When based on known absorption cross sections, CRDS is an absolute
concentration measurement technique. However, a number of corrections are
necessary to convert measured ring-down times into volume mixing ratios of
absorbing trace gases. Some corrections are related to the physical
construction of the cavities, and others are related to chemical processes in
the inlets and cavities that can bias the concentrations measured. The size
and accuracy of these corrections contribute significantly to the overall
uncertainty of the measurements, and are discussed in detail below. The more
straightforward corrections needed are related to the effective absorption
path length (l-to-d ratio), losses of NO3 radicals in inlets and
filters as previously described (Schuster et al., 2009; Thieser et al., 2016)
and formation of NO2 during the sampling time through
O3 + NO reaction. For this instrument, the effective l-to-d
ratio is 0.69±0.02 for the channels detecting at 405 nm, and
0.77±0.04 for the channels detecting at 662 nm. Based on several
laboratory experiments, the transmission of NO3 in the filter/filter
holder is 70±3 %. The 3 % uncertainty was obtained by repeated
measurements under laboratory conditions. During the NOTOMO campaign (see
Sect. 4), no discontinuities in the NO3 signal were observed after
hourly filter changes, which implies that there was no measurable change in
transmission over the hour of exposure. In highly polluted environments, or
those with highly reactive aerosol, this may not be the case, and more frequent
filter changes may be necessary to avoid loss of NO3. No losses of
N2O5 on passage through the filter holder are observed under
laboratory conditions, and similarly to NO3, no evidence was obtained
for losses on aged filters during the NOTOMO campaign. The transmission of
NO3 in the NO3 and N2O5 channels is 97.4±2.5 % and 90.2±5 %, respectively. Details and results of the
experiments to derive l-to-d ratios and correction factors for
NO3 losses are given in the Supplement. Corrections
related to NO oxidation by O3 are described in Sect. 3.2.5.
NO2, ΣPNs and ΣANs channels
Losses of NO2 in the heated ΣPNs and ΣANs inlets
The transmission of NO2 through the hot glass inlets (with frit and
packed with glass beads) of the ΣPNs and ΣANs channels was
investigated by sampling NO2 in synthetic air (up to 10 ppbv)
into the three 405 nm channels simultaneously. NO2 is depleted
slightly in the hot inlets with a transmission of 98.5±2 % and
95.5±2 % found for the ΣPNs and ΣANs inlets,
respectively (see Fig. 5). The NO2 transmission was measured before,
during and after the field deployment of this instrument (corresponding to a
period of several months), and was found to be constant and also independent
of relative humidity.
Reaction of radicals with NO / NO2 in the ΣPNs
channel (448 K)
The thermal decomposition of PNs leads to the formation of organic radicals
and NO2. In an ideal situation, in which PNs decompose 100 % to
NO2 and when NO2 does not undergo any further production or
loss reaction, the total [NO2] measured in the ΣPNs channel is
equal to the sum of ambient NO2 and PNs. Under the operating
conditions of the CRDS instrument described here, the total [NO2]
signal in the ΣPNs channel can, however, be biased by a number of
reactions initiated by the organic radicals. The influence of these reactions
has been described in the literature (Day et al., 2002; Paul and Osthoff,
2010; Thieser et al., 2016). Taking the example of peroxyacetyl nitrate
(PAN), subsequent to its thermal decomposition (Reaction R1a), there are
processes that lead both to the removal of NO2 (Reactions R1b, R5)
and to its formation (Reactions R2, R3, R4).
CH3C(O)O2NO2+M→CH3C(O)O2+NO2+MCH3C(O)O2+NO2+M→CH3C(O)O2NO2+M
CH3C(O)O2+NO(+O2)→NO2+CH3O2+CO2CH3O2+NO(+O2)→HCHO+HO2+NO2HO2+NO→OH+NO2OH+NO2+M→HNO3+M
Reactions (R2)–(R4) show that, in the most unfavourable scenario, if all
organic radicals react with NO, each peroxyacetyl radical can lead to the
formation of three NO2 molecules, biasing the result by the same factor.
Likewise, the presence of very high NO2 could conceivably result in
complete reformation of PAN (Reaction R1b). The size and sign of the bias
thus depends on the relative concentrations of NO and NO2 and, most
importantly, on the rate of loss of organic radicals to the reactor walls or
via thermal decomposition. The bias resulting from these reactions can be
reduced by minimising the residence time between thermal dissociation and
detection and by making pressure-dependent recombination reactions inefficient
by working at low pressures, e.g. by using LIF detection of NO2 (Day
et al., 2002; Wooldridge et al., 2010). As CRDS instruments operate at higher
pressures, to maintain sufficient sensitivity (typically from 0.5 to
1 bar), we have taken a different approach and optimised the surface
losses of the organic radicals by modifying the surface-to-volume ratio in
the heated inlets.
