Despite the broad consensus on the negative long-term effects of carbon dioxide (CO2) emissions and the efforts to reduce these emissions, the atmospheric CO2 concentrations continue to rise. During the course of 2018, the average CO2 concentration increased from 407 to 410 ppm at the Mauna Loa observatory, representing the fourth-highest annual growth ever recorded at that observatory . CO2 emissions from localized point sources represent a large fraction of the CO2 emitted into the atmosphere. The International Energy Agency (IEA) recently reported that emissions from coal-fired power plants exceeded 10 GtCO2yr-1 for the first time in 2018, hence accounting for approximately 30 % of the global CO2 emissions , mainly due to the continued growth of coal use in Asia and other emerging economies. Figure  depicts the global distribution of reported and estimated annual CO2 emissions from power plants for the year 2009, as provided by the CARMA (Carbon Monitoring for Action) v3.0 database , together with the corresponding cumulative distribution of the power plant emissions. The emission total from 16 898 individual power plants, where exact or approximate coordinates are available, adds up to 9.9 GtCO2yr-1. A large fraction of power plant emissions originates from a relatively small number of large to medium-sized power plants. The CARMA data show that 153 large power plants (>10 MtCO2yr-1) accounted for 24 % of the total annual power plant CO2 emissions, whereas 2111 large and medium-sized power plants (>1 MtCO2yr-1) accounted for as much as 88 % of the power plant CO2 emission budget, clearly demonstrating the significant contribution from medium-sized power plants (1–10 MtCO2yr-1) to the global CO2 emission budget.

To advance towards emission accounting and reduction measures, agreed upon in the Paris Agreement in force since 2016, the independent verification of reported emissions is of high importance. To this end, spaceborne instruments provide a suitable platform with which continuous long-term measurements can potentially be combined with near-global coverage with no geopolitical boundaries.

Most of the currently operating, planned and proposed instruments for passive CO2 observations from space measure the reflected shortwave infrared (SWIR) solar radiation in several spectral windows covering the oxygen A (O2 A) band near 750 nm as well as the weak and strong CO2 absorption bands near 1600 and 2000 nm, respectively, e.g., GOSAT Greenhouse Gases Observing Satellite;, OCO-2 Orbiting Carbon Observatory-2;, TanSat , GOSAT-2 , OCO-3 , MicroCarb , GeoCarb , CarbonSat and G3E Geostationary Emission Explorer for Europe;. These instruments and instrument concepts further rely on a comparatively high spectral resolution on the order of approximately 0.05–0.3 nm, representing resolving powers (ratio of wavelength over the full-width half-maximum of the instrument spectral response function) ranging from approximately 3600 for the strong CO2 absorption bands near 2000 nm for CarbonSat up to >20000 for the OCO and GOSAT instruments. Such advanced instruments, like GOSAT and OCO-2 that have been operating since 2009 and 2014, respectively, generally target an accuracy and coverage sufficient to study the natural CO2 cycle on a regional to continental scale e.g., but have also been used to observe and quantify CO2 gradients on the regional scale caused by anthropogenic CO2 emissions in urban areas . OCO-2 data have further been used to observe strong CO2 plumes from localized natural and anthropogenic CO2 sources like volcanoes and coal-fired power plants , demonstrating the capabilities of imaging spectrometers to monitor CO2 from space. The spatial resolution of OCO-2 and similar instruments like OCO-3, TanSat and the planned Copernicus CO2 Monitoring mission CO2M (on the order of 2–4 km2) does, however, pose a difficulty for the routine monitoring of localized power plant CO2 emissions, since the plume is usually only sampled by a handful of pixels in which CO2 plume enhancements cannot be fully separated from the background, making quantitative CO2 emission rate estimates difficult and vulnerable to cloud contamination and instrument noise propagating into CO2 retrieval errors. For this reason CO2M will target isolated large power plants (≳10 MtCO2yr-1) and large urban agglomerations (≳ Berlin) , and thus a large fraction of the emission total will be missed.

