Trace Gas Exchange Measurements with Standard Infrared Analyzers

Transcription

1 Practical Environmental Measurement Methods Trace Gas Exchange Measurements with Standard Infrared Analyzers Last change of document: February 23, 2007 Supervisor: Charles Robert Room no: S 4381 ph: 4352 Lab: outside NW1, TBA Introduction Trace gases are introduced into or removed from the lower atmosphere by a number of different processes, such as direct emission/deposition or in-situ production/destruction. Emission and deposition are processes that most often happen near the earth s surface, in the planetary boundary layer. As this is also the atmospheric layer almost all human activity takes place, it comes as no surprise that anthropogenic activities can lead to large trace gas emissions, and that we are interested to know about these processes as they can directly effect our health (can you think of a good example?), as well as indirectly by altering atmospheric parameters that are important to us, such as temperature, humidity, and visibility. When a trace gas is directly emitted into the troposphere from the ground it will exhibit an exponential gradient with elevation (why?). If this gradient is steep enough and our trace gas measurement principle is sufficiently precise, we can measure that gradient. In other words: A large trace gas emission from or deposition to the ground exhibits a steep gradient that can easily be measured (again: can you think of an example?). Small trace gas fluxes (= mass transport per unit time and area; can be emission or deposition) of trace gases between the ground and the boundary layer can often not be measured by a gradient or a related technique. One often has to rely on enclosure techniques for flux measurements. Enclosures (chambers, boxes) are fixed volumes of air separated from their environment by flexible or rugged walls. Any trace gas source or sink inside that volume causes a measurable change of that trace gas concentration that can directly be related to the trace gas flux. A variety of different chambers has been used to enclose green plants or soils to measure trace gas exchanges of H 2 O, CO 2, CO, CH 4, higher hydrocarbons, etc. In the experiment described below, you will learn how to set up an experiment to measure the fluxes of CO 2 and CO between soils and the atmosphere. You will use standard infrared absorption analyzers for trace gas analysis and low-cost soil chambers. We will discuss the measurement principles before the experiments and you will deploy the chambers and carry out several tests outside the NW1 building to determine the role of soils for the trace gases CO 2 and CO. Measurement principles Both the measurement of carbon dioxide and carbon monoxide can be based on the absorption of light in the infrared wavelengths ( heat waves) of the spectrum. Light absorption is mathematically treated using Beer s Law (also called Lambert-Beer s Law) I(λ) = I 0 (λ) exp[- ε(λ) c L] (1) where I 0 (λ) is the initial intensity before absorption at wavelength λ, ε(λ) is the specific absorption coefficient of the investigated gas at wavelength λ, a constant, L is the path-length 1

