MAY 2010
KALNAJS AND AVALLONE
869
A Novel Lightweight Low-Power Dual-Beam Ozone Photometer Utilizing
Solid-State Optoelectronics
LARS E. KALNAJS AND LINNEA M. AVALLONE
Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado
(Manuscript received 16 July 2009, in final form 3 December 2009)
ABSTRACT
Recent advances in semiconductor materials and fabrication techniques have allowed the development of
light-emitting diodes (LEDs) with wavelengths extending down into the UV-C region (l , 280 nm). A new
ozone photometer has been developed utilizing these novel light sources. The application of solid-state
technology to the proven dual-beam UV absorption technique has improved instrument performance while
reducing power consumption and weight compared to existing instrumentation. The newly developed instrument is expected to have an accuracy of 1% at surface level pressure, a resolution better than 1 ppb, and
measurement rates up to 1 Hz over the range of ozone mixing ratios encountered from the earth’s surface to
the middle stratosphere. Size, weight, and power consumption have also been significantly reduced, with
a mass of 3 kg and a power consumption of less than 5 W. Initial development is focused on an instrument
suitable for measurements from autonomous platforms and in harsh environments; however, the technology
is highly adaptable to other applications.
1. Introduction
Ozone is one of the most important gases present in
our environment. It is critical to all life as a filter of
harmful solar ultraviolet (UV) light in the stratosphere
and it is a significant component of anthropogenic pollution and a health hazard in the troposphere. Accurate
and reliable measurements of ozone concentrations are
vital to understanding the protective layer of stratospheric ozone, diagnosing its depletion, and monitoring
its subsequent recovery. Ozone is also the benchmark
species for quantifying urban air pollution, and accurate
measurements of ozone levels related to anthropogenic
pollution are critical from both regulatory and health
perspectives.
Although there are several techniques and technologies in existence for the measurement of ozone, there
is no single method that is suitable for all measurement
locations and requirements. In particular, there is no single instrument capable of making high-resolution measurements over a long time scale (months or longer) in
Corresponding author address: Lars E. Kalnajs, Laboratory for
Atmospheric and Space Physics, University of Colorado, 1234 Innovation Dr., Boulder, CO 80303.
E-mail: kalnajs@colorado.edu
DOI: 10.1175/2009JTECHA1362.1
Ó 2010 American Meteorological Society
a lightweight (under 5 kg) and low-power (under 5 W)
form factor. The instrument described herein is designed
to meet the requirements for long-duration autonomous
ozone monitoring on a remote and battery- or renewable energy–powered platform. The intended application is long-duration stratospheric ballooning; however,
the design is equally applicable to other platforms such
as uncrewed aerial vehicles (UAVs) or to ground-based
autonomous measurement stations. Additional design
features allow the instrument to be adaptable to situations that require a response rate as fast as 10 Hz.
a. Background
Because of its highly reactive and unstable nature and
large temporal and spatial variability, ozone presents
a significant measurement challenge. There are three
commonly used techniques for in situ detection and
quantification of atmospheric ozone: detection of chemiluminescent radiation resulting from the reaction of ozone
with nitric oxide (or similar chemiluminescent processes);
measurement of the current produced from the reaction
of ozone with iodide in a liquid potassium iodide (KI)
concentration cell; and absorption of the ultraviolet
mercury-vapor (Hg-vapor) line at 254 nm. Each method
has a particular set of advantages and drawbacks for the
determination of ozone.
870
JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY
Chemiluminescence is typically used when there is a
requirement for high temporal resolution (e.g., Eastman
and Stedman 1977). There are two chemiluminescent
reactions that are used for the determination of ozone:
the reaction between ozone and nitric oxide (NO), which
produces an electronically excited state of NO2 that emits
light in the infrared (830 nm), and the reaction between
ozone and ethylene, which produces photons near 400 nm.
The latter is the U.S. federal reference method for the
determination of ozone (Title 40, CFR Part 50, Appendix D; available online at http://ecfr.gpoaccess.gov).
Although these and other chemiluminescent techniques
offer fast time response and high sensitivity, they have
the significant drawback that they require an expendable and toxic reagent and, in the case of the nitric oxide
technique, a cryogen to cool the infrared detector. In
addition, chemiluminescent techniques do not offer an
absolute measurement of ozone and require careful and
frequent laboratory calibration to obtain accurate results.
