SPIE Abstract – RAPCD White Paper

Vertical profiling of air pollution at RAPCD
Michael J. Newchurch1a, Kirk A. Fullerb, David A. Bowdleb
Steven Johnsonc, Richard T. Mcnidera Kevin Knuppa, Bill Lapentac,
Noor Gillanib, Arastoo Biazarb,
Global Hydrology and Climate Center
National Space Science and Technology Center, 320 Sparkman Drive NW, Huntsville, AL 35805
a
Atmospheric Science Department, University of Alabama in Huntsville
b
Earth Systems Science Center, University of Alabama in Huntsville
c
Earth Systems Science Division, NASA Marshall Space Flight Center
and John Burris
Atmospheric Chemistry and Dynamics Branch, Goddard Space Flight Center
ABSTRACT
Local and regional pollution interact at the interface between the Planetary Boundary Layer and the Free
Troposphere. The vertical distributions of ozone, aerosols, and winds must be measured with high temporal and
vertical resolution to characterize this interchange and ultimately to accurately forecast ozone and aerosol pollution.
To address this critical issue, the Regional Atmospheric Profiling Center for Discovery (RAPCD) was built and
instrumented in the National Space Science and Technology Center on the UAH campus. The UV DIAL ozone lidar,
Nd:YAG aerosol lidar, and 2-micron Doppler wind lidar, along with balloon-borne ECC ozonesondes, form the core
of the RAPCD instrumentation for studying this problem. Instrumentation in the associated Mobile Integrated
Profiling (MIPS) laboratory includes a 915Mhz profiler, sodar, and ceilometer. The collocated Applied Micro-particle
Optics and Radiometry (AµOR) laboratory hosts the FTIR, MOUDI, and optical particle counter. Using MODELS-3
analysis by colleagues, and cooperative ventures with the co-located National Weather Service Forecasting Office in
Huntsville, AL, we are developing a unique facility for advancing the state-of-the-science in pollution forecasting.
Keywords: atmospheric chemistry, Doppler wind lidar, ozone, trace gases, aerosols, winds, clouds, air pollution,
spectroscopy, ozone forecasting, polarimetry, sensors, photonics.
1. SCIENCE CONTEXT
1.1 Importance of air quality
High concentrations of ozone, particulate matter (PM), and other pollutants affect populations in large parts of the
United States and other countries. These effects include pulmonary distress, asthma, reduced visibility, crop damage,
and degradation of other environmental conditions, with significant socioeconomic impacts. Because of concern over
these deleterious effects, the US government requires state and local governments to develop air pollution mitigation
plans that meet strict air-quality standards, and also imposes significant penalties on areas that violate those standards.
1.2 Multi-scale processes and impacts
With the advent of new National Air-Quality Standards for ozone (0.08 ppb averaged over 8 hours), the physical
linkage between continental, regional, and local ozone and precursor levels has become extremely important. While
most regional transport occurs in the free troposphere (FT) above the planetary boundary layer (PBL), the exchange
between the FT and the PBL provides a mechanism by which regional and local air quality can affect each other. This
FT/PBL interface experiences significant diurnal variation as the PBL builds during the day and collapses at night.
Additional turbulent processes are important in the convective daytime PBL, while nocturnal jets and valley flow
regimes become important in the stable nocturnal boundary layer (NBL). This diurnal behavior is also influenced by
synoptic conditions, local orography, and land surface conditions.
1
mike@nsstc.uah.edu; phone: 256-961-7825; fax 256-961-7755, http://vortex.nsstc.uah.edu/atmchem
As an example of atmospheric ozone variability, Fig. 1 shows tropospheric measurements of ozone mixing ratios
and relative humidity, obtained during 2-hr ozonesonde flights on a weekly basis from 1999 to 2002 over Huntsville,
AL. Most flights occurred during mid-day (~1300 hours local time). Intense vertical variability is evident during each
flight. In particular, ozone concentrations (and relative humidity) often differ markedly between the PBL (1-2 km) and
the overlying FT. Weekly variability is usually much smaller than vertical variability. These weekly snapshots clearly
reveal the gross vertical structure and the broad range of variability in atmospheric ozone, but they cannot diagnose
high-frequency temporal variability. Independent measurements with higher temporal resolution (daily, for 30 days, on
three occasions) show similar variability over shorter time intervals.
While the local FT interacts directly with the PBL,
the regional FT is influenced by continental and even
intercontinental transport of pollutants and precursors
(1-5). A number of extensive field campaigns have
addressed continental inflow and outflow: the New
England Air Quality Study (NEAQS) 2002, Aerosol
Characterization Experiment (ACE) I and II, ACE Asia,
ICARTT 2004, and TRACE-P. These studies show that
trans-Pacific transport of Asian trace gases and aerosols
can sometimes reach as far as North America. These
intercontinental processes affect regional backgrounds,
which then serve as sources for local processes.
1.3 Multi-level scientific & programmatic response
In order to diagnose pollution sources and resulting
pollution levels, the US Environmental Protection
Agency (EPA) has measured ozone and PM levels at
the surface for many years. Although the Weather
Service measures winds, temperature, and humidity
aloft regularly (twice a day) from radiosonde stations
roughly a few hundred kilometers apart, information
about ozone and aerosols (PM) aloft is extremely
sparse. Other than during sporadic field campaigns, the
only ozone measurements above the surface occur only
at four sites in the US and only at weekly intervals (6).
Measurements of aerosols aloft occur sporadically in a
few places in the US.
Figure 1 False color time-height cross-sections from 1999 to
Beginning in the summer of 2004, the National
2002 and from the surface to 17 km MSL, for simultaneous
Weather Service (NWS), in cooperation with EPA and
measurements of ozone and water vapor from balloon-borne
NASA, will be responsible for issuing routine air
ozonesondes over Huntsville, AL. Upper panel: ozone mixing
quality forecasts. With only a surface-measurement
ratio (parts per billion by volume, ppbv), using false color
infrastructure contributing to this important task, the
scale on right side. Lower panel: relative humidity (%), using
outlook is analogous to making weather forecasts from
lower half of false color scale. Triangles on each panel show
surface observations, without upper-air temperature,
sonde launch dates (approximately weekly). Black trace in top
panel and red trace in bottom panel represent height of thermal
pressure, or winds. No weather forecaster could hope
tropopause (separation between troposphere and stratosphere).
to produce accurate forecasts without upper-air
information on flow fields, convergence/divergence
zones, jet-stream characteristics, etc. However, the nation will expect the NWS to make air pollution forecasts with no
initial conditions or boundary conditions on pollution fields beyond a weekly climatology of current or past ozone
amounts or precursor amounts above the surface.
