Removing Water Vapor Interference in Peroxy Radical Chemical

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Removing water vapor interference in peroxy radical chemical amplification with a large diameter Nafion® dryer Chengqiang Yang, Weixiong Zhao, Bo Fang, Xuezhe Xu, Yang Zhang, Yan-Bo Gai, Wei-Jun Zhang, Dean S Venables, and Weidong Chen Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b04830 • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 14, 2018

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Analytical Chemistry

Removing

water

vapor

interference

in

peroxy

radical

chemical

amplification with a large diameter Nafion® dryer Chengqiang Yang,†,‡ Weixiong Zhao,*,† Bo Fang,† Xuezhe Xu,† Yang Zhang,† Yanbo Gai,† Weijun Zhang,*,†,‡ Dean S. Venables,§ Weidong Chen∥ † Laboratory of Atmospheric Physico-Chemistry, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei, 230031, Anhui, China ‡ School of Environmental Science and Optoelectronic Technology, University of Science and Technology of China, Hefei, 230026, Anhui, China § Department of Chemistry and Environmental Research Institute, University College Cork, Cork, Ireland ∥ Laboratoire de Physicochimie de l'Atmosphère, Université du Littoral Côte d'Opale, 59140 Dunkerque, France

Corresponding Author * Tel.: +86-551-65591961. Fax: +86-551-65591560 E-mail: [email protected], [email protected]

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Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: The chemical amplification (PERCA) method has been widely used for measuring peroxy radicals concentrations in the troposphere. The accuracy and sensitivity of the method is critically dependent on the chain length (CL) – that is, the number of radical amplification cycles. However, CL decreases strongly with higher relative humidity (RH). So far, there does not appear to be a method to overcome this impact. Here we report the development of a Nafion® dryer based dual-channel PERCA instrument. The large diameter Nafion® dryer efficiently removes water vapor in milliseconds and minimally affects the sample. The low losses of peroxy radicals on the NafionTM membrane make it an attractive tool for raising the CL, and thereby the measurement accuracy and sensitivity of PERCA systems. The reported instrument demonstrates this promising and simple method to minimize water vapor interference.

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INTRODUCTION Peroxy radicals (HO2 and RO2) play significant roles in the degradation mechanism of volatile organic compounds (VOCs) and the production of ozone (O3) and secondary organic aerosols (SOA).1-5 The PERCA (PEroxy Radical Chemical Amplification) technique is widely used for peroxy radical measurement in atmospheric chemistry field studies, in which peroxy radical concentrations are determined by measuring the concentration of NO2 formed by the reaction of NO with peroxy radicals in a flow reactor:

HO2 + NO → OH + NO2

(1)

RO2 + NO → RO + NO2

(2)

Chemical amplification of peroxy radicals occurs through chain reactions sustained by a mixture of CO and O2: 6-9 M OH + CO + O2  → CO2 + HO2

RO + O2 → HO2 + organic products

(3) (4)

PERCA is an indirect method. A critical parameter for the accuracy and sensitivity of PERCA instruments is the chain length (CL), that is, the number of radical amplification cycles before being lost. The concentration of total peroxy radical RO2* (RO2* = HO2 + ΣRO2) can then be obtained by dividing the measured (amplified) NO2 concentration by CL. For an accurate measurement of RO2*, contributions from all sources of uncertainty should be well characterized.8,9 These uncertainties include the interference of background pollutants, radical partitioning, NO2 measurement, CL determination, and the influence of water vapor on CL. Some of these issues have been effectively dealt with in the literature. Dual-channel PERCA instruments effectively resolve problems arising from rapid fluctuations in ambient pollutants like NO2 and O3;8-11 the accuracy in the NO2 measurement can be improved by replacing luminol chemiluminescence detectors with optical detectors;9,12-16 and denuding methods have been developed for the selective measurement of atmospheric HO2 and RO2.17,18 So far, however, there does not appear to be a method to overcome the adverse effect of ambient water vapor on the CL.19 Irreconcilable differences between two PERCA instruments in the airborne measurements during the AMMA (African 3



