Free troposphere as a major source of CCN for the equatorial pacific boundary layer: Long-range transport and teleconnections
Airborne aerosol measurements in the central equatorial Pacific during\nPASE (Pacific Atmospheric Sulfur Experiment) revealed that cloud\ncondensation nuclei (CCN) activated in marine boundary layer (MBL)\nclouds were strongly influenced by entrainment from the free troposphere\n(FT). About 65% entered at sizes effective as CCN in MBL clouds, while\nsimilar to 25% entered the MBL too small to activate but subsequently\ngrew via gas to particle conversion. The remaining similar to 10% were\ninferred to be sea salt aerosol.\nFT aerosols at low carbon monoxide (CO) mixing ratios (<63 ppbv) were\nmostly volatile at 360 degrees C with a number mode peak of around 30-40\nnm dry diameter and tended to be associated with cloud outflow from\ndistant (3000 km or more) deep convection. Higher CO concentrations were\ncommonly associated with trajectories from South America and the Amazon\nregion (ca. 10 000 km away) and occurred in layers indicative of\ncombustion sources (biomass burning season) partially scavenged by\nprecipitation. These had number modes near 60-80 nm dry diameter with a\nlarge fraction of CCN.2 (those activated at 0.2% supersaturation and\nrepresentative of MBL clouds) prior to entrainment into the MBL. Flight\naveraged concentrations of CCN.2 were similar for measurements near the\nsurface, below the inversion and in the FT just above the inversion,\nconfirming that subsidence and entrainment of FT aerosol strongly\ninfluenced MBL CCN.2. Concurrent flight-to-flight variations of CCN.2 at\nall altitudes below 3 km also imply MBL CCN.2 concentrations were in\nquasi-equilibrium with the FT over a 2-3 day timescale.\nThe observed FT transport over thousands of kilometers indicates\nteleconnections between MBL CCN and cloud-scavenged sources of both\nnatural and/or residual combustion origin. Nonetheless, in spite of its\nimportance, this source of CCN number is not well represented in most\ncurrent models and is generally not detectable by satellite because of\nthe low aerosol scattering in such layers as a result of cloud\nscavenging. In addition, our measurements confirm nucleation in the MBL\nwas not evident during PASE and argue against a localized linear\nrelation in the MBL between dimethyl sulfide (DMS) and CCN suggested by\nthe CLAW hypothesis. However, when the FT is not impacted by long-range\ntransport, sulfate aerosol derived from DMS pumped aloft in the ITCZ\n(Inter-Tropical Convergence Zone) can provide a source of CCN to the\nboundary layer via FT teleconnections involving more complex non-linear\nprocesses.