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Nov 27, 2017 - Although others had significant NO injection (OFR-iNO), most such experiments were actually not under practically viable high-NO condit...
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Article Cite This: ACS Earth Space Chem. XXXX, XXX, XXX−XXX

Model Evaluation of New Techniques for Maintaining High-NO Conditions in Oxidation Flow Reactors for the Study of OH-Initiated Atmospheric Chemistry Zhe Peng,† Brett B. Palm,† Douglas A. Day,† Ranajit K. Talukdar,† Weiwei Hu,† Andrew T. Lambe,‡,§ William H. Brune,∥ and Jose L. Jimenez*,† †

Cooperative Institute for Research in Environmental Sciences and Department of Chemistry, University of Colorado, Boulder, Colorado 80309, United States ‡ Chemistry Department, Boston College, Chestnut Hill, Massachusetts 02467, United States § Aerodyne Research, Inc., Billerica, Massachusetts 01821, United States ∥ Department of Meteorology, Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *

ABSTRACT: Oxidation flow reactors (OFRs) efficiently produce OH radicals using low-pressure Hg-lamp emissions at λ = 254 nm (OFR254) or both λ = 185 and 254 nm (OFR185). OFRs under most conditions are limited to studying low-NO chemistry (where RO2 + HO2 dominates RO2 fate), even though substantial amounts of initial NO may be injected. This is due to very fast NO oxidation by high concentrations of OH, HO2, and O3. In this study, we model new techniques for maintaining high-NO conditions in OFRs, that is, continuous NO addition along the length of the reactor in OFR185 (OFR185-cNO), recently proposed injection of N2O at the entrance of the reactor in OFR254 (OFR254-iN2O), and an extension of that idea to OFR185 (OFR185-iN2O). For these techniques, we evaluate (1) fraction of conditions dominated by RO2 + NO while avoiding significant nontropospheric photolysis and (2) fraction of conditions where reactions of precursors with OH dominate over unwanted reactions with NO3. OFR185-iN2O is the most practical for general high-NO experiments because it represents the best compromise between experimental complexity and performance upon proper usage. Short lamp distances are recommended for OFR185-iN2O to ensure a relatively uniform radiation field. OFR185-iN2O with low O2 or using Hg lamps with higher 185 nm-to-254 nm ratio can improve performance. OFR185-iN2O experiments should generally be conducted at higher relative humidity, higher UV, lower concentration of non-NOy external OH reactants, and percent-level N2O. OFR185-cNO and OFR185-iN2O at optimal NO precursor injection rate (∼2 ppb/s) or concentration (∼3%) would have satisfactory performance in typical field studies where ambient air is oxidized. Exposure estimation equations are provided to aid experimental planning. This work enables improved high-NO OFR experimental design and interpretation. KEYWORDS: oxidation flow reactor, high-NO chemistry, kinetic modeling, nontropospheric organic photolysis, VOC oxidation by NO3, experimental design

1. INTRODUCTION Large environmental chambers have long been used for research in atmospheric chemistry.1−5 They allow the study of the reaction system of interest free from interferences due to uncontrollable meteorological condition changes and emissions in field studies and enable a focus on processes at the gas and particle level, for example, volatile organic compound (VOC) oxidation and secondary organic aerosol (SOA) formation.6−9 The former is closely related to the removal of air pollutants10 and the latter has impacts on human health11 and on climate via radiative effects of aerosols.12 Despite the wide applications of chambers, the photochemical age that chambers are able to reach is limited by the attainable OH concentration and residence time. The former (106−108 molecules cm−3) is generally not much higher than typical ambient OH concentrations (106−107 molecules cm−3);13,14 © XXXX American Chemical Society

the latter is intrinsically limited by time scales (typically minutes to hours) of the partitioning of gases and particles to Teflon chamber walls and the sample gas flow rate out of the chamber.1,15−17 Consequently, chambers are generally not suitable for simulating ambient processes occurring over multiple days of photochemical aging.4,18−21 Besides, the typically large volumes of chambers also lead to poor portability to the field. Oxidation flow reactors (OFRs) have emerged as an alternative to environmental chambers. They are typically smaller reactors (with a volume of liters) equipped with low-pressure Hg lamps. The UV emissions from those lamps at 254 nm Received: Revised: Accepted: Published: A

June 20, 2017 November 21, 2017 November 27, 2017 November 27, 2017 DOI: 10.1021/acsearthspacechem.7b00070 ACS Earth Space Chem. XXXX, XXX, XXX−XXX

