In this section, the results are presented in two parts, i.e., first for the
simulations with well-known pathways only and secondly with all significant
pathways, as proposed in Sect. 2.2. Then based on the results and their
comparison with the atmosphere and chamber experiments, we propose guidelines
for OFR operation to ensure atmospherically relevant RO2 chemistry,
as well as other chemistries already discussed in the previous studies
(Peng et al., 2016, 2018), in OFRs.
Simulations with well-known pathways (RO2+HO2,
RO2+RO2, RO2+NO and RO2+NO2)
Due to the significantly different reactivities of non-acyl and acyl
RO2, the results of these two types of RO2 are shown
separately.
Non-acyl RO2
In this case non-acyl RO2 radicals have only three fates, i.e.,
RO2+HO2, RO2+NO and RO2+RO2. The relative
importance of these three fates can be shown in a triangle plot (Fig. 1). The
figure includes data points of OFR185 (including OFR185-iN2O) and
OFR254-70 (including OFR254-70-iN2O), as well as several typical ambient
and chamber studies, including two pristine remote area cases (P1 and
P2) from the ATom-1 study (Wofsy et al., 2018), two forested area cases
(F1 and F2) from the BEACHON-RoMBAS and GoAmazon campaigns,
respectively (Ortega et al., 2014; Martin et al., 2016, 2017), an urban area
case (U) from the CalNex-LA campaign (Ryerson et al., 2013), and five typical
chamber experiment cases (C1–C5) from the FIXCIT study
(Nguyen et al., 2014). These typical cases shown in Fig. 1 bring to light
several interesting points.
In all ambient and chamber cases, medium and slower RO2+RO2
contribute negligibly to the RO2 fate. This confirms a common
impression that self- and cross-reactions of many RO2 radicals do not
significantly affect RO2 fates.
However, if RO2 self- and cross-reacts rapidly, RO2+RO2
can be the most important loss pathway among RO2+RO2,
RO2+HO2 and RO2+NO even in pristine regions with
higher VOC (e.g., P1 in Fig. 1) compared to an average pristine region
case (P2). Note that the P1 case is still very clean compared to
typical forested and urban areas (Table 2).
Forested areas located in the same region as pollution sources are not as
“low NO” as one may expect (points F1 and F2 in Fig. 1).
RO2+NO contributes ∼ 20 %–50 % to RO2 loss, as NO and
HO2 concentrations are of the same order of magnitude in these
cases.
RO2+NO dominates over RO2+RO2 and RO2+HO2 in
almost all urban areas. Even in relatively clean urban areas such as Los
Angeles during CalNex-LA in 2010 (point U in Fig. 1), average NO is ∼1 ppb, still sufficiently high to ensure the dominance of RO2+NO among the
three pathways.
Various chamber cases in the FIXCIT campaign (low to high OHRext; low to
high NO; points Cx in Fig. 1) are able to represent specific
RO2 fates that appear in different regions in the atmosphere.
On these plots, points for bad conditions (in terms of non-tropospheric
photolysis) are not shown because of the lack of experimental
interest. The triangle plots for OFR254-7 (including OFR254-7-iN2O) in
the same form (Supplement Fig. S1a, b) show no qualitative differences from the results
of OFR254-70, implying that initial O3 in OFR254 modes has only
minor impacts on RO2 fate. We see this result not only for
well-known non-acyl RO2 fate, but also for the aspects discussed in
the following sections. The similarity between OFR254 modes can be explained
by the minor effects of a lower O3 on HOx at
relatively low OHRext (Peng et al., 2015). Cases at higher
OHRext often have stronger non-tropospheric photolysis (Peng et
al., 2016) and hence are more likely to be under bad conditions and are not
shown in Figs. 1 and S1a, b. For simplicity, this similarity is not discussed
further.
Triangle plots of RO2 fate by RO2+HO2,
RO2+RO2 and RO2+NO (without RO2+OH and
RO2 isomerization considered in the model) for RO2 with
the medium self- and cross-reaction rate constant
(1×10-13 cm-3 molecule-1 s-1)
in (a) OFR185 (including OFR185-iN2O) and (c) OFR254-70 (including
OFR254-70-iN2O) and for RO2 with the fast
self- and cross-reaction rate constant
(1×10-11 cm-3 molecule-1 s-1) in (b) OFR185 (including
OFR185-iN2O) and (d) OFR254-70
(including OFR254-70-iN2O). Inclined tick values on an axis indicate the
grid lines that should be followed (in parallel to the inclination) to read
the corresponding values on this axis. The OFR data points are colored by the
logarithm of the exposure ratio between 254 nm photon flux and OH, a measure
of badness of OFR conditions in terms of 254 nm organic photolysis. Several
typical ambient and chamber cases (see Table 2 for details on these cases)
are also shown for comparison.
Several typical ambient and chamber (the FIXCIT campaign) cases that
are compared to OFR cases.
Type
Label
Case
OHRVOC (s-1)
OH
NO
HO2
Ambient
P1
Pristine (Pacific Ocean, high RO2)a
1.9
0.39 ppt
1.9 ppt
11 ppt
P2
Pristine (Pacific Ocean, typical)a
1
0.25 ppt
3 ppt
25 ppt
F1
Forested (Rocky Mountains)b
N/Ac
1 ppt
60 ppt
100 ppt
F2
Forested (Amazon, wet season)d
9.6
1.2×106 molecules cm-3
37 ppt
5.1×108 molecules cm-3
U
Urban (Los Angeles)e
25f
1.5×106 molecules cm-3g
1.5 ppbe
1.5×108 molecules cm-3g
Chamber (FIXCIT)
C1
Exp. no. 25h
30.5i
3×106 molecules cm-3
15 ppt
150 ppt
C2
Exp. no. 17h
116i
1.2×106 molecules cm-3
10 ppt
50 ppt
C3
Exp. no. 26h
32i
2×107 molecules cm-3
3.5 ppb
230 ppt
C4
Exp. no. 22h
147i
2.3×106 molecules cm-3
430 ppb
4.3 ppb
C5
Exp. no. 16h
45.7i
4×106 molecules cm-3
80 ppt
8 ppt
a Wofsy et al. (2018) for the Atom-1 campaign.
