Size distribution of atmospheric aerosols at Maitri, Antarctica Vimlesh Pant 1 , Devendraa Siingh * , A.K. Kamra Indian Institute of Tropical Meteorology, Pune, India article info Article history: Received 11 January 2011 Received in revised form 26 May 2011 Accepted 10 June 2011 Keywords: Antarctic aerosols Aerosol size distribution Nucleation mode particles Marine aerosols abstract Measurements of the concentration and size distribution of the atmospheric aerosol particles in the size range of 0.003e20 mm diameter have been made at Maitri (70  45 0 52 00 S, 11  44 0 03 00 E) during JanuaryeFebruary, 2005. The measured particle size ranges extended from 0.5 to 20 mm throughout the period, from 0.016 to 0.7 mm in January and 0.01 to 0.4 mm in February. For short intervals of time, comprising a total period of 210 h, the measurements were made for particles in the size range of 0.003e0.16 mm. Total particle number concentrations of coarse and fine particles vary from 0.1 to 0.8 and from 100 to 2000 particles cm 3 , respectively. The fine particle concentration undergoes a diurnal variation with values remaining low (300e400 cm 3 ) during low sun periods and increasing up to w750 cm 3 at noontime. The monthly-averaged number size distributions show maxima in accu- mulation mode at 0.772  0.023 mm, in Aitken mode at 0.089  0.005 mm in January which shifts to 0.03  0.003 mm in February, and in nucleation mode at 0.018  0.002 mm. The hourly-averaged curves can have one mode each in coarse, accumulation, and nucleation size ranges, and two modes in Aitken size range of particles. Total number concentration of particles in coarse mode is higher in oceanic than in continental air masses. Further, while the oceanic air masses have nucleation mode at 0.01 mm and Aitken mode at 0.024 mm, continental air masses have nucleation mode at 0.017 mm. Intermixing of the two air masses at coastal site results in multi-modal size distributions. It is inferred that while in continental air masses the nucleation mode particles are aged, in oceanic air masses these are likely to be transported from the upper troposphere under subsidence of cyclonic storms revolving around the continent of Antarctica. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Atmospheric aerosol particles can significantly affect the radia- tive balance of the Earth, both directly, through scattering and absorption of the short and long-wave radiations (Charlson et al., 1992) and indirectly, by acting as condensation nuclei, thereby enhancing the liquid water content and lifetime of the clouds (Twomey, 1974; Squires, 1958; Albrecht, 1989). They may also influence the heterogeneous chemistry of the atmosphere (Schwartz et al., 1996). Over the continent of Antarctica where w98% of the surface is covered with snow and ice and the surface albedo can exceed 0.85, the aerosols, especially the absorbing particles sus- pended over the bright surface can make significant changes in radiative forcing in those regions (Chylek and Coakley, 1974; Randles et al., 2004). Large spatial and temporal variabilities in the compo- sition and the physical and chemical characteristics of aerosols over the continent of Antarctica further adds to the complications of estimating such changes in radiative forcing. Understanding these changes to estimate the radiation budget of the atmosphere has become important in recent times in view of the decreasing polar albedo due to decrease in sea-ice and snow cover areas (de la Mare, 1997; Holland et al., 2006). Intermixing of the continental and marine air-masses makes it difficult to understand the physical and chemical characteristics of the atmospheric aerosols over a coastal station. Temporal and spatial variabilities of air circulation over such regions further complicate the problem of understanding aerosol properties and in identifying the sources and sinks of aerosols. The complexity of the problem is somewhat reduced if such measurements are made at a coastal station in Antarctica where effects of anthropogenic pollution can be neglected, atmosphere is much cleaner and the air circulation is more organized and persistent (e.g. Parish, 1988; Ito, 1993). At an Antarctic coastal station, the atmospheric aerosols are mainly generated by the gas-to-particle conversion of oxidation products of precursor gases emitted by the ocean and by the * Corresponding author. E-mail addresses: devendraasiingh@tropmet.res.in, dvendraasiingh@gmail.com (D. Siingh). 1 Present address: Aryabhatta Research Institute of Observational Sciences, Nainital, India. Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv 1352-2310/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2011.06.028 Atmospheric Environment 45 (2011) 5138e5149