Studies of stratospheric ozone levels are generally
performed with the potassium iodide concentration cell,
which is commonly referred to as the electrochemical
concentration cell (ECC; Komhyr 1967), which is similar
in principle to the Brewer–Mast instrument (Brewer and
Milford 1960). The ECC technique introduces ambient
air into a potassium iodide solution that forms half of an
electrochemical cell. The other half of the cell contains
a reference potassium iodide solution of known concentration. Ozone introduced into the sample cell increases
the concentration of free iodine (I2) and induces an
electrical potential between the cells. This potential is
proportional to the amount of ozone introduced to the
sample cell. The primary advantage of the ECC technique is the low weight (;600 g), minimal power consumption (;2 W), and low cost (approximately $600 U.S.
as of 2009) of the instrumentation. It is the technique of
choice for stratospheric soundings because the instrument
can be launched on a small helium balloon and does not
need to be recovered. The drawback to the ECC technique is the use of a liquid reagent that is prone to
freezing and evaporation and the need for premeasurement calibration. Its use is largely limited to atmospheric
soundings with durations of 2–4 h.
The most common technique for routine ozone monitoring, which is the one applied here, is ultraviolet absorption. This technique is a direct application of the
Beer–Lambert law [Eq. (1)], where the ozone number
density n can be determined from the change in transmitted light I/Io, the pathlength l, and the wavelengthspecific ozone absorption cross section s at 254 nm. The
UV light source used in almost all instruments of this type
up to now has been the low-pressure mercury-vapor lamp
that has an atomic emission line at 253.7 nm. The ozone
VOLUME 27
absorption cross section at this wavelength has been extensively researched in laboratory studies and is well
understood at room temperature (Rinsland et al. 2003)
as well as over a significant range of atmospherically relevant temperatures (Malicet et al. 1995; Voigt et al. 2001).
Early implementations of this technique used a single
absorption cell containing the air to be analyzed and a
solar-blind UV detector at the opposite end of the cell to
measure UV transmission (an example of this design is
the 1003-AH by Dasibi Environmental Corporation).
The cell alternately contains sample air and ozone-free
air. In this configuration, the maximum measurement
rate is twice the time taken to switch from ambient to
ozone-free air, which in practice is 20–30 s.
A refinement of this method is the dual-cell technique
that is implemented in the Thermo Environmental Instruments model 49 (TEI 49) and similar instruments.
These instruments use two absorption cells; at any given
instant, one cell contains air scrubbed of ozone and the
other contains ambient air. The cells are illuminated by
the same Hg-vapor lamp and the impact of variations in
lamp intensity (which can be significant) can be minimized by taking the ratio of the intensities measured at
the ends of the two cells. Ozone-free air is generated internal to the instrument from ambient air that is passed
through an ozone-specific scrubber, commonly MnO2.
The use of an ozone-specific scrubber is advantageous
because any species besides ozone that has a significant
absorption cross section at 254 nm is not removed by the
scrubber and will appear in both cells, thus not affecting
the ratio of transmission through the two cells. By alternating which cell receives scrubbed air and which receives
sample air, the impact of variations in detector response
and lamp emission patterns can be minimized. Typically,
the maximum measurement rate is equal to the time required to alternate cells, which in practice is on the order
of 10 s. Dual-cell UV absorption is the most commonly
used technique for environmental and air quality ozone
monitoring and is an approved U.S. federal equivalent
method (Title 40, CFR Part 53) for the determination of
ozone. The dual-cell absorption method is also commonly
used as transfer standard; a detailed discussion of the
sources of error and the relative uncertainty associated
with the method can be found in Viallon et al. (2006).
The main drawbacks to this technique arise from the
use of Hg-vapor lamps. There is a significant power and
weight penalty to thermally stabilize the optical bench
and to generate the high voltages necessary to produce
plasma in the Hg-vapor lamp. Additionally, because of
the nature of the plasma lamp, there can be significant
variability in lamp output both in terms of overall intensity and the emission geometry, which can affect the
ratio between the cells:
MAY 2010
KALNAJS AND AVALLONE
I
5e
Io
snl
.
(1)
b. Motivation and design goals
Although the gamut of distinct requirements for specific ozone measurements has largely been covered by
one or more of the measurement techniques and technologies discussed above, there is a pressing need to
address a subset of these requirements with a single instrument. For many applications, an ideal instrument
would be capable of making fast, accurate, and precise
measurements autonomously under nonideal conditions
for extended periods of time while consuming minimal
power in a compact and lightweight form factor. Only
some of these requirements can currently be met by any
existing instrument at any one time. The motivation to
develop a new UV absorption ozone instrument is to
achieve a larger subset of these design goals and to produce a versatile instrument with operating parameters
that can be reconfigured to suit a variety of applications.
The specific application that this instrument is designed
to meet is long-duration ballooning in the Antarctic
stratosphere. The critical parameters for this application
are low mass, low power consumption, high reliability,
and good precision. Instrument mass directly affects the
size and cost of the balloon as well as the maximum
achievable altitude. Particularly for operations in the wintertime polar stratosphere, power is limited by the amount
of available solar radiation for photovoltaic collectors.
Finally, the instrument must be able to function under the
harsh conditions of the polar stratosphere for periods of
up to six months, necessitating a robust design as well as
redundancy in case of a single component failure.