NO2 loss in the heated inlets of the ΣPNs and
ΣANs channels. Black points represent the NO2 mixing ratio
measured in the ΣPNs channel ([NO2]448 K)
normalised to the NO2 mixing ratio measured in the NO2
channel ([NO2]Amb). Red points represent the NO2
mixing ratio measured in the ΣANs channel
([NO2]648 K) normalised to the NO2 signal measured
in the NO2 channel.
First experiments on PAN samples using glass wool as a radical scavenger
resulted in the desired reduction in the rate of recombination of
CH3C(O)O2 with NO2. Glass wool was, however, observed to
greatly enhance (rather than reduce) the oxidation of NO to NO2. This
is presumably the result of a surface-catalysed process as previously
observed on powdered, aluminium silicate mineral dust samples (Hanisch and
Crowley, 2003), and may be related to the formation of oxidised surface sites
that can react with NO. This led us to test glass beads, which presumably
have less reactive “defective” sites than glass wool but a sufficient surface
area to remove a large fraction of the organic radicals (see below). We
conducted a series of experiments with different mixtures of PAN and
NO2 / NO (from 1 ppmv in synthetic air gas
bottles) and analysis of the observations by numerical simulation, similar to
that presented in Thieser et al. (2016).
The set of chemical reactions used in the numerical simulations is
essentially the same as that described in Thieser et al. (2016). One
exception is formation of NO2 via the reaction between O3 and
NO, which Thieser et al. (2016) treated as occurring independently of other
chemical processes. Here, this reaction is treated more rigorously by including
it in the chemical simulation. The other difference is the use of two
different values for a rate constant (ks) used to calculate the
kinetic limitation on the radical uptake coefficient (γ, see below).
In order to simulate all laboratory data sets, different values of
ks (one for sections A and B and one for sections C and D; see
Fig. S1 in Supplement) are required to account for the temperature gradient
in the heated sections of the inlets. The heterogeneous loss of radicals to
the reactor walls is based on a Langmuir–Hinshelwood mechanism, with the
first-order rate constant (kw) given by Eq. (2).
kw=γc‾A4
The uptake coefficient, γ, depends on both kinetic and diffusive
limitations as described in detail by Thieser et al. (2016). Here we focus on
the role of glass beads in enhancing the rate of uptake and reaction of
organic radicals by increasing the surface area available for reaction (A).
Without the glass beads, the value of A in the heated inlets is
3.5 cm2cm-3 (corresponding to a cylinder with internal diameter
1.2 cm). The presence of the glass beads, with an average diameter of
0.5 mm, results in a surface-to-volume ratio of
≈ 100 cm2cm-3. The exact determination of the
temperature profile in the inlet is possible in the case of a classic
cylindrical reactor, but is difficult to achieve with the presence of the
glass beads and glass frit. An approximate profile in section D of the
reactor (part of the cylindrical reactor between the fritted glass and the
cavity inlet; see Fig. S1) was obtained by measuring the temperature between
the fritted glass and the entrance of the cavity. It is assumed that the
temperature in section B (containing the glass beads) increases from ambient
to 448 K at the point where the gas enters section C (corresponding
to the fritted glass section). This profile is also used to calculate the
residence time in the ΣPNs channel of 1.3 s. To take into
account the variations of temperature, pressure, specific surface area and
diffusion radius along the glass reactor, a profile for each of these
parameters is used as input in the numerical simulation.
Figure 6a shows results of a set of four experiments in which different amounts
of PAN are mixed with different amounts of NO2 and monitored in the
three channels. The y axis shows the total [NO2] measured in the
ΣPNs channel ([NO2]448K) minus NO2
measured in NO2 channel ([NO2]Amb). The negative
slopes of these data indicate that NO2 from PAN decomposition is lost
at high mixing ratios of added NO2. A similar set of data, but with
added NO rather than NO2, is displayed in Fig. 6b. In this case, the
positive slopes indicate that NO2 is being formed from the reaction of
organic radicals with NO. Note that the solid lines are the results of the
numerical simulations (with identical mechanism) of NO2 formation and
loss in both experiments. The mixing ratios of PAN listed are those required
in the model to match the experimental data. In both data sets the slopes are
weakest at low PAN concentrations, reflecting the fact that radical-NO or
radical-NO2 reaction rates will be dependent on the radical
concentrations. The mixing ratios of PAN cannot be derived by simple back
extrapolation to zero NO2 or NO as this does not take into account
the NO2 formed when PAN itself decomposes. At the highest mixing
ratios of NO2, up to 25 % of the peroxyacetyl radical recombines
with NO2 while the other 75 % is lost to the walls or did not
react with NO2.
