Soot particles released in the atmosphere have long been investigated for their ability to affect the radiative forcing. Although freshly emitted soot particles are generally considered to yield only positive contributions to the radiative forcing, atmospheric aging can activate them into efficient cloud condensation or ice nuclei, which can trigger the formation of persistent clouds and ultimately provide a negative contribution to the radiative forcing. Depending on their residence time in the atmosphere, soot particles can undergo several physical and chemical aging processes that affect their chemical composition, particle size distribution and morphology, and ultimately their optical and hygroscopic properties. The impact of the physical-chemical aging on the properties of soot particles is still difficult to quantify, as well as their effect on the radiative forcing of the atmosphere.This work investigates the hygroscopic properties of chemically aged soot particles obtained from the combustion of aviation fuel, and in particular the interplay between aging mechanisms initiated by two widespread atmospheric oxidizers (O-3 and SO2). Activation is measured in water supersaturation conditions using a cloud condensation nuclei counter. Once particle morphology and size distribution are taken into account, the hygroscopicity parameter kappa is derived using kappa-K & ouml;hler theory and correlated to the change of the chemical composition of the particles aged in a simulation chamber. While fresh soot particles are poor cloud condensation nuclei (kappa < 10(-4)) and are not significantly affected by either O-3 or SO2 at the timescale of the experiments, rapid activation is observed when they are simultaneously exposed to both oxidizers. Activated particles become efficient cloud condensation nuclei, comparable to the highly hygroscopic particulate matter typically found in the atmosphere (kappa = 0.2-0.6 at RH = 20 %). Statistical analysis reveals a correlation between the activation and sulfur-containing ions detected on the chemically aged particles that are absent from the fresh particles.

Wildfire is a major source of biomass burning aerosols, which greatly impact Earth climate. Tree species in North America (NA) boreal forests can support high-intensity crown fires, resulting in elevated injection height and longer lifetime (on the order of months) of the wildfire aerosols. Given the long lifetime, the properties of aged NA wildfire aerosols are required to understand and quantify their effects on radiation and climate. Here we present comprehensive characterization of climatically relevant properties, including optical properties and cloud condensation nuclei (CCN) activities of aged NA wildfire aerosols, emitted from the record-breaking Canadian wildfires in August 2017. Despite the extreme injection height of similar to 12 km, some of the wildfire plumes descended into the marine boundary layer in the eastern North Atlantic over a period of similar to 2 weeks, owing to the dry intrusions behind mid-latitude cyclones. The aged wildfire aerosols have high single scattering albedos at 529 nm (omega(529); 0.92-0.95) while low absorption Angstrom exponents (angstrom(abs)) at 464 nm/648 nm (0.7-0.9). In comparison, angstrom(abs) of fresh/slightly aged ones are typically 1.4-3.5. This low angstrom(abs) indicates a nearly complete loss of brown carbon, likely due to bleaching and/or evaporation, during the long-range transport. The nearly complete loss suggests that on global average, direct radiative forcing of BrC may be minor. Combining Mie calculations and the measured aerosol hygroscopicity, volatility and size distributions, we show that the high omega(529) and low angstrom(abs) values are best explained by an external mixture of non-absorbing organic particles and absorbing particles of large BC cores (> similar to 110 nm diameter) with thick non-absorbing coatings. The accelerated descent of the wildfire plume also led to strong increase of CCN concentration at the supersaturation levels representative of marine low clouds. The hygroscopicity parameter, kappa(CCN), of the aged wildfire aerosols varies from 0.2 to 0.4, substantially lower than that of background marine boundary layer aerosols. However, the high fraction of particles with large diameter (i.e., within accumulation size ranges, similar to 100-250 nm) compensates for the low values of., and as a result, the aged NA wildfire aerosols contribute more efficiently to CCN population. These results provide direct evidence that the long-range transported NA wildfires can strongly influence CCN concentration in remote marine boundary layer, therefore the radiative properties of marine low clouds. Given the expected increases of NA wildfire intensity and frequency and regular occurrence of dry intrusion following mid-latitude cyclones, the influence of NA wildfire aerosols on CCN and clouds in remote marine environment need to be further examined.

Radiative forcing by aerosol particles containing black carbon (BC) may be positive or negative depending on specific atmospheric conditions. Black carbon itself absorbs solar radiation and thereby heats the surrounding environment. On the other hand, as a result of atmospheric aging, BC-containing particles may become hydrophilic due to oxidation, condensation, and/or coagulation of water-soluble material. The aged particles can act as cloud condensation nuclei (CCN) and contribute to cloud formation that may result either in cooling or heating. In this work, through a series of laboratory experiments, we investigate the transformation of soot particles from hydrophobic to hydrophilic and estimate the atmospheric residence time required for this transformation. Ethylene flame-generated soot particles were size-selected and exposed to OH radicals in a Potential Aerosol Mass flow reactor. Aging was simulated via OH exposures equivalent to atmospheric lifetimes over a range from hours to multiple days. The chemical composition of the organic coatings as a function of OH exposure was monitored with an Aerodyne Aerosol Mass Spectrometer. The CCN activity of the aged soot particles was measured as a function of OH exposure and chemical composition. Experimental measurements indicate that heterogeneous OH oxidation of initially CCN-inactive nascent soot produces CCN-active particles. Critical supersaturations at integrated OH exposures equivalent to 0.4, 2, and 10 days are 2.1%, 0.82%, and 0.40%. Corresponding values for the effective hygroscopicity parameter, K-eff,, ranged from K-eff < 8 x 10(-4) to K-eff = 0.09 as a function of OH exposure. Condensation of hydrophilic organic or inorganic coatings (produced from oxidation of gas-phase precursors introduced to the flow reactor) on soot particles speeds up the CCN activation by a factor of 6-50 depending on the nature and thickness of the coating. The results suggest that CCN activation of atmospheric BC-containing particles is primarily due to secondary coatings. (C) 2014 Elsevier Ltd. All rights reserved.