HO2 concentration measurements are widely accomplished by chemical conversion of HO2 to OH including reaction with NO and subsequent detection of OH by laser-induced fluorescence. RO2 radicals can be ...converted to OH via a similar radical reaction sequence including reaction with NO, so that they are potential interferences for HO2 measurements. Here, the conversion efficiency of various RO2 species to HO2 is investigated. Experiments were conducted with a radical source that produces OH and HO2 by water photolysis at 185 nm, which is frequently used for calibration of LIF instruments. The ratio of HO2 and the sum of OH and HO2 concentrations provided by the radical source was investigated and was found to be 0.50 ± 0.02. RO2 radicals are produced by the reaction of various organic compounds with OH in the radical source. Interferences via chemical conversion from RO2 radicals produced by the reaction of OH with methane and ethane (H-atom abstraction) are negligible consistent with measurements in the past. However, RO2 radicals from OH plus alkene- and aromatic-precursors including isoprene (mainly OH-addition) are detected with a relative sensitivity larger than 80 % with respect to that for HO2 for the configuration of the instrument with which it was operated during field campaigns. Also RO2 from OH plus methyl vinyl ketone and methacrolein exhibit a relative detection sensitivity of 60 %. Thus, previous measurements of HO2 radical concentrations with this instrument were biased in the presence of high RO2 radical concentrations from isoprene, alkenes or aromatics, but were not affected by interferences in remote clean environment with no significant emissions of biogenic VOCs, when the OH reactivity was dominated by small alkanes. By reducing the NO concentration and/or the transport time between NO addition and OH detection, interference from these RO2 species are suppressed to values below 20 % relative to the HO2 detection sensitivity. The HO2 conversion efficiency is also smaller by a factor of four, but this is still sufficient for atmospheric HO2 concentration measurements for a wide range of conditions.
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.
As part of the 2008 Campaign of Air Quality Research in Beijing and Surrounding Regions (CAREBeijing 2008), measurements of gaseous sulfuric acid (H2 SO4 ) have been conducted at an urban site in ...Beijing, China from 7 July to 25 September 2008 using atmospheric pressure ion drift - chemical ionization mass spectrometry (AP-ID-CIMS). This represents the first gaseous H2 SO4 measurements in China. Diurnal profile of sulfuric acid is strongly dependent on the actinic flux, reaching a daily maximum around noontime and with an hourly average concentration of 5 × 106 molecules cm-3 . Simulation of sulfuric acid on the basis of the measured sulfur dioxide concentration, photolysis rates of ozone and nitrogen dioxide, and aerosol surface areas captures the trend of the measured H2 SO4 diurnal variation within the uncertainties, indicating that photochemical production and condensation onto preexisting particle surface dominate the observed diurnal H2 SO4 profile. The frequency of the peak H2 SO4 concentration exceeding 5 × 106 molecules cm-3 increases by 16 % during the period of the summer Olympic Games (8-24 August 2008), because of the implementation of air quality control regulations. Using a multivariate statistical method, the critical nucleus during nucleation events is inferred, containing two H2 SO4 molecules (R2 = 0.85). The calculated condensation rate of H2 SO4 can only account for 10-25 % of PM1 sulfate formation, indicating that either much stronger sulfate production exists at the SO2 source region or other sulfate production mechanisms are responsible for the sulfate production.
Ozone concentrations in the Po Valley of northern Italy often exceed international regulations. As both a source of radicals and an intermediate in the oxidation of most volatile organic compounds ...(VOCs), formaldehyde (HCHO) is a useful tracer for the oxidative processing of hydrocarbons that leads to ozone production. We investigate the sources of HCHO in the Po Valley using vertical profile measurements acquired from the airship Zeppelin NT over an agricultural region during the PEGASOS 2012 campaign. Using a 1-D model, the total VOC oxidation rate is examined and discussed in the context of formaldehyde and ozone production in the early morning. While model and measurement discrepancies in OH reactivity are small (on average 3.4 ± 13%), HCHO concentrations are underestimated by as much as 1.5 ppb (45%) in the convective mixed layer. A similar underestimate in HCHO was seen in the 2002–2003 FORMAT Po Valley measurements, though the additional source of HCHO was not identified. Oxidation of unmeasured VOC precursors cannot explain the missing HCHO source, as measured OH reactivity is explained by measured VOCs and their calculated oxidation products. We conclude that local direct emissions from agricultural land are the most likely source of missing HCHO. Model calculations demonstrate that radicals from degradation of this non-photochemical HCHO source increase model ozone production rates by as much as 0.6 ppb h−1 (12%) before noon.
