• Energy Research

Abstract. This paper presents the re-construction of the 80 year time series of daily global shortwave downward radiation (SDR) at the subtropical high-mountain Izaña Atmospheric Observatory (IZO, Spain). For this purpose, we combine SDR estimates from sunshine duration (SD) data using the Ångström–Prescott method over the 1933/1991 period, and SDR observations directly performed by pyranometers between 1992 and 2013. Since SDR measurements have been used as a reference, a strict quality control has been applied, when it was not possible data have been re-calibrated by using the LibRadtran model. By comparing to high quality SDR measurements, the precision and consistency over time of SDR estimations from SD data have successfully been documented. We obtain a overall root mean square error (RMSE) of 9.2% and an agreement between the variances of SDR estimations and SDR measurements within 92% (correlation coefficient of 0.96). Nonetheless, this agreement significantly increases when the SDR estimation is done considering different daily fractions of clear sky (FCS). In that case, RMSE is reduced by half, up to about 4.5%, when considering percentages of FCS > 40% (90% of days in the testing period). Furthermore, we prove that the SDR estimations can monitor the SDR anomalies in consistency with SDR measurements and, then, can be suitable for re-constructing solar radiation time series. The re-constructed IZO global SDR time series between 1933 and 2013 confirms discontinuities and periods of increases/decreases of solar radiation at Earth's surface observed at a global scale, such as the early brightening, dimming and brightening. This fact supports the consistency of the IZO SDR time series presented in this work, which may be a reference for solar radiation studies in the subtropical North Atlantic region.

Abstract. Accurate accounting of emissions and removals of CO2 is critical for the planning and verification of emission reduction targets in support of the Paris Agreement. Here, we present a pilot dataset of country-specific net carbon exchange (NCE; fossil plus terrestrial ecosystem fluxes) and terrestrial carbon stock changes aimed at informing countries’ carbon budgets. These estimates are based on "top-down" NCE outputs from the v10 Orbiting Carbon Observatory (OCO-2) modeling intercomparison project (MIP), wherein an ensemble of inverse modeling groups conducted standardized experiments assimilating OCO-2 column-averaged dry-air mole fraction (XCO2) retrievals (ACOS v10), in situ CO2 measurements, or combinations of these data. The v10 OCO-2 MIP NCE estimates are combined with "bottom-up" estimates of fossil fuel emissions and lateral carbon fluxes to estimate changes in terrestrial carbon stocks, which are impacted by anthropogenic and natural drivers. These flux and stock change estimates are reported annually (2015–2020) as both a global 1° × 1° gridded dataset and as a country-level dataset. Across the v10 OCO-2 MIP experiments, we obtain increases in the ensemble median terrestrial carbon stocks of 3.29–4.58 PgCO2 yr-1 (0.90–1.25 PgC yr-1). This is a result of broad increases in terrestrial carbon stocks across the northern extratropics, while the tropics generally have stock losses but with considerable regional variability and differences between v10 OCO-2 MIP experiments. We discuss the state of the science for tracking emissions and removals using top-down methods, including current limitations and future developments towards top-down monitoring and verification systems.

Spectral measurements of direct solar ultraviolet irradiance are very important for many applications in the field of atmospheric sciences. Despite its usefulness, few UV monitoring sites include such measurements in their regular observational programs. Standardization of measurement methodologies and calibration techniques is required in order to reach the quality standard of global irradiance measurements. This study presents preliminary results from an intercomparison campaign of seven UV spectroradiometers of different types that took place at the high altitude site of Izana (28.3°N, 16.5°W, 2367 m above sea level), in Tenerife, Canary Islands in June 2005. The campaign is focused primarily on spectral measurements of direct solar irradiance. Among the objectives is to improve the quality of direct solar irradiance spectral measurements, through instrumental modifications and standardization of calibration techniques, as well as to assess the significance of the differences in the field of view of the spectroradiometers with respect to aerosols and to solar zenith angle. Under the low aerosol conditions prevailing during this campaign, we aimed to establish the differences among the various instruments under "ideal" conditions. Moreover, continuous measurements under stable total ozone and aerosol optical depth will be used to determine the extraterrestrial solar flux, through the application of the Langley extrapolation method. A first comparison of sky radiance measurements of the zenith light and of various directions on the sky show effects of sensitivity to polarization of one type of instruments and the variability of the provisional radiance calibration of 4 instruments.

