amt-9-4561-2016.pdf

Atmos. Meas. Tech., 9, 4561–4568, 2016

www.atmos-meas-tech.net/9/4561/2016/

doi:10.5194/amt-9-4561-2016

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

Correspondence to:

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

Received: 13 June 2016 – Published in Atmos. Meas. Tech. Discuss.: 16 June 2016

Revised: 12 August 2016 – Accepted: 15 August 2016 – Published: 16 September 2016

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 measure-

ments 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 hu-

mid 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 (relative humidity

of 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 ob-

tained on the 6 September 2013 flight of the Studies of

Emissions and Atmospheric Composition, Clouds and Cli-

mate Coupling by Regional Surveys (SEAC

RS) campaign.

Prompt ISOPOOH conversion to HCHO was the source of

< 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 the

flight, was conservatively estimated to be < 10 % of observed

HCHO, and is significant only when high ISOPOOH sam-

pling 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 com-

pounds (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 emis-

sions of non-methane hydrocarbons (NMHCs), such as iso-

prene 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 un-

derstanding of biogenic emissions and chemistry (Kaiser et

al., 2015; Wolfe et al., 2016).

Published by Copernicus Publications on behalf of the European Geosciences Union.

4562

J. M. St. Clair et al.: Potential HCHO measurement artifact from ISOPOOH

Measurements

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 un-

derstand biogenic oxidative environments (e.g., Karl et

al., 2007). The instrumental techniques used to measure

MVK and MACR, e.g., proton transfer reaction-mass spec-

trometry (PTR-MS) and gas chromatography (GC), have

recently been shown to convert the major first-generation

-dominated isoprene oxidation product, isoprene hy-

droxyhydroperoxides (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 instru-

ments. 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 sam-

ple 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 fluo-

ropolymer (PFA tubing) or coated in fluoropolymer (Fluoro-

Pel, Cytonix). Results for the Harvard formaldehyde instru-

ment (DiGangi et al., 2011; Hottle et al., 2009) will be pub-

lished 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 investi-

gated. A slow HCHO time constant, unique to the ISOPOOH

conversion experiments and caused by the experimental con-

ditions, is discussed. Finally, the experimental results for

ISOPOOH conversion to HCHO and the laboratory time con-

stant are discussed in the context of data from the Studies of

Emissions and Atmospheric Composition, Clouds and Cli-

mate Coupling by Regional Surveys (SEAC

RS) campaign

(Toon et al., 2016).

OOH

(1,2)

(4,3)

MVK

MACR

urface

+HCHO

Figure 1.

The major isomers of ISOPOOH and the observed hy-

drocarbon products of their instrument surface-mediated decom-

position: methacrolein (MACR), methyl vinyl ketone (MVK), and

formaldehyde (HCHO).

Table 1.

List of experiments.

Experiment

Compound

Description

(1,2)-ISOPOOH

Dry

(1,2)-ISOPOOH

Dry, with and with-

out untreated stain-

less tubing

(1,2)-ISOPOOH

Dry, ISAF inlet tu-

bing temperature

(4,3)-ISOPOOH

Dry, with untreated

stainless tubing

(4,3)-ISOPOOH

Dry

(4,3)-ISOPOOH

Dry, ISAF inlet tu-

bing temperature

(1,2)-ISOPOOH

Humid

(4,3)-ISOPOOH

Humid

(4,3)-ISOPOOH

Dry

E10

(1,2)-ISOPOOH

Dry

2 Experimental

In total, 10 experiments were conducted and are listed

in Table 1. Experiments were performed for both ma-

jor 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 nuclear magnetic resonance spectroscopy,

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 conver-

sion experiments and one humid air conversion experiment

were performed. The effect of temperature and tubing mate-

rial 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

fluori-

nated ethylene propylene chamber. ISOPOOH was intro-

duced into the chamber by flowing 20 standard liters per

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J. M. St. Clair et al.: Potential HCHO measurement artifact from ISOPOOH

4563

minute (sL min

−

) of dry air over droplets of ISOPOOH for

10–30 min, and the remaining chamber volume was filled

with dry air. The sampling setup is shown in Fig. 2. A sample

line (6.35 mm OD PFA) connects the chamber to the instru-

ments, with the sample flow determined by the sum of the

individual instrument sample flows (

∼

10 sL min

−

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 (Ca-

zorla et al., 2015) measured HCHO.

