Iodide CIMS and <i>m</i>∕<i>z</i> 62: the detection of HNO<sub>3</sub> as NO<sub>3</sub><sup>−</sup> in the presence of PAN, peroxyacetic acid and ozone

Abstract. Chemical ionisation mass spectrometry (CIMS) using I− (the iodide anion), hereafter I-CIMS, as a primary reactant ion has previously been used to measure NO3 and N2O5 both in laboratory field experiments. We show that reports of large daytime mixing ratios (both usually present detectable amounts only at night) are likely be heavily biased by the ubiquitous presence HNO3 troposphere lower stratosphere. demonstrate series experiments CIMS detection m/z 62 ions is efficient peroxy acetyl nitric anhydride (PAN) or peroxyacetic acid (PAA) especially O3. have characterised dependence sensitivity on acetate anions (CH3CO2-, 59, from either PAN PAA). The loss CH3CO2- via conversion NO3- may represent significant bias I-CIMS measurements PAA which continuous calibration (e.g. addition isotopically labelled PAN) not carried out. greatest achieved ambient levels O3 whereby thermodynamically disfavoured, direct reaction with form bypassed formation IOx-, reacts form, for example, iodic NO3-. ozone humidity was applied daytime, airborne good agreement I−(HNO3) cluster (specific detection) obtained. At high ratios, we concentration our ion–molecule reactor (IMR) significantly depleted. This reflected changes measured signal 127 IOx- formed does survive passage through instrument but detected after fragmentation I−. result trace gases stratospheric air masses unless gas continuously added impact characterised.