Christos Matsoukas

The interaction of biomass burning aerosols and especially their absorbing components like black carbon (BC) and brown carbon (BrC) with solar radiation is a source of significant uncertainty in estimations of the present and future climate. We investigate the three-dimensional aerosol clear-sky direct radiate effect (DRE) over the Euro-Mediterranean region during the summertime 2012 PEGASOS campaign, during which intense wildfires took place in Europe. Aerosol data from the 3-D chemical transport model PMCAMx-SR were used along with the FORTH radiative transfer model. The DREs are estimated at the Earth's surface, within the atmosphere and at the top of the atmosphere (TOA), at a high spatial (36 km × 36 km grids, 27 vertical layers) and temporal (hourly) resolution. Biomass burning aerosols significantly warms the atmosphere and cool the surface by up to 30 Wm-2 on a monthly basis near major wildfires (e.g. the Iberian Peninsula), thus impacting significantly the summertime solar radiative budget over the Euro-Mediterranean. Owing to their relatively higher absorption ability, biomass burning BC contributes more than BrC to both the atmospheric warming and surface cooling effects. The biomass burning BrC to BC DRE ratios range between 0.08 and 0.17 within the atmosphere and from 0.14 to 0.25 at the Earth's surface. Biomass burning aerosols induce a top-of-the-atmosphere warming effect up to 3.1 Wm-2 over regions with relatively high surface albedo and a planetary cooling effect up to 3.8 Wm-2 when they are advected over the sea. The biomass burning aerosol radiative effects exhibit significant hour-to-hour and day-to-day variability due to the heterogeneity of their emissions and the meteorological variability. The instantaneous atmospheric warming and surface cooling of biomass burning aerosols during a strong wildfire event near Valencia, Spain, both reached 600 Wm-2, inducing atmospheric heating rates as high as 50 K d-1 close to their source region. Ignoring BrC absorptivity and photobleaching induces errors in DRE calculations especially for the top-of-the-atmosphere DRE over high surface albedo regions. In these regions, the planetary warming is underestimated by up to 30 % when the absorption of BrC is neglected and is overestimated by up to 20% when BrC photobleaching is ignored.

Black carbon (BC) and brown carbon (BrC) are light-absorbing carbonaceous aerosol components that can contribute to radiative forcing and thus affect the climate. In this study, we focus on the modification of aerosol optical properties associated with BrC emissions from biomass burning. BrC is simulated with the introduction of three new species in the three-dimensional chemical transport model PMCAMx-SR, two primary-absorbing (inert and reactive BrC) species, and one "photobleached" BrC species. 10% of the emitted BrC is assumed to be inert, and the rest to be reactive and able to undergo photobleaching. The scattering and absorption coefficients of the aerosol for different wavelengths are estimated by using Mie theory. BrC causes a 5-15% increase of the aerosol optical depth at 550 nm in regions affected by fires and can increase light absorption by up to 12% compared to when there is no BrC. During major biomass burning events, the absorption of BrC can reach up to 13% of that of BC at 550 nm and 25% at 440 nm. The ability of the model to reproduce measured absorption is improved when BrC is added to the light-absorbing components.

In the present study, the all-sky Dust Direct Radiative Effect (DDRE) is estimated during intense Dust Aerosol (DA) episodes that took place from 2005 to 2018 over the broader Mediterranean Basin (MB). DA episodes are identified using a satellite algorithm, which is based on aerosol optical properties. The DDREs are estimated using a vertically resolved version of the FORTH spectral radiative transfer model (RTM). The RTM ran on a 3-hourly temporal and 0.5° × 0.625° horizontal spatial resolution for each of the 162 identified DA episode days (DAEDs), initialized with 3D dust and total (all) aerosol optical properties, namely optical depth, single scattering albedo, and asymmetry parameter from the MERRA-2 reanalysis. Cloud data (cloud amount, optical depth, and top pressure) are taken from the ISCCP-H. The RTM computes the upwelling and downwelling solar fluxes at TOA, at the surface, and at 50 levels in the atmosphere. The runs were made with all aerosols, and all but DA. The DDREs were calculated by the difference between the two sets of output fluxes. According to the model results averaged over the 162 studied episodes, DA reduces the net surface solar radiation over the MB, causing a cooling effect as large as −77 W/m² (climatological mean value at 12:00 UTC) over areas with high dust optical depth. On the contrary, they increase the atmospheric absorbed solar radiation, resulting in considerable (up to 79 W/m²) atmospheric radiative heating. At TOA, either heating (up to 30 W/m² over desert areas) or cooling (as strong as −21 W/m² over oceanic areas) are found, depending on the significantly different underlying surface albedos. DDREs are even higher during single episodes, with a surface cooling effect as large as −200 W/m² and a corresponding atmospheric heating effect of up to +187 W/m².

