Analysis of levels of nitrates and derivatives of ammonia in an urban atmosphere

To estimate the various ammoniacal compounds in the atmosphere and determine the role of nitrogen oxides and ammonia in the formation of nitrates as a function of temperature and relative humidity, a statistical model based on multiple linear regression was devised. The model was developed using the results from atmospheric samples of both primary (ammonia, sulfur dioxide, nitrogen dioxide, nitric oxide) and secondary pollutants (ammonium, nitrates, sulfates and chlorides) taken over a 1-year period from three monitoring stations in a large French city. To identify the ammoniacal compounds, NH4+, the dependent variable, was examined as a function of several explanatory variables: NO3- SO42- and Cl-. From the equations in the model, the presence of ammonium nitrate and ammonium hydrogen sulfate were predicted in the Spring-Summer. In the Winter, derivatives of nitrous acid were predicted. It was shown that there was a deficit in ammonium ions, related to the presence of unstable NH4Cl. To study the formation of atmospheric nitrates, NO3-, the dependent variable was examined as a function of several explanatory variables: NO, NO2, NH3, temperature and humidity. Two mechanisms emerged for the formation of nitrates, one in the gas phase during the Spring-Summer period, the other in the liquid phase during the Autumn-Winter period, in which nitrogen dioxide appeared to play a major role. In both cases, temperature and the concentration of ammonia were statistically significant factors. Since secondary pollutants tend to be spread evenly throughout the urban area, in contrast to the more localized gaseous and unstable pollutants, the identification of the main atmospheric particles provides further evidence for epidemiological investigations. (C) 1997 Elsevier Science B.V.