Investigation of a potential HCHO measurement artifact from ISOPOOH

Investigation of a potential HCHO measurement artifact from

ISOPOOH

Jason M. St. Clair

1,2

Jean C. Rivera-Rios

John D. Crounse

Eric Praske

Michelle J.

Kim

Glenn M. Wolfe

1,2

Frank N. Keutsch

3,6

Paul O. Wennberg

4,7

, and

Thomas F. Hanisco

Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center,

Greenbelt, MD, 20771, USA

Joint Center for Earth Systems Technology, University of Maryland Baltimore County, Baltimore,

MD, 21228, USA

Department of Chemistry, University of Wisconsin-Madison, Madison, WI, 53706, USA

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA,

91125, USA

Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena,

CA, 91125, USA

Paulson School of Engineering and Applied Sciences and Department of Chemistry and

Chemical Biology, Harvard University, Cambridge MA, 02138, USA

Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA,

91125, USA

Abstract

Recent laboratory experiments have shown that a first generation isoprene oxidation product,

ISOPOOH, can decompose to methyl vinyl ketone (MVK) and methacrolein (MACR) on

instrument surfaces, leading to overestimates of MVK and MACR concentrations. Formaldehyde

(HCHO) was suggested as a decomposition co-product, raising concern that in situ HCHO

measurements may also be affected by an ISOPOOH interference. The HCHO measurement

artifact from ISOPOOH for the NASA In Situ Airborne Formaldehyde instrument (ISAF) was

investigated for the two major ISOPOOH isomers, (1,2)-ISOPOOH and (4,3)-ISOPOOH, under

dry and humid conditions. The dry conversion of ISOPOOH to HCHO was 3±2% and 6±4% for

(1,2)-ISOPOOH and (4,3)-ISOPOOH, respectively. Under humid (RH= 40-60%) conditions,

conversion to HCHO was 6±4% for (1,2)-ISOPOOH and 10±5% for (4,3)-ISOPOOH. The

measurement artifact caused by conversion of ISOPOOH to HCHO in the ISAF instrument was

estimated for data obtained on the 2013 September 6 flight of the Studies of Emissions and

Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC

RS)

Correspondence to

: Jason M. St. Clair (jason.m.stclair@nasa.gov).

Supplementary material related to this article is available.

Data availability

SEAC

RS campaign data used in this paper can be obtained via the following DOI: 10.5067/Aircraft/SEAC4RS/Aerosol-TraceGas-

Cloud

NASA Public Access

Author manuscript

Atmos Meas Tech

. Author manuscript; available in PMC 2018 April 08.

Published in final edited form as:

Atmos Meas Tech

. 2016 ; 9(9): 4561–4568. doi:10.5194/amt-9-4561-2016.

NASA Author Manuscript

campaign. Prompt ISOPOOH conversion to HCHO was the source for <4% of the observed

HCHO, including in the high-isoprene boundary layer. Time-delayed conversion, where previous

exposure to ISOPOOH affects measured HCHO later in flight, was conservatively estimated to be

< 10% of observed HCHO and is significant only when high ISOPOOH sampling periods

immediately precede periods of low HCHO.

1 Introduction

Formaldehyde (HCHO) in the atmosphere is predominantly a product of the gas phase

oxidation of volatile organic compounds (VOCs). Oxidation of methane produces a global

background of HCHO that accounts for ~80% of the global HCHO production (

Fortems-

Cheiney et al., 2012

). The emissions of non-methane hydrocarbons (NMHCs), such as

isoprene and its oxidation products, are more localized and create spatial heterogeneity in

HCHO production due to their shorter atmospheric lifetime. HCHO is one of the few VOCs

that can be measured from orbit (e.g.,

Chance et al., 2000

) and is used to estimate global

isoprene emissions from space (

Marais et al., 2012

;

Millet et al., 2008

;

Palmer et al., 2003

2006

). Thus, accurate in situ HCHO measurements in isoprene-rich environments are crucial

for refining our understanding of biogenic emissions and chemistry (

Kaiser et al., 2015

;

Wolfe et al., 2016

Measurements of the major first generation NO-dominated isoprene oxidation products,

methyl vinyl ketone (MVK) and methacrolein (MACR), are frequently used in combination

with isoprene measurements to understand biogenic oxidative environments (e.g.,

Karl et al.,

2007

). The instrumental techniques used to measure MVK and MACR, e.g. proton transfer

reaction-mass spectrometry (PTR-MS) and gas chromatography (GC), have recently been

shown to convert the major first generation HO

-dominated isoprene oxidation product,

isoprene hydroxyhydroperoxides (ISOPOOH), into MVK or MACR with varying yields

(

Liu et al., 2013

;

