Measurements of atmospheric nitrous acid and nitric acid

A highly sensitive technique for the measurement of atmospheric HONO and HNO3 is reported. The technique is based on aqueous scrubbing using two coil samplers, followed by conversion of HNO3 to nitrite, derivatization of nitrite to a highly fight-absorbing azo dye with sulfanilamide (SA) and N-(1-naphthyl) ethylenediamine (NED), and high performance liquid chromatography (HPLC) analysis. HNO3 concentration was obtained by the difference of the two channels. Two scrubbing solutions were used for sampling the two species: a 1-mM phosphate buffer solution (PH 7) for the measurement of HONO and a 180 mM NH4Cl/NH3 buffer solution (pH 8.5) for the measurement of HONO + HNO3. The scrubbing solution flow rate was 0.24 ml min(-1) and the gas sampling flow rate was 21 min(-1). HNO3 in the NH4Cl/NH3 buffer solution was quantitatively reduced to nitrite along an on-line 0.8-cm Cd reductor column. Nitrite in both channels was derivatized with 2mM SA and 0.2mM NED in 25mM HCl. Quantitative derivatization was achieved within 5 min at 55degreesC. The azo dye derivative was then separated from the SA/NED reagent by reversed-phase HPLC and detected with a UV-vis detector at 540 nm. With an on-line SEP-PAK C-18 cartridge for the reagent purification, the method detection limit is estimated to be better than 1 pptv for HONO and about 20 pptv for HNO3. The sample integration time was about 2 min and the sampling frequency is every 10 min. Data collected in downtown Albany and Whiteface Mountain, NY, are shown as examples of applications of this technique in both urban and remote clean environments. (C) 2002 Elsevier Science Ltd. All rights reserved.