Ice out of Fire : Ice and cloud condensation nucleation of aerosol particles emitted from controlled soot generation and combustion of renewable fuels

Abstract: Aerosol particles, small solid or liquid particles suspended in a gas, are found everywhere in the atmosphere. Air pollution from such particles, mainly soot from combustion-derived activity is a leading cause of premature deaths in the world and have a large effect on the climate. Aerosol particles can interact with climate-related processes in the atmosphere either directly by absorbing or reflecting incoming solar radiation and thus have a heating or cooling effect respectively, or indirectly by influencing clouds which themselves interact with radiation. In the atmosphere all droplets of a cloud form from a subset of existing aerosol particles called cloud condensation nuclei (CCN), and depending on concentration and CCN ability they influence cloud formation and structure, and in turn the climate. Clouds may also consist partly or entirely of ice crystals, which also influences cloud properties and are of special relevance to mixed-phase clouds. For water to freeze at temperatures above -38 °C, an ice nucleating particle (INP) is required. However, the ability of soot to act as such an INP has long eluded the scientific community, and it is still an open question as to why some soot is more efficient as INP than others.Two laboratory studies were performed on soot generated with a miniCAST soot generator and investigated links between a range of soot physicochemical properties such as chemical composition, soot maturity, optical properties, size and effective density. In the first study, an on-line Continuous Flow Diffusion Chamber (CFDC) was used to investigate immersion and condensation ice nucleation (IN) activity of a wide range of soot maturities from freshly emitted particles. In the other study, an off-line droplet-on-substrate method was used to infer immersion IN activity for a smaller range of soot maturities of fresh and photochemically aged particles. The studies found no significant correlations between soot properties and IN activity, and the results gave indications to that IN related soot particle properties may not be possible to capture with those ensemble measurement methods used.In a separate laboratory study, immersion and condensation IN activity of combustion emissions from a heavy-duty diesel engine burning low-sulfur fossil diesel, hydrotreated vegetable oil, and rapeseed methyl ester was studied using a CFDC. The IN activity was tested both on freshly emitted as well as photochemically aged soot particles. The study found low to no IN activity from any of the fuels, in line with the current literature.Two studies were performed on the IN and CCN activity of combustion emissions from various combinations of cook stoves and biomass fuels related to sub-Saharan Africa. Immersion and condensation freezing IN activity was measured with a CFDC, and was moderately IN active in temperatures relevant to mixed-phase clouds. The CCN study showed that primary emission factors have high variability depending on what stove-biomass fuel combination is used, and furthermore depend strongly on the potassium content in the fuel. The results from the study indicate that biomass burning from cook stoves may be an underrepresented source of CCN.

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