Measurement-Procedure-Standardisations.md

@@ -87,10 +87,9 @@ Now the
 | vacuum ultraviolet resonance fluorescence (VUF) | Fluorescence is the emission of light (luminescence) by a substance that has absorbed light or other electromagnetic radiation (excitation). In most cases, the emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation. Vacuum Ultraviolet Resonance Fluorescence is a method where a reasonance lamp excited by reasonance fluorescence discharge (in combiantion with an optical filter) produces photons in the ultraviolet which react with a sample gas, inducing fluorescence in the vacuum ultraviolet (UV radiaion 10nm-200nm), subsequently detected by a photomultiplier tube. This method can be employed for CO, with the reasonance lamp emitting UV light between 145nm-151nm, with fluorescence occurring between 160nm-190nm. | Water vapour, drifts in lamp intensity, continuum raman scattering by O2. | |
 | cavity ringdown spectroscopy (CRDS) | Based on absorption spectroscopy, Cavity Ringdown Spectroscopy works by attuning light rays to the unique molecular fingerprint of the sample species. By measuring the time it takes the light to fade or "ring-down", you receive an accurate molecular count in milliseconds. The time of light decay, in essence, provides an exact, non-invasive, and rapid means to detect contaminants in the air, in gases, and even in the breath. The method is typically employed to measure CO and other greenhouse gases (e.g CO2, CH4, H2O). | Water vapour, CO2 and particulates | |
 | cavity attenuated phase shift spectroscopy (CAPS) | Operates on the principle that a specific species efficiently absorbs light at a known wavelength. This is the case for NO2, at 450nm. The degree to which the light is absorbed by a specific species is directly related to the species concentration as described by the Beer-Lambert Law (A = εLC; A = Absorbance (mol litres-1), ε = Molar absorptivity (litres mol-1 cm-1), L = mean optical path length of cell, C = species concentration). Emitted light in an optical cell is reflected back and forth between two mirrors, building intensity and running a very long path length. The long path length extends the “time” or “life” of the photon, thus providing ample time to measure absorbance when a species is present. The method is typically employed for direct measurement of NO2. | direct spectral interference with photochemically produced 1,2-dicarbonyl species (e.g., glyoxal, methylglyoxal) | |
-| differential optical absorption spectroscopy (DOAS) | The basic principle used in Differential optical absorption spectroscopy (DOAS) is absorption spectroscopy. DOAS allows the quantitative determination of multiple atmospheric trace gas concentrations by recording and evaluating the characteristic absorption structures (lines or bands) of the trace gas molecules along an absorption path of known length in the open atmosphere, following the Beer-Lambert law (I = Io e−KLC; K = molecular absorption coefficient at STP, L = optical path length of cell, C = species concentration , I = light intensity of sample gas, Io = light intensity of sample without measured species (reference gas) ). The wavelength of light where a distinct absorption peak occurs is determined for analyte. A wavelength on either side of the absorption peak is next determined. The intensity of a light source at wavelength is measured and then the intensity is measured again after the light passes through the analyte. The difference of the intensities is proportional to the concentration to the analyte. DOAS is a long path measuring technique. Measurements can be made in an optical pathway from 1 to 10 kilometers. The method measures the average concentration of a species along the path length, not for any single molecule. In the real atmosphere, multiple effects contribute to the overall attenuation of the light. In particular, aerosols and clouds scatter light and thereby reduce the intensity of the direct beam while increasing intensities measured in other directions. Also, there rarely is only one absorber relevant at a given wavelength and this also needs to be accounted for. The solution to this problem lies in the use of measurements at several wavelengths. Each molecule has a characteristic absorption spectrum (its spectral fingerprint) and therefore, simultaneous measurements at different wavelengths enable the separation of the contributions of the different absorbers. This is what DOAS does. Scattering by aerosols also needs accounting for. Their extinction cross-sections can be approximated by power laws (λ**-4 for Rayleigh scattering and λ**-1..0 for Mie scattering), the coefficients of which can be determined in the fit. The basic principle behind the separation of aerosol extinction and trace gas absorption is that the latter are identified using those parts of their absorption cross-sections that vary rapidly with wavelength. The more slowly varying parts of the absorption can not be separated from extinction by aerosols which is why the absorption cross-sections are often high pass filtered before use in a DOAS retrieval. In addition to their effect on the spectral distribution of the intensity measured, aerosols can also have a significant impact on the light path of scattered light. This has to be modelled explicitly when computing the airmass factors for the light path correction. The DOAS technique is characterised by the following: 
-(a) measuring the transmitted light intensity over a relatively (compared to the width of an absorption band) broad spectral interval; 
-(b) high‐pass filtering of the spectra to obtain a differential absorption signal and eliminating broad‐band extinction processes such as Rayleigh and Mie scattering (RS and MS); 
