Cecilia Arsene

Photochemical reactions in the tropospheric aqueous phase and on particulate matter

This paper is a tutorial review in the field of atmospheric chemistry. It describes some recent developments in tropospheric photochemistry in the aqueous phase and on particulate matter. The main focus is regarding the transformation processes that photochemical reactions induce on organic compounds. The relevant reactions can take place both on the surface of dispersed particles and within liquid droplets (e.g. cloud, fog, mist, dew). Direct and sensitised photolysis and the photogeneration of radical species are the main processes involved. Direct photolysis can be very important in the transformation of particle-adsorbed compounds. The significance of direct photolysis depends on the substrate under consideration and on the colour of the particle: dark carbonaceous material shields light, therefore protecting the adsorbed molecules from photodegradation, while a much lower protection is afforded for the light-shaded mineral fraction of particulate. Particulate matter is also rich in photosensitisers (e.g. quinones and aromatic carbonyls), partially derived from PAH photodegradation. These compounds can induce degradation of other molecules upon radiation absorption. Interestingly, substrates such as methoxyphenols, major constituents of wood-smoke aerosol, can also enhance the degradation of some sensitisers. Photosensitised processes in the tropospheric aqueous phase have been much less studied: it will be interesting to assess the photochemical properties of Humic-Like Substances (HULIS) that are major components of liquid droplets. The main photochemical sources of reactive radical species in aqueous solution and on particulate matter are hydrogen peroxide, nitrate, nitrite, and Fe(iii) compounds and oxides. The photogeneration of hydroxyl radicals can be important in polluted areas, while their transfer from the gas phase and dark generation are usually prevailing on an average continental scale. The reactions involving hydroxyl radicals can induce very fast transformation of compounds reacting with (*)OH at a diffusion-controlled rate (10(10) M(-1) s(-1)), with time scales of an hour or less. The hydroxyl-induced reactivity in solution can be faster than in the gas phase, influencing the degradation kinetics of water-soluble compounds. Moreover, photochemical processes in fog and cloudwater can be important sources of secondary pollutants such as nitro-, nitroso-, and chloro-derivatives.

Inhibition vs. enhancement of the nitrate-induced phototransformation of organic substrates by the •OH scavengers bicarbonate and carbonate

Contrary to common expectations, the hydroxyl scavengers, carbonate and bicarbonate, are able to enhance the phototransformation by nitrate of a number of substituted phenols. Carbonate and bicarbonate, in addition to modifying the solution pH, are also able to induce a considerable formation of the carbonate radicals upon nitrate photolysis. The higher availability of less-reactive species than the hydroxyl radical would contribute to substantially enhance the photodegradation of the phenols/phenolates that are sufficiently reactive toward the carbonate radical. This phenomenon has a potentially important impact on the fate of the relevant compounds in surface waters. In contrast, the degradation of compounds that are not sufficiently reactive toward CO(3)(-*) is inhibited by carbonate and bicarbonate because of the scavenging of *OH.

Photostability and photolability of dissolved organic matter upon irradiation of natural water samples under simulated sunlight

Abstract.In this study the photostability/photolability of Dissolved Organic Matter (DOM) was assessed in both lake and groundwater. DOM in groundwater can undergo significant irradiation when drawn to the surface for agricultural purposes. DOM was generally photostable in lake and photolabile in groundwater, with a more elevated rate constant of DOM disappearance in groundwater samples with higher Non-Purgeable Organic Carbon (NPOC). DOM in lake water became photolabile upon acidification. The parallel decrease of both Total Organic Carbon (TOC) and NPOC suggests that actual photomineralisation took place in the samples. The •OH radicals play a secondary role into DOM photomineralisation in lake water, despite the fact that their generation rate considerably increases at acidic pH. The role of •OH is also minor in the photomineralisation of DOM contained in nitrate-rich groundwater.

