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Global Hawk in NASA colors

O3 Photometer - UAS (NOAA)

Ozone (O3) in the lower stratosphere (LS) is responsible for absorbing much of the biologically damaging ultraviolet (UV) radiation from the sunlight, and thus plays a critical role in protecting Earth's environment. By absorbing UV light, O3 heats the surrounding air, leading to the vertical stratification and dynamic stability that define the stratosphere. Manmade halogen compounds, such as CFCs, cause significant damage to the O3 layer in the LS and lead to the formation of the Antarctic ozone hole. Accurate measurement of O3 in the LS is the first step toward understanding and protecting stratospheric O3. The UAS Ozone Photometer was designed specifically for autonomous, precise, and accurate O3 measurements in the upper troposphere and lower stratosphere (UT/LS) onboard the NASA Global Hawk Unmanned Aircraft System (GH UAS). With a data rate of 2 Hz, the instrument can provide high-time-resolution, detailed information for studies of O3 photochemistry, radiation balance, stratosphere-troposphere exchange, and air parcel mixing in the UT/LS. Furthermore, its accurate data are used for satellite validations. The quality of the data produced by the UAS Ozone Photometer, combined with the long range and endurance of the GH UAS, make it particularly valuable for satellite measurement validation. Contacts: Ru-Shan Gao, David Fahey, Troy Thornberry, Laurel Watts, Steve Ciciora

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Unmanned Aerial System Laser Hygrometer

ULH measures water vapor at high accuracy in the upper troposphere and lower stratosphere to meet the following science objectives:

1. validation and scientific collaboration with NASA earth-monitoring satellite missions, principally the Aura satellite, http://aura.gsfc.nasa.gov/

2. observations of stratospheric trace gases in the upper troposphere and lower stratosphere from the mid-latitudes into the tropics,

3. sampling of polar stratospheric air and the break-up fragments of the air that move into the mid-latitudes, The ULH flights on Global Hawk will advance the state of the art technologically with remote command and control. ULH will provide real-time in-situ stratospheric water vapor measurements from Global Hawk. Additionally, ULH will make continuous measurements during long-duration flights up to 33 hours, which would be impossible with manned aircraft.

The advantages of ULH over other hygrometers are:

• Small and lightweight instrument package,
• No outgassing (achieved by mounting the open-path optical cell in the free air stream),
• Fast time response measurements in and out of clouds, without contamination,
• Accurate with a low detection limit <1 ppmv.

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Dual-Beam UV-Absorption Ozone Photometer

The NOAA-O3 instrument consists of a mercury lamp, two sample chambers that can be periodically scrubbed of ozone, and two detectors that measure the 254-nm radiation transmitted through the chamber. The ozone absorption cross-section at this wavelength is accurately known; hence, the ozone number density can be easily calculated. Since the two absorption chambers are identical, virtually continuous measurements of ozone are made by alternating the ambient air sample and ozone scrubbed sample between the two chambers. At a one-second data collection rate, the minimum detectable concentration of ozone (one standard deviation) is 1.5 x 10 10 molecules/cm 3 (0.6ppbv at STP).

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Nuclei-Mode Aerosol Size Spectrometer

The nucleation-mode aerosol size spectrometer (NMASS) measures the concentration of particles as a function of diameter from approximately 4 to 60 nm. A sample flow is continuously extracted from the free stream using a decelerating inlet and is transported to the NMASS. Within the instrument, the sample flow is carried to 5 parallel condensation nucleus counters (CNCs) as shown in Fig. 1. Each CNC is tuned to measure the cumulative concentration of particles larger than certain diameter. The minimum detectable diameters for the 5 CNCs are 4.0, 7.5, 15, 30 and 55 nm, respectively. An inversion algorithm is applied to recover a continuous size distribution in the 4 to 60 nm diameter range.

The NMASS has been proven particularly useful in measurements of nucleation-mode size distribution in environments where concentrations are relatively high and fast instrumental response is required. The instrument has made valuable measurements vicinity of cirrus clouds in the upper troposphere and lower stratosphere (WAM), in the near-field exhaust of flying aircraft (SULFUR 6), in newly created rocket plumes (ACCENT), and in the plumes of coal-fired power plants (SOS ’99). The instrument has flown on 3 different aircraft and operated effectively at altitudes from 50 m to 19 km and ambient temperatures from 35 to -80ºC.