To contribute to closing this gap and expanding on future CO2 monitoring from space, we present the concept and a first performance assessment of a spaceborne imaging spectrometer that could be deployed for the dedicated monitoring of localized CO2 emissions. By targeting power plants with an annual emission rate down to approximately 1 MtCO2yr-1, a substantial fraction of the CO2 emissions from power plants and hence a significant part of the global man-made CO2 emission budget in total could be resolved (given global coverage through a fleet of instruments). As shown in Fig. , it is of key importance to also cover medium-sized power plants (1–10 MtCO2yr-1) as they alone contributed to approximately 64 % of the CO2 emissions from power plants in 2009 according to the CARMA v3.0 data. To achieve this, the proposed instrument has an envisaged spatial resolution of 50×50 m2. With such a high spatial resolution and large amount of ground pixels per unit area, averaging plume enhancements and background concentration fields is avoided. This leads to an enhanced contrast compared to a coarser spatial resolution. To increase the number of collected photons and hence the signal-to-noise ratio (SNR) and relative precision of the CO2 concentration retrievals, such a high spatial resolution has to be compensated for with a rather coarse spectral resolution. To further compensate for the limited spatial coverage of a single instrument, a comparatively compact and low-cost instrument design is an important aspect, as it would allow for a fleet of instruments to be deployed, increasing the spatial coverage.

recently demonstrated that atmospheric CO2 concentrations can be retrieved with an accuracy <1 % using such a comparatively simple spectral setup with one single spectral window and a relatively coarse spectral resolution of approximately 1.3–1.4 nm (resolving power of 1400–1600). demonstrated the ability to resolve and quantify methane (CH4) plumes, which pose a similar remote sensing challenge as CO2, using data from the spaceborne Hyperion imaging spectrometer with a spectral and spatial resolution of 10 nm (resolving power around 230) and 30 m, respectively. The observation of emission plumes, from plume detection to enhancement quantification and flux estimation, using imaging spectroscopy with a single narrow spectral window and a spectral resolution as coarse as 5 to 10 nm (resolving power around 200–500) has further been repeatedly demonstrated using airborne imaging spectroscopy data for both CO2 and CH4 . For an airborne instrument primarily dedicated to the quantitative imaging of CH4, but also CO2 plumes, proposed a single spectral window and a spectral resolution of 1.0 nm (resolving power around 2000–2400), again coarse enough to reach a spatial resolution on the order of 10–100 m. The commercial instrument GHGSat-D operated by the Canadian company GHGSat Inc. was launched in 2016 as a demonstrator for a satellite constellation concept targeting the detection of CH4 plumes from individual point sources within selected ≈10×10 km2 target regions at a spectral and spatial resolution of 0.1 nm (resolving power around 16 000) and 50 m, respectively . recently showed how anomalously large CH4 point sources can be discovered with GHGSat-D observations.

Given the results from previous studies and the technology at hand, we are confident that the proposed instrument concept presented here could be realized and that it would be an important complement to the fleet of current and planned spaceborne CO2 instruments, allowing for the routine quantitative monitoring of CO2 emissions from large and medium-sized power plants. The proposed instrument concept would also serve as a good complement and companion to CO2M by also targeting medium-sized power plants and providing high-resolution images with finer CO2 plume structures. The added value of such an instrument would be of interest both in terms of advancing science and providing independent emission estimates that could be used to verify reported CO2 emission rates at the facility level and inform policy makers on the progress of reducing man-made CO2 emissions. The proposed instrument concept is described in Sect. , followed by a description of the instrument noise model in Sect. . A global performance assessment addressing instrument noise and the errors introduced by atmospheric aerosol is presented in Sect. . The ability to resolve single CO2 emission plumes at an urban scale is further simulated in Sect. . A short summary and our concluding remarks are finally presented in Sect. .

The instrument concept presented in this paper is based on a spaceborne push-broom imaging grating spectrometer measuring spectra of reflected solar radiation in one single SWIR spectral window, from which the column-averaged dry-air mole fraction of CO2 (XCO2) can be retrieved. With an expected instrument mass of approximately 90 kg, it is suitable for deployment on small satellite buses. Since the proposed instrument targets the quantification of localized CO2 emissions from, e.g., coal-fired power plants, a high spatial resolution of 50×50 m2 is envisaged. The instrument is designed to fly in a sun-synchronous orbit at an altitude of 600 km and a local equatorial crossing time at 13:00 LT. This orbit is chosen in order to have a well-developed boundary layer at overpass together with good radiometric performance (high SNR).

