Abstract. We investigate the CCN activity of freshly emitted biomass burning particles and their hygroscopic growth at a relative humidity (RH) of 85 %. The particles were produced in the Mainz combustion laboratory by controlled burning of various wood types. The water uptake at sub-and supersaturations is parameterized by the hygroscopicity parameter, κ (c.f. Petters and Kreidenweis, 2007). For the wood burns, κ is low, generally around 0.06. The main emphasis of this study is a comparison of κ derived from measurements at sub-and supersaturated conditions (κ G and κ CCN ), in order to see whether the water uptake at 85 % RH can predict the CCN properties of the biomass burning particles. Differences in κ G and κ CCN can arise through solution nonidealities, the presence of slightly soluble or surface active compounds, or non-spherical particle shape. We find that κ G and κ CCN agree within experimental uncertainties (of around 30 %) for particle sizes of 100 and 150 nm; only for 50 nm particles is κ CCN larger than κ G by a factor of 2. The magnitude of this difference and its dependence on particle size is consistent with the presence of surface active organic compounds. These compounds mainly facilitate the CCN activation of small particles, which form the most concentratedCorrespondence to: U. Dusek (u.dusek@uu.nl) solution droplets at the point of activation. The 50 nm particles, however, are only activated at supersaturations higher than 1 % and are therefore of minor importance as CCN in ambient clouds. By comparison with the actual chemical composition of the biomass burning particles, we estimate that the hygroscopicity of the water-soluble organic carbon (WSOC) fraction can be represented by a κ WSOC value of approximately 0.2. The effective hygroscopicity of a typical wood burning particle can therefore be represented by a linear mixture of an inorganic component with κ ∼ = 0.6, a WSOC component with κ ∼ = 0.2, and an insoluble component with κ = 0.

Ultrafine particle (UFP) number and size distributions were simultaneously measured at five urban and rural sites during the summer of 2007 in Ontario, Canada as part of the Border Air Quality and Meteorology Study (BAQS-Met 2007). Particle formation and growth events at these five sites were classified based on their strength and persistence as well as the variation in geometric mean diameter. Regional nucleation and growth events and local short-lived strong nucleation events were frequently observed at the near-border rural sites, upwind of industrial sources. Surprisingly, the particle number concentrations at one of these sites were higher than the concentrations at a downtown site in a major city, despite its high traffic density. Regional nucleation and growth events were favored during intense solar irradiance and in less polluted cooler drier air. The most distinctive regional particle nucleation and growth event during the campaign was observed simultaneously at all five sites, which were up to 350 km apart. Although the ultrafine particle concentrations and size distributions generally were spatially heterogeneous across the region, a more uniform spatial distribution of UFP across the five areas was observed during this regional nucleation event. Thus, nucleation events can cover large regions, contributing to the burden of UFP in cities and potentially to the associated health impacts on urban populations. Local short-lived nucleation events at the three near-border sites during this summer three-week campaign were associated with high SO₂, which likely originated from US and Canadian industrial sources. Hence, particle formation in southwestern Ontario appears to often be related to anthropogenic gaseous emissions but biogenic emissions at times also contribute. Longer-term studies are needed to help resolve the relative contributions of anthropogenic and biogenic emissions to nucleation and growth in this region

We characterized particulate emissions from vegetation fires by burning Indonesian and German peat and other biomass fuels in a controlled laboratory setting. By measuring cloud condensation nuclei (CCN) both as a function of particle diameter (dp) and supersaturation (S), we discovered particles in peat smoke that were not activated to cloud droplets at high S (1.6%). These hydrophobic particles were present predominantly in the size range of dp > 200 nm, where typical wood burning particles are activated at S < 0.3%. Ambient measurements during the 1997 Indonesian peat fires suggested that peat smoke particles are highly soluble and therefore efficient CCN. Our CCN measurements performed on fresh smoke from peat samples of the same area suggest that these Indonesian smoke particles probably acquired soluble material through chemical processing in the atmosphere. Freshly emitted peat smoke particles are at least partially not very efficient CCN.

[1] We present the first results on optical properties (l $ 540 nm) of fresh aerosols from the combustion of Indonesian peat, German peat and other types of biomass, measured under controlled laboratory conditions. The mass scattering and mass absorption efficiencies for Indonesian and German peat aerosols are in the range of 6.0-8.1 and 0.04 -0.06 m 2 g À1 , respectively. A very high single scattering albedo (0.99) is observed for the peat smoke aerosols, reflecting the smoldering burning conditions (emission ratio, DCO/DCO 2 = 19 -50%). The relative increase in light scattering (f(RH)) due to an increase in relative humidity (RH) from 15% to 90% is very low (i.e., f(90) = 1.05) for both Indonesian and German peat aerosols. This value is considerably smaller than for aged Indonesian peat smoke particles (f(80) = 1.65) [Gras et al., 1999]. This suggests that atmospheric aging processes may be an important factor for aerosol hygroscopicity.