Nucleation events for the formation of charged aerosol particles at a tropical station - Preliminary results

Simultaneous measurements of atmospheric ions in the mobility range of 3.16-0.00133 cm2 v− 1 s− 1 (diameter range 0.46-50 nm) made with a Neutral Cluster and Air Ion Spectrometer (NAIS) at a tropical station, Pune (18° 31′ N, 73° 55′ E) over a period of 47 days, revealed that nucleation events for the formation of ions occurred on 17 days. These events generally occurred between 0800 and 1000 LT and enhanced concentrations of Aitken particles continued to be observed up to 1700-1800 LT. During these events, particles grew at rates of 3.1 ± 0.8 to 11.2 ± 3.5 nm h− 1 for different particle size ranges. Three cases of nucleation events, two with large and one with small particle concentrations, are examined and differences in ion characteristics between the different cases are pointed out. Characteristics and contributions of opposite polarity of ions to the increase in total ion concentration have been examined. A unique type of nucleation event in which particles grew to a size of ~ 41 nm at 0800 LT diameter and then decreased to a size of 8.64 nm diameter, is also reported. In this event, increase in total ion concentration occurred due to the increase of intermediate-, light large-, and heavy large-ions, but the decrease was mainly due to the decrease in only light and heavy large ions. Ratio of the positive to negative ions decreased during the period of this unique event as compared to before and after the event. Sulfur dioxide (SO2) concentration is positively correlated with the concentrations of intermediate, light large and heavy large ions but negatively correlated with the concentration of cluster ions.