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Abstract. Nitrous acid (HONO) plays an important role in troposphericoxidation chemistry as it is a precursor to the hydroxyl radical (OH).Measurements of HONO have been difficult historically due to instrumentinterferences and difficulties in sampling and calibration. The traditionalcalibration method involves generation of HONO by reacting hydrogen chloridevapor with sodium nitrite followed by quantification by various methods(e.g., conversion of HONO to nitric oxide (NO) followed by chemiluminescencedetection). Alternatively, HONO can be generated photolytically in thegas phase by reacting NO with OH radicals generated by H2O photolysis.In this work, we describe and compare two photolytic HONO calibrationmethods that were used to calibrate an iodide adduct chemical ionizationmass spectrometer (CIMS). Both methods are based on the water vaporphotolysis method commonly used for OH and HO2 (known collectively asHOx) calibrations. The first method is an adaptation of the common chemicalactinometry HOx calibration method, in which HONO is calculated based onquantified values for [O3], [H2O], and [O2] and the absorptioncross sections for H2O and O2 at 184.9 nm. In the second, novelmethod HONO is prepared in mostly N2 ([O2]=0.040 %) and issimply quantified by measuring the NO2 formed by the reaction of NOwith HO2 generated by H2O photolysis. Both calibration methodswere used to prepare a wide range of HONO mixing ratios between∼400 and 8000 pptv. The uncertainty of the chemicalactinometric calibration is 27 % (2σ) and independent of HONOconcentration. The uncertainty of the NO2 proxy calibration isconcentration-dependent, limited by the uncertainty of the NO2measurements. The NO2 proxy calibration uncertainties (2σ)presented here range from 4.5 % to 24.4 % (at [HONO] =8000 pptv and[HONO] =630 pptv, respectively) with a 10 % uncertainty associatedwith a mixing ratio of ∼1600 pptv, typical of valuesobserved in urban areas at night. We also describe the potential applicationof the NO2 proxy method to calibrating HOx instruments (e.g., LIF,CIMS) at uncertainties below 15 % (2σ).