The development of a new high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) is reported. The high-resolution capabilities of this instrument allow the direct separation of most ions from inorganic and organic species at the same nominal m/z, the quantification of several types of organic fragments (CxHy, CxHyOz, CxHyNp, CxHyOzNp), and the direct identification of organic nitrogen and organosulfur content. This real-time instrument is field-deployable, and its high time resolution (0.5 Hz has been demonstrated) makes it well-suited for studies in which time resolution is critical, such as aircraft studies. The instrument has two ion optical modes: a single-reflection configuration offers higher sensitivity and lower resolving power (up to approximately 2100 at m/z 200), and a two-reflectron configuration yields higher resolving power (up to approximately 4300 at m/z 200) with lower sensitivity. The instrument also allows the determination of the size distributions of all ions. One-minute detection limits for submicrometer aerosol are <0.04 microg m(-3) for all species in the high-sensitivity mode and <0.4 microg m(-3) in the high-resolution mode. Examples of ambient aerosol data are presented from the SOAR-1 study in Riverside, CA, in which the spectra of ambient organic species are dominated by CxHy and CxHyOz fragments, and different organic and inorganic fragments at the same nominal m/z show different size distributions. Data are also presented from the MIRAGE C-130 aircraft study near Mexico City, showing high correlation with independent measurements of surrogate aerosol mass concentration.

The Aerodyne aerosol mass spectrometer (AMS) was used to characterize physical and chemical properties of secondary organic aerosol (SOA) formed during ozonolysis of cycloalkenes and biogenic hydrocarbons and photo-oxidation of m-xylene. Comparison of mass and volume distributions from the AMS and differential mobility analyzers yielded estimates of "effective" density of the SOA in the range of 0.64-1.45 g/cm3, depending on the particular system. Increased contribution of the fragment at m/z 44, C02+ ion fragment of oxygenated organics, and higher "delta" values, based on ion series analysis of the mass spectra, in nucleation experiments of cycloalkenes suggest greater contribution of more oxygenated molecules to the SOA as compared to those formed under seeded experiments. Dominant negative "delta" values of SOA formed during ozonolysis of biogenics indicates the presence of terpene derivative structures or cyclic or unsaturated oxygenated compounds in the SOA. Evidence of acid-catalyzed heterogeneous chemistry, characterized by greater contribution of higher molecular weight fragments to the SOA and corresponding changes in "delta" patterns, is observed in the ozonolysis of alpha-pinene. Mass spectra of SOA formed during photooxidation of m-xylene exhibit features consistent with the presence of furandione compounds and nitro organics. This study demonstrates that mixtures of SOA compounds produced from similar precursors result in broadly similar AMS mass spectra. Thus, fragmentation patterns observed for biogenic versus anthropogenic SOA may be useful in determining the sources of ambient SOA.

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The uptake of gas-phase ammonia by aqueous surfaces was measured as a function of temperature, gas liquid interaction time, and pH in the range 0-13. Uptake measurements at low pH yielded values of the mass accommodation coefficient (R) as a function of temperature. The mass accommodation coefficient increases as the temperature decreases, from 0.08 at 290 K to 0.35 at 260 K. Time dependence of the uptake yielded values for the Henry's law constant. Uptake measurements at high pH indicate that an ammonia surface complex is formed at the interface. Codeposition studies in which an aqueous surface, initially at pH ) 4, was simultaneously exposed to both gas-phase ammonia and SO 2 were also performed. In such a codeposition experiment, the species entering the liquid neutralize each other and as a result the uptake of each species is enhanced. Modeling calculations indicate that the uptake of each species is in accord with bulk liquid-phase kinetics. IntroductionAmmonia in the atmosphere originates primarily from ground sources including decaying organic matter and chemical fertilizers. Significant amounts of NH 3 (0.1-100 ppbv) are found in both clean and polluted atmospheres as well as in cloud and fog droplets. 1 Since ammonia is the only soluble base found in the atmosphere in significant quantities, it plays a principal role in neutralizing acidic aerosols (H 2 SO 4 , HNO 3 , and HCl) converting them to new nonvolatile or semivolatile components; (NH 4 ) 2 SO 4 , NH 4 HSO 4 , NH 4 NO 3 , NH 4 Cl. 2 The process of neutralization influences the aqueous oxidation rates of S(IV) species. A recent study by Meng et al. 3 found that atmospheric ammonia is an important precursor for aerosol formation in the Los Angeles area.Gas-phase reactions involving NH 3 are slow. 4 Tropospheric lifetime for reaction with OH radical for example, is typically about 3 months, and tropospheric photolysis is negligible. 5 Therefore, uptake by aerosols and liquid droplets is the principal tropospheric sink for gaseous ammonia and heterogeneous interactions of NH 3 are of significant interest to atmospheric chemists.The uptake of gas phase ammonia by water has been previously studied in a limited range of acidities by Ponche et al. 6 at 17°C, and Bongartz et al. 7 at 25°C. We have completed a series of NH 3 -liquid water and the NH 3 -sulfuric acid uptake measurements in two independent studies using separate droplet train apparatuses. The water studies were done as a function of pH (0-13) and temperature in the range 20°C to -10°C. The sulfuric acid studies were done in the range 10 to 70 wt % H 2 -SO 4 and as a function of temperature in the range 20°C to -25°C. The time dependence of the uptake was measured by varying the gas-liquid interaction time from 2 to 15 ms. Uptake measurements yielded values of the mass accommodation coefficient (R) and provided information about interactions of

We report here a new approach for determining frequency-dependent real and imaginary components of the refractive index of crystalline solids directly from the observation of the infrared spectra of the corresponding aerosols. The interplay between scattering and absorption observed in large (> 0.5-pm-radius) particles allows us to properly scale the imaginary component, determined from the absorption spectrum of small (around 0.3-pm-radius) aerosols, using calculations based on the Mie scattering theory. Once the imaginary indices are properly scaled, the corresponding real indices are determined through a Kramers-Kronig analysis. The method is applied to the study of water ice aerosols, and comparisons with previous measurements confirm that the method is sound and accurate, Reported here is a detailed study of the temperature dependence of the refractive index of ice. Experiments are reported over the temperature range from 130 to 210 K, which includes the region of interest for the study of polar stratospheric clouds (PSC's).

