Three instruments that use different techniques to measure gaseous formaldehyde (HCHO) concentrations were compared in experiments in the atmospheric simulation chamber SAPHIR at Forschungszentrum ...Jülich. One instrument (AL4021, Aero-Laser GmbH) detects HCHO using the wet-chemical Hantzsch reaction (for efficient gas-phase stripping), chemical conversion and fluorescence measurement. An internal HCHO permeation source allows for daily calibrations. This instrument was characterized by sulfuric acid titration (overall accuracy 8.6 %) and yields measurements with a time resolution of 90 s and a limit of detection (3σ) of 0.3 ppbv. In addition, a new commercial instrument that makes use of cavity ring-down spectroscopy (CRDS) determined the concentrations of HCHO, water vapour, and methane (G2307, Picarro, Inc.). Its limit of detection (3σ) is specified as 0.3 ppbv for an integration time of 300 s, and its accuracy is limited by the drift of the zero signal (manufacturer specification 1.5 ppbv). A custom-built high-resolution laser differential optical absorption spectroscopy (DOAS) instrument provided HCHO measurements with a limit of detection (3σ) of 0.9 ppbv and an accuracy of 7 % using an optical multiple reflection cell. The measurements were conducted from June to December 2019 in experiments in which either ambient air flowed through the chamber or the photochemical degradation of organic compounds in synthetic air was investigated. Measured HCHO concentrations were up to 8 ppbv. Various mixtures of organic compounds, water vapour, nitrogen oxides and ozone were present in these experiments. Results demonstrate the need to correct the baseline in measurements performed by the Hantzsch instrument to compensate for drifting background signals. Corrections were equivalent to HCHO mixing ratios in the range of 0.5–1.5 ppbv. The baseline of the CRDS instrument showed a linear dependence on the water vapour mixing ratio with a slope of (-11.20±1.60) ppbv %−1 below and (-0.72±0.08) ppbv %−1 above a water vapour mixing ratio of 0.2 %. In addition, the intercepts of these linear relationships drifted within the specification of the instrument (1.5 ppbv) over time but appeared to be equal for all water mixing ratios. Regular zero measurements are needed to account for the changes in the instrument zero. After correcting for the baselines of measurements by the Hantzsch and the CRDS instruments, linear regression analysis of measurements from all three instruments in experiments with ambient air indicated good agreement, with slopes of between 0.98 and 1.08 and negligible intercepts (linear correlation coefficients R2>0.96). The new small CRDS instrument measures HCHO with good precision and is accurate if the instrument zero is taken into account. Therefore, it can provide measurements with similar accuracy to the DOAS instrument but with slightly reduced precision compared to the Hantzsch instrument.
The oxidation of Δ3-carene and one of its main oxidation products, caronaldehyde, by the OH radical and O3 was investigated in the atmospheric simulation chamber SAPHIR under atmospheric conditions ...for NOx mixing ratios below 2 ppbv.
Within this study, the rate constants of the reaction of Δ3-carene with OH and O3 and of the reaction of caronaldehyde with OH were determined to be (8.0±0.5)×10-11 cm3 s−1 at 304 K, (4.4±0.2)×10-17 cm3 s−1 at 300 K and (4.6±1.6)×10-11 cm3 s−1 at 300 K, in agreement with previously published values.
The yields of caronaldehyde from the reaction of OH and ozone with Δ3-carene were determined to be 0.30±0.05 and 0.06±0.02, respectively. Both values are in reasonably good agreement with reported literature values. An organic nitrate (RONO2) yield from the reaction of NO with RO2 derived from Δ3-carene of 0.25±0.04 was determined from the analysis of the reactive nitrogen species (NOy) in the SAPHIR chamber. The RONO2 yield of the reaction of NO with RO2 derived from the reaction of caronaldehyde with OH was found to be 0.10±0.02. The organic nitrate yields of Δ3-carene and caronaldehyde oxidation with OH are reported here for the first time in the gas phase. An OH yield of 0.65±0.10 was determined from the ozonolysis of Δ3-carene.
Calculations of production and destruction rates of the sum of hydroxyl and peroxy radicals (ROx=OH+HO2+RO2) demonstrated that there were no unaccounted production or loss processes of radicals in the oxidation of Δ3-carene for conditions of the chamber experiments.
