Search

Copyright of Adeptus is the property of Polish Academy of Sciences, Institute of Slavic Studies and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Formaldehyde (HCHO) and hydrogen peroxide (H2O2) play a key role in atmospheric oxidation processes. They act as sources and sinks for HOx radicals (OH + HO2), with OH as the primary oxidant that governs the atmospheric self-cleaning capacity. Measurements of these species allow evaluation of global chemistry-transport models which need to account for multifarious source distributions, transport and mixing, complex photochemical reaction pathways and deposition processes. HCHO is an intermediate produced during the oxidation of VOCs and is an indicator of photochemical activity and combustion related emissions. Due to its many production pathways and its rather short lifetime of only several hours at noon, accurate modelling of this species is challenging. In this study, we use in situ observations in the marine boundary layer (MBL) to evaluate results of the general circulation model EMAC (ECHAM5/MESSy2 Atmospheric Chemistry). The dataset was obtained during the AQABA ship campaign around the Arabian Peninsula in summer 2017. This region is characterized by high mixing ratios of photochemical air pollution, high humidity and strong solar irradiation, especially in the area around the Suez Canal and the Arabian Gulf. We find that EMAC fails to predict absolute mixing ratios of HCHO, especially during high pollution events, but it reproduces most of the HCHO variability seen in the different regions, while it systematically overestimates H2O2. This is mainly attributed to missing primary VOC emissions and the overestimation of HOx radicals, and also related to the models coarse spatial resolution. [ABSTRACT FROM AUTHOR]

Strongly enhanced tropospheric ozone mixing ratios have been reported in the Arabian Basin, a region with intense solar radiation and high concentrations of ozone precursors such as nitrogen oxides and volatile organic compounds. To analyze photochemical ozone production in the marine boundary layer (MBL) around the Arabian Peninsula, we use ship-borne observations of NO, NO2, O3, OH, HO2, HCHO, actinic flux, water vapor, pressure and temperature obtained during the summer 2017 Air Quality and Climate in the Arabian Basin (AQABA) campaign, compare them to simulation results of the ECHAM-MESSy atmospheric chemistry (EMAC) general circulation model. Net ozone production rates (NOPR) were greatest over the Gulf of Oman, the Northern Red Sea and the Arabian Gulf with median values of 14 ppbv day−1, 16 ppbv day−1 and 28 ppbv day−1, respectively. NOPR over the Mediterranean, the Southern Red Sea and the Arabian Sea did not significantly deviate from zero; however, results for the Arabian Sea indicate weak net ozone production of 5 ppbv day−1, and net ozone destruction over the Mediterranean and the Southern Red Sea with −2 ppbv day−1 and −4 ppbv day−1, respectively. Constrained by measured HCHO/NO2-ratios, our photochemistry calculations show that net ozone production in the MBL around the Arabian Peninsula occurs mostly in a transition regime between NOx and VOC-limitation with a tendency towards NOx-limitation except over the Northern Red Sea and the Oman Gulf. [ABSTRACT FROM AUTHOR]

Shipborne measurements of nitryl chloride (ClNO2), hydrogen chloride (HCl) and sulphur dioxide (SO2) were made during the AQABA (Air Quality and climate change in the Arabian BAsin) ship campaign in summer 2017. The dataset includes measurements over the Mediterranean Sea, the Suez Canal, the Red Sea, the Gulf of Aden, the Arabian Sea, the Gulf of Oman and the Arabian Gulf (also known as Persian Gulf) with observed ClNO2 mixing ratios ranging from the limit of detection to ≈ 600 pptv. We examined the regional variability in the generation of ClNO2 via the uptake of dinitrogen pentoxide (N2O5) to Cl-containing aerosol and its importance for Cl-atom generation in a marine boundary layer under the (variable) influence of emissions from shipping and oil industry. The yield of ClNO2 formation per NO3 radical generated was generally low (median of ≈ 1–5 % depending on the region), mainly as a result of gas-phase loss of NO3 dominating over heterogeneous loss of N2O5, the latter being disfavoured by the high temperatures found throughout the campaign. The contributions of ClNO2 photolysis and OH-induced HCl oxidation to Cl-radical formation were derived and their relative contributions over the diel cycle compared. The results indicate that over the northern Red Sea, the Gulf of Suez and the Gulf of Oman the formation of Cl-atoms will enhance the oxidation rates of some VOCs, especially in the early morning. [ABSTRACT FROM AUTHOR]

In the atmospheric boundary layer (ABL), incomplete mixing (i.e., segregation) results in reduced chemical reaction rates compared to those expected from mean values and rate constants derived under well mixed conditions. Recently, segregation has been suggested as a potential cause of discrepancies between modelled and measured OH radical concentrations, especially under high isoprene conditions. Therefore, the influence of segregation on the reaction of OH radicals with isoprene has been investigated by modelling studies and one ground-based and one aircraft campaign. In this study, we measured isoprene and OH radicals with high time resolution in order to directly calculate the influence of segregation in a low-NOx and high-isoprene environment in the central Amazon basin. The calculated intensities of segregation (Is) at the Amazon Tall Tower Observatory (ATTO) above canopy top are in the range of values determined at a temperate deciduous forest (ECHO-campaign) in a high-NOx low-isoprene environment, but stay below 10%. To establish a more general idea about the causes of segregation and their potential limits, further analysis was based on the budget equations of isoprene mixing ratios, the variance of mixing ratios, and the balance of the intensity of segregation itself. Furthermore, it was investigated if a relation of Is to the turbulent isoprene surface flux can be established theoretically and empirically, as proposed previously. A direct relation is not given and the amount of variance in Is explained by the isoprene flux will be higher the less the influence from other processes (e.g., vertical advection) is and will therefore be greater near the surface. Although ground based values of Is from ATTO and ECHO are in the same range, we could identify different dominating processes driving Is. For ECHO the normalized variance of isoprene had the largest contribution, whereas for ATTO the different transport terms expressed as a residual were dominating. To get a more general picture of Is and its potential limits in the ABL, we also compared these ground based measurements to ABL modelling studies and results from an aircraft campaign. The ground based measurements show the lowest values of the degree of inhomogenous mixing (<20%, mostly below 10%). These values increase if the contribution of lower frequencies is added. Values integrated over the whole boundary layer (modelling studies) are in the range from 10% to 30% and aircraft measurements integrating over different landscapes are amongst the largest reported. This presents evidence that larger scale heterogeneities in land surface properties contribute substantially to Is. [ABSTRACT FROM AUTHOR]

