Pico-SDLA (Spectromètre à Diode Laser Accordable) is an hygrometer developed by the Division Technique of the Institut National des Sciences de l'Univers (DT-INSU) and the Groupe de Spectrométrie Moléculaire et Atmosphérique (GSMA) to measure in situ water vapour, CO2, and methane under small balloons in the upper troposphere (UT) and the lower stratosphere (LS). Is a smaller/lighter version of the former instruments SDLA and micro-SDLA. The main difference with its predecesors is that it uses a one-meter long path for the open cell instead of multi-pass cells. The H2O version uses two different spectral lines (the more intense line is chosen for the stratosphere) to perform measurement up to 25 km.
In the figure at left we can see a scheme of the mounting of the laser sensor. The principle of the sensor is simple: the laser beam is propagated in the open atmosphere where it is partially absorbed by ambient H2O molecule, then, the amount of absorbed energy is related to the molecular density using the Beer-Lambert law. The laser diode is mounted in a small mechanical module made of aluminum and Teflon. Great care was taken in the thermal design of this laser module because of the large variation of the ambient temperature encountered during a balloon flight. Two fiber carbon tubes are used to maintain the laser diode and the 1 mm diameter InAs detector at a one meter distance apart. Two Sapphire lenses with a focal length of 25 mm and anti-reflection coated are used to ensure the optical combination. Simultaneously to the in situ absorption spectra, the atmospheric pressure and temperature are recorded using a small pressure gauge and three meteorological thermistors. To prevent pollution by H2O outgassing from the balloon envelope, a long flight chain is used.
The PicoSDLA is capable of fully unattended operation; a Pentium type processor controls the data acquisition and storage, the interfacing with telemetry and GPS-GSM subsystems as well as the driving of the laser diode. A GPS device gives information on the position of the sensor. The GPS device is coupled to a GSM (global system for mobile communication) subsystem; once the sensor has landed, several SMS (short message service) messages detailing position information are sent regularly to permit the recovery of the gondola. The weight of the realized laser sensor and all sub-systems is of roughly 800 g which makes the instrument perfect to be used with small balloons.
There were developed also other variants with small changes of the original design of the instrument adapted to specific measurements. Pico-SDLA CO2 is a similar version of pico-SDLA H2O except that it uses a 0,5 m long path, and absorption at 2.68 µm. A third version of pico-SDLA is dedicated to the measurement of methane. In contrast to the H2O and the CO2 sondes, the Pico-SDLA-CH4 is based on the use of a Compact Difference Frequency Generation (CDFG) laser source which emits at 3.24 µm allowing measurements of CH4 in the middle atmosphere with a optical path length of 3.6 m.
Also in this flight was included LOAC (Light Optical Aerosols Counter) an optical particle/sizer counter developed by Laboratoire de Physique et Chimie de l'Environnement et de l'Espace (LPCE-CNRS) from France. The system is light and compact enough to perform measurements not only at the surface but under all kinds of balloons in the troposphere and in the stratosphere. It performs observations at two scattering angles. The first one is around 12°, and is almost insensitive to the nature of the particles; the second one is around 60° and is strongly sensitive to the refractive index of the particles. By combining measurement at the two angles, it is possible to retrieve accurately the size distribution and to estimate the nature of the dominant particles (droplets, carbonaceous, salts and mineral particles) in several size classes. This speciation is based on calibration charts obtained in the laboratory.
LOAC is a modular instrument, for which some parts can be changed depending on the measurements conditions. For measurements under balloon or on the ground in low wind conditions, the aerosols are collected by a metal profiled inlet designed to optimize the sampling conditions when oriented in the wind direction. The particles are drawn up to the optical chamber through an isostatic tube by a small pump. The pump is connected to the exit of the optical chamber by a flexible plastic tube. For measurements in windy and rainy conditions, the inlet can be replaced by a total suspended particulate or TSP inlet rejecting rain droplets and particles greater than 100 µm. For long-duration measurements, the small pump can be replaced by a robust pump. The sampled air crosses a laser beam of 25 mW working at the wavelength 650 nm. The scattered light is recorded by two photodiodes at scattering angles of respectively 12º and 60º. Instead of using lenses to collect the light, the photons travel directly to the photodiodes through pipes, providing fields of view with a few degrees. The collecting area of the photodiodes is larger than the diameter of the pipes. This system prevents optical misalignment problems in case of vibrations and strong temperature variations like those encountered during atmospheric balloon flights.
To minimize its weight, the optical chamber is encased in plastic Delrin. The weight, including the pump, is of 350 grams.
Balloon launched on: 3/13/2012 at 22:03 utc
Launch site: Meteorological Research Institute, Baurú, Sao Paulo, Brazil
Balloon launched by:
Balloon manufacturer/size/composition: Zero Pressure Balloon 500 m3
End of flight (L for landing time, W for last contact, otherwise termination time): 3/13/2012
This flight was part of the TRO-Pico campaign performed in Brazil between 2012 and 2013. It aims at studying deep convection in the tropics and more precisely, the impact of overshooting convection in the lower stratosphere on the stratospheric water budget, from the local to the continental scale.
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