As part of the ICARTT experiment in the summer of 2004, approximately ten ozonesonde stations will participate
in an extended series of coordinated soundings to define the areal and vertical distribution of upper-air ozone. These
stations extend from Canada to Alabama, with many stations on the east coast and one in Boulder, CO. Several
stations were created solely for this field campaign, the first time so many ozonesondes have been flown
simultaneously over such an extended area. The observed ozone fields will guide flight planning for airborne
investigations of gases, aerosols, and meteorological processes that affect pollution outflows from North America.
1.4 Science questions
The primary RAPCD science question is the following: Can we accurately forecast surface ozone and aerosol
concentrations? Figure 2 shows that our current ability to model, much less forecast, ozone amounts is unsatisfactory
at the surface and worsens rapidly with altitude. We group the science issues that address the primary question into the
following categories: 1) Exchange between the Surface, Boundary-Layer, Free Troposphere, Upper Troposphere, and
Lower Stratosphere; 2) Life cycles of non-precipitating cumulus clouds; and 3) Aerosol characteristics and detection.
Figure 2 False-color time-height cross-sections for measured and modeled tropospheric ozone over central Tennessee. Left
panel: daily ozonesonde profiles indicate intense vertical and temporal variability. Right panel: EPA Models-3 regulatory model
captures only half of the variance in the PBL, much less in the free troposphere, and none in the stratosphere.
1.4.1
Surface/PBL/FT/UT/LS Exchange and Mixing
• Can we quantify the effects on local air quality due to vertical and long-range transport processes (PBL transport,
PBL import/export, FT intrusions)?
• Can we quantify the diurnal variations of the vertical exchange processes and the mass & energy fluxes between
the PBL and the FT? Nocturnal transport processes are especially enigmatic.
• Can we model accurately the air-quality impacts of large point sources, such as power plant plumes?
• What are the effects of land surface conditions, such as moisture content, heterogeneity, and heat capacity, on
boundary layer temperature, winds, and PBL heights, and ultimately on air quality?
• Can we quantify the effects on air quality from cloud-induced spatial and temporal variations in photolysis rates?
1.4.2
Cloud Life Cycle
• What are the typical structures of convective roots for non-precipitating or weakly precipitating cumulus clouds?
• How are these convective roots linked to large eddy circulations and systematic convergence zones in the PBL?
• What are the typical life cycles of these structures and their dynamic linkages?
• How do clouds and their convective roots interact with adjacent clouds, and with land surface effects (surface
heterogeneity), including orography and soil moisture?
• What are the effects of cloud-driven circulations on the entrainment of water vapor, aerosols, and trace gases into
cloud bases? What are the effects of the entrained trace materials on cloud properties?
• Do clouds and their convective roots exhibit satellite-observable signatures that could be used to improve cloud
parameterizations (dynamic, microphysical, chemical) in chemical transport models?
• How do cloud processes, including lightning, differ from clear-air processes in terms of their chemical effects?
1.4.3
Aerosol Optics, Radiation, and Microphysics
• What are the composition and optical properties of atmospheric aerosols?
• What are the climatologies, variability, and correlations of the 4-D fields for ozone, aerosols and water vapor?
• What is the character of complex aerosols (mixtures of inorganics, organics, soil dust, and water)?
•
•
•
•
•
•
•
What are the effects of water uptake on aerosol physical, chemical, and optical properties?
What role does heterogeneous chemistry play in local and regional air quality?
What are the urban – rural – regional climate impacts of atmospheric aerosols?
What are the roles of Biogenic Volatile Organic Compounds (BVOC) in ozone and aerosol production?
What is the aerosol information content from spectrometric remote sensing?
What role can spectropolarimetry play in the remote detection and characterization of nonspherical particles?
What optical properties could provide distinctive spectropolarimetric signatures for bioaerosol detection?
1.4.4
Laser beam propagation through the atmosphere
• How well can we predict laser energy propagation between the surface and aircraft or orbital altitude?
• How well can we calculate the degradation in laser beam quality due to atmospheric scintillation?
• How well can we determine the atmospheric effects on laser beam propagation at low elevation angles?
1.5 Science strategy
RAPCD will form the nucleus of a test bed for air quality forecasting in Huntsville, AL. In collaboration with the
National Weather Service (NWS), NASA, the University of Alabama in Huntsville, and other partners in the National
Space Science and Technology Center (NSSTC), we will bring our observational, modeling, and operational
forecasting assets to bear on the goal of air quality forecasting. This goal requires research, and implementation of
research results, in meteorology, gas chemistry, aerosol and cloud physicochemistry, and atmospheric radiation.
1.5.1
Surface/PBL/FT/UT/LS Exchange and Mixing
Current RAPCD instrumentation can address exchange and mixing processes that occur below about 6 km, with
multiple instruments measuring the PBL. We will seek support to extend our altitude range to the lower stratosphere.
We will adopt the conceptual framework of (7, 8) for the synoptic scale and analyze individual cases for processes that
occur on the local scale. Using the UAH LES-Chem model (9) nested within the MM5 assimilating satellite-measured
skin temperatures (10), we will compute the temporal evolution of surface, PBL, and FT meteorological properties.
Comparison of these 4-D modeled fields (in particular, wind, turbulence, aerosols, and H2O) to direct measurements
by the 915Mhz radar, sodar, ceilometer, ozone/temperature/humidity sonde, aerosol lidar, and 2-µm Doppler wind
lidar will diagnose the modeling accuracy. Analyses of regimes with a variety of conditions will delineate those
conditions that are well modeled and those that are poorly modeled. Focusing on events that mix or transport air into
and out of the PBL will elucidate those processes that promote interactions between regional and local air masses.
Occasional field campaigns, including several Doppler wind lidars simultaneously triangulating 3-D winds over the
Huntsville valley, will result in the first experiment to measure the 4-D wind field with multiple Doppler lidars.
