CAPS: 2007 Publications

Ammonium sulfate particles were generated by atomization and introduced into a smog chamber where they were coated with secondary organic aerosol from ozonolysis of limonene or -pinene. These mixed particles were then sampled with a humidified Tandem-DMA system where a monodisperse aerosol population was selected, humidified, and dried to observe the relative humidity (RH) at which the particles returned to the original dry diameter. The volume fraction of secondary organic aerosol (SOA) in the mixed particles ranged from 0.59 to 0.94 for limonene SOA and 0.54 to 0.63 for a-pinene SOA. Efflorescence RHs for our mixed aerosols were in the range of 28-34%, similar to our observation of 32% ERH for pure ammonium sulfate nanoparticles. These findings indicate that the effect of SOA on the ERH of inorganic salts in the atmosphere may be negligible.

Isoprene, the most abundant non-methane hydrocarbon emitted into the troposphere, has generally not been considered a major source of SOA due to the relatively high volatility of its oxidation products. In this study, the SOA formed from the oxidation of isoprene is predicted using a three-dimensional chemical transport model, PMCAMx, across the eastern U.S. for July, October, January, and April 2001-2002. The variability of the measured SOA yields in the available smog chamber studies is captured by combining the base case scenario with upper and lower bound estimates of the measurements. For the base case simulation, the predicted annual average isoprene SOA concentration in the southeast is 0.09 ug m-3 (bounds 0.04-0.23 ug m-3). Isoprene is predicted to produce 70% less SOA across the entire domain for spring and fall than during the summer and negligible amounts of SOA during the winter. During the summer, the average concentrations in the northeast are predicted to be 0.11 ug m-3 (bounds 0.04- 0.31 ug m-3) and in the southeast 0.19 ug m-3 (bounds 0.11- 0.58 ug m-3). PMCAMx predictions are compared to available measurements of some isoprene SOA components in North Carolina and New York State. These modeling results suggest that on an annual basis isoprene oxidation is a small but non-negligible organic aerosol source in the eastern U.S. Its contribution is relatively more important during the summer and in the southeast U.S.

An improved thermodenuder is used to measure the volatility of secondary organic aerosol (SOA) produced during a-pinene/O3 and a-pinene/NOx photooxidation. The thermodenuder allows a wide range of aerosol residence times in the heated zone compared to existing systems avoiding the complications due to slow mass and heat transfer processes. The performance of the thermodenuder was tested using mono-disperse ammonium sulfate particles. The volatility of SOA was investigated in the 50-220 oC temperature range. Almost 98% of the SOA volume generated from the a-pinene/O3 reaction evaporated at 75 oC after 15.8 s in the heated zone. However, more than 50% of the particle mass did not volatilize at 100 oC when the residence time was reduced to 1.6 s. The SOA obtained from a-pinene/NOx photooxidation showed similar volatility characteristics even after 10 h of “aging” in the smog chamber. The measured remaining aerosol mass after the particles pass through the thermodenuder is quite sensitive to their residence time in the heated zone of the system, for residence times of the order of seconds. Interpreting the remaining aerosol mass as non-volatile even when the thermodenuder operates at temperatures above 200 oC may be erroneous if low residence times (less than a few seconds) are used.

A source-resolved model has been developed to predict the contribution of different sources to primary organic aerosol concentrations. The model was applied to the eastern US during a 17 day pollution episode beginning on 12 July 2001. Primary organic matter (OM) and elemental carbon (EC) concentrations are tracked for eight different sources: gasoline vehicles, non-road diesel vehicles, on-road diesel vehicles, biomass burning, wood burning, natural gas combustion, road dust, and all other sources. Individual emission inventories are developed for each source and a three-dimensional chemical transport model (PMCAMx) is used to predict the primary OM and EC concentrations from each source. The sourceresolved model is simple to implement and is faster than existing source-oriented models. The results of the source-resolved model are compared to the results of chemical mass balance models (CMB) for Pittsburgh and multiple urban/rural sites from the Southeastern Aerosol Research and Characterization (SEARCH) network. Significant discrepancies exist between the source-resolved model and the CMB model predictions for some of the sources. There is strong evidence that the organic PM emissions from natural gas combustion are overestimated. It also appears that the OM and EC emissions from wood burning and off-road diesel are too high in the Northeastern US. Other similarities and discrepancies between the source-resolved model and the CMB model for primary OM and EC are discussed along with problems in the current emission inventory for certain sources.

