Abstract
Abstract. Airborne observations over the Amazon Basin showed high aerosol particle concentrations in the upper troposphere (UT) between 8 and 15 km altitude, with number densities (normalized to standard temperature and pressure) often exceeding those in the planetary boundary layer (PBL) by 1 or 2 orders of magnitude. The measurements were made during the German–Brazilian cooperative aircraft campaign ACRIDICON–CHUVA, where ACRIDICON stands for Aerosol, Cloud, Precipitation, and Radiation Interactions and Dynamics of Convective Cloud Systems and CHUVA is the acronym for Cloud Processes of the Main Precipitation Systems in Brazil: A Contribution to Cloud Resolving Modeling and to the GPM (global precipitation measurement), on the German High Altitude and Long Range Research Aircraft (HALO). The campaign took place in September–October 2014, with the objective of studying tropical deep convective clouds over the Amazon rainforest and their interactions with atmospheric trace gases, aerosol particles, and atmospheric radiation. Aerosol enhancements were observed consistently on all flights during which the UT was probed, using several aerosol metrics, including condensation nuclei (CN) and cloud condensation nuclei (CCN) number concentrations and chemical species mass concentrations. The UT particles differed sharply in their chemical composition and size distribution from those in the PBL, ruling out convective transport of combustion-derived particles from the boundary layer (BL) as a source. The air in the immediate outflow of deep convective clouds was depleted of aerosol particles, whereas strongly enhanced number concentrations of small particles (< 90 nm diameter) were found in UT regions that had experienced outflow from deep convection in the preceding 5–72 h. We also found elevated concentrations of larger (> 90 nm) particles in the UT, which consisted mostly of organic matter and nitrate and were very effective CCN. Our findings suggest a conceptual model, where production of new aerosol particles takes place in the continental UT from biogenic volatile organic material brought up by deep convection and converted to condensable species in the UT. Subsequently, downward mixing and transport of upper tropospheric aerosol can be a source of particles to the PBL, where they increase in size by the condensation of biogenic volatile organic compound (BVOC) oxidation products. This may be an important source of aerosol particles for the Amazonian PBL, where aerosol nucleation and new particle formation have not been observed. We propose that this may have been the dominant process supplying secondary aerosol particles in the pristine atmosphere, making clouds the dominant control of both removal and production of atmospheric particles.
Highlights
The ACRIDICON–CHUVA flights covered most of the Amazon Basin, reaching from the Atlantic coastal waters in the east to near the Colombian border in the west, and from the Guyanese border in the north to the arc of deforestation in the south
The cloud that was sampled on this flight appears to have been a pyrocumulus that had been ingesting fresh biomass smoke, as suggested by the strongly elevated carbon monoxide (CO) during the cloud passages. This flight will be discussed as a separate case study below (Sect. 3.6.). While these results show that the high particle concentrations we observed in the upper troposphere (UT) were not directly released from the cloud tops, they do not rule out the possibility that new particle formation had already started in the clouds or anvils
As part of the ACRIDICON–CHUVA 2014 aircraft campaign, we investigated the characteristics and sources of aerosols in the upper troposphere over the Amazon Basin
Summary
Aircraft measurements in the upper troposphere (UT) have consistently shown large regions with very high aerosol particle number concentrations, typically in the tens of thousands of particles per cm, with the strongest enhancements reported in tropical and subtropical regions (Clarke et al, 1999; Andreae et al, 2001; de Reus et al, 2001; Krejci et al, 2003; Lee et al, 2003; Young et al, 2007; Ekman et al, 2008, 2012; Yu et al, 2008; Froyd et al, 2009; Weigelt et al, 2009; Borrmann et al, 2010; Clarke and Kapustin, 2010; Mirme et al, 2010; Waddicor et al, 2012; Reddington et al, 2017; Rose et al, 2017). Twohy et al (2002) observed particle concentrations up to 45 000 cm−3 in the UT over North America and suggested that they had been formed in situ from gas-phase precursors brought up by deep convection. Weigel et al (2011) found similar concentrations in the UT over tropical America, Africa, and Australia, which they attributed to new particle formation from sulfuric acid and possibly organics. Weigel et al (2011) found similar concentrations in the UT over tropical America, Africa, and Australia, which they attributed to new particle formation from sulfuric acid and possibly organics Most of these elevated aerosol concentrations are in the nucleation- and Aitken-mode size ranges, i.e., at particle diameters smaller than about 90 nm, with maxima typically between 20 and 60 nm (e.g., de Reus et al, 2001; Lee et al, 2003; Weigel et al, 2011; Waddicor et al, 2012). 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, 2013; J. Wang et al, 2016) 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
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