1.5.2
Cloud Life Cycle
The goal of the proposed Cloud Life Cycle Experiment is to improve modeling of vertical transport and chemical
process associated with non-precipitating (or shallow precipitating) cumulus clouds. This experiment will focus on the
humid continental boundary layer over Huntsville, Alabama with coordinated remote sensing of clouds, winds,
aerosols, and trace gases for the identification and parameterization of correlated satellite observables. Large Eddy
Simulation (LES) modeling and mesoscale modeling (MM5/Models-3/CMAQ) of meteorology and chemistry will
form the theoretical framework for interpreting the observations. The proposed instrumentation complement comprises
multiple Doppler wind lidars, sodars, and radars; aerosol and ozone lidars; microwave radiometer for water vapor
profiles; a 915 MHz microwave vertical profiler, surface observations; and satellite observations from GOES, AQUA,
AURA, and CALIPSO.
1.5.3
Aerosol Microphysics, Optics, and Radiation
The Applied Microparticle Optics and Radiometry (AµOR) laboratory in RAPCD has its primary focus on
aerosols in humid, polluted, ozone-laden summertime airmasses over the Huntsville basin. The secondary focus is on
the seasonal variation of aerosol properties. This science area will include atmospheric and laboratory measurements
of aerosol optical, radiative, and microphysical properties. Aerosol physical, chemical, and optical properties will be
probed with an optical particle counter, and with FTIR spectroscopy, using a Mueller Matrix Polarimetry Module, an
Infrared Aerosol Analyzer, and a Micro-Orifice Uniform Deposit Impactor (MOUDI). These instruments can sample
laboratory specimens as well as ambient aerosols. The vertical structure and temporal variability of PBL aerosols will
be probed with pulsed lidars at multi-wavelengths: ozone-DIAL lidar (0.285 µm), Nd:YAG aerosol lidar (0.532 µm),
ceilometer (0.9 µm), and Doppler wind lidar (2 µm).
The AµOR program also promotes instrument development for improved measurements of aerosol absorption
spectra, standoff-detection of biological and chemical threats, and ultrahigh-sensitivity fluorescent and Raman
spectroscopy. Empirical and theoretical studies of the optical properties for particles with complex morphologies and
compositions provide an important complement to the above measurements and device development.
1.5.4
Laser radiative transfer through the atmosphere
Realistic simulations of laser ranging and tracking require accurate characterization of the absorption, scattering,
and scintillation properties of the atmosphere between the laser source and the tracked object, such as an aircraft or
satellite. Atmospheric extinction is a strong function of aerosol optical depth, intervening absorbing gases, temperature
fluctuations, and beam geometry. The lidars and spectrometers in the RAPCD laboratory are able to characterize the
tropospheric aerosol, ozone, and dynamic or refractive turbulence, for accurate simulations of laser beam propagation.
2. SCIENCE RESOURCES
2.1 Location
2.1.1
Geophysical
RAPCD is located in the National Space Science
and Technology Center in Huntsville, AL. This
northern Alabama location is exposed to a wide variety
of meteorological and air chemistry conditions that
range from cold, dry, clean air from Canada to hot,
moist, clean air from the Gulf of Mexico. This location
also experiences pollution inflows from the west, north,
and east. Regional power plants emit significant
amounts of NOx in their plumes that travel hundreds of
kilometers before dispersing in the PBL or FT. Natural
Figure 3 Annual average atmospheric aerosol extinction
and anthropogenic sources of Volatile Organic
coefficient (10-6 m-1) at surface. From: National Park Service.
Compounds (VOCs) are also plentiful, providing an
ideal natural laboratory to study air chemistry. Figure 3
maps the surface climatology of atmospheric aerosols, in terms of the annual average extinction coefficient at various
National Parks. This climatology clearly shows that northern Alabama is located in the area of most concentrated
aerosol optical depths. Water uptake by hydrophilic aerosols plays a significant role in this aerosol distribution.
Additionally the variability of the aerosol concentrations is quite large in this region.
Figure 4 National Space Science and Technology Center
(NSSTC), on campus of University of Alabama in Huntsville.
2.1.2
Organizational
RAPCD is one of several laboratories in the
National Space Science and Technology Center
(NSSTC, Fig. 4), a research organization with its core
facility in Huntsville, Alabama. A partnership between
NASA's Marshall Space Flight Center, Alabama
universities, federal agencies, and industry, the Center
is a laboratory for cutting-edge research in selected
scientific and engineering disciplines. Not only does the
NSSTC enable basic and applied research, it also
fosters the education of the next generation of scientists
and engineers. Undergraduate and graduate students
participate in the cooperative research and experience is
provided for educators. Research ranges from pure
science to technology development - with spacecraft,
sounding rockets, balloons, lidars, and aircraft, as well
as laboratory experiments being used to perform this
research.
Another research laboratory in the NSSTC is the NASA Short-term Prediction Research and Transition (SPoRT)
Center, which seeks to accelerate the infusion of NASA Earth Science Enterprise (ESE) observations, data
assimilation, and modeling research into NWS forecast operations and decision-making at the regional and local level.
These experimental products focus on the regional scale, and emphasize forecast improvements on time scales less
than 24 hr. Numerical weather prediction (NWP) models at the SPoRT Center include the Pennsylvania State
University/ National Center for Atmospheric Research (PSU/NCAR) Mesoscale Model Version 5 (MM5) and the
next-generation Weather Research and Forecast (WRF) model, both of which provide NWS forecasters with
supplemental information on temperature, wind, and precipitation that can be used in forecasting and decision making.
Research scientists at the SPoRT Center are improving the quality of numerical forecasts by assimilating remotely
sensed data from satellites and ground-based radars. The NWP infrastructure, along with expertise in remote sensing
and modeling, facilitates collaboration between RAPCD and SPoRT scientists. Additional information on SPoRT can
be obtained at: http://weather.msfc.nasa.gov/sport/.
2.2 Instruments
Table 1 lists the instruments, measured parameters and responsible scientists for the current RAPCD and MIPS
instruments. These instruments are grouped roughly as follows: (a) aerosol sizing, collection, and generation; (b)
aerosol spectroscopy (c) pulsed lidars; and (d) trace gases and meteorological variables; in order of increasing
background gray scale. Infrastructure is also in place for many other instruments in the laboratory and on the roof.
