A21H-01 INVITED
Terrestrial biogenic sources of organics in the atmosphere
Terrestrial ecosystems produce an impressive array of organic compounds, some of which are volatile or semi-volatile and are emitted into the atmosphere. Some of these sources of atmospheric constituents have been investigated for many years while others have only recently been discovered. The importance of biogenic organic emissions into the atmosphere was recognized decades ago but the magnitude of these emissions and their biological and atmospheric impact remains controversial in many cases. A quantitative understanding of organic gas and aerosol emissions from terrestrial ecosystems is needed for earth system and climate studies and for assessments of present and future air quality. Emissions of a few compounds are now routinely included in atmospheric chemistry and transport modeling studies but the associated uncertainties are substantial. Other important compounds are completely missing from these models. This presentation will review the progress in identifying, understanding and quantifying terrestrial ecosystem emissions beginning with the discoveries and insights of early investigators and concluding with the outstanding gaps in our current knowledge. The technical, observational and theoretical advances that led to our current understanding will be described as well as those required for future advances. Biogenic emissions are very sensitive to changes in climate, land management and atmospheric chemical composition. The response of biogenic emissions to potential future scenarios will be assessed and the implications for future air quality and climate considered.
A21H-02
Global observations of oxygenated Volatile Organic Compounds from space
Formaldehyde (HCHO), the smallest aldehyde of the atmosphere and glyoxal (CHO.CHO), the smallest a- dicarbonyl compound, are key intermediate products of the oxidation of volatile organic compounds (VOCs). Due to their short lifetime they are expected to provide valuable information on the global identification of the photochemical hot spots which are attributed to the various emission sources of anthropogenic, biogenic and biomass burning origin. This study presents the global composite maps of both HCHO and CHO.CHO vertical column densities as obtained, for the first time, from 2 different sensors the SCIAMACHY and the GOME-2 on board of the ENVISAT and METOP satellites, respectively. HCHO slant column densities (SCDs) were retrieved in the UV spectral region and the CHO.CHO in the VIS by applying the differential optical absorption spectroscopy technique (DOAS). Finally, the vertical column densities (VCDs) were calculated after taking into account the air mass factors computed with the radiative transfer model, SCIATRAN. These data sets of the VCDHCHO and VCDCHOCHO, covering the extended time period of 01.01.03 - 31.08.08, have been used to study the global seasonal and multi-annual behavior of both species. It was found that the highest values of these oVOCs, depending on the season, are observed above regions where anthropogenic activities, biogenic processes and biomass burning take place. South America, Africa, India, Indonesia and Asia (mainly South-eastern China) are among the dominant regions where high annual mean values of VCDHCHO (>1.0.1016molec.cm-2) and VCDCHO.CHO (>5.0.1014molec.cm-2) are computed. At higher latitudes, moderate annual mean values of VCDHCHO and VCDCHO.CHO are discernible, for example above North America, Europe and Australia. Notably, high column amounts of CHO.CHO are also observed over the tropical oceans and close to upwelling areas.
A21H-03
Top-down constraints on the atmospheric acetaldehyde budget
Acetaldehyde plays a key role in the atmosphere as a source of PAN, HCHO, and HOx. We present the first 3D global model assessment of the atmospheric acetaldehyde budget, using GEOS-Chem, and apply airborne (INTEX-A, INTEX-B, ITCT-2K4, MILAGRO), ground-based and marine observations to test current understanding of its sources and sinks. We find that direct biogenic emissions (28 Tg/y) and secondary production in the atmosphere are the main sources of atmospheric acetaldehyde, while plant decay (6 Tg/y), direct anthropogenic (4 Tg/y), and biomass burning emissions (3 Tg/y) play smaller roles. Previous work has shown that acetaldehyde measurements in background air may be suspect, and we compare simulated and observed values in the context of concurrent measurements (e.g., PAN and NOy) and present chemical understanding. Acetaldehyde is also produced photochemically in the surface ocean from the photolysis of dissolved organic matter and subsequently emitted to the atmosphere, and we evaluate the importance of this source for the atmospheric budget.
A21H-04
Oligomers, organosulfates, and nitroxy organosulfates identified in rainwater
Wet deposition is an important removal mechanism for atmospheric organic matter, and a potentially important input for receiving ecosystems, yet less than 50 percent of rainwater organic matter is considered chemically characterized. Precipitation samples collected in New Jersey, USA, were analyzed by negative ion ultra-high resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). We document the presence of 552 unique compounds in the rainwater over a mass range of 50-500 Da, in four compound classes (i.e., CHO, CHOS, CHON, and CHONS). The presence of oligomers, organosulfates, nitroxy organosulfates, organic acids, and linear alkylbenzene sulfonates is reported. Some compounds detected have distinct primary sources; however, the composition of the bulk of this material suggests it is formed in the atmosphere and composed of known contributors to secondary organic aerosol. For example, eight oligomer series known to form through aqueous photooxidation of methylglyoxal and organosulfate compounds known to form from 4 precursors in smog chamber experiments were identified in the rainwater samples. The oligomers, organosulfates, and nitroxy organosulfates detected in the rainwater could all contribute to the HULIS fraction of atmospheric organic matter.
