Photochemical Loss Research Articles

Photochemical reaction of tricresyl phosphate (TCP) in aqueous solution: Influencing factors and photolysis products

Organophosphate triesters (OPEs) have caused great concern as a class of emerging environmental contaminants due to their widespread use and their toxicity to organisms. However, the phototransformation behavior of OPE is still not fully understood, which is important for understanding their environmental fate. In the present study, the photodegradation of tricresyl phosphate (TCP), one of the most widely detected OPEs in aqueous environments, was investigated including the direct photolysis and in the presence of several natural water factors, NO2−, Fe3+ and humic acid. The degradation process followed the pseudo-first-order kinetics, with rate constant increasing slightly with increasing initial TCP concentration. The presence of NO2− and Fe3+ was observed to promote the photochemical loss of TCP, while humic acid played a negative role on TCP transformation. Electron spin resonance (EPR) analysis showed that carbon-centered radical was produced in the photolysis process of TCP, and hydroxyl radical contributed to the promotion of rate constant for Fe3+ and NO2−. Four photolysis products were tentatively identified by HPLC-LTQ-Orbitrap MS analysis, and the possible degradation pathways of TCP were proposed. These findings provide a meaningful reference for the fate and transformation of OPEs in natural water.

Diurnal variability, photochemical production and loss processes of hydrogen peroxide in the boundary layer over Europe

Abstract. Hydrogen peroxide (H2O2) plays a significant role in the oxidizing capacity of the atmosphere. It is an efficient oxidant in the liquid phase and serves as a temporary reservoir for the hydroxyl radical (OH), the most important oxidizing agent in the gas phase. Due to its high solubility, removal of H2O2 due to wet and dry deposition is efficient, being a sink of HOx (OH+HO2) radicals. In the continental boundary layer, the H2O2 budget is controlled by photochemistry, transport and deposition processes. Here we use in situ observations of H2O2 and account for chemical source and removal mechanisms to study the interplay between these processes. The data were obtained during five ground-based field campaigns across Europe from 2008 to 2014 and bring together observations in a boreal forest, two mountainous sites in Germany, and coastal sites in Spain and Cyprus. Most campaigns took place in the summer, while the measurements in the south-west of Spain took place in early winter. Diel variations in H2O2 are strongly site-dependent and indicate a significant altitude dependence. While boundary-layer mixing ratios of H2O2 at low-level sites show classical diel cycles with the lowest values in the early morning and maxima around local noon, diel profiles are reversed on mountainous sites due to transport from the nocturnal residual layer and the free troposphere. The concentration of hydrogen peroxide is largely governed by its main precursor, the hydroperoxy radical (HO2), and shows significant anti-correlation with nitrogen oxides (NOx) that remove HO2. A budget calculation indicates that in all campaigns, the noontime photochemical production rate through the self-reaction of HO2 radicals was much larger than photochemical loss due to reaction with OH and photolysis, and that dry deposition is the dominant loss mechanism. Estimated dry deposition velocities varied between approximately 1 and 6 cm s−1, with relatively high values observed during the day in forested regions, indicating enhanced uptake of H2O2 by vegetation. In order to reproduce the change in H2O2 mixing ratios between sunrise and midday, a variable contribution from transport (10 %–100 %) is required to balance net photochemical production and deposition loss. Transport is most likely related to entrainment from the residual layer above the nocturnal boundary layer during the growth of the boundary layer in the morning.

Up in smoke

Atmospheric Chemistry Extensive and intense wildfires in the Pacific Northwest of the United States in 2017 injected large quantities of smoke into the stratosphere. Yu et al. used satellite observations and modeling to characterize the history and chemistry of that smoke. The smoke rose to altitudes between 12 and 23 kilometers within 2 months owing to solar heating of black carbon. The smoke then remained in the stratosphere for more than 8 months. Photochemical loss of organic carbon resulted in a smoke lifetime 40% shorter than expected. Science , this issue p. [587][1] [1]: /lookup/doi/10.1126/science.aax1748

Black carbon lofts wildfire smoke high into the stratosphere to form a persistent plume.