Rivera-Rios et al., 2014

). Conversion of ISOPOOH to MVK/MACR likely

occurs via cleavage of the peroxy bond and is catalyzed by metal surfaces in instrument gas

sampling systems (

Rivera-Rios et al., 2014

). The alkoxy radicals generated from breaking

the peroxy bond subsequently decompose, in the presence of O

, to MVK (from (1,2)-

ISOPOOH) or MACR (from (4,3)-ISOPOOH), HCHO, and HO

. Figure 1 shows the two

major isomers of ISOPOOH and the observed hydrocarbon products of their instrument

surface-mediated decomposition.

The equivalent amount of HCHO produced with MVK/MACR has led to some concern in

the community that ISOPOOH causes a measurement artifact in HCHO instruments. The

degree of conversion will be highly dependent on instrument configuration, including the

nature of exposed surfaces, gas temperatures, and flow rates. This study was conducted to

investigate the potential HCHO measurement interference from ISOPOOH for the NASA

ISAF (In Situ Airborne Formaldehyde) HCHO instrument (

Cazorla et al., 2015

). The design

of the ISAF instrument minimizes sample volume to maximize the instrument time

response, and the small volume also minimizes surface area available for ISOPOOH

conversion to HCHO. In addition, most exposed surfaces in the sample inlet and instrument

are either fluoropolymer (PFA tubing) or coated in fluoropolymer (FluoroPel, Cytonix).

St. Clair et al.

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Results for the Harvard formaldehyde instrument (

DiGangi et al., 2011

;

Hottle et al., 2009

)

will be published separately as part of a larger study (FIXCIT/SOAS).

Experiments were conducted for both major ISOPOOH isomers, (1,2)-ISOPOOH and (4,3)-

ISOPOOH, under both dry and humid conditions. The influence of inlet temperature and the

composition of exposed surfaces were also investigated. A slow HCHO time constant,

unique to the ISOPOOH conversion experiments and caused by the experimental conditions,

is discussed. Finally, the experimental results for ISOPOOH conversion to HCHO and the

laboratory time constant are discussed in the context of data from the Studies of Emissions

and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys

(SEAC

RS) campaign (

Toon et al., 2016

2 Experimental

In total, 10 experiments were conducted and are listed in Table 1. Experiments were

performed for both major ISOPOOH isomers, (1,2)-ISOPOOH (ISOPBOOH) and (4,3)-

ISOPOOH (ISOPDOOH), which were synthesized for these experiments (

Rivera-Rios et al.,

2014

). Sample purity, as determined by NMR, was 78% for 1,2 ISOPOOH and 89% for 4,3

ISOPOOH with water, H

, and diethyl ether as the main impurities. For each isomer, two

dry air ISOPOOH to HCHO conversion experiments and one humid air conversion

experiment were performed. The effect of temperature and tubing material was investigated

for each isomer with two experiments for each, one with varied ISAF inlet tubing

temperatures and one with varied bare stainless steel tubing temperatures.

All experiments were conducted using a ~1 m

FEP chamber. ISOPOOH was introduced

into the chamber by flowing 20 standard liters per minute (sLm) of dry air over droplets of

ISOPOOH for 10-30 minutes, and the remaining chamber volume was filled with dry air.

The sampling set-up is shown in Fig. 2. A sample line (6.35 mm OD PFA) connects the

chamber to the instruments, with the sample flow determined by the sum of the individual

instrument sample flows (~10 sLm typical). Two Caltech Chemical Ionization Mass

Spectrometers (CIT-CIMS) instruments provided measurements of ISOPOOH (

Crounse et

al., 2006

;

Paulot et al., 2009

;

St. Clair et al., 2010

;

Nguyen et al., 2015

) and the ISAF

instrument (

Cazorla et al., 2015

) measured HCHO.

The ISAF flight inlet (

Cazorla et al., 2015

) was represented in the lab experiments by a 30.5

cm length of 6.35 mm OD silica-steel tubing (Fig. 2, item B) coated with a fluoropolymer

(FluoroPel, Cytonix) and attached to a thermally controlled piece of aluminum using thermal

epoxy. The equivalent tubing in the NASA DC-8 ISAF inlet is heated by two zones of

heaters, each controlled by a thermostat: the first zone is controlled to 43-54° C (Honeywell

3100U-31440) and the second zone is controlled to 27-38° C (Honeywell 3100U-31437).

