John C. Wenger

Prolonged stratospheric ozone loss in the 1995–96 Arctic winter

It is well established that extensive depletion of ozone, initiated by heterogenous reactions on polar stratospheric clouds (PSCs) can occur in both the Arctic and Antarctic lower stratosphere. Moreover, it has been shown that ozone loss rates in the Arctic region in recent years reached values comparable to those over the Antarctic,. But until now the accumulated ozone losses over the Arctic have been the smaller, mainly because the period of Arctic ozone loss has not—unlike over the Antarctic—persisted well into springtime. Here we report the occurrence—during the unusually cold 1995–96 Arctic winter—of the highest recorded chemical ozone loss over the Arctic region. Two new kinds of behaviour were observed. First, ozone loss at some altitudes was observed long after the last exposure to PSCs. This continued loss appears to be due to a removal of the nitrogen species that slow down chemical ozone depletion. Second, in another altitude range ozone loss rates decreased while PSCs were still present, apparently because of an early transformation of the ozone-destroying chlorine species into less active chlorinenitrate. The balance between these two counteracting mechanisms is probably a fine one, determined by small differences in wintertime stratospheric temperatures. If the apparent cooling trend in the Arctic stratosphere is real, more dramatic ozone losses may occur in the future.

A Study of Ozone Laminae Using Diabatic Trajectories, Contour Advection and Photochemical Trajectory Model Simulations.

In this paper, we show that the rate of ozone loss in both polar and mid-latitudes, derived from ozonesonde and satellite data, has almost the same vertical distribution (although opposite sense) to that of ozone laminae abundance. Ozone laminae appear in the lower stratosphere soon after the polar vortex is established in autumn, increase in number throughout the winter and reach a maximum abundance in late winter or spring. We indicate a possible coupling between mid-winter, sudden stratospheric warmings (when the vortex is weakened or disrupted) and the abundance of ozone laminae using a 23-year record of ozonesonde data from the World Ozone Data Center in Canada combined with monthly-mean January polar temperatures at 30 hPa.Results are presented from an experiment conducted during the winter of 1994/95, in phase II of the Second European Stratospheric And Mid-latitude Experiment (SESAME), in which 93 ozone-enhanced laminae of polar origin observed by ozonesondes at different time and locations are linked by diabatic trajectories, enabling them to be probed twice or more. It is shown that, in general, ozone concentrations inside laminae fall progressively with time, mixing irreversibly with mid-latitude air on time-scales of a few weeks. A particular set of laminae which advected across Europe during mid February 1995 are examined in detail. These laminae were observed almost simultaneously at seven ozonesonde stations, providing information on their spatial scales. The development of these laminae has been modelled using the Contour Advection algorithm of Norton (1994), adding support to the concept that many laminae are extrusions of vortex air. Finally, a photochemical trajectory model is used to show that, if the air in the laminae is chemically activated, it will impact on mid-latitude ozone concentrations. An estimate is made of the potential number of ozone molecules lost each winter via this mechanism.

The use of polar organic compounds to estimate the contribution of domestic solid fuel combustion and biogenic sources to ambient levels of organic carbon and PM2.5 in Cork Harbour, Ireland

PM(2.5) samples collected at Cork Harbour, Ireland during summer, autumn, late autumn and winter, 2008-2009 were analyzed for polar organic compounds that are useful markers for aerosol source characterization. The determined compounds include tracers for biomass burning primary particles, fungal spores, markers for secondary organic aerosol (SOA) from isoprene, α-/β-pinene, and d-limonene. Seasonal and temporal variations and other characteristic features of the detected tracers are discussed in terms of aerosol sources and processes. The biogenic species were detected only during the summer period where the contributions of isoprene SOA and fungal spores to the PM(2.5) organic carbon (OC) were estimated to be 1.6% and 1% respectively. The biomass burning markers, and in particular levoglucosan, were present in all samples and attributed to the combustion of cellulose-containing fuels including wood, peat, bituminous and smokeless coal. The contribution of domestic solid fuel (DSF) burning to the measured OC mass concentration was estimated at 10.8, 50, 66.4 and 74.9% for summer, autumn, late autumn and winter periods, respectively, based on factors derived from a series of burning experiments on locally available fuels. Application of an alternative approach, namely principal component analysis-multiple linear regression (PCA-MLR), to the measured concentrations of the polar organic marker compounds used in conjunction with real-time air quality data provided similar trends and estimates for DSF combustion during all seasons except summer. This study clearly demonstrates that, despite the ban on the sale of bituminous coal in Cork and other large urban areas in Ireland, DSF combustion is still the major source of OC during autumn and winter periods and also makes a significant contribution to PM(2.5) levels. The developed marker approach for estimating the contribution of DSF combustion to ambient OC concentrations can, in principle, also be applied to other locations.

Near-ultraviolet absorption cross sections of nitrophenols and their potential influence on tropospheric oxidation capacity.