In 2017, western Canadian wildfires injected smoke into the stratosphere that was detectable by satellites for more than 8 months. The smoke plume rose from 12 to 23 kilometers within 2 months owing to solar heating of black carbon, extending the lifetime and latitudinal spread. Comparisons of model simulations to the rate of observed lofting indicate that 2% of the smoke mass was black carbon. The observed smoke lifetime in the stratosphere was 40% shorter than calculated with a standard model that does not consider photochemical loss of organic carbon. Photochemistry is represented by using an empirical ozone-organics reaction probability that matches the observed smoke decay. The observed rapid plume rise, latitudinal spread, and photochemical reactions provide new insights into potential global climate impacts from nuclear war.

Chemistry and Photochemistry of Pyruvic Acid Adsorbed on Oxide Surfaces.

The surface chemistry and photochemistry of gas-phase pyruvic acid (CH3COCOOH) on two oxides, Al2O3 and TiO2, have been investigated using transmission Fourier transform infrared spectroscopy and mass spectrometry. At 298 K, the carboxylic acid group within pyruvic acid is found to react with surface hydroxyl groups (M-OH, M = Al, Ti) to yield pyruvate as a predominant adsorbed organic species. Upon broad-band UV irradiation (λ > 280 nm), there is a loss of adsorbed pyruvate with the concomitant formation of new products. The photochemical loss of pyruvate is higher on TiO2 than on Al2O3 indicating that the photochemistry is enhanced on the surface of a semiconductor oxide, TiO2, compared with an insulator oxide, Al2O3. Analysis of products extracted from the surface with mass spectrometry shows the formation of several new compounds. This includes zymonic acid, which is found to be present under both dark and light conditions, and other higher-molar-mass oligomeric species such as parapyruvic acid, acetolactic acid, and 2,4-dihydroxy-2-methyl-5-oxohexanoic acid that form only under irradiation. Although this study shows that there are some parallels between the aqueous-phase photochemistry of pyruvic acid and the photochemistry of adsorbed pyruvic acid in terms of the products that form, there are also distinct differences, with several other new photoproducts observed on these oxide surfaces, including lactic acid dimers and trimers as well as significant amounts of even larger oligomeric species not seen in the aqueous phase. Because of the role of pyruvic acid, the simplest of the α-keto acids, in the atmosphere and in metabolic pathways, these results have implications for the chemistry that occurs in both indoor and outdoor environments and under prebiotic Earth conditions. Overall, this study provides insights into the surface chemistry and photochemistry of pyruvic acid on different oxides (Al2O3 and TiO2).

One-Year Characterization and Reactivity of Isoprene and Its Impact on Surface Ozone Formation at A Suburban Site in Guangzhou, China

Isoprene has a potentially large effect on ozone (O3) formation in the subtropical, highly polluted city of Guangzhou. Online measurements of isoprene in Guangzhou city are scarce; thus, isoprene levels were monitored for one year at the Guangzhou Panyu Atmospheric Composition Station (GPACS), a suburban site in Guangzhou, using an online gas chromatography-flame ionization detector (GC–FID) system to investigate the characterization and reactivity of isoprene and its effect on the O3 peak profile in different seasons. The results showed that the daily average mixing ratios of isoprene at GPACS were 0.40, 2.20, 1.40, and 0.13 mixing ratio by volume (ppbv) in spring, summer, autumn, and winter, respectively. These values were considerably higher than the mixing ratios of isoprene in the numerous other subtropical and temperate cities around the world. Furthermore, isoprene ranked first with regard to O3 formation potential (OFP) and propylene-equivalent mixing ratio among 56 measured non–methane hydrocarbons (NMHCs). The ratios of isoprene to cis-2-butene, an exhaust tracer, were determined to estimate the fractions of biogenic and anthropogenic emissions. The results revealed a much greater contribution from biogenic than anthropogenic factors during the daytime in all four seasons. In addition, night-time isoprene emissions were mostly associated with vehicles in winter, and the residual isoprene that remained after photochemical loss during the daytime also persisted into the night. The high levels of isoprene in summer and autumn may cause the strong and broad peaks of the O3 profile because of its association with the most favorable meteorological conditions (e.g., high temperature and intense solar radiation) and the highest OH mixing ratio, which could affect human health by exposing people to a high O3 mixing ratio for prolonged periods. The lower mixing ratios of isoprene resulted in a weak and sharp peak in the O3 profile in both spring and winter. The high level of isoprene in the subtropical zone could accentuate its large impact on atmospheric oxidant capacity and air quality in Guangzhou city.