b Fry et al. (2013) for the BEACHON-RoMBAS campaign.
c RO2 concentration was given in Fry et al. (2013) (50 ppt)
so that OHRVOC is not needed for RO2 fate estimation.
d Daun Jeong and Saewung Kim, personal communication (2018), for the
GoAmazon campaign (Martin et al., 2016, 2017).
e Typical case in the CalNex-LA campaign (Ryerson et al., 2013).
f Estimated (Peng et al., 2016).
g Typical ambient value (Mao et al.,
2009; Stone et al., 2012).
h Data from Nguyen et al. (2014).
i Initial value.
An important feature confirmed in Fig. 1 is that OFR-iN2O modes
effectively realize conditions of experimental interest with variable
relative importance of RO2+NO in RO2 fate (Lambe et al.,
2017; Peng et al., 2018). Tuning initially injected N2O can achieve
this goal (Fig. 2). While it is possible to reduce RO2+HO2 in
OFR185-iN2O to negligible compared to RO2+NO by increasing
N2O, this is not possible in OFR254-70-iN2O due to fast NO
oxidation by the large amounts of O3 added in the reactor.
Nevertheless, OFR254-70-iN2O can still make RO2+NO dominate over
RO2+HO2 in RO2 fate. OFR and chamber cases span a
range of ∼ 0 %–∼ 100 % in relative importance of RO2+NO in
RO2 fate (Fig. 2), suggesting that both chambers and OFRs are able
to ensure the atmospheric relevance of RO2+NO in RO2 fate.
Frequency distributions of the relative importance of RO2+NO in
the fate of RO2 (with medium self- and cross-reaction rate constant and
without RO2+OH and RO2 isomerization considered) for
OFR185 (including OFR185-iN2O) and OFR254-70 (including
OFR254-70-iN2O). Distributions for several different N2O
levels are shown. Only good and risky conditions (in terms of
non-tropospheric organic photolysis) are included in the distributions. Also
shown is the relative importance of RO2+NO for several typical ambient
and chamber cases (see Table 2 for details on these cases).
Another important feature that can be easily seen in Fig. 1 is that medium-rate RO2+RO2 (and hence also RO2+RO2 slower than
10-13 cm3 molecule-1 s-1) is of negligible importance
in the fate of RO2 (Fig. 1a, c) in OFR185 (including
OFR185-iN2O), OFR254-70 (under most conditions, including
OFR254-70-iN2O), chambers and the atmosphere. Thus, a very large subset
of RO2 types have only a minor or negligible contribution from
RO2+RO2 to their fate. This is already known for ambient
RO2 fate (Ziemann and Atkinson, 2012). The reason why this is also
true in OFRs is that while OH is much higher than ambient levels,
HO2 and NO (high-NO conditions only) are also higher. One can
easily verify that steady-state RO2 concentrations (see Appendix B
for details) would not deviate from ambient levels by orders of magnitude.
The reactive fluxes of RO2+RO2 in OFRs are thus not
substantially different than in the atmosphere, while RO2+HO2
and RO2+NO (high-NO conditions only) are both faster in OFRs
because of higher HO2 and NO. The combined effect is a
reduced relative importance of RO2+RO2 in RO2
fate in OFRs compared to the atmosphere. The only exception in OFRs occurs at
very high VOC precursor concentrations (OHRext significantly
>100 s-1) in OFR254 (Fig. S2), in which OH levels are not
substantially suppressed due to large amounts of O3 (Peng et al.,
2015). As a result, RO2 concentration is remarkably increased by
strong production, and RO2+RO2 relative importance increases
roughly quadratically and becomes significant.
The generally lower relative importance of RO2+RO2 in OFRs
than in the atmosphere is more obvious for the fate of RO2 with
fast RO2+RO2 rate constants (Figs. 1b, d and 3). Although OFRs
can reasonably reproduce RO2 fates in typical low- and
moderate-OHRext ambient environments (e.g., typical pristine and forested
areas; Figs. 1b, d and 3) and low-OHRext chambers, OFR185 cannot achieve
a relative importance of RO2+RO2 significantly larger than
50 %, such as found in remote environments with higher VOC (e.g., P1 in
Fig. 1) and high-OHRext chamber experiments (e.g., C2 and C5 in
Fig. 1; the distribution for C2 is also shown in Fig. 3). In OFR254-70,
a relative importance of RO2+RO2 as high as ∼90 % may be
attained (Fig. S3). However, this requires very high OHRext, which leads
to medium (and slower) RO2+RO2 showing higher-than-ambient
relative importance. In reality, fast RO2+RO2 reactions all involve
substituted RO2, which almost certainly arises from and coexists with
unsubstituted RO2 (with slower self- and cross-reactions). Therefore,
very high OHRext in OFR254 is not really suitable for attaining dominant
RO2+RO2 conditions. In OFR185, a higher OHRext generally
also results in a higher RO2+RO2 relative importance because
of higher RO2 production (Fig. S3). Nevertheless, higher
OHRext is more likely to lead to risky or bad conditions (Fig. 3;
Peng et al., 2016). It should be noted that although it is difficult to
reliably achieve RO2+RO2 with a relative importance larger
than 50% in RO2 fate in OFRs, the distributions of
RO2+RO2 relative importance in OFRs seem to be within a
factor of 2 of those of field and aircraft campaigns (Fig. 3).
