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<p>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 H<span class="inline-formula">₂</span>O 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 HO<span class="inline-formula">₂</span> (known collectively as HO<span class="inline-formula">_<i>x</i></span>) calibrations. The first method is an adaptation of the common chemical actinometry HO<span class="inline-formula">_<i>x</i></span> calibration method, in which HONO is calculated based on quantified values for [O<span class="inline-formula">₃</span>], [H<span class="inline-formula">₂</span>O], and [O<span class="inline-formula">₂</span>] and the absorption cross sections for H<span class="inline-formula">₂</span>O and O<span class="inline-formula">₂</span> at 184.9 nm. In the second, novel method HONO is prepared in mostly N<span class="inline-formula">₂</span> ([O<span class="inline-formula">₂]=0.040</span> %) and is simply quantified by measuring the NO<span class="inline-formula">₂</span> formed by the reaction of NO with HO<span class="inline-formula">₂</span> generated by H<span class="inline-formula">₂</span>O photolysis. Both calibration methods were used to prepare a wide range of HONO mixing ratios between <span class="inline-formula">∼400</span> and 8000 pptv. The uncertainty of the chemical actinometric calibration is 27 % (<span class="inline-formula">2<i>σ</i>)</span> and independent of HONO concentration. The uncertainty of the NO<span class="inline-formula">₂</span> proxy calibration is concentration-dependent, limited by the uncertainty of the NO<span class="inline-formula">₂</span> measurements. The NO<span class="inline-formula">₂</span> proxy calibration uncertainties (<span class="inline-formula">2<i>σ</i>)</span> presented here range from 4.5 % to 24.4 % (at [HONO] <span class="inline-formula">=8000</span> pptv and [HONO] <span class="inline-formula">=630</span> pptv, respectively) with a 10 % uncertainty associated with a mixing ratio of <span class="inline-formula">∼1600</span> pptv, typical of values observed in urban areas at night. We also describe the potential application of the NO<span class="inline-formula">₂</span> proxy method to calibrating HO<span class="inline-formula">_<i>x</i></span> instruments (e.g., LIF, CIMS) at uncertainties below 15 % (<span class="inline-formula">2<i>σ</i>)</span>.</p>