Table 1 RAPCD and MIPS* Instruments
Instrument
MOUDI 10-stage impactor
Optical particle counter
Aerosol atomizer
VOAG (aerosol generator)
UV-NIR absorption meter
Shadowband radiometer
FTIR spectrometer
Infrared aerosol analyzer
Infrared spectropolarimeter
Aerosol lidar, 0.532 µm
Aerosol ceilometer, 0.906 µm*
Doppler lidar, 2 µm
YAG-pumped DIAL O3 lidar
Ozonesondes
2 kHz Doppler sodar*
915 MHz Doppler radar*
12-ch microwave radiometer*
Radio accoustic sounding syst.*
Measurement
Aerosol collection
Aerosol sizing
Polydisperse test aerosols
Monodisperse calib. aerosols
ID & quantify soot, dust
Direct/diffuse flux, opt. depth
Aerosol composition
Aerosol composition
Aerosol morphology
Aerosol vertical profile
Aerosol vertical profile
Wind, aerosol, 35-m gates
O3, aerosol, vertical profile
O3 vertical profile, 100 m bins
Winds
Precipitation, winds
T, P, RH
T
Range
0.06-18 µm
0.1 to 10 µm
0.1-10 µm
1-20 µm
0.4-0.9 µm
0.4-1.0 µm
2 to 25 µm
2 to 25 µm
3 to 14 µm
0.2 to 5 km
0.1 to 7 km
0.12 to 5 km
50 m gates
0-35 km
50 to 500 m
0.12 to 4 km
0 to 10 km
0.1-1.5 km
Rate
6 hr
NRT
N/A
N/A
NRT
NRT
N/A
NRT
N/A
60 s
1 Hz
7 Hz
RT
wkly
20 s
60 s
600 s
1 Hz
Investigator
Fuller/Bowdle
Fuller/Bowdle
Fuller/Bowdle
Bowdle
Fuller
Fuller/Fix
Fuller/Bowdle
Bowdle/Fuller
Fuller/Bowdle
Newchurch
Knupp
Johnson
Newchurch/Burris
Newchurch
Knupp
Knupp
Knupp
Knupp
2.2.1
Regional Atmospheric Profiling Center for Discovery (RAPCD) Lidar Laboratory
The RAPCD infrastructure includes 1400 square feet of laboratory space on the top floor of the NSSTC annex
with six vibration-isolated optical benches, nine remote-sensing chimneys through the roof, and 1900 square feet of
platform space on the roof . Remote-sensing instrumentation includes four pulsed lidars and a Fourier transform
infrared (FTIR) spectrometer, for measuring vertical profiles of ozone, winds, aerosols, and other trace gases.
Infrastructure also exists to mount a scanning telescope inside a rotating astronomical dome. Figure 5a shows the plan
locations of the nine chimneys that extend from the laboratory through the roof. The four chimneys on the left side are
for zenith viewing only. Three of the chimneys on the right side are designed for scanners or turning mirrors (e.g.,
heliostat or pointing mirror). The elevation schematic in Fig. 5b shows the staggered heights of the chimney tops for
solar tracking, azimuth-elevation pointing, and Doppler wind lidar hemispherical scanner, respectively. The highest
point is the mount for a scanning lidar or DOAS under the anticipated observatory dome.
2.2.2
RAPCD Doppler wind lidar (DWL)
The RAPCD DWL (Fig. 6) is a hemispherically
scanning pulsed lidar, with coherent (heterodyne)
detection. The DWL measures aerosol backscatter at 2
µm wavelength, and radial wind velocity at 1 m/s
resolution, within a 10-km range from the RAPCD
facility. The DWL transmitter is a chromium- and
thulium-doped, yttrium alumninum garnet (Cr:Tm:YAG)
laser; operating at 2.017 µm, in a flashlamp-pumped,
injection-seeded, Q-switched, master oscillator-slave
Figure 5a Roof plan showing four zenith chimneys (left side),
three elevated chimneys (center) and two additional zenith
chimneys (right side). Telescope pier and infrastructure for
enclosing an astronomical dome are also in place (top center).
oscillator configuration. The laser transmitter delivers
50 mJ pulses at 6.7 Hz, with a FWHM pulse length of
700 ns (105-m range), a nominal FWHM pulse width
corresponding to 1m/s velocity, and a typical chirp of
0.5 MHz/µs (~0.5 m/s total). The DWL uses a
monostatic coaxial telescope, with a clear aperture of
0.1 m and a 1/e2 beam diameter of 0.08 m. The
telescope is an off-axis paraboloid, to maximize
Figure 5b Elevation schematic of RAPCD rooftop chimneys,
aperture area and minimize stray light contamination.
and anticipated dome enclosing an existing telescope pier.
This telescope provides near-diffraction-limited beam
quality over focal ranges between __ m and collimated.
The expanded and collimated beam is eye safe at any range.
During atmospheric measurements, the horizontally polarized laser output beam passes through a Brewster plate,
which serves as a transmit/receive isolator. The
linearly polarized beam then traverses a quarter wave
plate imposing a circular polarization. The circularly
polarized beam passes through the telescope, travels
horizontally to a 45o folding mirror, deflects vertically
to an alt-azimuth hemispheric scanner, propagates
through the atmosphere to the desired target, and
scatters back from the target. The backscattered beam
experiences a polarization shift to circular, as well as a
Doppler shift due to target motion along the line of
sight (LOS) relative to the transceiver. The return
beam propagates back through the atmosphere to the
scanner, folding mirror, and telescope. At the quarter
wave plate, the polarization becomes vertical. The
vertically polarized beam reflects off the Brewster
plate, and passes to a heterodyne signal detector,
where it mixes with a frequency-offset local oscillator.
Figure 6 RAPCD pulsed 2µm Doppler wind lidar (MSFC)
The DWL optical bench was hardened for field
deployment, using fiber optic coupling, robust optical mounts, and a robust high power laser head for the master
oscillator. The optical train includes a boresighted HeNe laser and a boresighted video camera, to simplify beam
positioning for distant targets. The electro-optical system is optimized to reduce the minimum range. The Q-switch
acousto-optic loss modulator in the laser resonator cavity, after turning off to initiate the pulse, turns on again at the
optimum time to sharply attenuate the pulse tail. Reflective surfaces near the shared transmit/receive optical path are
optically shielded to minimize stray light contamination from the outgoing pulse. The RF receiver electronics are
configured to minimize saturation and ensure rapid recovery from residual stray light in the outgoing pulse. These
features provide a minimum range of ~150 m, significant chirp suppression, and shot-noise-limited receiver operation.
