Simultaneous measurements of ammonia and nitric acid in ambient air at Agra (27°10′N and 78°05′E) (India)

Simultaneous measurements of ammonia and nitric acid in ambient air were conducted at Dayalbagh, Agra using the mist chamber technique. The sampling site is located near a cattle shed. A total of 120 samples were collected during the period July–September and November–February (1997–1998). Sampling was performed during six different times a day. Gas-phase HNO3 was estimated as NO3− using ion chromatographic technique while ammonia was determined colorimetrically as NH4+ using indophenol blue method. The mean levels of NH3 and HNO3 for the entire data set were 16.3±2.8 and 1.6±1.4 ppbv, respectively. In the monsoon, mean values for NH3 and HNO3 averaged to 16.4±3.5 and 0.9±0.7 ppbv while the winter means were 11.8±4.4 and 2.1±1.2 ppbv, respectively. Concentration of both the species (NH3 and HNO3) did not show any significant diurnal behaviour in both the seasons. However, concentration of both NH3 and HNO3 were lower at dawn than the previous nightʹs value. This has been ascribed to their removal through dew. Concentrations of HNO3 are observed to increase during the daytime, consistent with its formation by photochemical reactions. Nitric acid and ammonia concentrations show a significant seasonal variation. Levels of HNO3 are higher in winter but lower in monsoon, while ammonia shows a reverse trend with higher monsoon and lower winter values. Observed trends in nitric acid and ammonia concentration are due to seasonal variation in emission sources, chemistry and meteorology. Gaseous ammonia and nitric acid are in equilibrium with NH4NO3 (solid or aqueous) in the atmosphere. The existence of this equilibrium was examined from simultaneous measurements of NH3 and HNO3 in the ambient air. It is found that for the monsoon data, measured concentrations are qualitatively below the predicted equilibrium value, while in the winter, concentration product ([NH3] [HNO3]) lies consistently above the predicted values. These deviations may be explained due to local sources of both [NH3] and [HNO3], presence of coarse nitrate particles and low-temperature and high-humidity conditions.