The chosen design is similar to the dual-cell UV absorption technique of Proffitt and McLaughlin (1983),
which is also used in the widely available TEI 49 ozone
analyzer and subsequent models (Thermo Scientific Corporation, Franklin, Massachusetts). The UV absorption
technique was chosen because it is an absolute measurement that requires infrequent calibration during operation. Significant changes have been made to the basic
design to increase reliability and to decrease weight
and power consumption while maintaining or improving the accuracy, precision, and time response over
existing UV absorption instruments. One of the most
significant changes is replacing the single low-pressure
mercury-vapor UV light source with a pair of solidstate UV light-emitting diodes (LEDs) with the same
center wavelength. The flow path has been redesigned to
improve reliability and provide complete redundancy.
The phototube UV detectors and detector optics have been
redesigned around silicon carbide (SiC) photodiodes to
871
provide greater resolution in a smaller and more robust
form factor. The electronics and control system has been
updated with higher-speed and higher-resolution data
acquisition and field-reconfigurable software-based instrument diagnostics and control.
2. Instrument description
a. Light source
The most novel and significant change to the dualbeam UV absorption ozone photometer described previously (Proffitt and McLaughlin 1983) is the substitution
of two solid-state LEDs in place of the customary Hgvapor lamp. Recent advances in materials science have
facilitated the reliable production of light-emitting and
light-detecting devices in the near-UV part of the spectrum based on wide band gap group III nitride materials
(AlGaN; Anceau et al. 2005). Among these devices are
LEDs with emission wavelengths as short as 250 nm. Two
LEDs with a peak emission at 254 nm are substituted for
the traditional Hg-vapor lamp to illuminate the sample
and reference cells (see Fig. 1). Each LED has an integrated feedback photodiode and thermistor within the
sealed LED package and is a variant of the UVTOP-255
UV LED produced by Sensor Electronic Technology
(Columbia, South Carolina). The feedback diode is operated in photovoltaic mode to give constant monitoring
of light output without the need for external beam splitters
or additional optics. This integrated photodiode provides
a reference value that can be used to correct for variability
in light intensity, which is one of the major limitations
in Hg-vapor lamp–based instruments. The output of the
photodiode is immediately amplified by an ultra-lownoise difet instrumentation amplifier [operational amplifier 129 (OPA129)] enclosed with each UV LED in a
shielded housing. The UV LED package also includes
a UV-transmissive hemispherical lens, which focuses the
output into a beam with a half-maximum width of 58.
This directed output dramatically increases the effective usable photon flux of the light source when compared to the essentially isotropic radiation pattern of
a Hg-vapor lamp.
It is difficult to meaningfully compare the power output of Hg-vapor lamps and UV LEDs because of the
differences in spectral widths and radiation geometries.
A more practical comparison is between the relative output intensities observed at the opposite end of the absorption cell as viewed with an appropriate UV detector,
which is a SiC photodiode in this case. The effective
intensity from a single LED driven with a nominal continuous wave (CW) current of 20 mA is approximately
600 nW mm22 compared to the effective intensity at the
872
JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY
VOLUME 27
FIG. 1. Instrument optical and flow diagram.
detector from a standard Hg-vapor lamp and the optical
assembly from a TEI 49C of 50 nW mm22. Short-term
lamp variability is a major limiting factor for the determination of ozone at a rate faster than the cell switching
rate. The LED has a 1s noise level of 50 ppm at 10 Hz
over a period of 100 s (Fig. 2) compared to ;500 ppm
for a typical Hg-vapor lamp.
The spectral power distribution of four sample LEDs
is shown in Fig. 3. The spectral full-width half-maximums
of the emission peaks range from 10.8 to 11.9 nm, with the
peak emission wavelength between 253.8 and 254.4 nm.
Examination of the absorption cross section of various
gases that are present at significant abundances in the
stratosphere has shown none with an absorption feature
that would fall in the 240–260-nm range without appearing at the Hg emission line at 253.7 nm. Thus, the broader
emission wavelength of the UV LEDs compared to the
narrow Hg-vapor line does not add any significant potential for interference.
It should also be noted that the ozone absorption cross
section is consistently large around 254 nm, changing by
less than 10% between 244 and 264 nm. However, the
absorption cross section needs to be treated somewhat
differently than with the Hg-vapor lamp, which produces
FIG. 2. Normalized UV LED output measured with SiC photodiode at 10 Hz, demonstrating
variations in intensity.
MAY 2010
KALNAJS AND AVALLONE
873
FIG. 3. Normalized UV LED emission spectra for four representative devices. Overlaid curve shows ozone
absorption cross section for comparison.
a narrow bandwidth atomic emission line. The effective
ozone absorption cross section is the integrated product
of the spectral emission of the UV LED with the ozone
absorption cross section as a function of wavelength.
