Iodide-CIMS and m/z 62: The detection of HNO 3 as NO 3- in the presence of PAN, peracetic acid and O 3

. Chemical Ionisation Mass Spectrometry (CIMS) using I - (the iodide anion) as primary reactant ion has previously been used to measure NO 3 and N 2 O 5 both in laboratory and field experiments. We show that reports of the large daytime mixing ratios of NO 3 and N 2 O 5 (usually only present in detectable amounts at night-time) are likely to be heavily biased by the ubiquitous presence of HNO 3 in the troposphere and lower stratosphere. We demonstrate in a series of laboratory 10 experiments that the CIMS detection of HNO 3 at m/z 62 using I - ions is efficient in the presence of PAN or peracetic acid (PAA) and especially O 3 . We have characterised the dependence of the sensitivity to HNO 3 detection on the presence of acetate anions (CH 3 CO 2- , m/z 59, from either PAN or PAA). The loss of CH 3 CO 2- via conversion to NO 3− in the presence of HNO 3 may represent a significant bias in I-CIMS measurements of PAN and CH 3 C(O)OOH. The largest sensitivity to HNO 3 at m/z 62 is achieved in the presence of ambient levels of O 3 whereby the thermodynamically disfavoured, direct reaction of 15 I - with HNO 3 to form NO 3- is bypassed by the formation of IO X– which react with HNO 3 to form e.g. iodic acid and NO 3- . The ozone and humidity dependence of the detection of HNO 3 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 - (HNO 3 ) cluster-ion (specific for HNO 3 detection) was obtained.

2017; Riva et al., 2019). In this work, we focus on the detection of two atmospherically important trace gases N2O5 and HNO3 using a CIMS operating with Ireactant ions.
Both N2O5 and HNO3 are formed in the atmosphere by the sequential oxidation of NO, which has both anthropogenic and natural sources. In a very well-known series of reactions, NO is oxidised (R1, R2) by reaction with O3 or peroxyl radicals (RO2) to NO2, which during the day, may be removed by reaction with OH to form HNO3 (R3) and during the night to form 35 N2O5 (R4, R5). Both HNO3 and N2O5 have important, non-gas loss processes such as uptake to particles and other surfaces. In addition, N2O5 can thermally dissociate back to NO3.
The chain of reactions to form N2O5 is broken during the day as NO3 is rapidly photolysed and also reacts with NO so that N2O5 is expected to be present at significant levels only at night-time. 45 The detection of N2O5 using Ireactant ions can be achieved by monitoring either the NO 3 − product at m/z 62 (see above) or the adduct ion at m/z 235 (Kercher et al., 2009). The former is reported to be more sensitive and less dependent on water vapour concentrations but less specific, with large and highly variable background signals potentially arising from trace gases such as NO3, ClONO2 and BrONO2. Despite this, night-time N2O5 has been monitored in ambient air (as NO3 -) using Ireactant ions, showing reasonable agreement with optical methods (Slusher et al., 2004;Dubé et al., 2006;Chang et al., 50 2011).
During a recent, airborne deployment of our I-CIMS, we monitored NO 3 − at m/z 62 in an attempt to detect N2O5 during two night-time flights. The air masses we investigated were mainly in the tropical free and upper troposphere and lower stratosphere and we did not expect significant interference from e.g. halogen nitrates at m/z 62. However, our airborne measurements (described in detail in section 4) revealed a large and variable signal at m/z 62 both during the day and night. 55 To illustrate this, raw signals obtained during daytime when the aircraft sampled air masses with varying degrees of stratospheric influence are displayed in Fig. S1. The signal at m/z 62 is large and highly variable and is not affected by addition of NO to the heated inlet, ruling out its assignment to either N2O5 or NO3 (see below). The great increase in signal when entering the lower stratosphere and the obvious correlation with O3 (Fischer et al., 1997;Popp et al., 2009) provided an early clue to the identity of the trace-gas detected at m/z 62 which we initially assigned to HNO3. Our results thus appeared 60 to contrast the conclusions of a previous observation of a large daytime signal at m/z 62 when deploying an I-CIMS (in this case in the boundary layer), which was interpreted as resulting (at least in part) from high levels of daytime NO3 and/or N2O5 https://doi.org/10.5194/amt-2021-57 Preprint. Discussion started: 18 March 2021 c Author(s) 2021. CC BY 4.0 License. (Wang et al., 2014). Based on complementary laboratory experiments, Wang et al. (2014) showed, in accord with earlier investigations (Fehsenfeld et al., 1975;Huey et al., 1995), that HNO3 is not detected sensitively at m/z 62 using I-CIMS.
