CCN closure results from Indian Continental Tropical Convergence Zone (CTCZ) aircraft experiment

In-situ aircraft measurements of cloud condensation nuclei (CCN) and aerosol size distribution were carried out over the region spanning from 24.78 °N to 29.5 °N and 78.1 °E to 85.0 °E from June 29 to July 3, 2009 during the Indian Continental Tropical Convergence Zone (CTCZ) campaign, consisting of total 9 flight sorties. CCN measurements were conducted at a constant supersaturation (SS) of 0.84%. It was found that at higher altitudes (4.7–6.7 km), 30 nm sized particles were dominating while at lower altitudes (0.6–3.7 km), 50 nm particles. Overall, CCN closure ratio (CCNpredicted/CCNmeasured) at different altitudes using Köhler theory and assuming pure ammonium sulfate was 1.375 (R2 = 0.80). For each sortie, the closure ratio varied with height and depended greatly on measured CN concentrations. A case study of Khajuraho sortie showed that the closure ratio increased from 1.07 to 1.40 as the CCN concentration increased from 1000 cm− 3 to 4000 cm− 3. Results of CCN closure improved significantly (overprediction improved by 37.5% and 34.6% for Pantnagar and Gaya, respectively) with the assumption of internally mixed aerosols composed of ammonium sulfate and insoluble organics. Hygroscopicity parameter calculated for these two sorties (κ = 0.51 and 0.5) indicates the presence of moderately hygroscopic organic species along with some inorganic content.