The thermostats are mounted on two aluminum blocks that enclose and heat the inlet tubing;

the inlet tubing and the sample flow are likely cooler than the control temperature. For the

lab tests, a Minco heater controller (model CT325) provided variable and stable control

temperatures, and a thermistor on the aluminum piece provided the temperature. In flight,

flow rates through the DC-8 inlet are 10-25 sLm, with ~2.5 sLm of the total inlet flow

subsampled by the instrument. During these experiments, the large inlet flow was not

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possible due to limited chamber volume. All of the inlet flow passed through the instrument

and ranged from 3-7 sLm.

All experiments started with dry air flowing through the sample line to measure the

background level of ISOPOOH and HCHO. Typical background mixing ratios were 120 and

150 pptv, respectively, and were influenced by recent experiments and sample tubing

flushing time. For humid experiments, the dry background was followed by a humid

background where the air was humidified by passing the dry air through a water bubbler.

Because of dissolved trace HCHO in the liquid water and possible wall exchange effects, the

background HCHO was typically higher under humid conditions than dry conditions by

~40-100 pptv. Any background HCHO that originated from a source other than the dry or

humid air (e.g. from the ISOPOOH sample) was not accounted for and would bias the

conversion rate high. After obtaining a background measurement, the instruments sampled

the chamber air diluted by dry (or humid) air provided by a mass flow controller as shown in

Fig. 2. By varying the amount of air provided by the flow controller, the instruments

sampled multiple [ISOPOOH] from the same chamber fill.

The effect of ISAF inlet temperature on ISOPOOH conversion to HCHO was investigated

by setting the inlet tube to multiple temperatures with a constant [ISOPOOH] and observing

the change in measured HCHO. To demonstrate the potential for heated, untreated stainless

steel to convert ISOPOOH to HCHO, another set of experiments was conducted where the

sampling line from the chamber included a 15.4 cm section of 6.35 mm OD stainless steel

tubing held inside a GC oven. The untreated tubing was inline between the chamber and the

instruments (Fig. 2, item A).

3 Results and Discussion

3.1 Treatment of pre-instrument ISOPOOH to HCHO conversion

The HCHO data at the beginning of each conversion experiment contains a rapid increase

followed by a multi-exponential decay. The ISOPOOH data exhibits a complementary time

response such that if the HCHO decay is added to the ISOPOOH data, the result is a sharp

transition from the background measurement to the final ISOPOOH mixing ratio. We infer

that this represents a possible ISOPOOH to HCHO conversion upstream of the instruments

and a correction should be applied to improve the calculation of the conversion rate within

the ISAF instrument.

The HCHO data for the first ISOPOOH mixing ratio of each experiment were fit with a

three term multi-exponential function to determine the appropriate time constants. The

function (Eq. (1)) is:

Function = A

× exp( − t/τ

) + A

× exp( − t/τ

) + A

× exp( − t/τ

) + c

(1)

All fits were constrained to require positive values for all fit parameters, and the data for

post-experiment zero data could not be negative after subtraction of the decay fit. These

constraints were imposed to prevent the fit from using one term from offsetting a non-

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physical value for another. The data were then fit using the selected three time constants

(100 s, 800 s, 2000 s) to obtain the best pre-exponential terms to describe all of the

experiments. The final fit was conducted with the ratio of the first three pre-exponential

terms fixed relative to each other: A

= A

and A

=0.25×A

. Two variables were optimized:

, and c. The goal of the preceding steps was to obtain a three term exponential function

that would reasonably describe the data from multiple experiments by fitting a single scaling

factor (representing [ISOPOOH] at t=0) and a constant (representing a background). The

ISOPOOH and HCHO used hereafter have the decay fit added and subtracted, respectively,

from the original data. Figure S1 shows the effect of the correction on experiment E7 data as

an example. Two experiments were not corrected for pre-instrument conversion: E5 and

E10. Experiment E5 did not require this correction because it was a continuation of another

experiment (E4) and so did not exhibit the same decay. E10 was excluded because HCHO

data for the first set point was close to stable before the ISOPOOH concentration was

changed, and the correction applied to the other experiments would be larger than the E10

data warrant.