Nitrophenols and methylnitrophenols have been identified as photolytic precursors of nitrous acid, HONO, but their gas-phase absorption has not previously been reported. In this study, the absorption cross sections of 2-nitrophenol, 3-methyl-2-nitrophenol, and 4-methyl-2-nitrophenol were measured from 320 to 450 nm using incoherent broad-band cavity-enhanced absorption spectroscopy (IBBCEAS). The benzaldehyde absorption spectrum was measured to validate the approach and was in good agreement with literature spectra. The nitrophenol absorption cross sections are large (ca. 10(-17) cm(2) molecule(-1)) and blue-shifted about 20 nm compared to previously measured solution spectra. Besides forming HONO, nitrophenol absorption influences other photochemistry by reducing the available actinic flux. The magnitudes of both effects are evaluated as a function of solar zenith angle, and nitrophenol absorption is shown to lower the photolysis rates of O(3) and NO(2).

Structure-activity relationship (SAR) for the gas-phase ozonolysis of aliphatic alkenes and dialkenes.

The configuration of alkyl substituents about carbon-carbon unsaturated bonds exerts a controlling influence on the rate of the ozonolysis reaction. Alkyl substituents can increase (via the inductive effect) and decrease (via the steric effect) the activity of unsaturated bonds, and an accurate description of this information ought to correlate with the ozonolysis rate coefficient. A strong linear relationship is observed (R2 = 0.94), providing the basis of our SAR method. SAR estimates were tested against literature measurements of ozonolysis rate coefficients for 48 aliphatic alkenes and dialkenes, and were found to be accurate to within a factor of 2.3 of the measured value for the entire dataset. This represents a significant improvement over methods reported in the literature, where quoted predictions are at best accurate to within a factor of 6.5. Rates of gas-phase ozonolysis of alkenes and dialkenes can now be predicted with unprecedented accuracy using a simple SAR. The SAR was then validated against new experimental data. Absolute rate coefficients for the gas-phase reaction of ozone with a series of alkenes were determined in a simulation chamber at 295 +/- 2 K and atmospheric pressure by monitoring the loss of ozone in the presence of excess alkene. The rate coefficients (in units of 1 x 10(-18) cm3 molecule(-1) s(-1)) are: 5.12 +/- 0.93 for 1-pentene, 2,3-dimethyl; 406 +/- 49 for 2-pentene, 2-methyl; 151 +/- 5 for (E)-2-hexene, 14.5 +/- 1.0 for 1,5-hexadiene and 20.7 +/- 3.1 for 1,5-hexadiene, 2-methyl. There is good agreement between the experimental and predicted values and the adjustable parameters of the SAR are shown to be insensitive to the inclusion of the new data. The use of the SAR in atmospheric chemical modelling is investigated. Ozonolysis and OH radical rate coefficients are estimated for each alkene and dialkene present in the MCM v3.1. The effects of error within predicted rate coefficients upon modelled concentrations of a number of key species, including O3, OH, HO2, NO and NO2 were rather small and is not in itself a major cause of uncertainty in modelled concentrations.

Rate Coefficients for the Gas-Phase Reaction of Hydroxyl Radicals with 2-Methoxyphenol (Guaiacol) and Related Compounds

2-Methoxyphenol (guaiacol) and its derivatives are potential marker compounds for wood smoke emissions in the atmosphere. To investigate the atmospheric reactivity of this type of compounds, rate coefficients for their reactions with hydroxyl (OH) radicals have been determined at 294 ± 2 K and 1 atm using the relative rate method with gas chromatography for chemical analysis. The rate coefficients (in units of cm³ molecule⁻¹ s⁻¹) are: 2-methoxyphenol, (7.53 ± 0.41) × 10⁻¹¹; 3-methoxyphenol, (9.80 ± 0.46) × 10⁻¹¹; 4-methoxyphenol, (9.50 ± 0.55) × 10⁻¹¹; 2-methoxy-4-methylphenol, (9.45 ± 0.59) × 10⁻¹¹; and methoxybenzene, (2.20 ± 0.15) × 10⁻¹¹. The estimated atmospheric lifetime for 2-methoxyphenol is ~2 h, indicating that it is too reactive to be used as a tracer for wood smoke emissions. The reactivity of the methoxyphenols is compared with other substituted aromatics and interpreted in relation to the type, number, and positions of the different substituents on the aromatic ring. The atmospheric implications of the reactions are also discussed.

