High OH Levels Research Articles

HONO Budget and Its Role in Nitrate Formation in the Rural North China Plain.

Nitrous acid (HONO) is a major precursor of tropospheric hydroxyl radical (OH) that accelerates the formation of secondary pollutants. The HONO sources, however, are not well understood, especially in polluted areas. Based on a comprehensive winter field campaign conducted at a rural site of the North China Plain, a box model (MCM v3.3.1) was used to simulate the daytime HONO budget and nitrate formation. We found that HONO photolysis acted as the dominant source for primary OH with a contribution of more than 92%. The observed daytime HONO could be well explained by the known sources in the model. The heterogeneous conversion of NO2 on ground surfaces and the homogeneous reaction of NO with OH were the dominant HONO sources with contributions of more than 36 and 34% to daytime HONO, respectively. The contribution from the photolysis of particle nitrate and the reactions of NO2 on aerosol surfaces was found to be negligible in clean periods (2%) and slightly higher during polluted periods (8%). The relatively high OH levels due to fast HONO photolysis at the rural site remarkably accelerated gas-phase reactions, resulting in the fast formation of nitrate as well as other secondary pollutants in the daytime.

The pollution levels, variation characteristics, sources and implications of atmospheric carbonyls in a typical rural area of North China Plain during winter

Atmospheric carbonyls were measured at a typical rural area of the North China Plain (NCP) from November 13 to December 24, 2017 to investigate the pollution characteristics, sources and environmental implications. Fifteen carbonyls were detected, and formaldehyde, acetaldehyde and acetone accounted for about 81% at most. The concentration of the total carbonyls in heavily polluted days was twice more than that in clean days. In contrast to other carbonyls, m-tolualdehyde exhibited relatively high concentrations in the clean days in comparison with the polluted days. The ratios of three principal carbonyls to CO showed similar daily variations at different pollution levels with significant daytime peaks. Multiple linear regression analysis revealed that the contributions of background, primary and secondary sources to three principal carbonyls showed similar variation trends from the clean level to the heavily polluted level. The OH formation rate of formaldehyde showed a similar variation trend to its photodegradation rate, reaching the peak value at noon, which is important to maintain relatively high OH levels to initiate the oxidation of various gas-phase pollutants for secondary pollutant formation at the rural site. OH radical consumption rate and ozone formation potential (OFP) calculations showed that formaldehyde and acetaldehyde were the dominant oxidative species among measured carbonyls. As for OH radical consumption, n-butyraldehyde and m-tolualdehyde were important contributors, while for ozone formation potential, n-butyraldehyde and propionaldehyde made significant contributions. In addition, the contribution of carbonyl compounds to secondary organic aerosol (SOA) formation was also important and needs further investigation.

Polybrominated diphenyl ethers and their hydroxylated and methoxylated derivatives in seafood obtained from Puget Sound, WA

Synthetic polybrominated diphenyl ethers (PBDEs) are ubiquitous environmental contaminants and known to occur in most food items. Consumer fish products have been identified as having some of the highest PBDE levels found in USA food sources. Natural formation of hydroxylated (OH-) and methoxylated (MeO-) PBDEs are also known to occur in simple marine organisms, which may be bioaccumulated by seafood. In this study, we report findings of an initial survey of PBDE, OH-PBDE and MeO-PBDE content in common seafood items available to residents living in the Puget Sound region of Washington State. Seafood samples were either purchased from local grocery stores or caught off the coast of SE Alaska and in Puget Sound. The edible portions of the seafood were analyzed, which for finfish was white muscle (skinless fillets) and for shellfish, either the entire soft tissue (bivalves) or processed meat (calamari, shrimp and scallops). Results indicated that finfish typically had higher levels of PBDEs compared to shellfish with BDE-47 and BDE-99 as the most common congeners detected. Among shellfish, bivalves (clams and mussels) were notable for having much higher levels of OH- and MeO-PBDEs compared to other types of seafood with 6′-OH-BDE-47 and 2′-MeO-BDE-68 being the more common OH- and MeO- congeners, respectively. Based on our results and recent updates to daily fish consumption rates, estimated intake rates for Washington State residents will be between 34 and 644ngPBDEs/day, depending on species consumed. For the OH- and MeO- forms, daily exposure is much more variable but typically would range between 15 and 90ng/day for most seafood types. If shellfish are primarily consumed, OH-PBDE intake could be as high as 350ng/day. These daily intake rates for PBDEs are higher than most dietary intake rates calculated for populations in other world regions.