Experimental analysis of development of compulsive behavioural disorders due to intravenous fentanyl self-administration

Organophosphate triesters (OPEs) have caused great concern as a class of emerging environmental contaminants due to their widespread use and their toxicity to organisms. However, the phototransformation behavior of OPE is still not fully understood, which is important for understanding their environmental fate. In the present study, the photodegradation of tricresyl phosphate (TCP), one of the most widely detected OPEs in aqueous environments, was investigated including the direct photolysis and in the presence of several natural water factors, NO2−, Fe3+ and humic acid. The degradation process followed the pseudo-first-order kinetics, with rate constant increasing slightly with increasing initial TCP concentration. The presence of NO2− and Fe3+ was observed to promote the photochemical loss of TCP, while humic acid played a negative role on TCP transformation. Electron spin resonance (EPR) analysis showed that carbon-centered radical was produced in the photolysis process of TCP, and hydroxyl radical contributed to the promotion of rate constant for Fe3+ and NO2−. Four photolysis products were tentatively identified by HPLC-LTQ-Orbitrap MS analysis, and the possible degradation pathways of TCP were proposed. These findings provide a meaningful reference for the fate and transformation of OPEs in natural water.

Photochemistry of 2-butenedial and 4-oxo-2-pentenal under atmospheric boundary layer conditions.

Unsaturated 1,4-dicarbonyl compounds, such as 2-butenedial and 4-oxo-2-pentenal are produced in the atmospheric boundary layer from the oxidation of aromatic compounds and furans. These species are expected to undergo rapid photochemical processing, affecting atmospheric composition. In this study, the photochemistry of (E)-2-butenedial and both E and Z isomers of 4-oxo-2-pentenal was investigated under natural sunlight conditions at the large outdoor atmospheric simulation chamber EUPHORE. Photochemical loss rates, relative to j(NO2), are determined to be j((E)-2-butenedial)/j(NO2) = 0.14 (±0.02), j((E)-4-oxo-2-pentenal)/j(NO2) = 0.18 (±0.01), and j((Z)-4-oxo-2-pentenal)/j(NO2) = 0.20 (±0.03). The major products detected for both species are a furanone (30-42%) and, for (E)-2-butenedial, maleic anhydride (2,5-furandione) (12-14%). The mechanism appears to proceed predominantly via photoisomerization to a ketene-enol species following γ-H abstraction. The lifetimes of the ketene-enol species in the dark from 2-butenedial and 4-oxo-2-pentenal are determined to be 465 s and 235 s, respectively. The ketene-enol can undergo ring closure to yield the corresponding furanone, or further unimolecular rearrangement which can subsequently form maleic anhydride. A minor channel (10-15%) also appears to form CO directly. This is presumed to be via a molecular elimination route of an initial biradical intermediate formed in photolysis, with an unsaturated carbonyl (detected here but not quantified) as co-product. α-Dicarbonyl and radical yields are very low, which has implications for ozone production from the photo-oxidation of unsaturated 1,4-dicarbonyls in the boundary layer. Photochemical removal is determined to be the major loss process for these species in the boundary layer with lifetimes of the order of 10-15 minutes, compared to >3 hours for reaction with OH.