The unexpected observation of a large daytime signal at m/z 62 during airborne operation led us to perform a series of 65 laboratory experiments to identify potential "interfering" trace gases at this mass-to-charge ratio when using I-CIMS. In contrast to the conclusions drawn from previous studies, our laboratory and airborne measurements conclusively show that, during daytime, the predominant contributor to m/z 62 when sampling ambient air (in the presence of ozone) is likely to be HNO3.

Experimental details 70
The I-CIMS we used in our laboratory and airborne investigations (see Fig. 1) is similar to that described by Slusher et al. (2004) and Zheng et al. (2011) and was originally constructed in collaboration with Georgia Tech as a prototype instrument of the company THS (http://thsinstruments.com). It is essentially a hybrid of the instruments described by Phillips et al. (2013) and Eger et al. (2019), the former using a 210 Po ion source, the latter an electrical discharge source but with improved (digital) control of the MS settings enabling different mass-to-charge ratios to be monitored using different potentials for the 75 collisional dissociation of cluster ions. For all the experiments described below, the 210 Po ion source was used to generate Ias this configuration has much better sensitivity for PAN, the main target trace gas during the deployment of the I-CIMS on the HALO aircraft (High Altitude Long range platform for atmospheric Observations). The set-up for PAN detection includes a heated inlet section (~170 °C, 100 mbar, residence time ~ 40 ms) to thermally dissociate PAN to CH3C(O)O2 which subsequently reacts with Ito form the acetate anion (CH3CO2 -) which is detected at m/z 59. At this inlet temperature 80 and pressure, the thermal decomposition rate constant for PAN is 380 s -1 implying a lifetime of ~2 ms. For N2O5, the rate coefficient for its thermal dissociation to NO2 and NO3 is 1940 s -1 (lifetime of ~0.5 ms) so that N2O5 is stoichiometrically converted to NO3 and the instrument measures the sum of N2O5 and NO3 at m/z 62. In order to separate PAN signals from those of peracetic acid (CH3C(O)OOH, also detected as CH3CO2at m/z 59) we periodically add NO to the inlet to remove CH3C(O)O2 and thus eliminate sensitivity to PAN. As NO reacts more rapidly with NO3 than with CH3C(O)O2 at 170 °C 85 (kNO+NO3 = 2.3 × 10 -11 cm 3 molecule -1 s -1 , kNO+CH3C(O)O2 = 1.4 × 10 -11 cm 3 molecule -1 s -1 ) the concentration of NO added is also sufficient to quantitatively titrate NO3 to NO2 and thus provides a measure of the "background" signal at m/z 62 in the absence of NO3 and N2O5.
During airborne operation on HALO, the dynamic pressure generated in a forward facing trace gas inlet (TGI) located on top of the aircraft (see Fig. 1) was used to create a flow of air through ¼ inch (OD) PFA tubing sampling at an angle of 90° to 90 the flight direction. The ¼ inch tubing was atached to a ½ inch (OD) PFA tube attached to an exhaust plate at the underside of the aircraft to create a fast "bypass" flow. The bypass flow was sub-sampled (again at 90° and by ¼ inch PFA tubing heated to 40°C) by the 1.4 L (STP) min -1 flow into the I-CIMS. Sub-sampling twice at 90° to the flow was helpful in reducing the number of large particles (e.g. cloud droplets) that could enter the thermal dissociation inlet and IMR. The thermal dissociation inlet of the I-CIMS is regulated to a pressure of 100 mbar, which results in a pressure in the ion-95 molecule reactor of 24 mbar. This way, a stable pressure in the thermal dissociation inlet and the Ion Molecule Reactor (IMR) was maintained at altitudes up to ~15 km. Prior to take off, the inlet line and TGI were flushed with nitrogen to prevent contamination by the high levels of pollutant trace gases at the airport. As described in Eger et al. (2019) negative ions exiting the IMR were declustered in passage through a collisional dissociation region (CDC, 0.6 mbar) before passing through an octopole ion-guide (6 × 10 -3 mbar) and a quadrupole for mass selection (9 × 10 -5 mbar) prior to detection using a 100 channeltron.
