Karena A. McKinney

Uptake of Epoxydiol Isomers Accounts for Half of the Particle-Phase Material Produced from Isoprene Photooxidation via the HO2 Pathway

The oxidation of isoprene is a globally significant source of secondary organic material (SOM) of atmospheric particles. The relative importance of different parallel pathways, however, remains inadequately understood and quantified. SOM production from isoprene photooxidation was studied under hydroperoxyl-dominant conditions for <5% relative humidity and at 20 °C in the presence of highly acidic to completely neutralized sulfate particles. Isoprene photooxidation was separated from SOM production by using two continuously mixed flow reactors connected in series and operated at steady state. Two online mass spectrometers separately sampled the gas and particle phases in the reactor outflow. The loss of specific gas-phase species as contributors to the production of SOM was thereby quantified. The produced SOM mass concentration was directly proportional to the loss of isoprene epoxydiol (IEPOX) isomers from the gas phase. IEPOX isomers lost from the gas phase accounted for (46 ± 11)% of the produced SOM mass concentration. The IEPOX isomers comprised (59 ± 21)% (molecular count) of the loss of monitored gas-phase species. The implication is that for the investigated reaction conditions the SOM production pathways tied to IEPOX isomers accounted for half of the SOM mass concentration.

The Green Ocean Amazon Experiment (GoAmazon2014/5) Observes Pollution Affecting Gases, Aerosols, Clouds, and Rainfall over the Rain Forest

AbstractThe Observations and Modeling of the Green Ocean Amazon 2014–2015 (GoAmazon2014/5) experiment took place around the urban region of Manaus in central Amazonia across 2 years. The urban pollution plume was used to study the susceptibility of gases, aerosols, clouds, and rainfall to human activities in a tropical environment. Many aspects of air quality, weather, terrestrial ecosystems, and climate work differently in the tropics than in the more thoroughly studied temperate regions of Earth. GoAmazon2014/5, a cooperative project of Brazil, Germany, and the United States, employed an unparalleled suite of measurements at nine ground sites and on board two aircraft to investigate the flow of background air into Manaus, the emissions into the air over the city, and the advection of the pollution downwind of the city. Herein, to visualize this train of processes and its effects, observations aboard a low-flying aircraft are presented. Comparative measurements within and adjacent to the plume followed t...

Isoprene photochemistry over the Amazon rainforest

Significance For isolated regions of the planet, organic peroxy radicals produced as intermediates of atmospheric photochemistry have been expected to follow HO2 rather than NO pathways. Observational evidence, however, has been lacking. An accurate understanding of the relative roles of the two pathways is needed for quantitative predictions of the concentrations of particulate matter, oxidation capacity, and consequent environmental and climate impacts. The results herein, based on measurements, find that the ratio of the reaction rate of isoprene peroxy radicals with HO2 to that with NO is about unity for background conditions of Amazonia. The implication is that sufficient NO emissions are maintained by natural processes of the forest such that both HO2 and NO pathways are important, even in this nominally low-NO region. Isoprene photooxidation is a major driver of atmospheric chemistry over forested regions. Isoprene reacts with hydroxyl radicals (OH) and molecular oxygen to produce isoprene peroxy radicals (ISOPOO). These radicals can react with hydroperoxyl radicals (HO2) to dominantly produce hydroxyhydroperoxides (ISOPOOH). They can also react with nitric oxide (NO) to largely produce methyl vinyl ketone (MVK) and methacrolein (MACR). Unimolecular isomerization and bimolecular reactions with organic peroxy radicals are also possible. There is uncertainty about the relative importance of each of these pathways in the atmosphere and possible changes because of anthropogenic pollution. Herein, measurements of ISOPOOH and MVK + MACR concentrations are reported over the central region of the Amazon basin during the wet season. The research site, downwind of an urban region, intercepted both background and polluted air masses during the GoAmazon2014/5 Experiment. Under background conditions, the confidence interval for the ratio of the ISOPOOH concentration to that of MVK + MACR spanned 0.4–0.6. This result implies a ratio of the reaction rate of ISOPOO with HO2 to that with NO of approximately unity. A value of unity is significantly smaller than simulated at present by global chemical transport models for this important, nominally low-NO, forested region of Earth. Under polluted conditions, when the concentrations of reactive nitrogen compounds were high (>1 ppb), ISOPOOH concentrations dropped below the instrumental detection limit (<60 ppt). This abrupt shift in isoprene photooxidation, sparked by human activities, speaks to ongoing and possible future changes in the photochemistry active over the Amazon rainforest.