Source apportionment of carbonaceous aerosols in Xi'an, China: insights from a full year of measurements of radiocarbon and the stable isotope <sup>13</sup>C

Abstract. Sources of organic carbon (OC) and elemental carbon (EC) in Xi'an, China, are investigated based on 1-year radiocarbon and stable carbon isotope measurements. The radiocarbon results demonstrate that EC is dominated by fossil sources throughout the year, with a mean contribution of 83±5 % (7±2 µg m−3). The remaining 17±5 % (1.5±1 µg m−3) is attributed to biomass burning, with a higher contribution in the winter (∼24 %) compared to the summer (∼14 %). Stable carbon isotopes of EC (δ13CEC) are enriched in winter (-23.2±0.4 ‰) and depleted in summer (-25.9±0.5 ‰), indicating the influence of coal combustion in winter and liquid fossil fuel combustion in summer. By combining radiocarbon and stable carbon signatures, relative contributions from coal combustion and liquid fossil fuel combustion are estimated to be 45 % (median; 29 %–58 %, interquartile range) and 31 % (18 %–46 %) in winter, respectively, whereas in other seasons more than one half of EC is from liquid fossil combustion. In contrast with EC, the contribution of non-fossil sources to OC is much larger, with an annual average of 54±8 % (12±10 µg m−3). Clear seasonal variations are seen in OC concentrations both from fossil and non-fossil sources, with maxima in winter and minima in summer because of unfavorable meteorological conditions coupled with enhanced fossil and non-fossil activities in winter, mainly biomass burning and domestic coal burning. δ13COC exhibited similar values to δ13CEC, and showed strong correlations (r2=0.90) in summer and autumn, indicating similar source mixtures with EC. In spring, δ13COC is depleted (1.1 ‰–2.4 ‰) compared to δ13CEC, indicating the importance of secondary formation of OC (e.g., from volatile organic compound precursors) in addition to primary sources. Modeled mass concentrations and source contributions of primary OC are compared to the measured mass and source contributions. There is strong evidence that both secondary formation and photochemical loss processes influence the final OC concentrations.

Case study of ozone anomalies over northern Russia in the 2015/2016 winter: measurements and numerical modelling

Abstract. Episodes of extremely low ozone columns were observed over the territory of Russia in the Arctic winter of 2015/2016 and the beginning of spring 2016. We compare total ozone columns (TOCs) from different remote sensing techniques (satellite and ground-based observations) with results of numerical modelling over the territory of the Urals and Siberia for this period. We demonstrate that the provided monitoring systems (including the new Russian Infrared Fourier Spectrometer IKFS-2) and modern three-dimensional atmospheric models can capture the observed TOC anomalies. However, the results of observations and modelling show differences of up to 20 %–30 % in TOC measurements. Analysis of the role of chemical and dynamical processes demonstrates that the observed short-term TOC variability is not a result of local photochemical loss initiated by heterogeneous halogen activation on particles of polar stratospheric clouds that formed under low temperatures in the mid-winter.

Photodegradation Rate Constants for Anthracene and Pyrene Are Similar in/on Ice and in Aqueous Solution.

Snowpacks contain a variety of chemicals, including organic pollutants such as toxic polycyclic aromatic hydrocarbons (PAHs). While PAHs undergo photodegradation in snow and ice, the rates of these reactions remain in debate. Some studies report that photochemical reactions in snow proceed at rates similar to those expected in a supercooled aqueous solution, but other studies report faster reaction rates, particularly at the air-ice interface (i.e., the quasi-liquid layer, or QLL). In addition, one study reported a surprising nonlinear dependence on photon flux. Here we examine the photodegradation of two common PAHs, anthracene and pyrene, in/on ice and in solution. For a given PAH, rate constants are similar in aqueous solution, in internal liquid-like regions of ice, and at the air-ice interface. In addition, we find the expected linear relationship between reaction rate constant and photon flux. Our results indicate that rate constants for the photochemical loss of PAHs in, and on, snow and ice are very similar to those in aqueous solution, with no enhancement at the air-ice interface.

