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Production and Measurement of Organic Particulate Matter in the Harvard Environmental Chamber

The production and the evolution of atmospheric organic particulate matter (PM) are insufficiently understood for accurate simulations of atmospheric ...

Harvard University

Chemical mechanisms for the production of secondary organic material (SOM) are developed in focused laboratory studies but widely used in the complex modeling context of the atmosphere. Given this extrapolation, a stringent testing of the mechanisms is important. In addition to particle mass yield as a typical standard for model-measurement comparison, particle composition expressed as O:C and H:C elemental ratios can serve as a higher dimensional constraint. A paradigm for doing so is developed herein for SOM production from a C(5)-C(10)-C(15) terpene sequence, namely isoprene, α-pinene, and β-caryopyhllene. The model MCM-SIMPOL is introduced based on the Master Chemical Mechanism (MCM v3.2) and a group contribution method for vapor pressures (SIMPOL). The O:C and H:C ratios of the SOM are measured using an Aerosol Mass Spectrometer (AMS). Detailed SOM-specific AMS calibrations for the organic contribution to the H(2)O(+) and CO(+) ions indicate that published O:C and H:C ratios for SOM are systematically too low. Overall, the measurement-model gap was small for particle mass yield but significant for particle-average elemental composition. The implication is that a key chemical pathway is missing from the chemical mechanism. The data can be explained by the particle-phase homolytic decomposition of organic hydroperoxides and subsequent alkyl-radical-promoted oligomerization.

Particle-phase chemistry of secondary organic material: modeled compared to measured O:C and H:C elemental ratios provide constraints.

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. 

Uptake of epoxydiol isomers accounts for half of the particle-phase material produced from isoprene photooxidation via the HO2 pathway.

The production of secondary organic material (SOM) by the reactive uptake of isoprene photooxidation products was investigated using partially to wholly neutralized sulfuric acid particles. The experiments were performed at a relative humidity (RH) of <5% and a temperature of 20 °C. The extent X of neutralization was adjusted from that of sulfuric acid (X = 0) to that of ammonium sulfate (X = 1). Significant SOM production was observed only for X < 0.7. The threshold of 0.7 corresponded to the transition point of the sulfate particles from aqueous to solid for <5% RH. The phase transition of inorganic sulfate therefore regulated the particle-phase reactions that produce isoprene SOM, at least for the investigated conditions. For aqueous particles, a decreasing extent of neutralization was associated with increasing production of SOM, including increased production of oligomers and organosulfates. These results can underpin treatments of phase-dependent SOM production within chemical transport models, thereby improving the accuracy of simulations of biogenic-anthropogenic interactions in the atmosphere and the associated impacts of aerosol particles on climate and air quality. 

Physical state and acidity of inorganic sulfate can regulate the production of secondary organic material from isoprene photooxidation products.

Gaseous species produced via the HO2 reaction pathways of isoprene photo-oxidation were reacted with liquid, partially neutralized sulfate aerosol particles at 293 ± 1 K and <5% relative humidity. Isoprene-derived epoxydiols (IEPOX) were taken up for all neutralizations so long as the liquid phase was maintained. By comparison, isoprene-derived hydroperoxides (ISOPOOH) were taken up only for low neutralization (i.e., high acidity). The release of product molecules to the gas phase increased for low neutralization, corresponding to the release of at least 60 product molecules for the uptake of 100 reactant molecules at the lowest neutralization. A major reaction pathway was hydroperoxide cleavage in the particle phase to produce volatile products. Product species larger than the C5 chain of isoprene were also released to the gas phase, implying that some accretion products in the particle phase were sufficiently volatile to partition to the gas phase. The study results show that the dependence of reactive uptake on neutralization varies by species. Furthermore, in addition to functionalization and accretion, decomposition and re-volatilization should be considered in mass balance formulations of reactive uptake by atmospheric particles. 

Uptake and release of gaseous species accompanying the reactions of isoprene photo-oxidation products with sulfate particles.

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.

Isoprene photochemistry over the Amazon rainforest.

Nitrogen oxides (NO  ) 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 NO  concentrations prevalent outside of polluted regions. Directly measuring oxidation capacity, represented primarily by hydroxyl radicals (OH), is challenging, and the span in NO  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 (NO  ), indicative of NO  exposure during the preceding few hours, spanned from 0.3 to 3.5 parts per billion. Accompanying the increase of NO  concentration, the inferred equivalent noontime OH concentrations increased by at least 250% from 0.6 × 10 to 1.6 × 10 cm. The conclusion is that, compared to background conditions of low NO  concentrations over the Amazon forest, pollution increased NO  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 NO  in sustaining OH concentrations.

Yingjun Liu

School of Engineering and Applied Sciences

Harvard University

Production and Measurement of Organic Particulate Matter in the Harvard Environmental Chamber