The product of the normalized emission from the four
LEDs in Fig. 3 and the ozone cross section at 293 K
(Burrows et al. 1998) is shown in Fig. 4. It should be
noted that the variability in the integrated effective cross
section for the four different devices falls within 0.5% of
the mean, and thus device-to-device variability is not
a significant source of uncertainty in the effective ozone
cross section.
Because of the large expected variation in operating
temperature for the instrument and the lack of thermal
control, the temperature dependence of the ozone absorption cross section must also be considered. There
are several published empirical models for the temperature dependence of the ozone absorption cross section
(Bass and Paur 1981; Burrows et al. 1998; Voigt et al.
2001). These models strive to capture the variation in the
ozone cross-section continuum from 240 to 790 nm. By
comparing the product of these models and the average
UV LED emission spectrum with the high-resolution
temperature-dependent data of Burrows et al. (1998), it
was determined that a simple linear fit described the
temperature variation of the effective ozone cross section
from 203 to 293 K over the relatively small range of
relevant wavelengths. The linear fit represents the temperature dependence of the integrated effective absorption cross section to better than 0.2% from 203 to 293 K.
This represents a smaller level of uncertainty than that
of the original spectral data used to calculate the temperature dependence, which is estimated to be between
0.7% and 2% (Orphal 2003). The combined uncertainty
from measurements of the ozone absorption cross section, device-to-device variability, and temperature dependence are estimated to be 1%.
The flux from the UV LED is also temperature dependent, with a negative intensity–temperature coefficient, operating with higher efficiency at low temperatures.
Although many commercial UV photometers employ
active temperature control of the light source, it is not
necessary with the UV LEDs, which can significantly
reduce power consumption. Using the integrated feedback photodiode, it is possible to account for the temperature coefficient in data processing or, if desired, to
control the drive current to the LED to maintain a constant output that is independent of temperature. An
additional consideration is the stability of spectral power
distribution of the UV LEDs with temperature variations. Figure 5 demonstrates that the wavelength of peak
spectral power and the full-width half-maximum spectral
874
JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY
VOLUME 27
FIG. 4. Effective ozone absorption cross section for four devices. Effective cross section is the convolution of the
normalized UV LED emission spectrum and room temperature ozone cross section. The numerical values listed give
the effective cross sections at 254.7 nm.
width do not significantly vary for temperatures ranging
from 2408 to 1408C.
A potentially serious limitation of the first generation
of deep-UV LEDs is their relatively short lifetime. The
manufacturer estimates that the useful lifetime of the
UVTOP-255 is between 300 and 500 h at a nominal drive
current of 20 mA. Although it is expected that future
generations of these devices will have significantly longer lifetimes, this limitation has been addressed in the
current instrument design. Unlike Hg-vapor lamps, UV
LEDs can be driven in a pulsed mode at frequencies in
excess of 1 kHz, which increases both the lifetime of the
LEDs and their peak intensity. The drive frequency is
limited by the detector analog-to-digital sampling rate;
however, frequencies of 10 Hz with a duty cycle of 0.05
are possible, which will extend the usable lifetime of the
UV LEDs to between 5000 and 10 000 h.
b. Detectors
The two primary UV detectors in this instrument are
SiC photodiodes operating in photovoltaic mode. A highgain, low-noise transimpedance amplifier is integrated
into the detector housing to minimize analog noise.
Similar to the light source, the photodiodes have an
integrated UV-transmissive lens that increases the effective sensitive area from 1 to 11 mm2 without introducing
additional electrical noise or added parasitic capacitance.
The half-maximum detector field of view is approximately 58, and sensitivity at 254 nm is approximately
0.13 nV nW21 mm22. The high performance amplifier,
radio frequency (RF) shielding, and a low-noise 24-bit
delta–sigma analog-to-digital converter give a total detector electrical noise level of 2 mV (1s at 1 Hz), which
corresponds to a maximum theoretical ozone measurement resolution of 0.1 ppb at 1 Hz. For comparison, the
phototube-based detector system used in the TEI 49
(TEI part 8592) has a maximum theoretical ozone resolution of 1 ppb at 1 Hz, limited by the 100-kHz voltageto-frequency converter.
c. Data acquisition and control system
Data acquisition and instrument control are accomplished with an embedded Advanced RISC Machine
(ARM9) single-board computer (SBC; TS-7260, Technologic Systems). This SBC has sufficient computing power
to run a reduced version of a desktop operating system,
in this case TS-Linux, for maximum software and interface configurability with a minimal footprint and power
MAY 2010
KALNAJS AND AVALLONE
875
FIG. 5. Normalized UV LED emission spectra as a function of operating temperature. Three
curves represent 2408 (top curve), 238 (middle curve), and 408C (bottom curve).
consumption (,0.25 W under standard load). In addition
to using a low-power clock-scalable processor (ARM
EP9302, Cirrus Logic), the TS-7260 SBC can de-energize
unused subsystems [e.g., universal serial bus (USB),
Ethernet, serial] through software control to further
reduce power consumption. The operating code is stored
in 32 MB of onboard flash memory, and additional data
storage and development code are contained on USB flash
memory drives. The user interface is a remote terminal
connected via Ethernet or RS-485 serial connection. The
main board provides a PC-104 bus for communication with
the data acquisition boards (Fig. 6).
