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