Comment pubs.acs.org/est
Introduction to the Focus Issue on Marine Boundary Layer: Ocean Atmosphere Interactions Processes
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concentrations of several such compounds, which help constrain our understanding of their air−sea partitioning behavior, with clear differences between the southern and northern hemisphere in terms of atmospheric concentrations. Extending air−water exchange gradient measurements to short-lived species is challenging as quantitative measurements of trace-gas concentrations and fluxes are inherently very difficult, and new and improved methodologies are required to fully understand these processes. Grilli et al. discuss such a novel instrument which demonstrates very high sensitivity toward some critically important species. This compact and robust spectrometer relies on an alternated injection of a frequency-doubled femtosecond radiation at 338 and 436 nm into two parallel high-finesse cavities, for measuring BrO, H2CO and IO, and NO2. Among the gas phase halogen species, I2 and IO have received much recent attention, as their sources from the sea surface are poorly understood. In two papers, O′Dowd and coworkers describe a series of controlled chamber experiments which characterize iodine and iodine-derived particle formation from marine bacteria, under various environmental conditions. In a laboratory study, Richards and Finlayson-Pitts investigated the potential synergisms between halide and nitrate ions, both present in sea-salt particles. Surprisingly, mixed solutions containing both halogen and nitrate anions showed different photochemistry than displayed by the nitrate anions alone. Such studies underscore the potential complexity of chemical interactions in sea-surface environment, involved in gaseous halogen precursor production. And indeed, unexpectedly large tropospheric chlorine concentrations have also received much attention in the past two years. In Riedel et al., measurements of ClNO2 and Cl2 within the polluted marine boundary layer are described. In the Los Angeles region, ClNO2 was more ubiquitous than Cl2 during most nights of the study period, implying distinct and varying sources. Such measurements provide key information to show the role of anthropogenic vs natural Cl sources to the atmosphere. Finally, a full understanding of marine boundary layer chemistry can only come about through accurate modeling of the relevant chemical processes. Sommariva and von Glasow present such a one-dimensional model to simulate the chemical evolution of air masses in the tropical Atlantic Ocean, with a focus on halogen chemistry as it introduces significant sinks for methane and ozone. The results also showed that this chemistry is very sensitive to cloud processing. Cloud formation is dependent on the CCN properties of particles which are produced from the ocean and thus strongly affected by the organics present in the sea surface microlayer. Obviously, as suggested above, atmosphere−ocean coupling introduces feedbacks into the system which need to be better understood. Studying processes in the marine boundary layer is
arth’s oceans form a coupled system with the atmosphere, exchanging heat, momentum, and water, as well as a large variety of chemical species at the air−sea interface. These exchanges exert a considerable and complex influence on the climate system, as well as having more local effects on regional air quality and marine biological productivity. The emission and deposition of trace chemical species and aerosol particles at the air−sea interface is the topic of this Special Issue. The “sea-surface microlayer” (SML), which comprises the uppermost tens to hundreds of micrometers of the water surface, has physical, chemical, and biological properties that differ from those of the underlying subsurface water. Its presence at the ocean−atmosphere boundary has important consequences for interfacial transport, due to its different and complex chemical nature, and for the formation and composition of sea-spray generated particles. Despite the importance of the sea−surface microlayer, its chemical composition is still not yet fully resolved. Pinxteren et al. present a detailed study of the composition of surface microlayer samples collected in the southern Baltic Sea. These samples were analyzed for dissolved organic carbon and dissolved total nitrogen, as well as for several organic nitrogen containing compounds and carbohydrates. SML enrichment factors (as compared to the composition of the underlying bulk water) were observed to strongly depend of the chemical nature of the compound, underlining the importance of single compound analysis for gaining better understanding. The ocean surface chemical composition certainly also affects sea-spray particle properties, as highlighted by King et al. through laboratory experiments using a wave tank under controlled water composition and spray formation conditions. Depending on the chemical nature of the surface, jet-produced particles showed a considerable amount of organic enrichment in the relative to sea bulk water, potentially impacting the cloud condensation nucleus (CCN) properties of sea-salt particles. The chemical composition of the surface microlayer is controlled by biogeochemical and physical processes in the ocean and at its surface. The biological productivity in the oceans is dependent on the availability of nutrients, such as N, P, and Fe. Many of these are provided by aerosol impingement upon the sea surface, releasing soluble species there. The Critical Review by Schultz and co-workers presents our current understanding of mineral dust deposition to the ocean. Mackey et al. discuss the interplay between aerosol composition and the rate of P release, from field incubation experiments with aerosol samples collected in the Sargasso Sea and Red Sea. These exhibited different dependences toward P as a nutrient, but highlighted a potential underestimation of atmospheric P deposition. Persistent organic pollutants (POPs), such as PCBs and other chlorinated compounds, have been detected in increased concentrations in the SML. Since they are very longlived, they are often found far from the presumed sources as they are transported through the Earth system. Lohmann and co-workers report field measurements of air and sea surface © 2012 American Chemical Society
Published: October 2, 2012 10383
dx.doi.org/10.1021/es303551b | Environ. Sci. Technol. 2012, 46, 10383−10384
Environmental Science & Technology
Comment
becoming an incredible playground for climate and atmospheric scientists originating from many different fields.
surfaces. More specifically, he studies photosensitized processes at interfaces in connection with organic and mineral aerosol chemistries. Such processes do affect the aging of aerosols. In the context of oceanatmosphere interactions, he studied (in collaboration with the Donaldson group) the photochemistry of organic compounds at the surface of the ocean. These processes form the link between ocean biogeochemistry, primary and secondary aerosol formation, evolution in the air aloft, and the climate impact of marine boundary layer aerosols.
D. J. Donaldson*,† Christian George*,‡ †
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Department of Chemistry, University of Toronto, 80 St. George St. Toronto, ON Canada M5S 3H6 ‡ Université de Lyon, Lyon, F-69626, France; Université Lyon 1, Lyon, F-69626, France; CNRS, UMR5256, IRCELYON, Institut de Recherches sur la Catalyse et l’Environnement de Lyon, Villeurbanne, F-69626, France
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (D. J. D.); christian.
[email protected] (C. G.). Notes
The authors declare no competing financial interest. Biographies
James Donaldson is Professor and Associate Chair for Graduate Studies in the Department of Chemistry, and Professor of Chemistry in the Department of Physical and Environmental Sciences at the University of Toronto. His research interests lie in the physical chemistry of environmental processes. In particular, he is studying the heterogeneous chemistry and photochemistry of surfaces exposed to the atmosphere. His interest in ocean surface chemistry was motivated by the discovery (in collaboration with the George group) of photosensitized halide oxidation at the air−water interface, using compounds present in the sea surface microlayer as chromophores.
Christian George is a senior scientist at the Centre National de la Recherche Scientifique (CNRS) and leads one of the research teams of the Research Institute on Catalysis and the Environment at Lyon. His research focuses on the physicochemical properties of aerosols and 10384
dx.doi.org/10.1021/es303551b | Environ. Sci. Technol. 2012, 46, 10383−10384
