Isoprene carries approximately half of the flux of non-methane volatile organic carbon emitted to the atmosphere by the biosphere. Accurate representation of its oxidation rate and products is essential for quantifying its influence on the abundance of the hydroxyl radical (OH), nitrogen oxide free radicals (NO ), ozone (O), and, via the formation of highly oxygenated compounds, aerosol. We present a review of recent laboratory and theoretical studies of the oxidation pathways of isoprene initiated by addition of OH, O, the nitrate radical (NO), and the chlorine atom. From this review, a recommendation for a nearly complete gas-phase oxidation mechanism of isoprene and its major products is developed. The mechanism is compiled with the aims of providing an accurate representation of the flow of carbon while allowing quantification of the impact of isoprene emissions on HO and NO free radical concentrations and of the yields of products known to be involved in condensed-phase processes. Finally, a simplified (reduced) mechanism is developed for use in chemical transport models that retains the essential chemistry required to accurately simulate isoprene oxidation under conditions where it occurs in the atmosphere-above forested regions remote from large NO emissions.

We report fluxes and dry deposition velocities for 16 atmospheric compounds above a southeastern United States forest, including: hydrogen peroxide (H 2 O 2 ), nitric acid (HNO 3 ), hydrogen cyanide (HCN), hydroxymethyl hydroperoxide, peroxyacetic acid, organic hydroxy nitrates, and other multifunctional species derived from the oxidation of isoprene and monoterpenes. The data suggest that dry deposition is the dominant daytime sink for small, saturated oxygenates. Greater than 6 wt %C emitted as isoprene by the forest was returned by dry deposition of its oxidized products. Peroxides account for a large fraction of the oxidant flux, possibly eclipsing ozone in more pristine regions. The measured organic nitrates comprise a sizable portion (15%) of the oxidized nitrogen input into the canopy, with HNO 3 making up the balance. We observe that water-soluble compounds (e.g., strong acids and hydroperoxides) deposit with low surface resistance whereas compounds with moderate solubility (e.g., organic nitrates and hydroxycarbonyls) or poor solubility (e.g., HCN) exhibited reduced uptake at the surface of plants. To first order, the relative deposition velocities of water-soluble compounds are constrained by their molecular diffusivity. From resistance modeling, we infer a substantial emission flux of formic acid at the canopy level (∼1 nmol m −2 ·s −1 ). GEOS−Chem, a widely used atmospheric chemical transport model, currently underestimates dry deposition for most molecules studied in this work. Reconciling GEOS−Chem deposition velocities with observations resulted in up to a 45% decrease in the simulated surface concentration of trace gases.biosphere−atmosphere exchange | isoprene | dry deposition | OVOCs | fluxes

The blood–brain barrier (BBB) restricts the entry of proteins as well as potential drugs to cerebral tissues. We previously reported that a family of Kunitz domain‐derived peptides called Angiopeps can be used as a drug delivery system for the brain. Here, we further characterize the transcytosis ability of these peptides using an in vitro model of the BBB and in situ brain perfusion. These peptides, and in particular Angiopep‐2, exhibited higher transcytosis capacity and parenchymal accumulation than do transferrin, lactoferrin, and avidin. Angiopep‐2 transport and accumulation in brain endothelial cells were unaffected by the P‐glycoprotein inhibitor, cyclosporin A, indicating that this peptide is not a substrate for the efflux pump P‐glycoprotein. However, competition studies show that activated α2‐macroglobulin, a specific ligand for the low‐density lipoprotein receptor‐related protein‐1 (LRP1) and Angiopep‐2 can share the same receptor. In addition, LRP1 was detected in glioblastomas and brain metastases from lung and skin cancers. Fluorescent microscopy also revealed that Alexa488‐Angiopep‐2 co‐localized with LRP1 in brain endothelial cell monolayers. Overall, these results suggest that Angiopep‐2 transport across the BBB is, in part, mediated by LRP1.

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[1] Aqueous extracts of secondary organic aerosols (SOA) generated from the ozonolysis of d-limonene were subjected to dissolution, evaporation, and re-dissolution in the presence and absence of ammonium sulfate (AS). Evaporation with AS at pH 4-9 produced chromophores that were stable with respect to hydrolysis and had a distinctive absorption band at 500 nm. Evaporation accelerated the rate of chromophore formation by at least three orders of magnitude compared to the reaction in aqueous solution, which produced similar compounds. Absorption spectroscopy and high-resolution nanospray desorption electrospray ionization (nano-DESI) mass spectrometry experiments suggested that the molar fraction of the chromophores was small (<2%), and that they contained nitrogen atoms. Although the colored products represented only a small fraction of SOA, their large extinction coefficients (>10 5 L mol À1 cm À1 at 500 nm) increased the effective mass absorption coefficient of the residual organics in excess of 10 3 cm 2 g À1 -a dramatic effect on the optical properties from minor constituents. Evaporation of SOA extracts in the absence of AS resulted in the production of colored compounds only when the SOA extract was acidified to pH $ 2 with sulfuric acid. These chromophores were produced by acid-catalyzed aldol condensation, followed by a conversion into organosulfates. The presence of organosulfates was confirmed by high resolution mass spectrometry experiments. Results of this study suggest that evaporation of cloud or fog droplets containing dissolved organics leads to significant modification of the molecular composition and serves as a potentially important source of light-absorbing compounds.