Possible controls on Arctic clouds by natural aerosols from long-range transport of biogenic emissions and ozone depletion events

During the PAMARCMiP 2018 campaign (March and April 2018) a proton-transfer-reaction mass spectrometer (PTR-MS) was deployed onboard the POLAR 5 research aircraft and sampled the High Arctic atmosphere under Arctic haze conditions. More than 100 compounds exhibited levels above 1 pmol/mol in at least 25% of the measurements. We used back trajectories and acetone mixing ratios to identify periods with and without long-range transport from continental sources.

Air masses with continental influence contained elevated levels of compounds associated with (aged) biogenic emissions or aged anthropogenic pollution (e.g., methanol, peroxyacetylnitrate (PAN), acetone, acetic acid, methylethylketone (MEK), proprionic acid, and pentanone), but benzene – a marker for primary pollution – was not enhanced. Almost half of all positively detected compounds (>100) in the High Arctic atmosphere can be associated with terpene oxidation products. This constitutes a strong signature of biogenic terpenes and their oxidation products on the High Arctic atmosphere. Many of these compounds will condense and produce biogenic secondary organic aerosol (SOA) – a natural source of organic aerosol (OA) in addition to the aerosols that can be associated with anthropogenic pollution. Therefore, we hypothesize that biogenic SOA exerted significant control over the complex system of aerosols, clouds and longwave radiation in the pre-industrial Arctic winter, even though their role is likely marginal under contemporary polluted Arctic haze conditions. However, biogenic SOA may become an important factor in the future again, if biogenic emissions are enhanced due to climate change and if polluting technologies are (hopefully) phased out in the near future.

During two flights, surface ozone depletion events (ODE) were observed that coincided with enhanced levels of acetone, and methylethylketone. There is evidence that ODEs may also be associated with the emission of biogenic ice-nucleating particles (INP) because the filter samples taken during these two flights exhibited enhanced levels of highly active ice-nucleating particles (INP).

Both these processes, INP production in association with ozone depletion events, and the transport of biogenic SOA could require corrections in estimates of preindustrial radiative forcing (RF). If preindustrial RF has been stronger, the Arctic amplification would be even stronger than currently assumed.