A24A-01 INVITED
Emissions and Secondary Organic Aerosol Production from Semivolatile and Intermediate Volatility Organic Compounds
Organic aerosols are a highly-dynamic system dominated by both variable gas-particle partitioning and chemical evolution. Important classes of organics include semivolatile and intermediate volatility organic compounds (SVOC and IVOC, respectively). SVOCs are compounds that exist in both the gas and particle phases at typical atmospheric conditions while IVOC are low-volatility vapors that exist exclusively in the gas phase. Both classes have saturation concentrations that are orders of magnitude lower than volatile organic compounds (VOC) that are the traditional subjects of atmosphere chemistry, such as monoterpenes, alkyl benzenes, etc. The SVOC and IVOC are poorly represented for in current atmospheric chemistry models. Source testing indicates that SVOC and IVOC emissions from biomass combustion, diesel engines and other sources exceed the primary organic aerosol emissions; thus the oxidation of these vapors could serve as a significant source of organic aerosol in the atmosphere. The formation of secondary organic aerosol (SOA) from the reactions between OH radicals and SVOCs and IVOCs was investigated in the Carnegie Mellon University smog chamber. Experiments were conducted with n-alkanes and emission surrogates (diesel fuel and lubricating oil). SVOC oxidation produces oxidized organic aerosol but little new organic aerosol mass. This behavior can be explained by the coupled effects of partitioning and aging. Oxidation of SVOC vapors creates low volatility species that partition into the condensed phase; this oxidation also reduces the SVOC vapor concentration which, in turn, requires particle-phase SVOC to evaporate to maintain phase equilibrium. In contrast, oxidation of IVOC results in sustained production of SOA consistent with a reaction with relatively slow kinetics and high mass yield. Aerosol Mass Spectrometer data indicates that the SOA formed from IVOC has a mass spectrum that is quite similar to the oxygenated organic aerosol factor observed in field studies.
A24A-02
Organic photochemistry of glyoxal (CHOCHO) in aerosol water: SOA formation from C2H2
The lightest Non Methane HydroCarbon (NMHC), i.e. acetylene (C2H2) is found to form secondary
organic aerosol (SOA). Contrary to current belief, the number of carbon
atoms, n, for a NMHC to act as SOA precursor is lowered to n=2 here. These our results present first direct
laboratory evidence that the widely established paradigm of SOA formation based on absorptive partitioning
of semivolatile products into an organic aerosol phase is incomplete, though widely used in most SOA models
on regional and global scale at present; according to this view, isoprene (n=5) was believed to be the
smallest NMHC that forms SOA.
The OH-radical initiated oxidation of C2H2 forms glyoxal (CHOCHO) as the highest yield product, and
>99% of the SOA from C2H2 is attributed to CHOCHO. We conclude that water soluble organic
carbon (WSOC) photochemistry of CHOCHO in aerosol water presents a viable alternative pathway to form
SOA.
Contrary to current belief, the SOA yields (YSOA) from C2H2 further did not correlate with the
organic mass portion of the seed, but increased linearly with liquid water content (LWC) of the seed. For fixed
LWC, YSOA varied by more than a factor of five. This seed effect on YSOA is attributed to
heterogeneous photochemistry of CHOCHO in aerosol water, and is difficult to explain based on the
established paradigm of SOA formation. It identifies a potential bias in the currently
available YSOA data also for other SOA precursor NMHCs. Our results demonstrate that SOA formation
via the aqueous phase is not limited to cloud droplets, but proceeds also in the absence of clouds, i.e. does
not stop once a cloud droplet evaporates. Atmospheric models need to be expanded to include SOA
formation from WSOC photochemistry of CHOCHO, and possibly other a-dicarbonyls,
in aqueous aerosols.
