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: Significance of Greenhouse Gas Measurement for Carbon Management Technologies

James R Whetstone

Introduction

Carbon management systems aim to reduce atmospheric warming and climate effects by controlling warming agent flows to the atmosphere. Atmospheric warming is primarily driven by its greenhouse gas concentrations. Therefore, greenhouse gases are a primary focus of carbon management efforts. The main greenhouse gases (GHGs) of interest are carbon dioxide (CO,), methane (CH4), and nitrous oxide (N.O), the halogenated hydrocarbon gases not covered by the Montreal Protocols (often termed the F-gases— fluorinated hydrocarbons), nitrogen tri-fluoride and sulfur hexa-fluoride. Reduction strategies are informed by estimation and measurement of greenhouse gas quantities emitted from and taken up by a wide range of processes and economic activities occurring at Earth's surface. Reliable quantitative information is critical to assessing the performance of reduction mitigation efforts. Greenhouse gas inventory reports are widely-accepted sources used as mitigation policy performance metrics to assess efforts often focused on energy production and usage or efficiency and on land use. The UN Framework Convention on Climate Change (UNFCCC) and the Task Force on National Inventories of the Intergovernmental Panel on Climate Change (IPCC, TFI) have established requirements and guidelines to provide a uniform and robust inventory reporting framework for quantifying greenhouse gas emissions (IPCC-TFI, 2006).

Carbon management strategies aim to reduce atmospheric greenhouse gas emissions both locally and globally. Mitigation actions are primarily aimed at reductions in energy usage and are most often implemented at regional and local scales (Gurney, 2011). Mitigation effort implementation will benefit from information at the scales over which carbon managers have purview and exercise control. Evaluation of the effectiveness of these efforts will benefit from more reliable and accurate information on both emissions and uptake across global to local geospatial scales. Independent emissions measurements are useful for demonstrating progress toward mitigation goals; they support fairness in trading, permitting, and regulation, and utilize and guide financially efficient decisions about potential reduction pathways. Additionally, improving the reliability of emissions and uptake information advances the reporting accuracy both nationally and sub-nationally. This chapter will survey measurement systems and approaches underpinning inventory data compilation and reporting, physical and chemical processes upon which measurement technologies rely, and their application from global to local scales. Measurement and estimation performance needs at sub-national, national, and international levels will be discussed in order to give context in carbon management quantification requirements.

The Atmosphere, Quantities, Measurements and Standards

Measurement systems use approaches and configurations reflecting then application, available scientific knowledge, and technological capabilities, and to some degree, then cost of realization and implementation. Emission and uptake processes result in greenhouse gas exchange flows, or fluxes, between Earth's surface and its atmosphere, from sources like power plants and vehicles and from uptake sinks like vegetation on the land and marine life and seawater in the ocean. Measurements to quantify exchange fluxes are heavily dependent upon their nature, i.e., the physical characteristics and conditions under which the processes function and within which measurement must be accomplished. For some systems, direct measurements of emission mass flows can be realized. In others, estimation models are needed in order to infer emissions based upon process characteristics and reference measurements or data. Here, quantification will be discussed in two methodological classes, termed “top-down” and “bottom-up” methods.

Bottom-up approaches quantify exchange flows with the atmosphere based upon emission process characteristics and activity parameters that are often tabulated by economic sector and found in publicly available datasets. This type of information is the basis for national emissions reports to the UNFCCC and is discussed later on in this chapter. Examples include refrigerant (fluorinated gas) leakage from air conditioning systems and from then production processes, carbon dioxide (CO,) from fossil fuel combustion systems used for applications ranging from electricity generation to heating buildings, and methane emissions from leakage in natural gas production, transportation, and distribution to users or enteric fermentation from cattle, to name a few. A variety of methods are used, ranging from emission- activity factor models and data to direct measurement.

Top-down approaches use quantification means based on observation of atmospheric properties and dynamics coupled with greenhouse gas concentration observations expressed as dry atmosphere concentration values. The remainder of this chapter will summarize some of the more widely-used methods, including brief process descriptions illustrating quantification techniques and discussion of quantification performance needs to inform carbon management efforts.

Quantities and units

In this chapter, the term carbon measurement relates to the determination of greenhouse gas amounts moving between Earth's surface and atmosphere. Quantification approaches are governed by the physical nature of emission sources and sinks and determine which of two quantification methods may be applicable. Here, the term "flow” will be applied to the relatively restricted case of gases moving within process structures and the rate of gas movement in them, e.g., flow of exhaust gases from fossil fuel combustion in power plants or vehicles. Combustion products, mainly CO, and water vapor, are confined within the process, such that measurement of exhaust gas constituency and bulk gas flowrate quantifies the amount of CO,, or other minor constituent, emitted. Flowrate units, generally mass per unit time, e.g., kilograms/second or kilograms/minute, are often converted to emission factors in order to be consistent with IPCC emission reporting methodologies, as discussed below. Although these measurement strategies are employed in power plants, they are not currently in practice for vehicle emissions. Characterization of vehicle and engine types or classes benefit from direct constituent and flow measurement over the range of operating conditions to quantify emissions and emission factors (NYFEL, 2019) used for emissions estimation.