A21H-05
Detailed Carbon Isotopic Characterization of Aerosol-Derived Organic Carbon Deposited to two Temperate Watersheds
Atmospheric deposition of carbonaceous aerosols can be a quantitatively significant flux in the carbon budgets of temperate watersheds. Characterizing the sources and fates of this material is therefore critical for assessing its role in carbon and organic matter cycling in these systems. Aerosol samples were collected in the Hudson and York River watersheds throughout 2006-2007 and analyzed for quantities and isotopic signatures (δ13C, Δ14C) of total and water-soluble organic carbon (TOC, WSOC, respectively). On average ~2.4 and 2.1 mg m-2 d-1 of aerosol TOC were deposited to the Hudson and York River watersheds, respectively, and nearly half of this material was water-soluble. δ13C analyses indicated that both the TOC and the WSOC were primarily terrestrial in nature. TOC Δ14C signatures covered a broad range for both watersheds, with calculated contributions from fossil sources (e.g., anthropogenic combustion of petroleum, coal, etc.) ranging from 0% for samples collected during the summer of 2007 to approximately 50% for samples collected in the winter of 2007. Aerosol-derived WSOC Δ14C values were less variable and were nearly always enriched in 14C with respect to the corresponding TOC, indicating that contemporary aerosol material tends to partition into the aqueous phase, while fossil-derived aerosol OC is more likely to remain insoluble. However, WSOC still often showed considerable contributions from fossil OC (up to 20%). Thus, some portion of the anthropogenic fossil-derived aerosol OC is relatively soluble and may be transported hydrologically through watersheds and aquatic systems. A subset of aerosol samples from each watershed was selected for more thorough isotopic analysis of operationally-defined components of the carbonaceous material. Isotopic signatures were obtained for TOC, WSOC, total solvent-extract, and the aliphatic, aromatic, and polar components. Isotopic information on these fractions allows us to determine which components contribute to the age and source characteristics of aerosols. This information will help refine our understanding of the role of aerosol OC, and specifically, anthropogenically-derived aerosol OC, at the atmosphere-land-water interfaces.
A21H-06
Insights Into Water-Soluble Organic Aerosol Sources From Carbon-13 Ratios of Size Exclusion Chromatography Fractions
Many sources of organic aerosols have been identified and quantified, and much of this work has used individual (mosty water-insoluble) compounds as tracers of primary sources. However, most organic aerosol cannot be molecularly characterized, and the water-soluble organic carbon (WSOC) in many aerosols is thought to originate from gaseous precursors (i.e., it is secondary in nature). It can therefore be difficult to infer aerosol sources, particularly of background (i.e., aged) aerosols, and of the relatively high-MW component of aerosols. The stable isotope ratios (δ13C) of organic aerosols have been used to distinguish between sources, with lighter values (-30‰ to -25‰) interpreted as having originated from fossil fuel combustion and C4 biogenic emission, and heavier values (-25‰ to - 20‰) indicating a marine or C3 biogenic source. Most published measurements were of either total suspended particulates or PM2.5, however, and it is unknown to what extent these fractions differ from submicron WSOC. We report δ13C for submicron WSOC collected at a variety of sites, ranging from marine to polluted to background continental. Bulk marine organic δ13C ranged from -30.4 to - 27.6‰, slightly lighter than previously published results. This could be due to the elimination of supermicron cellular material or other biogenic primary emissions from the sample. Continental WSOC δ13C ranged from -19.1 to -29.8‰, with heavier values (-19.8 ± 1.0‰) in Oklahoma and lighter values at Great Smoky Mountain National Park in Tennessee (-25.8 ± 2.6‰) and Illinois (-24.5 ± 1.0‰). This likely results from the greater proportional of C3 plant material in the Oklahoma samples. In addition to bulk samples, we used size exclusion chromatography (SEC) to report δ13C of organic aerosols as a function of hydrodynamic diameter. Variability and magnitude of hydrodynamic diameter was greatest at low SEC pH, indicative of the acidic character of submicron WSOC. Tennessee aerosols included neutral compounds not observed at other sites, and in general showed more variability in both hydrodynamic diameter and δ13C. This suggests that secondary formation processes were most active at this site.
A21H-07
Is Carbonyl Sulfide a Tracer for the Emissions and Oxidation Products of Biogenic VOCs?
Carbonyl sulfide (OCS) is a relatively inert sulfur species with mostly diffuse sources in the troposphere including emissions from the ocean and production from CS2. A large sink in the troposphere is the uptake by vegetation, governed by the same physical parameters as the uptake of carbon dioxide. As a result, there is recent interest in the use of carbonyl sulfide to study the carbon cycle. Here, we investigate whether the relative absence of OCS in an air mass is correlated with the presence of biogenic volatile organic compounds (VOCs) and their oxidation products, including organic aerosol and peroxyacyl nitrates (PANs). Airborne data obtained with the NOAA WP-3D during ICARTT (International Consortium for Atmospheric Research on Transport and Transformation) in 2004 are used for this purpose. It will be shown that the lowest mixing ratios of OCS in the northeastern U.S. were observed in the continental boundary layer. High levels of biogenic VOCs were usually observed in parallel with low OCS. The opposite was not always true, caused by the difference in lifetime of biogenic VOCs and OCS, and possibly by a difference in vegetation types responsible for biogenic emissions and OCS uptake. The correlation between OCS and water-soluble organic carbon (WSOC), a subset of the organic carbon in secondary organic aerosol (SOA), was very weak in the boundary layer. These findings suggest that the variability in organic aerosol in the northeastern U.S. is only very weakly determined by biogenic VOC emissions.