A44B-01 16:00h
Regional scale modeling of particle nucleation in the lower atmosphere over the Eastern United States
It is well recognized that atmospheric particles affect regional air quality, human health, and climate, and these effects depend strongly on particle number size distributions and compositions. The number size distributions of atmospheric aerosols are determined by a number of microphysical processes, of which the nucleation process is critical but least understood. To understand the key microphysics controlling the formation and fate of secondary particles and to represent these processes in the models are important to improve the predictive understanding of the environmental and climatic impacts of atmospheric aerosols. In this study, we investigate the new particle formation in the lower continental atmosphere over the Eastern United States, using a nucleation sub-model coupled with the Comprehensive Air Quality Model with extensions (CAMx). The evolution of freshly formed particles at selected locations is also simulated with a detailed aerosol microphysics model driven by the time-dependent variables predicted with the CAMx. Our nucleation sub-model includes three different nucleation mechanisms: binary homogeneous nucleation (BHN) of H$_{2}$SO$_{4}$-H$_{2}$O, ternary homogeneous nucleation (THN) of H$_{2}$SO$_{4}$-H$_{2}$O-NH$_{3}$, and ion-mediated nucleation (IMN) of H$_{2}$SO$_{4}$-H$_{2}$O. The CAMx is driven by the meteorological fields from the meteorological model and uses the gridded emission inventory prepared with Sparse Matrix Operator Kernel Emissions (SMOKE) Modeling System. We find that the temporal and spatial variations of nucleation rates predicted by different nucleation theories are quite different. BHN theory predicts negligible nucleation in the lower atmosphere over Eastern United States. Based on the IMN theory, the plumes from Ohio River Valley and some urban centers are significant source of secondary particles. In such plumes, though the pre-existing particle concentration is high, the high production rate of H$_{2}$SO$_{4}$ as a result of high SO$_{2}$ concentration leads to relative high H$_{2}$SO$_{4}$(g) concentration which promotes nucleation. The IMN rates are limited by ionization rates (up to 20 ions cm$^{-3}$s$^{-1}$ in continental boundary layer). The nucleation rates predicted with the THN theory depend strongly on ammonia concentration and THN rates frequently exceed 10$^{6}$ cm$^{-3}$s$^{-1}$ in the regions with high NH3 concentration. The frequency and nucleation rates predicted with the IMN theory are consistent with the results from the Pittsburgh Air Quality Study (from July 2001 to June 2002) which indicate that nucleation occurred on 50% of the study days and the average nucleation rates during the nucleation periods never exceeded ~10 cm$^{-3}$s$^{-1}$. The power plant plumes frequently across Pittsburgh area, and in sunny days the IMN theory predicts significant nucleation in such plumes. In our simulations of the evolution of particle number size distributions, the condensation of precursor gases (including sulfuric acid and condensable organic species from both anthropogenic and biogenic sources) and the coagulation scavenging are taken into account. Our simulations indicate that a significant fraction of the freshly formed particles can grow to the sizes in the range of 30-100 nm within 10 hours. These particles may contribute to the regional haze formation and may serve as cloud condensation nuclei.
A44B-02 16:15h
A Parameterization of Ion Induced Nucleation of Sulphuric Acid and Water
Aerosol is ubiquitous in Earth's atmosphere, and affects health, visibility, atmospheric chemistry, and climate. Particle nucleation that can not be explained with classical binary nucleation theory has been observed in the lower and middle troposphere. Ion induced nucleation (IIN), involving the negative ion H$_{2}$SO$_{4}$/H$_{2}$O mechanism, has been shown to be an effective source of new particles in atmosphere. This paper describes a 5 dimensional parameterization of IIN that covers the complete range of conditions relevant to the lower atmosphere, based on the kinetic aerosol model SAWNUC (Sulphuric Acid and Water Nucleation). SAWNUC is a kinetic model that is based on experimental thermodynamics of small ion clusters of H$_{2}$SO$_{4}$ and H$_{2}$O. A nested Do Loop version of SAWNUC was created to cover the complete range of input variables required for parameterization. Six output variables generated from SAWNUC were (1) particle nucleation rate (cm$^{-3}$ s$^{-1}$), (2) H$_{2}$SO$_{4}$ nucleation rate (cm$^{-3}$ s$^{-1}$), (3) number of H$_{2}$SO$_{4}$ in average nucleating cluster, (4) number of H$_{2}$O in average nucleating cluster, (5) radius (nm) of average nucleating cluster, and (6) first order loss of H$_{2}$SO$_{4}$ to particles (s$^{-1}$). The parameterized dataset consisted of 76800 points. The input parameters for parameterization are (1) Temperature-T (190-300 K), (2) Relative Humidity-RH (0.05-0.95), (3) number concentration of H$_{2}$SO$_{4}$ (10$^{5}$-10$^{8}$ cm$^{-3}$), (4) First order loss of H$_{2}$SO$_{4}$ to particles (0.00009-0.0245 s$^{-1}$), and (5) Ion Source rate (1-50 ion pairs cm$^{-3}$ s$^{-1}$). The parameterization generally reproduces the modeled nucleation rate to within a factor of two over the whole range of conditions, except when the nucleation rate is very low ($<$ 10$^{-6}$ cm$^{-3}$ s$^{-1}$), which corresponds to less than 0.1 particle day$^{-1}$ cm$^{-3}$. If the ratio of model value to the parameterized value falls between 0.5 and 2.0, it is treated as efficient output of parameterization. Efficiencies exceeding 85 percent are obtained. The standard deviation of the ratio of model value to the parameterization value is used as a measure of the accuracy of the parameterization. For the present IIN parameterization, the standard deviation falls between 0.2 and 0.5. This parameterization speeds up IIN calculations by a factor of $\sim$10$^{6}$, as compared to the original SAWNUC model.
