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.
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.
Several previous field studies have reported unexpectedly large concentrations of hydroxyl and hydroperoxyl radicals (OH and HO.sub.2, 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, HO.sub.2 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 HO.sub.2 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 HO.sub.2 radicals, OH reactivity (k.sub.OH ), 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 HO.sub.2 radical concentrations found in previous field studies.
Abstract Tropospheric ozone (O 3 ) is an important greenhouse gas that is also hazardous to human health. The formation of O 3 is sensitive to the levels of its precursors NO x (≡NO + NO 2 ) and ...peroxy radicals, for example, generated by the oxidation of volatile organic compounds (VOCs). A better understanding of this sensitivity will show how changes in the levels of these trace gases could affect O 3 levels today and in the future, and thus air quality and climate. In this study, we investigate O 3 sensitivity in the tropical troposphere based on in situ observations of NO, HO 2 and O 3 from four research aircraft campaigns between 2015 and 2023. These are OMO (Oxidation Mechanism Observations), ATom (Atmospheric Tomography Mission), CAFE Africa (Chemistry of the Atmosphere Field Experiment in Africa) and CAFE Brazil, in combination with simulations using the EMAC atmospheric chemistry—climate model. We use the metric α (CH 3 O 2 ) together with NO to investigate the O 3 formation sensitivity. We show that O 3 formation is generally NO x ‐sensitive in the lower and middle tropical troposphere and is in a transition regime in the upper troposphere. By distinguishing observations impacted by lightning or not we show that NO from lightning is the most important driver of O 3 sensitivity in the tropics. NO x ‐sensitive chemistry predominates in regions without lightning impact, with α (CH 3 O 2 ) ranging between 0.56 and 0.82 and observed average O 3 levels between 35 and 55 ppbv. Areas affected by lightning exhibit strongly VOC‐sensitive O 3 chemistry with α (CH 3 O 2 ) of about 1 and average O 3 levels between 55 and 80 ppbv.
Plain Language Summary Ozone (O 3 ) in the troposphere is both an air pollutant and a greenhouse gas. It is formed from nitrogen oxides (NO x ) and volatile organic compounds (VOCs). The formation can be sensitive to either of these precursors depending on their abundance. Considering the high relevance of O 3 in regard to human health and global warming, it is important to understand this sensitivity of O 3 formation, which allows to predict future changes in O 3 . Here, we investigate O 3 formation sensitivity toward NO x and VOCs in the tropical troposphere based on aircraft measurements during four research campaigns between 2015 and 2023, and a global model. We include observations of NO, HO 2 (hydroperoxyl radicals) and O 3 over South America, the Middle East and the Pacific, Atlantic and Indian Ocean. We find that O 3 formation is sensitive to NO x in the lower tropical troposphere. In the upper tropical troposphere, lightning events control O 3 chemistry and promote strong VOC‐sensitive O 3 formation.
Key Points α (CH 3 O 2 ) correlated with NO is a powerful metric for indicating O 3 sensitivity and is valid throughout the troposphere O 3 chemistry in the remote tropical lower troposphere is found to be NO x ‐sensitive NO emissions from lightning drive O 3 sensitivity in the tropical upper troposphere and induce highly VOC‐sensitive chemistry
The first wintertime in situ measurements of hydroxyl (OH), hydroperoxy (HO.sub.2) and organic peroxy (RO.sub.2) radicals (RO.sub.x = OH + HO.sub.2 + RO.sub.2) in combination with observations of ...total reactivity of OH radicals, k.sub.OH 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 2.4Ã10.sup.6 cm.sup.-3 in severely polluted air (k.sub.OH â¼ 27 s.sup.-1) to 3.6Ã10.sup.6 cm.sup.-3 in relatively clean air (k.sub.OH â¼ 5 s.sup.-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 RO.sub.x radicals was dominated by the photolysis of nitrous acid accounting for 46 % of the identified primary production pathways for RO.sub.x 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, HO.sub.2 and RO.sub.2 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 HO.sub.2 and RO.sub.2 concentrations by factors up to 5 during pollution episodes. The HO.sub.2 and RO.sub.2 observed-to-modeled ratios increased with increasing NO concentrations, indicating a deficit in our understanding of the gas-phase chemistry in the high NO.sub.x 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.
Ye
et al
. have determined a maximum nitrous acid (HONO) yield of 3% for the reaction HO
2
·H
2
O + NO
2
, which is much lower than the yield used in our work. This finding, however, does not affect ...our main result that HONO in the investigated Po Valley region is mainly from a gas-phase source that consumes nitrogen oxides.
comprehensive field campaign was carried out in summer 2014 in Wangdu, located in the North China Plain. A month of continuous OH, HO.sub.2 and RO.sub.2 measurements was achieved. Observations of ...radicals by the laser-induced fluorescence (LIF) technique revealed daily maximum concentrations between (5-15)â¯âÃâ10.sup.6 â¯cm.sup.-3, (3-14)â¯âÃâ10.sup.8 â¯cm.sup.-3 and (3-15)â¯âÃâ10.sup.8 â¯cm.sup.-3 for OH, HO.sub.2 and RO.sub.2, respectively. Measured OH reactivities (inverse OH lifetime) were 10 to 20â¯s.sup.-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 1â¯ppbv, but systematic discrepancies are observed in the afternoon for NO mixing ratios of less than 300â¯pptv (the model-measurement ratio is between 1.4 and 2 in this case). If additional OH recycling equivalent to 100â¯pptv NO is assumed, the model is capable of reproducing the observed OH, HO.sub.2 and RO.sub.2 concentrations for conditions of high volatile organic compound (VOC) and low NO.sub.x concentrations. For HO.sub.2, good agreement is found between modeled and observed concentrations during day and night. In the case of RO.sub.2, the agreement between model calculations and measurements is good in the late afternoon when NO concentrations are below 0.3â¯ppbv. A significant model underprediction of RO.sub.2 by a factor of 3 to 5 is found in the morning at NO concentrations higher than 1â¯ppbv, which can be explained by a missing RO.sub.2 source of 2â¯ppbvâh.sup.-1 . As a consequence, the model underpredicts the photochemical net ozone production by 20â¯ppbv per day, which is a significant portion of the daily integrated ozone production (110â¯ppbv) derived from the measured HO.sub.2 and RO.sub.2 . The additional RO.sub.2 production from the photolysis of ClNO.sub.2 and missing reactivity can explain about 10â¯% and 20â¯% of the discrepancy, respectively. The underprediction of the photochemical ozone production at high NO.sub.x 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 NO.sub.x conditions.
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.
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 HO.sub.2 and RO.sub.2 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 308â¯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 (NO.sub.3, N.sub.2 O.sub.5 ), 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 0.05âÃâ10.sup.6 â¯cm.sup.-3 OH for a mixing ratio of 5â¯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 1âÃâ10.sup.6 â¯cm.sup.-3 is observed when 5.8â¯ppbv limonene reacts with 600â¯ppbv ozone. Experiments with the nitrate radical NO.sub.3 reveal a small interference signal in the OH, HO.sub.2, and RO.sub.2 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 NO.sub.3 giving equivalent radical concentrations of 1.1âÃâ10.sup.5 â¯cm.sup.-3 OH, 1âÃâ10.sup.7 â¯cm.sup.-3 HO.sub.2, and 1.7âÃâ10.sup.7 â¯cm.sup.-3 RO.sub.2 per 10â¯pptv NO.sub.3.
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