Frequency distributions of the relative importance of
RO2+RO2 in the fate of RO2 (with fast self- and cross-reaction rate constant and without RO2+OH and RO2
isomerization considered) for OFR185 (including OFR185-iN2O), OFR254-70
(including OFR254-70-iN2O), and a chamber experiment and in the
atmosphere (a couple of different environments). The OFR distributions for
good and risky conditions (in terms of 254 nm organic photolysis; see Table
S1 for the definitions of these conditions) are shown separately. Also shown
is the relative importance of RO2+RO2 for several typical
chamber cases (see Table 2 for details on these cases). The range of the
RO2+RO2 relative importance for most high-NO conditions is
highlighted in cyan.
In the case of very fast RO2+RO2, all features for fast
RO2+RO2 discussed above are still present (Fig. S1c, d). The
only major difference between the results for fast RO2+RO2 and
very fast RO2+RO2 is the significantly higher relative
importance of RO2+RO2 in RO2 fate in the latter
case, which is expected. In summary, fast RO2+RO2 is not
perfectly reproduced in OFRs in terms of relative importance in RO2
fate, but it is significant when this pathway is also important in the
atmosphere.
The HOx recycling ratio β (see Sect. 2.3) is one of
the key factors determining HO2 in the OFR model, yet it is not
well constrained. Although we make reasonable assumptions for it in the model
input (see Sect. 2.3 for details), a sensitivity study to explore its
effects is also performed here. For RO2 with the fast
self- and cross-reaction rate constant, we perform simulations with the
HOx recycling ratios fixed to a number of values from 0
(radical termination) to 2 (radical proliferation) in lieu of those
calculated under the assumptions described in Sect. 2.3. As expected, the
contribution of RO2+RO2 to RO2 fate increases
monotonically between β=2 and β=0 (Fig. S4) as the recycling
of the competing reactant HO2 decreases. Nevertheless, the change
in the average RO2+RO2 relative importance from β=0 to
β=2 is generally within a factor of 2. Thus, it still holds that the
RO2+RO2 relative importance in OFRs is generally lower than in
the atmosphere. Only at β∼0 may OFR185 theoretically attain a
relative importance of RO2+RO2 of ∼70 %, as in the
P1 case (pristine, but relatively high VOC; Fig. S5). Note that β=0 for all VOC oxidation (including oxidation of intermediates) is extremely
unlikely. In OFR254, even if RO2+RO2 may contribute up to ∼100 % to RO2 fate at very high OHRext at β=0, these
conditions still also lead to significant RO2+RO2 in the fate
of RO2 that self- and cross-reacts more slowly, which is not
atmospherically relevant.
Acyl RO2
As described in Sect. 2.1, the generic acyl RO2 modeled in this
study has the same loss pathways as RO2 with the fast
self- and cross-reaction rate constant, except for RO2+NO2, which
can be a significant acyl RO2 loss pathway in OFRs as well as both
chambers and the atmosphere. When this reaction is included in the simulations of
acyl RO2, it is a minor or negligible loss pathway of RO2
at low N2O, while it can be the dominant fate of acyl RO2
at high N2O (Fig. 4). In general, the RO2+NO2
relative importance increases with initial N2O. This is always true
in OFR254-70-iN2O between N2O = 0.02 % and
N2O = 20 %, while in OFR185-iN2O, the average relative
contribution of RO2+NO2 to RO2 fate starts to
decrease at N2O ∼10 % because RO2+NO regains some
importance. This results from the HOx suppression caused by
high NOy and strong NO production at high N2O.
Strong NO production increases its concentration and suppresses
HOx under these conditions, limiting the conversion of NO to
NO2. Because of the strong OH suppression by high
NOy at N2O ≥10 %, these conditions are not
desirable (Peng et al., 2018).
Average relative importance of RO2+NO2 in acyl
RO2 fate (RO2+OH and RO2 isomerization not
considered) in OFR185 (including OFR185-iN2O) and OFR254-70 (including
OFR254-70-iN2O). The averages are calculated based on good and risky
conditions (in terms of non-tropospheric organic photolysis) only.
The only difference between the simulations of acyl RO2 and of the
fast self- and cross-reacting non-acyl RO2 is the quasi-irreversible
reaction RO2+NO2→RO2NO2 at room temperature,
whose effects are revealed by a comparison of the triangle plots of the
RO2 fates in each case (Figs. 1b, d and S6). RO2+NO2
is clearly dominant in acyl RO2 fate in OFRs as long as
RO2+NO plays some role (not necessarily under high-NO conditions). In
OFR185-iN2O, the relative importance of RO2+RO2 in the
sum of the HO2, NO and RO2 pathways is reduced (Fig.
S6a) compared to that of non-acyl RO2 with fast
RO2+RO2 (Fig. 1b) because RO2+NO2 decreases
acyl RO2 concentration. Such a decrease is not significant in
OFR254-70-iN2O (Fig. S6b, compared to Fig. 1d), since for non-acyl
RO2, it is already stored in the form of RO2NO2
as an RO2 reservoir. In other words, the high initial O3
greatly accelerates NO-to-NO2 oxidation and shifts the equilibrium
RO2+NO2↔RO2NO2 far to the right
even for non-acyl RO2.
RO2+NO2 is an inevitable and dominant sink of most acyl
RO2 in high-NOx OFRs, though the extent of this
dominance differs substantially among the different OFR operation modes. In
OFR254-70-iN2O, RO2+NO makes a minor or negligible
contribution to acyl RO2 fate because the required high
O3 very rapidly oxidizes NO to NO2 and leads to very low
NO-to-NO2 ratios (e.g., ∼0.003–0.03; see Fig. S7). In
OFR185-iN2O, the contribution of RO2+NO can be somewhat
significant, with typical NO-to-NO2 of ∼0.03–0.4. (Fig. S7).
