Ambient measurements of nitryl chloride (ClNO2) were
performed at a rural site in Germany, covering three periods in winter,
summer, and autumn 2019, as part of the JULIAC campaign (Jülich
...Atmospheric Chemistry Project) that aimed to understand the
photochemical processes in air masses typical of midwestern Europe.
Measurements were conducted at 50 m aboveground, which was mainly located
in the nocturnal boundary layer and thus uncoupled from local surface
emissions. ClNO2 is produced at night by the heterogeneous reaction of
dinitrogen pentoxide (N2O5) on chloride (Cl−) that contains
aerosol. Its photolysis during the day is of general interest, as it produces chlorine (Cl) atoms that react with different atmospheric trace gases to form radicals. The highest-observed ClNO2 mixing ratio
was 1.6 ppbv (parts per billion by volume; 15 min average) during the night of 20 September. Air masses reaching the measurement site either originated from long-range transport from the southwest and had an oceanic influence or circulated in the nearby region and were influenced by anthropogenic activities. Nocturnal maximum ClNO2 mixing ratios were around 0.2 ppbv if originating from long-range transport in nearly all seasons, while the values were higher, ranging from 0.4 to 0.6 ppbv for regionally influenced air. The chemical composition of long-range transported air was similar in all investigated seasons, while the regional air exhibited larger differences between the seasons. The N2O5 necessary for ClNO2 formation comes from the reaction of nitrate radicals (NO3) with nitrogen dioxide (NO2), where NO3
itself is formed by a reaction of NO2 with ozone (O3). Measured
concentrations of ClNO2, NO2, and O3 were used to quantify
ClNO2 production efficiencies, i.e., the yield of ClNO2 formation
per NO3 radical formed, and a box model was used to examine the
idealized dependence of ClNO2 on the observed nocturnal O3 and
NO2 concentrations. Results indicate that ClNO2 production
efficiency was most sensitive to the availability of NO2 rather than
that of O3 and increased with decreasing temperature. The average
ClNO2 production efficiency was highest in February and September, with values of 18 %, and was lowest in December, with values of 3 %. The average ClNO2 production efficiencies were in the range of 3 % and 6 % from August to November for air masses originating from long-range transportation. These numbers are at the high end of values reported in the literature, indicating the importance of ClNO2 chemistry in rural environments in midwestern Europe.
The photooxidation of pinonaldehyde, one product of the α-pinene degradation, was investigated in the atmospheric simulation chamber SAPHIR under natural sunlight at low NO concentrations (<0.2 ppbv) ...with and without an added hydroxyl radical (OH) scavenger. With a scavenger, pinonaldehyde was exclusively removed by photolysis, whereas without a scavenger, the degradation was dominated by reaction with OH. In both cases, the observed rate of pinonaldehyde consumption was faster than predicted by an explicit chemical model, the Master Chemical Mechanism (MCM, version 3.3.1). In the case with an OH scavenger, the observed photolytic decay can be reproduced by the model if an experimentally determined photolysis frequency is used instead of the parameterization in the MCM. A good fit is obtained when the photolysis frequency is calculated from the measured solar actinic flux spectrum, absorption cross sections published by Hallquist et al. (1997), and an effective quantum yield of 0.9. The resulting photolysis frequency is 3.5 times faster than the parameterization in the MCM. When pinonaldehyde is mainly removed by reaction with OH, the observed OH and hydroperoxy radical (HO2) concentrations are underestimated in the model by a factor of 2.
