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

Abstract

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.

What do we learn from long-term cloud condensation nuclei number concentration, particle number size distribution, and chemical composition measurements at regionally representative observatories?

“…At the coastal sites in the Mediterranean (FIK) and Atlantic (MHD), 30 the non-refractory submicron aerosol particle mass is driven by inorganic components, predominantly sulfate. However, increased organic particle mass is observed during the biomass burning season at FIK (Bougiatioti et al, 2016), when κ reaches a minimum, and in springtime at MHD, as has been observed previously (Ovadnevaite et al, 2014 generally high owing to the influence of sea salt, but at the same time is also very variable owing to the mixed influences of marine organic aerosol and anthropogenic air pollution. Fig.…”

“…At BRW, the influence of Arctic Haze is evident from the higher CCN number concentrations in late winter and spring (Barrie, 1986). Also at FIK, the seasonal cycle is characterized by pollution events occurring in summer that are 5 associated with long-range transport of biomass burning aerosol containing larger size particles and absence of precipitation (Bougiatioti et al, 2016). The coastal sites at the Atlantic (MHD) and Pacific (NOT) show relatively large variability in all measured parameters without exhibiting a distinct seasonal cycle.…”

“…In the Mediterranean environment the distribution is not bimodal, although it exhibits a small plateau for slightly more hygroscopic particles, while the majority of particles are on average less hygroscopic than in the other coastal areas. This might be due to European pollution outflow and biomass burning plumes (Bougiatioti et al, 2016). At NOT, despite the influence of two Atmos.…”

“…The coastal environments of FIK in the Mediterranean and NOT in the Pacific Ocean exhibit generally higher concentrations due to particular pollution influences which for example include long-range transport of NE European pollution and biomass burning plumes (Bougiatioti et al, 2016) and long-range 5 transport of East Asian pollutions plumes (Iwamoto et al, 2016), respectively. In terms of CCN number concentrations, the NOT site is in fact similar to the European rural background sites MEL and CES, which experience higher concentrations than the higher latitude continental background site in VAV and the substantially cleaner boreal forest environment (SMR).…”

“…Bougiatioti et al, 2009;Crosbie et al, 2015;Cubison et al, 2008;Ervens et al, 2010;Paramonov et al, 2015;Rose et al, 2011;Whitehead et al, 2014;Wong et al, 2011). Most of these observations focus on relatively short time periods and some attempt to capture specific circumstances such as biomass burning events (e.g., Bougiatioti et al, 2016) or focus on the hygroscopicity of specific aerosol particle components such as black carbon (e.g., Schwarz et al, 2015) or organic carbon (e.g., Frosch et al, 2011). While such studies provide detailed insights into CCN activation processes and 5 contribute to our comprehensive understanding of aerosol-cloud interactions, they cannot address questions of regional and temporal CCN variability.…”