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This paper presents the measurements of gaseous SO2, NO2, HNO3 and NH3 and particulate NH4+, NO3- and SO42- at Rampur, a rural site of semi-arid region of India and annual mean concentrations are 3.7+/-2.2, 7.3+/-3.7, 0.7+/-0.6 and 6.7+/-4.2 and 1.0+/-0.4, 1.1+/-1.3 and 2.6+/-1.6 mug m(-3), respectively. Seasonal variation with higher concentration in winter is observed for gaseous SO2, NO2, and NH3 and particulate NH4+. The concentration of HNO3 and particulate NO3- and SO42- are higher in summer. Summer to winter ratio of HNO3 and particulate NO3- are 3.8 and 1.8, respectively, showing seasonal variation is more pronounced in case of HNO3 than particulate NO3-. The ratio of gaseous NH3 to particulate NH4+ (NH3/NH4+) is more than 1 probably due to basic nature of aerosol. Examination of equilibrium between gaseous NH3 and HNO3 and particulate NH4NO3 shows observed equilibrium constant is lower than theoretical equilibrium constant in summer and vice versa in winter. (C) 2003 Elsevier Ltd. All rights reserved.