Urban NO-to-NO2 ratios vary widely, for example (roughly, and
excluding significant tails in the frequency distributions) 0.02–1 for
Barcelona and 0.007–0.7 for Los Angeles and Pittsburgh (see Fig. S7). Given
these variations among different urban areas, RO2+NO and
RO2+NO2 for acyl RO2 in OFR185-iN2O can be
regarded as relevant to urban atmospheres. Exceptions to the relevance of
OFR185-iN2O occur during morning rush hours (e.g., see the high
NO-to-NO2 tail for the Pittsburgh case in Fig. S7), near major NO
sources and/or in urban atmospheres with stronger NO emission intensity
(e.g., Beijing, especially in winter; Fig. S7). In these cases,
NO-to-NO2 ratios may significantly exceed 1, and RO2+NO
may be the dominant acyl RO2 loss pathway. Such high-NO conditions
appear difficult to simulate in OFRs with the current range of techniques.
Acyl RO2 is not the dominant type among RO2 types under most
conditions in OFRs, chambers and the atmosphere, since their formation
usually requires multistep (at least 2 steps) oxidation via specific pathways
leading to an oxidized end group (i.e., aldehyde and then acylperoxy).
However, simulations using the GECKO-A model in urban (Mexico City) and
forested (Rocky Mountains) atmospheres (Fig. S8) show that acyl
RO2 can still be a major (very roughly 1/3) component of
RO2 at ages of several hours or more. Therefore, acyl
RO2 chemistry in a high-NO OFR can significantly deviate from that in
an urban atmosphere with NO dominating NOx and can be
relevant to an urban atmosphere with NO2 dominating
NOx. On the other hand, a few theoretical studies suggested
that H abstraction by the acylperoxy radical site from hydroperoxy groups
close to the acylperoxy site in multifunctional acyl RO2 may be
extremely fast (Jørgensen et al., 2016; Knap and Jørgensen, 2017). If
these theoretical predictions are sufficiently accurate, these acyl
RO2 types may exclusively undergo an intramolecular H shift to form non-acyl
RO2 or other radicals and prevent RO2+NO2 from
occurring even at very high (ppm level) NO2. However, this type of
RO2 is structurally specific and may not have strong impacts on the
overall acyl RO2 chemistry.
Simulations with all significant pathways
Since RO2 isomerization does not significantly affect the generic
RO2 concentration, the two RO2 fates that were recently
found to be potentially important, i.e., RO2+OH and RO2
isomerization, can be discussed separately.
RO2+OH
In the troposphere, RO2+OH is a minor (at low NO) or negligible (at
high NO) RO2 loss pathway (Fittschen et al., 2014; Assaf et al.,
2016; Müller et al., 2016), as its rate constant is roughly
an order of magnitude higher than that of RO2+HO2 (Table 1),
while the ambient OH concentration is on average 2 orders of magnitude lower than
that of HO2 (Mao et al., 2009; Stone et al., 2012; Fig. 5). We
will not discuss RO2+OH in the high-NO cases in detail. Simply put,
the relative importance of RO2+OH is generally negatively
correlated with input N2O in OFR-iN2O, as NOx
suppresses OH and the relative importance of RO2+NO increases. Below, we
focus on low-NO (actually, for simplicity, zero-NO) conditions.
Frequency distributions of (a) the HO2-to-OH ratio and
(b) the relative importance of RO2+OH in the fate of RO2
(with medium self- and cross-reaction rate constant) for OFR185 (including
OFR185-iN2O), OFR254-70 (including OFR254-70-iN2O), and a chamber
experiment and in the atmosphere (a couple of different environments). The
OFR distributions for lower (F185 <3.16×1012 photons cm-2 s-1; F254 <5.95×1014 photons cm-2 s-1) and higher
UV (F185 ≥3.16×1012 photons cm-2 s-1; F254 ≥5.95×1014 photons cm-2 s-1) are shown separately. Only good
and risky conditions (in terms of non-tropospheric organic photolysis) are
included in the distributions for OFRs. Also shown are the
HO2-to-OH ratio and the relative importance of RO2+OH for an OFR
experiment with ambient air input in a field study (BEACHON-RoMBAS; Palm et al., 2016).
At N2O =0, it would be ideal if an HO2-to-OH ratio
identical to the ambient values was realized in OFRs. In OFR185 cases with
medium RO2+RO2, an HO2-to-OH ratio around 100 occurs at a
combination of low H2O (of the order of 0.1%), low F185 (of the
order of 1011 photons cm-2 s-1) and medium
OHRext (10–100 s-1) and also at medium F185 (∼1012 photons cm-2 s-1) combined with very high
OHRext (∼1000 s-1, Fig. S9). Under both sets of
conditions, relatively high external OH reactants suppress OH, whose
production is relatively weak, and convert some OH into HO2 through
HOx recycling in organic oxidation (e.g., via alkoxy radical
chemistry). The reason why such an OH-to-HO2 conversion is needed
to attain an ambient-like HO2-to-OH ratio is that OFR185 is unable
to achieve this via the internal (mainly assisted by O3)
interconversion of HOx. This inability is most evident when
F185 (1013–1014 photons cm-2 s-1) and H2O
(of the order of 1 %) are high and OHRext is low (<∼10 s-1; Fig. S9). Under these conditions, OH production by
H2O photolysis is so strong that the HO2-to-OH ratio is
lowered to ∼1, since OH and H (which recombines with O2 to
form HO2) are produced in equal amounts from H2O
photolysis. As the RO2+OH rate constant is only roughly
1 order of magnitude higher than that for RO2+HO2, slightly
lower HO2-to-OH ratios (e.g., ∼30) suffice to keep
RO2+OH minor in this case. A combination of UV and H2O
that are not very high and a moderate OHRext that is able to
convert some OH to HO2 and somewhat elevate the HO2-to-OH
ratio results in minor relative importance for RO2+OH (Figs. S9 and
S10).