Although anucleated, blood platelets are highly organized cells rich in different types of organelles. Three specific granule populations store different types of constituents, some of which are at ...high concentrations. Platelets thus transport some specific compounds through the whole body. During circulation, platelets are reactive to various stimuli and release the materials stored in the specific granules. This 'release reaction' is an important step of primary haemostasis. Energy and messengers required for platelet reactivity are provided by mitochondria and the dense tubular system. Each granule population has specific properties concerning both the structure and the role played by the released constituents. Dense granules contain small non-protein molecules that are secreted to recruit other platelets. f -Granules contain large adhesive and healing proteins. Lysosomes contain hydrolases able to eliminate the circulating platelet aggregate. The extrusion of storage granules' content to the platelet's environment occurs according to regulated secretion events: movements of granules, apposition and fusion of granule and plasma membranes. Typical platelet disorders resulting from a storage granule abnormality are referred to as a storage pool defect and are characterized by a prolonged bleeding time.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Airborne measurements of solar spectral radiance reflected by cirrus are performed with the HALO-Solar Radiation (HALO-SR) instrument onboard the High Altitude and Long Range Research Aircraft (HALO) ...in November 2010. The data are used to quantify the influence of surface albedo variability on the retrieval of cirrus optical thickness and crystal effective radius. The applied retrieval of cirrus optical properties is based on a standard two-wavelength approach utilizing measured and simulated reflected radiance in the visible and near-infrared spectral region. Frequency distributions of the surface albedos from Moderate resolution Imaging Spectroradiometer (MODIS) satellite observations are used to compile surface-albedo-dependent lookup tables of reflected radiance. For each assumed surface albedo the cirrus optical thickness and effective crystal radius are retrieved as a function of the assumed surface albedo. The results for the cirrus optical thickness are compared to measurements from the High Spectral Resolution Lidar (HSRL). The uncertainty in cirrus optical thickness due to local variability of surface albedo in the specific case study investigated here is below 0.1 and thus less than that caused by the measurement uncertainty of both instruments. It is concluded that for the retrieval of cirrus optical thickness the surface albedo variability is negligible. However, for the retrieval of crystal effective radius, the surface albedo variability is of major importance, introducing uncertainties up to 50%. Furthermore, the influence of the bidirectional reflectance distribution function (BRDF) on the retrieval of crystal effective radius was investigated and quantified with uncertainties below 10%, which ranges below the uncertainty caused by the surface albedo variability. The comparison with the independent lidar data allowed for investigation of the role of the crystal shape in the retrieval. It is found that if assuming aggregate ice crystals, the HSRL observations fit best with the retrieved optical thickness from HALO-SR.
Besides isoprene, monoterpenes are the non-methane volatile organic compounds (VOCs) with the highest global emission rates. Due to their high reactivity towards OH, monoterpenes can dominate the ...radical chemistry of the atmosphere in forested areas. In the present study the photochemical degradation mechanism of β-pinene was investigated in the Jülich atmosphere simulation chamber SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction Chamber). One focus of this study is on the OH budget in the degradation process. Therefore, the SAPHIR chamber was equipped with instrumentation to measure radicals (OH, HO2, RO2), the total OH reactivity, important OH precursors (O3, HONO, HCHO), the parent VOC β-pinene, its main oxidation products, acetone and nopinone and photolysis frequencies. All experiments were carried out under low-NO conditions ( ≤ 300 ppt) and at atmospheric β-pinene concentrations ( ≤ 5 ppb) with and without addition of ozone. For the investigation of the OH budget, the OH production and destruction rates were calculated from measured quantities. Within the limits of accuracy of the instruments, the OH budget was balanced in all β-pinene oxidation experiments. However, even though the OH budget was closed, simulation results from the Master Chemical Mechanism (MCM) 3.2 showed that the OH production and destruction rates were underestimated by the model. The measured OH and HO2 concentrations were underestimated by up to a factor of 2, whereas the total OH reactivity was slightly overestimated because the model predicted a nopinone mixing ratio which was 3 times higher than measured. A new, theory-derived, first-generation product distribution by Vereecken and Peeters (2012) was able to reproduce the measured nopinone time series and the total OH reactivity. Nevertheless, the measured OH and HO2 concentrations remained underestimated by the numerical simulations. These observations together with the fact that the measured OH budget was closed suggest the existence of unaccounted sources of HO2. Although the mechanism of additional HO2 formation could not be resolved, our model studies suggest that an activated alkoxy radical intermediate proposed in the model of Vereecken and Peeters (2012) generates HO2 in a new pathway, whose importance has been underestimated so far. The proposed reaction path involves unimolecular rearrangement and decomposition reactions and photolysis of dicarbonyl products, yielding additional HO2 and CO. Further experiments and quantum chemical calculations have to be made to completely unravel the pathway of HO2 formation.