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ACS Earth and Space Chemistry

manner and provided an experimental demonstration of the method. In this study, we investigate three new techniques that may maintain high-NO conditions, including the one proposed by Lambe et al.40 using the box model of Peng and Jimenez.39 We clarify when good high-NO conditions can be achieved by using these new techniques and also if they can lead to other experimental issues, for example, substantial VOC consumption by NO3, and/or RO2 suppression through RO2 + NO2 → RO2NO2.39 The results can help to extend the applicability of OFRs to high-NO conditions and provide insights into the design and interpretation of past and future OH-oxidation OFR experiments at high NO.

(OFR254) or both 185 and 254 nm (OFR185) lead to production of large amounts of OH radicals (equivalent to photochemical ages of hours to weeks) within a residence time of minutes.18,19,22−26 The strong capability of OH production and good portability are remarkable advantages of OFRs, particularly for field measurements, when high photochemical ages and/or experimental throughput are required. As a result of these advantages, OFRs have been increasingly used in field,25,27−29 source,24,30−32 and laboratory studies.22,33−35 As OFRs have been gaining popularity in experiments, modeling studies have characterized OFR radical chemistry. Li et al.36 and Peng et al.37 investigated the HOx radical chemistry in OFRs using box models, which predicted OH exposure (OHexp) and O3 concentration (abbreviated O3 hereafter) in good agreement with measurements. They showed that external OH reactivity [OHRext = ∑kici, i.e., the sum of the products of reaction rate constants of externally introduced species with OH (ki) and concentrations of those species (ci)] can cause OH suppression (decrease in OH concentration) in OFRs under many conditions, reaching several orders-ofmagnitude in some cases. Peng et al.38 identified OFR operation conditions where VOC consumption by non-OH/nontropospheric species could be important compared to that by OH. Simply, high OHRext and/or low water vapor mixing ratio (abbreviated H2O hereafter) favor non-OH pathways playing a role, because high OHRext suppresses OH and low H2O limits OH production but typically neither affects the important nonOH pathways. Among the non-OH pathways, 185 and 254 nm photons are particularly undesirable for VOC fate, as they are “non-tropospheric” (not existing in the troposphere) and so energetic that VOC photolysis at these wavelengths may lead to different products than in the troposphere. Peng et al.38 also proposed several approaches to avoid non-OH/nontropospheric chemistry. Almost all past OFR experimental studies using OH oxidation of VOCs operated under low-NO conditions where organic peroxy radicals, RO2, preferentially reacted with HO2 over NO. Many of them were conducted at no/low input NOx. Although others had significant NO injection (OFR-iNO), most such experiments were actually not under practically viable high-NO conditions. In general, NO and major oxidants (OH, HO2, and O3) are antagonistic and suppress each other at high levels. When NO is not sufficiently high (though it may still be higher than typical ambient levels), it is rapidly oxidized by large amounts of OH, HO2, and O3 in OFRs (e.g., the OFR experiments in the CalNex-Los Angeles campaign25 and in an urban tunnel).24 Thus, even in those polluted environments OFR chemistry proceeds mainly via RO2 + HO2 and not RO2 + NO. When NO is sufficiently high, it, together with other external OH reactants, can heavily suppress OH and create several types of experimental artifacts, including nontropospheric VOC photolysis, significant VOC oxidation by NO3, suppression of RO2 by very high NO2 (usually ppm level), and alteration of OH-aromatic adduct fate due to competition between O2 and NO2.39 Though not impossible, ensuring both the dominance of RO2 + NO over RO2 + HO2 and minor nontropospheric VOC photolysis (“good high-NO conditions”) in OFRs with initial NO injection was found to be feasible only in a very narrow range of conditions.39 This highlights the need for improved techniques to maintain highNO conditions in OFR. Lambe et al.40 recently proposed an injection of percent-level N2O in OFR254 to maintain high NO in a more controllable

