Week 6: Instrumentation for air quality measurements

Monday, 1 October, Wednesday, 3 October, and Friday, 5 October 2007, lecture I (pdf), lecture II (pdf)

Overhead sources are
Chemistry of the upper and lower atmosphere, Finlayson-Pitts and Pitts, Academic Press, 2000
and various small sources and manuals

Instrument Manuals

Points/Topics to remember from this weeks classes:

• Most air quality measurements are based on “simple”, automated, optical measurement methods, using a closed (light) path
• CO₂, CO, and O₃ (and SO₂) are measured in absorption, using Lambert-Beer’s Law

1. broadband light sources are used

2. CO₂ and CO are measured using IR light from a heated filament, ozone is measured using UV light from a mercury vapor lamp

3. only short path lengths (<0.4 m) are necessary for CO₂ and O₃ due to their strong absorptivity, a folded path length of several meters is used for CO

4. I is measured on ambient air, I₀ is measured by selectively removing the trace gas of interest from the light absorption cell (CO₂, O₃), or by selectively removing the spectrum of the trace gas of interest from the cell (CO), called the Gas Filter Correlation (GFC) method

5. specialized photo-multipliers can be used as detectors, generally after selection of a particular absorption peak through a wavelength band filter

• Nitrogen oxides have too low abundance for absorption techniques. They are measured with a chemiluminescence method
• NO is reacted with excess ozone (pseudo-first order reaction) in a completely dark cavity. The formed NO₂ is in an (electronically) exited state, the fluorescence quantum yield of which is enhanced by reducing the pressure (and temperature) inside the cavity
• emitted blue photons are “counted” with a photo-multiplier-tube (PMT), which creates a small current proportional to the fluorescence intensity
• NO₂ (and potentially other NO_y species) is (are) measured via converting it to NO before entering the ozone plus NO reaction inside the cavity. The commercial catalysts (proprietary Au and MoO₃ surfaces) for this reaction convert NO₂, but also PAN, HNO₃, and other peroxy- and alkylnitrates to NO
• selective NO_x measurements can be performed by converting NO₂ photolytically to NO via a high intensity UV-lamp
• Ozone has (historically) been measured using Dobson (spectrophoto)meters, and the iodine method. The latter, a wet chemical method, proceeds via

1. I^- + O₃ → IO₃^- (iodate) (in acidic H₂O solution)

2. IO₃^- + 5 I^- + 6 H⁺ → 3 I₂ + 3 H₂O (called a syn-proportionation)

3a. I₂ + starch → intense blue color (using colometry to calculate O₃)

3b. I₂ + 2 e- → 2 I^- (measuring the electrode current precisely)

Today, basically only reaction 3b is used any more in commercial instruments. Historically, starch and acidic iodide impregnated paper discs were used to monitor ozone as far back as 1900

• Methane is measured most often by gas chromatography (see below), but also via IR absorption

• Spectroscopic methods are generally preferred analytical methods for trace gas measurements, because they are highly specific, and often very sensitive and precise. They are also the only way of remotely sensing the atmosphere
• Differential Optical Absorption Spectroscopy (DOAS) in the UV-vis is a preferred and frequently used spectroscopic method for atmospheric trace gas measurements. An open path (through the atmosphere) is used with natural (sun, moon) or artificial (strong xenon-arc lamps) broad-band light sources and retro-reflectors. Wavelength scans are performed with a grating that has a finite wavelength resolution, then send to a detector. I is measured on-peak, I₀ is measured off-peak
• DOAS is essentially the method used by most instruments measuring trace gases from space. Back-scattered light from the atmosphere is used for I, I₀ is taken directly from the sun or measured off-peak. The pathlength has to be estimated from a radiative transfer model. As back-scattered light comes from higher in the atmosphere (as opposed to the surface), the stratosphere is what can be monitored accurately from space
• FTIR (not covered in class) uses fast scans with a Michelson Interferometer (see also http://en.wikipedia.org/wiki/Infrared_spectroscopy) to record a full infrared spectrum within seconds. This method is used in laboratory and sometimes field experiments for high resolution of infrared-active trace gas species ranging from CO and CO₂ to complex mixtures of organics

• (Column) Chromatography (http://en.wikipedia.org/wiki/Chromatography) is the next most widely used analytical technique in the atmospheric sciences. It is less expensive in general compared to optical multi-species methods such as DOAS or FTIR but generally more time- and labor-intensive. Chromatography is based on equilibrium partitioning of an analyte between a mobile and a stationary phase. Small differences in the partition coefficient between different analytes lead to differences in retention on a chromatographic column, which ultimately explains the high resolving power of this method. Partition coefficients vary with type of phase and interaction, and with temperature, which is why chromatographic columns are almost always enclosed in an oven. After eluting from the column several different detection methods can to be used for analyte-specific or non-specific signal creation

o gas chromatography (GC) uses an inert carrier gas as mobile phase and either a liquid or solid as the stationary phase. The column can be temperature-controlled from sub-ambient to over 400 °C for optimum analyte retention and resolution. Several different GC methods are widely used to measure atmospheric trace gases from permanent gases to very rare atmospheric constituents in the ppt range. An air sample can be analyzed from anywhere between 2 minutes and 2 hours depending on species and method applied

o liquid chromatography (LC) uses an inert solvent or solvent mixture (including water) as the mobile phase and a solid (polymer or mineral surface) as stationary phase. It is widely used to measure atmospheric constituents with high polarity and (therefore) water solubility, and low volatility species, both otherwise not easily measured with a GC method

o GC detectors include flame ionization detection (FID) for hydrocarbons and other VOCs, thermal conductivity detection (TCD) for permanent gases, and electron capture detection (ECD) for electronegative gases such as N₂O and all halogenated trace gases. LC detectors (not discussed) include UV-vis light absorption (including spectral information for compound identification) and conductivity detection (CD) for ionic species (called ion chromatography)

o detector signals are recorded versus elapsed time since sample injection, called the chromatogram, showing signal peaks, every time a separated compound elutes from the chromatographic column. Peaks are electronically (software-) integrated for quantification

o all chromatographic methods need to be calibrated, i.e. elution order needs to be established (often via tabulated retention indices for most widely used columns, and through injection of the actual analyte in test runs) and detector-response evaluated for quantification