Light-absorbing properties of ambient black carbon and brown carbon from fossil fuel and biomass burning sources

The optical properties of ambient black carbon-containing particles and the composition of their associated coatings were investigated at a downtown site in Toronto, Canada, for 2 weeks in June 2013. The objective was to assess the relationship between black carbon (BC) coating composition/thickness and absorption. The site was influenced by emissions from local vehicular traffic, wildfires in Quebec, and transboundary fossil fuel combustion emissions in the United States. Mass concentrations of BC and associated nonrefractory coatings were measured using a soot particle–aerosol mass spectrometer (SP-AMS), while aerosol absorption and scattering were measured using a photoacoustic soot spectrometer (PASS). Absorption enhancement was investigated both by comparing ambient and thermally denuded PASS absorption data and by relating absorption data to BC mass concentrations measured using the SP-AMS. Minimal absorption enhancement attributable to lensing at 781 nm was observed for BC using both approaches. However, brown carbon was detected when the site was influenced by wildfire emissions originating in Quebec. BC coating to core mass ratios were highest during this period (~7), and while direct absorption by brown carbon resulted in an absorption enhancement at 405 nm (>2.0), no enhancement attributable to lensing at 781 nm was observed. The efficiency of BC coating removal in the denuder decreased substantially when wildfire-related organics were present and may represent an obstacle for future similar studies. These findings indicate that BC absorption enhancement due to lensing is minimal for downtown Toronto, and potentially other urban locations, even when impacted by long-range transport events.

A kinetic and mechanistic study of the gas-phase reactions of OH radicals and Cl atoms with some halogenated acetones and their atmospheric implications

Rate coefficients for the reactions of hydroxyl radicals and chlorine atoms with a series of halogenated acetones of the type CX3COCH3 (X = H, Cl, F) have been determined using a photolytic relative-rate technique at T = 298 K and at 760 Torr total pressure. The reactions studied and the rate coefficients obtained are shown in the table. Reaction Reaction number Rate coefficient/cm3 molecule−1 s−1 OH + CH3COCH3 → products (1) (2.2 ± 0.5) × 10−13 OH + CH2ClCOCH3 → products (2) (4.2 ± 0.8) × 10−13 OH + CHCl2COCH3 → products (3) (3.8 ± 0.8) × 10−13 OH + CCl3COCH3 → products (4) (1.5 ± 0.3) × 10−14 OH + CH2FCOCH3 → products (5) (2.1 ± 0.4) × 10−13 OH + CF3COCH3 → products (6) (6.9 ± 1.3) × 10−15 Cl + CH3COCH3 → products (7) (2.2 ± 0.4) × 10−12 Cl + CH2ClCOCH3 → products (8) (2.0 ± 0.2) × 10−12 Cl + CHCl2COCH3 → products (9) (1.7 ± 0.3) × 10−13 Cl + CCl3COCH3 → products (10) (1.7 ± 0.3) × 10−14 Cl + CH2FCOCH3 → products (11) (8.2 ± 1.6) × 10−13 Cl + CF3COCH3 → products (12) (8.0 ± 1.6) × 10−15 The errors quoted reflect an estimate of the absolute uncertainty in the measured rate coefficients of ±20%. For reactions (7)–(12), Fourier transform infrared spectroscopy was used to identify products. Qualitative ultra-violet absorption spectra were also recorded for most of the halogenated species investigated in this study, and have been used together with the kinetic data to derive atmospheric lifetimes for these species.

Determination of Arrhenius parameters for thereactions of ozone with cycloalkenes

The kinetics of the gas-phase reactions of ozone with a series of cycloalkenes have been investigated using a conventional static system. Ozone loss was monitored in the presence of excess cycloalkene and rate data measured over the temperature range 240–331 K. Rate constants for the reactions of ozone with C 5 –C 8 cycloalkenes and several substituted cyclopentenes and cyclohexenes at 298 K were determined and Arrhenius parameters were also calculated from the experimental data. The rate parameters obtained in this study are compared with previous literature data and discussed in terms of structure–reactivity relationships.

Enhanced Volatile Organic Compounds emissions and organic aerosol mass increase the oligomer content of atmospheric aerosols

Secondary organic aerosol (SOA) accounts for a dominant fraction of the submicron atmospheric particle mass, but knowledge of the formation, composition and climate effects of SOA is incomplete and limits our understanding of overall aerosol effects in the atmosphere. Organic oligomers were discovered as dominant components in SOA over a decade ago in laboratory experiments and have since been proposed to play a dominant role in many aerosol processes. However, it remains unclear whether oligomers are relevant under ambient atmospheric conditions because they are often not clearly observed in field samples. Here we resolve this long-standing discrepancy by showing that elevated SOA mass is one of the key drivers of oligomer formation in the ambient atmosphere and laboratory experiments. We show for the first time that a specific organic compound class in aerosols, oligomers, is strongly correlated with cloud condensation nuclei (CCN) activities of SOA particles. These findings might have important implications for future climate scenarios where increased temperatures cause higher biogenic volatile organic compound (VOC) emissions, which in turn lead to higher SOA mass formation and significant changes in SOA composition. Such processes would need to be considered in climate models for a realistic representation of future aerosol-climate-biosphere feedbacks.