In OFR254-70, it is more difficult to reach an HO2-to-OH ratio of
∼100, which can only be realized at a combination of very low H2O
and F254 (∼0.07 % and ∼5×1013 photons cm-2 s-1,
respectively) and very high OHRext (∼1000 s-1). This is mainly
due to high O3 in OFR254-70, which controls the
HOx interconversion through HO2+O3→OH+2O2 and OH+O3→HO2+O2 and makes both
OH and HO2 more resilient to changes due to OHRext
(Peng et al., 2015). Even without H2O photolysis at 185 nm as a
major HO2 source, the HOx interconversion
controlled by O3 in OFR254-70 still brings the HO2-to-OH
ratio to ∼1 in the case of minimal external perturbation (see the region
at the highest H2O and UV and OHRext=0 in the OFR254-70 part of
Fig. S9). This ratio cannot be easily elevated in OFR254-70 because of the
resilience of OH to suppression for this mode (Peng et al., 2015).
Thus, this ratio is relatively low (<30) under most conditions (Fig.
S9), and consequently (and undesirably) RO2+OH is a major
RO2 fate in OFR254-70. There is an exception at relatively low
H2O and UV with very high OHRext (Fig. S10); however, these
conditions are undesirable in terms of non-tropospheric organic photolysis
(Peng et al., 2016).
Only the results of RO2 with medium RO2+RO2 are
discussed in this section. Those of RO2 with the fast
RO2+RO2 are not shown as they are not qualitatively different.
In OFR185, for the fast self- and cross-reacting RO2,
RO2+RO2 is relatively important at high OHRext
(>∼100 s-1; Fig. S3), while RO2+OH is a
major RO2 fate at low OHRext (generally of the order of
10 s-1 or lower) and relatively high H2O and UV (Fig. S10).
These two ranges of conditions are relatively far away from each other, and
hence there is no condition under which RO2+RO2 and
RO2+OH are both major pathways that compete, which simplifies
understanding RO2 fate. However, in OFR254-70, some conditions may
lead to both significant RO2+RO2 (for the
fast self- and cross-reacting RO2) and RO2+OH (e.g.,
H2O ∼0.5 %, F254 ∼1×1015 photons cm-2 s-1 and OHRext∼100 s-1). Nevertheless, as long as RO2+OH plays a major
role, these conditions do not bear much experimental interest and thus do not
need to be discussed in detail.
RO2 isomerization
RO2 isomerization is a first-order reaction. For this type of
reaction to occur, RO2 does not need any other species but only a
sufficiently long lifetime against all other reactants combined, as most
RO2 isomerization rate constants are <10 s-1. Radical
(OH, HO2, NO, etc.) concentrations in OFRs are much higher than
ambient levels and may shorten RO2 lifetimes compared to those in
the troposphere. Possibly reduced RO2 lifetimes naturally raise
concerns over the potentially diminished importance of RO2
isomerization in OFRs.
In this section we examine generic RO2 lifetimes against all
reactions (calculated without RO2 isomerization taken into account)
in OFR (including OFR-iN2O) cases (for the medium RO2+RO2
case) and compare them with the RO2 lifetimes in recent major
field and aircraft campaigns in relatively clean environments and a field
campaign in an urban area (CalNex-LA), as well as a low-NO chamber experiment
(Fig. 6). Indeed, RO2 lifetimes in clean ambient cases and in
chambers with near-ambient radical levels are generally much longer than
those in OFRs. The RO2 lifetime distribution of the explored good
and risky cases in OFR254-70 (including OFR254-70-iN2O) barely overlaps
the ambient and chamber cases, while in OFR185 (including
OFR185-iN2O), the RO2 lifetime can be as long as ∼10 s, which
is longer than in urban areas and roughly at the lower end of the range of
ambient RO2 lifetimes in clean environments (Fig. 6). The longest
RO2 lifetime in OFR185 occurs at very low F185 (of the order of
1011 photons cm-2 s-1) and H2O (∼0.1 %; Fig. S11)
when HOx is low. In OFR254-70, for RO2 to survive
for ∼10 s, in addition to very low UV and H2O, high OHRext is
also needed (Fig. S11). High-OHRext conditions in OFR254-70 cause OH
suppression and a decrease in HOx concentration and hence
result in relatively long RO2 lifetimes. However, the strong OH
suppression is likely to create bad conditions (high contribution of
non-tropospheric photolysis) (Peng et al., 2016). Low-OHRext
conditions do not lead to long RO2 lifetimes in OFR254-70 even at
very low F254 and H2O, since O3-assisted HOx
recycling prevents a very low HOx level even if
HOx primary production is low (Peng et al., 2015)
(a) Same format as Fig. 5, but for RO2 lifetime
(RO2 isomerization included in the model but excluded from lifetime
calculation). (b) Relative contribution of isomerization to
RO2 fate as a function of RO2 isomerization rate constant
in several model cases for OFR experiments in the BEACHON-RoMBAS campaign
(Palm et al., 2016), in a chamber experiment and in two ambient cases.
Isomerization rate constants of several RO2 types (Crounse et al.,
2013; Praske et al., 2018) are also shown.