Accuracy. The instrument is calibrated using condensationally generated particles that are singly charged and classified by differential electrical mobility. Absolute counting efficiencies are determined by comparison with an electrometer. Monte carlo simulations of the propagation of uncertainties through the numerical inversion algorithm and comparison with established laboratory techniques are used to establish accuracies for particular size distributions, and may vary for different particle size distributions. A study of uncertainties in aircraft plume measurements demonstrated a combined uncertainty (accuracy and precision) of 38%, 36% and 38% for number, surface and volume, respectively.

Precision. The precision is controlled by particle counting statistics for each channel. If better precision is desired, it is necessary only to accumulate over longer time intervals.

Response Time: Data are recorded with 10 Hz resolution, and the instrument has demonstrated response times of this speed in airborne sampling. However the effective response time depends upon the precision required to detect the change in question. Small changes may require longer times to detect. Plume measurements with high concentrations of nucleation-mode particles may be processed at 10 Hz.

Specifications: Weight is approximately 96 lbs, including an external pump. External dimensions are approximately 15”x16”x32”. Power consumption is 350 W at 28 VDC, including the pump.

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Microwave Temperature Profiler

The Microwave Temperature Profiler (MTP) is a passive microwave radiometer, which measures the natural thermal emission from oxygen molecules in the earth’s atmosphere for a selection of elevation angles between zenith and nadir. The current observing frequencies are 55.51, 56.65 and 58.80 GHz. The measured "brightness temperatures" versus elevation angle are converted to air temperature versus altitude using a quasi-Bayesian statistical retrieval procedure. The MTP has no ITAR restrictions, has export compliance classification number EAR99/NLR. An MTP generally consists of two assemblies: a sensor unit (SU), which receives and detects the signal, and a data unit (DU), which controls the SU and records the data. In addition, on some platforms there may be a third element, a real-time analysis computer (RAC), which analyzes the data to produce temperature profiles and other data products in real time. The SU is connected to the DU with power, control, and data cables. In addition the DU has interfaces to the aircraft navigation data bus and the RAC, if one is present. Navigation data is needed so that information such as altitude, pitch and roll are available. Aircraft altitude is needed to perform retrievals (which are altitude dependent), while pitch and roll are needed for controlling the position of a stepper motor which must drive a scanning mirror to predetermined elevation angles. Generally, the feed horn is nearly normal to the flight direction and the scanning mirror is oriented at 45-degrees with respect to receiving feed horn to allow viewing from near nadir to near zenith. At each viewing position a local oscillator (LO) is sequenced through two or more frequencies. Since a double sideband receiver is used, the LO is generally located near the "valley" between two spectral lines, so that the upper and lower sidebands are located near the spectral line peaks to ensure the maximum absorption. This is especially important at high altitudes where "transparency" corrections become important if the lines are too "thin." Because each frequency has a different effective viewing distance, the MTP is able to "see" to different distances by changing frequency. In addition, because the viewing direction is also varied and because the atmospheric opacity is temperature and pressure dependent, different effective viewing distances are also achieved through scanning in elevation . If the scanning is done so that the applicable altitudes (that is, the effective viewing distance times the sine of the elevation angle) at different frequencies and elevation angles are the same, then inter-frequency calibration can also be done, which improves the quality of the retrieved profiles. For a two-frequency radiometer with 10 elevation angles, each 15-second observing cycle produces a set of 20 brightness temperatures, which are converted by a linear retrieval algorithm to a profile of air temperature versus altitude, T(z). Finally, radiometric calibration is performed using the outside air temperature (OAT) and a heated reference target to determine the instrument gain. However, complete calibration of the system to include "window corrections" and other effects, requires tedious analysis and comparison with radiosondes near the aircraft flight path. This is probably the most important single factor contributing to reliable calibration. For stable MTPs, like that on the DC8, such calibrations appear to be reliable for many years. Such analysis is always performed before MTP data are placed on mission archive computers.

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Meteorological Measurement System

The Meteorological Measurement System (MMS) is a state-of-the-art instrument for measuring accurate, high resolution in situ airborne state parameters (pressure, temperature, turbulence index, and the 3-dimensional wind vector). These key measurements enable our understanding of atmospheric dynamics, chemistry and microphysical processes. The MMS is used to investigate atmospheric mesoscale (gravity and mountain lee waves) and microscale (turbulence) phenomena. An accurate characterization of the turbulence phenomenon is important for the understanding of dynamic processes in the atmosphere, such as the behavior of buoyant plumes within cirrus clouds, diffusions of chemical species within wake vortices generated by jet aircraft, and microphysical processes in breaking gravity waves. Accurate temperature and pressure data are needed to evaluate chemical reaction rates as well as to determine accurate mixing ratios. Accurate wind field data establish a detailed relationship with the various constituents and the measured wind also verifies numerical models used to evaluate air mass origin. Since the MMS provides quality information on atmospheric state variables, MMS data have been extensively used by many investigators to process and interpret the in situ experiments aboard the same aircraft.