Using measured HO2 as a model constraint brings modeled and measured OH concentrations into agreement. This suggests that the chemical mechanism includes all relevant OH-producing reactions but is missing a source for HO2. The missing HO2 source strength of (0.8 to 1.5) ppbv h−1 is similar to the rate of the pinonaldehyde consumption of up to 2.5 ppbv h−1. When the model is constrained by HO2 concentrations and the experimentally derived photolysis frequency, the pinonaldehyde decay is well represented. The photolysis of pinonaldehyde yields 0.18 ± 0.20 formaldehyde molecules at NO concentrations of less than 200 pptv, but no significant acetone formation is observed. When pinonaldehyde is also oxidized by OH under low NO conditions (maximum 80 pptv), yields of acetone and formaldehyde increase over the course of the experiment from 0.2 to 0.3 and from 0.15 to 0.45, respectively. Fantechi et al. (2002) proposed a degradation mechanism based on quantum-chemical calculations, which is considerably more complex than the MCM scheme and contains additional reaction pathways and products. Implementing these modifications results in a closure of the model–measurement discrepancy for the products acetone and formaldehyde, when pinonaldehyde is degraded only by photolysis. In contrast, the underprediction of formed acetone and formaldehyde is worsened compared to model results by the MCM, when pinonaldehyde is mainly degraded in the reaction with OH. This shows that the current mechanisms lack acetone and formaldehyde sources for low NO conditions like in these experiments. Implementing the modifications suggested by Fantechi et al. (2002) does not improve the model–measurement agreement of OH and HO2.
The photooxidation of methyl vinyl ketone (MVK) was investigated in
the atmospheric simulation chamber SAPHIR for conditions at which organic
peroxy radicals (RO2) mainly reacted with NO (“high
NO” ...case) and for conditions at which other reaction channels could
compete (“low NO” case). Measurements of trace gas concentrations
were compared to calculated concentration time series applying the Master
Chemical Mechanism (MCM version 3.3.1). Product yields of methylglyoxal and
glycolaldehyde were determined from measurements. For the high NO
case, the methylglyoxal yield was (19 ± 3) % and the glycolaldehyde yield
was (65 ± 14) %, consistent with recent literature studies. For the low
NO case, the methylglyoxal yield reduced to (5 ± 2) % because
other RO2 reaction channels that do not form methylglyoxal became
important. Consistent with literature data, the glycolaldehyde yield of
(37 ± 9) % determined in the experiment was not reduced as much as
implemented in the MCM, suggesting additional reaction channels producing
glycolaldehyde. At the same time, direct quantification of OH radicals
in the experiments shows the need for an enhanced OH radical
production at low NO conditions similar to previous studies
investigating the oxidation of the parent VOC isoprene and methacrolein, the
second major oxidation product of isoprene. For MVK the
model–measurement discrepancy was up to a factor of 2. Product yields and
OH observations were consistent with assumptions of additional
RO2 plus HO2 reaction channels as proposed in literature for
the major RO2 species formed from the reaction of MVK with
OH. However, this study shows that also HO2 radical
concentrations are underestimated by the model, suggesting that additional
OH is not directly produced from RO2 radical reactions, but
indirectly via increased HO2. Quantum chemical calculations show that
HO2 could be produced from a fast 1,4-H shift of the second
most important MVK derived RO2 species (reaction rate constant
0.003 s−1). However, additional HO2 from this reaction
was not sufficiently large to bring modelled HO2 radical
concentrations into agreement with measurements due to the small yield of
this RO2 species. An additional reaction channel of the major
RO2 species with a reaction rate constant of
(0.006 ± 0.004) s−1 would be required that produces concurrently
HO2 radicals and glycolaldehyde to achieve model–measurement
agreement. A unimolecular reaction similar to the
1,5-H shift reaction
that was proposed in literature for RO2 radicals from MVK
would not explain product yields for conditions of experiments in this study.
A set of H-migration reactions for the main RO2 radicals were
investigated by quantum chemical and theoretical kinetic methodologies, but
did not reveal a contributing route to HO2 radicals or
glycolaldehyde.