Some emission quantification methodologies, discussed later in this chapter, are based on quantification of flows of greenhouse gases to the atmosphere, based upon observation of greenhouse gas dry air concentration, measured as mole fraction, and combined with methods to estimate their atmospheric dynamics. A flux parameter involves the movement of items per unit area per unit time. For example, the mass of a greenhouse gas per meter squared-second is a commonly used unit, i.e., kg/(m2- sec) used to describe flows of atmospheric trace gases, e.g., greenhouse gases. Concentration is a measure of the amount of one substance mixed with others. Common usage in the atmospheric and greenhouse gas observing communities is the statement of greenhouse gas mole fraction values in units of pmol/mol (10“* moles/mole). Mole fraction is inextricably connected to the mole, the unit of the amount of substance of the International System of Units, the SI, and denoted as mol (BIPM, 2019). Just as one dozen consists of 12 things, one mole contains 6.022 140 76 x 1023 elementary entities, i.e., Avogadro's Constant of elementary entities, such as molecules of COr Greenhouse gases often have atmospheric mole fraction values in this and smaller ranges relative to diy ah. The convention of stating mole fraction values on a dry air basis is used to properly account for variations in atmospheric conditions. Perhaps as importantly, or more so, this is a means to exclude effects of the highly variable dilution of water vapor in the atmosphere, which varies from near zero to к 4%. As shown in the box above (NASA, 2019; NOAA, 2019), greenhouse gas mole fractions are considerably smaller than daily water vapor variations and have been shown to remain unchanged as atmospheric water vapor content varies.

Dry Air Constituent Mole Fractions

Nitrogen

78.08%

Oxygen

20.95%

Argon

0.9%

Carbon Dioxide (CO,)

405-411 pmol/mol

Methane (CH4)

1.858-1.867 pmol/mol

Nitrous Oxide (N,0)

к 330 nmol/mol

F Gases

~ 1 to 600 pmol/mol

Radiative physics, atmospheric warming, and greenhouse gas measurements

Earth’s atmosphere transmits and absorbs both solar radiation and the thermal radiation emitted by Earth’s surface. The atmosphere is composed primarily of the two-atom molecules, nitrogen (N,) and oxygen (O,). The remaining ~ 1% of the diy atmosphere is largely composed of chemically inert atomic argon, the long-lived, 3- and 4-atom greenhouse gas molecules, the large molecule halocarbons, and the chemically reactive species impacting human health. Incoming solar radiation, occurring across the ultraviolet, visible, and infrared (IR) spectral regions, is partially reflected back to space, absorbed by atmospheric components, or is absorbed by Earth’s surface, warming the planet to levels that support life. Radiative models of the Earth predict that absorption of incoming solar radiation by Earth's surface alone warms it on average to к -20 “Celsius (~ 0 °F). Earth’s wanned surface emits thennal radiation across the infr ared region at wavelengths ranging from ~ 1 to 20 micrometers (pm). This additional warming, supporting life on Earth, is due to retention of part of the thennal energy radiated by Earth's surface by some atmospheric gases, i.e., the greenhouse gases. The selective absorption of GHGs retains thennal energy in the atmosphere. The outsized capability of low concentration greenhouse gases to warm the atmosphere results from their capacity to absorb thennal radiation much more strongly than oxygen and nitrogen, as discussed later in this chapter, a capacity several orders of magnitude greater. The thennal energy absorbed by GHGs is transferred as kinetic energy, temperature, through molecular collision processes to the atmosphere's most abundant constituents, nitrogen and oxygen, thereby increasing atmospheric temperatures.

Measurement of gr eenhouse gas diy air mole fraction (moles of a greenhouse gas per mole of diy air), in or sampled from the atmosphere or from processes on Earth's surface, is a central observable parameter for quantifying exchange fluxes in top-down or bottom-up methods. A variety of measurement methods are used. Then selection for a particular application is dependent upon considerations of sensitivity, accuracy, application conditions, and cost, among others. Mole fraction measurement methods utilize several of the chemical or radiative properties of greenhouse gases. Application areas range fr om the standards laboratory environments to those used in field observation. The highest accuracy capabilities of laboratoiy environments are needed in order to assign mole fr action values to standard gas mixtures.

Field applications may involve direct measurements of samples drawn from the atmosphere or an emitting process stream. Atmospheric sampling occurs, continuously, daily, and weekly while sampling of power generation stacks occurs on a near real-time basis. These observations are often referenced to standard gases of known and certified composition as a means of ensuring consistency and accuracy in measurement results.

 
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