The first wintertime in situ measurements of hydroxyl (OH), hydroperoxy (HO2) and organic peroxy (RO2) radicals (ROx=OH+HO2+RO2) in combination with observations of total reactivity of OH radicals, ...kOH in Beijing are presented. The field campaign “Beijing winter finE particle STudy – Oxidation, Nucleation and light Extinctions” (BEST-ONE) was conducted at the suburban site Huairou near Beijing from January to March 2016. It aimed to understand oxidative capacity during wintertime and to elucidate the secondary pollutants' formation mechanism in the North China Plain (NCP). OH radical concentrations at noontime ranged from 2.4×106cm-3 in severely polluted air (kOH∼27s-1) to 3.6×106cm-3 in relatively clean air (kOH∼5s-1). These values are nearly 2-fold larger than OH concentrations observed in previous winter campaigns in Birmingham, Tokyo, and New York City. During this campaign, the total primary production rate of ROx radicals was dominated by the photolysis of nitrous acid accounting for 46 % of the identified primary production pathways for ROx radicals. Other important radical sources were alkene ozonolysis (28 %) and photolysis of oxygenated organic compounds (24 %). A box model was used to simulate the OH, HO2 and RO2 concentrations based on the observations of their long-lived precursors. The model was capable of reproducing the observed diurnal variation of the OH and peroxy radicals during clean days with a factor of 1.5. However, it largely underestimated HO2 andRO2 concentrations by factors up to 5 during pollution episodes. The HO2 and RO2 observed-to-modeled ratios increased with increasing NO concentrations, indicating a deficit in our understanding of the gas-phase chemistry in the high NOx regime. The OH concentrations observed in the presence of large OH reactivities indicate that atmospheric trace gas oxidation by photochemical processes can be highly effective even during wintertime, thereby facilitating the vigorous formation of secondary pollutants.
Traditionally, tropospheric radical chemistry is discussed in terms of the daytime photochemically produced hydroxyl radical (OH). Radicals, however, are also important during nighttime: this is ...especially true for ozone and the nitrate radical (NO3), which both act as key initiators of the degradation of alkenes such as biogenic monoterpenes. These reactions lead to the formation of peroxy radicals (HO2 and RO2) and hydroxyl radicals at night. We present recent observations of nighttime concentrations of NO3, RO2, HO2, and OH by differential optical absorption spectroscopy (DOAS), matrix isolation electron spin resonance (MIESR), laser‐induced fluorescence (LIF), and a chemical amplifier (CA) in the framework of the Berliner Ozonexperiment (BERLIOZ) campaign at Pabstthum, Germany, together with modeling studies of nocturnal radical chemistry. Modeled RO2 mixing ratios reached 40 ppt while the measured ROx level went up to 22 ppt at the same time. Modeled and measured HO2 mixing ratios were up to 6 and 4 ppt, respectively. In the case of OH, a nocturnal concentration of (1.85 ± 0.82) × 105 cm−3 was measured during one night. At this time, the model yielded an OH level of (4.1 ± 0.7) × 105 cm−3. This overestimation by the model could point to a missing nocturnal sink of OH. Nitrate radical reactions with terpenes were found responsible for producing 77% of the RO2 radicals, 53% of the HO2, and 36% of the OH radicals during night. Nighttime ozonolysis formed 12% of the RO2, 47% of the HO2, and 64% of the OH radicals. Another 11% of the RO2 radicals were formed by OH–volatile organic compound (VOC) reactions. A positive linear correlation of RO2 and NO3 was observed and could be reproduced in model calculations originating from the loss of both radicals by reaction with NO and the NO3‐initiated RO2 production. The contribution of nighttime OH to the atmosphere's oxidation capacity (oxidation rate of VOCs, CO, and CH4) was found negligible (<0.5%).
A comprehensive field campaign was carried out in summer 2014 in Wangdu, located in the North China Plain. A month of continuous OH, HO2 and RO2 measurements was achieved. Observations of radicals by ...the laser-induced fluorescence (LIF) technique revealed daily maximum concentrations between (5-15) × 106cm-3, (3-14) × 108cm-3 and (3-15) × 108cm-3 for OH, HO2 and RO2, respectively. Measured OH reactivities (inverse OH lifetime) were 10 to 20s-1 during daytime. The chemical box model RACM 2, including the Leuven isoprene mechanism (LIM), was used to interpret the observed radical concentrations. As in previous field campaigns in China, modeled and measured OH concentrations agree for NO mixing ratios higher than 1ppbv, but systematic discrepancies are observed in the afternoon for NO mixing ratios of less than 300pptv (the model-measurement ratio is between 1.4 and 2 in this case). If additional OH recycling equivalent to 100pptv NO is assumed, the model is capable of reproducing the observed OH, HO2 and RO2 concentrations for conditions of high volatile organic compound (VOC) and low NOx concentrations. For HO2, good agreement is found between modeled and observed concentrations during day and night. In the case of RO2, the agreement between model calculations and measurements is good in the late afternoon when NO concentrations are below 0.3ppbv. A significant model underprediction of RO2 by a factor of 3 to 5 is found in the morning at NO concentrations higher than 1ppbv, which can be explained by a missing RO2 source of 2ppbv h-1. As a consequence, the model underpredicts the photochemical net ozone production by 20ppbv per day, which is a significant portion of the daily integrated ozone production (110ppbv) derived from the measured HO2 and RO2. The additional RO2 production from the photolysis of ClNO2 and missing reactivity can explain about 10% and 20% of the discrepancy, respectively. The underprediction of the photochemical ozone production at high NOx found in this study is consistent with the results from other field campaigns in urban environments, which underlines the need for better understanding of the peroxy radical chemistry for high NOx conditions.