2 of absorption, c is the concentration of the absorbing gas, and I(λ) is the resulting light intensity after absorption. You will derive this formula as part of your experimental protocol. In practice, the path-length L is determined by the instrumental setup and is constant. The gas to be measured is pumped through the absorption cell at a constant speed, and constant pressure and temperature (why is that important?). The instrument measures the reference intensity I 0 either via a separate path with zero gas, or by selectively removing the gas to be measured periodically from the gas stream for example using a catalyst. I/I 0 is electronically linearized, producing a linear signal with concentration c. In standard operating procedure, one calibrates the instrument by introducing several known mixing ratios to the absorption cell, including a zero, which ultimately produces a calibration line. You will do this calibration prior to the experiment. You might wonder what role the specific absorption ε plays. ε is a function of λ and basically characterizes the absorption spectrum of the trace gas you want to measure. As the light source in infrared analyzers is usually not monochromatic but broadband, one has to optically filter (λ selection) the portion that provides good overlap with an appropriate trace gas absorption feature. If the respective feature is not unique to the specific trace gas, interferences from other trace gases can complicate the measurement. The most common interference stems from water vapor, which has a large number of spectral features in the infrared. The CO 2 /water analyzer we use automatically corrects for this interference. The water vapor interference in the CO infrared analyzer is very small as it applies the gas filter correlation method using a chopper. We will discuss this at the beginning of the experiment. Experiment We want to determine how CO 2 and CO are exchanged between soils and the atmosphere using an enclosure technique measurement. There are essentially two ways to establish such a measurement: via a static or via a dynamic method. 1. The static method places the enclosure on the ground and isolates the volume inside of it (t 0 ) after measuring the trace gas composition. At subsequent times t 1, t 2, etc., we repeat the measurement and watch the trace gas mixing ratio development. A graph of trace gas mixing ratio versus time then determines the trace gas flux (what other parameter is needed?). Assuming this flux comes from the soil, it is generally referenced to the soil area covered by the enclosure (why not refer to soil mass? what if the flux comes from plants in the enclosure?). 2. The dynamic method, or flow-through method is an equilibrium method where gas from outside the chamber is constantly flushing the inside volume, thereby transporting any emitted gases away before they can accumulate significantly, as in the case of the static method. This way, the environment (temperature, humidity, trace gas composition, etc.) inside the enclosure is only slightly different from the outside. The higher the flow, the smaller the trace gas mixing ratio differences between the in-flowing air and the out-flowing air. However, as the trace gas flux F is determined from F = V (c out - c in ) / A (2) where A is the reference soil area, and c out and c in are the trace gas concentrations of the outflowing and in-flowing air, respectively, one has to regulate the flow, V, such that a measurable concentration difference is achieved. One has generally preferred the dynamic over the static method when investigating life plants (why?), such as in plant physiology studies. The static method is used when the trace gas flux is too small to use the dynamic method, such as for measuring soil methane, CH 4, fluxes. 2

3 Experimental Setup and Tasks Before you start with the measurements, you will have to calibrate the instruments. For that purpose one generally carries out a dynamic dilution of a gas standard of known concentration, c cal = c std V std / (V std + V dil ) (3) where V std and V dil are the flows of the standard and the diluting air, respectively. A calibration curve is recorded by changing the ratio V std / (V std + V dil ), generally by changing V dil only. Equipment and procedure are described below. To guide you in carrying out this soil trace gas flux experiment yourself, look at the following schematic of a possible setup. Infrared analyzers flow meter pump to flow air into chamber flow meter gas lines 3-way valve soil chamber Figure 1: Schematic of soil trace gas flux measurement setup For a static method measurement, the gas flow through the analyzer(s) has to be returned to the chamber (dotted line in Figure 1), while the knowledge of the exact flow is not that important. However, which precautions do you have to take? For a dynamic method measurement, flow return is not necessary, but the in-flow to the chamber has to be measured carefully. Also the out-flow should be measured, as V in should always be larger than V out. Why? The following items will be at the experimental site (outside NW1) when you get there: - one CO infrared analyzer, one open-path CO 2 infrared analyzer - three mechanical flow controllers/-meters - a mixed CO 2 and CO standard in a can, N 2 as diluting zero gas - a chart recorder or a similar data logging device - several meters of flexible Teflon tubing and several connectors - one mechanical 3-way valve - one plexiglass soil chamber, consisting of a bottom frame and a lid - an extra pump, a hand-held temperature probe, a tape measure, and trash bags 3