Formation of Organobrominated Compounds in the Presence of Bromide under Simulated Atmospheric Aerosol Conditions

Photobromination of phenol takes place upon UV/Vis irradiation of FeIII and bromide under acidic conditions, and most likely involves the brominating agent Br2(-*). Bromination is also observed in the presence of nitrate and bromide under UV irradiation, most likely involving Br2(-*) formed upon oxidation of bromide by *OH. Moreover, quantitative bromination of phenol is observed in the dark in the presence of hydrogen peroxide and bromide. This process is strongly favored under acidic conditions, but a residual, pH-independent bromination pathway is also present. The rates and yields of bromination (up to 100%) are considerably higher than those reported for chlorination under comparable conditions, suggesting that the higher activity of bromine species could compensate for the lower concentration of bromide ions in aerosol compared to chlorides. The reported processes are potent tial sources of reactive bromine species (Br2(-*), HBrO) and aromatic bromo derivatives in atmospheric aerosols, in particular after the acidification process linked with aerosol aging.

SAMPLE PREPARATION FOR TRACE ANALYSIS BY CHROMATOGRAPHIC METHODS

The determination of trace analytes in complex natural matrices often requires extensive sample extraction and preparation prior to chromatographic analysis. Correct sample preparation can reduce analysis time, sources of error, enhance sensitivity, and enable unequivocal identification, confirmation, and quantification. This overview considers general aspects on sample preparation techniques for trace analysis in various matrices. The discussed extraction/enrichment techniques cover classical methods, such as Soxhlet and liquid-liquid extractions along with more recently developed techniques like pressurized liquid extraction, liquid phase microextraction (LPME), accelerated microwave extraction, and ultrasound-assisted extraction. This overview also deals with more selective methodologies, such as solid phase extraction (SPE), solid phase microextraction (SPME), and stir bar sorptive extraction (SBSE). The adopted approach considers the equilibriums involved in each technique. The applicability of each technique in environmental, food, biological, and pharmaceutical analyses is discussed, particularly for the determination of trace organic compounds by chromatographic methods.

GC × GC-MS HYPHENATED TECHNIQUES FOR THE ANALYSIS OF VOLATILE ORGANIC COMPOUNDS IN AIR

Comprehensive two-dimensional gas chromatography (GC × GC) and its direct applications to measurement of volatile and semivolatile organic compounds in air are reviewed and discussed. The paper includes a brief discussion of the instrumental set-up and theory for the comprehensive GC × GC hyphenated with different detection techniques. Several reviewed types of modulators demonstrate that the applications of comprehensive GC × GC are still under development, underlying the flexibility of the system as well. The fundamental differences between one-dimensional and two-dimensional gas chromatography, regarding their potential to provide both qualitative and quantitative information, are also presented. The present article focuses on reported applications dealing with the analysis of volatile and semivolatile organic compounds from air (gas and particles related), but some data related to other sample types analyzed with comprehensive GC × GC are also briefly presented. The paper supports the idea that there is a good reason for interest in comprehensive GC × GC, which seems to be a suitable technique for applications in the separation of complex mixtures of volatile and semivolatile compounds.

Applications of Liquid Chromatographic Techniques in the Chemical Characterization of Atmospheric Aerosols

This paper presents a comprehensive literature review on liquid chromatography (LC) techniques, singular and/or in tandem with mass spectrometry (MS), applied to identify and quantify inorganic and organic chemical constituents in atmospheric aerosols. Significant contributions in the field of aerosol chemical composition, obtained either by field measurements or laboratory investigations, are also highlighted. The two major LC techniques that have been used to date to identify aerosol chemical constituents are ionic chromatography (IC) and high performance liquid chromatography (HPLC). They have been used more often in off-line than in on-line mode and both of them can be coupled with several types of mass spectrometry (MS). The two techniques seem to be suitable to obtain information on water-soluble inorganic and organic ions with low molecular weight (IC) or on organic compounds with higher molecular mass (HPLC). LC–MS with electrospray ionization is among the most powerful LC techniques to elucidate possible major contributors to the so-called “unidentified substances” fraction, and it is often reported in studies aimed to investigate the chemical composition of the aerosols. The information generated by liquid chromatography, especially related to different organic compounds into the aerosol particles, helped to elucidate some reaction pathways or improve some postulated mechanisms that may be responsible for the formation of secondary organic aerosols (SOA). However, despite the efforts made to elucidate the aerosols chemical composition, a significant part still remains highly unclear. It is expected that new studies carried out by using complementary analytical techniques will help answering such questions, but further development is needed to make such techniques applicable to large-scale analysis all over the world.