The goal of this modeling study is to determine how concentrations of ozone respond to changes in climate over the eastern USA. The sensitivities of average ozone concentrations to temperature, wind speed, absolute humidity, mixing height, cloud liquid water content and optical depth, cloudy area, precipitation rate, and precipitating area extent are investigated individually. The simulation period consists of July 12-21, 2001, during which an ozone episode occurred over the Southeast. The ozone metrics used include daily maximum 8 h average O3 concentration and number of grid cells exceeding the US EPA ambient air-quality standard. The meteorological factor that had the largest impact on both ozone metrics was temperature, which increased daily maximum 8 h average O3 by 0.34 ppb K1 on average over the simulation domain. Absolute humidity had a smaller but appreciable effect on daily maximum 8 h average O3 (0.025 ppb for each percent increase in absolute humidity). While domain-average responses to changes in wind speed, mixing height, cloud liquid water content, and optical depth were rather small, these factors did have appreciable local effects in many areas. Temperature also had the largest effect on air-quality standard exceedances; a 2.5 K temperature increase led to a 30increase in the area exceeding the EPA standard. Wind speed and mixing height also had appreciable effects on ozone air- quality standard exceedances.

A three-dimensional chemical transport model (PMCAMx) is used to simulate PM mass and composition in the eastern United States for a July 2001 pollution episode. The performance of the model in this region is evaluated, taking advantage of the highly time and size-resolved PM and gas-phase data collected during the Pittsburgh Air Quality Study (PAQS). PMCAMx uses the framework of CAMx and detailed aerosol modules to simulate inorganic aerosol growth, aqueous- phase chemistry, secondary organic aerosol formation, nucleation, and coagulation. The model predictions are compared to hourly measurements of PM2.5 mass and composition at Pittsburgh, as well as to measurements from the AIRS and IMPROVE networks. The performance of the model for the major PM2.5 components (sulfate, ammonium, and organic carbon) is encouraging (fractional errors are in general smaller than 50%). Additional improvements are possible if the rainfall measurements are used instead of the meteorological model predictions. The modest errors in ammonium predictions and the lack of bias for the total (gas and particulate) ammonium suggest that the improved ammonia inventory used is reasonable. The significant errors in aerosol nitrate predictions are mainly due to difficulties in simulating the nighttime formation of nitric acid. The concentrations of elemental carbon (EC) in the urban areas are significantly overpredicted. This is a problem related to both the emission inventory but also the different EC measurement methods that have been used in the two measurement networks (AIRS and IMPROVE) and the actual development of the inventory. While the ability of the model to reproduce OC levels is encouraging, additional work is necessary to confirm that that this is due to the right reasons and not offsetting errors in the primary emissions and the secondary formation. The model performance against the semi-continuous measurements in Pittsburgh appears to be quite similar to its performance against daily average measurements in a wide range of stations across the Eastern US. This suggests that the skill of the model to reproduce the diurnal variability of PM2.5 and its major components is as good as its ability to reproduce the daily average values and also the significant value of high temporal resolution measurements for model evaluation.

The temperature-dependence of secondary organic aerosol (SOA) concentrations is measured using a temperature-controlled smog chamber. Aerosols are generated from reaction of α-pinene (14-150 ppb) and ozone at a constant temperature of 22 ( 2^oC in the presence of the OH-scavenger 2-butanol. After the reactions are completed the chamber is heated or cooled in a range from 20 to 40 ^oC. SOA volume concentrations increase at temperatures below the initial formation temperature and decrease at elevated temperatures. The response to the temperature change as measured by percent mass change per degree ranged from -0.4 to -3.6% K-1, for a total mass reduction of 5-60% upon heating from 22 to 35 ^oC. The reported range is due to two factors: (1) experimental uncertainty, arising mainly from uncertainty in evaporation and condensation behavior of particles lost to the chamber wall; (2) differences in the temperature response from experiment to experiment. Aerosol temperature sensitivity was also measured by tandem differential mobility analysis (TDMA) where similarly generated SOA were heated from 20 to 25 ^oC to 30-40 ^oC with residence times of 0.5-1.5 min, resulting in particle volume reductions of up to 20%. The TDMA experiments indicate that evaporation of the SOA particles in this system occurs with a potentially significant mass transfer limitation (e.g., accommodation coefficient <0.1).

