Top Down GHG estimations

Top-Down GHG emission methods are based on atmospheric measurements and atmospheric models estimations. It consists of linking emissions with atmospheric concentrations using atmospheric transport (and chemistry) models, often referred as inverse modeling. A large number of scientific studies demonstrate that inverse modeling can be used to check the consistency between bottom-up emission inventories and GHG concentrations measured in the atmosphere.

However, the accuracy of emissions' estimations derived from inverse modeling, and the spatial scales at which the emissions can be estimated, depend on the quality and density of measurements and the quality of the atmospheric models. Furthermore, inverse modeling provides estimates of total emissions, including both anthropogenic and natural sources. The World Meteorological organization(WMO) has initiated the Integrated Global Greenhouse gas Information System (IG3IS) with aim of promoting top-down methods.

CO2 monitoring from space can provide important additional information and identify CO2 hotpots. For instance, Nassar et al., used Orbiting Carbon Observatory 2 (OCO-2) satellite retrievals to quantify, CO2 emissions from large point sources in close agreement with reported daily emission values. This study suggests that future CO2 imaging satellites, optimised for point sources, could monitor emissions from individual power plants, which will be important for areas that lack detailed emission information. Recent improvements in satellite retrievals are encouraging and various studies investigate the potential to use satellite data to quantify CO2 emissions from large cities and point sources.

In addition to satellite data, ground based carbon monitoring remains indispensable and will require a significant expansion of surface monitoring stations, such as the "Integrated carbon Observation System" (ICOS) network over Europe, and national and international networks, including "Total Carbon Column Observing Network" (TCCON) for validating satellite data. Furthermore, measurement programs closer to emission sources, which can quantify emissions at facility scale, should be further expanded. Such facility scale measurements can provide more representative emission factors and allow to directly improve emission inventories.

• How we measure CO2 concentrations in the atmosphere? The measured quantity of CO2 by measurement stations is described by the chemical term "mole fraction", defined as the number of carbon dioxide molecules in a given number of molecules of aire, after removal of water vapor. For example, 413 parts per million of CO2 (abbrieved as ppm) means that in every million molecules of (dry) air there are on average 413 CO2 molecules. More details of CO2 measurement methods are available in this document provided by Global Monitoring Laboratory.

• Orbiting Carbon Observatory 2 (OCO-2): OCO-2 is a CO2 observing satellite used to study carbon dioxide concentrations and distributions in the atmosphere. The OCO-2 project objectives are to collect the space-based measurements needed to quantify variations in the column averaged atmospheric dioxide (CO2) dry air mole fraction, with the precision, resolution, and coverage needed toi improve our understanding of surface CO2 sources and sinks on regional scales (> 1000km). The entire OCO-2 data records can be obtained from the Nasa earth data portal.

• The Total Carbon Column Observing Network (TCCON): TCCON is a network of ground-based Fourier Transform Spectrometers recording direct solar spectra in the near-infrared spectral region. From these spectra, accurate and precise column-averaged abundance of CO2, CH4, N2O, HF, CO, H2O, and HDO are retrieved.

• The Global Greenhouse Gas Reference Network: The Global Greenhouse Gas Reference Network measures the atmospheric distribution and trends of the three main long-term drivers of climate change, carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), as well as carbon monoxide (CO) which is an important indicator of air pollution. The Reference Network is a part of NOAA's Global Monitoring Laboratory in Boulder, Colorado. The carefully calibrated and documented measurements data are provided freely through the Global Monitoring Laboratory data portal.

• Copernicus CO2 Monitoring Task Force

• Integrated carbon Observation System

• Greenhouse gas monitoring

Greenhouse gas monitoring is the direct measurement of greenhouse gas emissions and levels. GHGs are measured from space such as by Orbiting Carbon Observatory and by the mean of networks of ground stations such as the Integreated Carbon Observation System. GHGs monitoring refers to tracking how much GHGs is produced by particular activity at a particular point in time.

NASA CArbon Monitoring System (CMS) is a climate research program that provides grants for climate research that measure carbon dioxide and methane emissions. Using instruments in satellites and ariplanes CMS funded research projects provide data to the United States and other countries that help track progress of individual nations regarding their emissions.

• Space-based measurements of GHG emissions

Space-based measurements of carbon dioxide (CO2) are used to help understanding Earht's carbon cycle. There are two high-precision CO2 observing satellites: GOSAT and OCO-2.