A43E-01 INVITED
The Formation and Growth of Atmospheric Aerosols: Recent Results from MILAGRO
In March, 2006, a team of researchers from NCAR and the University of Minnesota carried out measurements pertinent to new particle formation and growth at the T1 ground-based site in Tecamac, Mexico, as part of the MILAGRO field campaign. Our goal was to understand the processes by which new particles form and grow in the polluted outflow of a megacity. Several important results have emerged from these measurements and the ensuing analyses. New particle formation and subsequent condensational growth occurred frequently outside of Mexico City, and were often the dominant processes affecting number concentrations in that area. Nucleation rates calculated from our measurements were found to have a squared dependence on sulfuric acid vapor concentrations, which is consistent with observations from diverse atmospheric environments including Boulder, Atlanta, Mauna Loa, and Idaho Hill (Rocky Mountains). Nucleation was dominated by neutral cluster formation, with ion-induced nucleation playing a relatively minor role. Finally, newly formed, sub-10 nm particles were composed of a complex mix of chemical compounds, with nitrate and organics appearing as the dominant species following strong new particle formation events while sulfate compounds remained relatively constant. This new information on nanoparticle composition is key to understanding why the growth rates of freshly nucleated particles are often significantly higher than can be explained by the condensation of sulfuric acid alone.
A43E-02 INVITED
The Role of Organics in Atmospheric New Particle Formation and Growth
Atmospheric aerosols impair visibility and human health, interfere with radiative transfer, and alter cloud formation. The major contributors include sulfate and organic aerosols from anthropogenic and biogenic activities, which are produced through a multitude of complex multiphase atmospheric processes by photochemical oxidation of emitted sulfur dioxide and volatile organic compounds (VOCs) into less volatile forms and gas-to-particle conversion. Aerosol nucleation events have been frequently observed under various tropospheric conditions and account for a major fraction of the total aerosol population, but the fundamental chemical processes responsible for aerosol nucleation and growth remain poorly understood. New particle formation occurs in two distinct stages, nucleation to form thermodynamically stable clusters and subsequent growth of the newly nucleated clusters by condensation or heterogeneous reactions to detectable particles that competes with capture and removal by coagulation with pre-existing particles. Sulfuric acid has been identified as a key species in new particle formation, but it is commonly recognized that binary nucleation of water and sulfuric acid is not efficient enough to explain measured nucleation events. Considerable uncertainty exists regarding the identity of other condensable species responsible for nucleation and growth of new particles. Currently available results of aerosol nucleation and growth from experimental, theoretical, and field studies are rather conflicting, hindering efforts to model formation and growth of secondary aerosols on the regional and global scales and to assess their atmospheric impacts. This talk will summarize recent progress in the understanding of the contributions of organics to atmospheric aerosol nucleation and growth. New experimental and theoretical results on this subject will be presented.
A43E-03 INVITED
Observations of Nighttime New Particle Formation in the Troposphere
We present atmospheric and laboratory observations which indicate efficient new particle formation during the nighttime in the troposphere under low condensation sinks, in contrast to the current prevailing assumption that aerosol nucleation takes places only during the daytime and typically from sulfuric acid. High concentrations of ultrafine particles with diameters from 4 to 9 nm (up to 1,000 per cc) were measured from the three days of nighttime observations in the upper troposphere during the NSF/NCAR GV Progressive Science Missions. Long- term ground-based observations of charged and neutral clusters and aerosols made in Tumbarumba, Australia, also showed surprisingly high frequency of nighttime new particle formation (30 - 64 %) with low condensation sinks. Laboratory studies also indicated that nucleation takes place at low concentrations of sulfuric acid without photochemical processes in the absence of UV and OH. These observations show that nighttime nucleation processes are more important than previously understood. Nighttime nucleation can be significant for global aerosol load and cloud condensation nuclei productions and thus need to be included in global climate models.
A43E-04
Global contribution of nucleation to aerosol number and cloud condensation nuclei
The contribution of new particle formation (nucleation) to the tropospheric burdens of aerosol number, or condensation nuclei (CN), and cloud condensation nuclei (CCN) is very uncertain. To evaluate these contributions, we perform global aerosol microphysical simulations to explore how uncertainties in nucleation mechanisms and nucleation rates affect the CN and CCN concentrations. The global model used is the GISS II- prime general circulation model with the size-resolved aerosol microphysics module, TOMAS (1-3). Various parameterizations of binary, ternary and ion-induced nucleation theories are tested in the model to determine how they affect the spatial and temporal distribution of CN and CCN (4-6). For example, a robust feature of global aerosol microphysics models is that binary nucleation parameterizations predict high nucleation rates in the upper troposphere, but the impact of these particles on boundary layer CCN concentrations requires investigation. A novel pseudo steady-state approximation is applied to gas phase sulfuric acid allowing for longer time steps during the calculation of nucleation and condensation and a significant decrease the computation time. (1) Hansen, J. et al., Mon. Weather Rev., 111, (1983). (2) Adams, P. J., and J. H. Seinfeld, J. Geophys. Res., 107, (2002). (3) Pierce, J. P., and Adams, P. J., J. Geophys. Res., 110, (2005) (4) Vehkamaki, H., et al., J. Geophys. Res., 107, (2002). (5) Napari, I., et al., J. Geophys. Res., 107, (2002). (6) Modgil, M. S., et al., J. Geophys. Res., 110, (2005).