In an OH-free experiment with added OH scavenger, the photolysis frequency of caronaldehyde was obtained from its photolytical decay. The experimental photolysis frequency was a factor of 7 higher than the value calculated from the measured solar actinic flux density, an absorption cross section from the literature and an assumed effective quantum yield of unity for photodissociation.
The oxidation of Î.sup.3 -carene and one of its main oxidation products, caronaldehyde, by the OH radical and O.sub.3 was investigated in the atmospheric simulation chamber SAPHIR under atmospheric ...conditions for NO.sub.x mixing ratios below 2 ppbv. Within this study, the rate constants of the reaction of Î.sup.3 -carene with OH and O.sub.3 and of the reaction of caronaldehyde with OH were determined to be (8.0±0.5)x10-11 cm.sup.3 s.sup.-1 at 304 K, (4.4±0.2)x10-17 cm.sup.3 s.sup.-1 at 300 K and (4.6±1.6)x10-11 cm.sup.3 s.sup.-1 at 300 K, in agreement with previously published values. The yields of caronaldehyde from the reaction of OH and ozone with Î.sup.3 -carene were determined to be 0.30±0.05 and 0.06±0.02, respectively. Both values are in reasonably good agreement with reported literature values. An organic nitrate (RONO.sub.2) yield from the reaction of NO with RO.sub.2 derived from Î.sup.3 -carene of 0.25±0.04 was determined from the analysis of the reactive nitrogen species (NO.sub.y) in the SAPHIR chamber. The RONO.sub.2 yield of the reaction of NO with RO.sub.2 derived from the reaction of caronaldehyde with OH was found to be 0.10±0.02. The organic nitrate yields of Î.sup.3 -carene and caronaldehyde oxidation with OH are reported here for the first time in the gas phase. An OH yield of 0.65±0.10 was determined from the ozonolysis of Î.sup.3 -carene. Calculations of production and destruction rates of the sum of hydroxyl and peroxy radicals (ROx=OH+HO2+RO2) demonstrated that there were no unaccounted production or loss processes of radicals in the oxidation of Î.sup.3 -carene for conditions of the chamber experiments. In an OH-free experiment with added OH scavenger, the photolysis frequency of caronaldehyde was obtained from its photolytical decay. The experimental photolysis frequency was a factor of 7 higher than the value calculated from the measured solar actinic flux density, an absorption cross section from the literature and an assumed effective quantum yield of unity for photodissociation.
The oxidation of Δ3-carene and one of its main oxidation products, caronaldehyde, by the OH radical and O3 was investigated in the atmospheric simulation chamber SAPHIR under atmospheric conditions ...for NOx mixing ratios below 2 ppbv. Within this study, the rate constants of the reaction of Δ3-carene with OH and O3 and of the reaction of caronaldehyde with OH were determined to be (8.0±0.5)×10-11 cm3s-1 at 304 K, (4.4±0.2)×10-17 cm3s-1 at 300 K and (4.6±1.6)×10-11 cm3s-1 at 300 K, in agreement with previously published values. The yields of caronaldehyde from the reaction of OH and ozone with Δ3-carene were determined to be 0.30±0.05 and 0.06±0.02, respectively. Both values are in reasonably good agreement with reported literature values. An organic nitrate (RONO2) yield from the reaction of NO with RO2 derived from Δ3-carene of 0.25±0.04 was determined from the analysis of the reactive nitrogen species (NOy) in the SAPHIR chamber. The RONO2 yield of the reaction of NO with RO2 derived from the reaction of caronaldehyde with OH was found to be 0.10±0.02. The organic nitrate yields of Δ3-carene and caronaldehyde oxidation with OH are reported here for the first time in the gas phase. An OH yield of 0.65±0.10 was determined from the ozonolysis of Δ3-carene. Calculations of production and destruction rates of the sum of hydroxyl and peroxy radicals (ROx=OH+HO2+RO2) demonstrated that there were no unaccounted production or loss processes of radicals in the oxidation of Δ3-carene for conditions of the chamber experiments. In an OH-free experiment with added OH scavenger, the photolysis frequency of caronaldehyde was obtained from its photolytical decay. The experimental photolysis frequency was a factor of 7 higher than the value calculated from the measured solar actinic flux density, an absorption cross section from the literature and an assumed effective quantum yield of unity for photodissociation.
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.
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.
PDF
