Microphysical and Chemical Aerosol Properties of Cloud Droplet Residual Particles Collected in Arctic Clouds during ACLOUD

Low and mid-level, supercooled and mixed-phase clouds are suspected to contribute considerably to the amplified warming of the Arctic. Therefore, a better understanding about the formation and persistence of these arctic clouds is needed. Closely related to the cloud formation is the question, on which arctic aerosol particles these clouds are formed and what are the sources of those cloud condensation nuclei (CCN). The CCN might be locally produced, emitted or advected within the boundary layer reaching the cloud base or might be long range transported through the free troposphere and entrained into the cloud from above (Tjernström et al. 2014). This again seems to depend mainly on the situation whether the cloud is capped by or extends into the inversion layer. Indeed, for two recent arctic field campaigns (ASCOS, SHEBA) that the latter was observed more frequently (Sedlar et al. 2012). Therefore the investigation of the microphysical and chemical aerosol properties of particles that were involved in arctic cloud formation, which should furthermore result in indications about their sources, was part of the aircraft-based field campaign ACLOUD (Arctic CLoud Observations Using airborne measurements during polar Day, part of the German arctic research project (AC)³) in May/June 2017. Research flights were carried out from Longyearbean (Svalbard) over open sea water, the marginal ice zone and sea ice, whereby supercooled cloud droplets of arctic low and mid-level clouds were sampled in-situ by means of a Counterflow Virtual Impactor (CVI) inlet (Ogren et al. 1985) aboard the Polar 6 aircraft. After evaporating the condensed water inside the CVI, dry cloud droplet residuals (CDR) which are considered as the true CCN, were released and characterized by different aerosol sensors. Below and above the clouds, the CVI was deployed as an aerosol inlet in order to sample the prevailing ambient particles (AP) and to determine the same aerosol properties. AP and CDR number concentration and size distribution was measured by a condensation particle counter (CPC), the ultra-high sensitivity aerosol spectrometer (UHSAS) and an optical particle counter (Sky-OPC), respectively. The AP and CDR mixing state of different aerosol types and the CDR number and mass concentration of black carbon is inferred from measurement with a single particle mass spectrometer (ALABAMA), a single particle soot photometer (SP2) and a particle soot absorption photometer (PSAP). Case studies of in-situ cloud transects during ACLOUD will be presented that show differences in the size distribution and chemical composition of the CDR, which will be discussed with regard to potential impact parameters. This implies cloud base height (low vs. mid-level clouds), cloud microphysical conditions, the underlying surface (open water, drift ice zone, sea ice, land), the aerosol properties of the aerosol below and above the clouds, meteorological conditions (boundary layer height, inversion layers), and weather situations (cold air outbreak, warm air intrusion). Such an in-situ cloud droplet sampling in the Arctic by means of a CVI inlet has been applied during the ISDAC campaign (Hiranuma et al. 2013). Due to a different aerosol instrumentation it was possible to characterize the CDR with a higher time and primarily higher spatial resolution during ACLOUD.

References:

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