Particle fluxes and condensational uptake over sea ice during COBRA

Abstract

Particle fluxes were measured over sea ice at Hudson Bay, Canada, during the COBRA experiment in February and March, 2008. Eddy covariance particle fluxes were measured using a condensation particle counter and an ultrasonic anemometer on a 2.5m mast on the sea ice. After applying appropriate corrections and filtering, the mean net deposition velocity was 0.120.11mms -1 for particles measured with a CPC 3776 (lower size threshold, D p50 =2.5nm) and was at the detection limit of the measurement system. No evidence of nucleation events was seen. Two optical particle counters (at heights 0.2 and 1.35m on the mast) allowed size segregated fluxes of particles in the accumulation and coarse mode diameter range 0.3-20m to be derived using the aerodynamic flux gradient method. Strong net emission fluxes were observed around midday, 3rd March, when winds increased to around 10ms -1, suggesting ice particle resuspension. The fluxes during this period had a significant influence on the derived condensational loss rate to the available particle surfaces, k t. Number fluxes were greatest in the smallest size channels, while the largest sizes dominated the mass flux. Number fluxes also increased with wind speed, and this relationship was strongest for the smaller sizes. Particle mass size distributions showed an enhanced mode around 400nm (dry size). Values of k t were well approximated by the molecular regime and were found to be much smaller and less variable than values derived for marine air. © 2012. American Geophysical Union. All Rights Reserved.