Monsoon Multidisciplinary Analysis) radical intercomparison exemplify the strong and inconsistent dependence upon relative humidity (RH) in PERCA measurements.6 CL is dependent upon the competition between chain propagating and terminating reactions, but is also influenced by the physical configuration and losses in the reactor and the experimental conditions.9 Among the chain terminating reactions, the wall losses of radicals and water contributed gas phase chemistry are most important; radical-radical reactions are negligible; and due to its relative small branching ratio, the nitrate forming channels of organic radicals generally make a minor contribution to termination processes.6,13,20,21 Significant deterioration in CL with increasing RH has been observed. 19-21 The value of CL at 40% RH is almost half of its value in dry air and decreases further at higher RH.8 However, accurate determination of the water sensitivity of CL for a particular instrument is a challenging task due to the complex influence of water vapor on the gas phase chemistry (including the influence of water vapor on reactions of HO2 with CO and NO, and on OH with CO).19 Very recently, Wood et al.22 reported a new PERCA instrument using C2H6 (instead of CO) for atmospheric peroxy radicals measurement by chemical amplification. Compared to CO based amplification, the reported system had a reduced water influence on CL (about 32% reduction at RH = 50% compared to dry conditions), but the CL was also about 7 times smaller (CL = 17 at a RH of 50%). Retaining high amplification while reducing the influence of water vapor on the CL is therefore still a desirable objective in PERCA instruments. In this work, we present a new Nafion® dryer based dual-channel PERCA system for accurate and sensitive measurement of ambient peroxy radicals. NafionTM is a copolymer that selectively removes water and is highly resistant to chemical attack.23 Unlike traditional micro-porous membrane tubing, water vapor in the sample can be removed by water-of-hydration absorption in milliseconds while only modestly affecting the sample. CO is used as the amplification gas to achieve a high CL for the instrument. A smaller, more linear water dependence was observed at inlet sample RH < 50%. The CL value at 48% inlet sample RH was ~ 81, a modest reduction of about 12% compared to dry conditions. We show that the Nafion® dryer based system shows promise as a simple and universal method to 4


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minimize the impact of water vapor on CL and improve the sensitivity and precision of peroxy radical measurements.

EXPERIMENTAL SECTION Figure 1 shows a schematic of the sample inlet of the Nafion® dryer based dual-channel PERCA instrument developed in the present work. The instrument works at atmospheric pressure and room temperature (~ 26 °C). A MD-700 type large diameter dryer (Perma Pure) is used to dry the sample. The dryer assembly is 70 cm long and encloses a 60 cm long, 18 mm inner diameter NafionTM tube. The total volume of the dryer is about 160 cm3. Sample inlet

Purge air exhaust 60 cm

Nafion® dryer

70 cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Dry purge air

Pre-reactor chamber Addition NO + CO point 1

NO + N2

Bypass flow

Reactors

Reactor 1 Amplification model

Addition point 2

N2

Reactor 2 Background model

CO

to NO2 detectors

Figure 1. Schematic diagram of the inlet of the Nafion® dryer based dual-channel PERCA.

The tubing was purged with dry zero air (6% RH) to remove water vapor from the sample. The dried sample was drawn into a conical pre-reactor chamber (volume ~ 430 cm3) made of aluminum and coated with halocarbon wax (Series 1500, Halocarbon Products Corporation). A bypass at the bottom of the chamber controlled the total flow rate at the sample inlet. The chamber output was split into two parallel reactors and then drawn to the NO2 detectors. Reactor 1 operated in amplification mode and reactor 2 operated in background mode. Both reactors consisted of a 32 cm long FEP (Fluorinated Ethylene 5