2. METHODS The reference design of the OFR [Potential Aerosol Mass (PAM) flow reactor] and the chemical kinetics box model used in this study have previously been described in detail.19,36−39 Here we provide a brief summary, together with information that is either new or critical to the work discussed below. 2.1. Potential Aerosol Mass Flow Reactor. The PAM reactor has several different versions, the first of which was introduced by Kang et al.19 The physical reactor vessel simulated in this study, used by almost all PAM groups, is an ∼13 L cylinder. Two types of low-pressure Hg lamps have been used with the PAM reactor. Our default simulations will use the BHK lamps (model no. 82-9304-03, BHK Inc.) from the Penn State design, which has been characterized in more detail.36 A sensitivity study for the recently introduced Aerodyne lamps is discussed later, as insufficient information on their emission properties exists at present for a detailed characterization. Both types of lamps emit photons at 185 and 254 nm that generate OH. These reactors are currently used by a number of groups in the field of SOA research.41 In “OFR185 mode”, both 185 and 254 nm photons are used in OH production. The 185 nm lights photolyze water vapor and O2. The former photolysis produces OH and HO2 while the latter generates O(3P). O(3P) rapidly recombines with O2, producing O3. Then O(1D) is formed through O3 photolysis at 254 nm and reacts with water vapor, which produces additional OH. When Hg lamps are mounted inside quartz sleeves, 185 nm photons are filtered (verified by the lack of O3 production) and only 254 nm photons (“OFR254 mode”) lead to OH production, by photolysis of externally formed and injected O3. As Peng et al.37 found, the amount of injected O3 (O3,in) is critical to the HOx chemistry under some conditions, and this amount (X ppm) is also included in the notation for OFR254 mode (OFR254-X). In this study, we investigate new techniques where N-containing species are injected in several different manners with the goal of reaching significant and sustained NO levels. Thus, we utilize “OFR185-iZ” to denote OFR185 with initially injected N-containing species Z. “OFR185-cZ” is taken to denote a mode of operation in which reactant Z is added continuously through the reactor at a constant rate. The same nomenclature is used for the OFR254 mode. For example, OFR254-iN2O denotes the technique of initial N2O injection into OFR254 demonstrated by Lambe et al.40 in this study. 2.2. Model Description. In the present work, we use the same chemical kinetic model as in our previous study,39 which includes reactions of HOx and NOy species for studying OFR chemistry and added the relevant reactions involving N2O. The rate constants of these reactions are taken from the JPL Chemical Kinetic Data Evaluation.42 The current model assumes plug flow conditions, because nonplug-flow models B

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model under some approximations (see Section S1 of Peng and Jimenez39 for more details of the approximations). High-NO conditions are defined as those with r(RO2 + NO)/r(RO2 + HO2) > 1. Good/risky/bad conditions are classified according to F185exp/OHexp and F254exp/OHexp, where F185exp (F254exp) are 185 (254) nm photon flux exposure, that is, product of F185 (F254) and time. These exposure ratios quantify the relative importance of nontropospheric (at 185 and 254 nm) VOC photolysis to their reactions with OH (see Figures 1 and 2 of Peng et al.38 for more details about the determination of the relative importance of nontropospheric photolysis of individual VOCs). The definitions of all condition types are summarized in Table 2. In general, under “good conditions,” nontropospheric photolysis is minor for most VOCs; under “bad conditions,” even if precursors are not photolabile at 185 or 254 nm, significant nontropospheric VOC photolysis can hardly be avoided due to photolysis of oxidation intermediates. Under “risky conditions,” some species that strongly absorb at 185 and/or 254 nm (e.g., toluene) or react with OH slowly (e.g., pyruvic acid) may have relatively important nontropospheric photolysis, while for other species of opposite properties (e.g., alkanes and alkenes), nontropospheric photolysis is minor. To evaluate how good (with little nontropospheric chemistry occurring) the new high-NO maintaining techniques are, the entire space of physical conditions for all VOCs (or at least a large representative subset) needs to be taken into account. The fact that a specific VOC has minor nontropospheric photolysis under a certain condition gives no indication on how good an OFR operation mode is, as other VOCs (including the products of the original VOC) may have significant nontropospheric photolysis under this conditions, and/or this VOC may have significant nontropospheric photolysis under other conditions. The criteria for F185exp/OHexp and F254exp/OHexp ensure that most VOCs have minor nontropospheric photolysis. Thus, the fraction of good conditions is naturally a clear indicator of how good an operation mode is.

