Search

The moisture sorption isotherms of Algerian bay leaves (Laurus nobilis) were determined experimentally in this work. The equilibrium moisture contents of the leaves were measured at 40, 50, and 60 °C using static gravimetric method. Six mathematical models were tested to fit the experimental data of sorption isotherms and predict the hygroscopic behavior during storage or drying. Peleg model was found to be the best fitting model for describing the sorption curves. The net isosteric heat of sorption was computed fromthe equilibrium data at different temperatures by applying the integrated form of the Clausius-Clapeyron equation. The net isosteric heat of sorption is inversely proportional to the equilibrium moisture content and isfound to be an exponential function of moisture content.

Despite the known biochemical production of a range of aromatic compounds by plants and the presence of benzenoids in floral scents, the emissions of only a few benzenoid compounds have been reported from the biosphere to the atmosphere. Here, using evidence from measurements at aircraft, ecosystem, tree, branch and leaf scales, with complementary isotopic labeling experiments, we show that vegetation (leaves, flowers, and phytoplankton) emits a wide variety of benzenoid compounds to the atmosphere at substantial rates. Controlled environment experiments show that plants are able to alter their metabolism to produce and release many benzenoids under stress conditions. The functions of these compounds remain unclear but may be related to chemical communication and protection against stress. We estimate the total global secondary organic aerosol potential from biogenic benzenoids to be similar to that from anthropogenic benzenoids (~10 Tg y-1), pointing to the importance of these natural emissions in atmospheric physics and chemistry.

The moisture sorption isotherms of Algerian bay leaves (Laurus nobilis) were determined experimentally in this work. The equilibrium moisture contents of the leaves were measured at 40, 50, and 60 °C using static gravimetric method. Six mathematical models were tested to fit the experimental data of sorption isotherms and predict the hygroscopic behavior during storage or drying. Peleg model was found to be the best fitting model for describing the sorption curves. The net isosteric heat of sorption was computed from the equilibrium data at different temperatures by applying the integrated form of the Clausius-Clapeyron equation. The net isosteric heat of sorption is inversely proportional to the equilibrium moisture content and is found to be an exponential function of moisture content.

We study the interactions between atmospheric boundary layer (ABL) dynamics and atmospheric chemistry using a mixed-layer model coupled to chemical reaction schemes. Guided by both atmospheric and chemical measurements obtained during the DOMINO (Diel Oxidant Mechanisms in relation to Nitrogen Oxides) campaign (2008), numerical experiments are performed to study the role of ABL dynamics and the accuracy of chemical schemes with different complexity: the Model for Ozone and Related chemical Tracers, version 4 (MOZART-4) and a reduced mechanism of this chemical system. Both schemes produce satisfactory results, indicating that the reduced scheme is capable of reproducing the O3-NOx-VOC-HOx diurnal cycle during conditions characterized by a low NOx regime and small O3 tendencies (less than 1 ppb per hour). By focusing on the budget equations of chemical species in the mixedlayer model, we show that for species like O3, NO and NO2, the influence of entrainment and boundary layer growth is of the same order as chemical production/loss. This indicates that an accurate representation of ABL processes is crucial in understanding the diel cycle of chemical species. By comparing the time scales of chemical reactive species with the mixing time scale of turbulence, we propose a classification based on the Damk¨ohler number to further determine the importance of dynamics on chemistry during field campaigns. Our findings advocate an integrated approach, simultaneously solving the ABL dynamics and chemical reactions, in order to obtain a better understanding of chemical pathways and processes and the interpretation of the results obtained during measurement campaigns.

This paper describes the background, instrumentation, goals, and the regional influences on the HUMPPA-COPEC intensive field measurement campaign, conducted at the Boreal forest research station SMEAR II (Station for Measuring Ecosystem-Atmosphere Relation) in Hyytiälä, Finland from 12 July-12 August 2010. The prevailing meteorological conditions during the campaign are examined and contrasted with those of the past six years. Back trajectory analyses show that meteorological conditions at the site in 2010 were characterized by a higher proportion of southerly flow than in the other years studied. As a result the summer of 2010 was anomalously warm and high in ozone making the campaign relevant for the analysis of possible future climates. A comprehensive land use analysis, provided on both 5 and 50 km scales, shows that the main vegetation types surrounding the site on both the regional and local scales are: coniferous forest (Scots pine and/or Norway spruce); mixed forest (Birch and conifers); and woodland scrub (e.g. Willows, Aspen); indicating that the campaign results can be taken as representative of the Boreal forest ecosystem. In addition to the influence of biogenic emissions, the measurement site was occasionally impacted by sources other than vegetation. Specific tracers have been used here to identify the time periods when such sources have impacted the site namely: biomass burning (acetonitrile and CO), urban anthropogenic pollution (pentane and SO2) and the nearby Korkeakoski sawmill (enantiomeric ratio of chiral monoterpenes). None of these sources dominated the study period, allowing the Boreal forest summertime emissions to be assessed and contrasted with various other source signatures.