The analysis of the measurements shows that (without correction) NO2
would be overestimated by approximately 40 % for 3 ppb of PAN
when sampling air containing 8 ppbv of NO. This is much lower that
the ≈ 150 % reported by Thieser et al. (2016) for similar
conditions in their ΣPNs channel, reflecting the increased loss of
radicals on the glass surfaces.
In both series of experiments, the model reproduces the data for the whole
range of [PAN], [NO2] and [NO] explored. A set of experiments was
carried out to investigate whether the formation (or loss) of NO2
described above could be influenced by the presence of water vapour acting, for example, as a quencher of a surface reaction. A mixture of PAN plus NO2
was humidified to 60 % RH, with other operating conditions similar to the
previous experiments. The numerical model reproduced the measurement data
without the need to modify the wall loss rates or add any chemical processes
involving H2O, suggesting that the gas and surface reactions taking
place are not significantly influenced by adsorbed water at these
temperatures. In order to correct field data for these biases, an iterative
fitting procedure is used and described in detail in Sect. 4.2.
Modelled (lines) and measured difference between the NO2
signal in the hot channels and ambient temperature channel for different PAN
samples and different added amounts of NO and NO2.
(a) Addition of NO2 to the 448 K (PNs) channel.
(b) Addition of NO to the 448 K (PNs) channel.
(c) Addition of NO2 to the 648 K (ANs) channel.
(d) Addition of NO2 to the 648 K (ANs) channel.
Reaction of radicals with NO / NO2 in the ΣANs
channel (648 K)
As first discussed by Thieser al. (2016), experimental and theoretical
studies show that the higher temperature (648 K) of the ΣANs
channel changes the chemical processes substantially compared to
448 K. ANs can now decompose to NO2 and an alkyl radical
fragment, whereas the peroxyacetyl radical (from PAN decomposition) is
thermally unstable. The NO2 generated in the ΣANs channel when
adding NO2 / NO to PAN samples is displayed in Fig. 6c and d. The
results show a greatly reduced effect of NO2 recombination or NO
oxidation compared to the ΣPNs channel (Fig. 6a, b), largely resulting
from the instability of the of CH3C(O)O2 radical. The model takes
into account two main pathways for the fate of the peroxyacetyl radical
(Reactions R6, R7) in which the acetyl radical (CH3CO) formed in
Reaction (R6) can react further with O2 to reform a peroxyacetyl
radical or an OH radical and an α-lactone (Reactions R8, R9) (Carr et
al., 2011). CH2C(O)OOH can also decompose to OH and an
α-lactone (Reaction R10) (Carr et al., 2007, 2011; Chen and Lee,
2010). If not lost to the walls, OH can react with NO2 to form
HNO3 according to Reaction (R5).
CH3C(O)O2+M→CH3CO+O2+MCH3C(O)O2→CH2C(O)OOHCH3CO+O2+M→CH3C(O)O2+MCH3CO+O2→OH+C2H2O2CH2C(O)OOH→OH+C2H2O2CH3CO+O2+M→CH3O2+CO+M
The acetyl radical can also form a methylperoxy radical CH3O2
through the formation of a CH3 radical and CO. The net reaction is
given by Reaction (R11). In the presence of NO, the peroxy radicals
(CH3C(O)O2, CH3O2 and HO2), if not lost to the
glass surface, can lead to the formation of NO2. To simulate the
reactions in the ΣANs channel, the temperature profile was determined
in the same way as the ΣPNs channel profile. Due to the higher gas
flow velocity at the elevated temperatures, the pressure profile is slightly
different than in the ΣPNs channel and gives a cavity pressure that is
about 10 mbar lower. The results of the experiments involving
addition of NO2 to PAN samples in the ANs channel are presented in
Fig. 6c. For the range of PAN concentrations covered, the loss of NO2
by chemical recombination is about 5 % of the initial PAN. The initial
PAN concentration used as input variable for the numerical simulations is,
for all four experiments, about 3–5 % higher than that needed to fit the
data obtained in the PNs channel (Fig. 6a), which is most probably related to
model uncertainties. Since this 3–5 % difference is the same for all
four experiments and is apparently not correlated with the amount of PAN, it
is added to the calculation of total uncertainty for the PNs measurement. The
results of the NO addition experiments are shown in Fig. 6d. The thermal
decomposition of the peroxyacetyl radical reduces the positive bias due to NO
oxidation so that a maximum factor of only 1.04 (compared to 1.40 in the
ΣPNs channel) is obtained. The initial PAN concentration required to
fit the NO addition data set at 648 K is 5–10 % higher than at
448 K, which is related to uncertainty in the [NO] mixing ratio used
or a bias in the simulation.