Iions were generated by combining flows of 4 cm 3 (STP) min -1 CH3I/N2 (400 ppmv) with 750 cm 3 (STP) min -1 N2 and passing the mixture through a 370 MBq 210 Po source. Under standard operating conditions (including airborne deployment), a constant amount of H2O was added to the IMR by flowing 50 sccm N2 (at 1 bar pressure) through a 30 cm length of waterpermeable 1/8-inch tubing (Permapure) immersed in water. The 50 sccm flow of N2 acquires a relative humidity close to 100 105 % in transit through the tubing and is subsequently mixed with the dry N2 flow and sample air. Under these conditions, the ratio of signals at m/z 145 (I -(H2O)) to that at m/z 127 (I -) was 0.068. By comparison with calibration curves (see Fig S2 and associated text) this indicates an H2O concentration in the IMR of ~4 × 10 14 molecule cm -3 . For laboratory tests, the amount of water in the IMR could be increased by reducing the pressure in the permeable tube (thus increasing the mole fraction of H2O) or set to zero by bypassing the humidifier. 110 Based on a (calculated) literature value for the free energy of formation of I -(H2O)1 of -6.1 kcal mol -1 (Teiwes et al., 2019) we derive an equilibrium constant (at 298 K) of K6 = 1.16 × 10 -15 cm 3 molecule -1 for the formation and thermal dissociation of I -(H2O)1 With an H2O concentration (in the IMR) of 3.9 × 10 14 molecule cm -3 this implies that the ratio [I -(H2O)1 ] / [I -] = 0.45. Our 115 measured ratio of signals at m/z 145 (I -(H2O)) / m/z 127 (I -) was a factor ~6 lower, reflecting the fact that, even when the declustering potential is reduced to its minimum value, most I -(H2O) ions do not survive the CDC region.
During extended operation of the CIMS, changes in sensitivity were captured by monitoring the primary ion signal (Iand its water cluster). Background signals at each of the mass-to-charge ratios monitored were obtained by passing the sampled air through a tubular scrubber (alluminium) filled with stainless-steel wool heated to 120 °C. 120

Laboratory Characterisation
As described above, our observations of a clear correlation between m/z 62 and O3 mixing ratios during the first HALO deployment of the I-CIMS strongly suggested that HNO3 was the origin of the signal although previous experiments had shown that Idoes not react with HNO3 to form NO3 -. In order to determine the sensitivity of our I-CIMS to HNO3 we constructed a permeation source in which a 20 cm 3 (STP) min -1 flow of zero air was passed through a 1m length of PFA 125 tubing (0.125 inch OD) which was formed into a coil and submerged in an aqueous solution of 65% HNO3 held at 50°C. The https://doi.org/10.5194/amt-2021-57 Preprint. Discussion started: 18 March 2021 c Author(s) 2021. CC BY 4.0 License. permeation rate was determined by passing the 20 cm 3 (STP) min -1 flow through an optical absorption cell and measuring the optical extinction at 185 nm where the absorption cross-section of HNO3 is well known (Dulitz et al., 2018). For the I-CIMS calibration, the 20 cm 3 (STP) min -1 output was dynamically diluted to generate a mixing ratio of between 5 and 50 ppbv.
Based on uncertainties in the absorption cross-section (5%), the reproducibility of the optical measurement and the dilution 130 factor, the uncertainty of the HNO3 mixing ratio is estimated as 15 %. Figure 2 shows the response of the I-CIMS at m/z 62 to addition of various amounts of HNO3. Throughout the paper when presenting raw data, we normalise the I-CIMS signal by dividing by the primary ion signal at m/z 127. The weak signal in the absence of O3 (blue data points) confirms the conclusions of previous studies that derive a low rate coefficient for reaction (R6). For comparison, approximate, relative sensitivities to PAN (m/z 59), N2O5 (m/z 62) and HNO3 (m/z 62), using 135 this instrument are 1, 0.1 and 5 × 10 -4 , respectively. Indeed, as written below, reaction (R7) is endothermic by ~43 kJ mol -1 (Goos et al., 2005).
In a further series of experiments, we measured the response of the I-CIMS to HNO3 when adding O3 to the zero-air. The results, also plotted in Fig. 2 (black symbols), indicate a factor ~250 increase in the signal at m/z 62 when ~500 ppbv ozone 140 was added. There are two possible explanations for this observation. The first involves conversion of NO2 impurity (that is present as a ~8 % impurity in the HNO3 permeation flow) to NO3 and N2O5 (R1, R4, R5) which are subsequently detected.