Evaluation of resistance and the role of some defense responses in wheat cultivars to Fusarium head blight

Abstract Fusarium graminearum and F. culmorum are the causal agents of Fusarium head blight (FHB) in cereal crops worldwide. Application of resistant cultivars is the most effective and economic method for management of FHB and reducing mycotoxin production in wheat. Understanding the physiological and biochemical mechanisms involved in basal resistance of wheat to FHB disease is limited. In this research, after screening resistance levels of eighteen wheat cultivars planted in Iran, Gaskozhen and Falat were identified as partially resistant and susceptible wheat cultivars against Fusarium spp., respectively. Also, we investigated the role of hydroxyl radical (OH−), nitric oxide (NO), callose deposition, lipid peroxidation and protein content in basal resistance of wheat to the hemi-biotrophic and necrotrophic Fusarium species causing FHB. Nitric oxide as a signaling molecule may be involved in physiological and defensive processes in plants. Our results showed that NO generation increased in seedlings and spikes of wheat cultivars after inoculation with Fusarium species. We observed earlier and stronger callose deposition at early time points after infection by Fusarium spp. isolates than in non-infected plants, which was positively related to the resistance levels in wheat cultivars. Higher levels of OH− and malondialdehyde (MDA) accumulation (as a marker of lipid peroxidation) were observed in the Falat than in the Gaskozhen cultivar, under non-infected and infected conditions. So, estimation of lipid peroxidation could be useful to evaluate cultivars’ susceptibility. These findings can provide novel insights for better recognition of physiological and biochemical markers of FHB resistance, which could be used for rapid screening of resistance levels in wheat cultivars against this destructive fungal disease.

Direct measurement of free radical levels in the brain after cortical ischemia induced by photothrombosis.

Tissue ischemia is connected with the production of free radicals (FR). This study was designed to directly measure of the amount of FR in rat brains related to a photothrombotic ischemic event shortly after establishing the lesion. A model of left hemisphere photothrombosis ischemia was used in the experiment. Brains of animals from the experimental group were removed and placed in liquid N(2) for 60 min after the green laser exposure, the control group brains, exposed to the photosensitive dye Rose Bengal (RB), were placed in liquid N(2) for 80 min after RB application, naive control brains were also briefly stored in liquid N(2). Spectroscopy of electron paramagnetic (spin) resonance was used to directly measure FR (hydroxyl (OH(.)) and nitroxyl (NO(.)). Compared to naive controls, both the ischemia and RB groups had significantly higher levels of OH(.), however, there were no differences between them. Comparison of hemispheres, i.e., with and without ischemia, in the experimental group did not show any significant difference in OH(.). NO(.) were elevated in the ischemia and RB groups compare to naive controls. Higher levels of NO(.) were found in hemispheres with ischemia compared to unexposed hemispheres. Increases in OH(.) were probably associated with the action of RB itself in this model of ischemia. Increases in NO(.) were closely related to the pathogenesis of photothrombotic ischemia and could be related to the activity of nitric oxide synthases.

SOA formation from the photooxidation of <i>α</i>-pinene: systematic exploration of the simulation of chamber data

