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

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 m/z 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 (CH3CO2-, m/z 59, from either PAN or PAA). The loss of CH3CO2- via conversion to NO3- 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 m/z 62 is achieved in the presence of ambient levels of O3 whereby the thermodynamically disfavoured, direct reaction of I− with HNO3 to form NO3- is bypassed by the formation of IOx-, which reacts with HNO3 to form, for example, iodic acid and NO3-. The ozone and humidity dependence of the detection of HNO3 at m/z 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 m/z 127 as the IOx- 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.

An Alternative Calibration Method for Measuring N2O5 with an Iodide-Chemical Ionization Mass Spectrometer and Influencing Factors

“…Despite the presence of two discernible peaks (62 and 235) in the determination of N 2 O 5 by I-CIMS, the value of NO 3 – remains highly uncertain owing to its susceptibility to interference from other nitrogen-containing species. Consequently, we employ I(N 2 O 5 ) − as a surrogate to denote the concentration of N 2 O 5 , which is comparatively more reliable. ,, The ensuing analysis will be based on the peak observed at 235 amu utilized for the quantification of N 2 O 5 employing I-CIMS N 2 O 5 + I − → normalI normalN normalO 2 + N O 3 − …”

“…An advantage of this technique is that it does not suffer from high and variable chemical interferences compared with TD-CIMS. In recent years, this method has been widely used to measure N 2 O 5 both in laboratory and field experiments ,− I − + N 2 O 5 → normalI ( normalN 2 normalO 5 ) − …”

Formation of Highly Oxidized Organic Compounds and Secondary Organic Aerosol from α-Thujene Ozonolysis

“…The other advantage of analyzing only M-NO 3 – and M-I – species is that we can have greater confidence that measured species were not fragmented by ionization. , Additionally, an important limitation of using iodide as a reagent ion is the interference of secondary ion chemistry. In the presence of O 3 , the I – ion can undergo secondary ion chemistry, forming iodide oxyanions (IO – , IO 2 – , IO 3 – ) that can cluster with oxidized organic compounds. , This is a source of uncertainty in the molecular formula assignment of oxidized organics with I – CIMS, but is not considered for our study, for which we assume all oxygen atoms measured with I – CIMS are attributed to oxidized organic compounds.…”

Supplementary material to "Night-time NO emissions strongly suppress chlorine and nitrate radical formation during the winter in Delhi"

“…This yields an estimated maximum sensitivity of 23 cps pptv -1 per million Icounts for the FIGAERO-CIMS using the X-ray ioniser, with an iodide flow of 1.5 lpm and an IMR pressure of 250 mbar. Previous studies indicate that the sensitivity of the IN2O5signal at m/z 234.886 and the IClNO2signal at m/z 207.867 will be influenced by the relative humidity within the instrument (Kercher et al, 2009;Dörich et al, 2021;Lee et al, 2014). During the campaign, the IH2Osignal observed by the FIGAERO-CIMS during gas-phase measurements was around 13% of the iodide signal.…”

“…During the campaign, the IH2Osignal observed by the FIGAERO-CIMS during gas-phase measurements was around 13% of the iodide signal. This suggests a mixing ratio of approximately 0.69 g kg -1 or a vapour pressure of 0.51 Torr within the IMR region (Dörich et al, 2021). It is therefore likely that, according to the results of laboratory experiments carried out by Kercher et al (2009), this theoretically-estimated maximum sensitivity of 23 cps pptv -1 per million Icounts is around three times too low for N2O5.…”