This can however be ruled out as the rate-limiting step in the formation of NO3 is the slow reaction between NO2 and O3 with k4 = 3.5 × 10 -17 cm 3 molecule -1 s -1 at room temperature (Atkinson et al., 2004). The addition of 1000 ppbv O3 (equivalent to a concentration of 2.4 × 10 13 molecule cm -3 ) would only convert an insignificant fraction of the NO2 to NO3 in 145 the ~40 ms reaction time available from the point of mixing to the IMR. This could be confirmed by adding NO (7.7 ppm) to the inlet which would remove any NO3 (see above) and observing no change in the signal at m/z 62.
The second explanation is that the presence of O3 results in the generation of further reagent ions that can react with HNO3.
In the presence of water vapour, Iis also present as a hydrate I -(H2O) (see above) for which, according to Teiwes et al. (2019), the rate coefficient for reaction with O3 (R13a, R13b) is a factor ~40 larger than k8 and results in the formation of IO2and I -: As R8 is rate limiting, this implies an increase in the amount of e.g. IO3formed in the IMR in the presence of water. In most regions of the troposphere and lower atmosphere ozone mixing ratios lie between 30 and >1000 ppbv. An ambient ozone concentration of 50 ppbv results in a concentration in the IMR of > 10 10 molecules cm -3 . The large rate coefficients for R9 and R10 and the reactions of IOand IO2with O2 result in the rapid inter-conversion of I -, IO -, IO2and IO3which results 170 (for a given RH and ozone concentration) in a quasi-equilibrium between IOXanions.
We explored the relevance of these reactions for our I-CIMS by carrying out a set of experiments in which varying amounts of O3 were added to the inlet and the mass-to-charge ratios corresponding to IO -(m/z 143), IO2 -(m/z 159) and IO3 -(m/z 175) were monitored; the results are depicted in Fig. 3.
First, we note that all three mass-to-charge ratios were indeed observed, but only under conditions where the CDC potential 175 was set to the lowest value at which ions still reach the detector. The dependence of the various IOXanions on the O3 mixing ratio is broadly as expected from the reaction scheme (R7-R11) listed above: The major contributor to IOXat low [O3] is IO -, which is converted to IO2and IO3more efficiently as O3 increases, while the total concentration of IOXincreases approximately linearly. At the maximum O3 mixing ratio used (577 ppbv) there are (following dilution) 375 ppbv in the IMR, which translates to a concentration (at 24 mbar and ~298 K) of 2.1 × 10 11 molecule cm -3 . This O3 concentration is 180 comparable with those used by Teiwes et al. (2018) (~1-4 10 11 molecule cm -3 ) or Bhujel et al. (2020) (~4 × 10 10 molecule cm -3 ) in their ion-trap based, kinetic investigations of the formation of IOXwhen reacting Iwith O3. Their observation that IO3is the dominant anion is however not consistent with our results, which indicate that IO3represents only ~35% of the total IOXsignal. The relative abundance of each IOXdepends not only on the O3 concentration but also on the reaction time, which, for both Teiwes et al. (2018) and Bhujel et al. (2020) was between 10-100 ms. Based on the flow into the IMR, 185 its volume (~50 cm 3 ) and the pressure we calculate a similar residence time (for neutrals) of about 25 ms. We also considered the possibility that the application in our I-CIMS of a potential difference between the entrance and exit of the IMR (to optimise ion-transmission) could result in a significantly shorter IMR-residence time for ions. This was assessed by calculating the drift-velocity (Vd) in the IMR from the electric field strength (E ~12 Vm -1 ) and the ion mobility (µ).