lead to limited changes under most conditions but much higher computational costs.37 SO2 is used as a surrogate for the reactivity of external OH reactants including CO and VOCs.37,38 This treatment is justified by the fact that total OHRext (including primary VOCs and oxidation intermediates can evolve significantly with OHexp43−46 and this variability cannot be accurately modeled even by mechanisms as explicit as Master Chemical Mechanism.44,46 We thus adopt the surrogate treatment for simplicity and efficiency, while representing a decay in OHRext that is slower than that of primary species to approximately account for later generation products. Note that in this work oxidized N-containing species (OxN ≡ N2O + NOy) are not surrogated but are treated explicitly in the model. Therefore, they are not counted in OHRext and OHRext only refers to non-OxN OHRext hereinafter, unless specifically stated otherwise. The model has been validated against field and laboratory OFR experiment results under both low- and high-NO conditions.36,37,40 Its outputs are within a factor of three of measurements in CalNex-LA,25 BEACHON-RoMBAS,28 and SOAS27 field campaigns, where ambient air was influenced by a variety of biogenic and anthropogenic gases.36 For laboratory experiments an even better model-measurement agreement can be achieved.36,37,40 As in the previous study,39 we assume typical temperature and atmospheric pressure in Boulder, CO, U.S.A. (295 K and 835 mbar) for all cases for the consistency with our previous modeling studies.36−39 Impacts of pressure on model outputs are minor.36 Residence time is always set to a typical value of 180 s. Model cases are explored over wide ranges of physical input conditions: water vapor mixing ratio (abbreviated H2O hereafter) of 0.07%−2.3%, that is, relative humidity of 2−71% at 295 K; 185 nm UV photon flux (abbreviated UV at 185 nm or F185) of 1.0 × 1011−1.0 × 1014 [corresponding 254 nm photon flux (UV at 254 nm or F254) of 4.2 × 1013−8.5 × 1015 photons cm−2 s−1]; OHRext of 1−1000 s−1; O3,in of 2.2−70 ppm for OFR254; operation mode-specific OxN injection amount/ rate. These cases are evenly spaced in a logarithmic scale in all dimensions except 254 nm UV, which is calculated from 185 nm UV according to the relationship reported by Li et al.36 Cases at OHRext = 0 are also explored. Two-character labels are utilized to denote several typical cases within the explored ranges (Table 1).

3. RESULTS AND DISCUSSIONS In this section, we investigate the OxN chemistry of the new techniques in this study, that is, OFR185-cNO, OFR254-iN2O, and OFR185-iN2O, and their performance on maintaining high NO while avoiding undesired reactions. We then propose operation guidelines for these new techniques based on our modeling results. 3.1. Techniques to Maintain High-NO Conditions. 3.1.1. OFR185-cNO. Peng and Jimenez39 pointed out that despite fast oxidation by OH, HO2, and O3, under good conditions (see definition in Section 2.2), NO that is simply initially injected into OFR185 (OFR185-iNO) still often has a sufficiently long presence (lifetime >10 s). In principle, an obvious improvement would thus be to implement several consecutive injections of NO along the reactor length to decrease the NO mixing time and enable the presence of significant NO concentrations over the entire OFR residence time. For modeling purposes, we will use an idealized version of this technique, that is, continuous addition of NO all along the flow (OFR185-cNO). This may be realized, for instance, by tubelike inlets along the flow with a large number of holes releasing NO with the hole size increasing along the reactor length (Figure S1a). We note that the implementation of “cNO” may alter the residence time distribution and the surface-to-volume ratio of the OFR, which may consequently influence the transmission of gas-phase species (e.g., Lambe et al.)26 and

Table 1. Code of the Labels of Typical Casesa water mixing ratio options

example

photon flux

L = low (1011 photons cm−2 s−1 at 185 nm; 4.2 × 1013 photons cm−2 s−1 at 254 nm) M = medium (1%) M = medium (1013 photons cm−2 s−1 at 185 nm; 1.4 × 1015 photons cm−2 s−1 at 254 nm) H = high (2.3%) H = high (1014 photons cm−2 s−1 at 185 nm; 8.5 × 1015 photons cm−2 s−1 at 254 nm) LM low water mixing ratio, medium photon flux L = low (0.07%)

a

A case label is composed of two characters denoting the water mixing ratio and the photon flux, respectively.