The DWL is also optimized for stable transmission and reference frequencies. A heterodyne resonance detector
senses the laser cavity length and determines the optimum time to trigger the Q-switch, as the cavity length ramps
through and returns back to resonance (the so-called “ramp-and-fire” method). A small fraction of the outgoing pulse
is split between a direct detection pulse detector and a heterodyne reference detector. The pulse detector serves as a
timing reference for the transmitted pulse, both by sensing the maximum slope on the rising pulse, and also by sensing
the pulse peak. The reference detector measures the spectral width and central frequency of the transmitted pulse.
These three detectors also use fiber coupling. The RF signal processing electronics samples the reference after the Qswitch, then switches to the signal detector. The RF switch occurs when pulse truncation begins, 650 µs after the pulse
peak. This switching point also marks the half-maximum for the rising signal from a point target at the new minimum
range. The reference detector and the signal detector use the same local oscillator reference for heterodyne detection.
Thus, the difference between the signal frequency and the reference frequency represents the Doppler frequency shift
due to target motion along the LOS, relative to the DWL. The low-velocity approximation to the well-known Doppler
formula gives the relative LOS velocity, ∆V, in terms of the measured frequency shift ∆ν and the wavelength λ, as
follows:
(1)
∆ V = − 0 . 5 λ ∆ν
Equation 1 is valid as long as the local oscillator frequency remains stable for longer the round-trip time between the
transceiver and atmospheric target. This condition is ensured by the short round-trip times, the inherent stability of the
local oscillator, and the presence of Faraday isolators in strategic fiber optical paths. A fixed frequency offset (95
MHz) in the local oscillator eliminates any ambiguity in the sign of the measured velocity.
Coherent Technologies, Inc. (CTI, in Lafayette, CO) built the DWL transceiver for NASA MSFC in 1993.
Other vendors supplied the telescope (Schwarz Electro-Optics, Inc.); the hemispheric scanner (DFM, Boulder CO) ;
and the RF signal processor. Performance optimization was implemented in-house.
2.2.3
RAPCD Scanning Aerosol Lidar
The scanning aerosol lidar (Fig. 7) uses elastic backscattering at 0.532-µm to characterize the spatial distribution
and optical properties of atmospheric aerosols between
~0.3 µm and ~1 µm diameter. The Nd:YAG laser
transmits short (~10ns) pulses at 50Hz and 50mJ per
pulse. The laser mounts on the side of a 26cm, f/10,
Cassegrain telescope. The laser beam is emitted
parallel to the telescope after going through a
periscope, so that the effective exit aperture is 41cm
from the center of the telescope. This separation
decreases the chance of near field detector saturation,
and decreases the dynamic range required of the
analog-to-digital conversion system. In the telescope
focal plane, a 3-mm diameter silicon avalanche
photodiode (APD) amplifies the light passing through
an interference filter. The laser power supply and
cooler are mounted in a single box, separate from the
laser/telescope assembly. The telescope-laser system
can scan rapidly through 200 degrees horizontally and
90 degrees vertically, using computer-controlled
Figure 7 RAPCD laboratory west section; with Nd:YAG
aerosol lidar (right rear), hemispheric scanner (left center) for
motors attached to the telescope. The lidar has a
Doppler wind lidar, and optical particle counter (right front).
maximum range of about 6~8km, and a range
resolution of 2.5 meters. Figure 8 shows 5 minutes of
lidar data from the mid-day planetary boundary layer.
2.2.4
Ozone Differential Absorption Lidar (DIAL)
The ozone DIAL technique (Figs. 9, 10) obtains
vertical profiles of atmosphere ozone density. This
technique measures the backscattered signal power from
two simultaneous collocated 4-5mJ ultraviolet light
pulses that are spectrally separated by a few nanometers
and differentially absorbed by ozone. Wavelengths are
Figure 9 False-color time-height cross section of aerosol
backscatter from Nd:YAG lidar, during 5-min observation
of the mid-day planetary boundary layer over Huntsville.
chosen such that λon is absorbed more by the ozone
molecule than λoff. However, the spectral separation is
small enough to minimize any interference due to
spectral differentials in Rayleigh and aerosol scattering.
We generate each UV pulse by frequency doubling the
output of a tunable dye laser, which is pumped by the
second harmonic of a pulsed Nd:YAG laser. The UV
pulses propagate upward into the atmosphere. A
coaligned zenith-viewing telescope collects and
separates the backscattered signals from each pulse. A
Figure 8 Differential Absorption Lidar (DIAL) for measuring
vertical profiles of tropospheric ozone.
Schematic of the OZONE Differential
Absorption Lidar (DIAL)
Measure vertical profiles of
ozone from the earth surface
to the free troposphere using
the difference in atmospheric
absorption at 285 and 291
nm.
YAG laser 1
λ=1064nm
Doubler
Separator
λ=532nm
λ=578nm
Dye laser 1
-On absorption-
Doubler & Separator
λ=582nm
λ=291nm
λ=598nm
λ=1064nm
YAG laser 2
λ=1064nm
Doubler
Separator
λ=532nm
Telescope
λ=1064nm
Telescope
Dye laser 2
-Off absorptionλ=570nm
Doubler & Separator
λ=285nm
Detection & data
acquisition
Mike3/papers/tropoz/aguf98
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10
pair of matched photomultiplier
tubes converts the incident photons
to electrical pulses. Downstream
electronics amplify the pulses. A
discriminator outputs a single pulse
for each input pulse above a preset
noise threshold. A digital counting
board accumulates the discriminator
pulses in successive 1µs bins. Each
temporal bin represents a given
altitude bin, based on the round-trip
time between pulse emission and
signal reception. The next set of UV
pulses repeats the process. The
ozone density in a given altitude bin
is derived from the signal power
ratio in the wavelength channels λon
and λoff. The current system provides
ozone profiles up to an altitude of
approximately 7 km.