3.2 Conversion under dry conditions

Two experiments for each of the ISOPOOH isomers (E1 and E10 for (1,2)-ISOPOOH; E5

and E9 for (4,3)-ISOPOOH) were conducted under dry conditions, and the results are listed

in Table 2. Figure 3 shows the HCHO time series for one (1,2)-ISOPOOH experiment, as

well as the linear regression of HCHO and ISOPOOH for the two experiments. The

background signal for both species was subtracted from the data in all regression plots. The

regressions give ~3% conversion of (1,2)-ISOPOOH to HCHO under dry conditions for both

experiments fit separately and as one data set. The equivalent plots for (4,3)-ISOPOOH are

shown in Fig. S2. For (4,3)-ISOPOOH, the fits for the two experiments differ significantly,

with one experiment giving ~4% conversion and the second giving ~7% conversion. The

second conversion experiments followed a humid experiment, and the prolonged exposure of

surfaces to water vapor may have affected the subsequent conversion efficiency despite the

sample line being purged with dry air for ~30 minutes in between experiments. The

conversion of (4,3)-ISOPOOH to HCHO under dry conditions is estimated to be ~6%.

3.3 Conversion under humid conditions

One experiment for each ISOPOOH isomer was conducted under humid conditions to

evaluate the influence of water vapor on ISOPOOH conversion to HCHO. The results are

displayed in Table 2, and the data from the (1,2)-ISOPOOH (E7) and (4,3)-ISOPOOH (E8)

experiments are shown in Figs. S3 and S4, respectively. Relative humidity was ~60% for E7

and ~40% for E8, and was determined using the ToF-CIMS CF

−

two water cluster signal

(m/z=121). The conversion rate under humid conditions is higher than under dry conditions,

and the HCHO-ISOPOOH relationship is not linear over the range of [ISOPOOH]. One

possible cause of the nonlinearity is that the conversion process becomes saturated with

respect to ISOPOOH at higher [ISOPOOH]. If saturation is occurring, the conversion rate

relevant to field operation will be the higher rate obtained from the lower [ISOPOOH] data.

Because of the uncertainty related to the higher background, surface equilibration, and

nonlinearity, the estimated uncertainty for the ISOPOOH conversion fraction is higher for

the humid experiments than the dry experiments. Using the lower [ISOPOOH] data where

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the relationship is linear, the (1,2)-ISOPOOH conversion to HCHO is determined to be 6%

and for (4,3)-ISOPOOH it is 10%.

3.4 Temperature dependence of conversion

The effect of tubing temperature on ISOPOOH conversion to HCHO was evaluated for an

ISAF inlet tubing proxy and for untreated stainless steel tubing. The data for both sets of

experiments are displayed in Fig. 4. The inlet tubing was heated to multiple temperature set

points, including near the upper limit of the thermostats that control the inlet heaters during

flight (54° C). The data indicated no discernable temperature dependence of ISOPOOH to

HCHO conversion for either ISOPOOH isomer. A section of untreated stainless steel tubing

was heated to much higher temperatures to demonstrate the potential for ISOPOOH

conversion on heated, untreated metal surfaces. The conversion fraction for a given

temperature was also found to be a function of the gas-tubing contact time, determined by

the length of exposed tubing and the sample flow rate, and should not be taken as

quantitatively applicable to other configurations. Room temperature conversion rates for

(4,3)-ISOPOOH were not significantly higher with the untreated stainless tubing in place

than the experiments (Table 2) without it. At elevated temperatures, however, the conversion

fraction steadily increased, reaching ~50% conversion for (1,2)-ISOPOOH at 160° C.

3.5 Instrument time constant

The inherent HCHO time response of the ISAF instrument is rapid, measured as 0.19 s in

the lab (

Cazorla et al., 2015

). The equilibration time indicated in the conversion experiments

is considerably longer, though the time constants in the correction term described in Sect. 3

removed some of the equilibration time. During the conversion experiments, the ISAF

instrument was exposed to ISOPOOH mixing ratios that are ~10 times higher than those

present in the real atmosphere. Due to small conversion efficiencies, such high levels of

ISOPOOH were necessary for discriminating signal changes over the chamber background.

The high ISOPOOH levels, however, thoroughly coated the surface sites in the instrument

that convert ISOPOOH to HCHO to a degree that is not relevant to the atmosphere, and

resulted in a longer term HCHO source from ISOPOOH conversion that is unlikely to

represent the instrument response in the real atmosphere. The potential interference from

this time constant is nonetheless investigated in the following section.