The preliminary optical design assumes a circular aperture (15 cm in diameter) and is based on a three-mirror anastigmat (TMA) telescope, combined with an Offner-type spectrometer, as shown in Fig. . The optic system relies on metal-based mirrors and is designed as an athermal configuration for a wide temperature range onboard the satellite. The three mirrors of the TMA are standard aspheres aligned on a single optical axis. The efficiency of the optical bench (throughput), including transmittance and grating efficiency, is estimated to 0.48, and the f number (fnum), equal to the ratio of focal length to aperture diameter, amounts to 2.4. The dispersed electromagnetic radiation is focused onto a two-dimensional array detector that captures the spatial across-track dimension and the spectral dimension of the incoming radiation. A detector with a pixel area of 900 µm2 and a quantum efficiency of 0.8 is assumed for this study. The quantum efficiency depends on the wavelength but is for now assumed constant for both spectral windows. These values are in line with typical values for a state-of-the-art detector.

In order to reach a sufficient SNR, the proposed spatial resolution only allows for a relatively coarse spectral resolution. used spectrally degraded GOSAT soundings to demonstrate the capability of retrieving XCO2 from a single spectral window at such a coarse spectral resolution using a spectral setup (in terms of spectral range, resolution and oversampling ratio) compact enough to fit onto 256 detector pixels. They evaluate two alternative spectral setups covering the spectral ranges 1559–1672 nm (hereafter also referred to as SWIR-1) and 1982–2092 nm (hereafter also referred to as SWIR-2) with a spectral resolution (full-width half-maximum – FWHM – of the instrument spectral response function) of 1.37 and 1.29 nm, respectively, and an oversampling ratio of 3. The resolving power of the SWIR-1 and SWIR-2 setups amounts to approximately 1200 and 1600, respectively. For optics design reasons, we use a spectral oversampling ratio of 2.5 in this study, resulting in a spectral sampling distance of 0.55 and 0.52 nm for SWIR-1 and SWIR-2, respectively. Simulated synthetic measurements of spectral radiances for the two prospective spectral setups are shown in Fig. , assuming a Gaussian instrument response function with an FWHM of 1.37 and 1.29 nm, respectively, as proposed by . The SWIR-1 window (Fig. a) exhibits two weak CO2 absorption bands around 1568–1585 nm and 1598–1615 nm and has the advantage of a stronger top-of-atmosphere (TOA) signal due to higher solar irradiance and surface albedo at these wavelengths. It also allows for the simultaneous retrieval of CH4 using the CH4 absorption band near 1666 nm. The SWIR-2 window, on the other hand, exhibits two stronger CO2 absorption bands around 1995–2035 and 2045–2080 nm and has higher sensitivity to atmospheric aerosol that can potentially be exploited during the XCO2 retrieval . showed similar performance for SWIR-1 and SWIR-2 but suspect SWIR-2 to be the favorable spectral setup given the stronger CO2 absorption bands, the ability to account for particle scattering and the lower radiance SNR required to reach sufficiently small XCO2 noise errors. In this paper, we further investigate the performance of the two spectral setups in order to finally conclude which is the more suitable one given the preliminary instrument design and realistic instrument SNR assumed here.

The instrument is designed to have a radiance SNR of 100 at the continuum for a reference scene with a Lambertian surface albedo of 0.1 and solar zenith angle (SZA) of 70∘. Given the altitude of 600 km and the corresponding orbital velocity of 7562 ms-1, the instrument traverses along one 50 m ground pixel in 7.2 ms. The number of photons collected over the course of 7.2 ms is, however, not enough to reach an SNR of 100. To increase the SNR, we suggest increasing the integration time to 70 ms. This would normally lead to elongated ground pixels (approx. 50×500 m2), but by using forward motion compensation (FMC), the instrument can be periodically altered in the along-track direction such that each ground pixel is sampled for a time period longer than the actual satellite overpass time see, e.g.,. FMC has the evident drawback that the coverage along the satellite track is discontinuous, since no data are sampled when the instrument returns to the starting forward position. A second disadvantage is the geometrical distortion of the ground pixels that increases with the maximum off-nadir angle. The baseline design assumes 1000 measurements to be made in the along-track dimension for each FMC repetition, leading to off-nadir angles up to approximately 20∘. Further assuming 1000 detector pixels in the spatial dimension would consequently result in observed tiles on the order of 50×50 km2.

Table  summarizes the preliminary mission concept and instrument design parameters assumed for this study. It should be clear that this is a preliminary baseline design used to demonstrate the CO2 monitoring abilities and added value of the proposed instrument concept. Alternative instrument designs will be further investigated, and the exact instrument design will most likely be subject to change before the instrument is realized. The continuum SNR for our reference scene should nevertheless remain at roughly 100, ensuring a similar performance as presented in this paper.