An RO2 lifetime (without RO2 isomerization included) of
10 s leads to a relative importance of isomerization of 50 % in the total
fate (including all loss pathways) of RO2 with an isomerization
rate constant of 0.1 s-1, which is a typical order of magnitude for
isomerization rate constants of multifunctional RO2 with hydroxyl
and hydroperoxy substituents (Fig. 6; Crounse et al., 2013; D'Ambro et
al., 2017; Praske et al., 2018). Although a 50 % relative importance of
isomerization under some OFR conditions is still lower than those in
relatively low-NO ambient environments and low-NO chambers, this relative
importance should certainly be deemed major and far from negligible as some
have speculated (Crounse et al., 2013). Other monofunctional RO2
(with peroxy radical site only) and bifunctional RO2 with a peroxy
radical site and a carbonyl group isomerize so slowly (∼0.001–0.01 s-1)
that their isomerizations are minor or negligible loss pathways in
the atmosphere, chambers and OFRs with RO2 lifetimes around 10 s
(Fig. 6). Isomerizations of other types of multifunctional RO2
(e.g., multifunctional acyl RO2 with hydroxyl and hydroperoxy
substituents at favorable positions) are extremely fast (rate constants up to
106 s-1; Jørgensen et al., 2016; Knap and Jørgensen,
2017) and always dominate in their fates in the relatively low-NO atmosphere
as well as in chambers and OFRs with RO2 lifetimes around 10 s.
In the discussion about RO2 isomerization above (as in the
RO2+OH exploration in Sect. 3.2.1), we only examine low-NO (or
zero-NO for simplicity) conditions with medium RO2+RO2. In
high-NO environments, e.g., polluted urban atmospheres with NO of at least
∼10 ppb and high-NO OFRs in the iN2O modes, the RO2 lifetime
is so short that isomerization is no longer a major fate for any but the most
rapidly isomerizing multifunctional RO2 types discussed above. NO
measured in Los Angeles during the CalNex-LA campaign (Ortega et al.,
2016) was only ∼1 ppb, which would to allow RO2 to survive for
a few seconds and isomerize (Fig. 6), even in an urban area.
The OFR simulations for the discussions about RO2 isomerization are
the same as those conducted to study RO2+OH, i.e., the ones with
medium RO2+RO2 and RO2+OH included. For fast
RO2 self- and cross-reaction cases, RO2 lifetimes may be
significantly shorter than for RO2 with the medium
self- and cross-reaction rate constant at high OHRext (>∼100 s-1) in OFR185 (Fig. S3). These high-OHRext conditions are likely
to be risky or bad (of little experimental interest) (Peng et al.,
2016) and thus do not need to be discussed further in detail. OFR254-70 (a
zero-NO mode) does not generate good or risky (of at least some experimental
interest in terms of non-tropospheric organic photolysis) conditions, also
leading to low-NO-atmosphere-relevant RO2 lifetimes (Fig. 6).
RO2 types with faster self- and cross-reaction rate constants have even
shorter lifetimes in OFR254-70 and will not be discussed further.
Guidelines for OFR operation
In this section we discuss OFR operation guidelines for atmospherically
relevant RO2 chemistry, with a focus on OFR185 and OFR254 (zero-NO
modes). Since RO2+HO2 and RO2+NO both can vary from
negligible to dominant RO2 fate in OFRs, chambers and the
atmosphere (Figs. 1 and 2), these two pathways are not a concern in OFR
atmospheric relevance considerations, and neither is RO2+RO2. Medium or slower RO2+RO2 is minor or negligible
in the atmosphere and chambers, as well as in OFRs, as long as high
OHRext is avoided in OFR254 (Fig. S2). Fast RO2+RO2 is
somewhat less important in OFRs than in the atmosphere (Figs. 1b, d and 3),
but is still qualitatively atmospherically relevant, given the uncertainties
associated with the HOx recycling ratios of various reactive
systems and the huge variety of RO2 types (and hence
RO2+RO2 rate constants).
Accordingly, we focus on the atmospheric relevance of RO2+OH and
RO2 isomerization, i.e., their relative contributions close to
ambient values. Under typical high-NO conditions, RO2+NO dominates
RO2 fate and RO2+OH is negligible. High NO also shortens
the RO2 lifetime enough to effectively inhibit RO2
isomerization. Both the dominance of RO2+NO and the inhibition of
RO2 isomerization also occur in the atmosphere and in chambers, so
high-NO OFR operation (typically NO >10 ppb) represents these
pathways realistically. Some care is, however, required with the
RO2+OH and RO2 isomerization pathways at low NO. Since
RO2+HO2 in OFRs is always a major RO2 fate at low NO
and RO2+RO2 is generally not problematic, RO2+OH
and RO2+HO2 can be kept atmospherically relevant as long as the
HO2-to-OH ratio is close to 100 (the ambient average). In addition,
the RO2 lifetime (calculated without RO2 isomerization taken
into account) should be at least around 10 s.
Practically, OH production should be limited to achieve this goal. Too-strong
OH production at high H2O and UV can elevate OH and HO2
concentrations, which shortens RO2 lifetime and decreases the
HO2-to-OH ratio to ∼1 (see Sect. 3.2.1). OH production is
roughly proportional to both H2O and UV (Peng et al., 2015), so
it can be limited by reducing either or both. However, H2O and UV have
different effects on non-tropospheric organic photolysis. At a certain
OHRext, the OH production rate roughly determines the OH concentration in OFRs.
Reducing UV decreases both OH and UV roughly proportionally (Peng et
al., 2015), and hence changes in F185exp / OHexp and
F254exp / OHexp are small (Peng et al., 2016);
i.e.,
non-tropospheric organic photolysis does not become significantly worse if UV
is reduced. By contrast, if H2O is reduced without also decreasing UV,
F185exp / OHexp and F254exp / OHexp both increase, signifying
a stronger relative importance of non-tropospheric photolysis. Therefore,
reducing UV is strongly preferred as an OH production limitation method and
is effective in making both RO2+OH and RO2 isomerization
more atmospherically relevant.