3.6 Insight from SEAC

RS for conversion and time constant importance

The Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by

Regional Surveys (SEAC

RS) campaign included flight tracks over the Ozark Mountains to

sample an isoprene rich, low-NO

region. Figure 5 shows data from the SEAC

RS flight (6

September 2013) with some of the highest ISOPOOH mixing ratios of the campaign. In the

top panel, ISAF HCHO and CIT ISOPOOH data are plotted as a time series along with the

possible HCHO from ISOPOOH conversion, assuming a conservative 8% prompt

conversion rate. The percentage of the ISAF HCHO measurement that is potentially from

prompt ISOPOOH conversion is shown in the bottom panel and does not exceed 4% of the

measured HCHO. For comparison, the stated calibration accuracy of ISAF is ±10%. The

conversion percent was set to zero for ISOPOOH values of ≤ 80 pptv to improve figure

clarity.

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While prompt conversion of ISOPOOH to HCHO was shown to have a negligible impact on

the HCHO measurement, a transition from a high ISOPOOH, high HCHO, sampling

environment to a low HCHO environment may result in a greater fractional influence of

ISOPOOH conversion on measured HCHO, if a significant amount of ISOPOOH conversion

continues after leaving the boundary layer. To illustrate the potential effects of delayed

ISOPOOH conversion to HCHO on field data, a section of the SEAC

RS 2013 September 6

flight was selected where the aircraft rapidly ascended from an isoprene-rich boundary layer

up to over 12 km (Fig. 6, bottom panel). The HCHO transitioned from ~4 ppbv to as low as

~100 pptv and ISOPOOH decreased from peaks of ~1.3 ppbv to ~0 pptv (Figs 5 and 6, top

panels).

The instrument response to a rapid decrease in ISOPOOH and HCHO mixing ratios was

modeled using the end of Experiment E8, where the sample line source was transferred from

the chamber to clean humid air and subsequently to dry air. The decay of the HCHO signal

was first normalized to the initial mixing ratio so that at time=0, HCHO=1. The sample line

response was then removed from the fractional HCHO decay by subtracting the fractional

ISOPOOH decay, which was assumed to be representative of the sample line response. The

remaining fractional HCHO decay represents only the instrument response.

The modeled long-term HCHO source from ISOPOOH conversion is shown in Fig. 6, top

panel. The HCHO decay started at time = 20.582 h UTC, which was chosen to coincide with

the rapid drop in ambient HCHO mixing ratio. The fractional decay of the HCHO from

ISOPOOH conversion from Experiment E8 was applied to the in situ data, with the starting

value of the in situ HCHO decay being the mean HCHO from rapid conversion (8% of

ISOPOOH) over the hour prior to leaving the boundary layer. The bottom panel of Fig. 6

shows the percentage of observed HCHO that would be from ISOPOOH conversion under

the modeled assumptions. The HCHO from ISOPOOH conversion accounts for less than

10% of the measured HCHO and is largely controlled by low ambient HCHO rather than

large amounts of HCHO from ISOPOOH conversion. The delayed HCHO source from

ISOPOOH is likely a function of exposure time in addition to ISOPOOH mixing ratio. The

HCHO decay from Experiment E8 followed exposure to 6-19 ppbv of ISOPOOH over ~ 3

hours, resulting in an integrated ISOPOOH exposure that greatly exceeds in situ sampling

conditions. As a result, the estimated error shown in Fig. 6, bottom panel, may be an upper

limit.

4 Summary and Conclusions

In the ambient atmosphere, (1,2)-ISOPOOH will be the predominant isomer due to first

generation isoprene RO

branching and isomerization (

Bates et al., 2014

;

Teng et al., in

preparation

), and more rapid reaction of (4,3)-ISOPOOH with OH (

St. Clair et al., 2015

Consequently, the laboratory experiments with (1,2)-ISOPOOH are most relevant to the

potential HCHO artifact in ISAF. Under dry conditions, only ~3% of the (1,2)-ISOPOOH

was converted to HCHO. Under humid conditions closer to typical high ISOPOOH sampling

conditions, the conversion is still only ~6% of ISOPOOH, albeit with higher uncertainty. For

comparison, the stated ISAF measurement uncertainty is ±10% for calibration and ±20 pptv

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for offset. Prompt conversion of ISOPOOH to HCHO in the ISAF instrument will cause a

negligible error in the measurement of ambient HCHO.