Propylene) tube with an inner diameter of 19 mm. The flow rate of each reactor was 1.5 L/min (SLPM, standard liter per minute), giving a residence time of ~ 3.6 s in each reactor. Operation of the developed PERCA instrument is similar to our previously described system.9 The reagent gases NO (100 ppmv in N2, 100 ml/min) and CO (99.995%, 100 ml/min) were added to reactor 1 at addition point 1 (Fig. 1) to amplify peroxy radicals. For reactor 2, CO was replaced with N2 to account for background NO2. A 0.45 µm Teflon particle filter was placed at the end of each reactor to remove the peroxy radicals and terminate the chain reaction. Below the reactors (addition point 2, Fig. 1), 100 ml/min CO was injected into reactor 2 to remove radicals remaining after the filter; 100 ml/min N2 was injected into reactor 1 to maintain the same flow conditions in the two reactors. The concentrations of NO2 at each outlet of the reactors were measured with identical BBCES (broadband cavity enhanced spectroscopy) detectors. Compared with our earlier PERCA-BBCES system,9 the NO2 detectors used in the current Nafion® dryer based instrument have been improved by using a custom instrumental cage system and a Kalman adaptive filter to retrieve concentrations.24 A mixing ratio precision of 40 pptv (1σ) in 21 s total acquisition time was achieved. The concentration of peroxy radicals was calculated by taking the difference in the measured NO2 concentrations from the two reactors (∆NO2) and dividing this value by CL. Since HO2 is more sensitive than RO2 to water vapor,7,18 we evaluated the performance of the Nafion® dryer based PERCA system for HO2 measurement. Thermal decomposition of H2O2 vapor up to 600 °C was used as a stable HO2 calibration source as in our previous work.9 The produced HO2 concentrations ranged from 1 to 7 ppbv. The RH of the sample was controlled between 3% to 95% using a Perma Pure FC125-240 series gas humidifier.

RESULTS AND DISCUSSION Several key issues must be addressed before applying the Nafion® dryer to PERCA measurements. (1) Losses of reactive radicals on the Nafion membrane: Wall losses on the membrane should be at acceptably low levels and (ideally) should not be influenced by the RH. (2) Moisture removal efficiency of the dryer: The effect of water vapor on CL is minimal at 6


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low RH (about 20-30%).9 The sample should be dried to a sufficiently low RH to remove most interferences from water vapor. (3) Maximizing CL: For ambient applications, CL should be as large as possible in order to achieve high-sensitivity measurements. These factors are evaluated and discussed below.

1. Performance of the dryer at different flow rates The moisture removal efficiency of the dryer was tested across a range of inlet flow rates and relative humidities. The performance curves are shown in Fig. 2. The performance test curve at a common flow rate of 5 SLPM provided by the manufacturer is also shown in the figure. The slopes of the measured curve were similar (ranging from 0.47 to 0.50) for the different flow rates (5, 6, 8, 10 SLPM), but the intercepts increased with the inlet flow rates. The manufacturer’s recommended inlet flow rate for this dryer is 3-8 SLPM. A flow rate of 5 SLPM was chosen for the performance evaluation in this work as a good balance between radical wall losses and moisture removal efficiency. At this flow rate, the sample can be dried to 47% and 35% RH at the outlet starting from initial inlet RHs of 95% and 72%, respectively.

55

5 SLPM (y = 1.509 + 0.474 x) 6 SLPM (y = 4.612 + 0.481 x) 8 SLPM (y = 6.618 + 0.498 x) 10 SLPM (y = 8.525 + 0.479 x) 5 SLPM (Manufacturer's data)

50 45

% RH at outlet

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40 35 30 25 20 15 10 20

30

40

50

60

70

80

90

100

% RH at inlet

Figure 2. Moisture removal performance of the MD-700 Nafion® dryer at different inlet flow rates and humidities. The manufacturer’s performance curve at 5 SLPM is shown for comparison.

7



2. HO2 loss rate coefficient due to the NafionTM membrane The time dependence of HO2 loss in the Nafion® dryer can be calculated from:8

[HO 2 ]t =[HO 2 ]0e- kloss t

(5)

where [HO2]t and [HO2]0 are respectively the HO2 mixing ratios at the inlet and the outlet of the dryer, kloss is the first order wall loss coefficient of HO2 and is governed by the reactions on the wall of the tubing and not by mass transfer,20 and t is the residence time of the sample in the Nafion® dryer. The HO2 mixing ratio was measured by adding a small quantity of NO (100 ppmv in N2, 100 ml/min) to the sample to form NO2, which was measured by the dual-channel BBCES NO2 detectors. The HO2 mixing ratio of the calibration source in this experiment was ~ 6 ppbv at a flow rate of 3 SLPM. A bypass dry zero air was introduced to change the flow rate to vary the sample residence time. The self-reaction of HO2 is negligible because it is over 3 orders of magnitude slower than the HO2 + NO reaction under our instrumentation conditions. No additional radical losses were observed upon dilution.19,20 Flow rate (SLPM) 24.00