Analog signals from the primary detectors and integrated LED-monitoring photodiodes are digitized using
a high-resolution, low-noise 24-bit S–D primary analog-todigital converter (PADC; MPC-624, Micro/Sys Corporation, based on the LT2440 converter, Linear Technologies).
The 24-bit converter has a maximum achievable resolution of 60 parts per trillion (ppt) and a practical total
electrical noise level better than 500 ppt when combined
with the detector system described previously. The S–D
architecture allows for software-definable sampling rate
and resolution. At the highest resolution [24.6 effective
number of bits (ENOB) or 3.9 ppt theoretical ADC resolution], the maximum sample rate is 6.875 Hz, which
yields a maximum ozone measurement rate of 1 Hz in
continuous mode and 0.1 Hz in low-power pulsed
mode. The fastest sampling rate is 3.52 kHz, which
corresponds to 17 ENOB or a resolution of 8 ppm and
a maximum ozone measurement rate of approximately
10 Hz, which is limited by the flow system. The primary
analog-to-digital converter is also designed for lowpower operation, and it can be instructed to enter a lowpower sleep state between measurements at lower data
rates.
Thermodynamic parameters necessary for calculation
of ozone mixing ratio (pressure and temperature) are
measured using solid-state piezoresistive pressure transducers (MPX2102 freescale semiconductor; 1 per cell)
and 2-kV precision thermistors (2 per cell). The specified accuracy of the MPX2102 is 1.5% with a precision
of 0.5%; however, with individual calibration, accuracies
of better than 0.5% can be achieved. The thermistors
were calibrated in a thermal chamber against a National
Institute of Standards and Technology (NIST)–traceable
thermometer (Dostmann P600). After a four-point calibration function is applied, the residual error in determining the air temperature is less than 0.58 over the
range of 2608 to 1608C. The flow through the instrument is determined using a solid-state differential pressure gauge (MPX5010 freescale semiconductor; labeled
‘‘dP’’ in Fig. 1) measuring the pressure drop across a slight
restriction on the instrument outlet. The differential
pressure gauge uses significantly less power (35 mW) than
commercially available thermal or paddle-wheel flow
meters, has no moving parts, and has been shown to work
down to 2408C. The flow measurement using the differential pressure gauges has a resolution of approximately
876
JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY
VOLUME 27
FIG. 6. Instrument electrical block diagram.
20 cm3 min21 over a range of 0–1000 cm3 min21. This
resolution is sufficient for this application, because flow
is a purely diagnostic quantity not directly used in the
calculation of mixing ratio. Analog signals from these
transducers are digitized by a secondary 8-channel 16-bit
analog-to-digital converter (TS-ADC16, Technologic Systems). The 16-bit converter (25-ppm resolution) is sufficient to match the precision of the attached transducers.
A custom-built module provides power regulation
and distribution for the instrument as well as digital
control over the instrument subsystems. Input power
(14–36 VDC) is converted to 5 VDC (CPU, analog system, and transducers) and 12 VDC (LEDs and pumps)
using DC–DC converters. The regulated power from
the DC–DC converters is distributed to the various
subsystems through solid-state relays. These relays are
controlled via digital input/output (I/O) from the computer, allowing individual subsystems to be de-energized
when not in use to reduce power consumption. This
module also provides power and digital control for two
miniature three-way solenoid valves that switch sample
and reference air between cells. These valves are magnetic latching valves (LHLX0500350B, the Lee Co.) that
require current only when switching, which results in an
average power consumption of less than 1 mW. The
wetted material in these valves (silicone and epoxy resin)
is not inert to ozone; however, laboratory tests have
shown that, because of the extremely small wetted area,
there is no measurable ozone loss to the valve surfaces
over a temperature range of 2408 to 408C and flow range
of 10–1000 cm3 min21.
d. Mechanical design
The primary requirements for the first proposed field
deployment of the instrument are low-power consumption and operation at low ambient temperature. Thus,
the flow system for the prototype instrument was designed
to meet these requirements. Pumping is provided by two
constant-volume Teflon reciprocating pumps (Ensci Corporation, Boulder, Colorado) operating in parallel. These
pumps are designed for use on ECC ozonesondes and
have a proven track record for operation in the stratosphere. Laboratory tests have shown that the pumps will
operate reliably and will start from a cold-soaked condition at temperatures down to 2388C and can run
continuously for periods of at least three months. Each
pump provides ;200 cm3 min21 of flow with a power
consumption of 1 W. The pumps are computer controlled
via solid-state relays. Two pumps are included for redundancy as well as to provide higher flow (400 cm3 min21)
for faster time response measurements. Future applications of this instrument could employ a different pumping
system chosen specifically for the given situation. To
achieve the maximum time resolution (10 Hz) of the
instrument, a higher flow rate (;10 L min21) would be
required.