To further explore the effects of UV reduction on the RO2+OH (Fig. 5)
and RO2 isomerization (Fig. 6) pathways, we divide our OFR case
distributions into higher UV and lower UV classes, with the boundary being
the midlevel (in logarithmic scale) UV in the explored range. The
distributions for lower UV conditions (solid lines in Figs. 5 and 6) are
clearly closer to the ambient cases (i.e., HO2-to-OH ratio closer to
100, smaller RO2+OH relative importance and longer RO2
lifetime).
Since OFR254 is unable to achieve both conditions with at least some
experimental interest (i.e., with sufficiently low non-tropospheric
photolysis) and an atmospherically relevant RO2 lifetime, we now
discuss preferable conditions for OFR185 only. As F185 close to or lower than
1012 photons cm-2 s-1 is needed for the RO2 lifetime to
be around 10 s or longer (Fig. S11), the OH concentration under preferable
conditions for atmospherically relevant RO2 chemistry (∼109 molecules cm-3 or lower) is much lower than the maximum
that OFR185 can physically reach (∼1010–1011 molecules cm-3).
Furthermore, lower OH production leads to higher susceptibility
to OH suppression by external OH reactants (Peng et al., 2015), which
can create non-tropospheric photolysis problems (Peng et al., 2016).
We thus recommend H2O as high as possible to maintain practically high
OH while allowing lower UV to limit the importance of non-tropospheric
organic photolysis.
(a) RO2 lifetime in the absence of isomerization,
(b) relative importance of RO2+OH in RO2 fate, and
(c) logarithm of the exposure ratio between 254 nm photon flux and OH
as a function of 185 nm photon flux and external OH reactivity for OFR185 at
N2O =0 and H2O =2.3 %. Three lines denoting conditions
leading to OH of 3.16×108, 1×109 and
3.16×109 molecules cm-3 are added in each panel. The thick and thin parts of
these lines correspond to good and risky conditions (in terms of 254 nm
organic photolysis, which is usually worse than 185 nm organic photolysis;
Peng et al., 2016), respectively.
The performance of various OFR185 conditions at high H2O (2.3 %) is
illustrated in Fig. 7 as a function of F185 and OHRext. The three
criteria for the performance, i.e., RO2 lifetime (calculated without
RO2 isomerization considered), relative importance of
RO2+OH and log(F254exp / OHexp) (a measure of 254 nm
non-tropospheric photolysis, which is usually worse than that at 185 nm;
Peng et al., 2016), are shown. At F185 of ∼1011–1012 photons cm-2 s-1
and OHRext around or lower than 10 s-1, all three criteria are satisfied. Since UV (and hence OH
production) is relatively low, a low OHRext (∼10 s-1) is
required to avoid heavy OH suppression and keep conditions good (green area
in Fig. 7c). Nevertheless, risky conditions
(log(F254exp / OHexp) <7; light red area in
Fig. 7c) may also bear some experimental conditions depending on the type
of VOC precursors (specifically on their reactivity toward OH, their
photolability at 185 and 254 nm, and the same quantities for their oxidation
intermediates; Peng et al., 2016; Peng and Jimenez, 2017). Thus,
higher OHRext (up to ∼100 s-1) may also be considered in OFR
experiments with some precursors (e.g., alkanes). In practice, the preferred
conditions may require F185 even lower than that our lowest simulated lamp
setting (Li et al., 2015). Such a low F185 may be realized, e.g., by
partially blocking 185 nm photons using nontransparent lamp sleeves with
evenly placed holes that allow for some 185 nm transmission.
Under these preferred conditions, OH concentration in OFR185 is ∼109 molecules cm-3, equivalent to a photochemical age of
∼1 eq. days for a typical residence time of 180 s. This is much
shorter than ages corresponding to the maximal oxidation capacity of OFRs
(usually eq. weeks or months; Peng et al., 2015), but it is similar
to the ages of the maximal organic aerosol
formation in OFRs processing ambient air (Tkacik et al., 2014; Ortega et al.,
2016; Palm et al., 2016). We show the maximal SOA formation case in the
OFR185 experiments in the BEACHON-RoMBAS campaign in the Rocky Mountains
(Palm et al., 2016) as an example (Figs. 5 and 6). During the campaign,
relative humidity was high (>60 % in most of the period),
OHRext was estimated to be relatively low (∼15 s-1)
in this forested area and UV in the OFR was limited in the case of the
maximal SOA formation age (∼0.7 eq. days). All these physical
conditions were favorable for atmospherically relevant RO2 fate
(Figs. 5 and 6). RO2+OH was minor in this case and the relative
importance of RO2 isomerization in RO2 fate in the OFR
was within a factor of ∼2 of that in the atmosphere for all
RO2 (regardless of the isomerization rate constant) during the
BEACHON-RoMBAS campaign (Fig. 6). The effect of UV on the relative importance
of RO2 isomerization for this example is also illustrated in
Fig. 6. In the sensitivity case with a lower age, lower UV results in a
larger contribution of isomerization to RO2 fate, while the
relative importance of RO2 isomerization is lower in a sensitivity
case with an age 3 times that of the maximal SOA formation. In an extreme
sensitivity case with the highest UV in the range of this study (with an age
of 4 eq. months), RO2 isomerization becomes minor or negligible for
all RO2 except extremely rapidly isomerizing ones.