Chemical characteristics of size resolved atmospheric aerosols in Iasi, north-eastern Romania. Nitrogen-containing inorganic compounds controlling aerosols chemistry in the area

This study assesses the atmospheric aerosol load and behaviour (size and seasonal dependent) of the major inorganic and organic aerosol ionic components (i.e., acetate, (C2H3O2), formate, (HCO2), fluoride, (F–), chloride, (Cl–), 15 nitrite, (NO2), nitrate, (NO3), phosphate, (PO4), sulfate, (SO4), oxalate, (C2O4), sodium, (Na+), potassium, (K+), ammonium, (NH4), magnesium, (Mg2+) and calcium (Ca2+), in Iasi urban area, north-eastern Romania. Continuous measurements were carried out over 2016 by means of a cascade Dekati Low-Pressure Impactor (DLPI) performing aerosol size classification in 13 specific fractions evenly distributed over the 0.0276 up to 9.94 μm size range. Fine particulate Cl–, NO3, NH4 and K+ exhibited clear minima during the warm seasons and clear maxima over the cold seasons, mainly 20 controlled by corroboration between factors such as enhancement in the emission sources, changes in the mixed layer depth and specific meteorological conditions. Fine particulate SO4 did not show much variation with respect to seasons. Particulate NH4 and NO3 ions were identified as critical parameters controlling aerosols chemistry in the area. The measured concentrations of particulate NH4 and NO3 in fine mode (PM2.5) aerosols were found to be in reasonable good agreement with modelled values for winter but not for summer, an observation reflecting actually the susceptibility of 25 NH4NO3 aerosols to be lost due to volatility over the warm seasons. Clear evidences have been obtained for the fact that in Iasi, north-eastern Romania, NH4 in PM2.5 is primarily associated with SO4 and NO3 but not with Cl–. However, indirect ISORROPIA-II estimations showed that the atmosphere in the investigated area might be ammonia-rich during both the cold and warm seasons, such as enough NH3 to be present to neutralize H2SO4, HNO3 and HCl acidic components and to generate fine particulate ammonium salts, in the form of (NH4)2SO4, NH4NO3 and NH4Cl. ISORROPIA-II runs allowed us estimating 30 that over the warm seasons ~ 35 % of the total analyzed samples presented pH values in the very strong acidity fraction (0–3 Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-1030 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 17 November 2017 c

Atmospheric Wet Deposition Monitoring in Iasi, Romania

The degradation of environmental quality in many regions of the world has accelerated during the past decades especially due to the industrial development, which has led to important changes in different compartments of the environment. Important atmospheric species are considered to be responsible for wide spread environmental effects including, changes in pH deposition, corrosion of buildings material etc. Deposition of air pollutants is an important loss process for most of the species present in the atmosphere that can cause severe damage to ecosystems. Air pollutants are deposited to the earth’s surface especially through wet and dry processes. Deposition rates are determined in order to estimate the impact of these pollutants on ecological systems. Deposition of pollutants by wet processes is relatively easy to determine through analysis of precipitation samples. However, it is well recognised that less is known about dry deposition, which is much more difficult to measure (estimated using measured air concentration and the deposition velocity concept) and which appears to predominate near strong emission sources with wet deposition predominating further downwind (Whelpdale et al., 1997). Direct measurement of pollutants deposition by dry processes is more difficult and requires extensive instrumentation and technical resources.

Determination of Photolysis Frequencies for Selected Carbonyl Compounds in the EUPHORE Chamber Environmental

Small carbonyl compounds are formed during the photochemical oxidation of many volatile organic compounds (VOC’s), in urban as well as in rural areas. Photolysis and reaction with the OH radical are the most important initiation reactions for the atmospheric degradation of these compounds, and lead to the formation of peroxy radicals in the former case and either stable molecules and/or free radicals in the latter case (Finlayson-Pitts and Pitts, 1999).