Each optical absorption cell is constructed of a 50-cmlong, 19-mm-diameter Teflon tube, with a 6-mm internal
MAY 2010
877
KALNAJS AND AVALLONE
diameter. These tubes are securely mounted inside an
aluminum channel to provide structural rigidity. The
thick-walled tube is used to accommodate threaded
holes for gas fittings. The internal diameter of the tube
matches the lens size of the detector and UV LED. Both
ends of each cell have been bored out to accept the body
of the detector or UV LED with a silicon O ring, ensuring a gas-tight seal, thus removing the need for UVtransmissive windows. The optical length of the cell is
determined by the distance between the detector and
LED lenses and is measured to be 48.8 cm. The Teflon
cells have matte finish, which minimizes reflections off of
the cell walls. The tubing on the inlet side of the absorption cells is Teflon-lined polyvinyl chloride (PVC).
All components are cleaned with de-ionized water and
methanol and are exposed to high ozone (.1 ppm) to
ensure passivation before use. Wall losses to the cells,
detectors, tubing, and fittings were measured by introducing large concentrations of ozone to the system at
low flow rates (;100 cm3 s21) and comparing the measured ozone mixing ratio at the inlet and outlet of the
instrument using a TEI 49C. For ozone mixing ratios
ranging from 100 to 1000 ppb, the difference between
inlet and outlet mixing ratios was smaller than could be
resolved with the precision of the TEI 49C and stability
of the ozone generator (;3 ppb).
Two ozone scrubbers containing manganese dioxide
screens (MnO2) are employed in the instrument. Because of the low flow rate through the instrument,
these scrubbers can be significantly smaller than the standard scrubbers used in commercial ozone instruments.
Each scrubber contains approximately 50 cm2 of MnO2coated copper screening. Testing has shown that the
scrubbers can catalyze up to 1 ppm of ozone to molecular
oxygen at surface pressure and temperatures down to
2508C. Two scrubbers are used for redundancy against
scrubber fouling or solenoid valve failure. The overall
dimensions of the instrument are 12 cm 3 10 cm 3
58 cm, with a mass of 2.98 kg.
e. Data processing and reduction
Ozone number density is calculated by applying the
Beer–Lambert law to the four primary analog signals
from the detectors and the feedback photodiodes. Depending on the data rate relative to the cell switching
period, the data-reduction methods are slightly different. In all cases, the primary and integrated UV LED–
monitoring detectors are zero calibrated by taking a
measurement for each detector with both UV LEDs off
and then subtracting these voltages from each detector
on subsequent measurements. When the data rate is the
same or lower than the flow switching rate (e.g., a cell
switching period of 10 s and a measurement period of
10 s), I [refer to Eq. (1)] is defined as the average of the
primary detector counts on the sample cell normalized
by the LED feedback detector on the sample cell over
an entire switching period. Similarly, Io is the average of
the primary detector signal on the ozone-scrubbed cell
normalized by the feedback detector on that same cell.
With knowledge of the cell length (l 5 48.8 cm) and the
integrated, temperature-dependent ozone absorption
cross section s over the emission spectrum of the UV
LED as a function of temperature, the number density
can be calculated using Eq. (2). A linear model for the
temperature variation of the ozone cross section (as
described in section 2a) is applied during data processing. Furthermore, the mixing ratio can be derived from
the ozone number density divided by the air density as
determined from the average of the cell pressures and
temperatures and the ideal gas law. For data assurance
or in the case of a component failure, it is possible to
accurately calculate the ozone concentration from each
cell individually at a rate of twice the switching period.
In this case, I and Io are calculated sequentially from
a single cell. However, there is a greater potential from
interference from rapidly varying species that absorb at
254 nm using this technique:
I I
LED
I
Io
5e
snl
.