http://www.atmos-chem-phys-discuss.net/8/14841/2008/acpd-8-14841-2008.html
A24A-03
Evidence for Secondary Organic Aerosol Formation Involving Liquid-Phase Partitioning to Haze Particles in Summertime Atlanta
Simultaneous online (10 min) measurements of water-soluble organic carbon in the particle (WSOCp) and gas (WSOCg) phase were made in Atlanta to characterize the gas/particle partitioning of WSOC in an urban environment. The data set is substantial (n=10994) spanning continuous measurements from May to September 2007. During this period WSOCp was a large component of PM2.5, and on average accounted for 70% of Organic Carbon (OC, gC per gC). The WSOC partitioning parameter, Fp, which represents the fraction of total WSOC (WSOCg + WSOCp) in the particle phase, was found to depend on aerosol liquid water content, and the WSOCp and NOx concentration. Overall, these results provide a detailed overview of WSOC partitioning behavior in the summertime in an urban region dominated by biogenic emissions. WSOC gas/particle partitioning showed a strong RH dependence that was attributed to particulate liquid water. At elevated RH (> 70%), a significant increase in WSOC partitioning to the particle phase was observed and followed the predicted water uptake by the fine particle inorganic components, resulting in an apparent linear dependence between WSOC partitioning and particle liquid water concentration. The increase in WSOCp concentrations from this effect was significant, and it offers compelling evidence that secondary organic aerosol formation involving partitioning to liquid water associated with fine haze particles is an important aerosol source. Partitioning was positively correlated with WSOCp for concentrations below roughly 4 μ g C m-3, but showed little dependence at higher WSOCp concentrations. In contrast, no relationship between Fp and total OC aerosol mass was found for any OC concentration. Chemical similarity between the absorbing organic phase and partitioning compounds appears to be important. NOx concentrations also affected Fp. On average, lower NOx concentrations were associated with higher Fp values, however, the Fp - NOx relationship may not have been linked to the gas/particle partitioning process, but may instead result from the influence NOx imparts on the product distribution of volatile organic compound oxidation.
A24A-04
Understanding Anthropogenic and Biogenic Primary and Secondary Aerosol Mixtures
Atmospheric organic matter can constitute 20-70% of the total fine aerosol mass, yet much of its chemical composition is not well understood nor are the mechanisms via which these compounds form. Continental organic aerosol can be directly emitted as primary aerosol from combustion sources or formed in the atmosphere via secondary process with both biogenic and anthropogenic hydrocarbon precursors. In urban air masses, the mixture of anthropogenic primary emissions and secondary organic aerosol (SOA) is ubiquitous. Changes due to the mixing of the different organic chemical compositions can affect the oxidation state and ageing properties of particulates. Hence the change in state can consequently impact air quality, public health and regional climate. The mixing state and the affinity of primary emissions with SOA not well understood: very little is known about the properties of these mixtures. In this a study, SOA formed in the Carnegie Mellon University environmental smog chamber is mixed with diesel exhaust and motor oil aerosol. The objective is to combine two aerosol classes in an external mixture and to then watch the dynamics as well as total mass balance as the mixtures come to equilibrium, possibly forming a single, internal mixture. The SOA is formed from monoterpene and sesquiterpene precursors, specifically α-pinene and β- caryophyllene SOA formed via dark ozonlysis. Primary aerosol is injected into the chamber once the biogenic nucleation and growth has completed and the mixture is allowed to age for three to four hours. A suite of instruments characterize the changes in size, volatility, and chemical composition of the aerosol mixtures. Scanning Mobility Particle Sizers record changes in the size distribution before and after mixing. An Aerodyne High Resolution Time of Flight Aerosol Mass Spectrometer measures the change in mass spectra and the composition of separate size modes both before and after the mixing event, and a thermodenuder is used to characterize changes in mixed aerosol volatility.
A24A-05
Nocturnal isoprene oxidation over the Northeast United States and its impact on reactive nitrogen partitioning and secondary organic aerosol
Isoprene is the largest single VOC emission to the atmosphere and is important to production of tropospheric oxidants and aerosol. Isoprene emissions are sunlight-dependent and undergo rapid photochemical oxidation during daylight hours. In regionally polluted areas, however, late-day isoprene emissions that remain in the atmosphere at sunset undergo oxidation by NO3, which is present only at night and whose production depends on the availability of NOx. These nighttime reactions provide a mechanism for degradation of biogenic VOC by an anthropogenic oxidant. The northeast United States is a region with large emissions of both isoprene and NOx. A recent aircraft study examined isoprene and its nocturnal oxidants in a series of night flights across this region. Substantial amounts of isoprene were observed after dark that were strongly anticorrelated with the presence of NO3. The products of photochemical oxidation of isoprene, methyl vinyl ketone and methacrolein, were more uniformly distributed, and could serve as tracers for the presence of isoprene at sunset, prior to its oxidation by NO3. Estimates of the mass of isoprene oxidized in darkness by NO3 based on these tracers shows that up to 20% of isoprene emissions in this regionally polluted area undergo nocturnal oxidation. The organic nitrates produced from the NO3 + isoprene reaction, though not directly measured, were estimated to account for 2 – 9% of total reactive nitrogen, and were large compared to other long-lived organic nitrates such as PAN. The mass of isoprene oxidized by NO3 was comparable to and correlated with the organic aerosol loading for flights with relatively low backgrounds of organic aerosol. On these flights, the contribution of nocturnally derived isoprene secondary organic aerosol was estimated at 1 – 15% of organic aerosol.