At the atmosphere simulation chamber SAPHIR in Jülich both Laser-Induced Fluorescence Spectroscopy (LIF) and Long-Path Differential Optical Laser Absorption Spectroscopy (DOAS) are operational for ...the detection of OH radicals at tropospheric levels. The two different spectroscopic techniques were compared within the controlled environment of SAPHIR based on all simultaneous measurements acquired in 2003 (13 days). Hydroxyl radicals were scavenged by added CO during four of these days in order to experimentally check the calculated precisions at the detection limit. LIF measurements have a higher precision (σ= 0.88×10^sup 6^ cm^sup -3^) and better time resolution (Δt = 60 s), but the DOAS method (σ= 1.24×10^sup 6^ cm^sup -3^, Δt = 135 s) is regarded as primary standard for comparisons because of its good accuracy. A high correlation coefficient of r = 0.95 was found for the whole data set highlighting the advantage of using a simulation chamber. The data set consists of two groups. The first one includes 3 days, where the LIF measurements yield (1 - 2) ×10^sup 6^ cm^sup -3^ higher OH concentrations than observed by the DOAS instrument. The experimental conditions during these days are characterized by increased NO^sub x^ concentration and a small dynamic range in OH. Excellent agreement is found within the other group of 6 days. The regression to the combined data of this large group yields unity slope without a significant offset.PUBLICATION ABSTRACT
The oxidation of nitric oxide to nitrogen dioxide by hydroperoxy (HO2) and organic peroxy radicals (RO2) is responsible for the chemical net ozone production in the troposphere and for the ...regeneration of hydroxyl radicals, the most important oxidant in the atmosphere. In Summer 2014, a field campaign was conducted in the North China Plain, where increasingly severe ozone pollution has been experienced in the last years. Chemical conditions in the campaign were representative for this area. Radical and trace gas concentrations were measured, allowing for calculating the turnover rates of gas-phase radical reactions. Therefore, the importance of heterogeneous HO2 uptake on aerosol could be experimentally determined. HO2 uptake could have suppressed ozone formation at that time because of the competition with gas-phase reactions that produce ozone. The successful reduction of the aerosol load in the North China Plain in the last years could have led to a significant decrease of HO2 loss on particles, so that ozone-forming reactions could have gained importance in the last years. However, the analysis of the measured radical budget in this campaign shows that HO2 aerosol uptake did not impact radical chemistry for chemical conditions in 2014. Therefore, reduced HO2 uptake on aerosol since then is likely not the reason for the increasing number of ozone pollution events in the North China Plain, contradicting conclusions made from model calculations reported in the literature.
Theoretical, laboratory, and chamber studies have shown
fast regeneration of the hydroxyl radical (OH) in the photochemistry of isoprene,
largely due to unimolecular reactions which were previously ...thought not to
be important under atmospheric conditions. Based on early field
measurements, nearly complete regeneration was hypothesized for a wide range
of tropospheric conditions, including areas such as the rainforest where
slow regeneration of OH radicals is expected due to low concentrations of
nitric oxide (NO). In this work the OH regeneration in isoprene
oxidation is directly quantified for the first time through experiments
covering a wide range of atmospherically relevant NO levels (between 0.15
and 2 ppbv – parts per billion by volume) in the atmospheric simulation
chamber SAPHIR. These conditions cover remote areas partially influenced by
anthropogenic NO emissions, giving a regeneration efficiency of OH close to 1, and areas like the Amazonian rainforest with very low NO, resulting in
a surprisingly high regeneration efficiency of 0.5, i.e. a factor of 2 to 3
higher than explainable in the absence of unimolecular reactions. The
measured radical concentrations were compared to model calculations, and the
best agreement was observed when at least 50 % of the total loss of
isoprene peroxy radicals conformers (weighted by their abundance) occurs via
isomerization reactions for NO lower than 0.2 ppbv. For these levels of NO,
up to 50 % of the OH radicals are regenerated from the products of the 1,6
α-hydroxy-hydrogen shift (1,6-H shift) of Z-δ-RO2 radicals through the photolysis of an unsaturated hydroperoxy aldehyde (HPALD)
and/or through the fast aldehydic hydrogen shift (rate constant
∼10 s−1 at 300 K) in di-hydroperoxy carbonyl peroxy
radicals (di-HPCARP-RO2), depending on their relative yield. The
agreement between all measured and modelled trace gases (hydroxyl,
hydroperoxy, and organic peroxy radicals, carbon monoxide, and the sum of
methyl vinyl ketone, methacrolein, and hydroxyl hydroperoxides) is nearly
independent of the adopted yield of HPALD and di-HPCARP-RO2 as both
degrade relatively fast (<1 h), forming the OH radical and CO among
other products. Taking into consideration this and earlier isoprene studies,
considerable uncertainties remain on the distribution of oxygenated products,
which affect radical levels and organic aerosol downwind of unpolluted
isoprene-dominated regions.