(2)
oLED
Using the same instrument, it is also possible to calculate ozone mixing ratios at a faster rate than the cell
switching rate for data rates up to 10 Hz. For this type of
measurement, the instrument is operated with a larger
external pump to achieve a sufficiently fast cell flushing
time. Additionally, the UV LEDs are driven in a pulsed
mode at 1 Hz with a 0.9 duty cycle. The 100 ms s21 dark
time is necessary to ensure that the detector baselines
are not drifting relative to each other, because this is not
inherently accounted for by the cell switching. The primary ADC is run at 440 Hz (21.8 ENOB) to gather 10
digital samples per detector per ozone data point. A high
sampling rate is necessary to ensure fast channel switching so that high-frequency variations in light output can
be effectively removed by normalizing the detectors relative to the LED-integrated detectors. There are several
caveats to data rates faster than the cell switching frequency. Although it is possible to account for detector
zero drift, detector span drift can only be accounted for
at the switching period and may influence measured data.
Also, running the LEDs continuously decreases the instrument lifetime by a factor of 10. Finally, the flows required to ensure a sufficient flushing rate for high-speed
878
JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY
VOLUME 27
FIG. 7. Instrument response (asterisk) to ozone step functions generated by TEI 49PS ozone
calibrator (line shows calibrator output). Ozone instrument data have been time shifted to
account for lags resulting from tubing and instrument flushing time.
measurements can introduce turbulence and pressure
fluctuations into the measurement cell.
3. Instrument performance
A prototype instrument was tested in the laboratory
using a TEI 49 primary standard (49PS) ozone calibrator. The calibrator can generate a known ozone mixing
ratio from an ozone-free zero-air source with an accuracy of 3 ppb and a precision and stability of 1 ppb. The
ozonated output from the calibrator was plumbed to the
prototype instrument using Teflon tubing, and an analog
mixing ratio output from the calibrator was logged using
a spare analog channel on the secondary ADC. Both the
TEI 49PS and the UV LED instrument were allowed to
warm up for 30 min prior to the test, and all tubing was
conditioned with high ozone (;300 ppb) to minimize
wall losses. Figure 7 compares the instrument (time shifted
to account for sampling mismatch and line delays) to the
reported mixing ratio from the ozone generator for
ozone set-point steps from 300, 200, 100, and 0 ppb with
a final step up to 500 ppb. Instrument noise level can be
estimated by measuring the variability in the measured
mixing ratio relative to the reported output from the
TEI 49PS for a fixed set point. However, a portion of this
variability may be due to real variations in the ozone
output from the 49PS at either submeasurement time
scales or subprecision magnitudes. Instrument accuracy
was determined by comparing the root-mean-square
(RMS) difference between ozone number density calculated using Eq. (1) (where Io is defined as the 10-s
mean detector signal for the reference periods before
and after a sample period and I is the 10-s mean detector
signal for a sample period) and the reported number
density from the ozone calibrator. The precision and
accuracy for the instrument calculated from these tests
are similar to the specified precision and accuracy for
the TEI 49PS; thus, these represent an upper bound for
these quantities as the variability cannot be attributed to
either the generator or instrument. Table 1 shows noise
and accuracy figures calculated from a longer-duration
comparison to the TEI 49PS as well as the theoretical
values calculated from the propagation of individual measurement uncertainties (described in section 2c) through
the data reduction algorithm. Because of the limitations
that the precision and accuracy of the ozone calibrator
place on the measurement, the theoretical values should
present a truer picture of the precision and accuracy
achievable at standard conditions.
Figure 8 demonstrates an intercomparison between
the UV LED–based instrument and an electrochemical
ozonesonde (Ensci 2Z) during a simulated atmospheric
sounding. The measurements were taken in a thermovacuum chamber with ozone introduced into a Teflon
mixing manifold at partial pressures chosen to be representative of real-world midlatitude ozone profiles. The
MAY 2010
KALNAJS AND AVALLONE
TABLE 1. Instrument noise levels and accuracy based on comparison to TEI 49PS reported mixing ratio at a range of set points
and theoretical accuracy determined by propagating sensor errors
through the data-reduction algorithm.
Calibrator set
point/theoretical
value (ppb)
0.1-Hz std
dev (ppb)
0.1-Hz RMS
accuracy (ppb)
Calculated from
components
0.1-Hz RMS
accuracy (ppb)
0
100
200
300
500
0.8
0.7
0.6
0.7
0.7
1.2
1.8
3.1
4.0
5.1
—
0.9
1.7
2.6
4.3
Measured values
results show a mean absolute error of 0.24-mPa ozone
partial pressure, which increases with altitude. This represents a mean percentage error of 4%. The UV LED
instrument shows a consistent bias toward lower ozone
mixing ratios, which can be partly explained by the slower
response time (20 s as opposed to 2 s for the ECC sonde)
and frequent positive excursions in the ambient ozone
partial pressure. At the altitudes at which the instrument
will operate in the upcoming balloon experiment (50–
80 hPa), the mean discrepancy between the two instru-
879
ments is less than 2%, which is within the estimated
accuracy of the ozonesonde.
