Abstract
Abstract. Airborne aerosol measurements in the central equatorial Pacific during PASE (Pacific Atmospheric Sulfur Experiment) revealed that cloud condensation nuclei (CCN) activated in marine boundary layer (MBL) clouds were strongly influenced by entrainment from the free troposphere (FT). About 65% entered at sizes effective as CCN in MBL clouds, while ~25% entered the MBL too small to activate but subsequently grew via gas to particle conversion. The remaining ~10% were inferred to be sea salt aerosol. FT aerosols at low carbon monoxide (CO) mixing ratios (< 63 ppbv) were mostly volatile at 360 °C with a number mode peak of around 30–40 nm dry diameter and tended to be associated with cloud outflow from distant (3000 km or more) deep convection. Higher CO concentrations were commonly associated with trajectories from South America and the Amazon region (ca. ~10 000 km away) and occurred in layers indicative of combustion sources (biomass burning season) partially scavenged by precipitation. These had number modes near 60–80 nm dry diameter with a large fraction of CCN.2 (those activated at 0.2% supersaturation and representative of MBL clouds) prior to entrainment into the MBL. Flight averaged concentrations of CCN.2 were similar for measurements near the surface, below the inversion and in the FT just above the inversion, confirming that subsidence and entrainment of FT aerosol strongly influenced MBL CCN.2. Concurrent flight-to-flight variations of CCN.2 at all altitudes below 3 km also imply MBL CCN.2 concentrations were in quasi-equilibrium with the FT over a 2–3 day timescale. The observed FT transport over thousands of kilometers indicates teleconnections between MBL CCN and cloud-scavenged sources of both natural and/or residual combustion origin. Nonetheless, in spite of its importance, this source of CCN number is not well represented in most current models and is generally not detectable by satellite because of the low aerosol scattering in such layers as a result of cloud scavenging. In addition, our measurements confirm nucleation in the MBL was not evident during PASE and argue against a localized linear relation in the MBL between dimethyl sulfide (DMS) and CCN suggested by the CLAW hypothesis. However, when the FT is not impacted by long-range transport, sulfate aerosol derived from DMS pumped aloft in the ITCZ (Inter-Tropical Convergence Zone) can provide a source of CCN to the boundary layer via FT teleconnections involving more complex non-linear processes.
Highlights
Ocean Science lar for measurements near the surface, below the inversion and in the free troposphere (FT) just above the inversion, confirming that subsidence and entrainment of FT aerosol strongly influenced marine boundary layer (MBL) cloud condensation nuclei (CCN)
Clarke et al.: CCN in the equatorial pacific boundary layer been driven by the need for greater understanding of associated atmospheric chemistry and general geochemical cycling of atmospheric constituents and by the need to quantify the impact of anthropogenic emissions on aerosol optical depth (AOD), cloud condensation nuclei (CCN) and clouds
Repeated flight patterns during PASE revealed the relation between FT and MBL aerosol in the equatorial Pacific and its variability
Summary
Ocean Science lar for measurements near the surface, below the inversion and in the FT just above the inversion, confirming that subsidence and entrainment of FT aerosol strongly influenced MBL CCN.. Because many combustion aerosols are a result of primary emissions they tend to be larger than those nucleated from the gas phase This increases their relevance to processes linked to light scattering and CCN, both of which are dominated by sizes larger than 50 nm. A review of more recent models and measurements (Quinn and Bates, 2011) support this process as an important source of “natural” aerosol for the MBL These often appear to dominate FT aerosol number in the Hadley circulation (Clarke and Kapustin, 2010). Volatile condensation nuclei are referred to here as CNvol and the non-volatile CN, remaining at 360 ◦C, as CNhot These properties and their links to gas phase measurements and CCN are used to characterize their relation to air mass types and trajectories encountered over the equatorial Pacific. We discuss characteristics of aerosol from combustion and cloud outflow, along with their transport in the equatorial FT, and their subsequent sizeresolved entrainment into the MBL where they provide the major source of CCN
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