Modifications of the transition flow reactor theory for the more accurate measurement of nitric acid and nitrate and ammonium ions

Sixty five samples were taken with a Transition Flow Reactor (TFR) for the measurement of nitric acid (HNO₃) and nitrate particulates (NO ₃ ⁻ ), in a central Athens street having a heavy traffic density, between February 1989 and February 1990. It was established that the TFR theory overestimated HNO₃ and underestimated NO ₃ ⁻ . This is caused by the retention of a fraction of the NO₂ and a fraction of the HNO₂ as NO ₂ ⁻ on the sampler's nylon linear. NO ₂ ⁻ is in turn oxidized to NO ₃ ⁻ and this is determined as HNO₃. Two modifications of the TFR theory are proposed for a better estimation of the HNO₃ and NO ₃ ⁻ . With the first modification HNO₃ is estimated as being equal to the smaller of two values: HNO_{3 TFR} and the value calculated if it is considered that 91% of HNO₃ is retained by the nylon filter. The second modification was based on the quantification of the effect of NO₂ on the concentrations of HNO₃ and NO ₃ ⁻ . This modification is an improvement on the first one and enables the calculation of a lower limit for the concentration of HNO₂. Both modifications improve the accuracy of the TFR sampler in measuring HNO₃ and NO ₃ ⁻ , without altering it structurally. It was also found that the nylon filter partially retains some NH₃, causing a negative error in the measurement of NH ₄ ⁺ with the TFR. The extent of NH₃ retention on the nylon filter is proportional to the HNO₃ concentration. This error must be taken into account whenever NH ₄ ⁺ is being determined.