The electrical mobility of Iwas calculated for our conditions (using the Mason-Schamp equation) as ~ 0.15 m 2 V -1 s -1 using a collision cross-section (for an I -/ N2 pair) of 9 × 10 -16 cm 2 molecule -1 (McCracken, 1952). Via equation (1), this results in a drift velocity of 1.8 m s -1 , or an ion residence time (in the ~8 cm long IMR) of 44 ms, which is comparable to the residence time of neutrals. Our observation that IO -(and not IO3 -) is the dominant ion-signal is thus unlikely to result from differences in reaction times, temperature (our IMR is at ~15°C above ambient temperature owing to the heated inlet) or O3 195 concentrations in the different set-ups but may be related to the higher pressure of O2 (> 2 mbar) in our IMR which converts IO3back to IOthus competing with further oxidation (via reaction with O3) to IO3 -. Additionally, the high IMR pressures (24 mbar) in our experiments are ~ six orders of magnitude higher than the ~10 -5 mbar available in the ion-trap experiments of Teiwes et al. (2018) and Bhujel et al. (2020) which will result in more rapid thermalization of the ions present and prevent potentially non-thermal reactions and thus bias to the rate coefficients derived. 200 The effect of adding H2O to the IMR was explored in a further set of experiments and the variation of the signals at mass-tocharge ratios corresponding to IOXwith [H2O] are displayed in Fig. 4. The experiments were carried out with the O3 mixing ratio fixed at either 70 or 120 ppbv, close to that typically found in the lower troposphere (~20-100 ppbv). At the lowest H2O Having established that all of the expected IOXanions are present in our IMR, we can propose a route for HNO3 detection as 210 NO3which involves transfer of a proton from HNO3 (a very strong acid) to the conjugate base of the respective iodine containing acids (hypoiodous-, iodous-and iodic-acid): Taking IO 3 − as an example, we see that the net reaction, (I -+ O3 + HNO3 →NO3 -+ HOIO2) is driven by the relative stability of iodic acid compared to O3, thus bypassing the thermodynamic barrier to direct formation of NO3from HNO3 + I -. As described above, the O3 dependence of the ion signals we observe for IO -, IO2and IO3are consistent with the sequential oxidation of Iby O3. However, the relative ion-abundance we observe at the detector does not necessarily reflect the relative concentration of the ions in the IMR and we cannot assign the individual contribution of any single IOXanion to HNO3 220 detection. We are unable to completely shut of collisional dissociation in our I-CIMS which may be a characteristic that is peculiar to our instrument as we do not detect weakly-bound I instruments utilising Ichemical ionisation (Lee et al., 2014). Hence, our relative sensitivity to the IOXcomponents is unknown.
In order to confirm that IOXis responsible for detection of HNO3 we examined the depletion of the signals at m/z 143, m/z 225 159 and m/z 175 when adding very large concentrations of HNO3 to the IMR. The results, summarised in Fig. 5, indicate that all three IOXions are removed when the HNO3 mixing ratio was increased from zero to 80 ppbv, but with different fractional changes. This can be understood if e.g. the individual IOXreact with HNO3 with different rate coefficients. The solid lines in Fig. 5 represent exponential decays of each ion, with rate coefficients of ~10 × 10 -10 cm 3 molecule -1 s -1 for HNO3 + IO3 -, ~7 × 10 -10 cm 3 molecule -1 s -1 for HNO3 + IO2and ~3 × 10 -10 cm 3 molecule -1 s -1 for HNO3 + IO -. These 230 approximate values were derived by converting the HNO3 mixing ratio into a concentration in the IMR and assuming pseudo-first-order behaviour (i.e. negligible depletion of HNO3) so that (using IO3as example): Where S(IO3 -)t and S(IO3 -)0 are the signals at m/z 175 after and prior to addition of HNO3, respectively. [HNO3]IMR is the concentration (molecule cm -3 ) of HNO3 in the IMR, k (cm 3 molecule -1 s -1 ) is the rate coefficient for reaction between HNO3 235 and IO3and t is the reaction time, which we assume to be 25 ms (see above). This analysis assumes that the re-establishment of equilibria between IOXis minimal on the time scale of the reaction between any single IOXand HNO3. The results indicate qualitatively that IO3is the most reactive of theIOXanions towards HNO3, but that all three contribute to HNO3 detection. The depletion of the summed IOXsignals versus the accompanying increase in signal due to NO3at m/z 62 is displayed in Fig. 6, which indicates a roughly linear relationship, confirming that IOXare mainly responsible for detection of 240 HNO3 in our I-CIMS. We note that the increase in signal at m/z 62 is about a factor 100 greater than the reduction in signal from IOX -, confirming that the detection of IO3in our instrument is inefficient.
While the reactions of IOXwith HNO3 represent the most likely route to HNO3 detection at m/z 62 in our CIMS other possibilities are the reactions of oxide, superoxide and ozone anions (OX -) and hydrated OXwith HNO3 as they have large rate coefficients (> 10 -9 cm 3 molecule -1 s -1 ) and form NO3 - (Huey, 1996;Wincel et al., 1996;Lengyel et al., 2020): 245 However, when adding O3 (up to 600 ppbv) to the IMR we saw no signal that could be attributable to any oxide anion OX -.
