Comparison of two photolytic calibration methods for nitrous acid

Nitrous acid (HONO) plays an important role in tropospheric oxidation chemistry as it is a precursor to the hydroxyl radical (OH). Measurements of HONO have been difficult historically 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 (known collectively as HOx) 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 [O-3], [H2O], and [O-2] and the absorption cross sections for H2O and O-2 at 184.9 nm. In the second, novel method HONO is prepared in mostly N-2 ([O-2]=0.040 %) and is simply quantified by measuring the NO2 formed by the reaction of NO with HO2 generated by H2O photolysis. Both calibration methods were used to prepare a wide range of HONO mixing ratios between similar to 400 and 8000 pptv. The uncertainty of the chemical actinometric calibration is 27 % (2 sigma) and independent of HONO concentration. The uncertainty of the NO2 proxy calibration is concentration-dependent, limited by the uncertainty of the NO(2 )measurements. The NO2 proxy calibration uncertainties (2 sigma) presented here range from 4.5 % to 24.4 % (at [HONO] =8000 pptv and [HONO] =630 pptv, respectively) with a 10 % uncertainty associated with a mixing ratio of similar to 1600 pptv, typical of values observed in urban areas at night. We also describe the potential application of the NO2 proxy method to calibrating HOx instruments (e.g., LIF, CIMS) at uncertainties below 15 % (2 sigma).