Abstract. Chemical mechanisms play an important role in simulating the atmospheric chemistry of volatile organic compound oxidation. Comparison of mechanism simulations with laboratory chamber data tests our level of understanding of the prevailing chemistry as well as the dynamic processes occurring in the chamber itself. α-Pinene photooxidation is a well-studied system experimentally, for which detailed chemical mechanisms have been formulated. Here, we present the results of simulating low-NO α-pinene photooxidation experiments conducted in the Caltech chamber with the Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A) under varying concentrations of seed particles and OH levels. Unexpectedly, experiments conducted at low and high OH levels yield the same secondary organic aerosol (SOA) growth, whereas GECKO-A predicts greater SOA growth under high OH levels. SOA formation in the chamber is a result of a competition among the rates of gas-phase oxidation to low-volatility products, wall deposition of these products, and condensation into the aerosol phase. Various processes – such as photolysis of condensed-phase products, particle-phase dimerization, and peroxy radical autoxidation – are explored to rationalize the observations. In order to explain the observed similar SOA growth at different OH levels, we conclude that vapor wall loss in the Caltech chamber is likely of order 10−5 s−1, consistent with previous experimental measurements in that chamber. We find that GECKO-A tends to overpredict the contribution to SOA of later-generation oxidation products under high-OH conditions. Moreover, we propose that autoxidation may alternatively resolve some or all of the measurement–model discrepancy, but this hypothesis cannot be confirmed until more explicit mechanisms are established for α-pinene autoxidation. The key role of the interplay among oxidation rate, product volatility, and vapor–wall deposition in chamber experiments is illustrated.

Production of hydroxyl radicals from Fe-containing fine particles in Guangzhou, China

Abstract Reactive oxygen species (ROS) production from ambient fine particles has been correlated with the soluble transition metal content of PM2.5, which also has clear association with particle-mediated cardiopulmonary toxicity. Hydroxyl radical ( OH) is the most harmful ROS species through chemical reactions of redox-active particle components. Atmospheric Fe, as the dominant species of the transition metals in the atmosphere, is associated with OH generation in ambient particle extracts. Our results revealed that Fe-containing particles (18,730 in total number) contributed approximately 3.7% on average to all detected particles throughout the summer and winter sampling period in Guangzhou, which was clustered into four distinct particle classes, including Fe-rich, Metal-rich, NaK-rich and Dust-rich. Fe-rich class was the dominant one with a fraction of 61%, followed by Dust-rich (14%), Metal-rich (13%). and NaK-rich (12%). The iron oxide was enriched in the Fe-rich class. OH generation induced by Fe-containing fine particles collected in Guangzhou (GZ) was quantified in a surrogate lung fluid (SLF), and it was found that Fe-containing fine particles were generally much reactive in generating OH in the presence of four antioxidants (200 μM ascorbate, 300 μM citrate, 100 μM reduced l -glutathione, and 100 μM uric acid). The annual average OH amount produced in our samples was 132.98 ± 27.43 nmol OH mg−1 PM2.5. OH production had a clear seasonal pattern with higher amount in summer and lower in winter. By measuring the amount of total and SLF-soluble metal in our PM2.5 samples using ICP-MS, we found that ROS activities were associated with the ionizable Fe through Fenton type reactions in the Guangzhou PM2.5. Expected burdens of PM2.5 derived OH in human lung lining fluid suggests that typical daily particulate matter exposure in Guangzhou is already a concern, and it could produce much higher levels of OH, leading to higher cytotoxicity.

Observation and modelling of OH and HO<sub>2</sub> concentrations in the Pearl River Delta 2006: a missing OH source in a VOC rich atmosphere