Tropospheric sources and sinks of gas-phase acids in the Colorado Front Range

Abstract. We measured organic and inorganic gas-phase acids in the Front Range of Colorado to better understand their tropospheric sources and sinks using a high-resolution time-of-flight chemical ionization mass spectrometer. Measurements were conducted from 4 to 13 August 2014 at the Boulder Atmospheric Observatory during the Front Range Air Pollution and Photochemistry Éxperiment. Diurnal increases in mixing ratios are consistent with photochemical sources of HNO3, HNCO, formic, propionic, butyric, valeric, and pyruvic acid. Vertical profiles taken on the 300 m tower demonstrate net surface-level emissions of alkanoic acids, but net surface deposition of HNO3 and pyruvic acid. The surface-level alkanoic acid source persists through both day and night, and is thus not solely photochemical. Reactions between O3 and organic surfaces may contribute to the surface-level alkanoic acid source. Nearby traffic emissions and agricultural activity are a primary source of propionic, butyric, and valeric acids, and likely contribute photochemical precursors to HNO3 and HNCO. The combined diel and vertical profiles of the alkanoic acids and HNCO are inconsistent with dry deposition and photochemical losses being the only sinks, suggesting additional loss mechanisms.

Characteristics and Source Apportionment of Volatile Organic Compounds in the Rainy Season of Guangzhou City

Atmospheric volatile organic compounds (VOCs) were measured in an urban area of Guangzhou on July 2016 using an on-line gas chromatography mass spectrometry/fire ion detector. Seventy-three VOCs were detected with an average concentration of (118.83±79.40) μg·m-3, a maximum concentration of 492.42 μg·m-3, and a minimum concentration of 10.54 μg·m-3 during the period. The peak value of the TVOC concentration appeared at about 07:00 in the morning, indicating that motor vehicle pollution had a significant contribution at the site. The minimum value appeared at about 14:00 in the afternoon, related to photochemical losses. High concentrations were also observed from 21:00 to 24:00, which was probably related to pollution emissions and boundary layer compression. Source analysis by PMF showed that the site was mainly affected by five VOC sources: vehicle exhaust, solvent use, fuel loss at fuel stations, plant emissions, and cooking exhaust, the contributions of which were 29.79%, 26.61%, 24.86%, 9.91%, and 8.84%, respectively. Vehicle exhaust was the largest source of VOCs during the daytime, while the contribution of plant emissions increased significantly at noon. The contribution of solvent uses and fuel loss at fuel stations rose during the night and became the main source of VOCs until early morning.

Modeling Martian Atmospheric Losses over Time: Implications for Exoplanetary Climate Evolution and Habitability

Abstract In this Letter, we make use of sophisticated 3D numerical simulations to assess the extent of atmospheric ion and photochemical losses from Mars over time. We demonstrate that the atmospheric ion escape rates were significantly higher (by more than two orders of magnitude) in the past at ∼4 Ga compared to the present-day value owing to the stronger solar wind and higher ultraviolet fluxes from the young Sun. We found that the photochemical loss of atomic hot oxygen dominates over the total ion loss at the current epoch, while the atmospheric ion loss is likely much more important at ancient times. We briefly discuss the ensuing implications of high atmospheric ion escape rates in the context of ancient Mars, and exoplanets with similar atmospheric compositions around young solar-type stars and M-dwarfs.

Interferometric Imaging of Titan’s HC3N, H13CCCN, and HCCC15N

Abstract We present the first maps of cyanoacetylene isotopologues in Titan’s atmosphere, including H13CCCN and HCCC15N, detected in the 0.9 mm band using the Atacama Large Millimeter/submillimeter array (ALMA) around the time of Titan’s (southern winter) solstice in 2017 May. The first high-resolution map of HC3N in its v 7 = 1 vibrationally excited state is also presented, revealing a unique snapshot of the global HC3N distribution, free from the strong optical depth effects that adversely impact the ground-state (v = 0) map. The HC3N emission is found to be strongly enhanced over Titan’s south pole (by a factor of 5.7 compared to the north pole), consistent with rapid photochemical loss of HC3N from the summer hemisphere combined with production and transport to the winter pole since the 2015 April ALMA observations. The H13CCCN/HCCC15N flux ratio is derived at the southern HC3N peak, and implies an HC3N/HCCC15N ratio of 67 ± 14. This represents a significant enrichment in 15N compared with Titan’s main molecular nitrogen reservoir, which has a 14N/15N ratio of 167, and confirms the importance of photochemistry in determining the nitrogen isotopic ratio in Titan’s organic inventory.