Where [O3] is the O3 mixing ratio in ppbv and B has a value of 1.515 × 10 -3 per ppbv of O3. As shown in Fig 7b, for a given [HNO3], the parameter A is dependent on the water vapour concentration (i.e. on ratio of signals at m/z 145 and m/z 127, (S145 / S127) over the range explored and can be parameterised as: 260 A = 0.138 + 0.929 × (S145 / S127) In these experiments, H2O was not added to the TD inlet ( The positive intercept in Fig. 7b indicates that there is significant sensitivity to HNO3 detection at m/z 62 in the absence of water in the IMR, implying that IOXanions can react directly with HNO3 to form NO3as written in R13-15. The increase in the sensitivity to HNO3 as the water vapour concentration is increased is consistent with the formation of I -(H2O) (m/z 145) which reacts more rapidly with O3 (to form IO2directly) than does I - (Teiwes et al., 2019), thereby increasing the abundance of IOXin the IMR (see above) and thus the instrument's sensitivity to HNO3. 270 The very strong sensitizing effect of ozone and H2O vapour can explain why similar instruments to ours observe large signals at m/z 62 when sampling ambient air. Indeed, both O3 and HNO3 are ubiquitous and generally present at much high levels than either NO3 or N2O5 and attempts to measure these traces gases using I-CIMS without TD-inlets and NO titration (to remove the HNO3 contribution) will likely result in erroneously high levels of both, especially during the day when lower-tropospheric O3 and HNO3 are often at their highest levels. It also explains why laboratory tests (generally carried out 275 without added O3 or H2O) have shown only low (or no) sensitivity to HNO3 at m/z 62.
We have also evaluated the potential for "unintentional" HNO3 detection at m/z 62 by its reaction with the acetate anion, CH3CO2 -: The CH3CO2anion is the conjugate base of a weak acid (CH3C(O)OH) has been utilised to monitor a number of trace gases 280 via proton transfer (Veres et al., 2008). While Veres et al. (2010)  Our results disagree with the conclusions of Wang et al. (2014) who saw no increase at m/z 62 when adding PAN to air containing HNO3 but are consistent with the use of CH3CO2as primary reactant ion to detect HNO3 at m/z 62 (Veres et al., 2008). Figure 8b indicates that the increase in signal at m/z 62 when adding HNO3 to a flow of CH3C(O)OOH in air is approximately proportional to the reduction in the ion-signal at m/z 59. This helps confirm that CH3CO2is the ion responsible for the detection of HNO3 but also indicates that the detection of PAN and CH3C(O)OOH via conversion to 295 CH3CO2can be compromised when HNO3 is present in the air sample. Indeed, in many air masses the concentration of HNO3 can be an order of magnitude greater than that of either PAN or CH3C(O)OOH and given that other abundant trace gases (e.g. organic acids) also react with CH3CO2 - (Veres et al., 2008) further reactions of CH3CO2in the ion-molecule reactor regions of I-CIMS instruments may result in a significant bias (to lower values) which would have to be analysed case-by-case for different instruments. 300 Wang et al. (2014) observed that the majority of the m/z 62 signal during the daytime could be removed by addition of NO (0.54 ppmv or 1.3 × 10 13 molecule cm -3 ) to the inlet. At their inlet temperature of 120-180 °C, NO reacts with O3 with a rate coefficient in the range 6-9 × 10 -14 cm 3 molecule -1 s -1 , which results in a half-life for O3 of 500 to 800 ms. (Wang et al., 2014) do not mention the residence time of air passing through their heated inlet, but it appears plausible that a substantial fraction of ambient O3 would have been removed during background measurement, thus decreasing (or removing) sensitivity towards 305 HNO3 via reactions involving O3 in the IMR, and leading the authors to conclude that NO3 was being detected above a lower background than truly present.
To illustrate the potential size of the bias due to HNO3 when monitoring N2O5 at m/z 62 in field measurements we take the relative sensitivities (at m/z 62) of our I-CIMS to N2O5 and to HNO3 (in the presence of typical boundary layer mixing ratios of O3 (50 ppbv) and at typical relative humidity (50%). Under these conditions, with N2O5 and HNO3 mixing ratios of 0.2 310 and 2 ppbv, respectively, we calculate that HNO3 would account for > 70% of the signal at m/z 62.