The discussions above indicate that the atmospheric relevance of gas-phase
RO2 chemistry in OFRs deteriorates as the photochemical age over
the whole residence time (180 s) increases. To reach longer ages, longer
residence times (with UV still being low) can be adopted. However, OFR
residence times >10 min tend to be limited by the increasing
importance of wall losses (Palm et al., 2016). As a result, longer
residence times can only increase photochemical age in OFRs up to about a
week. This implies that in OFR cases with ages much higher than that of
maximal SOA formation (corresponding to the heterogeneous oxidation stage of
SOA), the atmospheric relevance of gas-phase RO2 chemistry in the
SOA formation stage (before the age of maximal SOA formation) often cannot be
ensured. However, under those conditions new SOA formation is typically not
observed, and the dominant process affecting OA is heterogeneous oxidation of
the preexisting OA (Palm et al., 2016). If the heterogeneous
oxidation of newly formed SOA is of interest, a two-stage solution may be
required. Lower UV can be used in the SOA formation stage to keep the
atmospheric relevance of the gas-phase chemistry, while high UV can be used
in the heterogeneous aging stage to reach a high equivalent age. The latter
approach is viable since heterogeneous oxidation of SOA by OH is slow and
particle-phase chemistry is not strongly affected by gas-phase species except
OH when OH is very high (Richards-Henderson et al., 2015, 2016; Hu et
al., 2016). This two-stage solution may be realized through a cascade-OFR
system or UV sources at different intensities within an OFR (e.g., spliced
lamps).
Praske et al. (2018) measured RO2 isomerization rate constants
at 296 and 318 K and observed an increase in the rate constants by a factor
of ∼5 on average. A 15 K temperature increase in OFRs would lead to
RO2 isomerization being accelerated by a factor of ∼3, while
other major gas-phase radical reactions have weak or no
temperature dependence (e.g., ∼7 %, ∼5 %, ∼6 %
and ∼19 % slowdowns for isoprene+OH, toluene + OH, typical
RO2+NO and RO2+HO2, respectively; Atkinson and Arey,
2003; Ziemann and Atkinson, 2012). As a consequence, the relative importance
of RO2 isomerization in RO2 fate in OFRs can be elevated
and closer to atmospheric values (Fig. 6). Nevertheless, a 15 K increase in
temperature may also result in some OA evaporation (Huffman et al., 2009;
Nault et al., 2018). Besides, the reduction of acylperoxy nitrate formation in
OFRs, which may be useful to mimic some urban environments where NO plays a
larger role in acyl RO2 fate (see Sect. 3.1.2), is unlikely to be
achieved by increasing OFR temperature. The O–N bond energy of acylperoxy
nitrates is ∼28 kcal mol-1 (Orlando and
Tyndall, 2012), which can be taken as an approximate reaction energy of their
decomposition. Then a 20 K temperature increase results in the equilibrium
constant of acyl RO2+NO2↔ acyl RO2NO2
being shifted toward RO2+NO2 by a factor of ∼20. However, this
shift is still too small relative to the equilibrium constant itself. It can
be deduced by a simple calculation that for the generic acyl RO2
in this study in an OFR at 318 K (20 K higher than room temperature) with
NO2 of 1012 molecules cm-3 (a relatively low level in
typical OFR-iN2O experiments; Peng et al., 2018), ∼0.1 % of
the total amount of acyl RO2+ acyl RO2NO2
will be present in the form of acyl RO2. Even if acylperoxy nitrate
decomposition is 20 times faster than at room temperature and the formed acyl
RO2 can irreversibly react with NO and decrease the acylperoxy nitrate
concentration, this effect is small: typically up to an approximate 20 % decrease
in acylperoxy nitrate and usually negligible changes in NO and NO2.
The minor effect is due to (i) an acylperoxy concentration that is still very
low, (ii) an NO concentration that is much lower than NO2 and (iii) an acylperoxy nitrate
decomposition lifetime that is still of the order of
minutes.
As discussed above, high H2O, low UV and low OHRext are recommended
for keeping the atmospheric relevance of RO2 chemistry in OFRs.
These three requirements are also part of the requirements for attaining good
high-NO conditions in OFR185-iNO (the OFR185 mode with initial NO injection;
Peng and Jimenez, 2017). In addition to these three, an initial NO of
several tens of ppb is also needed to obtain a good high-NO condition in
OFR185-iNO. Under these conditions, RO2+NO dominates over
RO2+HO2 and hence RO2+OH; UV is low, the photochemical
age is typically ∼1 eq. days and the RO2 lifetime can be a
few seconds. Therefore, these conditions are a good fit for studying the
environments in relatively clean urban areas, such as Los Angeles during
CalNex-LA (Ortega et al., 2016), where NO is high enough that the dominant
bimolecular fate of RO2 is RO2+NO but low enough to
maintain RO2 lifetimes that allow for the most common RO2
isomerizations.
As RO2 fate in OFRs is a highly complex problem and it can be
tricky to find suitable physical conditions to simultaneously achieve
experimental goals and keep the atmospheric relevance of the chemistry in OFRs,
we provide here an OFR RO2 Fate Estimator (in the Supplement) to
qualitatively aid experimental planning. The OFR RO2 Fate Estimator
couples the OFR Exposure Estimator (Peng et al., 2016, 2018) to a general
RO2 fate estimator (also in the Supplement; see Fig. S12 for a
screenshot of its layout). The OFR Exposure Estimator updated in this study
also contains estimation equations for the HO2-to-OH ratio in
OFR185 (in OFR254, RO2 fate is always atmospherically irrelevant at
low NO, while at high NO, RO2+NO dominates and a detailed
RO2 fate analysis is no longer needed). In the general
RO2 fate estimator, all RO2 reactant concentrations and
all RO2 loss pathway rate constants can be specified. Thus, the
general RO2 fate estimator can also be applied to the atmosphere
and chamber experiments, in addition to OFRs. When applied to OFRs, the
general RO2 fate estimator is provided by the OFR RO2
Fate Estimator with quantities estimated in the OFR Exposure Estimator (e.g.,
OH and NO). RO2 concentration and fate are calculated according to
Appendix B in the RO2 fate estimators.