Aqueous multivariate phototransformation kinetics of dissociated tetracycline: implications for the photochemical fate in surface waters.

Antibiotics are ubiquitous pollutants in aquatic systems and can exist as different dissociated species depending on the water pH. New knowledge of their multivariate photochemical behavior (i.e., the photobehavior of different ionized forms) is needed to improve our understanding on the fate and possible remediation of these pharmaceuticals in surface and waste waters. In this study, the photochemical degradation of aqueous tetracycline (TC) and its dissociated forms (TCH20, TCH-, and TC2-) was investigated. Simulated sunlight experiments and matrix calculations indicated that the three dissociated species had dissimilar photolytic kinetics and photooxidation reactivities. TC2- photodegraded the fastest due to apparent photolysis with a kinetic constant of 0.938 ±0.021min-1, followed by TCH- (0.020 ±0.005min-1) and TCH20 (0.012 ±0.001min-1), whereas TCH- was found to be the most highly reactive toward •OH (105.78 ± 3.40M-1s-1), and TC2- reacted the fastest with 1O2 (344.96 ± 45.07M-1s-1). Water with relatively high pH (e.g., ~ 8-9) favors the dissociated forms of TCH- and TC2- which are most susceptible to photochemical loss processes compared to neutral TC. The calculated corresponding environmental half-lives (t1/2,E) in sunlit surface waters ranged from 0.05h for pH = 9 in midsummer to 3.68h for pH = 6 in midwinter at 45° N latitude. The process was dominated by apparent photolysis (especially in summer, 62-91%), followed by 1O2 and •OH oxidation. Adjusting the pH to slightly alkaline conditions prior to UV or solar UV light treatment may be an effective way of enhancing the photochemical removal of TC from contaminated water. Graphical abstract Aqueous multiple photochemical behavior of dissociated tetracycline (TCH20, TCH-, and TC2-) is first comprehensively reported on revealing the phototransformation kinetics and implications for the fate in surface waters.

Reconciling discrepancies in the source characterization of VOCs between emission inventories and receptor modeling

Emission inventory (EI) and receptor model (RM) are two of the three source apportionment (SA) methods recommended by Ministry of Environment of China and used widely to provide independent views on emission source identifications. How to interpret the mixed results they provide, however, were less studied. In this study, a cross-validation study was conducted in one of China's fast-developing and highly populated city cluster- the Pearl River Delta (PRD) region. By utilizing a highly resolved speciated regional EI and a region-wide gridded volatile organic compounds (VOCs) speciation measurement campaign, we elucidated underlying factors for discrepancies between EI and RM and proposed ways for their interpretations with the aim to achieve a scientifically plausible source identification. Results showed that numbers of species, temporal and spatial resolutions used for comparison, photochemical loss of reactive species, potential missing sources in EI and tracers used in RM were important factors contributed to the discrepancies. Ensuring the consensus of species used in EIs and RMs, utilizing a larger spatial coverage and longer time span, addressing the impacts of photochemical losses, and supplementing emissions from missing sources could help reconcile the discrepancies in VOC source characterizations acquired using both approaches. By leveraging the advantages and circumventing the disadvantages in both methods, the EI and RM could play synergistic roles to obtain robust SAs to improve air quality management practices.

The effect of pH, nitrate, iron (III) and bicarbonate on photodegradation of oxytetracycline in aqueous solution

Photodegradation is a very important elimination pathway of Oxytetracycline (OTC) in aquatic environments. The photochemical behavior of OTC in absence/presence of nitrate, iron (III) and bicarbonate, especially their combinations, was systematically studied in aqueous solution by employing a Suntest-CPS+ sunlight simulator in this work. Meanwhile, the effect of solution pH (4.8–9.1) and irradiation intensity (250–500 W/m2) on the removal efficiency of OTC have also been assessed. The results showed that OTC degradation followed a pseudo-first-order kinetics in most conditions except for irradiation in neutral/alkaline solutions and in presence of NO3−, where a two-step pseudo-first-order kinetics was obeyed. OTC degradation was highly pH-dependent and increased significantly with increasing pH. The presence of NO3− and HCO3− promoted the photochemical loss of OTC in aqueous solution by generating hydroxyl radicals and adjusting solution pH, respectively; whereas iron (III) was found to play a negative role under the conditions studied. The influence of NO3− coupled with Fe3+ was not a simple additive effect and the overall performance was to inhibit OTC degradation. In addition, combination of HCO3− and NO3− exhibited an antagonistic effect. Results from irradiation in co-presence of NO3−, Fe3+ and HCO3− suggested that direct photolysis of OTC was much more efficient than its indirect photolysis. These results provide a meaningful reference for understanding the fate and transformation of OTC in natural water systems.

