Statistical analysis of the atmospheric ion concentrations and mobility distributions at a tropical station, Pune

Abstract

Measurements of ion mobility spectra in the range of 0.00133–3.16 cm2 s−1 volt−1 were made at a tropical station, Pune, India from March 2010 to December 2012. A total of 12 004 hourly averaged ion mobility spectra were obtained over the entire period. The average mobility spectrum shows three distinct peaks for the traditional categories of small, intermediate and large ions. However, to understand the seasonal variability and statistical characteristics of ions that justify their further division into five different categories, we have divided our data into five mobility ranges of small cluster, big cluster, intermediate, light large and heavy large ions. Our results show that small ions in atmospheric conditions at Pune can be further divided into two classes with the boundary at a mobility of 1.78 cm2 s−1 volt−1 (diameter 0.66 nm) where relative standard deviation of ion concentrations starts increasing instead of decreasing with mobility. Some but not all small cluster ions of negative polarity are likely to grow to the intermediate ion size. Further, the growth from small to big cluster ions is faster for negative than positive ions. In contrast to the observations of higher mean values of ion concentration in warm season than in winter at a midlatitude station, our observations show higher mean values of ion concentration in winter than in the warm season at this tropical station. This contrast in observations at the two sites is explained by trapping of radioactive emanations by snow at the ground during winter at the midlatitude site.Our observations indicate that intermediate ions are generated by two independent processes, probably by growth of big cluster ions and by direct nucleation from trace gases. Further, the results indicate that the sources for light and heavy large ions are different.