Photochemical processes in ambient air were studied using the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich, Germany. Ambient air was continuously drawn into the chamber through a ...50 m high inlet line and passed through the chamber for 1 month in each season throughout 2019. The residence time of the air inside the chamber was about 1 h. As the research center is surrounded by a mixed deciduous forest and is located close to the city Jülich, the sampled air was influenced by both anthropogenic and biogenic emissions. Measurements of hydroxyl (OH), hydroperoxyl (HO2), and organic peroxy (RO2) radicals were achieved by a laser-induced fluorescence instrument. The radical measurements together with measurements of OH reactivity (kOH, the inverse of the OH lifetime) and a comprehensive set of trace gas concentrations and aerosol properties allowed for the investigation of the seasonal and diurnal variation of radical production and destruction pathways. In spring and summer periods, median OH concentrations reached 6 × 106 cm-3 at noon, and median concentrations of both HO2 and RO2 radicals were 3 × 108 cm-3. The measured OH reactivity was between 4 and 18 s-1 in both seasons. The total reaction rate of peroxy radicals with NO was found to be consistent with production rates of odd oxygen (Ox= NO2+ O3) determined from NO2 and O3 concentration measurements. The chemical budgets of radicals were analyzed for the spring and summer seasons, when peroxy radical concentrations were above the detection limit. For most conditions, the concentrations of radicals were mainly sustained by the regeneration of OH via reactions of HO2 and RO2 radicals with nitric oxide (NO). The median diurnal profiles of the total radical production and destruction rates showed maxima between 3 and 6 ppbv h-1 for OH, HO2, and RO2. Total ROX (OH, HO2, and RO2) initiation and termination rates were below 3 ppbv h-1. The highest OH radical turnover rate of 13 ppbv h-1 was observed during a high-temperature (max. 40 ∘C) period in August. In this period, the highest HO2, RO2, and ROX turnover rates were around 11, 10, and 4 ppbv h-1, respectively. When NO mixing ratios were between 1 and 3 ppbv, OH and HO2 production and destruction rates were balanced, but unexplained RO2 and ROX production reactions with median rates of 2 and 0.4 ppbv h-1, respectively, were required to balance their destruction. For NO mixing ratios above 3 ppbv, the peroxy radical reaction rates with NO were highly uncertain due to the low peroxy radical concentrations close to the limit of NO interferences in the HO2 and RO2 measurements. For NO mixing ratios below 1 ppbv, a missing source for OH and a missing sink for HO2 were found with maximum rates of 3.0 and 2.0 ppbv h-1, respectively. The missing OH source likely consisted of a combination of a missing inter-radical HO2 to OH conversion reaction (up to 2 ppbv h-1) and a missing primary radical source (0.5–1.4 ppbv h-1). The dataset collected in this campaign allowed analyzing the potential impact of OH regeneration from RO2 isomerization reactions from isoprene, HO2 uptake on aerosol, and RO2 production from chlorine chemistry on radical production and destruction rates. These processes were negligible for the chemical conditions encountered in this study.
Photochemical processes in ambient air were studied using the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich, Germany. Ambient air was continuously drawn into the chamber through a ...50 m high inlet line and passed through the chamber for 1 month in each season throughout 2019. The residence time of the air inside the chamber was about 1 h. As the research center is surrounded by a mixed deciduous forest and is located close to the city Jülich, the sampled air was influenced by both anthropogenic and biogenic emissions. Measurements of hydroxyl (OH), hydroperoxyl (HO.sub.2 ), and organic peroxy (RO.sub.2) radicals were achieved by a laser-induced fluorescence instrument. The radical measurements together with measurements of OH reactivity (k.sub.OH, the inverse of the OH lifetime) and a comprehensive set of trace gas concentrations and aerosol properties allowed for the investigation of the seasonal and diurnal variation of radical production and destruction pathways. In spring and summer periods, median OH concentrations reached 6 x 10.sup.6 cm.sup.