Abstract. The heterogeneous reaction of OH radicals with sub-micron squalane particles, in the presence of O2, is used as a model system to explore the fundamental chemical mechanisms that control the oxidative aging of organic aerosols in the atmosphere. Detailed kinetic measurements combined with elemental mass spectrometric analysis reveal that the reaction proceeds sequentially by adding an average of one oxygenated functional group per reactive loss of squalane. The reactive uptake coefficient of OH with squalane particles is determined to be 0.3±0.07 at an average OH concentration of ~1×1010 molecules·cm−3. Based on a comparison between the measured particle mass and model predictions it appears that significant volatilization of a reduced organic particle would be extremely slow in the real atmosphere. However, as the aerosols become more oxygenated, volatilization becomes a significant loss channel for organic material in the particle phase. Together these results provide a chemical framework in which to understand how heterogeneous chemistry transforms the physiochemical properties of particle phase organic matter in the troposphere.

Abstract. Simulated primary organic aerosols (POA), as well as other particulates and trace gases, in the vicinity of Mexico City are evaluated using measurements collected during the 2006 Megacity Initiative: Local and Global Research Observations (MILAGRO) field campaigns. Since the emission inventories, transport, and turbulent mixing will directly affect predictions of total organic matter and consequently total particulate matter, our objective is to assess the uncertainties in predicted POA before testing and evaluating the performance of secondary organic aerosol (SOA) treatments. Carbon monoxide (CO) is well simulated on most days both over the city and downwind, indicating that transport and mixing processes were usually consistent with the meteorological conditions observed during MILAGRO. PreCorrespondence to: J. D. Fast (jerome.fast@pnl.gov) dicted and observed elemental carbon (EC) in the city was similar, but larger errors occurred at remote locations since the overall CO/EC emission ratios in the national emission inventory were lower than in the metropolitan emission inventory. Components of organic aerosols derived from Positive Matrix Factorization of data from several Aerodyne Aerosol Mass Spectrometer instruments deployed both at ground sites and on research aircraft are used to evaluate the model. Modeled POA was consistently lower than the measured organic matter at the ground sites, which is consistent with the expectation that SOA should be a large fraction of the total organic matter mass. A much better agreement was found when modeled POA was compared with the sum of "primary anthropogenic" and "biomass burning" components derived from Positive Matrix Factorization (PMF) on most days, especially at the surface sites, suggesting that the overall magnitude of primary organic particulates released was reasonable. However, simulated POA Published by Copernicus Publications on behalf of the European Geosciences Union. 6192 J. Fast et al.: Evaluating simulated primary anthropogenic and biomass burning organic aerosols from anthropogenic sources was often lower than "primary anthropogenic" components derived from PMF, consistent with two recent reports that these emissions are underestimated. The modeled POA was greater than the total observed organic matter when the aircraft flew directly downwind of large fires, suggesting that biomass burning emission estimates from some large fires may be too high.

Secondary organic aerosol (SOA) and oxidized primary organic aerosol (OPOA) were produced in laboratory experiments from the oxidation of fourteen precursors representing atmospherically relevant biogenic and anthropogenic sources. The SOA and OPOA particles were generated via controlled exposure of precursors to OH radicals and/or O₃ in a Potential Aerosol Mass (PAM) flow reactor over timescales equivalent to 1–20 days of atmospheric aging. Aerosol mass spectra of SOA and OPOA were measured with an Aerodyne aerosol mass spectrometer (AMS). The fraction of AMS signal at <i>m/z</i> = 43 and <i>m/z</i> = 44 (<i>f</i>₄₃, <i>f</i>₄₄), the hydrogen-to-carbon (H/C) ratio, and the oxygen-to-carbon (O/C) ratio of the SOA and OPOA were obtained, which are commonly used to characterize the level of oxidation of oxygenated organic aerosol (OOA). The results show that PAM-generated SOA and OPOA can reproduce and extend the observed <i>f</i>₄₄&ndash;<i>f</i>₄₃ composition beyond that of ambient OOA as measured by an AMS. Van Krevelen diagrams showing H/C ratio as a function of O/C ratio suggest an oxidation mechanism involving formation of carboxylic acids concurrent with fragmentation of carbon-carbon bonds. Cloud condensation nuclei (CCN) activity of PAM-generated SOA and OPOA was measured as a function of OH exposure and characterized as a function of O/C ratio. CCN activity of the SOA and OPOA, which was characterized in the form of the hygroscopicity parameter &kappa;_org, ranged from 0.003 to 0.28 over measured O/C ratios ranging from 0.05 to 1.42. This range of &kappa;_org and O/C ratio is significantly wider that has been previously obtained. To first order, the &kappa;_org-to-O/C relationship is well represented by a linear function of the form &kappa;_org = (0.17 &plusmn; 0.04) &times; O/C + 0.04, suggesting that a simple, semi-empirical parameterization of OOA hygroscopicity and oxidation level can be defined for use in chemistry and climate models