water vapor density profile retrieved
from AATS-14 and UW-PH measurements during Pelican flight tf15 on July
8, 1997.

Figure 4: Aerosol optical depth and extinction profile retrieved from
AATS-14 (=525 nm), from size distribution measurements (Caltech OPC,
=525 nm), and by combining scattering and absorption measurements of 3
nephelometers and the PSAP instrument during Pelican flight tf15 on July
8, 1997.

Figure 6: Spectral aerosol optical depth for the MBL (31 -- 1111 m)
during Pelican flight tf15 on July 8, 1997.

Figure 7: Spectral aerosol optical depth for part of the Dust layer
(2722 -- 3852 m) during Pelican flight tf15 on July 8, 1997.

Figure 8: Same as Figure 1 but for NOAA-14 on July 17 1997.

Figure 9: Flight pattern for Pelican flight tf20 on July 17, 1997.

Figure 10: Aerosol optical depth and extinction profile retrieved from
AATS-14 measurements during Pelican flight tf20 on July 17, 1997. Only 4
of 13 aerosol channels are shown.

Figure 11: Columnar water vapor and water vapor density profile
retrieved from AATS-14 and UW-PH measurements during Pelican flight tf20
on July 17, 1997.

Figure 12: Same as Figure 4 but for Pelican flight tf20 on July 17,
1997.

Figure 13: Same as Figure 5 but for the profiles shown in Figure 12.

Figure 14: Spectral aerosol optical depth for the MBL (64-1121m) during
Pelican flight tf20 on July 17, 1997.

Figure 15: Spectral aerosol optical depth for part of the dust layer
(1844--3891 m) during Pelican flight tf20 on July 17, 1997.

Figure 16: Comparison of MBL area size distributions from in-situ
measurements and from inverted AATS-14 spectral extinction measurements
during Pelican flight tf20 on July 17, 1997

Figure 17: Same as Figure 16 but for elevated dust layer.

Figure 18: AOD (=0.63µm) retrieved from AVHRR radiances. Compare with
Figure 8 to localize areas of cloud contamination.

Figure 19: Comparison of AOD as retrieved by AVHRR and AATS-14. For
explanations of error bars see text.

Figure 20: AOD profiles from lidar (=524 nm) and AATS-14 (=524.8 nm)
above Izaña. The lidar profile is constrained to match the Cimel AOD.
Also shown the MFR AOD at nearby Teide at 3570 m a.s.l. (for Cimel and
MFR the AODs are interpolated to =524 nm).

Figure 21: Spectral AOD from airborne AATS-14 on the Pelican aircraft
compared to spectral AOD from ground based MFR on Teide (3570 m), when
Pelican was flying at identical altitude.

![](media/image1.png){width="4.65625in" height="4.677083333333333in"}

Figure 1: NOAA-12 AVHRR Channel 1 (0.63µm center wavelength) image of
the Canary Islands, July 8, 1997. The islands are (from left to right):
Hierro, La Palma, Gomera, Tenerife, Gran Canaria, Fuerteventura, and
Lanzarote. The African coastline can be seen in the lower right corner
of the picture. The dimensions of the image (x×y) are 286×563 km.

![](media/image2.pct){width="6.372916666666667in" height="4.7875in"}

Figure 2: Aerosol optical depth and extinction profile retrieved from
AATS-14 measurements during Pelican flight tf15 on July 8, 1997. Only 4
of 13 aerosol channels are shown. The dashed lines indicate the smoothed
AOD profiles used to derive extinction.

![](media/image2.pct){width="6.372916666666667in" height="4.7875in"}

Figure 3: Columnar water vapor and water vapor density profile retrieved
from AATS-14 and UW-PH measurements during Pelican flight tf15 on July
8, 1997.

![](media/image2.pct){width="6.372916666666667in" height="4.7875in"}

Figure 4: Aerosol optical depth and extinction profile retrieved from
AATS-14 (λ=525 nm), from size distribution measurements (Caltech OPC,
λ=525 nm), and by combining scattering and absorption measurements of 3
nephelometers and the PSAP instrument during Pelican flight tf15 on July
8, 1997.

