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. Aerosol–cloud interactions contribute to the large uncertainties in currentestimates of climate forcing. We investigated the effect of aerosol particleson cloud droplet formation by model calculations and aircraft measurementsover the Amazon and over the western tropical Atlantic during theACRIDICON–CHUVA campaign in September 2014. On the HALO (HighAltitude Long Range Research) research aircraft,cloud droplet number concentrations (Nd) were measured near thebase of clean and polluted growing convective cumuli using a cloud combinationprobe (CCP) and a cloud and aerosol spectrometer (CAS-DPOL). An adiabaticparcel model was used to perform cloud droplet number closure studies forflights in differently polluted air masses. Model input parameters includedaerosol size distributions measured with an ultra-high sensitive aerosolspectrometer (UHSAS), in combination with a condensation particle counter(CPC). Updraft velocities (w) were measured with a boom-mounted Rosemountprobe. Over the continent, the aerosol size distributions were dominated byaccumulation mode particles, and good agreement between measured and modeledNd values was obtained (deviations ≲ 10 %)assuming an average hygroscopicity of κ∼0.1, which is consistentwith Amazonian biomass burning and secondary organic aerosol. Above theocean, fair agreement was obtained assuming an average hygroscopicity ofκ∼0.2 (deviations ≲ 16 %) and furtherimprovement was achieved assuming different hygroscopicities for Aitken andaccumulation mode particles (κAit=0.8, κacc=0.2; deviations ≲ 10 %), which may reflectsecondary marine sulfate particles. Our results indicate that Aitken modeparticles and their hygroscopicity can be important for droplet formation atlow pollution levels and high updraft velocities in tropical convectiveclouds.

Cloud droplet formation at the base of tropical convective clouds: closure between modeling and measurement results of ACRIDICON–CHUVA