Secondary aerosol formation and identification of regional source locations by PSCF analysis in the Indo-Gangetic region of India

Secondary aerosol formation was studied at Allahabad in the Indo-Gangetic region during a field campaign called Land Campaign-II in December 2004 (northern winter). Regional source locations of the ionic species in PM10 were identified by using Potential Source Contribution Function (PSCF analysis). On an average, the concentration of water soluble inorganic ions (sum of anions and cations) was 63.2 mu gm(-3). Amongst the water soluble ions, average NO (3) (-) concentration was the highest (25.0 mu gm(-3)) followed by SO (4) (2-) (15.8 mu gm(-3)) and NH (4) (+) (13.8 mu gm(-3)) concentrations. These species, contributed 87% of the total mass of water soluble species, indicating that most of the water soluble PM10 was composed of NH4NO3 and (NH4)(2)SO4/NH4HSO4 or (NH4)(3)H(SO4)(2) particles. Further, the concentrations of SO (4) (2-) , NO (3) (-) , and NH4(+) aerosols increased at high relative humidity levels up to the deliquescence point (similar to 63% RH) for salts of these species suggesting that high humidity levels favor the conversion and partitioning of gaseous SO2, NOx, and NH3 to their aerosol phase. Additionally, lowering of ambient temperature as the winter progressed also resulted in an increase of NO (3) (-) and NH (4) (+) concentrations, probably due to the semi volatile nature of ammonium nitrate. PSCF analysis identified regions along the Indo-Gangetic Plain (IGP) including Northern and Central Uttar Pradesh, Punjab, Haryana, Northern Pakistan, and parts of Rajasthan as source regions of airborne nitrate. Similar source regions, along with Northeastern Madhya Pradesh were identified for sulfate.