A43E-05
Estimating the Importance of Boundary Layer Nucleation on CCN With a Global Aerosol Model
Boundary layer nucleation events have been observed frequently at many locations around the world. Recent observations suggest that, at least locally, the nucleation events can significantly contribute to the number of Cloud Condensation Nuclei (CCN). However, the global importance of nucleation events to CCN is still largely unquantified. While the mechanisms governing atmospheric nucleation are still poorly understood, new observation-based parameterizations for atmospheric nucleation enable us to use a global aerosol microphysics model (GLOMAP) to make a first estimate of the total contribution of nucleation events to CCN. We find that nucleation can increase surface-level CCN concentrations over large continental regions by as much as 50%. A factor 1000 uncertainty in the nucleation rate derived from observations translates into a factor 3 uncertainty in CCN change. Our results show that boundary layer nucleation is an important global source of CCN regardless of the uncertainty in the calculated nucleation rates.
A43E-06
Multivariate analysis of homogeneous nucleation measurements: Application to the p-toluic acid/sulfuric acid/water system
Recent kinetic extensions of the nucleation theorem (KNT) suggest that the logarithm of the steady-state nucleation rate has strong multi-linear dependence on the log concentrations of condensable species present in the vapor phase. A further remarkable result is that the coefficients of this linear dependency provide a direct determination of the molecular content of the critical nucleus itself. Building on these results, the powerful utility of multivariate statistical methods is demonstrated here for physically-based parameterization and interpretation of nucleation rate measurements. The new approach is applied to recent measurements by Zhang and co- workers on the p-toluic acid/ sulfuric acid/water ternary vapor system. A parameterization for nucleation rate dependence on vapor composition, accurate over the range of the measurements, is obtained and estimates of critical nucleus molecular composition are presented. We show that a single molecule of p-toluic acid present in the critical nucleus is sufficient to trigger a ternary nucleation event. Efforts underway to apply the new methods to analysis of new particle formation in the atmosphere are described.
A43E-07
Aerosol Nucleation Frequency and Hygroscopicity at a Forested Site in the Southeastern United States
This paper examines how aerosol hygroscopicity and nucleation frequency, measured with a tandem differential mobility analyzer (TDMA) at a rural site in the southeastern United States, vary with air mass source regions and transport pathways. We also examine an observed enhancement in aerosol hygroscopicity during nucleation events. During October through December, 2006, a TDMA was used to measure submicron aerosol size distributions and size-resolved hygroscopicity at a forested site in the southeastern U.S. Measurements were conducted above a mixed deciduous forest at the Virginia Forest Research Facility, a newly operational permanent micrometeorological facility located approximately 20 km east of the foothills of the Blue Ridge Mountains. The site features a 40-m walkup scaffolding tower above a 20-m mixed deciduous forest with excellent fetch, and a climate controlled lab for housing instruments and data acquisition systems. Hygroscopic growth factor distributions were measured for eight dry diameters between 0.013 and 0.400 μm. Cluster analysis of back trajectories was applied to identify characteristic source regions and air mass pathways during the study period. Significant differences were found in the frequency of aerosol nucleation events and hygroscopicity amongst the five distinct pathways identified by the cluster analysis. Nucleation occurred most frequently when fast moving air masses originated from the northern central U.S. and lower central Canada. In these cases existing aerosol surface area was low, and the majority of trajectories passed directly over the Ohio River valley. Hygroscopicity generally increased with increasing dry diameter, except for the largest size bin, which had a slightly lower hygroscopicity than the next smallest size. Two clusters with short, stagnant trajectories contained aerosols with enhanced hygroscopicity at dry diameters less than 0.100 μm. Aerosol hygroscopicity was significantly enhanced, especially at small diameters, during new particle formation events.
A43E-08
On the contribution of H2SO4-H2O binary homogeneous and ion-mediated nucleation to new particle formation in the global troposphere
New particle formation frequently observed throughout the troposphere is an important source of atmospheric cloud condensation nuclei and is one of key processes that need to be accurately represented in future generations of climate models. The number of parameters controlling the nucleation rates and the dependence of nucleation rates on these parameters based on different nucleation mechanisms vary significantly. Therefore, it is important to identify the right nucleation mechanisms so that the aerosol radiative forcing can be confidently projected based on the future emission scenarios and climate change. We have generated nucleation rate look- up tables for both H2SO4-H2O binary homogeneous nucleation (BHN) and ion-mediated nucleation (IMN) based on the updated kinetic nucleation models which incorporate new thermodynamic data and physical algorithm (Yu, Atmos. Chem. Phys., 6, 5193-5211, 2006; Yu, J. Chem. Phys., 127, 054301, 2007). The BHN and IMN look-up tables have been integrated into a global chemical transport model (GEOS-Chem) to study BHN and IMN contribution to new particle formation in the global troposphere. Based on the kinetic BHN model which is constrained by multiple independent laboratory measurements, BHN is negligible except in the tropical upper troposphere (above around 200 hPa) and a few hot spots in middle and lower troposphere. In contrast, we find that IMN, which appears to be supported by recent measurements of the overcharging of newly formed particles in the boundary layer, can lead to significant new particle formation not only in the upper troposphere but also in the lower troposphere. We have further compared the simulated annual mean results to a comprehensive set of data relevant to new particle formation around the globe. We show that predicted annual spatial patterns of IMN rates agree reasonably well with land-, ship-, and aircraft-based observations. The implications of our simulations and future research needs will be discussed. http://www.albany.edy/~yfq