Is nucleation capable of explaining observed aerosol integral number increase during southerly transport over Scandinavia?

Abstract

A B S T R A C T Using a pseudo-Lagrangian approach, changes in aerosol size distribution was investigated during southerly transport under clear sky conditions from Finnish Lapland to Hyytiälä. Seventy-nine individual transport cases were considered. The mean transport distance was 700 km and mean transport time 66 h. On average, a sevenfold increase in Aitken mode number concentration could be observed. An increase in number concentration was observed in virtually all the cases. Several of the studied cases were associated with indications of nucleation at the receptor site. Six of the cases were simulated in detail utilizing a box-model approach. Aerosol dynamics was evaluated using the University of Helsinki Multi-component Aerosol model. Particle formation was assumed to be controlled by a kinetic nucleation mechanism. Growth of particles was suggested to be controlled by, except water and ammonia, sulphuric acid, and some unknown species with saturation vapour pressure of 3 × 10 6 cm −3 . This product was supposed to derive from terpene oxidation by hydroxyl radical, ozone, and nitrate radical.The investigation strongly suggests nucleation events occurring over large scales to be responsible for the observed number increase during transport under modelled conditions. Using a simplified two layer structure of the lowermost troposphere, we highlight the role of vertical exchange. Modelled growth rates were found to be in agreement with observational data, in the order of 1-2 nm h -1 . In order to reproduce the observed growth rates, a molar yield of condensable products from terpene oxidation of 10% was required. Concentration of sulphuric acid and condensable organic vapours were on average 3 × 10 6 and 1.5 × 10 7 cm −3 , respectively.

Enhanced organic mass fraction and decreased hygroscopicity of cloud condensation nuclei (CCN) during new particle formation events

“…The k value of 0.1 derived for the condensing organic material corresponds to typical k values of fresh SOA observed in chamber studies. The hypothesis that SOA compounds are closely associated with the growth of new aerosol particles has been brought forward for forested remote regions [Tunved et al, 2006] and has been recently substantiated with a comprehensive set of measurements [Laaksonen et al, 2008]. Using a very different set of measurements, we find evidence for the close association of SOA with NPF events also in the relatively polluted, forested Feldberg region.…”

“…Nevertheless, particle number and size are expected to remain the major predictors for the variability of CCN concentration [Dusek et al, 2006;Rose et al, 2008b;Gunthe et al, 2009]. Reduced hygroscopicity of newly formed particles can be expected where SOA plays a major role in particle growth, e.g., over boreal and tropical forests [Tunved et al, 2006;Gunthe et al, 2009]. In other regions, such as polluted megacities, high proportions of inorganic compounds may enhance the hygroscopicity of CCN [Wiedensohler et al, 2009].…”

“…Many proposed nucleation mechanisms involve sulfuric acid and other highly soluble species [Curtius, 2006;Kuang et al, 2008]. Moreover, there is strong evidence that organic acids promote particle nucleation [Zhang et al, 2004] and that in remote forested locations biogenic secondary organic aerosol (SOA) plays an important role in the growth of new particles to larger sizes [Laaksonen et al, 2008;Tunved et al, 2006]. The composition of the condensing vapors may impact the CCN properties of the newly formed particles.…”

Subarctic atmospheric aerosol composition: 3. Measured and modeled properties of cloud condensation nuclei

“…Since Abisko and the Stordalen nature reserve are located about 200 km north of the Arctic Circle, midnight sun in summer can drive the biogenic emissions as well as the photochemical processes at almost any time of the day. Particle nucleation and growth, possibly from condensation of biogenic vapors [Tunved et al, 2006] can be measured frequently, also at night [Svenningsson et al, 2008].…”

Boreal forests, aerosols and the impacts on clouds and climate

“…Although the role of forest emissions in instigating formation events is unclear, it is well understood that the oxidized BVOCs can increase particle growth rates and thereby influence the production of CCN from the newly formed particles. In remote continental environments, sulphuric acid explains only a fraction (generally less than 10%) of the observed growth of newly formed particles (Kulmala et al 2001(Kulmala et al , 2004aBoy et al 2003Boy et al , 2005Tunved et al 2006b), with the rest being attributed to SOA material from organic vapours emitted from the forest (O'Dowd et al 2002;Allan et al 2006;Tunved et al 2006a;Laaksonen et al 2008). Tunved et al (2006a) studied the evolution of aerosol in air that was transported from clean oceanic regions across the boreal forest to field stations in Finland.…”