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

Abstract. Aerosol–cloud interactions contribute to the large uncertainties in current estimates of climate forcing. We investigated effect aerosol particles on cloud droplet formation by model calculations and aircraft measurements over Amazon western tropical Atlantic during ACRIDICON–CHUVA campaign September 2014. On HALO (High Altitude Long Range Research) research aircraft, number concentrations (Nd) were measured near base clean polluted growing convective cumuli using a combination probe (CCP) spectrometer (CAS-DPOL). An adiabatic parcel was used perform closure studies for flights differently air masses. Model input parameters included size distributions with an ultra-high sensitive (UHSAS), condensation particle counter (CPC). Updraft velocities (w) boom-mounted Rosemount probe. Over continent, dominated accumulation mode particles, good agreement between modeled Nd values obtained (deviations ≲ 10 %) assuming average hygroscopicity κ∼0.1, which is consistent Amazonian biomass burning secondary organic aerosol. Above ocean, fair κ∼0.2 16 further improvement achieved different hygroscopicities Aitken (κAit=0.8, κacc=0.2; deviations %), may reflect marine sulfate particles. Our results indicate that their can be important at low pollution levels high updraft clouds.