A24A-06
Aerosol Coupling in the Earths System (ACES): Linking Chemical Composition of Secondary Organic Aerosol to Hygroscopic Properties.
The formation of secondary organic aerosol from the oxidation of reactive biogenic emissions is thought to be an important factor in global climate regulation. Biogenic Secondary Organic Aerosol (BSOA) can contribute to indirect radiative forcing by acting as cloud condensation nuclei. The potential of particles to act as CCN depends on their composition and hygroscopic properties. Sesquiterpenes, such as beta-caryophyllene, have been proposed as SOA precursors due to their low volatility and highly reactive nature. One of the main problems with using smog chamber simulations to study SOA formation and properties is the need to use high concentrations of reactive organic species. This can result in higher SOA yields and a different composition than in the real atmosphere. As part of the Aerosol coupling in the Earth System (ACES) project, a series of novel experiments were carried out at the Manchester Aerosol Chamber. By using a high volume pump and a collapsible chamber, the entire contents of the chamber can be sampled rapidly onto a filter. Recent improvements in the sensitivity of analytical techniques have allowed us to use lower VOC precursor concentrations (20 ppb) than previous studies. Repetitive experiments were carried out and filter samples taken at different experiment times, 2, 4 and 6 hours, allowing the evolution of individual SOA components to be evaluated. Complementary proton transfer mass spectrometry was used to study the evolution of gas phase oxidation products. The composition of beta-caryophyllene SOA was studied using liquid chromatography coupled to mass spectrometry. Twelve components were identified based on MS2 fragmentation patterns. The SOA composition was found to be much simpler than seen for monoterpenes and no oligomers were found. Experiments at 42 ppb and 210 ppb, indicate that the distribution of products varied depending on the starting VOC concentration. Low concentrations resulted in a higher proportion of more polar species such as nor-caryophyllenic acid (172 g mol-1), indicating the importance of carrying out smog chamber studies as close to ambient concentrations as possible. Results obtained using an Aerodyne aerosol mass spectrometer showed that SOA formed using lower initial precursor concentration contained a higher fraction of m/z 44 (a typical marker for highly oxygenated organic molecules) compared to SOA produced using high precursor concentration. Ageing resulted in an increasingly complex OVOC profile compared to the simpler, early generation products, seen in the initial stages of the experiment. The hygroscopic properties of the beta-caryophyllene SOA were investigated using a hygroscopicity tandem differential mobility analyser (HTDMA) and a CCN counter. The evolution of the hygroscopic growth factor and the CCN activity were compared to the aerosol composition, giving an insight into how the oxidation processes altered the ability of the aerosol to take up water.
A24A-07
Observations of organic aerosol mass (OA) growth downwind of urban and industrial source in Houston Area
During TexAQS-2006 field study, organic aerosol mass (OA)downwind of Houston urban center and the petrochemical industries along the Houston Ship Channel was measured aboard the NOAA WP-3D aircraft, using a compact time-of-flight aerosol mass spectrometer (C-ToF-AMS). We have used on-board measurements of CO, benzene, and SO2 to identify the different plumes transected on several days in order to characterize the growth of OA with transport downwind of the sources. The results show that the OA growth in urban plume is similar to other urban centers in NE U.S. However, the observed growth in OA downwind of Houston Ship Channel exceeds those observed downwind of urban centers. We also present results from a chemical box model that was developed to simulate secondary organic aerosol (SOA) formation in the urban and industrial plumes, using absorbing- mass dependent and the most recent NOx dependent aerosol formation yields. Furthermore, we used a transport model, FLEXPART, along with the EPA BEIS biogenic emission inventory, to estimate isoprene and monoterpene surface contributions to the air masses sampled on two flights. Using constant aerosol formation yields, the amount of OA that could be formed from these biogenic precursors was estimated to be less than 1.5 ug m-3, even during a flight to the north of Houston where high biogenic emissions are observed. The tight correlation between the observed OA and CO on this flight also indicates that anthropogenic sources play the dominant role in OA formation in Houston area.