The oxidation of organics in aerosol particles affects the physical properties of aerosols through a process known as aging. Atmo- spheric particles compose a huge set of specific organic compounds, most of which have not been identified in field measurements. Lab- oratory experiments inevitably address model systems of reduced complexity to isolate critical chemical phenomena, but growing ev- idence suggests that composition effects may play a central role in the atmospheric aging of organic particles. In this review we seek to address the connections between recent laboratory studies and recent field campaigns addressing the aging of organic aerosols. We review laboratory studies on the uptake of oxidants, the evolution of particle-water interactions, and the evolution of particle density with aging. Finally, we review field data addressing condensed-phase lifetimes of organic tracers. These data suggest that although matrix effects identified in the laboratory have taken a step toward rec- onciling laboratory-field disagreements, further work is needed to understand the actual aging rates of organics in ambient particles.

Limona ketone was synthesized to explore the secondary organic aerosol (SOA) formation mechanism from limonene ozonolysis and also to test group-additivity concepts describing the volatility distribution of ozonolysis products from similar precursors. Limona ketone SOA production is indistinguishable from a-pinene, confirming the expected similarity. However, limona ketone SOA production is significantly less intense than limonene SOA production. The very low vapor pressure of limonene ozonolysis products is consistent with full oxidation of both double bonds in limonene and furthermore with production of products other than ketones after oxidation of the exo double bond in limonene. Mass-balance constraints confirm that ketone products from exo double-bond ozonolysis have a minimal contribution to the ultimate product yield. These results serve as the foundation for an emerging framework to describe the effect on volatility of successive generations of organic compounds in the atmosphere.

[1] Organic aerosols in the atmosphere are exposed to oxidants, but the oxidation kinetics are largely unknown. We investigate the decay of organic species in laboratory-generated organic aerosols exposed to atmospherically relevant ozone concentrations in a smog chamber. The experiments were conducted using five different organic aerosols, varying in complexity from three to twelve components. These mixtures include alkenoic acids, alkanoic acids, alkanedioic acids, n-alkanes, and sterols and are designed to simulate meat cooking emissions. A relative rate constants approach was used to compare reaction rates of individual organic species and to compare the reaction rates of the aerosol species to gas phase tracers. Significant decay was observed for all species (except for the n-alkanes) in at least one of the experimental systems. By relating the decomposition of condensed phase alkenoic acids to gas phase alkenes, we show that the reaction rate constants of oleic acid and palmitoleic acid evolve as the aerosol is processed, decreasing by a factor of 10 over the course of a 4-hour experiment. The decay rate constants of cholesterol, oleic acid, and palmitic acid all depend strongly on aerosol composition, with more than an order of magnitude change in the effective rate constants depending on mixture composition. Effects of aerosol composition are likely to be even more significant in atmospheric aerosol, where particle compositions are highly variable. The data presented here indicate these mixture effects are complicated, making it difficult to extrapolate from simple laboratory systems to atmospherically relevant conditions.

We present a novel method for continuous, stable OH radical production for use in smog chamber studies, especially those focused on organic aerosol aging. Our source produces OH radicals from the reaction of 2,3-dimethyl-2- butene and ozone and is unique as a method that requires neither NOx nor UV photolysis of a radical precursor. Typical radical concentrations are in the range of (4-8) x 10⁶ molec cm-3 and are easily sustainable over experimental time scales of several hours. We discuss design considerations, radical production capability under different operating conditions, and the core source chemistry. As a proof of concept we present preliminary results from oxidation of n-hexacosane aerosol observed with an Aerodyne Aerosol Mass Spectrometer. The extent of hexacosane oxidation is sufficient to significantly change the organic aerosol mass spectrum by virtue of fast heterogeneous uptake of OH radicals at the particle surface, with a calculated uptake coefficient γ= 1.04 0.21.

