Free troposphere as a major source of CCN for the equatorial pacific boundary layer: long-range transport and teleconnections

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. <br><br> 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. <br><br> 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

Aerosol characteristics and particle production in the upper troposphere over the Amazon Basin

“…Largescale entrainment of UT and MT air into the boundary layer has been suggested as the major source of new particles in marine regions (Raes, 1995;Katoshevski et al, 1999;Clarke et al, 2013). Over Amazonia with its high degree of convective activity, downdrafts are likely to play a more important role.…”

“…They generally occur as layers of a few hundred to a few thousand meters in thickness, often extending over large horizontal distances, and are found over continents as well as over the most remote oceanic regions. The high concentrations of these aerosols in the UT are of great significance for the climate system, because they make this region an important reservoir of particles for the transport both downward into the planetary boundary layer (PBL) (Clarke et al, 1999(Clarke et al, , 2013 and upward into the tropical transition layer (TTL) and the lower stratosphere (Brock et al, 1995;Weigel et al, 2011;Randel and Jensen, 2013), where they can grow into the optically and cloud-microphysically active size range.…”

“…When enhanced UT particle concentrations in the accumulation mode (larger than about 90 nm) have been observed, the enrichment was frequently attributed to sources of sulfur dioxide (SO 2 ) and other combustion emissions, especially biomass burning (BB), based on correlations with combustion tracers, such as carbon monoxide (CO), and air mass trajectories (e.g., Andreae et al, 2001;Clarke and Kapustin, 2010;Weigel et al, 2011;Clarke et al, 2013). After having been lofted to the UT by deep convection, particles in this size range can be transported over hemispheric distances, because removal processes are very inefficient at these altitudes Clarke and Kapustin, 2010).…”

Modification, calibration, and performance of the Ultra-High Sensitivity Aerosol Spectrometer for particle size distribution and volatility measurements during the Atmospheric Tomography Mission (ATom) airborne campaign

“…Several measurements and modeling studies (Clarke and Kapustin, 2002;Korhonen et al, 2008;Mericanto et al, 2010;Quinn et al, 2017;Raes, 1995) suggest that secondary particles formed in the FT play an important role in governing CCN abundance in the MBL, despite the presence of sea salt. It is possible that sea salt may dominate aerosol mass in the MBL, but that CCN concentrations may be controlled by secondary processes, even those occurring in the FT above (Clarke et al, 2013;Raes, 1995;Twomey, 1977;Quinn and Bates, 2011).…”

“…Hence, the size distribution data gathered by the UHSAS instruments over altitudes between ∼ 0.2 and ∼ 13 km will improve our understanding of global aerosol characteristics in the under-sampled regions of the atmosphere that closely resemble natural conditions minimally perturbed by pollution. These new measurements may be placed in the context of similar data gathered over more than 2 decades by Clarke (1991), Clarke et al (1997Clarke et al ( , 1998Clarke et al ( , 2013, Clarke andKapustin (2002, 2010), and others (e.g., Anderson et al, 1996) to help fill gaps in knowledge of aerosol properties, processes, sources, sinks, and aerosolcloud-climate interactions.…”