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.
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.
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.
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.
The changes in cadmium ion bioabsorptive properties were investigated for the brown microalgae Isochrysis galbana (T-Iso) in various saline solutions (0, 10, 20, 30, 40, and 50 parts per thousand, or ...ppt) and pH environments (pH 3, 4, 5, 6, 7, and 8) modeling those of coastal and intertidal waters. Optimal saline growth conditions for T-Iso were observed at 30 ppt. Under these optimal saline conditions, the effective concentration at 50% lethality for cadmium ion for T-Iso is 2.3 parts per million (ppm), with a maximum intracellular absorption of 8.6 fg/cell. T-Iso maximal surfacial cadmium binding was determined from Langmuir isotherm plots; Qmax = 98 mg/g (pH 6.00, 0 ppt), falling to 19 mg/g in higher salinities (pH 6.00, 50 ppt). The Freundlich constants n and Kf followed a similar cadmium binding trend: at 0 ppt, Kf = 16.6 and n = 2.83, while at higher salinities (50 ppt), the values dramatically decreased to Kf = 3.64 and n = 1.34. Direct relationships between ln Qmax (98, 70, 45, 28, 23, and 19) and ln Kf (16.6, 12.8, 8.36, 4.63, and 3.64) versus respective salinity (0, 10, 20, 30, 40, and 50 ppt) produced linear correlations. The pseudo-second-order binding kinetic rate in distilled water is 8.5 × 10−3 g/mg/min, while in saline conditions (30 ppt) the rate decreased to 4.4 × 10−3 g/mg/min. The percentage of adsorption loading capacity of the algae in the water column increases with initial cadmium exposure: the highest percentage of adsorbance (C0 = 25 ppm) in 0 ppt is 92%, while under equivalent conditions in saline water (50 ppt), adsorption falls to 31%.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NMLJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Ye et al. have determined a maximum nitrous acid (HONO) yield of 3% for the reaction HO2·H2O + NO2, 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.
This paper presents the measurements of OH and HO
2
radical concentrations as well as photolysis frequencies of different molecules during the Berliner Ozone (BERLIOZ) field experiment in July/August ...1998 at the rural site Pabstthum about 50 km NW of Berlin. Radical concentrations were measured using laser‐induced fluorescence (LIF) spectroscopy, while filter radiometers and a scanning spectroradiometer were used to obtain photolysis frequencies. The radical data set covers the time period from 20 July to 6 August and consists of more than 6000 simultaneous measurements of OH and HO
2
with a typical time resolution of about 90 s. The maximum OH and HO
2
daytime concentrations were 8 × 10
6
and 8 × 10
8
cm
−3
, respectively. While nighttime values of OH were usually below the detection limit of our instrument (3.5 × 10
5
cm
−3
), HO
2
did show significant concentrations throughout most of the nights (on average 3 × 10
7
cm
−3
). The OH concentration was mainly controlled by solar UV radiation and showed a high linear correlation with J(O
1
D). A deviation from this general behavior was observed around dawn and dusk, when OH concentrations well above the detection limit were observed, although J(O
1
D) was essentially zero. A comparison with data sets from previous campaigns revealed that even though the linear correlation is found in other environments as well the slope OH/J(O
1
D) differs significantly. The diurnal cycles of HO
2
were less dependent on the solar actinic flux but were predominantly influenced by NO. During episodes of high NO, HO
2
remained below the detection limit (1 × 10
7
cm
−3
) but started to rise rapidly as soon as NO started to decrease.
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