Size distribution of PM<Subscript>10</Subscript> mass aerosols and its ionic characteristics were studied for 2 years from January 2006 to December 2007 at central Delhi by employing an 8-stage Andersen Cascade Impactor sampler. The mass of fine (PM<Subscript>2.5</Subscript>) and coarse (PM<Subscript>10−2.5</Subscript>) mode particles were integrated from particle mass determined in different stages. Average concentrations of mass PM<Subscript>10</Subscript> and PM<Subscript>2.5</Subscript> were observed to be 306 ± 182 and 136 ± 84 μg m<Superscript>−3</Superscript>, respectively, which are far in excess of annual averages stipulated by the Indian National Ambient Air Quality Standards (PM<Subscript>10</Subscript>: 60 μg m<Superscript>−3</Superscript> and PM<Subscript>2.5</Subscript>: 40 μg m<Superscript>−3</Superscript>). The highest concentrations of PM<Subscript>10−2.5</Subscript> (coarse) and PM<Subscript>2.5</Subscript> (fine) were observed 505 ± 44 and 368 ± 61 μg m<Superscript>−3</Superscript>, respectively, during summer (June 2006) period, whereas the lower concentrations of PM<Subscript>10−2.5</Subscript> (35 ± 9 μg m<Superscript>−3</Superscript>) and PM<Subscript>2.5</Subscript> (29 ± 13 μg m<Superscript>−3</Superscript>) were observed during monsoon (September 2007). In summer, because of frequent dust storms, coarse particles are more dominant than fine particles during study period. However, during winter, the PM<Subscript>2.5</Subscript> contribution became more pronounced as compared to summer probably due to enhanced emissions from anthropogenic activities, burning of biofuels/biomass and other human activities. A high ratio (0.58) of PM<Subscript>2.5</Subscript>/PM<Subscript>10</Subscript> was observed during winter and low (0.24) during monsoon. A strong correlation between PM<Subscript>10</Subscript> and PM<Subscript>2.5</Subscript> (r <Superscript>2</Superscript> = 0.93) was observed, indicating that variation in PM<Subscript>10</Subscript> mass is governed by the variation in PM<Subscript>2.5</Subscript>. Major cations (NH<Subscript>4</Subscript> <Superscript>+</Superscript>, Na<Superscript>+</Superscript>, K<Superscript>+</Superscript>, Ca<Superscript>2+</Superscript> and Mg<Superscript>2+</Superscript>) and anions (F<Superscript>−</Superscript>, Cl<Superscript>−</Superscript>, SO<Subscript>4</Subscript> <Superscript>2−</Superscript> and NO<Subscript>3</Subscript> <Superscript>−</Superscript>) were analyzed along with pH. Average concentrations of SO<Subscript>4</Subscript> <Superscript>2−</Superscript> and NO<Subscript>3</Subscript> <Superscript>−</Superscript> were observed to be 12.93 ± 0.98 and 10.33 ± 1.10 μg m<Superscript>−3</Superscript>, respectively. Significant correlation between SO<Subscript>4</Subscript> <Superscript>2−</Superscript> and NO<Subscript>3</Subscript> <Superscript>−</Superscript> in PM<Subscript>1.0</Subscript> was observed indicating the major sources of secondary aerosol which may be from thermal power plants located in the southeast and incomplete combustion by vehicular exhaust. A good correlation among secondary species (NH<Superscript>+</Superscript>, NO<Subscript>3</Subscript> <Superscript>−</Superscript> and SO<Subscript>4</Subscript> <Superscript>2−</Superscript>) suggests that most of NH<Subscript>4</Subscript> <Superscript>+</Superscript> is in the form of ammonium sulfate and ammonium nitrate in the atmosphere. During winter, the concentration of Ca<Superscript>2+</Superscript> was also higher; it may be due to entrainment of roadside dust particles, traffic activities and low temperature. The molar ratio (1.39) between Cl<Superscript>−</Superscript> and Na<Superscript>+</Superscript> was observed to be close to that of seawater (1.16). The presence of higher Cl<Superscript>−</Superscript> during winter is due to western disturbances and probably local emission of Cl<Superscript>−</Superscript> due to fabric bleaching activity in a number of export garment factories in the proximity of the sampling site. Copyright Springer Science+Business Media Dordrecht 2013