The ISAF flight inlet (Cazorla et al., 2015) was repre-

sented 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 fluo-

ropolymer (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 (Hon-

eywell 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 con-

troller (model CT325) provided variable and stable control

temperatures, and a thermistor on the aluminum piece pro-

vided the temperature. In flight, flow rates through the DC-

8 inlet are 10–25 sL min

−

, with

∼

2.5 sL min

−

of the total

inlet flow subsampled by the instrument. During these exper-

iments, the large inlet flow was not possible due to limited

chamber volume. All of the inlet flow passed through the in-

strument and ranged from 3 to 7 sL min

−

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 exper-

iments and sample tubing flushing time. For humid experi-

ments, the dry background was followed by a humid back-

ground 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 condi-

tions than dry conditions by

∼

40–100 pptv. Any background

HCHO that originated from a source other than the dry or hu-

mid air (e.g., from the ISOPOOH sample) was not accounted

for and would bias the conversion rate high. After obtain-

ing 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 con-

version 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

TQ-

CIMS

oF-

CIMS

ISAF

HCHO

~1 m

chamber

6.35 mm OD PF

A sample line

contr

oller

Figure 2.

Experimental setup. During experiments with untreated

stainless tubing, the sample line passed through the tubing (item A)

before proceeding to the instruments. The ISAF inlet tubing loca-

tion is noted by item B.

the potential for heated, untreated stainless steel to convert

ISOPOOH to HCHO, another set of experiments was con-

ducted 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 in-line 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 ex-

periment contain a rapid increase followed by a multi-

exponential decay. The ISOPOOH data exhibit a comple-

mentary 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 cor-

rection 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 as follows.

Function

exp

(

−

t/τ

)

exp

(

−

t/τ

)

exp

(

−

t/τ

)

(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 nonphysical value for another. The

data were then fit using the selected three time constants

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

to describe all of the experiments. The final fit was con-

ducted with the ratio of the first three pre-exponential terms

fixed relative to each other:

and

0.25

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J. M. St. Clair et al.: Potential HCHO measurement artifact from ISOPOOH

Table 2.

Conversion Fraction of ISOPOOH to HCHO in ISAF.

Room temperature

Conversion

Estimated

measurements

fraction

uncertainty*

(1,2)-ISOPOOH, dry

0.03

0.04

(4,3)-ISOPOOH, dry

0.06

0.03

(1,2)-ISOPOOH, humid

0.06

0.02

(4,3)-ISOPOOH, humid

0.10

0.04

* The uncertainty was conservatively estimated using data without

applying the pre-instrument conversion correction. The ratio of the HCHO

data/ISOPOOH data, both background-subtracted, was plotted for each

experiment. The highest value of the ratio, taken as a mean for each data

section selected for the conversion fits (red dots in Fig. 3, top panel), was

determined for each experiment and was averaged if there were two of that

experiment type. For the dry experiments, the uncertainty was set to the

difference between the highest value and the fit value. For the humid

experiments, the uncertainty was taken as twice the difference.

Two variables were optimized:

and

. The goal of the

preceding steps was to obtain a three-term exponential func-

tion that would reasonably describe the data from multiple

experiments by fitting a single scaling factor (representing

[ISOPOOH] at

0) and a constant (representing a back-

ground). The ISOPOOH and HCHO used hereafter have the

decay fit added and subtracted, respectively, from the origi-

nal data. Figure S1 in the Supplement shows the effect of the

correction on experiment E7 data as an example. Two exper-

iments were not corrected for pre-instrument conversion: E5

and E10. Experiment E5 did not require this correction be-

cause 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 were close to stable before

the ISOPOOH concentration was changed, and the correc-

tion 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 regres-

sion 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 % con-

version 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 conver-

sion experiments followed a humid experiment, and the pro-

longed exposure of surfaces to water vapor may have affected

the subsequent conversion efficiency despite the sample line

being purged with dry air for

∼

30 min in between experi-

UTC

HCHO (ppbv)

0.2

0.4

0.6

0.8

All data

Selected for fit

Background

01:16

01:36

02:00

02:24

02:48

(1,2)-ISOPOOH (ppbv)

HCHO (ppbv)

0.2

0.4

0.6

0.8

E10

E1 fit: 0.024 x [ISOPOOH] +0.02

E10 fit: 0.031 x [ISOPOOH] +0.15

Combined fit: 0.028 x [ISOPOOH] +0.08

Figure 3.

Top panel: time series of dry air conversion experiment

E10 with (1,2)-ISOPOOH at three ISOPOOH mixing ratios. Bot-

tom panel: linear fits to experiments E1 (dashed blue line) and E10

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

ments. 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 CIT-

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 sat-

urated 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 frac-

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J. M. St. Clair et al.: Potential HCHO measurement artifact from ISOPOOH

4565

Temperature (°C)

Conversion fraction

0.2

0.4

0.6

0.8

16 ppb (1,2) ISOPOOH, bare stainless

28 ppb (4,3) ISOPOOH, bare stainless

16 ppb (1,2) ISOPOOH, no bare stainless

18 ppb (1,2) ISOPOOH, heated ISAF inlet

15 ppb (4,3) ISOPOOH, heated ISAF inlet

100

120

140

160

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.