Abstract. Ambient OH and HO2 concentrations were measured by laser induced fluorescence (LIF) during the PRIDE-PRD2006 (Program of Regional Integrated Experiments of Air Quality over the Pearl River Delta, 2006) campaign at a rural site downwind of the megacity of Guangzhou in Southern China. The observed OH concentrations reached daily peak values of (15–26) × 106 cm−3 which are among the highest values so far reported for urban and suburban areas. The observed OH shows a consistent high correlation with j(O1D) over a broad range of NOx conditions. The correlation cannot be reproduced by model simulations, indicating that OH stabilizing processes are missing in current models. The observed OH exhibited a weak dependence on NOx in contrast to model predictions. While modelled and measured OH agree well at NO mixing ratios above 1 ppb, a continuously increasing underprediction of the observed OH is found towards lower NO concentrations, reaching a factor of 8 at 0.02 ppb NO. A dependence of the modelled-to-measured OH ratio on isoprene cannot be concluded from the PRD data. However, the magnitude of the ratio fits into the isoprene dependent trend that was reported from other campaigns in forested regions. Hofzumahaus et al. (2009) proposed an unknown OH recycling process without NO, in order to explain the high OH levels at PRD in the presence of high VOC reactivity and low NO. Taking a recently discovered interference in the LIF measurement of HO2 into account, the need for an additional HO2 → OH recycling process persists, but the required source strength may be up to 20% larger than previously determined. Recently postulated isoprene mechanisms by Lelieveld et al. (2008) and Peeters and Müller (2010) lead to significant enhancements of OH expected for PRD, but an underprediction of the observed OH by a factor of two remains at low NO (0.1–0.2 ppb). If the photolysis of hydroperoxy aldehydes from isoprene is as efficient as proposed by Peeters and Müller (2010), the corresponding OH formation at PRD would be more important than the primary OH production from ozone and HONO. While the new isoprene mechanisms need to be confirmed by laboratory experiments, there is probably need for other, so far unidentified chemical processes to explain entirely the high OH levels observed in Southern China.

Involvement of protein kinase C in lignin peroxidase expression in oxygenated cultures of the white rot fungus Phanerochaete chrysosporium

Hydroxyl radicals (OH ) were previously hypothesized as the principal ROS triggering lip-H2 expression in liquid cultures of Phanerochaete chrysosporium, through signal transduction. We therefore focused on determining the relationship between protein kinase C (PKC), ROS and LIP-H2. Significantly lower PKC activity was measured in high-LIP-producing (oxygenated) vs. low-LIP-producing (aerated—grown with free exchange of atmospheric air) cultures. In oxygenated cultures, inactivation of PKC activity by calphostin C, staurosporin or H7 caused significant elevation in lip-H2 and also in MnSOD1 expression. Stimulation of PKC by phorbol 12-myristate 13-acetate (PMA) caused the reverse effect. Significantly low levels of lip-H2 expression were detected when O 2 −, H 2O 2 or OH were scavenged by 4,5-dihydroxy-1,3-benzene disulfonic acid (Tiron), pyruvate or dimethylsulfoxide (DMSO), whether the level of PKC was normal, stimulated or inactivated. In situ generation of OH , via addition of Fenton reagents to aerated cultures, reduced pkc expression. In contrast, OH scavenging stimulated pkc expression. This work suggests that due to high OH levels, fungal cells activate a complex defensive system which regulates the levels of Fenton components by repressing PKC and stimulating LIP, MnSOD1 and catalase.

Atmospheric oxidation capacity in the summer of Houston 2006: Comparison with summer measurements in other metropolitan studies

Both similarities and differences in summertime atmospheric photochemical oxidation appear in the comparison of four field studies: TEXAQS2000 (Houston, 2000), NYC2001 (New York City, 2001), MCMA2003 (Mexico City, 2003), and TRAMP2006 (Houston, 2006). The compared photochemical indicators are OH and HO 2 abundances, OH reactivity (the inverse of the OH lifetime), HO x budget, OH chain length (ratio of OH cycling to OH loss), calculated ozone production, and ozone sensitivity. In terms of photochemical activity, Houston is much more like Mexico City than New York City. These relationships result from the ratio of volatile organic compounds (VOCs) to nitrogen oxides (NO x ), which are comparable in Houston and Mexico City, but much lower in New York City. Compared to New York City, Houston and Mexico City also have higher levels of OH and HO 2, longer OH chain lengths, a smaller contribution of reactions with NO x to the OH reactivity, and NO x -sensitivity for ozone production during the day. In all four studies, the photolysis of nitrous acid (HONO) and formaldehyde (HCHO) are significant, if not dominant, HO x sources. A problematic result in all four studies is the greater OH production than OH loss during morning rush hour, even though OH production and loss are expected to always be in balance because of the short OH lifetime. The cause of this discrepancy is not understood, but may be related to the under-predicted HO 2 in high NO x conditions, which could have implications for ozone production. Three photochemical indicators show particularly high photochemical activity in Houston during the TRAMP2006 study: the long portion of the day for which ozone production was NO x -sensitive, the calculated ozone production rate that was second only to Mexico City's, and the OH chain length that was twice that of any other location. These results on photochemical activity provide additional support for regulatory actions to reduce reactive VOCs in Houston in order to reduce ozone and other pollutants.