The instrument surfaces that converted ISOPOOH to HCHO in ISAF were thoroughly

coated with ISOPOOH by long exposure to high ISOPOOH concentrations, and background

HCHO was subsequently elevated for a time period orders of magnitude longer than

suggested by the instrument time constant (0.19 s). The slow time response may be an

artifact unique to the laboratory experiments, or it may be an indication of a delayed

conversion of ISOPOOH to HCHO that could cause an error in the measured HCHO after

exposure to ISOPOOH. The delayed conversion would manifest for upper tropospheric

measurements that follow closely after extended boundary layer sampling in an isoprene-

rich area. For the SEAC

RS campaign example given, the error was still within the stated

ISAF measurement uncertainty.

Heating of the treated ISAF inlet tubing over the possible operation range had no effect on

the conversion rate, while heating untreated stainless steel tubing converted a substantial

fraction (~50% at 160° C) of ISOPOOH to HCHO with high (>80%) molar conversion of

ISOPOOH to HCHO, as determined by dividing the change in ISOPOOH by the change in

HCHO. The high conversion rate on the untreated stainless steel tubing suggests attributes

that should be avoided in instrument and inlet design to minimize conversion of

hydroperoxide compounds. These experiments suggest that instruments with any amount of

metal surface uncoated with a fluoropolymer, particularly if the surface is heated, are likely

susceptible to conversion of organic peroxides such as ISOPOOH. The most likely site for

conversion in ISAF is the MKS pressure controller upstream of the detection cell. As a step

toward eliminating the small conversion in ISAF, this component will be replaced with a

version that has smaller unflushed volumes and fluoropolymer-coated surfaces.

Future work will extend the testing of instrumentation towards interferences from additional

reactive peroxides that can be abundant in the boundary layer, e.g. hydroxymethyl hydrogen

peroxide.

Supplementary Material

Refer to Web version on PubMed Central for supplementary material.

Acknowledgments

FNK and JCR were supported by the National Science Foundation (AGS 1628491, 1628530, 1247421, 1321987).

JMSC, GMW, and TFH were supported by NASA (NNH12ZDA001N-UACO). MJK was supported by the National

Science Foundation (AGS PRF 1524860).

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Figure 1.

The major isomers of ISOPOOH and the observed hydrocarbon products of their instrument

surface-mediated decomposition: methacrolein (MACR), methyl vinyl ketone (MVK), and

formaldehyde (HCHO).

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Figure 2.

Experimental set up. During experiments with untreated stainless tubing, the sample line

passed through the tubing (item A) before proceeding to the instruments. The ISAF inlet

tubing location is noted by item B.

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Figure 3.

(Top Panel) Time series of dry air conversion experiment E10 with (1,2)-ISOPOOH at three

ISOPOOH mixing ratios. (Bottom Panel) Linear fits to experiments E1 (dashed blue line)

and E10 (dashed red line) individually and together (solid black line).

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Figure 4.

Temperature dependence of ISOPOOH conversion to HCHO for the ISAF inlet and

untreated stainless steel tubing. Data shown with symbols (E2, E4) are mean values, and

data shown with lines (E3, E6) are 5° C wide binned mean values.

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Figure 5.

SEAC

RS flight on 6 September 2013. Top panel: Time series of ISAF HCHO (black line),

CIT ISOPOOH (blue line), and potential HCHO signal from prompt ISOPOOH conversion

(red line). Bottom panel: Percent of the HCHO observed that might be from prompt

ISOPOOH conversion (red line) and aircraft altitude (black line).

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Figure 6.

Hypothesized HCHO signal from delayed ISOPOOH conversion, shown for part of

SEAC

RS flight on 6 September 2013. Top panel: Time series of ISAF HCHO (black line),

CIT ISOPOOH (blue line), potential HCHO signal from prompt ISOPOOH conversion (red

line), and potential HCHO signal from delayed ISOPOOH conversion (cyan line). Bottom

panel: Altitude (black line) and percentage of HCHO that might be from delayed ISOPOOH

conversion (red line).

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Table 1

List of Experiments

Experiment

Compound

Description

(1,2)-ISOPOOH

Dry

(1,2)-ISOPOOH

Dry, with and without untreated stainless tubing

(1,2)-ISOPOOH

Dry, ISAF inlet tubing temperature

(4,3)-ISOPOOH

Dry, with untreated stainless tubing

(4,3)-ISOPOOH

Dry

(4,3)-ISOPOOH

Dry, ISAF inlet tubing temperature

(1,2)-ISOPOOH

Humid

(4,3)-ISOPOOH

Humid

(4,3)-ISOPOOH

Dry

E10

(1,2)-ISOPOOH

Dry

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