Flow reactor and triple quadrupole mass spectrometer investigations of negative ion reactions involving nitric acid: Implications for atmospheric HNO3 detection by chemical ionization mass spectrometry

Abstract

Atmospheric nitric acid measurements by ACIMS (Active Chemical Ionization Mass Spectrometry) are based on ion-molecule reactions of CO3-(H2O)n and NO3-(H2O)n with HNO3. We have studied these reactions in the laboratory using a flow tube apparatus with mass spectrometric detection of reactant and product ions. Both product ion distributions and rate coefficients were measured. All reactions were investigated in an N2-buffer (1–3 hPa) at room temperature. The reaction rate coefficients of OH-, O2-, O3-, CO4-, CO3-, CO3-H2O, NO3-, and NO3-H2O were measured relative to the known rate k=3.0×10-9 cm3 s-1 for the reaction of O- with HNO3. The main product ion of the reaction of CO3-H2O with HNO3 was found to be (CO3HNO3)- supporting a previous suggestion made on the basis of balloon-borne ACIMS measurements. For the reaction of bare CO3- with HNO3 three product ions were observed, namely NO3-, (NO3OH)-, and (CO3HNO3)-. The reaction rate coefficients for CO3-H2O (1.7×10-9 cm3 s-1) and NO3-H2O (1.6×10-9 cm3 s-1) were found to be close to the collision rate. The measured k values for bare CO3- (1.3×10-9 cm3 s-1) and NO3- (0.7×10-9 cm3 s-1) are somewhat smaller. The collisional dissociations of CO3-(H2O)n, NO3-(H2O)n (n=1, 2), (CO3HNO3)- and (NO3HNO3)-, occasionally influencing ACIMS measurements, were also studied. Fragment ion distributions were measured using a triple quadrupole mass spectrometer. The results showed that previous stratospheric nitric acid measurements were unimpaired from collisional dissociation processes whereas these processes played a major role during previous tropospheric measurements leading to an underestimation of nitric acid concentrations. Previous ACIMS HNO3 detection was also affected by the conversion of CO3-(H2O)n to NO3-(H2O)n due to ion source-produced neutral radicals. A novel ACIMS ion source was developed in order to avoid these problems and to improve the ACIMS method.