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Multiple-Angle Aerosol Spectrometer Probe

The Multiple-Angle Aerosol Spectrometer Probe (MASP) determines the size and concentration of particles from about 0.3 to 20 microns in diameter and the index of refraction for selected sizes. Size is determined by measuring the light intensity scattered by individual particles as they transit a laser beam of 0.780µm wavelength. Light scattered from particles into a cone from 30 to 60 degrees forward and 120 to 150 degrees backwards is reflected by a mangin mirror through a condensing lens to the detectors. A comparison of the signals from the open aperture detector and the masked aperture detector is used to accept only those particles passing through the center of the laser beam. The size of the particle is determined from the total scattered light. The index of refraction of particles can be estimated from the ratio of the forward to back scatter signals. A calibration diode laser is pulsed periodically during flight to ensure proper operation of the electronics. The shrouded inlet minimizes angle of attack effects and maintains isokinetic flow through the sensing volume so that volatilization of particles is eliminated.

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Cloud Physics Lidar

The Cloud Physics Lidar, or CPL, is a backscatter lidar designed to operate simultaneously at 3 wavelengths: 1064, 532, and 355 nm. The purpose of the CPL is to provide multi-wavelength measurements of cirrus, subvisual cirrus, and aerosols with high temporal and spatial resolution. Figure 1 shows the entire CPL package in flight configuration. The CPL utilizes state-of-the-art technology with a high repetition rate, low pulse energy laser and photon-counting detection. Vertical resolution of the CPL measurements is fixed at 30 m; horizontal resolution can vary but is typically about 200 m. The CPL fundamentally measures range-resolved profiles of volume 180-degree backscatter coefficients. From the fundamental measurement, various data products are derived, including: time-height crosssection images; cloud and aerosol layer boundaries; optical depth for clouds, aerosol layers, and planetary boundary layer (PBL); and extinction profiles. The CPL was designed to fly on the NASA ER-2 aircraft but is adaptable to other platforms. Because the ER-2 typically flies at about 65,000 feet (20 km), onboard instruments are above 94% of the earth’s atmosphere, allowing ER-2 instruments to function as spaceborne instrument simulators. The ER-2 provides a unique platform for atmospheric profiling, particularly for active remote sensing instruments such as lidar, because the spatial coverage attainable by the ER-2 permits studies of aerosol properties across wide regions. Lidar profiling from the ER-2 platform is especially valuable because the cloud height structure, up to the limit of signal attenuation, is unambiguously measured.

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Focused Cavity Aerosol Spectrometer

The FCAS II sizes particles in the approximate diameter range from 0.07 mm to 1 mm. Particles are sampled from the free stream with a near isokinetic sampler and are transported to the instrument. They are then passed through a laser beam and the light scattered by individual particles is measured. Particle size is related to the scattered light. The data reduction for the FCAS II takes into account the water which is evaporated from the particle in sampling and the effects of anisokinetic sampling (Jonsson et al., 1995).

The FCAS II and its predecessors have provided accurate aerosol size distribution measurements throughout the evolution of the volcanic cloud produced by the eruption of Mt. Pinatubo. (Wilson et al., 1993). Near co-incidences between FCAS II and SAGE II measurements show good agreement between optical extinctions calculated from FCAS size distributions and extinctions measured by SAGE II.

Accuracy: The instrument has been calibrated with monodisperse aerosol carrying a single charge. The FCAS III and the electrometer agree to within 10%. Sampling errors may increase the uncertainty but a variety of comparisons suggests that total uncertainties in aerosol surface are near 30% (Jonsson, et al., 1995).

Precision: The precision equals 1/ÖN where N is the number of particles counted. In many instances the precision on concentration measurements may reach 7% for 0.1 Hz data. If better precision is desired, it is necessary only to accumulate over longer time intervals.

Response Time: Data are processed at 0.1 Hz. However, the response time depends upon the precision required to detect the change in question. Small changes may require longer times to detect. Plume measurements may be processed with 1 s resolution.

Weight: Approximately 50 lbs.

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