4 Proceed as follows, making careful written notes as you go along: 0.1 Turn on the infrared analyzers, so they can warm up before the experiment. To bridge this time, we will have a look inside the CO measurement instrument and discuss its internal setup and operation. So be prepared for that and look at the schematic in the Appendix! 0.2 Prepare the dynamic dilution by connecting the CO 2 /CO standard to the small flow meter, and nitrogen to one of the larger ones. Combine the two flows behind the meters with a T-connector (called TEE), and provide at least another 1 m of tubing for mixing. Connect to another TEE from which you will extract air into the CO analyzer (not yet!), and connect the bypass to the CO 2 analyzer. Start the chart recorder, speed mm min We will now open the nitrogen flow carefully and regulate it to ~2 L min -1. NOW we connect the CO analyzer. This will be the zero-measurement on both instruments, and we save it on paper using the chart recorder. Notice how long the instruments take to deliver a stable signal. Estimate the noise level! 0.4 We now open the small standard tank and regulate the flow between 1 and 10 ml min -1. You should record at least four calibration points (including the zero), starting from a low point of ~1/2000 flow ratio. After the highest point, return to 1 ml standard flow, but increase the nitrogen flow to 4 L min -1 and record this point as well. 0.5 Stop the Standard flow and re-record the zero-measurement. Any changes? Close the nitrogen flow and disconnect the calibration setup. 1.1 Choose an appropriate piece of soil surface close to the measurement table and characterize it verbally (what s on it? is it wet or dry, in sun or shade? others??) 1.2 Place the lower part of the chamber on that spot and secure its position. Make the necessary gas line connections for a static chamber measurement 1.3 Place the temperature probe in the upper five centimeters of the soil inside the chamber 1.4 When the infrared analyzers are ready, put the chamber lid on and note the first measurement value. This is your t 0 measurement 1.5 Continue monitoring the actual CO, CO 2, and temperature values every minute. Looking at them carefully, what conclusions can you draw already now? 1.6 Terminate the experiment when you think it is finished, or latest after 30 minutes 2.1 Move the chamber to another location, that in your opinion should be different to the first location, and repeat the experiment 2.2 While monitoring the results: How is the second spot different in terms of the trace gas measurements? How could the answer to that question be related to your choice of soil location and characteristics? 2.3 Terminate the experiment when you think it is finished, or latest after 30 minutes. 3.1 Change the gas line set up of the experiment such that you can carry out a dynamic chamber measurement, and set an in-flow of approximately 4 L min -1 to the chamber 3.2 Repeat the experiment at the same soil location, terminate when finished, i.e. when a more or less stable CO and CO 2 -reading has been reached 3.3 Reduce the in-flow to the chamber to 2 L min -1 and monitor the changes 3.4 Increase the in-flow to the chamber to 6 L min -1 and again monitor the changes 4.1 If there is enough time, move the chamber back to the first location and repeat the flux measurement with the dynamic method there 4.2 Cover the chamber with the black plastic bags we provide and monitor any changes in temperature or trace gas composition 4

5 Data Analysis After the experiment you should have a paper-recorded data sheet with notes on temperatures ready. Enter it into an appropriate Spreadsheet program (MS Excel, Lotus, etc.), one page per table. Create two additional pages to 1. answer the questions that I posed above (pre- and during experiment), and 2. answer the ones below (post-experiment). The complete file will be your protocol, which is going to be due two weeks after the experiment, via to crobert@iup.physik.uni-bremen.de From the data tables and your notes, your tasks are to 1. plot the calibration line for both instruments and calculate the measurement error. Which errors do you have to consider? Did you write down the mixing ratios in the Standard? 2. calculate the concentrations/mixing ratios (including their errors!) of CO and CO 2 throughout the experiments, using your previously determined calibration lines 3. calculate the approximate chamber-air exchange time from experiments calculate the fluxes (including their errors!) of CO and CO 2 at both soil locations with both methods, and compare these fluxes to literature values You should further be able to answer the following questions: 1. Which trace gas measurement, CO or CO 2, appears more accurate, and why? 2. Which flux, CO or CO 2, was higher? Why? Which enclosure method is the better one for CO, CO 2 flux measurements? 3. If the fluxes between the two measurement locations were different, what do your notes tell you about the possible reasons, i.e. how were the fluxes related to soil condition? 4. After studying your results and reading through the literature, please describe the major processes that lead to CO and CO 2 exchange between soils and the atmosphere. Are these processes and their magnitude of any relevance for CO and CO 2 in the atmosphere? Why? Literature: Peter Warneck, Chemistry of the Atmospheres Raich, J. W., and W. H. Schlesinger The global carbon dioxide flux in soil respiration and its relationship to vegetation and climate. Tellus 44 B: Gary M. King, Characteristics and significance of atmospheric carbon monoxide consumption by soils, Chemosphere - Global Change Science 1 (1-3), 1999, Duration of Experiment: ~4 hours Home/PC-work: also 3-4 hours 5

6 Appendix: CO instrument internal setup 6

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