In order to investigate the role of organic (alkyl) radicals generated from
the thermal decomposition of ANs, a set of experiments was conducted in which
a sample of i-propyl nitrate (C3H7ONO2) in synthetic air was mixed
with different amounts of NO and NO2. Thermal decomposition of
i-propyl nitrate (Reaction R12) is followed by Reactions (R13) and (R14)
that generate HO2 and CH3O2, which can convert NO to
NO2 and sequester NO2 as HO2NO2 and
CH3O2NO2.
C3H7ONO2→C3H7O+NO2C3H7O+O2→CH3C(O)CH3+HO2C3H7O+M→CH3+CH3CHO+M
Figure 7a and b show that the addition of NO increases the amount of
NO2 formed per AN, whereas the presence of NO2 results in a
negative bias to the data. These effects are captured well for both data sets
by the model simulations (blue lines).
Modelled (lines) and measured difference between the NO2
signal in the 648 K channels and ambient temperature channel for i-propyl
nitrate (IPN) samples and different added amounts of NO2 (a)
or NO (b).
Influence of O3 pyrolysis. Loss of NO2 (in ppbv) in
the heated ΣANs channel compared to the NO2 channel vs. the
product of the NO2 and O3 mixing ratios.
Effect of thermal decomposition of O3
At high temperatures, O3 decomposes according to Reaction (R15).
Although most of the O atom produced reacts with O2 to reform ozone
(Reaction R16), it can form NO+O2 in the presence of NO2
(Reaction R17).
O3→O2+OO+O2+M→O3+MO+NO2→O2+NO
The importance of this process is strongly dependent on the operating
conditions of the instrument, recent studies showing that higher temperatures
or pressures may result in a significant, negative bias from the ozone-pyrolysis-initiated reduction of NO2 (Lee et al., 2014; Thieser et
al., 2016). Assuming that the steady-state concentration of the O atom in the
heated inlets is dominated by Reactions (R15) and (R16)
(k16[O2]≫k17[NO2]), the loss of NO2 can be
approximated as follows:
-dNO2=NO2O3k15k17k16O2t.
The term in brackets is constant at constant temperature, pressure and flow
rate through the heated inlet, and can be determined experimentally by
sampling different mixtures of NO2 and O3 simultaneously
through the ambient temperature channel and the two hot channels. Figure 8
shows the difference [NO2]Amb-[NO2]648 K
(in ppbv) as a function of [NO2]×[O3] for three
different levels of ozone (50, 115 and 185 ppbv). A linear fit
through the data points yields a slope of 2.54±0.26×10-4 ppbv-1, which is < 50 % of the value found by
Thieser et al. (2016). The difference can be attributed mainly to the use of
a lower oven temperature (648 K in this work instead of
723 K). No reduction of NO2 is observed in the ΣPNs
channel because thermal decomposition of O3 at 448 K is too
slow. To illustrate the impact of the thermal dissociation of O3 in
the ANs channel, we note that 5 ppbv of total NO2 in the
presence of 50 ppbv of O3 results in the removal of
75 pptv NO2. Since the amount of O3 decomposed and NO
produced in this process is negligible compared to ambient amounts, it does
not affect the input conditions for the numerical simulations, and can
therefore be treated separately using Eq. (3).
Production of NO2 from the O3+NO reaction when
sampling into the three 405 nm channels with different amounts of NO and
O3. The numbers in parentheses are the slopes of least-squares fits
(solid lines), which correspond to the product of the effective second-order
rate constant and the reaction time.
Effect of NO oxidation by O3
A positive bias to the measurement of NO2 may result from NO
oxidation by O3 during the time it takes for the sampled air to flow
through the inlets and cavities of the instrument (Reaction R18).