Field Measurements
Having shown that HNO3 is detected by our I-CIMS with reasonable sensitivity when sufficient O3 is present in ambient air

CAFÉ-Africa
Here we examine the results obtained during a HALO flight as part of the CAFE-Africa mission. The flight in question was the transfer from Sal airport on the Cape Verde islands (which served as base-station during the mission) back to Germany. 320 During the flight the aircraft flew mainly at high altitudes (13-15 km) so that stratospheric air was sampled at higher latitudes but also made two dives into the free-troposphere. The flight track is displayed in Fig S4.   Figure 9 shows a time-series of ozone mixing ratios during the flight (in red, panel a) along with the I-CIMS signal at m/z 62 (in red, panel b). In air masses with stratospheric influence (i.e. O3 values > 100 ppb, 12:20 -15:10 UTC) there is an obvious, strong co-variance between these two parameters. However, once corrected for the dependence of the sensitivity of the I-325 CIMS to O3 (equations 2 and 3) we obtain the black line representing the mixing ratios of HNO3 and the covariance is greatly reduced. We also note that, apart from some significant increases at ~11:30 and ~16:00 the HNO3 mixing ratio decreases slowly throughout the flight, which is the result of HNO3 generation in the 210 Po source leading to an initially large background signal. The formation of HNO3 in the 210 Po source has been documented previously (Ji et al., 2020); its level can be reduced by permanently flushing N2 through the source while keeping the mass-spectrometer under operational vacuum. 330 This was not possible during the CAFE missions on HALO as continuous operation of the instrument (i.e. overnight between flights) was not possible. A roughly exponential decay of the HNO3 background signal was observed in all of the flights in which m/z 62 was monitored, which presumably reflects depletion of the initially large HNO3 reservoir which was built up when the I-CIMS was switched off.
A rough correction of the dataset was thus undertaken by subtracting an exponentially decaying background from the total 335 HNO3 signal. The resulting HNO3 mixing ratios are depicted as the blue line in Fig. 9b and plotted against the O3 mixing ratio in Fig. 9c. Considering only the high altitude data for which O3 mixing ratios were > 100 ppbv (stratospheric influence, black data points) we derive a slope of HNO3 / O3 = (3 ± 0.5) × 10 -3 (the uncertainty is 2 σ, statistical only) which is consistent with previously reported values obtained in airborne measurements of HNO3 and O3 in the lower stratosphere (see Popp et al. (2009) and references therein). We stress that deriving accurate mixing ratios of HNO3 is not possible with this 340 data set and the values obtained are strongly dependent on the background correction. Here, we merely wish to indicate that, while most of the variability in our m/z 62 signal is related to the central role of ozone in the detection scheme (i.e. formation of IOX), some covariance between HNO3 and O3 remains after correction of the raw data and the slope is roughly in line with that expected. We also do not propose that the correlation of m/z 62 with O3 proves that the signal can be attributed entirely to HNO3. This aspect will be covered in section 4.2. 345 Examining Fig. 9b reveals sharp increases in the (background corrected) HNO3 mixing ratio when sampling at lower altitudes, noticeably at 11:30-12:00 (3.9 km altitude) and at 15:45-16:10 (4.7 km altitude) and at the end of the flight during descent to Oberpfaffenhofen in Bavaria, Germany. In all cases, these periods of enhanced HNO3 coincided with higher levels of particles. Back trajectories (HYSPLIT) indicated that, in the 10 days prior to interception by HALO, the air mass sampled at 11:30 had passed over the West African continent (Mauritania, Mali and Niger), whereas the air masses sampled after 16:00 were of European origin. The large, coincidental increase in the HNO3 mixing ratio and particle mass was a recurrent feature of the CAFE-Africa flights. It is conceivable that the HNO3 measured by the I-CIMS was a mixture of gas-phase HNO3 and HNO3 associated with particles that desorb HNO3 when passing through the thermal dissociation inlet at 180 °C.
This temperature would be sufficient to thermally convert ammonium nitrate to HNO3 (and NH3) as well as to result in the desorption of HNO3 that was physi-sorbed e.g. on chemically aged black-carbon or mineral-dust particles. As we do not 355 know the efficiency with which particles of various diameters enter the TD-inlet of the CIMS, we cannot estimate the relative contribution of gas-phase and particulate nitrate to the signal at m/z 62 but indicate that a similar phenomenon may occur in ground-based measurements using TD-inlets and may represent an additional source of bias during ambient measurements of NO3 and/or N2O5 at m/z 62.  Southern Germany to the Atlantic (west of Ireland) and back at various altitudes (for flight track see Fig S6). Figure 10a plots the raw signals measured by the I-CIMS at m/z 62 and m/z 190 as well as the O3 mixing ratio. Similar to the CAFE-Africa data-set, the signal at m/z 62 covaries strongly with the O3 mixing ratios, which were between ~40 and ~700 ppbv.

