Ice nucleating ability of particulate emissions from solid biomass-fired cookstoves: an experimental study

Abstract. Ice nucleating abilities of particulate emissions from solid-fuel burning cookstoves were studied using a portable ice nuclei counter SPIN (SPectrometer for Ice Nuclei) as part of the SUSTAINE (Salutary Umea STudy of Aerosols IN Biomass Cookstove Emissions) laboratory experiment campaign. The emissions were generated from two traditional cookstove types commonly used for household cooking in sub-Saharan Africa, and two advanced gasifier stoves which are under research to promote sustainable development alternatives. The studied solid fuels included biomass from two different African tree species, Swedish softwood and agricultural residue products relevant to the region. Measurements were performed with a modified version of the standard water boiling test on (1) polydisperse samples from flue gas during burning and (2) size-selected accumulation mode (250–500 nm) soot particles from a 15-m3 aerosol-storage chamber, from which the particles were introduced to water-supersaturated freezing conditions in the SPIN. We observed that accumulation mode soot particles generally produced an ice-activated fraction of 10−3 in temperatures that were 1–1.5 °C higher than what was required for homogeneous freezing at fixed RHwater = 115 %. Five special experiments where the combustion performance of one cookstove was intentionally modified were also performed, which led to a significant increase in the ice nucleating ability of the particles in two experiments, resulting in 10−3 ice activation at up to 5.9 °C higher temperatures than homogeneous freezing. Moreover, six different physico-chemical properties of the emission particles were investigated but we did not find a clear correlation between them and increasing ice-nucleating ability. We conclude that in general, the studied freshly emitted combustion aerosols only facilitate immersion freezing at temperatures moderately above where homogeneous freezing occurs.