NO+O3→NO2
The rate coefficient of this reaction is temperature-dependent, and the effect
is larger at 448 and 648 K than at room temperature. To investigate
this interference, different mixtures of O3 and NO were sampled
simultaneously through the three channels. The results are shown in Fig. 9 where
data from four experiments with different O3 mixing ratios (41, 62,
80 and 131 ppbv) are plotted. The solid lines are fits to the data,
the slopes of which are second-order rate constants multiplied by a reaction
time (cm3molecule-1). The rate constant for the gas-phase
reaction between NO and O3 is listed as kO3+NO=9×10-19exp(-850/T)×T2.25 cm3molecule-1s-1
(Atkinson et al., 2004). For the ΣPNs channel (448 K), using
this expression results in an underestimation of the effective (measured)
production of NO2 by a factor 1.06. For the ΣANs
(648 K) channel the equivalent factor is 1.52. The most likely reason
for this discrepancy is that some NO is oxidised in a surface catalysed
process, presumably involving surface sites that are activated following
interaction with O3. To account for this extra source of NO2,
the O3 plus NO reaction is implemented in the chemical model (with
modified rate expressions) for the 448 and 648 K data corrections (see
Sect. 3.2.2 and 3.2.3), and corrected manually for the NO2 channel.
Detection of other species as NO2 after thermal
dissociation
Atmospheric trace gases other than PNs and ANs can potentially be detected as
NO2 in this instrument. As thermal decomposition of N2O5 to
NO3 and NO2 has been shown to be 100 % efficient in our
383 K inlet in the 662 nm channels, we expect 100 % dissociation at the
higher temperatures of the ΣPNs and ΣANs channels. This may
represent a significant source of bias during the night when N2O5
mixing ratios can be large. However, as the five-channel instrument
simultaneously monitors N2O5, this can be easily corrected. Here we
consider possible HNO3 and ClNO2 interferences in the
ΣANs channel.
HNO3 is a major reservoir of tropospheric NOx. Although
HNO3 is thermally stable at temperatures below 700 K, Wild et
al. (2014) report ≈ 95 % conversion of HNO3 to
NO2 at 648 K, whereas Thieser et al. (2016) found about
10 % at 723 K. In order to test for unwanted detection of
HNO3 when sampling from the hot inlets (with glass beads) in the
five-channel instrument, we injected a sample of HNO3 in synthetic air
directly prior to the flow division before entering the three 405 nm
channels. In this way, possible losses of HNO3 in the inlet in front
of the heated sections of the instrument were minimised. HNO3 was
generated in a custom-built permeation source, which was calibrated by
absorption spectroscopy at 184.85 nm, where HNO3 absorbs
strongly. The permeation source generated several ppmv of HNO3 in a
flow of 10 sccm, which, following dilution, provided a mixing ratio
of ≈ 30 ppbv at the CRDS inlet. The permeation source also
generated about 3 ppbv of NO2. Figure 4 (open circles) shows
that NO2 arising from HNO3 decomposition could not be
observed in either heated inlet, enabling us to set an upper limit to the
decomposition efficiency of HNO3 to NO2 of < 0.5 %.
This is in broad agreement with the previous measurement by Thieser et
al. (2016), who saw no evidence for HNO3 decomposition below
650 K, and is in stark contrast to that reported by Wild et
al. (2014). While we have no rigorous explanation of this, we note that the
gas-phase thermal dissociation of HNO3 to NO2 at the
temperature of these experiments is too slow to explain its formation, which
suggests that it is probably surface-catalysed in the experiments of Wild et
al. (2014), implying that different glass types or chemical history of the
surface may influence the formation of NO2 significantly.
As reported previously, ClNO2, formed in the atmosphere in the
reaction between N2O5 and chloride-containing particles, can be
detected as NO2 in TD-CRDS instruments (Thaler et al., 2011; Wild et
al., 2014; Thieser et al., 2016) The ClNO2 decomposition efficiency
as a function of the oven temperature is plotted in Fig. 4. The ClNO2
sample was generated by flowing Cl2 in synthetic air over sodium
nitrite crystals. In normal operating conditions (at 648 K), 90±3 % of the ClNO2 is decomposed to NO2. As ClNO2
mixing ratios are highly variable and can approach ppbv levels, ClNO2
may thus represent a serious limitation to ANs measurements, unless
independent measurements are available to enable correction.