Ship-borne measurements have been made in air over the remote South Atlantic and Southern Oceans in January-March 2007. This cruise encountered a large-scale natural phytoplankton bloom emitting reactive hydrocarbons (e. g. isoprene); and a high seas squid fishing fleet emitting NOx (NO and NO2). Using an atmospheric chemistry box model constrained by in-situ measurements, it is shown that enhanced ozone production ensues from such juxtaposed marine biogenic and anthropogenic emissions. The relative impact of shipping and phytoplankton emissions on ozone was examined on a global scale using the EMAC model. Ozone in the marine boundary layer was found to be over ten times more sensitive to NOx emissions from ships, than to marine isoprene in the region south of 45 degrees. Although marine isoprene emissions make little impact on the global ozone budget, co-located ship and phytoplankton emissions may explain the increasing ozone reported for the 40-60 degrees S southern Atlantic region.

We report here isoprene emission rates determined from various phytoplankton cultures incubated under PAR light which was varied so as to simulate a natural diel cycle. Phytoplankton species representative of different phytoplankton functional types (PFTs) namely: cyanobacteria, diatoms, coccolithophorides, and chlorophytes have been studied. Biomass normalised isoprene emission rates presented here relative to the chlorophyll-a (Chl-a) content of the cultures showed that the two cyanobacteria (Synechococcus and Trichodesmium) were the strongest emitters with emission rates in the range of 17 to 28 mu g C(5)H(8) g(-1) Chl-a h(-1). Diatoms produced isoprene in a significantly lower emission range: 3 to 7.5 mu g C(5)H(8) g(-1) Chl-a h(-1) and Dunaliella tertiolecta was by far the lowest emitter of our investigated plankton cultures. Despite the group specific differences observed, a high emission rate variance was observed to occur within one phytoplankton group. However, a combination of literature and our own data showed a clear relationship between the actual cell volume and the isoprene emission rates. This relationship could be a valuable tool for future modelling approaches of global isoprene emissions.

We have combined the first satellite maps of the global distribution of phytoplankton functional type and new measurements of phytoplankton-specific isoprene productivities, with available remote marine isoprene observations and a global model, to evaluate our understanding of the marine isoprene source and its impacts on organic aerosol abundances. Using satellite products to scale up data on phytoplankton-specific isoprene productivity to the global oceans, we infer a mean "bottom-up" oceanic isoprene emission of 0.31 +/- 0.08 (1 sigma) Tg/yr. By minimising the mean bias between the model and isoprene observations in the marine atmosphere remote from the continents, we produce a "top-down" oceanic isoprene source estimate of 1.9 Tg/yr. We suggest our reliance on limited atmospheric isoprene data, difficulties in simulating in-situ isoprene production rates in laboratory phytoplankton cultures, and limited knowledge of isoprene production mechanisms across the broad range of phytoplankton communities in the oceans under different environmental conditions as contributors to this difference between the two estimates. Inclusion of secondary organic aerosol (SOA) production from oceanic isoprene in the model with a 2% yield produces small contributions (0.01-1.4%) to observed organic carbon (OC) aerosol mass at three remote marine sites in the Northern and Southern Hemispheres. Based on these findings we suggest an insignificant role for isoprene in modulating remote marine aerosol abundances, giving further support to a recently postulated primary OC source in the remote marine atmosphere.

Tropical forests are a strong source of biogenic volatile organic compounds (BVOCs) to the atmosphere which can potentially impact the atmospheric oxidation capacity. Here we present airborne and ground-based BVOC measurements representative for the long dry season covering a large area of the northern Amazonian rainforest (6-3° N, 50-59° W). The measurements were conducted during the October 2005 GABRIEL (Guyanas Atmosphere-Biosphere exchange and Radicals Intensive Experiment with the Learjet) campaign. The vertical (35 m to 10 km) and diurnal (09:00-16:00) profiles of isoprene, its oxidation products methacrolein and methyl vinyl ketone and methanol and acetone, measured by PTR-MS (Proton Transfer Reaction Mass Spectrometry), have been used to empirically estimate their emission fluxes from the forest canopy on a regional scale. The mixed layer isoprene emission flux, inferred from the airborne measurements above 300 m, is 5.7 mg isoprene m−2 h−1 after compensating for chemistry and ~6.9 mg isoprene m−2 h−1 taking detrainment into account. This surface flux is in general agreement with previous tropical forest studies. Inferred methanol and acetone emission fluxes are 0.5 mg methanol m−2 h−1 and 0.35 mg acetone m−2 h−1, respectively. The BVOC measurements were compared with fluxes and mixing ratios simulated with a single-column chemistry and climate model (SCM). The inferred isoprene flux is substantially smaller than that simulated with an implementation of a commonly applied BVOC emission algorithm in the SCM.