-(c) quantitative determination of trace column densities by matching the observed spectral signatures to prerecorded (reference) spectra by, for instance, least‐squares methods. DOAS instruments are often divided into two main groups: passive and active ones. The active DOAS system such as longpath(LP)-systems and cavity-enhanced(CE) DOAS systems have their own light-source, whereas passive ones use the sun as their light source, e.g. MAX(Multi-axial)-DOAS. Also the moon can be used for night-time DOAS measurements, but here usually direct light measurements need to be done instead of scattered light measurements as it is the case for passive DOAS systems such as the MAX-DOAS. | Interference by miscellaneous atmospheric constituents. Heavy rain and fog, and even high humidity. Atmospheric turbulence, such as that from thermal-induced effects, can distort reflections. Anything that interrupts the path of the laser will cause some interference (i.e., animals, cars, planes, etc.). | |
+| differential optical absorption spectroscopy (DOAS) | The basic principle used in Differential optical absorption spectroscopy (DOAS) is absorption spectroscopy. DOAS allows the quantitative determination of multiple atmospheric trace gas concentrations by recording and evaluating the characteristic absorption structures (lines or bands) of the trace gas molecules along an absorption path of known length in the open atmosphere, following the Beer-Lambert law (I = Io e−KLC; K = molecular absorption coefficient at STP, L = optical path length of cell, C = species concentration , I = light intensity of sample gas, Io = light intensity of sample without measured species (reference gas) ). The wavelength of light where a distinct absorption peak occurs is determined for analyte. A wavelength on either side of the absorption peak is next determined. The intensity of a light source at wavelength is measured and then the intensity is measured again after the light passes through the analyte. The difference of the intensities is proportional to the concentration to the analyte. DOAS is a long path measuring technique. Measurements can be made in an optical pathway from 1 to 10 kilometers. The method measures the average concentration of a species along the path length, not for any single molecule. In the real atmosphere, multiple effects contribute to the overall attenuation of the light. In particular, aerosols and clouds scatter light and thereby reduce the intensity of the direct beam while increasing intensities measured in other directions. Also, there rarely is only one absorber relevant at a given wavelength and this also needs to be accounted for. The solution to this problem lies in the use of measurements at several wavelengths. Each molecule has a characteristic absorption spectrum (its spectral fingerprint) and therefore, simultaneous measurements at different wavelengths enable the separation of the contributions of the different absorbers. This is what DOAS does. Scattering by aerosols also needs accounting for. Their extinction cross-sections can be approximated by power laws (λ**-4 for Rayleigh scattering and λ**-1..0 for Mie scattering), the coefficients of which can be determined in the fit. The basic principle behind the separation of aerosol extinction and trace gas absorption is that the latter are identified using those parts of their absorption cross-sections that vary rapidly with wavelength. The more slowly varying parts of the absorption can not be separated from extinction by aerosols which is why the absorption cross-sections are often high pass filtered before use in a DOAS retrieval. In addition to their effect on the spectral distribution of the intensity measured, aerosols can also have a significant impact on the light path of scattered light. This has to be modelled explicitly when computing the airmass factors for the light path correction. The DOAS technique is characterised by the following: (a) measuring the transmitted light intensity over a relatively (compared to the width of an absorption band) broad spectral interval; (b) high‐pass filtering of the spectra to obtain a differential absorption signal and eliminating broad‐band extinction processes such as Rayleigh and Mie scattering (RS and MS); (c) quantitative determination of trace column densities by matching the observed spectral signatures to prerecorded (reference) spectra by, for instance, least‐squares methods. DOAS instruments are often divided into two main groups: passive and active ones. The active DOAS system such as longpath(LP)-systems and cavity-enhanced(CE) DOAS systems have their own light-source, whereas passive ones use the sun as their light source, e.g. MAX(Multi-axial)-DOAS. Also the moon can be used for night-time DOAS measurements, but here usually direct light measurements need to be done instead of scattered light measurements as it is the case for passive DOAS systems such as the MAX-DOAS. | Interference by miscellaneous atmospheric constituents. Heavy rain and fog, and even high humidity. Atmospheric turbulence, such as that from thermal-induced effects, can distort reflections. Anything that interrupts the path of the laser will cause some interference (i.e., animals, cars, planes, etc.). | |
+| electrochemical membrane diffusion | Methodology developed specifically for measurement of NH3: https://www.sciencedirect.com/science/article/pii/S0003267003012650?via%3Dihub. Gas is sampled in a sampler comprising two opposite channels separated by a gas permeable, water repellent polypropylene membrane. Subsequently, the acid sample solution is pumped into a selector where an alkaline solution is added to ionize all sampled ambient acid gasses, resulting in an enhanced selectivity. In the selector, the ammonia can diffuse through a second membrane into a purified water stream where an electrolyte conductivity sensor quantifies the resulting ammonium concentration. The realized system is shown to be selective enough not to be influenced by normal ambient carbon dioxide concentrations. Experiments with a gas flow of 3 ml/min, containing ammonia concentrations ranging from 9.8 to 0.3 ppm in a nitrogen carrier flow, into a 15 μl/min sample solution flow and finally into a 5 μl/min purified water stream have been carried out and show that the system is sensitive to ammonia concentration below 1 ppmv. | CO2 | |
+