AµOR: FTIR
Spectroscopy Suite
Dye lasers, which produce yellow light doubled to UV light at 291 and 285 nm.
A
research-grade ThermoThese UV wavelengths are differentially absorbed by ozone, yielding a
Nicolet
Nexus
870 series Fourier
measurement of the ozone concentration from the backscattered UV signals.
Transform
Infrared
(FTIR)
Spectrometer (Fig. 11) forms the heart of our spectroscopy suite. The spectrometer is highly extensible through
Figure 10 In the tropospheric ozone DIAL system, two Nd:YAG lasers pump two
2.2.5
internal and external accessory modules, and is capable of covering an expanded range from 20–25000 cm-1. The
Nexus 870 includes several advanced features, such as step scan, rapid scan, slow scan, and triggered scan. A multiangular reflectance accessory enables reflectance spectroscopy from near normal incidence to near glancing incidence.
Major modules, such as Gas Chromatography (GC-IR) and FT-Raman, can be added to further extend the capabilities
of this FTIR spectrometer.
2.2.6
AµOR: Mueller Matrix Infrared Spectropolarimeter
The AµOR Mueller Matrix Infrared Spectropolarimeter (Fig. 11) is an external accessory built around the FTIR
spectroscopy suite. This unique module measures the complete Mueller matrix over the spectral range 2.5-14 µm, for
samples in transmission, backscatter, or any other scattering angle. This module acquires 60 polarization-modulated
intensity spectra. Calibration and data reduction procedures extract the Mueller matrix spectrum from these
oversampled data. The derived Mueller matrix spectrum can be further analyzed to provide other spectrally mapped
polarization properties, such as depolarization,
polarizance, retardance, diattenuation, etc.
2.2.7
AµOR: Micro-Orifice Uniform
Deposit Impactor (MOUDI)
The Micro-Orifice Uniform Deposit Impactor
(MOUDI) is a 10-stage cascade impactor that provides
high sampling flow rate, sharp cut-point characteristics,
and low wall loses. The impaction surfaces rotate to
achieve near uniform particle deposition on the
impaction surface. The MOUDI enables aerosol
sampling, size classification, and subsequent chemical
or spectroscopic analysis. This instrument can be used
in roof-top monitoring of local air quality, exhaust
sampling from automobile engines, and a variety of
other environments and applications.
Figure 11 Mueller Matrix Infrared Spectropolarimeter (right
side), with Fourier Transform Infrared (FTIR) spectrometer
(Nicolet Nexus, Model 870, center).
2.2.8
AµOR: Passive Cavity Aerosol Spectrometer
Probe (PCASP) Optical Particle Counter
The PCASP (Fig. ) provides real-time measurements of
aerosol concentration and size distribution with high size
resolution, using 32 size bins for particle diameters between
0.1 and 10.0 µm.
2.2.9
Ozonesondes
The Huntsville UAH/CMDL ozonesonde station is part
of the NOAA/CMDL network of worldwide ozonesonde
sites. It is located in the NSSTC at 35.3N 86.6W and
commenced operation on 20 April 1999. Balloon borne
sondes are launched weekly with additional flights for field
campaigns. We launched daily sondes for the Nashville 99,
TexAQS 2000, and Chesapeake Lighthouse AIRS 2003
validation campaigns. The instrument package includes a KI
ECC ozonesonde instrument and a Vaisala temperaturehumidity sensor. Flights typically extend up to 35 km,
although the humidity measurements are valid only in the
troposphere. Figure 12 displays typical profiles of ozone,
temperature, and humidity.
Figure 12 Vertical profiles obtained from the UAH
ozonesonde. Left panel shows ozone concentration (mPa,
blue curve, bottom axis, temperature (°C, red curve, top
axis), and relative humidity (green curve, bottom axis
10x). Right panel shows ozone mixing ratio (ppbv);
bottom curve refers to bottom axis; top curve refers to
expanded scale on top axis.
2.2.10 Mobile Integrated Profiling System (MIPS)
The MIPS suite of instruments includes a 12-channel microwave radiometer H2O profiler, Doppler sodar, pulsed
scanning Doppler radars, 915Mhz zenith profiler, ceilometer, and pyranometer to produce temperature profile,
moisture profile, winds, turbulence, aerosols, and broadband solar insolation. Fig 13 shows the time-height cross
section of backscatter power from the sodar, 915 MHz profiler and ceilometer. The trio of panels in Fig 14 presents
915 spectral moments over a 2-h period centered near bore passage at 0430 UTC. Ceilometer cloud base and aerosol
backscatter features (regions of maxima and minima) are
superimposed.
Note that the bore passage is
accompanied by an increase in water vapor, and a cloud
field of ~40 min duration (about 30 km in horizontal
extent). The bore passage was accompanied by a
significant enhancement in aerosol backscatter at low
levels. Low-level drying occurred after bore passage,
but a return of water vapor, frontal like in character, was
measured after 0800 by the radiometer. This return flow
was also accompanied by an increase in ceilometer
backscatter. The pair of panels in Fig 15 shows MPR
measurements of temperature (top) and water vapor
density (bottom) for the same period, 0200-1000 UTC.
The dotted line in the temperature panel is an estimate
of cloud base height determined from the temperature
profile. s(From Knupp and Karan 2004.)
Figure 14. Sodar backscatter at low levels indicates
stabilization of the NBL prior to bore arrival at 0430 UTC
(marked by the dashed line). 915 and ceilometer backscatter
reveal an upward motion of the ABL top as the bore
approached. The stable layer remained displaced upwards
above 2 km after bore passage. The ceilometer backscatter also
shows a significant increase in aerosol concentration/size after
Figure 13. Time vs. height section of MIPS measurements of a
bore passage during IHOP on 21 June 2002. The trio of panels
portrays backscatter power from the sodar, 915 MHz profiler,
and lidar ceilometer.
Figure 15. A significant perturbation in water vapor (lower
panel) accompanied the bore. Low level drying occurred after
passage, and then water vapor values rebounded after 0800
UTC. A relatively minor temperature perturbation
accompanied the bore passage.