-3 at noon, and median concentrations of both HO.sub.2 and RO.sub.2 radicals were 3 x 10.sup.8 cm.sup.-3 . The measured OH reactivity was between 4 and 18 s.sup.-1 in both seasons. The total reaction rate of peroxy radicals with NO was found to be consistent with production rates of odd oxygen (O.sub.x = NO.sub.2 + O.sub.3) determined from NO.sub.2 and O.sub.3 concentration measurements. The chemical budgets of radicals were analyzed for the spring and summer seasons, when peroxy radical concentrations were above the detection limit. For most conditions, the concentrations of radicals were mainly sustained by the regeneration of OH via reactions of HO.sub.2 and RO.sub.2 radicals with nitric oxide (NO). The median diurnal profiles of the total radical production and destruction rates showed maxima between 3 and 6 ppbv h.sup.-1 for OH, HO.sub.2, and RO.sub.2 . Total RO.sub.X (OH, HO.sub.2, and RO.sub.2) initiation and termination rates were below 3 ppbv h.sup.-1 . The highest OH radical turnover rate of 13 ppbv h.sup.-1 was observed during a high-temperature (max. 40 .sup." C) period in August. In this period, the highest HO.sub.2, RO.sub.2, and RO.sub.X turnover rates were around 11, 10, and 4 ppbv h.sup.-1, respectively. When NO mixing ratios were between 1 and 3 ppbv, OH and HO.sub.2 production and destruction rates were balanced, but unexplained RO.sub.2 and RO.sub.X production reactions with median rates of 2 and 0.4 ppbv h.sup.-1, respectively, were required to balance their destruction. For NO mixing ratios above 3 ppbv, the peroxy radical reaction rates with NO were highly uncertain due to the low peroxy radical concentrations close to the limit of NO interferences in the HO.sub.2 and RO.sub.2 measurements. For NO mixing ratios below 1 ppbv, a missing source for OH and a missing sink for HO.sub.2 were found with maximum rates of 3.0 and 2.0 ppbv h.sup.-1, respectively. The missing OH source likely consisted of a combination of a missing inter-radical HO.sub.2 to OH conversion reaction (up to 2 ppbv h.sup.-1) and a missing primary radical source (0.5-1.4 ppbv h.sup.-1). The dataset collected in this campaign allowed analyzing the potential impact of OH regeneration from RO.sub.2 isomerization reactions from isoprene, HO.sub.2 uptake on aerosol, and RO.sub.2 production from chlorine chemistry on radical production and destruction rates. These processes were negligible for the chemical conditions encountered in this study.
Photochemical processes in ambient air were studied using the atmospheric
simulation chamber SAPHIR at Forschungszentrum Jülich, Germany. Ambient
air was continuously drawn into the chamber through a ...50 m high inlet line
and passed through the chamber for 1 month in each season throughout 2019.
The residence time of the air inside the chamber was about 1 h. As the
research center is surrounded by a mixed deciduous forest and is located
close to the city Jülich, the sampled air was influenced by both
anthropogenic and biogenic emissions. Measurements of hydroxyl (OH),
hydroperoxyl (HO2), and organic peroxy (RO2) radicals were achieved
by a laser-induced fluorescence instrument. The radical measurements
together with measurements of OH reactivity (kOH, the inverse of the OH
lifetime) and a comprehensive set of trace gas concentrations and aerosol
properties allowed for the investigation of the seasonal and diurnal
variation of radical production and destruction pathways. In spring and
summer periods, median OH concentrations reached 6 × 106 cm−3 at noon, and median concentrations of both HO2 and RO2
radicals were 3 × 108 cm−3. The measured OH reactivity
was between 4 and 18 s−1 in both seasons. The total reaction rate of
peroxy radicals with NO was found to be consistent with production rates of
odd oxygen (Ox= NO2 + O3) determined from NO2 and
O3 concentration measurements. The chemical budgets of radicals were
analyzed for the spring and summer seasons, when peroxy radical
concentrations were above the detection limit. For most conditions, the
concentrations of radicals were mainly sustained by the regeneration of OH
via reactions of HO2 and RO2 radicals with nitric oxide (NO). The
median diurnal profiles of the total radical production and destruction
rates showed maxima between 3 and 6 ppbv h−1 for OH, HO2, and
