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    Chemistry of new particle growth in mixed urban and biogenic emissions - Insights from CARES
    (München : European Geopyhsical Union, 2014) Setyan, A.; Song, C.; Merkel, M.; Knighton, W.B.; Onasch, T.B.; Canagaratna, M.R.; Worsnop, D.R.; Wiedensohler, A.; Shilling, J.E.; Zhang, Q.
    Regional new particle formation and growth events (NPEs) were observed on most days over the Sacramento and western Sierra foothills area of California in June 2010 during the Carbonaceous Aerosols and Radiative Effect Study (CARES). Simultaneous particle measurements at both the T0 (Sacramento, urban site) and the T1 (Cool, rural site located ~40 km northeast of Sacramento) sites of CARES indicate that the NPEs usually occurred in the morning with the appearance of an ultrafine mode at ~15 nm (in mobility diameter, Dm, measured by a mobility particle size spectrometer operating in the range 10-858 nm) followed by the growth of this modal diameter to ~50 nm in the afternoon. These events were generally associated with southwesterly winds bringing urban plumes from Sacramento to the T1 site. The growth rate was on average higher at T0 (7.1 ± 2.7 nm h−1) than at T1 (6.2 ± 2.5 nm h−1), likely due to stronger anthropogenic influences at T0. Using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS), we investigated the evolution of the size-resolved chemical composition of new particles at T1. Our results indicate that the growth of new particles was driven primarily by the condensation of oxygenated organic species and, to a lesser extent, ammonium sulfate. New particles appear to be fully neutralized during growth, consistent with high NH3 concentration in the region. Nitrogen-containing organic ions (i.e., CHN+, CH4N+, C2H3N+, and C2H4N+) that are indicative of the presence of alkyl-amine species in submicrometer particles enhanced significantly during the NPE days, suggesting that amines might have played a role in these events. Our results also indicate that the bulk composition of the ultrafine mode organics during NPEs was very similar to that of anthropogenically influenced secondary organic aerosol (SOA) observed in transported urban plumes. In addition, the concentrations of species representative of urban emissions (e.g., black carbon, CO, NOx, and toluene) were significantly higher whereas the photo-oxidation products of biogenic VOCs (volatile organic compounds) and the biogenically influenced SOA also increased moderately during the NPE days compared to the non-event days. These results indicate that the frequently occurring NPEs over the Sacramento and Sierra Nevada regions were mainly driven by urban plumes from Sacramento and the San Francisco Bay Area, and that the interaction of regional biogenic emissions with the urban plumes has enhanced the new particle growth. This finding has important implications for quantifying the climate impacts of NPEs on global scale.
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    The chemistry of OH and HO2 radicals in the boundary layer over the tropical Atlantic Ocean
    (München : European Geopyhsical Union, 2010) Whalley, L.K.; Furneaux, K.L.; Goddard, A.; Lee, J.D.; Mahajan, A.; Oetjen, H.; Read, K.A.; Kaaden, N.; Carpenter, L.J.; Lewis, A.C.; Plane, J.M.C.; Saltzman, E.S.; Wiedensohler, A.; Heard, D.E.
    Fluorescence Assay by Gas Expansion (FAGE) has been used to detect ambient levels of OH and HO2 radicals at the Cape Verde Atmospheric Observatory, located in the tropical Atlantic marine boundary layer, during May and June 2007. Midday radical concentrations were high, with maximum concentrations of 9 ×106 molecule cm−3 and 6×108 molecule cm−3 observed for OH and HO2, respectively. A box model incorporating the detailed Master Chemical Mechanism, extended to include halogen chemistry, heterogeneous loss processes and constrained by all available measurements including halogen and nitrogen oxides, has been used to assess the chemical and physical parameters controlling the radical chemistry. The model was able to reproduce the daytime radical concentrations to within the 1 σ measurement uncertainty of 20% during the latter half of the measurement period but significantly under-predicted [HO2] by 39% during the first half of the project. Sensitivity analyses demonstrate that elevated [HCHO] (~2 ppbv) on specific days during the early part of the project, which were much greater than the mean [HCHO] (328 pptv) used to constrain the model, could account for a large portion of the discrepancy between modelled and measured [HO2] at this time. IO and BrO, although present only at a few pptv, constituted ~19% of the instantaneous sinks for HO2, whilst aerosol uptake and surface deposition to the ocean accounted for a further 23% of the HO2 loss at noon. Photolysis of HOI and HOBr accounted for ~13% of the instantaneous OH formation. Taking into account that halogen oxides increase the oxidation of NOx (NO → NO2), and in turn reduce the rate of formation of OH from the reaction of HO2 with NO, OH concentrations were estimated to be 9% higher overall due to the presence of halogens. The increase in modelled OH from halogen chemistry gives an estimated 9% shorter lifetime for methane in this region, and the inclusion of halogen chemistry is necessary to model the observed daily cycle of O3 destruction that is observed at the surface. Due to surface losses, we hypothesise that HO2 concentrations increase with height and therefore contribute a larger fraction of the O3 destruction than at the surface.