To assess the performance of the proposed instrument concept with respect to retrieving XCO2 and resolving localized CO2 emissions, the expected instrument noise levels that accompany the measurements have to be quantified. To this end a numerical instrument noise model that calculates the instrument's SNR is developed, following a similar approach as, e.g.,  and . The SNR is given by 1SNR=Sσtot, where S is the signal, i.e., the number of photons emerging from a 50×50 m2 ground pixel that generate a charge in the detector, and σtot is the corresponding instrument noise. The signal S is calculated as 2S=Lλ⋅π⋅Adet4⋅fnum2⋅η⋅Qe⋅Δλ⋅tint, where Lλ is the simulated reflected solar spectral radiance at the telescope, Adet the detector pixel area, fnum the instrument's f number, η the efficiency of the optical bench, Qe the detector's quantum efficiency, Δλ the wavelength range covered by a single detector pixel and tint the integration time between the detector pixel readouts. Following the thin lens equation (for large distances between a lens and an object) and the magnification formula, the term π⋅Adet4⋅fnum2 can also be expressed as Aap⋅Ω, where Aap is the area of the aperture (=πr2 with r=7.5 cm) and Ω the instrument's solid angle, i.e., the squared ratio of the ground sampling distance (50 m) over the orbit altitude (600 km). Apart from Lλ that is calculated for each scene using a forward radiative transfer model, all quantities in Eq. () and their corresponding values were introduced in Sect. .

The total noise σtot in Eq. () accounts for the noise contribution from five separate instrument noise sources: 3σtot=σss2+σbg2+σdc2+σro2+σqz2, where σss=S is the signal shot noise, σbg is the noise due to thermal background radiation incident on the detector, σdc is the noise due to dark current in the detector, σro is the noise upon detector readout and σqz the quantization noise that arises when the analog signal is digitized. The thermal background signal per detector pixel is approximated as 4Sbg=Adet⋅Qe⋅tint⋅EBB, where EBB is the thermal black-body irradiance incident on the detector. EBB is determined by integrating the black-body spectral radiance Lλ,BB(Tbg) emitted by the background over the detector's cutoff wavelengths λ1 and λ2 and hemispheric opening angle, 5EBB=π∫λ1λ2Lλ,BB(Tbg)dλ. For this study, detector cutoff wavelengths of 900 and 2500 nm are assumed, and the background temperature Tbg is estimated to 200 K. The thermal background noise is then calculated as σbg=Sbg. Similarly, the dark current noise is given by σdc=Sdc, where Sdc=Idc⋅tint⋅Q is the per-pixel detector signal due to dark current. While Q=6.242×1018 electrons C-1 is constant, the dark current Idc strongly depends on the detector's operating temperature and is estimated to 1.6 fA pix-1 s-1 (assuming 150 K detector temperature), yielding a dark current signal of approximately 10000 electrons (e-) per detector pixel and second. Finally, the readout noise (σro) and quantization noise (σqz) are estimated to 100 and 40 e-, respectively. These noise levels are preliminary estimates used to test and evaluate the instrument concept but are comparable to those of state-of-the-art detectors for space applications.

Figure a shows the continuum SNR (calculated with Eqs. –) as a function of the scene brightness for the two prospective spectral setups SWIR-1 and SWIR-2. The scene brightness describes the conversion from incident solar irradiance to reflected solar radiance and is calculated as the product of the surface albedo and the cosine of the SZA divided by π, hence assuming a Lambertian surface. For the reference scene (albedo 0.1, SZA 70∘), the continuum SNR is approximately 180 and 100 for SWIR-1 and SWIR-2, respectively. The consistently higher SNR for SWIR-1 compared to SWIR-2 is mainly the result of higher solar radiance (see Fig. ) and generally higher surface albedo (see, e.g., Fig. 7 in ) in SWIR-1. Looking at the individual contributions from the different instrument noise sources in Fig. b, it is clear that the readout noise and signal shot noise are the major contributors, whereas the noise arising from quantization errors, dark current and thermal background radiation has a small or even negligible contribution in comparison. The signal shot noise is, however, smaller than the dark current, readout noise and quantization noise inside the CO2 absorption bands, in which the signal, and hence the signal shot noise, decreases. Note that all noise terms except for the signal shot noise, σss, are constant.

