23 This study addresses, through two types of experiments, the potential for the oceans to act as a 24 source of atmospheric ice-nucleating particles (INPs). The INP concentration via deposition 25 mode nucleation was measured in situ at a coastal site in British Columbia in August 2013. The 26 INP concentration at conditions relevant to cirrus clouds (i.e., -40°C and relative humidity with 27 respect to ice, RHice=139%) ranged from 0.2 L -1 to 3.3 L -1 . Correlations of the INP 28 concentrations with levels of anthropogenic tracers (i.e., CO, SO2, NOx, and black carbon) and 29 numbers of fluorescent particles do not indicate a significant influence from anthropogenic 30 sources or submicron bioaerosols, respectively. Additionally, the INPs measured in the 31 deposition mode showed a poor correlation with the concentration of particles with sizes larger 32 than 500 nm, which is in contrast with observations made in the immersion freezing mode. To 33 investigate the nature of particles that could have acted as deposition INP, laboratory 34 experiments with potential marine aerosol particles were conducted under the ice-nucleating 35 conditions used in the field. At -40°C, no deposition activity was observed with salt aerosol 36 particles (sodium chloride and two forms of commercial sea salt: Sigma-Aldrich and Instant 37 Ocean), particles composed of a commercial source of natural organic matter (Suwannee River 38 humic material), or particle mixtures of sea salt and humic material. In contrast, exudates from 39 three phytoplankton (Thalassiosira pseudonana, Nanochloris atomus, and Emiliania huxleyi) 40 and one marine bacterium (Vibrio harveyi) exhibited INP activity at low RHice values, down to 41 below 110%. This suggests that the INPs measured at the field site were of marine biological 42 origins, although we cannot rule out other sources, including mineral dust. 43 44 thank the University of Denver faculty start-up fund and PROF grant for 463 partial financial support. 464 465 References 466 Alpert, P.A., Aller, J.Y., and Knopf, D.A., 2011a. Ice nucleation from aqueous NaCl droplets with and 467 without marine diatoms, Atmos. Chem. Phys, 11, 5539-5555.
469Alpert, P. A., Aller, J. Y., and Knopf, D. A., 2011b. Initiation of the ice phase by marine biogenic surfaces 470 in supersaturated gas and supercooled aqueous phases, Phys.