A unique method for laboratory quantification of gaseous nitrous acid (HONO) using the reaction HONO+HCl -> ClNO+H2O

Although the formation and reactions of gaseous nitrous acid (HONO) in the atmosphere are of great interest, it is difficult to accurately measure HON O both in the atmosphere and in laboratory systems. We report a new techniq ue for quantifying gaseous HOMO in laboratory systems. The method utilizes the reaction of gas phase HOMO with an excess of HCl gas to produce nitrosy l chloride (ClNO), which is readily quantified using FTIR, HONO was formed by flowing N-2 over the surface of an aqueous HCl solution and through a be d of NaNO2, then directly into a 561 L chamber. An excess of gaseous HCl wa s added to the chamber to initiate the reaction in N-2 at room temperature and 1 atm total pressure. The loss of HONO was followed by DOAS and FTIR an d the formation of ClNO was measured by FTIR. While direct measurement of H ONO by FTIR is Limited by uncertainties in the available infrared absorptio n cross sections, calibration for ClNO is readily carried out since ClNO ca n be synthesized with high purity. The stoichiometry for ClNO formed to HON O reacted was determined to be 0.9 +/- 0.2 (1 sigma). The concentration-tim e profiles for both HONO and ClNO were fitted with a kinetics model which g ave a rate constant for the reaction HONO + HCl (k1)--> ClNO + H2O of k(1) less than or equal to (1.9 +/- 1.3) x 10(-19) cm(3) molecule(-1) s(-1) (2 s igma) at 297 K. This should be taken as the upper limit for the gas phase r eaction since some contribution from heterogeneous reaction at the chamber walls cannot be conclusively ruled out.