Detection limit and total uncertainty for NO2, PNs and
ANs
The detection limit for the 405 nm channels can be estimated in a similar
manner to that described for the 662 nm channels. The 2σ standard
deviation for consecutive zeros for the NO2, ΣPNs and
ΣANs channels are respectively 59, 74 and 54 pptv. Using these
values, we obtain detection limits for [NO2], [PNs] and [ANs] of 59,
94 and 80 pptv. The detection limits for [PNs] and [ANs] are obtained
by error propagation on the NO2 and PNs channels and NO2 and
ANs channel (see Sect. 4).
The total uncertainty associated with the [NO2] measurement is a
combination of the uncertainties in the l-to-d ratio, humidity matching
of the zero and ambient air and the correction for the NO+O3 reaction
on the absorption cross section of NO2 and on N2O5
decomposition. The total uncertainty on the absorption cross section is
estimated to 6 %, taking into account the error on the reference
cross section (Voigt et al., 2002) and fluctuation in the laser emission
spectrum. This value, when combined with the uncertainty associated with the
l-to-d ratio, results in 6.5 % uncertainty. The uncertainty
associated with the correction for NO2 formation in the NO+O3 reaction depends on ambient ozone and NO levels. Considering 10 %
uncertainty for kO3+NO and 5 % uncertainty for [O3],
[NO] and [NO2], [NO] and [O3] ambient levels of 1, 0.5 and
50 ppbv, we obtain an uncertainty for this correction of 0.1 % on
the final [NO2] value. NO2 can be formed at room temperature
by the slow thermal decomposition of N2O5. Taking a decomposition
rate constant of 4.4×10-2 s-1 at 303 K for
N2O5 (Atkinson et al., 2016) and a residence time in the NO2
channel of 1.7 s, we calculate that 0.08 of the N2O5 can
decompose. The largest N2O5-to-NO ratio measured during NOTOMO was
0.17, which results in a maximum contribution of ≈ 1.5 %
NO2 from N2O5 decomposition. As mentioned in Sect. 2.2, the
upper limit for bias caused by humidity matching errors when zeroing is
10 pptv, which results in a final uncertainty for [NO2] of
8 % + 10 pptv.
The uncertainty of the total NO2 detected in the ΣPNs and
ΣANs channels is a combination of the same uncertainties involved in
the [NO2] measurement plus the uncertainty of the NO2
transmission through the heated glass beads. Taking the values listed in
Sect. 3.2.1, we obtain an uncertainty for total NO2 detected of
7 %pptv for both ΣPNs and ΣANs channels. To
correct for the radical plus NOx biases, the total NO2
signals measured in the two hot channels are used to constrain the
chemical model described earlier. The uncertainty arising from these
calculations depends on the total [NO2] measured in each channel, and
is thus highly variable. Note that uncertainty arising from the humidity
difference is cancelled out by the subtraction to obtain [PNs] and [ANs]. The
uncertainty added by the calculation of the ambient [PNs] and [ANs] depends
on the model uncertainties, as well as on the uncertainty associated with the
trace gas concentrations involved in this calculation. The uncertainty
associated with [ΣPNs] and [ΣANs] is related to ambient
[NO2], [O3] and [NO]. The fact that the corrections rely on
PAN and IPN chemistry led Thieser et al. (2016) to estimate the uncertainty
associated with the model-derived corrections to be 30 % (max) of the
overall correction factor. The final uncertainties on the [ΣPNs] and
[ΣANs] value depend on the total NO2 signal in all three
channels, and can vary significantly. Below, we discuss the correction factors
required for the NO2, ΣPNs and ΣANs data sets obtained
during the first field deployment of the instrument.
(a) Mixing ratios of NO2, NO3,
N2O5, ΣPNs and ΣANs measured by the TD-CRDS instrument
between 30 June and 7 July 2015 at the Kleiner Feldberg observatory, Germany,
as part of the NOTOMO campaign. For the three lowest panels in
([NO2], [NO3] and [N2O5], the black points represent
the final corrected data. For the ΣPNs panel, the black points
correspond to uncorrected data obtained by subtracting [NO2] measured
in the NO2 channel from the NO2 measured in the ΣPNs
channel. The red points have been corrected using the chemical model with the
iterative fitting procedure. For the ΣANs panel, the black points
correspond to the uncorrected data (obtained by subtracting the NO2
signal measured in the ΣPNs channel from the NO2 signal
measured in the ΣANs channel). The red points have been corrected
using the chemical model plus iterative fitting procedure. Panels
(b) and (c) are frequency distributions for the correction
factors applied using the iterative numerical simulations.