During recent field campaigns, hydroxyl radical (OH) concentrations that were measured by laser-induced fluorescence (LIF) were up to a factor of ten larger than predicted by current chemical models ...for conditions of high OH reactivity and low NO concentration. These discrepancies, which were observed in forests and urban-influenced rural environments, are so far not entirely understood. In summer 2011, a series of experiments was carried out in the atmosphere simulation chamber SAPHIR in Jülich, Germany, in order to investigate the photochemical degradation of isoprene, methyl-vinyl ketone (MVK), methacrolein (MACR) and aromatic compounds by OH. Conditions were similar to those experienced during the PRIDE-PRD2006 campaign in the Pearl River Delta (PRD), China, in 2006, where a large difference between OH measurements and model predictions was found. During experiments in SAPHIR, OH was simultaneously detected by two independent instruments: LIF and differential optical absorption spectroscopy (DOAS). Because DOAS is an inherently calibration-free technique, DOAS measurements are regarded as a reference standard. The comparison of the two techniques was used to investigate potential artifacts in the LIF measurements for PRD-like conditions of OH reactivities of 10 to 30 s−1 and NO mixing ratios of 0.1 to 0.3 ppbv. The analysis of twenty experiment days shows good agreement. The linear regression of the combined data set (averaged to the DOAS time resolution, 2495 data points) yields a slope of 1.02 ± 0.01 with an intercept of (0.10 ± 0.03) × 106 cm−3 and a linear correlation coefficient of R2 = 0.86. This indicates that the sensitivity of the LIF instrument is well-defined by its calibration procedure. No hints for artifacts are observed for isoprene, MACR, and different aromatic compounds. LIF measurements were approximately 30–40% (median) larger than those by DOAS after MVK (20 ppbv) and toluene (90 ppbv) had been added. However, this discrepancy has a large uncertainty and requires further laboratory investigation. Observed differences between LIF and DOAS measurements are far too small to explain the unexpected high OH concentrations during the PRIDE-PRD2006 campaign.
Hydroxyl radicals (OH) are the most important reagent for the oxidation of trace gases in the atmosphere. OH concentrations measured during recent field campaigns in isoprene-rich environments were ...unexpectedly large. A number of studies showed that unimolecular reactions of organic peroxy radicals (RO2) formed in the initial reaction step of isoprene with OH play an important role for the OH budget in the atmosphere at low mixing ratios of nitrogen monoxide (NO) of less than 100 pptv. It has also been suggested that similar reactions potentially play an important role for RO2 from other compounds. Here, we investigate the oxidation of methacrolein (MACR), one major oxidation product of isoprene, by OH in experiments in the simulation chamber SAPHIR under controlled atmospheric conditions. The experiments show that measured OH concentrations are approximately 50% larger than calculated by the Master Chemical Mechanism (MCM) for conditions of the experiments (NO mixing ratio of 90 pptv). The analysis of the OH budget reveals an OH source that is not accounted for in MCM, which is correlated with the production rate of RO2 radicals from MACR. In order to balance the measured OH destruction rate, 0.77 OH radicals (1σ error: ± 0.31) need to be additionally reformed from each reaction of OH with MACR. The strong correlation of the missing OH source with the production of RO2 radicals is consistent with the concept of OH formation from unimolecular isomerization and decomposition reactions of RO2. The comparison of observations with model calculations gives a lower limit of 0.03 s−1 for the reaction rate constant if the OH source is attributed to an isomerization reaction of MACR-1-OH-2-OO and MACR-2-OH-2-OO formed in the MACR + OH reaction as suggested in the literature (Crounse et al., 2012). This fast isomerization reaction would be a competitor to the reaction of this RO2 species with a minimum of 150 pptv NO. The isomerization reaction would be the dominant reaction pathway for this specific RO2 radical in forested regions, where NO mixing ratios are typically much smaller.
Since the passage of the Clean Water Act in 1972, the United States has made great strides to reduce the threats to its rivers, lakes, and wetlands from pollution. However, despite our obvious ...successes, nearly half of the nation’s surface water resources remain incapable of supporting basic aquatic values or maintaining water quality adequate for recreational swimming. The Clean Water Act established a significant federal presence in water quality regulation by controlling point and non-point sources of pollution. Point-sources of pollution were the major emphasis of the Act, but Section 208 specifically addressed non-point sources of pollution and designated silviculture and livestock grazing as sources of non-point pollution. Non-point source pollutants include runoff from agriculture, municipalities, timber harvesting, mining, and livestock grazing. Non-point source pollution now accounts for more than half of the United States water quality impairments. To successfully improve water quality, restoration practitioners must start with an understanding of what ecosystem processes are operating in the watershed and how they have been affected by outside variables. A watershed-based analysis template developed in the Pacific Northwest can be a valuable aid in developing that level of understanding. The watershed analysis technique identifies four ecosystem scales useful to identify stream restoration priorities: region, basin, watershed, and site. The watershed analysis technique is based on a set of technically rigorous and defensible procedures designed to provide information on what processes are active at the watershed scale, how those processes are distributed in time and space. They help describe what the current upland and riparian conditions of the watershed are and how these conditions in turn influence aquatic habitat and other beneficial uses. The analysis is organized as a set of six steps that direct an interdisciplinary team of specialists to examine the biotic and abiotic processes influencing aquatic habitat and species abundance. This process helps develop an understanding of the watershed within the context of the larger ecosystem. The understanding gained can then be used to identify and prioritize aquatic restoration activities at the appropriate temporal and spatial scale. The watershed approach prevents relying solely on site-level information, a common problem with historic restoration efforts. When the watershed analysis process was used in the Whitefish Mountains of northwest Montana, natural resource professionals were able to determine the dominant habitat forming processes important for native fishes and use that information to prioritize, plan, and implement the appropriate restoration activities at the watershed scale. Despite considerable investments of time and resources needed to complete an analysis at the watershed scale, the results can prevent the misdiagnosis of aquatic problems and help ensure that the objectives of aquatic restoration will be met.
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