CAFE-Europa
The signal at m/z 190 does not show any correlation with O3 and the raw signals at m/z 62 and m/z 190 bear little resemblance to each other. 370 Using the calibrations parameters described in section 3 and (for m/z 190) in Fig. S4, the signals at m/z 62 and m/z 190 were converted to HNO3 mixing ratios, depicted in Fig. 10b. Despite the greatly divergent raw-signals, the HNO3 mixing ratios obtained using the different mass-to-charge ratios are in good agreement, both displaying a gradual decrease after take-off at ~08:00 UTC. The high initial level of HNO3 is largely the result of HNO3 being formed in the 210 Po source during overnight instrument shut-down (see section 4.1). The HNO3 mixing ratios observed at m/z 62 and m/z 190 both increase when the 375 aircraft sampled stratospheric air (11:00 to 13:00 and 15:10-15:30 UTC). In Fig. 10c HNO3 mixing ratios derived at m/z 62 and m/z 190 are plotted in a correlation diagram. The slope (0.98 ± 0.05) and intercept (-0.01 ± 0.28) indicate good agreement even when the raw signals are highly divergent at high levels of O3. The provides strong evidence that, in many if not most air masses, m/z 62 provides a measure of HNO3 rather than NO3 and N2O5.

Conclusions 380
A series of laboratory experiments investigating the origin of signal at m/z 62 when using an I-CIMS has revealed unexpected sensitivity to HNO3 at this mass-to-charge ratio in the presence of O3 or peracetic acid (PAA) or PAN. The ozone effect is related to the formation of IO X − which react rapidly with HNO3 to form NO3thus bypassing the thermodynamic barrier to formation of NO3by direct reacton of HNO3 with I -. The presence of O3 at a mixing ratio of 500 ppbv results in a 250-fold increase in sensitivity to HNO3 at m/z 62. The sensitivity to HNO3 at this mass-to-charge ratio was 385 also found to be highly dependent on the concentration of H2O in the ion-molecule reactor as this aids formation of IOX -. The sensitivity to HNO3 at m/z 62 in the presence of PAA is a result of the presence of acetate anions (CH3CO2 -) as demonstrated previously (Veres et al., 2008). We suggest that measurements of PAN using I-CIMS may be biased to low values if large mixing ratuios of HNO3 (or organic acids) are present. Our laboratory experiments indicate that measurements of atmospheric NO3 and N2O5 at m/z 62 can be heavily biased by the presence of HNO3, and may explain reports of 390 unexpectedly high daytime mixing ratios of N2O5. The relaive sensitivity at m/z 62 to HNO3 and N2O5 / NO3 will vary from one I-CIMS instrumet to the next and must thus be analysed case-by-case.
We have examined signals at m/z 62 during two periods of operation of the I-CIMS on the HALO-aircraft, one over the Atlantic west of the African coast and one over Europe. During the flights over Europe HNO3 mixing ratios derived from m/z 62 (NO 3 − ) and at m/z 190 (I -(HNO3)) were in very good agreement. The data obtained over the Atlantic indicated that 395 measurements at m/z 62 using a thermal dissociation inlet can be strongly influenced by particulate nitrate that can thermally dissociate (or desorb) to gas-phase HNO3.

Data availability
Data measured during the flight campaign CAFE campaigns are available to all scientists agreeing to the CAFE data protocol. The laboratory data underlying the Figures is availlable upon request to the authors. 400

Author contributions
RD conducted the laboratory experiments, carried out the airborne measurements with assistance from PE and JC and analysed the laboratory with assistance from JC. The manuscript was written by JC and RD with contributions from all other authors. JL designed and helped plan the airborne operations.

Competing interests 405
The authors declare that they have no conflict of interest.     Correlation of the HNO3 mixing ratios derived from the two masses. The red line is a bivariate fit (slope 0.98 ± 0.05, intercept -0.01 ± 0.28). A large fraction of the HNO3 measured stems from the polonium source, especially at the beginning of the flight. 580