Clouds containing supercooled water are important for both climate and weather, but our knowledge of which aerosol particle types nucleate ice in these clouds is far from complete. Combustion aerosols have strong anthropogenic sources, and if these aerosol types were to nucleate ice in clouds, they might exert a climate forcing. Here, we quantified the atmospheric ice-nucleating particle (INP) concentrations during the United Kingdom's annual Bonfire Night celebrations, which are characterized by large amounts of combustion aerosol from bonfires and fireworks. We used three immersion mode techniques covering more than 6 orders of magnitude in INP concentration over the temperature range from −10°C to homogeneous freezing. We found no observable systematic change in the INP concentration on three separate nights, despite more than a factor of 10 increase in aerosol number concentrations, up to a factor of 10 increase in PM 10 concentration, and more than a factor of 100 increase in black carbon (BC) mass concentration relative to pre-event levels. This implies that BC and other combustion aerosol such as ash did not compete with the INPs present in the background air. Furthermore, the upper limit of the ice-active site surface density, n s (T), of BC generated in these events was shown to be consistent with several other recent laboratory studies, showing a very low ice-nucleating activity of BC. We conclude that combustion aerosol particles similar to those emitted on Bonfire Night are at most of secondary importance for the INP population relevant for mixed-phase clouds in typical midlatitude terrestrial locations. Plain Language Summary Liquid water droplets found in clouds can cool to well below 0°C while remaining in the liquid phase (this is known as supercooling). These supercooled droplets can remain liquid down to below around −33°C without freezing, unless there is a certain type of aerosol particle present: an ice-nucleating particle (INP). Hence, INPs have the potential to drastically change the properties and lifetime of clouds, but the sources of INP in the atmosphere are poorly defined. In this study we measured the INP concentration before, during, and after a major combustion aerosol event in the United Kingdom, Bonfire Night. This celebration is characterized by bonfires (primarily made of waste wood, but also containing garden and household waste) and fireworks. We found that aerosol particles emitted during the celebration are not as effective at nucleating ice as aerosol particle already present in the atmosphere. We conclude that aerosol particles emitted from combustion processes such as those observed on Bonfire Night are not an important source of INPs.