RO2. Total ROX (OH, HO2, and RO2) initiation and
termination rates were below 3 ppbv h−1. The highest OH radical
turnover rate of 13 ppbv h−1 was observed during a high-temperature
(max. 40 ∘C) period in August. In this period, the highest
HO2, RO2, and ROX turnover rates were around 11, 10, and 4 ppbv h−1, respectively. When NO mixing ratios were between 1 and 3 ppbv,
OH and HO2 production and destruction rates were balanced, but
unexplained RO2 and ROX production reactions with median rates of
2 and 0.4 ppbv h−1, respectively, were required to
balance their destruction. For NO mixing ratios above 3 ppbv, the peroxy
radical reaction rates with NO were highly uncertain due to the low peroxy
radical concentrations close to the limit of NO interferences in the
HO2 and RO2 measurements. For NO mixing ratios below 1 ppbv, a
missing source for OH and a missing sink for HO2 were found with
maximum rates of 3.0 and 2.0 ppbv h−1, respectively. The
missing OH source likely consisted of a combination of a missing
inter-radical HO2 to OH conversion reaction (up to 2 ppbv h−1) and
a missing primary radical source (0.5–1.4 ppbv h−1). The dataset
collected in this campaign allowed analyzing the potential impact of OH
regeneration from RO2 isomerization reactions from isoprene, HO2
uptake on aerosol, and RO2 production from chlorine chemistry on
radical production and destruction rates. These processes were negligible
for the chemical conditions encountered in this study.
Several previous field studies have reported unexpectedly large concentrations of hydroxyl and hydroperoxyl radicals (OH and HO2, respectively) in forested environments that could not be explained by ...the traditional oxidation mechanisms that largely underestimated the observations. These environments were characterized by large concentrations of biogenic volatile organic compounds (BVOC) and low nitrogen oxide concentration. In isoprene-dominated environments, models developed to simulate atmospheric photochemistry generally underestimated the observed OH radical concentrations. In contrast, HO2 radical concentration showed large discrepancies with model simulations mainly in non-isoprene-dominated forested environments. An abundant BVOC emitted by lodgepole and ponderosa pines is 2-methyl-3-butene-2-ol (MBO), observed in large concentrations for studies where the HO2 concentration was poorly described by model simulations. In this work, the photooxidation of MBO by OH was investigated for NO concentrations lower than 200 pptv in the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich. Measurements of OH and HO2 radicals, OH reactivity (kOH), MBO, OH precursors, and organic products (acetone and formaldehyde) were used to test our current understanding of the OH-oxidation mechanisms for MBO by comparing measurements with model calculations. All the measured trace gases agreed well with the model results (within 15 %) indicating a well understood mechanism for the MBO oxidation by OH. Therefore, the oxidation of MBO cannot contribute to reconciling the unexplained high OH and HO2 radical concentrations found in previous field studies.
Precise and accurate hydroxyl radical (OH) measurements are essential to investigate mechanisms for oxidation and transformation of trace gases and processes leading to the formation of secondary ...pollutants like ozone (O3) in the troposphere. Laser-induced fluorescence (LIF) is
a widely used technique for the measurement of ambient OH radicals and was
used for the majority of field campaigns and chamber experiments. Recently,
most LIF instruments in use for atmospheric measurements of OH radicals
introduced chemical modulation to separate the ambient OH radical
concentration from possible interferences by chemically removing ambient OH
radicals before they enter the detection cell (Mao et al., 2012; Novelli
et al., 2014a). In this study, we describe the application and
characterization of a chemical modulation reactor (CMR) applied to the
Forschungszentrum Jülich LIF (FZJ-LIF) instrument in use at the atmospheric simulation chamber
SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction Chamber). Besides dedicated experiments in
synthetic air, the new technique was extensively tested during the
year-round Jülich Atmospheric Chemistry Project (JULIAC) campaign, in