Environmental context. Laboratory incubation experiments and shipboard measurements in the Southern Atlantic Ocean have provided the first evidence for marine production of monoterpenes. Nine marine phytoplankton monocultures were investigated using a GC-MS equipped with an enantiomerically-selective column and found to emit monoterpenes including (–)-/(+)-pinene, limonene and p-ocimene, all of which were previously thought to be exclusively of terrestrial origin. Maximum levels of 100–200 pptv total monoterpenes were encountered when the ship crossed an active phytoplankton bloom. Abstract. Laboratory incubation experiments and shipboard measurements on the Southern Atlantic Ocean have provided the first evidence for marine production of monoterpenes. Nine marine phytoplankton monocultures were investigated using a GC-MS equipped with an enantiomerically-selective column and found to emit at rates, expressed as nmol C10H16 (monoterpene) g [chlorophyll a]–1 day–1, from 0.3 nmol g [chlorophyll a]–1 day–1 for Skeletonema costatum and Emiliania huxleyi to 225.9 nmol g [chlorophyll a]–1 day–1 for Dunaliella tertiolecta. Nine monoterpenes were identified in the sample and not in the control, namely: (–)-/(+)-pinene, myrcene, (+)-camphene, (–)-sabinene, (+)-3-carene, (–)-pinene, (–)-limonene and p-ocimene. In addition, shipboard measurements of monoterpenes in air were made in January–March 2007, over the South Atlantic Ocean. Monoterpenes were detected in marine air sufficiently far from land as to exclude influence from terrestrial sources. Maximum levels of 100–200 pptv total monoterpenes were encountered when the ship crossed an active phytoplankton bloom, whereas in low chlorophyll regions monoterpenes were mostly below detection limit.

Methyl iodide has been measured during two field campaigns in the Atlantic region in 2002. The first took place in July–August at 2300 m on the island of Tenerife, while the second was a shipborne, east–west crossing of the Tropical Atlantic at 10°N in October–November of the same year. Both campaigns were periodically impacted by dust, advected from Africa in trade winds. Unexpectedly, during these dust events, methyl iodide mixing ratios were observed to be high relative to other times. Backward calculations with the particle dispersion model FLEXPART show the origin of the dust storms as Mauritania and southern Algeria for the ground- and ship-based campaigns, respectively. The dust-laden air traveled from its source above the marine boundary layer to the measurement region. On the basis of the field data correlations and the simulations, we suggest that dust-stimulated emission of methyl iodide has occurred. To test this hypothesis, dust samples were collected from the identified source regions and added to filtered Atlantic seawater. This rapidly produced methyl iodide. Further tests established that the addition of iodide increased the yield and that iodide with H2O2 was greater still. This was found for both sterilized and nonsterilized samples. We conclude that there is an abiotic methyl iodide production mechanism that can occur via substitution, analogous to those in soil, rather than radical recombination. This may occur when dust contacts seawater containing iodide or when marine water vapor condenses on dust containing iodide. This hypothesis appears to be consistent with recent long-term methyl iodide data sets from Tasmania and may help resolve current uncertainties in the iodine cycle.

We present a comparison of different Lagrangian and chemical box model calculations with measurement data obtained during the GABRIEL campaign over the tropical Atlantic Ocean and the Amazon rainforest in the Guyanas, October 2005. Lagrangian modelling of boundary layer (BL) air constrained by measurements is used to derive a horizontal gradient (¿5.6 pmol/mol km¿1) of CO from the ocean to the rainforest (east to west). This is significantly smaller than that derived from the measurements (16-48 pmol/mol km¿1), indicating that photochemical production from organic precursors alone cannot explain the observed strong gradient. It appears that HCHO is overestimated by the Lagrangian and chemical box models, which include dry deposition but not exchange with the free troposphere (FT). The relatively short lifetime of HCHO implies substantial BL-FT exchange. The mixing-in of FT air affected by African and South American biomass burning at an estimated rate of 0.12 h¿1 increases the CO and decreases the HCHO mixing ratios, improving agreement with measurements. A mean deposition velocity of 1.35 cm/s for H2O2 over the ocean as well as over the rainforest is deduced assuming BL-FT exchange adequate to the results for CO. The measured increase of the organic peroxides from the ocean to the rainforest (¿0.66 nmol/mol d¿1) is significantly overestimated by the Lagrangian model, even when using high values for the deposition velocity and the entrainment rate. Our results point at either heterogeneous loss of organic peroxides and/or their radical precursors, underestimated photodissociation or missing reaction paths of peroxy radicals not forming peroxides in isoprene chemistry. We calculate a mean integrated daytime net ozone production (NOP) in the BL of (0.2±5.9) nmol/mol (ocean) and (2.4±2.1) nmol/mol (rainforest). The NOP strongly correlates with NO and has a positive tendency in the boundary layer over the rainforest.