Reactions of ozone with alkenes can be a significant source of hydroxyl radicals in the atmosphere. In the present paper, the formation of OH radicals in the ozonolysis of selected alkenes under ...atmospheric conditions was directly observed. The experiments were carried out in the European photoreactor EUPHORE (Valencia, Spain). OH radicals were quantitatively detected by means of laser-induced fluorescence (LIF) using a new analytical instrument, which has been constructed on the basis of an existing setup already established in field studies. The OH radicals observed resulted directly from the reaction of ozone with the corresponding alkene. There was no indication that OH radicals were produced in the system by secondary processes. The experimentally observed concentration−time profiles of OH and ozone were excellently described by chemical modeling using explicit reaction mechanisms. The following OH yields were derived: 2,3-dimethyl-2-butene: (1.00 ± 0.25); 2-methyl-2-butene: (0.89 ± 0.22); trans-2-butene: (0.75 ± 0.19); α-pinene: (0.91 ± 0.23). In addition, the experiments carried out were modeled using the Regional Atmospheric Chemistry Mechanism (RACM), an established condensed chemical model applied in tropospheric chemistry. For 2,3-dimethyl-2-butene, 2-methyl-2-butene, and trans-2-butene the calculated concentration−time profiles of OH and ozone are in quite good agreement with the experimental data. However, in the case of α-pinene, the model fails for the simulation of OH due to the high grade of mechanism condensation, which results in a poor characterization of the primary reaction products.
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 limonene by the hydroxyl (OH) radical
and ozone (O3) was investigated in the atmospheric simulation chamber
SAPHIR (Simulation of Atmospheric
PHotochemistry In a large Reaction ...Chamber) in experiments performed at different nitric oxide (NO) mixing ratios from nearly 0 up to 10 ppbv. For the experiments dominated by OH
oxidation, the formaldehyde (HCHO) yield was experimentally determined and found to be (12 ± 3), (13 ± 3), and (32 ± 5) % for
experiments with low (∼ 0.1 ppbv), medium (∼ 0.3 ppbv), and high NO (5 to 10 ppbv), respectively. The yield in an
ozonolysis-only experiment was (10 ± 1) %, which agrees with previous
laboratory studies. The experimental yield of the first-generation organic nitrates from limonene–OH oxidation is calculated as (34 ± 5) %,
about 11 % higher than the value in the Master Chemical Mechanism (MCM),
which is derived from structure–activity relationships (SARs). Time series of measured radicals, trace-gas concentrations, and OH reactivity are compared
to results from zero-dimensional chemical box model calculations applying
MCM v3.3.1. Modeled OH reactivity is 5 to 10 s−1 (25 % to 33 % of the OH reactivity at the start of the experiment) higher than measured
values at the end of the experiments under all chemical conditions investigated, suggesting either that there are unaccounted loss processes of
limonene oxidation products or that products are less reactive toward OH. In
addition, model calculations underestimate measured hydroperoxyl radical
(HO2) concentrations by 20 % to 90 % and overestimate organic
peroxyl radical (RO2) concentrations by 50 % to 300 %. The largest deviations are found in low-NO experiments and in the ozonolysis experiment.
An OH radical budget analysis, which uses only measured quantities, shows
that the budget is closed in most of the experiments. A similar budget
analysis for RO2 radicals suggests that an additional RO2 loss
rate constant of about (1–6) × 10−2 s−1 for
first-generation RO2 is required to match the measured RO2
concentrations in all experiments. Sensitivity model runs indicate that
additional reactions converting RO2 to HO2 at a rate constant of
about (1.7–3.0) × 10−2 s−1 would improve the
model–measurement agreement of NOx, HO2, and RO2 concentrations and OH reactivity. Reaction pathways that could lead to the production of
additional OH and HO2 are discussed, which include isomerization reactions of RO2 from the oxidation of limonene, different branching
ratios for the reaction of RO2 with HO2, and a faster rate
constant for RO2 recombination reactions. As the exact chemical
mechanisms of the additional HO2 and OH sources could not be
identified, further work needs to focus on quantifying organic product
species and organic peroxy radicals from limonene oxidation.
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 (HO2) radical concentrations are approximately 25 % lower than measured values. Vereecken et al. (2007) suggested a shift of the initial organic peroxy radical (RO2) distribution towards RO2 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 RO2 distribution, but could also be done by implementing alternative reaction channels of RO2 species that currently lead to the formation of pinonaldehyde in the model.
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