We adopt the same definitions of the OFR operating condition types (good/risky/bad, high/low-NO) as our OFR modeling study with high-NO input.39 High- and low-NO conditions are distinguished by a comparison between the total reactive flux of RO2 + NO [r(RO2 + NO)] over the whole residence time and that of RO2 + HO2 [r(RO2 + HO2)]. The ratio between the two reactive fluxes is calculated by the C

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Figure 1. Schematics of main N-containing species and their major interconversion pathways under typical input conditions for (a) OFR185-cNO, (b) OFR254-5-iN2O, and (c) OFR185-iN2O. Species average concentrations (in molecules cm−3) are shown in black beside species names. Arrows denote directions of the conversions. Average reaction fluxes (in units of 109 molecules cm−3 s−1) are calculated according to the production rate and shown on or beside the corresponding arrows and in the same color. Within each schematic, the thickness of the arrows is a measure of their corresponding species flux. Multiple arrows in the same color and pointing to the same species should be counted only once for reaction flux on a species. All concentrations and fluxes are average ones over the residence time and have two significant digits.

Figure 1a shows typical reactive fluxes for an OFR185-cNO case at typical H2O, UV, and OHRext, and with a continuous NO injection rate (rNOin) of 6.2 × 1010 molecules cm−3 s−1 (i.e., a 3 ppb/s increase in NO concentration in the absence

careful attention to mixing of the injected NO is needed. These potential issues should be considered in future realistic experiments employing a cNO method, but are not considered in this chemical modeling study. D

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Figure 2. Frequency occurrence distributions of good/risky/bad conditions over logarithm of r(RO2 + NO)/r(RO2 + HO2) for (a) OFR185-cNO, (b) OFR254-iN2O, and (c) OFR185-iN2O. Also shown are those for three variants of OFR185-iN2O, i.e., (d) N2Oin = 3%, (e) O2 = 0.2%, and (f) F185/F254 = 5%, and for (g) OFR185-cNO and (h) OFR185-iN2O cases with some ambient inputs (temperature, H2O, and OHRext) in the SOAS27 field campaign. Refer to Table 2 for the definitions of different condition types. See Section S1 for more details of the cases with ambient inputs.

of chemistry. For simplicity, rNOin will only be expressed in ppb/s hereinafter). The total amount of injected NO is of the same order as that recommended for OFR185-iNO.39 The OxN chemistry in Figure 1a is also similar to that in OFR185-iNO (see Figure 1a of Peng and Jimenez39) for a typical OFR185-iNO case). NO is mainly oxidized to NO2 by HO2 and O3. OH also contributes to NO oxidation, resulting in the formation of HONO, which can also be oxidized to NO2 by OH. Reaction with OH to produce HNO3 is the main sink of NO2. The interconversion between NO2 and HO2NO2 also plays a role in NO2 fate, but HO2NO2 acts as a reservoir rather than a sink, as it converts back to NO2 through rapid thermal decomposition and reaction with OH. Contrary to OFR185-iNO, where high-NO conditions that are useful for OFR experiments are very difficult to achieve, high NO relative to HO2 is effectively obtained in the OFR185cNO mode, although little of the basic OxN chemistry is changed compared to OFR185-iNO. Under the conditions of Figure 1a, NO/HO2 > 10 is maintained over most of the residence time

(Figure S2a,b). This translates to an r(RO2 + NO)/r(RO2 + HO2) of ∼30, clearly indicating a high-NO condition. Nontropospheric photolysis is also minor. Although high-NO conditions could be effectively achieved, we find that only ∼5% of the conditions explored over the above-mentioned wide range of H2O, UV, and OHRext and an rNOin range of 0.001−1000 ppb/s are good high-NO conditions (Figure 2a). Risky high-NO conditions may also be of experimental interest for some precursors (see Section 2.2). Combined good and risky high-NO conditions comprise ∼30% of the explored cases, while good (good + risky) high-NO conditions are only ∼1% (∼10%) of the explored cases for OFR185-iNO in Peng and Jimenez.39 Because the explored spaces for OFR185-iNO and OFR185-cNO cover the same H2O, UV, and OHRext ranges and similar injected NO ranges, higher fractions of good and risky high-NO conditions in OFR185-cNO clearly demonstrate the potential usefulness of the continuous NO addition method. As with OFR185-iNO, good high-NO conditions in OFR185cNO are favored at higher H2O, lower UV, low OHRext, and E

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ACS Earth and Space Chemistry Table 2. Definition of Condition Types in This Study (Good/Risky/Bad High/Low-NO) condition criterion

good F185exp/OHexp < 3 × 103 cm s−1 and F254exp/OHexp < 4 × 105 cm s−1 condition criterion

risky F185exp/OHexp < 1 × 105 cm s−1 and F254exp/OHexp < 1 × 107 cm s−1 (excluding good conditions) high-NO r(RO2 + NO) r(RO2 + HO2)

