droplet diameters, which affects the extent and lifetime of clouds. The uncertainty in estimating the indirect radiative forcing is due to incomplete description of spatial and vertical distributions of anthropogenic and other type of aerosols.
Physico-chemical processes involved in the activation of particles into CCN are governed by the droplet curvature and solute effects on the water vapour pressure. The Köhler theory, composed of both these competing effects, is the theoretical basis to predict CCN activity. It determines the “critical” supersaturation (Sc in %) required by a particle to behave as a CCN and finally activate into cloud droplets. For a given aerosol particle, Sc depends on two important parameters, the dry diameter and chemical composition. Comparison of modelled CCN with the observed CCN concentration is the ultimate test of Köhler theory. Recently, some modifications to Köhler theory have been proposed to include additional effects that organic compounds, soluble, or insoluble, have on surface tension of droplet solution. CCN closure study has been performed for many years with varying success rates all over the world which depends on the aerosol characteristics and ambient conditions.
In India, efforts have been made to study aerosol microphysical and optical properties in various environments, but no study was carried out to understand CCN behaviour and closure study. This is the first study dealing with CCN properties of aerosols in India. In this study, inter and intra-seasonal variation of CCN concentrations, role of long-range transported aerosols and CCN closure study over Kanpur covering all major seasons was presented for two years 2008 and 2009. In addition, in-situ aircraft observations were carried out during the early monsoon season of 2009 across the Indian Continental Tropical Convergence
Zone region (CTCZ) to understand the vertical and spatial variation of key aerosol parameters.
Results
The ground based CCN study revealed that there is considerable intra seasonal variability of CCN and CCN/CN ratio (NCCN/NCN) due to differences in the directions of air masses in all the seasons. For both winter and pre-monsoon seasons, highest CCN concentrations were found for eastern continental transport routes whereas the highest CCN/CN ratio (0.30) was found in winter when the air masses came from central and eastern continental parts of India while in year 2009, the CCN/CN ratio was 0.53 for the similar air mass trend. Strong seasonal variability of CCN was found with highest CCN concentrations in winter and lowest in monsoon season due to cloud and precipitation scavenging. Supersaturation depletion corrections were applied to the seasons with very high concentration to reduce the uncertainty in CCN prediction. CCN closure study done using organics as insoluble fraction showed average over-prediction of 21±18% at all SS measured
(with under prediction of 5±30% for lower SS (0.13%) and over prediction of higher SS (11±32% and 47±38% at SS of 0.33% and 0.64%, respectively) (Figure 1). The prediction error is due to also be due to lack of size dependent chemical composition and assumption of internal mixing. On the other hand, in-situ aircraft measurements of CCN concentration at SS (0.84%) carried out at 6 sorties at different altitudes (0.6-6.7 km) during the CTCZ aircraft campaign. It was observed that aerosol CCN activation depends not only on the solute volume fraction but also on the number concentration and size distribution of aerosols obtained from SMPS, and their mixing state. The closure analysis assuming aerosols is composed of pure ammonium sulfate showed a very good agreement between the CCN measured and CCN predicted concentrations with an average closure ratio of 1.375, n = 229 and R2= 0.80. For these sorties, the CCN closure ratio varied greatly with altitude suggesting that the sampled air mass has different characteristics at different altitudes.
Figure 1: (CCN predicted versus CCN measured at three SS (0.13%, 0.33%, 0.64%).
