Reply on RC2

Abstract

Nitrous acid (HONO) plays an important role in tropospheric oxidation chemistry as it is a precursor to the hydroxyl radical. Measurements of HONO have been historically difficult due to instrument interferences and difficulties in sampling and calibration. The traditional calibration method involves generation of HONO by reacting hydrogen chloride vapor with sodium nitrite followed by quantification by various methods (e.g., conversion of HONO to nitric oxide (NO) followed by chemiluminescence detection). Alternatively, HONO can be generated photolytically in the gas-phase by reacting NO with OH radicals generated by H2O photolysis. In this work, we describe and compare two photolytic HONO calibration methods that were used to calibrate an iodide adduct chemical ionization mass spectrometer (CIMS). Both methods are based on the water vapor photolysis method commonly used for OH and HO2 calibrations. The first method is an adaptation of the common chemical actinometry HOx calibration method, in which HONO is calculated based on quantified values for [O3], [H2O], [O2], and the absorption cross sections for H2O and O2 at 184.9 nm. In the second, novel method the HONO concentration is simply determined based on the simultaneous measurements of NO2 formed by the reaction of NO with HO2 from the H2O photolysis. This second, novel approach generally has an improved (lower) calibration uncertainty and is simpler to apply. Calibration uncertainties are typically 30 to 36 % (2σ) for the actinometric method and as low as 9 % (2σ) for the NO2 proxy method, limited by the uncertainty of the NO2 measurements.