12.00

8.00

6.00

4.80

1.20

1.60

2.00

1.00 0.95

0.90

Relative HO2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.85

0.80

y = e- 0.158 (± 0.002) x

0.75

0.70 0.40

0.80

Residence time (s)

Figure 3. Measurement of wall losses of HO2 in the Nafion® dryer. Relative HO2 is the ratio of HO2 concentrations measured at the outlet and at the inlet of the dryer. The RH of the samples was kept at 5%. Changes in residence time were achieved by changing the inlet flow rate. The corresponding flow rates are shown on the top axis.

Figure 3 shows the time dependence of wall losses of HO2 in the MD-700 Nafion® dryer 8


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at 26 °C and 5% RH. About 26% HO2 was lost to the dryer with 5 SLPM inlet flow rate (corresponding to a residence time of ~1.92 s). By increasing the flow rate to 21 SLPM (~ 0.95 s residence time), the HO2 loss decreased to 5%. Negligible changes in wall losses were observed across a wide range of RH (supporting information Figure S1). An exponential (first order) fit was used to determine kloss.25,26 In this experiment, kloss in 18 mm diameter Nafion tubing was determined to be 0.158 ± 0.002 s-1. This rate is comparable to the NO3 radical loss rate in a 1/2 in. PFA (Perfluoroalkoxy) tube (0.2 s-1),25 but significantly smaller than the HO2 loss rate in a 1/4 in. PFA tube as reported by Mihele et al. (2.8 s-1 for HO2 and 0.8 s-1 for CH3O2 and C2H5O2).20 The sticking coefficient (γ) of HO2 on the wall of the NafionTM membrane can be determined from the observed kloss by:20,26

γ=

2 Rkloss c

(6)

where R is the radius of the Nafion tube (0.9 cm), and c is the average molecular speed of HO2 (440 m s-1) derived from:27

c=

8RgT

(7)

πM

where Rg, T, and M are the gas constant, temperature, and molar mass of the molecule, respectively. γ was determined to be 6.5×10-6 for the NafionTM membrane, which is about 5 times smaller than that in a PFA tube (3.1×10-5) and about 50 times smaller than that in glass (3×10-4),20 suggesting that Nafion is well suited for use in PERCA instruments. According to Eq. (6), kloss is proportional to the sticking coefficient and inversely proportional to the tube diameter. The sticking coefficient is determined by the surface physics of a material and depends on tubing surface structure, temperature, and the kinetic energy of the adsorbing molecules.28 Previously, a small diameter Nafion tube (2.2 mm external diameter) was used for selective removal of HO2 radicals.18 This tube diameter was more than 8 times smaller than ours and may have led to different sticking coefficient values and large wall losses for the HO2 radical. The reported loss ratio in Ref. 18 was about 94.6% for HO2 after passing through a 35 cm long tube. In contrast, HO2 wall losses were small over 9



our 60 cm long, 18 mm inner diameter Nafion® tube.

3. Sensitivity of sample RH on CL determination A calibration of CL is shown in Fig. 4. The flow rates of NO and CO were set at 100 ml/min. The corresponding NO and CO mixing ratios in the flow reactors were 5.9 ppmv and 5.9%, respectively. The HO2 mixing ratio determined from ((S2’-S1’)-(S2-S1)) was about 1.2 ppbv with a CL value of 92.5. This CL value is comparable with other PERCA systems.13,15 With improved NO2 detectors, the difference in S2’ and S2 is readily apparent and the uncertainty in CL was reduced to 4%. This is a 2 to 3 fold reduction in measurement uncertainty compared to some other PERCA systems.8,9 The measurement precision (1σ, 21s) was about 0.4 pptv for the HO2 radical.