2.2.11 Satellite-based measurements
Satellite-measured surface temperatures provide improved boundary conditions for surface energy budgets in
MM5 PBL models. Assimilation of these measured skin temperatures significantly improves the predictive accuracy
of mesoscale models (§2.3). Satellite measurements of chemical constituents (ozone, CO, CH4) provide useful
boundary conditions for photochemical models.
RAPCD is not only a user of satellite data; it also provides a national resource for validating satellite-based
measurements of aerosols and gases over heterogeneous land surfaces and complex terrain. The tropospheric ozone
lidar is designed and located to provide validation data to AIRS, OMI, and TES for tropospheric ozone measurements.
Aerosol lidar observations provide data for atmospheric corrections to the ozone measurements and validation for the
satellite-borne aerosol measurements made by MISR and MODIS and anticipated from CALIPSO. The ground-based
RAPCD Doppler wind measurements provide a resource for validating airborne and spaceborne wind measurements.
2.3 Models
The main focus of our research has been the understanding of the role and impact of the physical atmosphere in
air quality, and thereby improving air quality forecasts by improving the representation of the physical atmosphere in
air quality models. For example, photochemical production in the atmosphere is closely tied to boundary layer
temperatures. This sensitivity strongly affects the thermal decomposition of organic nitrates, which frees NO2 for
subsequent and faster photochemical production. Biogenic hydrocarbon emissions and anthropogenic evaporative
emissions are also strongly related to air temperatures. Additionally, the concentration of ozone and precursors is often
tied to the PBL depth, which, in turn, is determined in part by surface heating. To study the impact of fine scale
turbulent mixing on the photochemistry, we have been using Large-Eddy Simulation with Chemistry (LES-Chem
(11)), which explicitly resolves the large-scale eddies in 3-D numerical models of the flow field.
Although the surface heating and cooling rate is related to many factors, such as insolation and albedo, two critical
parameters – surface moisture availability and bulk heat capacity – are highly uncertain in their model specification.
Moisture availability (the combination of soil moisture and plant evapotranspiration) is not a measured quantity, so
modelers have often used this parameter as a tuning parameter. However, the general applicability of this tuning over
large areas is suspect. The other parameter, bulk heat capacity, enters as a direct inverse multiplier of the surface
fluxes in land surface models that predict the surface heating rate, and is thus potentially a very critical parameter in
realistic predictions of surface temperature. A priori specifications of surface heat capacity are complex and highly
uncertain in a model grid that represents different vegetation types, bare soil, surface water, and man-made objects.
Using a special satellite retrieval technique within MM5, we use satellite observations of morning skin tendencies
to infer moisture availability on the model grid scale. In a similar manner, we use satellite observations of evening skin
temperature to retrieve heat capacity. These derived boundary conditions, combined with solar insolation from satellite
observations, have produced a physical atmosphere that is more consistent with observations (12-14). This model
configuration provides significant improvements in the diurnal temperature cycle, and especially in the nocturnal
cooling rate.
We have also used Geostationary satellite observations of cloud placement, cloud top and cloud transmissivity
within CMAQ to correct the estimated photolysis rate, which is a key component of air quality modeling. Clouds can
have considerable impact on photochemistry by modifying the solar insolation in the spectral bands that affect
photolysis rates. Unfortunately, most current models have major problems with estimating the cloud corrections to
photolysis fields. First, model estimation of cloud transmissivity is highly parameterized and therefore introduces a
large uncertainty in cloud radiative properties, such as transmission and reflection. Second, but probably most
important, the cloud information for photochemical models is provided by a mesoscale model, which has difficulty
with the spatial/temporal placement and vertical extent of clouds. Rather than model predicted clouds, our method uses
observed clouds and satellite-retrieved transmissivity to eliminate these parameterizations.
2.4 Collaborations
2.4.1
Intramural
Several earth science research groups within the NSSTC provide opportunities for collaborative science. Relevant
research areas with significant expertise include numerical weather prediction (both fundamental and applied research,
as well as operational forecasting at the National Weather Service Huntsville Office), atmospheric radiative transfer,
land surface measurement and effects (including the Urban Heat Island group), lightning measurement and processes,
and information technology.
2.4.2
Interdisciplinary
Research groups in Huntsville outside of the NSSTC include the Center for Applied Optics at UAH and the Optics
Branch at MSFC. These groups provide expertise in optical design, nanoparticles, laser development, and hardware
fabrication. Very productive collaborations with the Biological and Physical Sciences Group at MSFC have aided
research in photonic sensing and device development. The Environmental Protection Agency and the City of
Huntsville Air Quality Division are collaborating with AµOR in aerosol measurement and analysis.
2.4.3
Extramural
Principal investigators in NSSTC benefit from extensive collaborations at major research institutions such as
Harvard, Cal Tech, Georgia Tech, St. Petersburg State University, Russia, National Center for Atmospheric Research,
GSFC, JPL, LaRC, NOAA Climate Monitoring and Diagnostics Laboratory, and National Severe Storms Laboratory.
3. SCIENCE PROJECTS
3.1 Ongoing
3.1.1
Tropospheric Lidars
The tropospheric ozone DIAL system is currently measuring ozone profiles from approximately 3-7 km with a
resolution of 30 min and 150 m. This system will be used both for satellite validation and tropospheric studies. We
anticipate extending the lower altitude limit to 0.5 km and the upper-altitude limit to the upper troposphere. In
addition, the aerosol lidar is measuring aerosol backscatter from approximately 1-5 km
3.1.2
Huntsville Ozonesonde Station
The ozonesonde station is part of the NOAA/CMDL ozonesonde network. This station has been operating since
1999, making weekly soundings of ozone, water vapor, and temperature over Huntsville. We have also participated in
field campaigns in Nashville, Chesapeake Bay, and Houston, where we obtained daily soundings. We are currently
participating in the INTEX-NA Ozonesonde Network System (IONS) field campaign, which features coordinated
sonde launches from twelve North American stations, in conjunction with aircraft measurements of pollution outflow.
3.1.3
TANdem INtegrating Cavity (TANIC) Absortion Meter (NSF)
A National Science Foundation grant has been obtained to build and test an innovative aerosol absorption meter,
using aerosol filter samples in a tandem integrating cavity. Work is in progress to build a prototype. When complete,
the TANIC will provide very accurate measurements of aerosol absorption, even in the presence of multiple scattering.