Atmospheric oxidation capacity sustained by a tropical forest

Terrestrial vegetation, especially tropical rain forest, releases vast quantities of volatile organic compounds (VOCs) to the atmosphere, which are removed by oxidation reactions and deposition of reaction products. The oxidation is mainly initiated by hydroxyl radicals (OH), primarily formed through the photodissociation of ozone. Previously it was thought that, in unpolluted air, biogenic VOCs deplete OH and reduce the atmospheric oxidation capacity. Conversely, in polluted air VOC oxidation leads to noxious oxidant build-up by the catalytic action of nitrogen oxides (NO(x) = NO + NO2). Here we report aircraft measurements of atmospheric trace gases performed over the pristine Amazon forest. Our data reveal unexpectedly high OH concentrations. We propose that natural VOC oxidation, notably of isoprene, recycles OH efficiently in low-NO(x) air through reactions of organic peroxy radicals. Computations with an atmospheric chemistry model and the results of laboratory experiments suggest that an OH recycling efficiency of 40-80 per cent in isoprene oxidation may be able to explain the high OH levels we observed in the field. Although further laboratory studies are necessary to explore the chemical mechanism responsible for OH recycling in more detail, our results demonstrate that the biosphere maintains a remarkable balance with the atmospheric environment.

Three‐dimensional climatological distribution of tropospheric OH: Update and evaluation

A global climatological distribution of tropospheric OH is computed using observed distributions of O3, H2O, NOt (NO2 +NO + 2N2O5 + NO3 + HNO2 +HNO4), CO, hydrocarbons, temperature, and cloud optical depth. Global annual mean OH is 1.16×106 molecules cm−3 (integrated with respect to mass of air up to 100 hPa within ±32° latitude and up to 200 hPa outside that region). Mean hemispheric concentrations of OH are nearly equal. While global mean OH increased by 33% compared to that from Spivakovsky et al. [1990], mean loss frequencies of CH3CCl3 and CH4 increased by only 23% because a lower fraction of total OH resides in the lower troposphere in the present distribution. The value for temperature used for determining lifetimes of hydrochlorofluorocarbons (HCFCs) by scaling rate constants [Prather and Spivakovsky, 1990] is revised from 277 K to 272 K. The present distribution of OH is consistent within a few percent with the current budgets of CH3CCl3 and HCFC‐22. For CH3CCl3, it results in a lifetime of 4.6 years, including stratospheric and ocean sinks with atmospheric lifetimes of 43 and 80 years, respectively. For HCFC‐22, the lifetime is 11.4 years, allowing for the stratospheric sink with an atmospheric lifetime of 229 years. Corrections suggested by observed levels of CH2Cl2 (annual means) depend strongly on the rate of interhemispheric mixing in the model. An increase in OH in the Northern Hemisphere by 20% combined with a decrease in the southern tropics by 25% is suggested if this rate is at its upper limit consistent with observations of CFCs and 85Kr. For the lower limit, observations of CH2Cl2 imply an increase in OH in the Northern Hemisphere by 35% combined with a decrease in OH in the southern tropics by 60%. However, such large corrections are inconsistent with observations for 14CO in the tropics and for the interhemispheric gradient of CH3CCl3. Industrial sources of CH2Cl2 are sufficient for balancing its budget. The available tests do not establish significant errors in OH except for a possible underestimate in winter in the northern and southern tropics by 15–20% and 10–15%, respectively, and an overestimate in southern extratropics by ∼25%. Observations of seasonal variations of CH3CCl3, CH2Cl2, 14CO, and C2H6 offer no evidence for higher levels of OH in the southern than in the northern extratropics. It is expected that in the next few years the latitudinal distribution and annual cycle of CH3CCl3 will be determined primarily by its loss frequency, allowing for additional constraints for OH on scales smaller than global.