which ambient air was continuously flowed into the SAPHIR chamber. It
allowed for performing OH measurement comparisons with differential optical
absorption spectroscopy (DOAS) and investigation of interferences in a large variety of chemical and meteorological conditions. Good agreement was
obtained in the LIF–DOAS intercomparison within instrumental accuracies (18 % for LIF and 6.5 % for DOAS) which confirms that the new chemical
modulation system of the FZJ-LIF instrument is suitable for measurement of
interference-free OH concentrations under the conditions of the JULIAC
campaign (rural environment). Known interferences from O3+H2O
and the nitrate radical (NO3) were quantified with the CMR in synthetic air in the chamber and found to be 3.0×105 and 0.6×105 cm−3, respectively, for typical ambient-air
conditions (O3=50 ppbv, H2O = 1 % and NO3=10 pptv). The interferences measured in ambient air during the JULIAC campaign in the summer season showed a median diurnal variation with a median maximum value of 0.9×106 cm−3 during daytime and a median minimum value of 0.4×106 cm−3 at night. The highest interference of 2×106 cm−3 occurred in a heat wave from 22 to 29 August, when the air temperature and ozone increased to 40 ∘C and 100 ppbv, respectively. All observed interferences could be fully explained by
the known O3+H2O interference, which is routinely corrected in FZJ-LIF measurements when no chemical modulation is applied. No evidence for an unexplained interference was found during the JULIAC campaign. A chemical model of the CMR was developed and applied to estimate the
possible perturbation of the OH transmission and scavenging efficiency by
reactive atmospheric trace gases. These can remove OH by gas phase reactions in the CMR or produce OH by non-photolytic reactions, most importantly by the reaction of ambient HO2 with NO. The interfering processes become relevant at high atmospheric OH reactivities. For the conditions of the JULIAC campaign with OH reactivities below 20 s−1, the influence on the
determination of ambient OH concentrations was small (on average: 2 %).
However, in environments with high OH reactivities, such as in a rain forest or megacity, the expected perturbation in the currently used chemical modulation reactor could be large (more than a factor of 2). Such
perturbations need to be carefully investigated and corrected for the proper
evaluation of OH concentrations when applying chemical scavenging. This
implies that chemical modulation, which was developed to eliminate
interferences in ambient OH measurements, itself can be subject to
interferences that depend on ambient atmospheric conditions.
Hydroxyl (OH) and peroxy radicals (HO.sub.2 and RO.sub.2) were measured in the Pearl River Delta, which is one of the most polluted areas in China, in autumn 2014. The radical observations were ...complemented by measurements of OH reactivity (inverse OH lifetime) and a comprehensive set of trace gases including carbon monoxide (CO), nitrogen oxides (NO.sub.x =NO, NO.sub.2) and volatile organic compounds (VOCs). OH reactivity was in the range from 15 to 80 s.sup.-1, of which about 50 % was unexplained by the measured OH reactants. In the 3 weeks of the campaign, maximum median radical concentrations were 4.5Ã10.sup.6 cm.sup.-3 for OH at noon and 3Ã10.sup.8 and 2.0Ã10.sup.8 cm.sup.-3 for HO.sub.2 and RO.sub.2, respectively, in the early afternoon. The completeness of the daytime radical measurements made it possible to carry out experimental budget analyses for all radicals (OH, HO.sub.2, and RO.sub.2) and their sum (RO.sub.x). The maximum loss rates for OH, HO.sub.2, and RO.sub.2 reached values between 10 and 15 ppbv h.sup.-1 during the daytime. The largest fraction of this can be attributed to radical interconversion reactions while the real loss rate of RO.sub.x remained below 3 ppbv h.sup.-1 . Within experimental uncertainties, the destruction rates of HO.sub.2 and the sum of OH, HO.sub.2, and RO.sub.2 are balanced by their respective production rates. In case of RO.sub.2, the budget could be closed by attributing the missing OH reactivity to unmeasured VOCs. Thus, the presumption of the existence of unmeasured VOCs is supported by RO.sub.2 measurements. Although the closure of the RO.sub.2 budget is greatly improved by the additional unmeasured VOCs, a significant imbalance in the afternoon remains, indicating a missing RO.sub.2 sink. In case of OH, the destruction in the morning is compensated by the quantified OH sources from photolysis (HONO and O.sub.3 ), ozonolysis of alkenes, and OH recycling (HO.sub.2 +NO). In the afternoon, however, the OH budget indicates a missing OH source of 4 to 6 ppbv h.sup.