3.1.4
Spectropolarimetry of Bioaerosols (DURIP)
A Defense University Research Instrumentation Program (DURIP) grant has been obtained to develop the AµOR
Infrared Spectropolarimeter, to study bioaerosol properties as a precursor to biodetection. This instrument is fully
operational, and experiments are being conducted in an attempt to characterize the polarimetric signatures of
bioaerosols, using polystyrene microspheres and b. Subtilis Var. Niger spores as bioaerosol surrogates. Collaboration
with the Biological and Physical Sciences group (NASA Marshall Space Flight Center, MSFC) allows us to produce
the microspheres with extremely accurate control of particle size. These and other particulate materials are used to
study morphological effects of on aerosol optical properties.
3.1.5
Homeland Security
The Mueller matrix of light scatterered from mineral dust and b. Subtilis spores is being studied to identify a set of
spectropolarimetric signatures that could be used in stand-off detection of airborne pathogens. Lab measurements,
theoretical modeling and analysis of the radiative properties of nonspherical particles such as soot, internally mixed
inhomogeneous particles, dust, ice, and (at longer wavelengths) snow are also in progress.
3.1.6
Air Quality Measurements using MOUDI
Our cascade impactor (MOUDI) is obtaining 24-hr aerosol samples on a weekly basis from the rooftop of the
Huntsville Air Quality Station (EPA AQS). This arrangement allows us to synchronize our sampler with the wellcharacterized AQS instrumentation suite. The MOUDI samples are analyzed for aerosol composition, using our FTIR
Spectrometer. The results of these measurements will be made available to the EPA.
3.1.7
Air Quality Modeling
The UAH modeling team has participated in several major air quality campaigns, including the Southern Oxidant
Study (SOS, 1995) and the Texas Air Quality Study (TexAQS, 2000). During 2001-2003, as part of the EPAsupported Metro EMPACT (Environmental Monitoring for Public Access and Community Tracking) program for
Birmingham, we developed a solid partnership with the Jefferson County Department of Health (JCDH) and the
Alabama Department of Environmental Management (ADEM). These partnerships will be invaluable in performing
and using the daily forecasts in a very timely and effective manner.
3.1.8
Sensor Web Enablement
In much the same way that HTML and HTTP standards enabled the exchange of any type of information on the
Web, the Open GIS Consortium’s (OGC) Sensor Web Enablement (SWE) initiative focuses on developing standards
that enable the discovery and exchange of sensor observations, as well as the tasking of sensor systems. Within the
SWE initiative, the enablement of such sensor webs is being pursued through the establishment of two encodings for
describing sensors (SensorML) and sensor observations (Observations & Measurements), and through three standard
interface definitions for web services (Sensor Observation Service, Sensor Planning Service, and Web Alert Services).
We have a vision of a Sensor Web that provides immediate discovery of sensor assets that meet our specific
needs, that allows easy access to sensor observations in a timely manner, and in some cases, provides the ability to task
online sensors to acquire strategic observations. We envision real-time access to a plethora of sensor data over wireless
connections, with real-time software that co-registers and synchronizes these observations within a 4D environment.
We further envision the enablement of autonomous sensor webs, in which sensors communicate with one another as
well as with humans, and in which both robotic actuators and sensors respond to alerts and observations published by
these sensors. We believe that the OGC SWE framework provides robust but easily adapted components for enabling
significant sensor web capabilities.
More information can be obtained from the following links:
• OpenGIS Consortium: http://www.opengis.org/
• OGC Sensor Web Enablement: http://www.opengis.org/functional/?page=swe
• SensorML: http://vast.uah.edu/SensorML
3.2 Pending
The 2-µm Doppler wind lidar will be operational after selected system upgrades. The flash lamps and selected
fiber optics will be replaced. The hemispheric scanner (DFM, Boulder CO), on loan from Simpson Weather Associates
(SWA, Charlottesville VA), will be installed in the DWL optical chimney on the rooftop of the RAPCD facility.
3.3 Planned
The RAPCD facility was designed to accommodate rooftop scanners, including a solar tracker, a scanning lidar
(gas or aerosol, potentially using depolarization or Raman scattering), and a Differential Absorption Spectrometer
(DOAS). The rooftop platform can also house sky-viewing spectrometers (e.g., Brewer). The AµΟR facility will be
used to develop a Muller matrix lidar, a Tandem Integrating Cavity (TANIC) shortwave spectrometer, single molecule
Raman spectroscopy, and biosensors for ultra-sensitive point detection of chemical and biological agents.
4. SUMMARY AND CONCLUSIONS
The Regional Atmospheric Profiling Center for Discovery, RAPCD, dedicated in 2003, is now hosting research in
profiling atmospheric gases and aerosols as well as laboratory measurements of aerosol characteristics. RAPCD
includes the UAH/CMDL ozonesonde station and enjoys a close relationship with the Mobile Integrated Profiling
System, MIPS. Although not all entirely operational, the comprehensive instrument suite includes the following
tropospheric profiling instruments: balloon borne ozonesonde, ozone UV DIAL lidar, 0.532-micron scanning aerosol
lidar, scanning 2-micron Doppler wind lidar, 0.906 µm ceilometer, 2 kHz Doppler sodar, 915 MHz Doppler radar, 12channel microwave radiometer, and radio acoustic sounding system (RASS). RAPCD laboratory facilities include
FTIR spectrometer with polarimetry, FTIR aerosol analyzer, Optical Particle Counter, MOUDI impactor, shadowband
radiometer, and UV-NIR absorption meter. RAPCD includes a modeling group that uses MM5 and CMAQ for both
meteorological forecasting and air quality assessment and prediction. Cooperation with the co-located National
Weather Service office provides a unique opportunity to apply the profiling capability and modeling expertise to attack
the problem of air quality forecasting. We invite interested collaborators to visit our facility and share these resources.
Acknowledgments
This research was supported in part by the following sponsors: NASA, NOAA, EPA, and DOD. The RAPCD
facility infrastructure was enabled by NASA, State of Alabama, Federal Highway Works Administration, Alabama
Department of Transportation, and UAH. The authors are grateful to Jack Kaye, Clyde Pearson, John Christy, Dave
Brown, and Gerry Grams for their invaluable contributions to the development of this unique facility.
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