The Total Carbon Column Observing Network (TCCON) and the Infrared Working Group of the Network for the Detection of Atmospheric Composition Change (NDACC-IRWG) are two ground-based networks that provide the retrieved concentrations of up to 30 atmospheric trace gases, using solar absorption spectrometry. Both networks provide reference measurements for the validation of satellites and models. TCCON concentrates on long-lived greenhouse gases (GHGs) for carbon cycle studies and validation. The number of sites is limited, and the geographical coverage is uneven, covering mainly Europe and the USA. A better distribution of stations is desired to improve the representativeness of the data for various atmospheric conditions and surface conditions and to cover a large latitudinal distribution. The two successive Fiducial Reference Measurements for Greenhouse Gases European Space Agency projects (FRM4GHG and FRM4GHG2) aim at the assessment of several low-cost portable instruments for precise measurements of GHGs to complement the existing ground-based sites. Several types of low spectral resolution Fourier transform infrared (FTIR) spectrometers manufactured by Bruker, namely an EM27/SUN, a Vertex70, a fiber-coupled IRCube, and a Laser Heterodyne spectro-Radiometer (LHR) developed by UK Rutherford Appleton Laboratory are the participating instruments to achieve the Fiducial Reference Measurements (FRMs) status. Intensive side-by-side measurements were performed using all four instruments next to the Bruker IFS 125HR high spectral resolution FTIR, performing measurements in the NIR (TCCON configuration) and MIR (NDACC configuration) spectral range. The remote sensing measurements were complemented by AirCore launches, which provided in situ vertical profiles of target gases traceable to the World Meteorological Organization (WMO) reference scale. The results of the intercomparisons are shown and discussed. Except for the EM27/SUN, all other instruments, including the reference TCCON spectrometer, needed modifications during the campaign period. The EM27/SUN and the Vertex70 provided stable and precise measurements of the target gases during the campaign with quantified small biases. As part of the FRM4GHG project, one EM27/SUN is now used as a travel standard for the verification of column-integrated GHG measurements. The extension of the Vertex70 to the MIR provides the opportunity to retrieve additional concentrations of N2O, CH4, HCHO, and OCS. These MIR data products are comparable to the retrieval results from the high-resolution IFS 125HR spectrometer as operated by the NDACC. Our studies show the potential for such types of spectrometers to be used as a travel standard for the MIR species. An enclosure system with a compact solar tracker and meteorological station has been developed to house the low spectral resolution portable FTIR systems for performing solar absorption measurements. This helps the spectrometers to be mobile and enables autonomous operation, which will help to complement the TCCON and NDACC networks by extending the observational capabilities at new sites for the observation of GHGs and additional air quality gases. The development of the retrieval software allows comparable processing of the Vertex70 type of spectra as the EM27/SUN ones, therefore bringing them under the umbrella of the COllaborative Carbon Column Observing Network (COCCON). A self-assessment following the CEOS-FRM Maturity Matrix shows that the COCCON is able to provide GHG data products of FRM quality and can be used for either short-term campaigns or long-term measurements to complement the high-resolution FTIR networks.

Abstract. Long-term ground-based ozone measurements are crucial to study the recovery of stratospheric ozone as well as the trends of tropospheric ozone. This study is performed in the context of the LOTUS (Long-term Ozone Trends and Uncertainties in the Stratosphere) and TOAR-II (Tropospheric Ozone Assessment Report, phase II) initiatives. We perform an intercomparison study of total column ozone and multiple partial ozone columns between the ground-based measurements available at the Lauder station from 2000 to 2022, which are the Fourier transform infrared (FTIR) spectrometer, Dobson Umkehr, ozonesonde, lidar, and the microwave radiometer. We compare partial columns, defined to provide independent information: one tropospheric and three stratospheric columns. The intercomparison is analyzed using the median of relative differences (the bias) of FTIR with each of the other measurements, the scaled Median Absolute deviation (MADs), and a trend of these differences (measurement drift). The total column shows a bias and strong scatter well within the combined systematic and random uncertainties respectively. There is however a drift of 0.6±0.5 %/decade if we consider the full time series. In the troposphere we find a low bias of -1.9 % with the ozonesondes. No drift is found between the three instruments in the troposphere, which is good for trend studies within TOAR-II. In both the lower and upper stratosphere, we get a negative bias for all instruments with respect to FTIR (between -1.2 % and -6.8 %), but all are within the range of the systematic uncertainties. In the middle stratosphere we seem to find a negative bias of around -5.2 to -6.6 %, pointing towards too high values for FTIR in this partial column. We find no significant drift in the stratosphere between ozonesonde and FTIR for all partial columns. We do observe drift between the FTIR and Umkehr partial columns in the lower and upper stratospheres (2.6±1.1 %/decade and -3.2±0.9 %/decade), with lidar in the midle and upper stratosphere (2.1±0.8 %/decade and -3.7±1.2 %/decade), and with MWR in the midle stratosphere (3.1±1.7 %/decade). These drifts point to the fact that the different observed trends in LOTUS are not due to different sampling, vertical sensitivity or time periods and gaps. However, the difference in trends in LOTUS is reduced by applying a new FTIR retrieval strategy, which changes inputs such as the choice of microwindows, spectroscopy from HITRAN2008 to HITRAN2020, and the regularization method.

Nitrous oxide (N2O) is the third most abundant anthropogenous greenhouse gas (after carbon dioxide and methane), with a long atmospheric lifetime and a continuously increasing concentration due to human activities, making it an important gas to monitor. In this work, we present a new method to retrieve N2O concentration profiles (with up to two degrees of freedom) from each cloud-free satellite observation by the Infrared Atmospheric Sounding Interferometer (IASI), using spectral micro-windows in the N2O ν3 band, the Radiative Transfer for TOVS (RTTOV) tools and the Tikhonov regularization scheme. A time series of ten years (2011–2020) of IASI N2O profiles and integrated partial columns has been produced and validated with collocated ground-based Network for the Detection of Atmospheric Composition Change (NDACC) and Total Carbon Column Observing Network (TCCON) data. The importance of consistency in the ancillary data used for the retrieval for generating consistent time series has been demonstrated. The Nitrous Oxide Profiling from Infrared Radiances (NOPIR) N2O partial columns are of very good quality, with a positive bias of 1.8 to 4% with respect to the ground-based data, which is less than the sum of uncertainties of the compared values. At high latitudes, the comparisons are a bit worse, due to either a known bias in the ground-based data, or to a higher uncertainty in both ground-based and satellite retrievals.