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Theoretical, laboratory and chamber studies have shown fast regeneration of hydroxyl radical (OH) in the photochemistry of isoprene largely due to previously disregarded unimolecular reactions. 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 the isoprene oxidation is directly quantified for the first time through experiments covering a wide range of atmospheric conditions (i.e. NO between 0.15 and 2 ppbv and temperature between 25 and 41 °C) 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 one, 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 parts per billion (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 photolysis of an unsaturated hydroperoxy aldehyde (HPALD) and/or through the fast aldehyde 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 on the adopted yield of HPALD and di-HPCARP-RO2 as both degrade relatively fast (< 1 h), forming OH radical and CO among other products. Taking into consideration this and earlier isoprene studies, considerable uncertainties remain on the oxygenated products distribution, which affect radical levels and organic aerosol downwind of unpolluted isoprene dominated regions. [ABSTRACT FROM AUTHOR]

Hydroxyl (OH) and peroxy radicals (HO2, RO2) 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 CO, NOx and VOCs. OH reactivity was in the range between 15 s-1 and 80 s-1, of which about 50 % was unexplained by the measured OH reactants. In the three weeks of the campaign, maximum median radical concentrations were 4.5 × 106 cm-3 for OH at noon, and 3 × 108 cm-3 and 2.0 × 108 cm-3 for HO2 and RO2, 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, HO2, and RO2) and their sum (ROx). The maximum loss rates for OH, HO2, and RO2 reached values between 10 ppbv/h and 15 ppbv/h during daytime. The largest fraction of this can be attributed to radical interconveresio reactions while the real loss rate of ROx remained below 3 ppbv/h. Within experimental uncertainties, the destruction rates of HO2 and the sum of OH, HO2, and RO2 are balanced by their respective production rates. In case of RO2, the budget can only be closed when the missing OH reactivity is attributed to unmeasured VOCs. Thus, the existence of unmeasured VOCs is directly confirmed by RO2 measurements. Although the closure of the RO2 budget is greatly improved by the additional unmeasured VOCs, a significant imbalance in the afternoon remains indicating a missing RO2 sink. In case of OH, the destruction in the morning is compensated by the quantified OH sources from photolysis (HONO, O3), ozonolysis of alkenes and OH recycling (HO2 + NO). In the afternoon, however, the OH budget indicates a missing OH source of (4-6) ppbv/h. The diurnal variation of the missing OH source shows a similar pattern as that of the missing RO2 sink so that both largely compensate each other in the ROx budget. These observations suggest the existence of a chemical mechanism that converts RO2 to OH without the involvement of NO. The photochemical net ozone production rate calculated from the reaction of HO2 and RO2 with NO yields a daily integrated amount of 102 ppbv ozone with daily integrated ROx primary sources being 22 ppbv in this campaign. This value 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 HO2 and all RO2 radicals would react with NO. However, the unknown RO2 loss (evident in the RO2 budget) causes 30 % less ozone production than would be expected from the VOC oxidation rate. [ABSTRACT FROM AUTHOR]

The first wintertime in situ measurements of hydroxyl (OH), hydroperoxy (HO2) and organic peroxy (RO2) radicals (ROxD OHCHO2 CRO2/ 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 2:4×106 cm-3 in severely polluted air (kOH ~ 27s-1/ to 3:6×106 cm-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 longlived 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 and RO2 concentrations by factors up to 5 during pollution episodes. The HO2 and RO2 observed-tomodeled 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 × 106 cm−3 in severely polluted air (kOH ~ 27 s−1) to 3.6 × 106 cm−3 in relatively clean air (kOH ~ 5 s−1). These values are nearly two-fold larger than OH concentrations observed in previous winter campaign 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 and RO2 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. [ABSTRACT FROM AUTHOR]

Several field studies reported unexpected large concentrations of hydroxyl and hydroperoxyl radicals (OH and HO2, respectively) in forested environments that could not be explained by the traditional oxidation mechanisms which 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 agree 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 reconcile the unexplained high OH and HO2 radical concentrations found in previous field studies. [ABSTRACT FROM AUTHOR]

In 2014, a large, comprehensive field campaign was conducted in the densely populated North China Plain. The measurement site was located in a botanic garden close to the smaller town Wangdu without major industry, but influenced by regional transportation of air pollution. The loss rate coefficient of atmospheric hydroxyl radicals (OH) was quantified by directmeasurements of the OH reactivity. Values ranged between 10 and 20 1/s for most of the daytime. Highest values were reached in the late night with maximum values around40 1/s. OH reactants mainly originated from anthropogenic activities as indicated (1) by a good correlation between measured OH reactivity and carbon monoxide, and (2) by a high contribution of nitrogen oxide species to the OH reactivity. Measured total OH reactivities can be wellexplained by measured trace gas concentrations including organic compounds, oxygenated organic compounds, CO and nitrogen oxides. Significant, unexplained OH reactivity was only observedduring nights, when biomass burning of agricultural waste occurred on surrounding fields. OH reactivity measurements also allow investigating the chemical OH budget. During this campaign, the OH destruction rate calculated from measured OH reactivity and measured OH concentration was balanced by the sum of OH production from ozone and nitrous acid photolysis and OH regeneration from hydroperoxyl radicals within the uncertainty of measurements. However, also a tendency for higher OH destruction compared to OH production at lower concentrations of nitric oxide is observed consistent with previous findings in field campaigns in China. [ABSTRACT FROM AUTHOR]

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 were achieved. Observations of radicals by laser induced fluorescence (LIF) technique gave daily maximum concentrations between (5-15) × 106 cm−3, (3-14) × 108 cm−3 and (3-15) × 108 cm−3 for OH, HO2 and RO2, respectively. Measured OH reactivities (inverse OH lifetimes) were 10 to 20 s−1 during daytime. A chemical box model constrained by trace-gas observations and based on a state-of-the-art chemical mechanism is used to interpret the observed radical concentrations. In general, the model can reasonably well reproduce measured radical concentrations during daytime. Like in previous field campaigns in China, modelled and measured OH concentrations agree for NO mixing ratios higher than 1 ppbv, but systematic discrepancies are observed in the afternoon for NO mixing ratios of less than 300 pptv (the model-measurement ratio is between 1.4 to 2 in this case). If additional OH recycling equivalent to 100 pptv NO is assumed, the model is also capable of reproducing the observed OH concentrations for conditions of high VOC and low NOx concentrations with good agreement in HO2 and RO2. Observed RO2 concentrations are underestimated in the morning hours by a factor of 3 to 5. This indicates that an additional chemical source of RO2 is missing in the model. The OH reactivity is also underpredicted in the early morning. Increasing VOC concentrations to match measured OH reactivity helps to reduce the discrepancy between modelled and measured RO2. The underprediction of RO2 coincides with high NO concentrations and therefore leads to a significant underestimation of the local ozone production rates determined from the peroxy radical (HO2 and RO2) reactions with NO. The underestimation corresponds to a daily integral ozone production of about 20 ppbv per day. [ABSTRACT FROM AUTHOR]

Direct detection of highly reactive, atmospheric hydroxyl radicals (OH) is widely accomplished by laserinduced 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. [ABSTRACT FROM AUTHOR]