In this section we conduct a first performance evaluation of the proposed instrument concept by assessing the XCO2 retrieval errors expected on a global scale. Such errors arise due to instrument noise and because of inadequate knowledge about the light path through the atmosphere due to scattering aerosol and cirrus particles. For this purpose we use a global trial ensemble with a large collection of geophysical scenarios with varying atmospheric gas concentrations, meteorological conditions, surface albedo, SZA, and aerosol and cirrus compositions that can be expected to be observed by a polar-orbiting instrument. The same methodology and dataset have been used in several previous studies to assess the greenhouse gas retrieval performance of different satellite instruments .

The global trial ensemble contains geophysical data representative for the months of January, April, July and October. Atmospheric gas concentrations stem from the CarbonTracker model CO2 for the year 2010;, the Tracer Model 4 CH4 for the year 2006; and the ECHAM5-HAM model H2O;. Surface albedo data representative for the SWIR-1 and SWIR-2 windows stem from the MODIS (Moderate Resolution Imaging Spectroradiometer) MCD43A4 product . Aerosol optical properties are calculated (assuming Mie scattering) for an aerosol size distribution superimposed from seven lognormal size distributions and five chemical types at 19 vertical layers, as provided by the ECHAM5-HAM model . Cirrus optical properties are calculated for randomly oriented hexagonal columns and plates following the ray-tracing model of and . In total the global trial ensemble consists of approximately 10 000 scenes with XCO2 ranging from 340 to 400 ppm with an average of 382 ppm, albedo ranging from 0 to 0.7 with an average of 0.13 (SWIR-2 window), aerosol optical thickness (AOT) ranging from 0 to 1.0 with an average of 0.05 (SWIR-2 window) and cirrus optical thickness (COT) ranging from 0 to 0.8 with an average of 0.13 (SWIR-2 window). Thus, the global trial ensemble contains challenging scenes with scattering loads that would be filtered out by current satellite retrievals, such as those applied to OCO-2 and GOSAT data, which typically screen scenes with scattering optical thickness greater than 0.3 (at the O2 A band around 760 nm). All data in the global trial ensemble are re-gridded to a spatial resolution of approximately 2.8∘ × 2.8∘. This is, of course, much coarser than the envisaged 50×50 m2, but for investigating the propagation of instrument noise into the target quantity XCO2 on a global scale, this dataset serves its purpose. See previous studies e.g., for further details on the content of the global trial ensemble.

The geophysical data for each scene are fed to the radiative transfer software RemoTeC in order to simulate corresponding synthetic measurements. The measurement noise is calculated by propagating the instrument's SNR (Sect. ) into a statistical error estimate according to the rules of Gaussian error propagation . Simulations are conducted globally for the 16th day of each of the four months January, April, July and October, hence covering SZA conditions ranging from 0 to 86∘.

By retrieving XCO2 from the simulated synthetic spectra, the range of XCO2 retrieval errors that can be expected with the proposed instrument concept can be estimated, as can the ability to account for atmospheric aerosol. The RemoTeC retrieval algorithm e.g., is based on a Philipps–Tikhonov regularization scheme that uses the first-order difference operator as a side constraint to retrieve the CO2 partial column profiles, from which XCO2 can be determined. Additional retrieval parameters are the total column concentrations of H2O and CH4 (only for SWIR-1), surface albedo (as a second-order polynomial), spectral shift, solar shift, and possibly information on scattering aerosol. Here we assume knowledge about the air mass (needed to calculate XCO2), but in reality meteorological and topography data would be required to estimate the air mass.

While the previous section assessed XCO2 errors for the range of geophysical conditions expected to be encountered on a global scale, this section evaluates the CO2 monitoring capabilities at an urban scale using high-resolution CO2 concentration and surface albedo data. Similar to Sect. , the high-resolution data are used to simulate synthetic measurements, from which synthetic XCO2 abundances can be retrieved in order to make a first assessment of the CO2 monitoring ability of the proposed instrument concept in terms of resolving CO2 emission plumes.

To follow the progress on reducing anthropogenic CO2 emissions worldwide, independent monitoring systems are of key importance. In this paper, we present the concept of a compact spaceborne imaging spectrometer with a high spatial resolution of 50×50 m2 targeting the monitoring of localized CO2 emissions. We further demonstrate how the instrument concept could resolve CO2 emission plumes from localized point sources like medium-sized power plants, thus having the potential to contribute to the independent large-scale verification of reported CO2 emissions at the facility level.

