In order to study the solar irradiance’s prediction over Greece, WRF-Chem model is applied, using three shortwave radiation parameterization schemes: Dudhia, Goddard and RRTMG which simulate differently the aerosol-radiation interaction. This study focuses on a typical summertime wind pattern, the Etesian outbreaks, during which polluted air masses are transported in Greek territory and therefore they affect incoming solar irradiance. The results indicate that schemes overall overestimate solar irradiance reaching the ground; Dudhia scheme by 9%, RRTMG by 13%, and Goddard by 17%. The performance of all schemes is improved when the aerosol-radiation interaction is considered at least by 1.5%, while local temperature changes, by up to 1.5°, are noticed.

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The spatial structure of the marine atmospheric boundary layer (MABL) over the Aegean Sea is investigated using the Weather Research and Forecasting (WRF) mesoscale model. Two ‘first-order’ non-local and five ‘1.5-order’ local planetary boundary-layer (PBL) parametrization schemes are used. The predictions from the WRF model are evaluated against airborne observations obtained by the UK Facility for Airborne Atmospheric Measurements BAe-14 research aircraft during the Aegean-GAME field campaign. Statistical analysis shows good agreement between measurements and simulations especially at low altitude. Despite the differences between the predicted and measured wind speeds, they reach an agreement index of 0.76. The simulated wind-speed fields close to the surface differ substantially among the schemes (maximum values range from 13 to 18ms-1 at 150-m height), but the differences become marginal at higher levels. In contrast, all schemes show similar spatial variation patterns in potential temperature fields. A warmer (1–2 K) and drier (2–3gkg-1) layer than is observed, is predicted by almost all schemes under stable conditions (eastern Aegean Sea), whereas a cooler (up to 2 K) and moister (1–2gkg-1) layer is simulated under near-neutral to nearly unstable conditions (western Aegean Sea). Almost all schemes reproduce the vertical structure of the PBL and the shallow MABL (up to 300 m) well, including the low-level jet in the eastern Aegean Sea, with non-local schemes being closer to observations. The simulated PBL depths diverge (up to 500 m) due to the different criteria applied by the schemes for their calculation. Under stable conditions, the observed MABL depth corresponds to the height above the sea surface where the simulated eddy viscosity reaches a minimum; under neutral to slightly unstable conditions this is close to the top of the simulated entrainment layer. The observed sensible heat fluxes vary from −40 to 25Wm-2, while the simulated fluxes range from −40 to 40Wm-2; however, all of the schemes’ predictions are close to the observations under unstable conditions. Finally, all schemes overestimate the friction velocity, although the simulated range (from 0.2 to 0.5ms-1) is narrower than that observed (from 0.1 to 0.7ms-1).@en

This study examines how new particle formation (NPF) in the eastern Mediterranean in summer affects CCN (cloud condensation nuclei) concentrations and cloud droplet formation. For this, the concentration and size distribution of submicron aerosol particles, along with the concentration of trace gases and meteorological variables, were studied over the central (Santorini) and southern Aegean Sea (Finokalia, Crete) from 15 to 28 July 2013, a period that includes Etesian events and moderate northern surface winds. Particle nucleation bursts were recorded during the Etesian flow at both stations, with those observed at Santorini reaching up to 1.5 × 10⁴ particles cm^-3; the fraction of nucleation-mode particles over Crete was relatively diminished, but a higher number of Aitken-mode particles were observed as a result of aging. Aerosol and photochemical pollutants covaried throughout the measurement period; lower concentrations were observed during the period of Etesian flow (e.g., 43-70 ppbv for ozone and 1.5-5.7 μg m^-3 for sulfate) but were substantially enhanced during the period of moderate surface winds (i.e., increase of up to 32 for ozone and 140 % for sulfate). We find that NPF can double CCN number (at 0.1% supersaturation), but the resulting strong competition for water vapor in cloudy updrafts decreases maximum supersaturation by 14 % and augments the potential droplet number only by 12 %. Therefore, although NPF events may strongly elevate CCN numbers, the relative impacts on cloud droplet number (compared to pre-event levels) is eventually limited by water vapor availability and depends on the prevailing cloud formation dynamics and the aerosol levels associated with the background of the region.

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The composition of the atmosphere over the Aegean Sea (AS) during an 'Etesian' outbreak under the influence of biomass burning (BB) activity is investigated. Simulations with the fully coupled WRF-Chem model during the Aegean-GAME campaign (29/8-9/9/2011) are used to examine the BB effect over the region. Two distinct Etesian flow patterns characterized by different transport conditions are analysed. The influence of the off-line calculated BB emissions on the atmospheric chemical composition over the AS under these conditions is estimated. In addition, sensitivity runs are used to examine the influence of the biogenic emissions calculated on-line and the realistic representation of the stratosphere-troposphere exchange processes are investigated through the time-varying chemical boundary conditions from the MOZART global chemical transport model. The horizontal and vertical distributions of gaseous and aerosol species are simulated under long-range transport conditions and interpreted in relation to the evolution of the Planetary Boundary Layer (PBL). In the case of a weaker synoptic system (medium-range transport conditions), even a small variability of meteorological parameters in limited areas become critical for the spatial distribution of gases and aerosols. The BB activity increases O₃, PM_2.5 and organic matter concentrations up to 5.5 ppb, 5.8 μg m^-3 and 3.3 μg m^-3, respectively. The spatial extent of the simulated BB plumes is further examined by comparison with airborne measurements of hydrogen cyanide (HCN). The estimated effect of biogenic emissions on O₃ and PM_2.5 concentrations is either positive or negative (±6 ppb for O₃ and up to ± 1 μg m^-3 for PM_2.5) depending on the emission algorithm employed. The realistic representation of the chemical boundary conditions reproduces an observed layer rich in O₃ above 4 km, but also increases O₃ concentrations inside the PBL by up to 40%.

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Long-range transportation of air pollutants from industrial and urban environments can significantly affect the quality of the air in remote regions. In this study, we investigate episodes of particulate transport (PT) from Eurasia to the remote environment of Northeastern Mediterranean, i.e., the region of the North Aegean Sea (NAS), during the summer when the synoptic Etesian wind conditions prevail. A temporary monitoring station was set up at a remote region on the island of Lemnos, which is located at the center of the NAS at a distance of ca. 250 km from the continent. Measurements of the aerosol particle size distributions, the total number and mass concentrations, as well as the chemical composition of the particles were conducted from 27 August to 10 September 2011. During this period, the wind speeds were high (typically higher than 5.5 ms-1) with a direction that mostly ranged from north to northeast (68% frequency). Winds having direction ranging from northwest to south were less frequent (7% frequency), while the rest of the cases were characterized as calm (i.e., wind speeds less than 1 m s-1; 25% frequency). Seven PT episodes were observed during the sampling period. When the wind direction was northeastern we observed up to a six-fold increase in particle number concentration of nucleation mode, while the peak size of the particles decreased from 100 to 20 nm. Interestingly, the nucleation-mode particles grew from ca. 15 to 25 nm with rates of ca. 9.0 nm h-1, which are representative of polluted areas. Analysis of the chemical composition of particle samples collected on filters during the PT episodes shows that the concentration of sulfates and nitrates increased by ca. 60%, while the OC/EC ratio increased by ca. 22% compared to the rest of the sampling period. Back-trajectory analysis for the period during the episodes shows that the air masses arriving at the station passed over the greater Istanbul area and the Black Sea 9 to 12 h before reaching our station. These observations provide strong evidence that the air quality in the remote region of the NAS can be significantly affected by the transportation of particulate pollution during the summer period, having potentially important effects upon human health and climate in the region.@en