In addition to tests of instrument measurement performance, instrument components were subjected to extended temperature tests in a thermal environmental
chamber to assess their suitability for use in the stratosphere. All components were functional for temperatures ranging from 1508 to 2388C. The limiting factor
for low-temperature operation is the Teflon sampling
pump. All electrical and optical components continued
to operate down to 2508C. This extended temperature
range (without supplemental heating) is a major advantage over Hg-vapor lamp–based instruments, which
require heating to 608C for operation. It is also predicted
that operation at lower temperatures will prolong instrument life, because the emission efficiency of the UV
LED is 4 times greater at 2358C than the emission efficiency at room temperature, allowing a smaller drive
current to be used to achieve the same intensity.
4. Conclusions
Advances in semiconductor materials and fabrication,
digital data acquisition, and low-power electronics have
FIG. 8. Comparison between UV LED instrument (asterisk) and Ensci 2Z ECC ozonesonde (line) during chamber
simulation of ozone sounding profile. Inset shows smoothed absolute difference between measurements as a function
of ambient pressure.
880
JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY
allowed the refinement of a proven ozone measurement
technique to provide new capabilities. Initial testing of
UV LEDs with output at a wavelength of 254 nm has
shown them to be a suitable replacement for low-pressure
mercury-vapor lamps. These LEDs provide greater usable intensity without the need for temperature control
and high-voltage power supplies. Additionally, LEDs
provide a more stable light source and can be used in
pulsed mode to further reduce energy consumption relative to Hg-vapor lamps. The combination of LEDs as
a light source with SiC photodiodes and high-resolution,
low-power data acquisition and control electronics facilitated the development of an instrument with measurement performance that exceeds that of conventional UV
photometers at size and power consumption closer to that
of electrochemical concentration cell instruments. The
high stability of the LED light source along with an integrated photodiode for intensity monitoring allows for the
measurement of ozone at rates previously only achievable
with chemiluminescent instruments. This new technology
will be highly applicable to measurements from small autonomous platforms such as UAVs and long-duration
balloons, as well as for high-precision measurements in
harsh remote or space-limited urban environments.
REFERENCES
Anceau, S., P. Lefebvre, T. Suski, L. Konczewicz, H. Hirayama,
and Y. Aoyagi, 2005: Enhancement of localization and confinement effects in quaternary group-III nitride multiquantum wells on SiC substrate. Phys. Status Solidi, 202A,
642–646.
VOLUME 27
Bass, A. M., and R. J. Paur, 1981: UV absorption cross-sections for
ozone—The temperature-dependence. J. Photochem., 17, 141.
Brewer, A. W., and J. R. Milford, 1960: The Oxford-Kew ozone
sonde. Proc. Roy. Soc. London, 256A, 470–495.
Burrows, J. P., A. Dehn, B. Deters, S. Himmelmann, A. Richter,
S. Voigt, and J. Orphal, 1998: Atmospheric remote-sensing
reference data from GOME: Part I. Temperature-dependent
absorption cross-sections of NO2 in the 231-794 nm range.
J. Quant. Spectrosc. Radiat. Transfer, 60, 1025–1031.
Eastman, J. A., and D. H. Stedman, 1977: Fast response sensor for
ozone eddy-correlation flux measurements. Atmos. Environ.,
11, 1209–1211.
Komhyr, W. D., 1967: Nonreactive gas sampling pump. Rev. Sci.
Instrum., 38, 981.
Malicet, J., D. Daumont, J. Charbonnier, C. Parisse, A. Chakir, and
J. Brion, 1995: Ozone UV spectroscopy. II. Absorption crosssections and temperature dependence. J. Atmos. Chem., 21,
263–273.
Orphal, J., 2003: A critical review of the absorption cross-sections
of O3 and NO2 in the ultraviolet and visible. J. Photochem.
Photobiol., 157A, 185–209.
Proffitt, M. H., and R. J. McLaughlin, 1983: Fast-response dualbeam UV-absorption ozone photometer suitable for use on
stratospheric balloons. Rev. Sci. Instrum., 54, 1719–1728.
Rinsland, C. P., and Coauthors, 2003: Spectroscopic parameters for
ozone and its isotopes: Recent measurements, outstanding
issues, and prospects for improvements to HITRAN. J. Quant.
Spectrosc. Radiat. Transfer, 82, 207–218.
Viallon, J., P. Moussay, J. E. Norris, F. R. Guenther, and
R. I. Wielgosz, 2006: A study of systematic biases and measurement uncertainties in ozone mole fraction measurements
with the NIST Standard Reference Photometer. Metrologia, 43,
441–450.
Voigt, S., J. Orphal, K. Bogumil, and J. P. Burrows, 2001: The
temperature dependence (203-293 K) of the absorption cross
sections of O3 in the 230-850 nm region measured by Fouriertransform spectroscopy. J. Photochem. Photobiol., 143A, 1–9.