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Introduction to Measurement Techniques in Environmental Physics University of Bremen, summer term 2006 In-situ Measurement Techniques Andreas Richter ( firstname.lastname@example.org ). Overview . some general thoughts on measurements of chemical species in the atmosphere
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University of Bremen, summer term 2006
In-situ Measurement Techniques
Andreas Richter (email@example.com)
How do we want to measure them?
kB = 1.38 1023 J mol-1 K-1
Beware: ppb = part per billion = 10-9 although European billon = 1012 !
Problem: Often, air samples have to be pre-treated to concentrate the species of interest or to remove unwanted interfering species
Filters: e.g. from Nylon or Teflon are used to extract species from airflow for later analysis
Problems: interference by particles, lack of specificity, change of collection efficiency
Denuders: removal of a gas from a laminar airflow by diffusion to the walls of a coated tube
Mist chamber and scrubber: air is passed through a chamber where a mist of water or other aqueous solution is used to scrub out a species of interest
Idea: Achieve highly specific absorption measurements by using gas of interest as filter in front of detector. Absorption (or emission) structures of the gas correlate 100% with the “filter”; any other absorption pattern is averaged out.
Application: CO, CO2, SO2, CH4
Problems: Only for one species, works best for low pressures (no pressure broadening), p and T must agree between measurement sample and cell.
In some exothermic reactions, part of the energy is released as photons that can be measured by a photomultiplier.
O3 + NO -> NO2* + O2
NO2* -> NO2 + h
NO2* + M -> NO2 + M
The emitted intensity depends on the effectiveness of quenching which is proportional to the pressure and the concentrations of [O3] and [NO].
If pressure and one concentration are kept constant, the intensity is proportional to the concentration of the other.
Chain Length CL
RO2 + NO→ NO2 + ... + RO
RO2 + O2→ R..COR.+ HO2
HO2 + NO→ NO2 + ... + OH
OH + CO→ HO2 + CO2
Titration of ozone in a potassium iodide (KI) solution according the redox reaction: 2 KI + O3 + H2O I2 + O2 + 2 KOH Measurement of "free" iodine (I2) in electrochemical reaction cell(s). The iodine makes contact with a platinum cathode and is reduced back to iodide ions by the uptake of 2 electrons per molecule of iodine: I2 + 2 e- on Pt 2 I- [cathode reaction]
Some References to sources used