We present version 4.0gmdd of the atmospheric chemistry box model CAABA/MECCA which now includes a number of new features: (i) skeletal mechanism reduction, (ii) the MOM chemical mechanism for volatile organic compounds, (iii) an option to include reactions from the Master Chemical Mechanism (MCM) and other chemical mechanisms, (iv) updated isotope tagging, and (v) improved and new photolysis modules (JVAL, RADJIMT, DISSOC). Further, when MECCA is connected to a global model, the new feature of coexisting multiple chemistry mechanisms (PolyMECCA/CHEMGLUE) can be used. Additional changes have been implemented to make the code more user-friendly and to facilitate the analysis of the model results. Like earlier versions, CAABA/MECCA-4.0gmdd is a community model published under the GNU General Public License. [ABSTRACT FROM AUTHOR]

Whenactivated with base, the iron(II) complexes with tetradentateamine(imine)diphosphine ligands, (S,S)-trans-[FeCl(CO)(PAr2-NH-N-PAr′2)]BF4(1: Ar, Ar′ = Ph; 2: Ar = Ph, Ar′ = 4-MeC6H4; 3: Ar, Ar′ = 3,5-Me2C6H3), are very active for the asymmetric transfer hydrogenation (ATH)of ketones in KOtBu/2-propanol. For ATH,better enantioselectivity, but lower catalytic activity, was observedin general when using catalyst precursors with the bulkier dixylylphosphinogroups compared to those with diphenylphosphino groups. The complexeswere much less active for the pressure hydrogenation of ketones, where 1and 2produced racemic product alcohols, while 3yielded chiral alcohols with an enantiomeric excess of upto 70% (R) at turnover frequencies up to 80 h–1and turnover numbers of 100 for a range of ketonesat 50 °C and 20 atm H2. This is a rare example ofasymmetric pressure hydrogenation using an iron complex. Unlike thecase of ATH, there is no effect on the rate upon the addition of KOtBu beyond the 2 equiv needed to convert theprecursor complex to the active amido(ene-amido) and amine(ene-amido)hydridoforms. Both AH and ATH reactions share the same iron hydride intermediateformed by reaction of the amido(ene-amido) iron complex witheither dihydrogen or 2-propanol. Kinetic studies on the H2hydrogenation of acetophenone catalyzed by 1, activatedby base in benzene, using the method of initial rates indicated thatthe heterolytic splitting of the dihydrogen at the amido(ene-amido)iron complex is the turnover-limiting step of the catalytic cyclefor hydrogenation. For 1in benzene at 323 K over theranges of concentrations [1] = (2.4–4.8) ×10–4M and [ketone] = (3.6–7.2) × 10–2M, and of H2pressures = 10–20atm, the rate law is rate = k[1][H2], with k= 0.16 ± 0.01 M–1s–1, ΔH⧧= 10.0 ± 0.2 kcal mol–1, and ΔS⧧= −31.0 ± 0.5 cal mol–1K–1. Detailed DFT calculationsalso support the finding that the barrier for H2splittingis the turnover-limiting step. The higher barrier for H2activation compared to isopropanol activation in order to generatethe active amine(ene-amido)hydrido form explains why this systemis biased toward ATH over AH. [ABSTRACT FROM AUTHOR]

The Middle East has exceptional environmental qualities, with intense solar radiation, extensive deserts and water scarcity that promotes dust mobilisation. Furthermore, the region is subject to high levels of air pollution due to emissions from traffic and petrochemical industry. The AQABA (Air Quality and climate in the Arabian BAsin) field campaign took place in summer 2017 covering a ship track from Toulon to Kuwait through the Mediterranean, around the Arabian Peninsula and back. As one of the most reactive oxidants in the atmosphere, the hydroxyl radical (OH) is a driving force in air chemistry. It is produced photochemically by UV radiation and can react with most VOC emitted by biogenic or anthropogenic sources. Its close chemical relative, the hydroperoxyl radical (HO2) is a major source of tropospheric ozone. HOx (OH and HO2) play a role in the production of aerosols via the formation of low volatility compounds like organic acids. Production and loss of HOx are impacted by dust through its influence on photolysis frequencies and uptake of HOx precursors. Additionally, uptake of HOx oxidation products on dust particles increases their hydrophilicity and therefore may change chemical and physical surface properties.During the AQABA campaign, HOx was measured using laser-induced fluorescence (LIF) to investigate the regional influences on HOx concentrations and budgets. This study contrasts HOx concentrations and OH recycling between different environmental conditions along the ship track. This includes the Arabian Gulf at very high temperatures, with dust events, petrochemical industries and ship traffic, compared to the pristine environment over the Arabian Sea, and European dominated air masses over the Mediterranean Sea. The measurements will be compared to calculations obtained using the CAABA/MECCA box model. [ABSTRACT FROM AUTHOR]