-1 . The diurnal variation of the missing OH source shows a similar pattern to that of the missing RO.sub.2 sink so that both largely compensate each other in the RO.sub.x budget. These observations suggest the existence of a chemical mechanism that converts RO.sub.2 to OH without the involvement of NO, increasing the RO.sub.2 loss rate during the daytime from 5.3 to 7.4 ppbv h.sup.-1 on average. The photochemical net ozone production rate calculated from the reaction of HO.sub.2 and RO.sub.2 with NO yields a daily integrated amount of 102 ppbv ozone, with daily integrated RO.sub.x primary sources being 22 ppbv in this campaign. The produced ozone can be attributed to the oxidation of measured (18 %) and unmeasured (60 %) hydrocarbons, formaldehyde (14 %), and CO (8 %). An even larger integrated net ozone production of 140 ppbv would be calculated from the oxidation rate of VOCs with OH if HO.sub.2 and all RO.sub.2 radicals react with NO. However, the unknown RO.sub.2 loss (evident in the RO.sub.2 budget) causes 30 ppbv less ozone production than would be expected from the VOC oxidation rate.
Direct detection of highly reactive, atmospheric hydroxyl radicals (OH) is widely accomplished by laser-induced fluorescence (LIF) instruments. The technique is also suitable for the indirect ...measurement of HO2 and RO2 peroxy radicals by chemical conversion to OH. It requires sampling of ambient air into a low-pressure cell, where OH fluorescence is detected after excitation by 308 nm laser radiation. Although the residence time of air inside the fluorescence cell is typically only on the order of milliseconds, there is potential that additional OH is internally produced, which would artificially increase the measured OH concentration. Here, we present experimental studies investigating potential interferences in the detection of OH and peroxy radicals for the LIF instruments of Forschungszentrum Jülich for nighttime conditions. For laboratory experiments, the inlet of the instrument was over flowed by excess synthetic air containing one or more reactants. In order to distinguish between OH produced by reactions upstream of the inlet and artificial signals produced inside the instrument, a chemical titration for OH was applied. Additional experiments were performed in the simulation chamber SAPHIR where simultaneous measurements by an open-path differential optical absorption spectrometer (DOAS) served as reference for OH to quantify potential artifacts in the LIF instrument. Experiments included the investigation of potential interferences related to the nitrate radical (NO3, N2O5), related to the ozonolysis of alkenes (ethene, propene, 1-butene, 2,3-dimethyl-2-butene,α-pinene, limonene, isoprene), and the laser photolysis of acetone. Experiments studying the laser photolysis of acetone yield OH signals in the fluorescence cell, which are equivalent to 0.05×106 cm-3 OH for a mixing ratio of 5 ppbv acetone. Under most atmospheric conditions, this interference is negligible. No significant interferences were found for atmospheric concentrations of reactants during ozonolysis experiments. Only for propene,α-pinene, limonene, and isoprene at reactant concentrations, which are orders of magnitude higher than in the atmosphere, could artificial OH be detected. The value of the interference depends on the turnover rate of the ozonolysis reaction. For example, an apparent OH concentration of approximately 1×106 cm-3 is observed when 5.8 ppbv limonene reacts with 600 ppbv ozone. Experiments with the nitrate radical NO3 reveal a small interference signal in the OH, HO2, and RO2 detection. Dependencies on experimental parameters point to artificial OH formation by surface reactions at the chamber walls or in molecular clusters in the gas expansion. The signal scales with the presence of NO3 giving equivalent radical concentrations of 1.1×105 cm-3 OH,1×107 cm-3 HO2, and 1.7×107 cm-3 RO2 per 10 pptv NO3.
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