Chemical composition of the NH3-HNO 3-H 2SO 4-NaCl system in the atmosphere of Athens, Greece.

This paper presents a procedure for estimating the distribution of ionic material in the NH3-HNO3-H2SO4-NaCl system and applies this procedure in a particular case. The data used were measurements of HNO3, NH3, NO 3 (-) , SO 4 (-) , NH 4 (+) Cl(-) and Na(+) performed during February 1989 - February 1990 in a central Athens street with high traffic density. According to the procedure, ions combine in the following manner: Na(+) combines preferentially with SO 4 (-) , then with NO 3 (-) , followed by NH 4 (+) with the remaining SO 4 (-) and then with the remaining NO 3 (-) to form bisulphates, sulphates and nitrates. The combination procedure showed that the main constituents of the NH3-HNO3-H2SO4-NaCl system are primarily (NH4)2SO4 and, to a lesser extent, NH4NO3 and NH4HSO4, with mean and maximum concentrations, during morning hours, (NH4)2SO4: 14.5 (max 46.8), NH4NO3: 2.97 (max 23) and NH4HSO4: 1.78 (max 40.6) µg m(-3). Lower concentations of Na2SO4, NaHSO4, NaNO3 and NH4Cl and very low concentrations of H2SO4 are also present, depending on the availability of NaCl. It became apparent from the ionic distribution that there is sufficient NH3 to neutralize the H2SO4 and HNO3. It was also shown that a significant fraction of the HNO3, especially on days with high pollution, occurs as aqueous NO 3 (-) . A number of empirical equations have been proposed, which enable the approximate estimation of the constituents of the NH3-HNO3-H2SO4-NaCl system from air pollution monitoring data and meteorological parameters.