NO2 mixing ratio (ppbv)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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140

Reaction Channel (with HO2 in the sample)

120

Reference Channel (without HO2 in the sample)

S3' = 131.87 ppbv

[HO2] = 1.2 ppbv

CL = 92.5

100

Zero air (without NO and CO in both reactors)

With NO and CO in both reactors

With NO only in both reactors

80 60 40 20

S2' = 13.28 ppbv

0

S2 = 12.08 ppbv 0

2

S3 = 20.75 ppbv

S1' = -0.007 ppbv S1 = -0.005 ppbv 4

6

8

10

12

14

Time (min)

Figure 4. Example calibration of CL at 5% RH. S1 and S1’ are the zero air measurements of the reference and reaction channels and are treated as baseline offsets for the two NO2 detectors. S2 and S3 correspond to background NO2 impurities in the NO reagent and in the sample in the reference channel. In the reaction channel, S2’ corresponds to the background NO2 impurity and NO2 formed by reaction (1) without chemical amplification, while S3’ includes the effect of peroxy radical amplification via reaction (3) by adding CO.

Figure 5 shows the RH effect on the reactor’s CL. When the Nafion® dryer was not used, our results are similar to those reported for other instruments.9,13,14 In particular, the chain length was sensitive to the sample inlet RH and fell off quickly between 20% and 40% RH. 10


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Moreover, CL was strongly reduced at moderate to high RH. For instance, at 48% inlet RH, CLwet was a third of its value under dry conditions (5% RH), while at 69% inlet RH, the CL was reduced by a factor of 4. The Nafion® dryer muted the influence of humidity on the CL. At 48% inlet RH, the value of CLwet (48%RH)/CLdry (5%RH) was ~ 0.88, which was about 3 times larger than that observed without using Nafion® dryer. A linear relationship between CLwet/CLdry and inlet RH was observed for inlet RH < 50%. The CL decrement shows a weak linear dependence on the water vapor and can be reliably corrected by using this linear function. Although not part of our experimental measurements, our data suggest that it would be possible to couple two Nafion® dryers in series and establish a linear relationship between CLwet/CLdry and inlet RH up to ca. 95% RH. The transmission of HO2 radicals in such a system would be expected to fall to 54%, but this reduction in efficiency would be an acceptable tradeoff because such a system would largely remove water vapor interference under most ambient conditions. The predicted CLwet/CLdry ratio for a two stage Nafion dryer is shown in the supporting information Figure S2. RH of the dried sample (%) 2

6

11

16

20

25

30

35

CLwet/CLdry = 1.0245 - 0.0029 × RHinlet

1.0

0.8

0.6 CLwet/CLdry = 1.0357 - 0.0063 × RHoutlet

1.0

CLwet / CLdry

CLwet / CLdry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.4

0.8 0.6 0.4

Without Nafion dryer With Nafion dryer Linear fit

0.2 0.2 0

10

0.0 0

10

20

30 40 50 RHoutlet (%)

20

60

Without Nafion dryer With Nafion dryer Linear fit of Nafion dryer result

70

30

40

50

60

70

RHinlet (%)

Figure 5. Influence of inlet RH on the ratio of CLwet/CLdry with and without dryer. The corresponding RH of the dried sample is shown on the top axis. The linear fit of the data below 50% inlet RH is also shown; the data point at RHinlet = 69% was not included in the fit. The insert shows the plot of the CL ratios as a function of the expected RH at the outlet of the dryer (see Fig. 2). The linear fit of the data below 25% RHoutlet is shown in blue. The agreement indicates that humidity remains the primary driver of the change in CL and that there are no obvious other losses associated with the dryer. 11