![](media/image2.pct){width="6.757638888888889in"
height="5.477083333333334in"}

Figure 5: Absolute (top) and relative differences (bottom) of aerosol
extinction at the green wavelength from Caltech computations (left),
from nephelometers and PSAP (right), relative to AATS-14 for the
profiles shown in Figure 4. The dashed lines depict the combined errors
of the compared results.

![](media/image2.pct){width="6.372916666666667in" height="4.7875in"}

Figure 6: Spectral aerosol optical depth for the MBL (31 -- 1111 m)
during Pelican flight tf15 on July 8, 1997.

![](media/image2.pct){width="6.372916666666667in" height="4.7875in"}

Figure 7: Spectral aerosol optical depth for part of the Dust layer
(2722 -- 3852 m) during Pelican flight tf15 on July 8, 1997.

![](media/image3.png){width="5.208333333333333in"
height="5.291666666666667in"}

Figure 8: Same as Figure 1 but for NOAA-14 on July 17 1997.

![](media/image2.pct){width="5.952083333333333in"
height="7.636111111111111in"}

Figure 9: Flight pattern for Pelican flight tf20 on July 17, 1997. The
asterisks denote the geographical location and altitude of the
observation sites at the Izaña observatory and near the summit of "El
Pico del Teide".

![](media/image2.pct){width="6.372916666666667in" height="4.7875in"}

Figure 10: Aerosol optical depth and extinction profile retrieved from
AATS-14 measurements during Pelican flight tf20 on July 17, 1997. Only 4
of 13 aerosol channels are shown. The dashed lines indicate the smoothed
AOD profiles used to derive extinction.

![](media/image2.pct){width="6.372916666666667in" height="4.7875in"}

Figure 11: Columnar water vapor and water vapor density profile
retrieved from AATS-14 and UW-PH measurements during Pelican flight tf20
on July 17, 1997.

![](media/image2.pct){width="6.372916666666667in" height="4.7875in"}

Figure 12: Same as Figure 4 but for Pelican flight tf20 on July 17,
1997.

![](media/image2.pct){width="6.757638888888889in"
height="5.477083333333334in"}

Figure 13: Same as Figure 5 but for the profiles shown in Figure 12.

![](media/image2.pct){width="6.372916666666667in" height="4.7875in"}

Figure 14: Spectral aerosol optical depth for the MBL (64-1121m) during
Pelican flight tf20 on July 17, 1997.

![](media/image2.pct){width="6.372916666666667in" height="4.7875in"}

Figure 15: Spectral aerosol optical depth for part of the dust layer
(1844--3891 m) during Pelican flight tf20 on July 17, 1997.

![](media/image2.pct){width="6.372916666666667in" height="4.7875in"}

Figure 16: Comparison of MBL area size distributions from in-situ
measurements and from inverted AATS-14 spectral extinction measurements
during Pelican flight tf20 on July 17, 1997. Dashed lines indicate
uncertainties of the Caltech results.

![](media/image2.pct){width="6.372916666666667in" height="4.7875in"}

Figure 17: Same as Figure 16 but for elevated dust layer.

![](media/image4.png){width="5.333333333333333in"
height="5.333333333333333in"}

Figure 18: AOD (λ=0.63µm) retrieved from AVHRR radiances. Compare with
Figure 8 to localize areas of cloud contamination.

![](media/image2.pct){width="6.135416666666667in"
height="4.601388888888889in"}

Figure 19: Comparison of AOD as retrieved by AVHRR and AATS‑14. For
explanations of error bars see text.

![](media/image5.wmf){width="6.479166666666667in" height="7.0in"}

Figure 20: AOD profiles from lidar (λ=524 nm) and AATS-14 (λ=524.8 nm)
above Izaña. The lidar profile is constrained to match the Cimel AOD.
Also shown the MFR AOD at nearby Teide at 3570 m a.s.l. (for Cimel and
MFR the AODs are interpolated to λ=524 nm).

![](media/image2.pct){width="6.135416666666667in"
height="4.601388888888889in"}

Figure 21: Spectral AOD from airborne AATS-14 on the Pelican aircraft
compared to spectral AOD from ground based MFR on Teide (3570 m), when
Pelican was flying at identical altitude.

[^1]: All particle sizes in this paper refer to diameters