tion is higher for the humid experiments than the dry experi-

ments. Using the lower [ISOPOOH] data where the relation-

ship 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 ex-

periments 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 tub-

ing was heated to much higher temperatures to demonstrate

the potential for ISOPOOH conversion on heated, untreated

metal surfaces. The conversion fraction for a given temper-

ature was also found to be a function of the gas-tubing con-

tact time, determined by the length of exposed tubing and

the sample flow rate, and should not be taken as quantita-

tively 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

◦

UTC

xing ratio (ppbv)

% of HCHO

from conversion

titude (km)

17:00

19:00

21:00

23:00

UTC

ISAF HCHO

ISOPOOH

HCHO from ISOPOOH

17:00

19:00

21:00

23:00

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).

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 experi-

ments is considerably longer, though the time constants in

the correction term described in Sect. 3.1 removed some of

the equilibration time. During the conversion experiments,

the ISAF instrument was exposed to ISOPOOH mixing ra-

tios 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 in-

strument 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 Composi-

tion, 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. Fig-

ure 5 shows data from the SEAC

RS flight (6 Septem-

ber 2013) with some of the highest ISOPOOH mixing ra-

tios of the campaign. In the top panel, ISAF HCHO and

CIT ISOPOOH data are plotted as a time series along with

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J. M. St. Clair et al.: Potential HCHO measurement artifact from ISOPOOH

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 com-

parison, the stated calibration accuracy of ISAF is

10 %.

The conversion percent was set to zero for ISOPOOH values

≤

80 pptv to improve figure clarity.

While prompt conversion of ISOPOOH to HCHO was

shown to have a negligible impact on the HCHO measure-

ment, a transition from a high ISOPOOH, high HCHO, sam-

pling 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 con-

version continues after leaving the boundary layer. To il-

lustrate the potential effects of delayed ISOPOOH conver-

sion to HCHO on field data, a section of the SEAC

RS 6

September 2013 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 pan-

els).

The instrument response to a rapid decrease in ISOPOOH

and HCHO mixing ratios was modeled using the end of Ex-

periment 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 de-

cay started at a time of 20:34:55 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 bot-

tom panel of Fig. 6 shows the percentage of observed HCHO

that would be from ISOPOOH conversion under the mod-

eled 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 de-

layed 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 h, resulting in an integrated

ISOPOOH exposure that greatly exceeds in situ sampling

UTC

Mix

ing ratio (pptv)

200

400

UTC

% of HCHO

from conversion

itude (km)

ISAF HCHO

CIT ISOPOOH

hort-term contamination

ong-term contamination

20:30

21:00

21:30

22:00

20:30

21:00

21:30

22:00

Figure 6.

Hypothesized HCHO signal from delayed ISOPOOH

conversion, shown for part of SEAC

RS flight on 6 Septem-

ber 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).

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.,

2016), and more rapid reaction of (4,3)-ISOPOOH with OH

(St. Clair et al., 2016). Consequently, the laboratory experi-

ments 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 hu-

mid 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 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 back-

ground 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 ar-

tifact 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 ex-

posure to ISOPOOH. The delayed conversion would mani-

fest for upper tropospheric measurements that follow closely

Atmos. Meas. Tech., 9, 4561–4568, 2016

www.atmos-meas-tech.net/9/4561/2016/

J. M. St. Clair et al.: Potential HCHO measurement artifact from ISOPOOH

4567

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 substan-

tial 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 un-

treated stainless steel tubing suggests attributes that should

be avoided in instrument and inlet design to minimize con-

version 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 per-

oxides such as ISOPOOH. The most likely site for conver-

sion in ISAF is the MKS pressure controller upstream of the

detection cell. As a step toward eliminating the small con-

version in ISAF, this component will be replaced with a ver-

sion that has smaller unflushed volumes and fluoropolymer-

coated surfaces.

Future work will extend the testing of instrumentation to-

wards interferences from additional reactive peroxides that

can be abundant in the boundary layer, e.g., hydroxymethyl

hydrogen peroxide.

5 Data availability

SEAC

RS campaign data (Hanisco, 2014; Wennberg, 2014)

used in this paper can be obtained from http://www-air.larc.

nasa.gov/cgi-bin/ArcView/seac4rs.

The Supplement related to this article is available online

at doi:10.5194/amt-9-4561-2016-supplement.

Acknowledgements.

Keutsch

and

Rivera-

Rios were supported by the National Science Foundation

(AGS 1628491, 1628530, 1247421, 1321987). J. M. St. Clair,

G. M. Wolfe, and T. F. Hanisco were supported by NASA

(NNH12ZDA001N-UACO). M. J. Kim was supported by the

National Science Foundation (AGS PRF 1524860).

Edited by: A. Hofzumahaus

Reviewed by: two anonymous referees

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