Despite a number of smog chamber studies of the alpha-pinene/O-3 system, the effect of temperature on alpha-pinene secondary organic aerosol (SOA) mass fractions (or yields) remains poorly understood. In this study, the temperature dependence of secondary organic aerosol mass fractions (AMF) during ozonolysis of apinene is investigated in a temperature controlled smog chamber. Experiments were performed with and without ammonium sulfate aerosol seeds at RH < 10% and at 0 degrees C, 15 degrees C, 20 degrees C, 30 degrees C and 40 degrees C. The initial alpha-pinene concentration varied from 3.5 to 50 ppb, and an excess of ozone was used. High time resolution secondary organic AMFs were obtained combining continuous gas-phase concentration measurements (using proton transfer reaction mass spectrometry, PTR-MS) with continuous aerosol concentration measurements (using a scanning mobility particle sizer, SMPS). The presence of inert aerosol seeds is often necessary to minimize experimental errors due to loss of semivolatile vapors to the walls of the chamber. The alpha-pinene secondary organic AMFs show a weak dependence on temperature in the 15 degrees to 40 degrees C range and stronger temperature dependence in the 0 degrees and 15 degrees C range. C1 Carnegie Mellon Univ, Dept Chem Engn, Pittsburgh, PA 15213 USA. Univ Iowa, Dept Engn, Iowa City, IA 52242 USA. Univ Patras, Dept Chem Engn, GR-26000 Patras, Greece.

Most primary organic-particulate emissions are semivolatile; thus, they partially evaporate with atmospheric dilution, creating substantial amounts of low-volatility gas-phase material. Laboratory experiments show that photo-oxidation of diesel emissions rapidly generates organic aerosol, greatly exceeding the contribution from known secondary organic-aerosol precursors. We attribute this unexplained secondary organic-aerosol production to the oxidation of low-volatility gas-phase species. Accounting for partitioning and photochemical processing of primary emissions creates a more regionally distributed aerosol and brings model predictions into better agreement with observations. Controlling organic particulate-matter concentrations will require substantial changes in the approaches that are currently used to measure and regulate emissions. C1 Carnegie Mellon Univ, Ctr Atmospher Particle Studies, Pittsburgh, PA 15213 USA. Univ Patras, Dept Chem Engn, Patras 26500, Greece.

Current regulation aimed at reducing inorganic atmospheric fine particulate matter (PM2.5) is focused on reductions in sulfur dioxide (SO2) and oxides of nitrogen (NOx equivalent to NO + NO2); however, controls on these pollutants are likely to increase in cost and decrease in effectiveness in the future. A supplementary strategy is reduction in ammonia (NH3) emissions, yet an evaluation of controls on ammonia has been limited by uncertainties in emission levels and in the cost of control technologies. We use state of the science emission inventories, an emission-based regional air quality model, and an explicit treatment of uncertainty to estimate the cost-effectiveness and uncertainty of ammonia emission reductions on inorganic particulate matter in the Eastern United States. Since a paucity of data on agricultural operations precludes a direct calculation of the costs of ammonia control, we calculate the ammonia savings potential, defined as the minimum cost of applying SO2 and NOx emission controls in order to achieve the same reduction in ambient inorganic PM2.5 concentration as obtained from a 1 ton decrease in ammonia emissions. Using 250 scenarios of NH3, SO2, and NOx emission reductions, we calculate the least-cost SO2 and NOx control scenarios that achieve the same reduction in ambient inorganic PM2.5 concentration as a decrease in ammonia emissions. We find that the lower-bound ammonia savings potential in the winter is $8,000 per ton NH3; therefore, many currently available ammonia control technologies are cost-effective compared to current controls on SO2 and NOx sources. Larger reductions in winter inorganic particulate matter are available at lower cost through controls on ammonia emissions. C1 Carnegie Mellon Univ, Dept Engn & Publ Policy, Pittsburgh, PA 15213 USA. Carnegie Mellon Univ, Dept Civil & Environm Engn, Pittsburgh, PA 15213 USA. Univ Patras, Dept Chem Engn, Patras 26500, Greece.