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    Development of a protocol for the auto-generation of explicit aqueous-phase oxidation schemes of organic compounds
    (Katlenburg-Lindau : EGU, 2019) Bräuer, Peter; Mouchel-Vallon, Camille; Tilgner, Andreas; Mutzel, Anke; Böge, Olaf; Rodigast, Maria; Poulain, Laurent; van Pinxteren, Dominik; Wolke, Ralf; Aumont, Bernard; Herrmann, Hartmut
    This paper presents a new CAPRAM-GECKOA protocol for mechanism auto-generation of aqueous-phase organic processes. For the development, kinetic data in the literature were reviewed and a database with 464 aqueousphase reactions of the hydroxyl radical with organic compounds and 130 nitrate radical reactions with organic compounds has been compiled and evaluated. Five different methods to predict aqueous-phase rate constants have been evaluated with the help of the kinetics database: gas-aqueous phase correlations, homologous series of various compound classes, radical reactivity comparisons, Evans-Polanyi-type correlations, and structure-activity relationships (SARs). The quality of these prediction methods was tested as well as their suitability for automated mechanism construction. Based on this evaluation, SARs form the basis of the new CAPRAM-GECKO-A protocol. Evans-Polanyi-type correlations have been advanced to consider all available H atoms in a molecule besides the H atoms with only the weakest bond dissociation enthalpies (BDEs). The improved Evans- Polanyi-type correlations are used to predict rate constants for aqueous-phase NO3 and organic compounds reactions. Extensive tests have been performed on essential parameters and on highly uncertain parameters with limited experimental data. These sensitivity studies led to further improvements in the new CAPRAM-GECKO-A protocol but also showed current limitations. Biggest uncertainties were observed in uptake processes and the estimation of Henry's law coefficients as well as radical chemistry, in particular the degradation of alkoxy radicals. Previous estimation methods showed several deficits, which impacted particle growth. For further evaluation, a 1,3,5-trimethylbenzene oxidation experiment has been performed in the aerosol chamber "Leipziger Aerosolkammer" (LEAK) at high relative humidity conditions and compared to a multiphase mechanism using the Master Chemical Mechanism (MCMv3.2) in the gas phase and using a methylglyoxal oxidation scheme of about 600 reactions generated with the new CAPRAM-GECKO-A protocol in the aqueous phase. While it was difficult to evaluate single particle constituents due to concentrations close to the detection limits of the instruments applied, the model studies showed the importance of aqueous-phase chemistry in respect to secondary organic aerosol (SOA) formation and particle growth. The new protocol forms the basis for further CAPRAM mechanism development towards a new version 4.0. Moreover, it can be used as a supplementary tool for aerosol chambers to design and analyse experiments of chemical complexity and help to understand them on a molecular level. © 2019 Author(s).
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    Relating particle hygroscopicity and CCN activity to chemical composition during the HCCT-2010 field campaign
    (München : European Geopyhsical Union, 2013) Wu, Z.J.; Poulain, L.; Henning, S.; Dieckmann, K.; Birmili, W.; Merkel, M.; van Pinxteren, D.; Spindler, G.; Müller, K.; Stratmann, F.; Herrmann, H.; Wiedensohler, A.
    Particle hygroscopic growth at 90% RH (relative humidity), cloud condensation nuclei (CCN) activity, and size-resolved chemical composition were concurrently measured in the Thüringer Wald mid-level mountain range in central Germany in the fall of 2010. The median hygroscopicity parameter values, κ, of 50, 75, 100, 150, 200, and 250 nm particles derived from hygroscopicity measurements are respectively 0.14, 0.14, 0.17, 0.21, 0.24, and 0.28 during the sampling period. The closure between HTDMA (Hygroscopicity Tandem Differential Mobility Analyzers)-measured (κHTDMA) and chemical composition-derived (κchem) hygroscopicity parameters was performed based on the Zdanovskii–Stokes–Robinson (ZSR) mixing rule. Using size-averaged chemical composition, the κ values are substantially overpredicted (30 and 40% for 150 and 100 nm particles). Introducing size-resolved chemical composition substantially improved closure. We found that the evaporation of NH4NO3, which may happen in a HTDMA system, could lead to a discrepancy in predicted and measured particle hygroscopic growth. The hygroscopic parameter of the organic fraction, κorg, is positively correlated with the O : C ratio (κorg = 0.19 × (O : C) − 0.03). Such correlation is helpful to define the κorg value in the closure study. κ derived from CCN measurement was around 30% (varied with particle diameters) higher than that determined from particle hygroscopic growth measurements (here, hydrophilic mode is considered only). This difference might be explained by the surface tension effects, solution non-ideality, gas-particle partitioning of semivolatile compounds, and the partial solubility of constituents or non-dissolved particle matter. Therefore, extrapolating from HTDMA data to properties at the point of activation should be done with great care. Finally, closure study between CCNc (cloud condensation nucleus counter)-measured (κCCN) and chemical composition (κCCN, chem) was performed using CCNc-derived κ values for individual components. The results show that the κCCN can be well predicted using particle size-resolved chemical composition and the ZSR mixing rule.