Chloride Oxidation by Ruthenium Excited-States in Solution.

Photodriven HCl splitting to produce solar fuels is an important goal that requires strong photo-oxidants capable of chloride oxidation. In a molecular approach toward this goal, three ruthenium compounds with 2,2'-bipyrazine backbones were found to oxidize chloride ions in acetone solution. Nanosecond transient absorption measurements provide compelling evidence for excited-state electron transfer from chloride to the Ru metal center with rate constants in excess of 1010 M-1 s-1. The Cl atom product was trapped with an olefin. This reactivity was promoted through pre-organization of ground-state precursors in ion pairs. Chloride oxidation with a tetra-cationic ruthenium complex was most favorable, as the dicationic complexes were susceptible to photochemical ligand loss. Marcus analysis afforded an estimate of the chlorine formal reduction potential E°(Cl•/-) = 1.87 V vs NHE that is at least 300 meV more favorable than the accepted values in water.

Volatile organic compounds (VOCs) in photochemically aged air from the eastern and western Mediterranean

Abstract. During the summertime CYPHEX campaign (CYprus PHotochemical EXperiment 2014) in the eastern Mediterranean, multiple volatile organic compounds (VOCs) were measured from a 650 m hilltop site in western Cyprus (34° 57′ N/32° 23′ E). Periodic shifts in the northerly Etesian winds resulted in the site being alternately impacted by photochemically processed emissions from western (Spain, France, Italy) and eastern (Turkey, Greece) Europe. Furthermore, the site was situated within the residual layer/free troposphere during some nights which were characterized by high ozone and low relative humidity levels. In this study we examine the temporal variation of VOCs at the site. The sparse Mediterranean scrub vegetation generated diel cycles in the reactive biogenic hydrocarbon isoprene, from very low values at night to a diurnal median level of 80–100 pptv. In contrast, the oxygenated volatile organic compounds (OVOCs) methanol and acetone exhibited weak diel cycles and were approximately an order of magnitude higher in mixing ratio (ca. 2.5–3 ppbv median level by day, range: ca. 1–8 ppbv) than the locally emitted isoprene and aromatic compounds such as benzene and toluene. Acetic acid was present at mixing ratios between 0.05 and 4 ppbv with a median level of ca. 1.2 ppbv during the daytime. When data points directly affected by the residual layer/free troposphere were excluded, the acid followed a pronounced diel cycle, which was influenced by various local effects including photochemical production and loss, direct emission, dry deposition and scavenging from advecting air in fog banks. The Lagrangian model FLEXPART was used to determine transport patterns and photochemical processing times (between 12 h and several days) of air masses originating from eastern and western Europe. Ozone and many OVOC levels were ∼ 20 and ∼ 30–60 % higher, respectively, in air arriving from the east. Using the FLEXPART calculated transport time, the contribution of photochemical processing, sea surface contact and dilution was estimated. Methanol and acetone decreased with residence time in the marine boundary layer (MBL) with loss rate constants of 0.74 and 0.53 day−1 from eastern Europe and 0.70 and 0.34 day−1 from western Europe, respectively. Simulations using the EMAC model underestimate these loss rates. The missing sink in the calculation is most probably an oceanic uptake enhanced by microbial consumption of methanol and acetone, although the temporal and spatial variability in the source strength on the continents might play a role as well. Correlations between acetone and methanol were weaker in western air masses (r2 = 0.68), but were stronger in air masses measured after the shorter transport time from the east (r2 = 0.73).