An ecosystem-scale perspective of the net land methanol flux: synthesis of micrometeorological flux measurements

Methanol is the second most abundant volatile organic compound in the troposphere and plays a significant role in atmospheric chemistry. While there is consensus about the dominant role of living plants as the major source and the reaction with OH as the major sink of methanol, global methanol budgets diverge considerably in terms of source/sink estimates reflecting uncertainties in the approaches used to model, and the empirical data used to separately constrain these terms. Here we compiled micrometeorological methanol flux data from eight different study sites and reviewed the corresponding literature in order to provide a first cross-site synthesis of the terrestrial ecosystem-scale methanol exchange and present an independent data-driven view of the land–atmosphere methanol exchange. Our study shows that the controls of plant growth on the production, and thus the methanol emission magnitude, and stomatal conductance on the hourly methanol emission variability, established at the leaf level, hold across sites at the ecosystem-level. Unequivocal evidence for bi-directional methanol exchange at the ecosystem scale is presented. Deposition, which at some sites even exceeds methanol emissions, represents an emerging feature of ecosystem-scale measurements and is likely related to environmental factors favouring the formation of surface wetness. Methanol may adsorb to or dissolve in this surface water and eventually be chemically or biologically removed from it. Management activities in agriculture and forestry are shown to increase local methanol emission by orders of magnitude; they are however neglected at present in global budgets. While contemporary net land methanol budgets are overall consistent with the grand mean of the micrometeorological methanol flux measurements, we caution that the present approach of simulating methanol emission and deposition separately is prone to opposing systematic errors and does not allow taking full advantage of the rich information content of micrometeorological flux measurements.

Isoprene photo-oxidation products quantify the effect of pollution on hydroxyl radicals over Amazonia

Observational evidence of isoprene oxidation shows that nitrogen oxides amplify the OH concentrations over the Amazon forest. Nitrogen oxides (NOx) emitted from human activities are believed to regulate the atmospheric oxidation capacity of the troposphere. However, observational evidence is limited for the low-to-median NOx concentrations prevalent outside of polluted regions. Directly measuring oxidation capacity, represented primarily by hydroxyl radicals (OH), is challenging, and the span in NOx concentrations at a single observation site is often not wide. Concentrations of isoprene and its photo-oxidation products were used to infer the equivalent noontime OH concentrations. The fetch at an observation site in central Amazonia experienced varied contributions from background regional air, urban pollution, and biomass burning. The afternoon concentrations of reactive nitrogen oxides (NOy), indicative of NOx exposure during the preceding few hours, spanned from 0.3 to 3.5 parts per billion. Accompanying the increase of NOy concentration, the inferred equivalent noontime OH concentrations increased by at least 250% from 0.6 × 106 to 1.6 × 106 cm−3. The conclusion is that, compared to background conditions of low NOx concentrations over the Amazon forest, pollution increased NOx concentrations and amplified OH concentrations, indicating the susceptibility of the atmospheric oxidation capacity over the forest to anthropogenic influence and reinforcing the important role of NOx in sustaining OH concentrations.

Influence of particle physical state on the uptake of medium-sized organic molecules

The uptake of medium-sized levoglucosan and 2,4-dinitrophenol to organic particles produced by α-pinene ozonolysis and to ammonium sulfate particles was studied from 10% to >95% relative humidity (RH). For aqueous sulfate particles, the water-normalized gas-particle partitioning coefficient of levoglucosan decreased from (1.0 ± 0.1) × 10-3 to (0.2 ± 0.1) × 10-3 (ng μg-1)particle/(ng m-3)gas from 40% to >95% RH, suggestive of a salting-in mechanism between levoglucosan and ionic ammonium sulfate solutions. For the organic particles, the levoglucosan partitioning coefficient increased from 10% to 40% RH and became invariant at (2.0 ± 0.4) × 10-3 (ng μg-1)/(ng m-3) above 40% RH. A kinetic limitation on uptake below 40% RH was implied, compared to a thermodynamic regime above 40% RH. The estimated diffusivity was 10-19±0.05 m2 s-1 at 40% RH. By comparison, the uptake of 2,4-dinitrophenol onto the organic particles was below detection limit, implying an upper limit on the partitioning coefficient of 6.8 × 10-6 (ng μg-1)/(ng m-3) at 80% RH. The results highlight that the molecular uptake of gases onto particles can be regulated by both kinetic and thermodynamic factors, either of which can limit the uptake of medium-sized organic molecules by atmospherically relevant particles.

A sampler for atmospheric volatile organic compounds by copter unmanned aerial vehicles

A sampler for volatile organic compounds (VOCs) was developed for deployment on a multicopter unmanned aerial vehicle (UAV). The sampler was designed to collect gasand aerosol-phase VOCs on up to four commercially available VOC-adsorbent cartridges for subsequent offline analysis by thermal-desorption gas chromatography. The sampler had a mass of 0.90 kg and dimensions of 19 cm × 20 cm×5 cm. Power consumption was <10 kJ in a typical 30 min flight, representing<3 % of the total UAV battery capacity. Autonomous sampler operation and data collection in flight were accomplished with a microcontroller. Sampling flows of 100 to 400 sccm were possible, and a typical flow of 150 sccm was used to balance VOC capture efficiency with sample volume. The overall minimum detection limit of the analytical method for a 10 min sample was 3 ppt and the uncertainty was larger than 3 ppt or 20 % for isoprene and monoterpenes. The sampler was mounted to a commercially available UAV and flown in August 2017 over tropical forest in central Amazonia. Samples were collected sequentially for 10 min each at several different altitude–latitude– longitude collection points. The species identified, their concentrations, their uncertainties, and the possible effects of the UAV platform on the results are presented and discussed in the context of the sampler design and capabilities. Finally, design challenges and possibilities for next-generation samplers are addressed.