Through radiative transfer simulations using a global trial ensemble, a preliminary yet realistic instrument design and an instrument noise model, we show that the expected instrument-noise-induced XCO2 errors are smaller than 1.1 and 2.0 ppm for 68 % and 95 % of the retrievals, respectively, using the SWIR-2 spectral setup covering the CO2 absorption bands near 2000 nm. For the SWIR-1 spectral setup covering the weaker CO2 absorption bands near 1600 nm, the instrument-noise-induced XCO2 errors are significantly higher, making it inadequate for the proposed instrument concept. Although the main focus in this paper is on the performance of the proposed CO2 monitoring instrument concept, we could also show that despite the usage of a single spectral window and a relatively coarse spectral resolution of 1.29 nm, scattering by highly complex atmospheric aerosol compositions can be partly accounted for during XCO2 retrievals on the global scale, limiting the deviation from the true XCO2 to at most 4.0 ppm for two-thirds of the retrievals. This gives us confidence that accurate two-dimensional fields of XCO2 enhancements could be retrieved from real spectra measured by the proposed instrument concept. A reasonable a priori state vector with respect to the aerosol properties (e.g., provided through models or a companion aerosol instrument; ) would, however, still be important. As an example, a multi-angle polarimeter instrument is planned to fly together with the CO2 instrument onboard the CO2M mission in order to minimize systematic XCO2 errors .

Using high-resolution CO2 emission data for the city of Indianapolis together with a Gaussian dispersion model, corresponding high-resolution albedo data and additional radiative transfer simulations, we have clearly demonstrated that the instrument is well suited for the task of the spaceborne CO2 monitoring of large and medium-sized power plants and can (only limited by its own instrument noise) resolve emission plumes from point sources with an emission source strength down to the order of 0.3 MtCO2yr-1. This is well below the target emission source strength of 1 MtCO2yr-1, hence leaving a significant margin for additional error sources and aspects not yet addressed here.

Given the results from this first performance assessment, the proposed instrument concept demonstrates clear potential for the independent quantification of CO2 emissions from medium-sized power plants (1–10 MtCO2yr-1), which are currently not targeted by other planned spaceborne CO2 monitoring missions. On the local scale (Indianapolis), we have constrained the present analysis to a day in July using a rather simplistic Gaussian dispersion model that assumes constant atmospheric stability and (unidirectional) horizontal wind speed. It might be that the ability to resolve CO2 emission plumes will become more, perhaps even too, challenging under certain more realistic conditions. Nevertheless, these first results are certainly promising and encourage further studies.

The high spatial resolution needed to resolve emission plumes from localized sources like medium-sized power plant does, however, imply limitations in terms of spatial coverage arising from the narrow swath (50 km assuming 1000 detector pixels in the spatial dimension) and the forward motion compensation. Hence, a single satellite with the proposed instrument concept could not quantify CO2 emissions at the local to regional scale with dense global coverage and high temporal resolution but would have to be restricted to some predefined targets. The relatively compact design with a single spectral window could, however, allow for the deployment of a fleet of instruments and hence the independent monitoring of localized CO2 emissions on a larger scale with high temporal resolution. As an alternative to a fleet of satellites, the proposed instrument concept could also prove useful in synergy with a spaceborne CO2 lidar e.g.,; the passive spectrometer would benefit from the lidar's accuracy and knowledge of the light path, and the lidar would benefit from the spectrometer's imaging capability.

With the successful demonstration in this paper, i.e., that CO2 emission plumes from medium-sized power plants can be resolved from space with a compact yet realistic instrument design, the next step will be to analyze the ability to quantify the corresponding CO2 emission rates from the two-dimensional fields of synthetically retrieved XCO2 enhancements. This follow-up study will be conducted for different seasons (with varying surface albedo and solar zenith angles), meteorological conditions and emission source strengths using large eddy, rather than Gaussian, modeling of the CO2 plume dispersion. Although the effect of aerosols has partly been assessed on the global scale in this study, information on the properties and distribution of aerosols should also be included in the local-scale simulations in order to better understand the instrument's ability to resolve and quantify localized CO2 emissions under more realistic conditions. Such an in-depth aerosol analysis is, however, the task of further future studies.