>1

bad F185exp/OHexp ≥ 1 × 105 cm s−1 or F254exp/OHexp ≥ 1 × 107 cm s−1 low-NO r(RO2 + NO) r(RO2 + HO2)

≤1

Figure 3. Fractional importance of the reaction rate of several species of interest with NO3 versus that with OH, as a function of the ratio of exposure to NO3 and OH. The curves of biogenics and phenols are highlighted by solid dots and squares, respectively. In the upper part of the figure, also shown are the modeled frequency distributions of ratios of NO3 exposure to OH exposure under good/risky/bad high/low-NO conditions for OFR185-cNO, OFR254-iN2O, and OFR185-iN2O. See Table 2 for the definitions of condition types. All curves, markers, and histograms in this figure share the same abscissa.

rNOin ∼ 0.1−10 ppb/s (Figure S3a). The reasons why this range of conditions is preferable can be understood in similar manner with OFR185-iNO, because the basic chemical regimes are very similar for both operation modes. A high H2O and a lower OHRext are beneficial to limiting the contribution of nontropospheric organic photolysis;38 lower UV avoids too fast production of OH, HO2, and O3, all of which consume NO. rNOin needs to be substantial so that NO concentrations can rival HO2 to obtain a high-NO condition, but too high rNOin is problematic, since OH is heavily suppressed by NOy species, leading to less good conditions. Peng and Jimenez39 also considered other experimental issues, for example, high NO2 suppressing RO2 and VOC reactions with NO3, in OFRs. OFR185-iNO and OFR185-cNO have comparable NO injection amounts for good high-NO conditions and very similar basic chemical regimes. Therefore, just like under good high-NO conditions in OFR185-iNO, the contribution of NO3 to VOC fate is minor for almost all VOCs (Figure 3). RO2 suppression by NO2 becomes substantial when NO2 is at ppm level or above. However, NO2 in the reactor is generally well below that level. Although total NO injection may exceed 1 ppm, not all NO is converted into NO2, which avoids experimental artifacts due to high NO2.

For OFR185-cNO, the population of conditions other than good high-NO is easy to understand. As H2O increases, conditions become better (bad → risky → good), while the shift of the boundary of high/low-NO conditions toward higher OHRext and/or rNOin is not substantial (Figure 4) as H2O does not have a direct impact on the OxN chemistry. As UV increases, conditions become better and but also increasingly low-NO, since more OH, HO2, and O3 are produced, which consume both non-OxN external OH reactants and NO more rapidly. 3.1.2. OFR254-iN2O. OFR254-iN2O is an alternative technique recently proposed by Lambe et al.40 to obtain high NO consistently throughout the reactor residence time. The key idea of this technique is the injection of a very large amount of N2O, which is nonreactive to HOx, RO2, and NOy but reacts with O(1D) to produce NO (Figure S1b). This reaction competes with that of H2O with O(1D) producing OH. An initial N2O mixing ratio (N2Oin) at the percent level is needed for a significant fraction of O(1D) to react with N2O, at typical H2O levels (∼1%). We investigate an OFR254-iN2O case at N2Oin = 2.5% and H2O = 1%, where O(1D) reacts with N2O and H2O at equal rates, as an example (Figure 1b). This example uses a typical UV, modest OHRext (10 s−1), and F

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Figure 4. Image plots of the condition types defined in Table 2 vs external OH reactivity (excluding N-containing species) and initial NO for several typical cases in OFR185-cNO (see Table 1 for the case label code). Brighter colors indicate low-NO conditions; shading indicates high-NO conditions.

