Twin Otter - CIRPAS - NPS

Langley Aerosol Research Group Experiment (LARGE).  The "classic" suite of instrumenation measures in-situ aerosol micrphysical and optical properties. The package can be tailored for specific science objectives and to operate on a variety of aircraft. Depending on the aircraft, measurments are made from either a shrouded single-diffuser "Clarke" inlet, from a BMI (Brechtel Manufacturing Inc.) isokinetic inlet, or from a HIML inlet. Primary measurements include:

1.) total and non-volatile particle concentrations (3nm and 10nm nominal size cuts),
2.) dry size distributions from 3nm to 5µm diameter using a combination of mobilty-optical-aerodynamic sizing techniques,
3.) dry and humidified scattering coefficients (at 450, 550, and 700nm wavelength), and
4.) dry absorption coefficients (470, 532, and 670nm wavelength).

LARGE derived products include particle size statistics (integrated number, surface area, and volume concentrations for ultrafine, accumulation, and coarse modes), dry and ambient aerosol extinction coefficients, single scattering albedo, angstrom exponent coefficients, and scattering hygroscopicity parameter f(RH).

In early 2000, the Ames Atmospheric Radiation Group completed the design and development of an all new Solar Spectral Flux Radiometer (SSFR). The SSFR is used to measure solar spectral irradiance at moderate resolution to determine the radiative effect of clouds, aerosols, and gases on climate, and also to infer the physical properties of aerosols and clouds. Additionally, the SSFR was used to acquire water vapor spectra using the Ames 25-meter base-path multiple-reflection absorption cell in a laboratory experiment. The Solar Spectral Flux Radiometer is a moderate resolution flux (irradiance) spectrometer with 8-12 nm spectral resolution, simultaneous zenith and nadir viewing. It has a radiometric accuracy of 3% and a precision of 0.5%. The instrument is calibrated before and after every experiment, using a NIST-traceable lamp. During field experiments, the stability of the calibration is monitored before and after each flight using portable field calibrators. Each SSFR consists of 2 light collectors, which are either fix-mounted to the aircraft fuselage, or on a stabilizing platform which counteracts the movements of the aircraft. Through fiber optic cables, the light collectors are connected to 2 identical pairs of spectrometers, which cover the wavelength range from (a) 350 nm-1000 nm (Zeiss grating spectrometer with Silicon linear diode array) and (b) 950 nm - 2150 nm (Zeiss grating spectrometer with InGaAs linear diode array). Each spectrometer pair covers about 95% of the incoming solar incident irradiance spectrum.

The DLH has been successfully flown during many previous field campaigns on several aircraft, most recently ACTIVATE (Falcon); FIREX-AQ, ATom, KORUS-AQ, and SEAC4RS (DC-8); POSIDON (WB-57); CARAFE (Sherpa); CAMP2Ex and DISCOVER-AQ (P-3); and ATTREX (Global Hawk). This sensor measures water vapor (H2O(v)) via absorption by one of three strong, isolated spectral lines near 1.4 μm and is comprised of a compact laser transceiver and a sheet of high grade retroflecting road sign material to form the optical path. Optical sampling geometry is aircraft-dependent, as each DLH instrument is custom-built to conform to aircraft geometric constraints. Using differential absorption detection techniques, H2O(v) is sensed along the external path negating any potential wall or inlet effects inherent in extractive sampling techniques. A laser power normalization scheme enables the sensor to accurately measure water vapor even when flying through clouds. An algorithm calculates H2O(v) concentration based on the differential absorption signal magnitude, ambient pressure, and temperature, and spectroscopic parameters found in the literature and/or measured in the laboratory. Preliminary water vapor mixing ratio and derived relative humidities are provided in real-time to investigators.