Articles | Volume 16, issue 11
Atmos. Chem. Phys., 16, 7389–7409, 2016

Special issue: BACCHUS – Impact of Biogenic versus Anthropogenic emissions...

Special issue: CHemistry and AeRosols Mediterranean EXperiments (ChArMEx)...

Atmos. Chem. Phys., 16, 7389–7409, 2016

Research article 14 Jun 2016

Research article | 14 Jun 2016

Biomass-burning impact on CCN number, hygroscopicity and cloud formation during summertime in the eastern Mediterranean

Aikaterini Bougiatioti1,2,3, Spiros Bezantakos4,5, Iasonas Stavroulas3, Nikos Kalivitis3, Panagiotis Kokkalis2,11, George Biskos6,7, Nikolaos Mihalopoulos3,7,10, Alexandros Papayannis2, and Athanasios Nenes1,8,9,10 Aikaterini Bougiatioti et al.
  • 1School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA
  • 2Laser Remote Sensing Unit, National Technical University of Athens, Zografou, Athens, Greece
  • 3ECPL, Department of Chemistry, University of Crete, Voutes, 71003 Heraklion, Greece
  • 4Department of Environment, University of the Aegean, Mytilene, 81100, Greece
  • 5Institute of Nuclear Technology and Radiation Protection, NCSR “Demokritos”, 15310 Ag. Paraskevi, Athens, Greece
  • 6Faculty of Civil Engineering and Geosciences, Delft University of Technology, Delft 2728 CN, the Netherlands
  • 7Energy Environment and Water Research Center, The Cyprus Institute, Nicosia 2121, Cyprus
  • 8School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA, USA
  • 9Institute of Chemical Engineering Sciences (ICE-HT), FORTH, Patras, Greece
  • 10IERSD, National Observatory of Athens, P. Penteli 15236, Athens, Greece
  • 11IAASARS, National Observatory of Athens, P. Penteli 15236, Athens, Greece

Abstract. This study investigates the concentration, cloud condensation nuclei (CCN) activity and hygroscopic properties of particles influenced by biomass burning in the eastern Mediterranean and their impacts on cloud droplet formation. Air masses sampled were subject to a range of atmospheric processing (several hours up to 3 days). Values of the hygroscopicity parameter, κ, were derived from CCN measurements and a Hygroscopic Tandem Differential Mobility Analyzer (HTDMA). An Aerosol Chemical Speciation Monitor (ACSM) was also used to determine the chemical composition and mass concentration of non-refractory components of the submicron aerosol fraction. During fire events, the increased organic content (and lower inorganic fraction) of the aerosol decreases the values of κ, for all particle sizes. Particle sizes smaller than 80 nm exhibited considerable chemical dispersion (where hygroscopicity varied up to 100 % for particles of same size); larger particles, however, exhibited considerably less dispersion owing to the effects of condensational growth and cloud processing. ACSM measurements indicate that the bulk composition reflects the hygroscopicity and chemical nature of the largest particles (having a diameter of  ∼  100 nm at dry conditions) sampled. Based on positive matrix factorization (PMF) analysis of the organic ACSM spectra, CCN concentrations follow a similar trend as the biomass-burning organic aerosol (BBOA) component, with the former being enhanced between 65 and 150 % (for supersaturations ranging between 0.2 and 0.7 %) with the arrival of the smoke plumes. Using multilinear regression of the PMF factors (BBOA, OOA-BB and OOA) and the observed hygroscopicity parameter, the inferred hygroscopicity of the oxygenated organic aerosol components is determined. We find that the transformation of freshly emitted biomass burning (BBOA) to more oxidized organic aerosol (OOA-BB) can result in a 2-fold increase of the inferred organic hygroscopicity; about 10 % of the total aerosol hygroscopicity is related to the two biomass-burning components (BBOA and OOA-BB), which in turn contribute almost 35 % to the fine-particle organic water of the aerosol. Observation-derived calculations of the cloud droplet concentrations that develop for typical boundary layer cloud conditions suggest that biomass burning increases droplet number, on average by 8.5 %. The strongly sublinear response of clouds to biomass-burning (BB) influences is a result of strong competition of CCN for water vapor, which results in very low maximum supersaturation (0.08 % on average). Attributing droplet number variations to the total aerosol number and the chemical composition variations shows that the importance of chemical composition increases with distance, contributing up to 25 % of the total droplet variability. Therefore, although BB may strongly elevate CCN numbers, the impact on droplet number is limited by water vapor availability and depends on the aerosol particle concentration levels associated with the background.

Short summary
BBOA from long-range transport exhibits increased CCN concentrations for particles larger than 100 nm. At the same time the hygroscopicity parameter decreased for all particle sizes, as sub-100 nm particles appear to be richer in less hygroscopic organic material, while larger particles become less hygroscopic due to condensation of less hygroscopic gaseous compounds. Finally, atmospheric processing of freshly emitted BBOA to more oxidized organic aerosol can result in a 2-fold increase of κ.
Final-revised paper