O3,in (5 ppm) is comparable to that in Lambe et al.40 Apart from the NO production pathway, the major OxN species and their interconversion pathways are similar to a typical OFR254-iNO case.39 Because of the large amount of injected O3 in OFR254-iN2O, NO loss is dominated by NO + O3 and HONO formation from NO + OH is no longer a major pathway as in OFR185. For the same reason, NO2 oxidation to NO3 emerges as one of major NO2 fates. Then the equilibrium between NO2 + NO3 and N2O5 also plays a role in the OxN chemistry (Figure 1b). In the case shown in Figure 1b, NO production is sustained but NO consumption is fast due to high O3. As a result, NO concentration is ∼1 ppb and r(RO2 + NO)/r(RO2 + HO2) is ∼0.6. The problem of relatively low r(RO2 + NO)/r(RO2 + HO2) is also observed elsewhere in the explored phase space (including an N2Oin span of 0.02−20%). As shown in Figure 2b, only ∼1% of the explored conditions are good highNO. Risky high-NO conditions are also of some experimental interest and comprise ∼20% of total explored conditions. Good high-NO conditions can mainly be achieved at UV > 1 × 1014 photons cm−2 s−1, H2O > 0.3%, OHRext < 10 s−1, O3,in > 10 ppm, and N2Oin > 2% (Figure S3b). High H2O and low OHRext always make conditions better; an N2Oin of several percent at least is necessary for high-NO conditions in OFR254-iN2O. Although a higher UV photon flux tends to make conditions worse,38 a threshold UV flux is critical for sufficient NO production in OFR254-iN2O (Figure 5), since NO production is roughly proportional to UV while HO2 production, buffered by O3,37 is less than proportional. In addition, a higher O3,in makes this buffer more effective and provides the

source of O(1D) that generates NO. To keep conditions good, UV and N2Oin cannot be both very high (a feature that is not clearly shown in Figure S3b) since then they can lead to very high NOx that suppresses OH. In OFR254-iN2O, as long as conditions are good, NO2 does not reach ppm level. Because of high concentrations of O3, NO3 formation is promoted in OFR254-iN2O. NO3exp/OHexp in OFR254-iN2O is ∼2 orders-of-magnitude higher than in OFR185-cNO for all condition types. Under good high-NO conditions of OFR254iN2O, resulting from high O3,in, this ratio is on the order of 10 and higher and may result in substantial consumption of phenols and many alkenes by NO3 (Figure 3). Even under good low-NO conditions, an NO3exp/OHexp of ∼1−10 could make reactions with NO3 major fates of dihydrofurans, phenols, and biogenics that are highly reactive toward NO3 (e.g., α-terpinene and terpinolene). Note that the distributions of NO3exp/OHexp for OFR254-iN2O in Figure 3 include cases with O3,in from 2 to 70 ppm. An O3,in close to the lower end of this range leads to slower NOx oxidation through the NO → NO2 → NO3 chain and a lower NO3exp/OHexp. 3.1.3. OFR185-iN2O. While Lambe et al.40 demonstrated the use of OFR254-iN2O, the same technique is in principle also applicable to OFR185. In OFR185-iN2O, O(1D), the key species to sustained NO production, is supplied by N2O photolysis at 185 nm. This makes initial O3 injection unnecessary. Despite being photolyzed, N2O concentration is hardly decreased and always uniform in the reactor. Even at the highest UV in this study and under the assumption of all O(1D) being reacted with N2O, N2O has a lifetime of ∼10 h. As a result, along the flow, NO concentration is also relatively uniform. As N2O is on G

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ACS Earth and Space Chemistry

Figure 5. Same format as Figure 4 but for OFR254−5-iN2O.

least some experimental interest (good/risky high-NO conditions) is ∼25%. The explored conditions for OFR185-iN2O can also be compared to those for OFR185-cNO, as the ranges of H2O, UV, and OHRext are identical and the ranges of the actual NO resulting from different OxN injection rates in both modes are almost the same. The fractions of good/risky high-NO conditions in both modes are also comparable. The OFR185cNO mode has higher experimental complexity than OFR185iN2O, as discussed above. Although OFR185-iN2O is the most practical method among the three investigated in this study, its performance over the entire physical condition space may not be always sufficient for experimental purposes. Good high-NO OFR185-iN2O experiments necessitate N2Oin > ∼0.7% (Figure S3c). Good high-NO conditions are more frequently present at higher H2O and lower OHRext in OFR185-iN2O, just as in OFR185-cNO. Nevertheless and contrary to OFR185-cNO, higher UV is favorable for good high-NO conditions in OFR185-iN2O. In other OFR185 modes [OFR185-iNO (see Peng and Jimenez39 and OFR185-cNO], a large production of O3 and HOx at high UV shortens NO lifetime. This is not problematic in OFR185-iN2O as NO production is also enhanced at high UV. In addition, high UV is necessary for conditions to be good in the presence of significant amounts of OH reactants, including NOx.38 This trend can also be observed in Figure 6. It is clearly illustrated that an increasing UV shifts the boundary between high- and low-NO conditions toward lower N2Oin. Also, a higher UV flux leads to good conditions being present at higher OHRext. Higher H2O also makes conditions better, which can be easily understood, as it reduces the fraction of high-NO conditions by producing more HOx.

