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.
PHOEBE was conducted during the Berlin Ozone Experiment (BERLIOZ) between 5 July and 7 August 1998 near the city of Berlin. It aimed at a quantitative understanding of the fast radical chemistry in ...rural and suburban air by simultaneous measurements of the major free radicals (OH, HO2, RO2, and NO3) and the chemical compounds and physical parameters that control the radical concentrations. Interpretation of the measurements involved models of different complexity. The methods deployed and the results are discussed in the subsequent publications of this special section. This paper outlines the aims of PHOEBE, describes the observational site at Pabstthum, and briefly summarizes the different measurements and instrument comparisons as well as the main results of the accompanying publications.
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.
For CareBeijing‐2006, two sites were established in urban and suburban regions of Beijing in summer 2006. Observations of O3 and its precursors together with meteorological parameters at both sites ...are presented. Gross ozone production rate P(O3) and sensitivity to nitric oxides (NOx) and volatile organic compounds (VOCs) were investigated using an observation‐based photochemical box model (OBM). P(O3) varied from nearly zero to 120 and 50 ppb h−1 for urban and suburban sites, respectively. These rates were greater than the accumulation rates of the observed oxidant (O3 + NO2) concentrations. The O3 episodes typically appeared under southerly wind conditions with high P(O3), especially at the urban site. Sensitivity studies with and without measured nitrous acid (HONO) as a model constraint suggested that the estimated P(O3) at both sites was strongly enhanced by radical production from HONO photolysis. Both NOx‐ and VOC‐sensitive chemistries existed over time scales from hours to days at the two sites. The variation in O3‐sensitive chemistry was relatively well explained by the ratio of the average daytime total VOC reactivity (kTVOC) to NO, with the transition chemistry corresponding to a kTVOC/NO value of 2–4 s−1 ppb−1. Pronounced diurnal variations in the O3 production regime were found. In the morning, conditions were always strongly VOC‐limited, while in the afternoon, conditions were variable for different days and different sites. The model‐calculated results were tested by measurements of H2O2, HNO3, total OH reactivity, and HOx radicals. The OBM was generally capable of correctly simulating the levels of P(O3), although it might tend to overpredict the VOC‐sensitive chemistry.
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.
The relationship between photolysis frequencies derived from spectroscopic measurements of actinic fluxes and irradiances was determined during a coordinated measurement campaign (International ...Photolysis Frequency Measurement and Modeling Intercomparison campaign (IPMMI)). When differences in viewing geometries are taken into account, the measurements are in close agreement. An empirical relationship, which is useful for high sun (noon) conditions or for daily integrals, was found to convert irradiance data to photolysis frequencies. For low‐sun conditions (large solar zenith angle), model calculations were shown to improve the accuracy. However, the input parameters to the model are site specific and the conversion depends on diffuse/direct ratios. During cloudy conditions, significant improvements in the conversion can be achieved by assuming the radiation field to comprise entirely diffuse isotropic radiation when the UVA transmission by cloud is less than 0.8. Changing cloud conditions remain the greatest limitation, but they tend to bias the results away from the clear‐sky case in a systematic way. Furthermore, although the cloud effects on the photolysis rates of nitrogen dioxide (J(NO2)) are rather large, they are much smaller for ozone photolysis (J(O3 → O(1D))), which is of prime importance in tropospheric chemistry. The study shows the potential for deriving historical and geographical differences in actinic fluxes from the extensive records of ground‐based measurements of spectral irradiance.
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.
The photooxidation of the most abundant monoterpene, α-pinene, by the hydroxyl radical (OH) was investigated at atmospheric concentrations in the atmospheric simulation chamber SAPHIR. ...Concentrations of nitric oxide (NO) were below 120 pptv. Yields of organic oxidation products are determined from measured time series giving values of 0.11±0.05, 0.19±0.06, and 0.05±0.03 for formaldehyde, acetone, and pinonaldehyde, respectively. The pinonaldehyde yield is at the low side of yields measured in previous laboratory studies, ranging from 0.06 to 0.87. These studies were mostly performed at reactant concentrations much higher than observed in the atmosphere. Time series of measured radical and trace-gas concentrations are compared to results from model calculations applying the Master Chemical Mechanism (MCM) 3.3.1. The model predicts pinonaldehyde mixing ratios that are at least a factor of 4 higher than measured values. At the same time, modeled hydroxyl and hydroperoxy (HO.sub.2) radical concentrations are approximately 25 % lower than measured values. Vereecken et al. (2007) suggested a shift of the initial organic peroxy radical (RO.sub.2) distribution towards RO.sub.2 species that do not yield pinonaldehyde but produce other organic products. Implementing these modifications reduces the model-measurement gap of pinonaldehyde by 20 % and also improves the agreement in modeled and measured radical concentrations by 10 %. However, the chemical oxidation mechanism needs further adjustment to explain observed radical and pinonaldehyde concentrations. This could be achieved by adjusting the initial RO.sub.2 distribution, but could also be done by implementing alternative reaction channels of RO.sub.2 species that currently lead to the formation of pinonaldehyde in the model.
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