Aerosol particles from tropical convective systems: 2. Cloud bases

Aerosol particles were collected at the altitudes of cloud bases during the Cirrus Regional Study of Tropical Anvils and Cirrus Layers–Florida Area Cirrus Experiment (CRYSTAL-FACE) and analyzed using transmission electron microscopy. The particles consist of ammonium sulfate (45–90% by number), sea salt (5–45%), mineral dust (1–20%), and anthropogenic materials such as soot and fly ash (<3%). Ammonium sulfate particles have rather uniform, submicron sizes (mostly 0.5 μm across). Sea-salt particles are larger, apparently having been deliquesced. However, submicron particles are also common. Many contain Na and mixed cation sulfates in addition to NaCl. Mineral dust consists largely of tabular clay particles. Samples from the 28 July flight contain much mineral dust, probably because of transport from the Saharan Desert. Aggregates of sea salt and mineral dust, ammonium sulfate, and soot particles are common. Such mixed aggregates are especially abundant in in-cloud samples. Cirrus samples from CRYSTAL-FACE contain many H₂SO₄ droplets (Kojima et al., 2004), but acidic sulfate particles are rare at the altitudes of cloud bases. H₂SO₄ probably formed at higher altitudes through oxidation of SO₂ in cloud droplets. Sea salt and mineral dust have been reported to be abundant in cloud particles collected using a counterflow virtual impactor (Cziczo et al., 2004), suggesting that these particles were incorporated into the convective systems from the cloud bases and akted as ice nuclei while being vertically transported.