the same order of magnitude as H2O in OFR-iN2O (Section 3.1.2) and the photoabsorption cross section of N2O at 185 nm is ∼2× that of H2O, N2O photolysis is a major O(1D)/OH source and generally promotes Ox and HOx chemistry in OFR185iN2O. While the OFR185-iN2O case in Figure 1c has a higher UV than in the OFR254-iN2O case in Figure 1b, O3 formed in the former case (∼3 ppm on average, Figure S2e,f) is less than that injected in the latter case. Therefore, NO lifetime is longer in the OFR185-iN2O case and NO concentration is 6 times that in the OFR254-iN2O case. Because the primary production of NOy species from N2O and the loss of NO2 due to HOx are both enhanced by a stronger O(1D) production in the latter case, NO2 levels are similar (within a factor of 2) in both cases. In the OFR185-iN2O case, the sink/reservoir species formed from HOx and NO2 (HNO3 and HO2NO2) are higher due to higher HOx production. The case shown in Figure 1c is “high-NO” [r(RO2 + NO)/ r(RO2 + HO2) ∼ 2], but it can barely be classified as “good”, as the measure for 254 nm organic photolysis is on the edge of good region [F185exp/OHexp ∼ 1 × 103 cm/s, F254exp/OHexp ∼ 4 × 105 cm/s]. The reason for high NO has already been discussed above. The barely good condition in terms of 254 nm photolysis results from OH suppression caused by high NOx, which corresponds to an OHRext of ∼30 s−1. The main species make the system resilient to OH suppression, O3,37 is only ∼3 ppm (corresponding to an OHRint of ∼5 s−1). From the analysis above, we can find that a low O3 favors high-NO conditions while disfavoring good conditions. It is thus not trivial to find appropriate physical inputs for good high-NO conditions. Overall, good high-NO conditions comprise ∼1% of the explored space (Figure 2c). The fraction of conditions of at H

DOI: 10.1021/acsearthspacechem.7b00070 ACS Earth Space Chem. XXXX, XXX, XXX−XXX

Article

ACS Earth and Space Chemistry

Figure 6. Same format as Figure 4, but for OFR185-iN2O.

high-NO fraction by a factor of 7. Similar results are observed for OFR185-cNO and OFR254-iN2O (not shown). Nonuniform UV Radiation Fields. Although other components of the gas flow in OFR (i.e., O2, O3, H2O2, VOCs, NOx etc.) cannot lead to absorption of most 185 and 254 nm photons under any operating conditions (at least for the PAM reactor and similar geometries), percent-level N2O (cross section at 185 nm: 1.43 × 10−19 photons cm−2 s−1)42 may absorb most UV at 185 nm. For an N2Oin of 5%, a 10 cm path (typical OFR radius) corresponds to an optical depth of 1.5. Therefore, the nonuniformity of the radiation field (RF) in OFR185-iN2O needs to be considered. In Figure 7, we compare two commonly used lamp arrangements in the PAM OFR, that is, two lamps and four lamps. In the case of no N2O, while the 4-lamp placement results in a slightly more uniform RF than the 2-lamp placement, both placements produce RFs across most of the reactor where the photon flux at 185 nm is ∼0.1−0.3 of that at lamp surface. In other words, practically both placements generate relatively uniform RFs. The nonuniformity is mainly due to the radial pattern of the lamp emissions (the photon flux from a lamp being inversely proportional to the distance to its center) but not to photoabsorption by gases. However, at N2Oin = 5%, 185 nm photons are strongly absorbed, leading to almost half the reactor cross section being devoid of 185 nm radiation in the 2-lamp placement. In contrast, the 4-lamp placement still keeps the RF relatively uniform with relative 185 nm photon flux to that at lamp surface being ∼0.1−0.2 in most of the reactor section. Thus, to avoid highly heterogeneous RFs that may produce undesirable results in OFR185-iN2O, one needs to conduct experiments with four lamps in a PAM reactor or with similar

The relative importance of NO3 reactions in VOC fate in OFR185-iN2O is higher than OFR185-cNO and lower than OFR254-iN2O (Figure 3). NO3exp/OHexp for good high-NO conditions in OFR185-iN2O ranges from ∼0.5 to ∼10. At the lower limit, the relative contribution of NO3 reactions to the loss of all VOCs is 2 or