Transport of isoprene and its oxidation products by deep convective clouds in the Amazon

<p>Transport of organic trace gases by deep convective clouds plays an important role for new particle formation (NPF) and particle growth in the upper atmosphere. Isoprene accounts for a major fraction of the global volatile organic vapor emissions and a significant fraction is emitted in the Amazon. We examined transport and chemical processing of isoprene and its oxidation products in a deep convective cloud over the Amazon using a box model. Trajectories of individual air parcels of the cloud derived from a large eddy simulation are used as input to the model. Our results show that there exist two main pathways for NPF from isoprene associated with deep convection. The first one is when the gas transport occurs through a cloud with low lightning activity and with efficient gas uptake of low-volatile oxidation products by ice particles. Some of the isoprene will reach the cloud outflow where it is further aged and produces low volatile species capable of forming and growing new particles. The second way is via transport through clouds with high lightning activity and with low gas uptake by ice. For this case, low volatile oxidation products will reach the immediate outflow in concentrations close to the values observed in the boundary layer. The efficiency of gas condensation on ice particles is still uncertain and further research in this direction is needed.</p>

Secondary Organic Aerosol Formation Enhanced by Organic Seeds of Similar Polarity at Atmospherically Relative Humidity

Secondary Organic Aerosol (SOA) forms in the atmosphere when semi-volatile oxidation products from biogenic and anthropogenic hydrocarbons condense onto atmospheric particulate matter. Climate models assume that oxidation products and preexisting organic aerosol form a well-mixed particle and enhance condensation, and, as a result, predict that future increases in anthropogenic primary organic aerosol (POA) will cause a significant increase in SOA. However, recent experiments performed at low humidity (<10%) demonstrate a single-phase particle does not always form, challenging the validity of model assumptions. In this work, we investigate the formation of SOA at atmospherically relevant humidities (55 - 65%) and examine this mixing assumption. We hypothesized that humidity leads to decreased viscosity and shorter mixing timescales, which is favorable for aerosol mixing. Here, α-pinene, a biogenic volatile organic compound is oxidized with ozone in a flow tube reactor in the presence of different organic aerosol seeds. Increased humidity did not enhance SOA formation with erythritol or squalane seed as hypothesized, implying that these compounds do not mix with α-pinene SOA in the range of humidities studied (55 – 65%). Yield enhancements were observed with tetraethylene glycol seed, demonstrating interaction between the SOA and seed. These observations suggest increased humidity does not promote mixing between the oxidation products and POA and highlight the need to fully understand the aerosol phase state in the atmosphere in order to better parameterize SOA formation and accurately predict future changes in air quality.

Impact of gas-to-particle partitioning approaches on the simulated radiative effects of biogenic secondary organic aerosol

The oxidation of biogenic volatile organic compounds (BVOCs) gives a range of products, from semi-volatile to extremely low-volatility compounds. To treat the interaction of these secondary organic vapours with the particle phase, global aerosol microphysics models generally use either a thermodynamic partitioning approach (assuming instant equilibrium between semi-volatile oxidation products and the particle phase) or a kinetic approach (accounting for the size dependence of condensation). We show that model treatment of the partitioning of biogenic organic vapours into the particle phase, and consequent distribution of material across the size distribution, controls the magnitude of the first aerosol indirect effect (AIE) due to biogenic secondary organic aerosol (SOA). With a kinetic partitioning approach, SOA is distributed according to the existing condensation sink, enhancing the growth of the smallest particles, i.e. those in the nucleation mode. This process tends to increase cloud droplet number concentrations in the presence of biogenic SOA. By contrast, an approach that distributes SOA according to pre-existing organic mass restricts the growth of the smallest particles, limiting the number that are able to form cloud droplets. With an organically mediated new particle formation mechanism, applying a mass-based rather than a kinetic approach to partitioning reduces our calculated global mean AIE due to biogenic SOA by 24 %. Our results suggest that the mechanisms driving organic partitioning need to be fully understood in order to accurately describe the climatic effects of SOA.

Impact of gas-to-particle partitioning approaches on the simulated radiative effects of biogenic secondary organic aerosol

The oxidation of biogenic volatile organic compounds (BVOCs) gives a range of products, from semi-volatile to extremely low-volatility compounds. To treat the interaction of these secondary organic vapours with the particle phase, global aerosol microphysics models generally use either a thermodynamic partitioning approach (assuming instant equilibrium between semi-volatile oxidation products and the particle phase) or a kinetic approach (accounting for the size-dependence of condensation). We show that model treatment of the partitioning of biogenic organic vapours into the particle phase, and consequent distribution of material across the size distribution, controls the magnitude of the first aerosol indirect effect (AIE) due to biogenic secondary organic aerosol (SOA). With a kinetic partitioning approach, SOA is distributed according to the existing condensation sink, enhancing the growth of the smallest particles, i.e., those in the nucleation mode. This process tends to increase cloud droplet number concentrations in the presence of biogenic SOA. By contrast, a thermodynamic approach distributes SOA according to pre-existing organic mass, restricting the growth of the smallest particles, limiting the number that are able to form cloud droplets. With an organically medicated new particle formation mechanism, applying a thermodynamic rather than a kinetic approach reduces our calculated global mean AIE due to biogenic SOA by 24%. Our results suggest that the mechanisms driving organic partitioning need to be fully understood in order to accurately describe the climatic effects of SOA.

Lipid Oxidation in Dispersive Systems with Monoacylglycerols

Model fat blends with a monoacylglycerol emulsifier with different acyl chain (C10, C12, C14, C16, C18, C18:1, C20, C22) were prepared and stored under oxygen atmosphere 8 weeks at temperature 20°C. Influence of monoacylglycerol on oxidation and oxidation stability of the model fat blends was studied. The model fat blends were prepared by mixing of fully hydrogenated structured fats that contained only palmitic and stearic acid (fully hydrogenated zero-erucic rapeseed oil and fully hydrogenated palmstearin) and half-refined soybean oil. Lipid oxidation was measured by determination of the peroxide value. Volatile oxidation products were detected by the solid phase microextraction in connection with gas chromatography-mass detector (SPME/GC-MS). The oxidative stability was measured by the Rancimat method. Lipid oxidation in model system with 1-octadecenoylglycerol (MAG18:1) was the most extended. On the other hand minimal lipid oxidation was found out in the presence of 1-tetradecanoylglycerol (MAG14) and 1-hexadecanoylglycerol (MAG16).

Quantification of Contaminant Removal by Evactron Cleaning Using Quartz Crystal Thickness Monitors

Hydrocarbon (HC) contamination is a persistent problem for users of electron microscopes (EMs), often leading to image distortion and interference with nanoprobing. The Evactron De-Contaminator (D-C) has been available for HC contamination removal in EMs since 1999. The Evactron D-C uses low power radio frequency (RF) generated plasma in order to produce oxygen radicals that clean the EM. The Oxygen Radical Source (ORS) is attached to the EM chamber, and a controlled leak of oxygen containing gas such as room air is passed through the plasma in order to produce oxygen radicals. The oxygen radicals chemically react with the HCs to form volatile oxidation products such as H2O, CO and CO2. These volatile compounds are pumped out of the EM chamber.