Abstract
Abstract. This work examines the effect of direct radiative forcing of aerosols in the eastern Mediterranean troposphere as a function of air mass composition, particle size distribution and hygroscopicity, and relative humidity (RH). During intensive field measurements on the island of Crete, Greece, the hygroscopic properties of atmospheric particles were determined using a Hygroscopicity Tandem Differential Mobility Analyzer (H-TDMA) and a Hygroscopicity Differential Mobility Analyzer-Aerodynamic Particle Sizer (H-DMA-APS). Similar to former studies, the H-TDMA identified three hygroscopic sub-fractions of particles in the sub-μm range: a more hygroscopic group, a less hygroscopic group and a nearly hydrophobic particle group. The average hygroscopic particle growth factors at 90 % RH were a significant function of particle mobility diameter (Dp): 1.42 (± 0.05) at 30 nm compared to 1.63 (± 0.07) at 250 nm. The H-DMA-APS identified up to three hygroscopic sub-fractions at mobility diameters of 1.0 and 1.2 μm. The data recorded between 12 August and 20 October 2005 were classified into four distinct synoptic-scale air mass types distinguishing between different regions of origin (western Mediterranean vs. the Aegean Sea) as well as the degree of continental pollution (marine vs. continentally influenced). The hygroscopic properties of particles with diameter Dp≥150 nm showed the most pronounced dependency on air mass origin, with growth factors in marine air masses exceeding those in continentally influenced air masses. Particle size distributions and hygroscopic growth factors were used to calculate aerosol light scattering coefficients at ambient RH using a Mie model. A main result was the pronounced enhancement of particle scattering over the eastern Mediterranean due to hygroscopic growth, both in the marine and continentally influenced air masses. When RH reached its summer daytime values around 70–80 %, up to 50–70 % of the calculated visibility reduction was due to the hygroscopic growth of the particles by water compared to the effect of the dry particles alone. The estimated aerosol direct radiative forcings for both, marine and continentally influenced air masses were negative indicating a net cooling of the atmosphere due to the aerosol. The radiative forcing ΔFr was nevertheless governed by the total aerosol concentration most of the time: ΔFr was typically more negative for continentally influenced aerosols (ca. −4 W m−2) compared to rather clean marine aerosols (ca. −1.5 W m−2). When RH occasionally reached 90 % in marine air masses, ΔFr even reached values down to −7 W m−2. Our results emphasize, on the basis of explicit particle hygroscopicity measurements, the relevance of ambient RH for the radiative forcing of regional atmospheres.
Highlights
In the accretive discussion on global climate change, atmospheric particles have been identified as a key uncertainty with respect to the global radiative balance
Different instrumental techniques including a Hygroscopicity Tandem Differential Mobility Analyzer (H-TDMA) instrument and a H-DMA-Aerodynamic Particle Sizer (APS) instrument were deployed at the Finokalia research station on Crete (Greece) to characterize the hygroscopic properties of the eastern Mediterranean aerosol fine and coarse particle size range
This paper presents physical and chemical parameters of aerosol particles collected between August and October 2005 in Finokalia on the northern coast of Crete, Greece
Summary
In the accretive discussion on global climate change, atmospheric particles (aerosols) have been identified as a key uncertainty with respect to the global radiative balance The understanding of the growth behaviour of particulate organic matter (POM) has been more patchy, due to the myriad of individual species involved (Saxena and Hildemann, 1996) This lack of understanding has hampered a better understanding of the role of POM on climate (Kanakidou et al, 2005). Different instrumental techniques including a H-TDMA instrument and a H-DMA-APS instrument were deployed at the Finokalia research station on Crete (Greece) to characterize the hygroscopic properties of the eastern Mediterranean aerosol fine and coarse particle size range. We further combine these data with simultaneous in-situ aerosol optical observations as well as particle number size distributions presented in an. The measured size-segregated particle hygroscopic growth factors were used in an optical radiation transfer model to investigate the influence of the prevailing ambient RH on the ambient particle scattering coefficients and therewith on the resulting aerosol direct radiative forcing
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