The interdecadal changes of the incident solar radiation at the Earth’s surface (SSR) are mainly driven by changes in clouds and aerosols. In order to investigate their contribution to the SSR changes (global dimming and brightening or GDB), the FORTH radiative transfer model (RTM) is used to compute the SSR fluxes. The cloud input data were taken from satellite observations of ISCCP-H, while aerosols and meteorological data were taken from the MERRA-2 reanalysis dataset. The RTM operates on a monthly basis and in 0.5° × 0.625° latitude-longitude spatial resolution. The GDB was also computed keeping constant at their initial 1984 values, each input parameter that was examined, resulting in a GDB with the ‘frozen’ parameter. The contribution of each parameter to the GDB is defined as the subtraction of the frozen GDB from the base-run GDB, and the positive/negative values of the contribution indicate that the interdecadal variability of the examined parameter increased/decreased the SSR. The aerosol optical depth (AOD) produced a dimming in India, Amazonia, and S. China, whereas it induced a brightening in Europe and Mexico. On the other hand, the total cloud cover (TCC) changes caused a dimming over the Arctic, Australia, and the South Ocean against a brightening in Europe, Mexico, the Middle East, and South America. The global mean contribution of changing AOD is 0.37 W/m², and for TCC, it is 4.7 W/m², indicating that globally, the counteraction of cloud cover to the overall global dimming is larger. Opposite contributions to GDB from AOD and TCC may occur over specific regions, highlighting the complexity of the causes of the GDB phenomenon.

In this study, an assessment of the FORTH radiative transfer model (RTM) surface solar radiation (SSR) as well as its interdecadal changes (Δ(SSR)), namely global dimming and brightening (GDB), is performed during the 35-year period of 1984–2018. Furthermore, a thorough evaluation of SSR and (Δ(SSR)) is conducted against high-quality reference surface measurements from 1193 Global Energy Balance Archive (GEBA) and 66 Baseline Surface Radiation Network (BSRN) stations. For the first time, the FORTH-RTM Δ(SSR) was evaluated over an extended period of 35 years and with a spatial resolution of 0.5° × 0.625°. The RTM uses state-of-the-art input products such as MERRA-2 and ISCCP-H and computes 35-year-long monthly SSR and GDB, which are compared to a comprehensive dataset of reference measurements from GEBA and BSRN. Overall, the FORTH-RTM deseasonalized SSR anomalies correlate satisfactorily with either GEBA (R equal to 0.72) or BSRN (R equal to 0.80). The percentage of agreement between the sign of computed GEBA and FORTH-RTM Δ(SSR) is equal to 63.5% and the corresponding percentage for FORTH-RTM and BSRN is 54.5%. The obtained results indicate that a considerable and statistically significant increase in SSR (Brightening) took place over Europe, Mexico, Brazil, Argentina, Central and NW African areas, and some parts of the tropical oceans from the early 1980s to the late 2010s. On the other hand, during the same 35-year period, a strong and statistically significant decrease in SSR (Dimming) occurred over the western Tropical Pacific, India, Australia, Southern East China, Northern South America, and some parts of oceans. A statistically significant dimming at the 95% confidence level, equal to −0.063 Wm⁻² year⁻¹ (or −2.22 Wm⁻²) from 1984 to 2018 is found over the entire globe, which was more prevalent over oceanic than over continental regions (−0.07 Wm⁻² year⁻¹ and −0.03 Wm⁻² year⁻¹, statistically significant dimming at the 95% confidence level, respectively) in both hemispheres. Yet, this overall 35-year dimming arose from alternating decadal-scale changes, consisting of dimming during 1984–1989, brightening in the 1990s, turning into dimming over 2000–2009, and brightening during 2010–2018.

In the present study, we used the FORTH deterministic spectral Radiation Transfer Model (RTM) to estimate detailed three-dimensional distributions of the Direct Radiative Effects (DREs) and their consequent modification of the thermal structure of the regional atmosphere during an intense dust episode that took place from 16 to 18 June 2016 over the Mediterranean Basin (MB). The RTM operated on a 3-hourly temporal and 0.5 × 0.625° spatial resolution, using 3-D aerosol optical properties (i.e., aerosol optical depth, single scattering albedo, and asymmetry parameter) and other surface and atmospheric properties from the MERRA-2 reanalysis and cloud properties (i.e., cloud amount, cloud optical depth, and cloud top height) from the ISCCP-H dataset. The model ran with and without dust aerosols, yielding the upwelling and downwelling solar fluxes at the top of the atmosphere, in the atmosphere, and at the Earth’s surface as well as at 50 levels in the atmosphere. The dust direct radiative effect (DDRE) was estimated as the difference between the two (one taking into account all aerosol types and one taking into account all except for dust aerosols) flux outputs. The atmospheric heating rates and subsequent convection induced by dust radiative absorption were calculated at 50 levels to determine how the DDRE affects the thermal structure and dynamics of the atmosphere. The results showed that such a great and intense dust transport event significantly reduces the net surface solar radiation over the MB (by up to 62 W/m² on a daily mean basis, and up to 200 W/m² on an hourly basis, at 12:00 UTC) while increasing the atmospheric solar absorption (by up to 72 W/m² daily and 187 W/m² hourly, at 12:00 UTC). At the top of the atmosphere, both heating (over desert areas) and cooling (over oceanic and other continental areas) are observed due to the significantly different surface albedos. Transported dust causes considerable heating of the region’s atmosphere, which becomes maximum at altitudes where the dust loadings are highest (0.14 K/3 h on 17 June 2016, 12:00 UTC, at 3–5 km above sea level). The dust solar absorption and heating induce a buoyancy as strong as 0.014 m/s², resulting in considerable changes in vertical air motions and possibly contributing to the formation of middle- and high-level clouds over the Mediterranean Basin.