Preprints
https://doi.org/10.5194/amt-2021-57
https://doi.org/10.5194/amt-2021-57

  18 Mar 2021

18 Mar 2021

Review status: this preprint is currently under review for the journal AMT.

Iodide-CIMS and m/z 62: The detection of HNO3 as NO3 in the presence of PAN, peracetic acid and O3

Raphael Dörich, Philipp Eger, Jos Lelieveld, and John N. Crowley Raphael Dörich et al.
  • Atmospheric Chemistry Department, Max Planck Institute for Chemistry, 55128, Mainz, Germany

Abstract. Chemical Ionisation Mass Spectrometry (CIMS) using I (the iodide anion) as primary chemi-ion has previously been used to measure NO3 and N2O5 both in laboratory and field experiments. We show that reports of the large daytime mixing ratios of NO3 and N2O5 (usually only present in detectable amounts at night-time) 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 PAN or peracetic 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 CH3C(O)OOH. The largest 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 react with HNO3 to form e.g. 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 very good agreement with measurements of the I(HNO3) cluster-ion (specific for HNO3 detection) was obtained.

Raphael Dörich et al.

Status: open (until 13 May 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on amt-2021-57', Anonymous Referee #1, 23 Mar 2021 reply
  • RC2: 'Comment on amt-2021-57', Anonymous Referee #2, 23 Mar 2021 reply
  • RC3: 'Comment on amt-2021-57', Anonymous Referee #3, 15 Apr 2021 reply
    • AC1: 'Reply on RC3', John N. Crowley, 15 Apr 2021 reply
    • AC5: 'Reply to RC3', John N. Crowley, 06 May 2021 reply
  • AC2: 'Supplementary information', John N. Crowley, 19 Apr 2021 reply

Raphael Dörich et al.

Raphael Dörich et al.

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Short summary
We demonstrate in laboratory experiments that the formation of IOx anions (formed in reactons of I with O3) or acetate anions (formed e.g. by the reaction of I with peracetic acid) results in unexpected sensitivity of an iodide chemical ionisation mass spectrometer (I-CIMS) to HNO3 at a mass-to-chanrge ratio of 62. This helps explain observations of apparent high daytime levels of N2O5. Airborne measurements using I-CIMS confirm these conclusions.