4. Ambient application of the improved PERCA To demonstrate instrument performance under ambient conditions, the improved PERCA system was deployed in a field campaign in November and December 2017 at the Guangzhou Institute of Geochemistry, Chinese Academy of Science (GIG, CAS). The observation site is located in an urban environment in the Pearl River Delta (PRD) region in Southern China. It is surrounded by residential buildings and is next to an Expressway. The instrument was installed in a temperature-controlled container on the roof of the office building with the sample inlet about 35 m above ground. Figure 6 shows the results of five-days of RO*2 measurements. Ambient RH was not measured during this experiment; only the RH of the dried sample in the pre-reactor chamber was measured to apply the humidity correction of the CL. The RH of the dried sample ranged from 11-25% and the corresponding values of CLwet/CLdry (from the linear fit of RHoutlet in Fig. 5) varied from 0.97 to 0.88. The wall loss-corrected RO2* concentrations with and without the application of humidity correction are shown in Fig. 6. By using a Nafion dryer, water interferences were effectively reduced. The concentration of RO2* is related to the local concentrations of NO and VOC precursors.6 Radical chemistry analysis and relationship between RO2* with other pollutants are not part of this work, but the diurnal variation of RO2* radicals we observed is similar to previously reported HO2 results in PRD region.29,30 The maximum peroxy radical concentration in this work was about 390 pptv, in agreement with the maximum value measured at Guangzhou Back Garden site (334 pptv, from figure 3 in Ref. 5, located in a rural environment). Our observations indicate that the improved PERCA system provides credible results in humid ambient air.

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0.40

25

RH(%) RO2* RO2* (humidity correction)

0.30 15 0.25 10

RHoutlet (%)

20

0.35

RO2* (ppbv)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.20 0.15

5

0.10 0 :00 00

:00 12

:00 00

:00 12

:00 00

:00 12

:00 00

:00 12

:00 00

:00 12

/26 /25 /25 /24 /24 /23 /23 /22 /22 /21 /21 /12 /12 /12 /12 /12 /12 /12 /12 /12 /12 /12 17 17 17 17 17 17 17 17 17 17 17 0 0 0 0 0 0 0 0 0 0 0 2 2 2 2 2 2 2 2 2 2 2

:00 00

Date & time (yyyy/mm/dd hh/mm)

Figure 6. Time series of the measured RH of the dried sample (blue line), peroxy radical concentrations with (red line), and without (black line) humidity corrections from 21 to 26 December 2017. 15 minimum average data are shown here. A strong diurnal variation is apparent.

CONCLUSIONS Our work reports the first application of a larger diameter Nafion® dryer to minimize the effect of water interference on accurate measurement of peroxy radicals. Our system performance was demonstrated using HO2 radical measurements, but the same results are expected to apply to RO2 radicals. The first order loss coefficient on Nafion was 0.158 s-1, which is ~ 5 times smaller than that for a 1/4 PFA tube that had previously been used as amplification reactor.9,13,14,19,20 This property makes Nafion tubes an attractive candidate for use as a reactor as it is expected to increase the chain length in chemical amplification and thereby improve the detection accuracy and sensitivity. With a 60 cm long dryer, the transmission of HO2 radicals was ~ 0.738 at a sample flow rate of 5 SLPM and was independent of sample RH. The sample can be dried to a RH value more than two times lower than the inlet RH with the Nafion® dryer. A weak linear 13



relationship between CLwet/CLdry and inlet RH at sample RH of < 50% was observed. This effect can be easily parameterized to reliably correct for the water effect on the CL in a simple configuration.

ASSOCIATED CONTENT Supporting Information Wall losses of HO2 in the Nafion® dryer at different sample RH and predicted behavior of the CLwet/CLdry ratio for a series Nafion dryer.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors wish to thank Dr. Shenzhen Zhou at Sun Yat-sen university and Dr. Wei Song at GIG, CAS for their support during the field measurement. This research is supported by the National Natural Science Foundation of China (41375127, 91544228), National Key Research and Development Program of China (2016YFC0202205), the Natural Science Foundation of Anhui Province (1508085J03), and the Youth Innovation Promotion Association CAS (2016383).

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15



For TOC only

1.0

CLwet / CLdry

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0.8 0.6 0.4

Without Nafion dryer With Nafion dryer

0.2 0

10

20

30

40

RH inlet (%)

16


50

60

70

Removing Water Vapor Interference in Peroxy Radical Chemical

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