http://home.iitk.ac.in/~snt/param
A44B-03 16:30h
Chemical Processing of an Urban Plume Crossing the Gulf of Maine
Aerosol and trace gas measurements were made aboard the NOAA RV Ronald H. Brown off the coast of New England during July/August 2004 as part of the New England Air Quality Study (NEAQS). Several urban plumes from Boston were tracked across the Gulf of Maine during the study to quantify the chemical processing of the gases and aerosols over time. On 1 August the ship tacked downwind to follow a power plant plume embedded within the urban Boston plume. During this 11 hour period, the ship crossed the plume 5 times over a downwind distance of 100 km. While not a Lagrangian experiment we were able to observe over time/distance 1) an "aging" of the hydrocarbon mixture, 2) a decrease in the NOx mixing ratio with a increase in the ozone, PAN, nitric acid and nitrate aerosol mixing ratios, 3) a decrease in the SO2 mixing ratio and the formation and growth of new particles, and 4) an increase in the organic aerosol concentration. These data will be used to quantify the chemical processing of the urban plume over the day.
A44B-04 16:45h
Physical and chemical processing of individual particles acquired during the New England Air Quality Study
The Particle Analysis by Laser Mass Spectrometry (PALMS) instrument acquired the mass spectra of over 600,000 individual tropospheric particles while participating in the New England Air Quality Study (NEAQS - summer 2004) on the WP-3D aircraft. The counterflow virtual impactor (CVI) was flown in front of the PALMS instrument allowing for sampling of cloud particles. A majority of the study focused on urban outflow from the Boston and New York areas and power plant plumes in the Ohio River Valley. Additionally, the experiment included several nighttime flights encompassing both sunset and sunrise. These measurements are the first individual particle spectra acquired from an aircraft platform during the night. The flight plans were arranged such that both initial and downwind sampling of urban areas and power plant plumes was achieved. We will examine the compositional changes of the mass spectra to assess the chemical processing with time from a selection of the environments sampled during this field experiment. Cloud and nighttime processing of the particles will also be studied.
A44B-05 17:00h
Preliminary Results of Aerosol Chemical Composition Measurements Above the Northeastern U. S. with an Aerodyne Aerosol Mass Spectrometer
The Aerodyne Aerosol Mass Spectrometer (AMS), which measures non-refractory components of aerosol particles with aerodynamic diameters between roughly 40 nm and 1.5 microns, provides mass spectra as well as organic, sulfate, ammonium, and nitrate mass distributions. Preliminary results will be shown from the Intercontinental Transport and Chemical Transformation - New England Air Quality Study (ITCT-NEAQS 2004) during July-August 2004, where an AMS was deployed aboard the NOAA WP-3D aircraft above the northeastern United States and 30-second averaged samples were collected. For the first time, a pressure controlled inlet was used to maintain a constant mass flow rate into the AMS for airborne sampling. By comparing the AMS total mass loadings to aerosol volume from optical measurements, the AMS aerodynamic focusing lens appeared to have significantly lower particle transmission efficiencies for particles with vacuum aerodynamic diameters larger than 700 nm (or around 500 nm optical diameter). A wide variety of air masses were sampled during the intensive period, including clean continental, fresh and aged urban, biomass burning, and power plant plumes. In general, the total AMS mass loadings were less than 10 micrograms per cubic meter and the composition was mostly organic and sulfate with lesser amounts of ammonium and nitrate. Furthermore, most of the organic material was oxidized, which is consistent with the organic material being aged and secondary in nature. In some power plant plumes, fresh particle formation was observed in both the sulfate mass and particle volume distributions. Variations in the relative amounts of sulfate and organic material were also measured in many sulfate plumes and the possibility of acid-catalyzed organic reactions of gas phase organic compounds with acidic sulfate particles will be explored.
A44B-06 17:15h
The Microphysical Coupling of Natural and Anthropogenic Aerosol Cycles
Aerosols are thought to play an important role in the global climate system. However, their effects on the radiation budget and even their global distribution and composition are not understood satisfactorily. A major uncertainty is the anthropogenic contribution of the global aerosol distribution and eventually the anthropogenic impact on aerosol radiative effects. Up to now, most multi-component aerosol modules in global circulation models approach this issue with a bulk approach predicting the mass of the aerosol components independently as external mixture. However, observations show that the mixing state of the global aerosol system is highly variable with a large internally mixed contribution. These results underscore that the aerosol components cannot be simulated independently and that aerosol aging processes need to be taken into account. The new aerosol-climate modelling system ECHAM5-HAM predicts size-distribution, composition, and mixing state of an ensemble of partly internally mixed modes for the components sulfate, black carbon, organic carbon, sea salt and dust. The microphysical approach includes the aging of aerosols via condensation, coagulation, and cloud processing. Results from a series of nudged simulations will be presented, in which we investigate the microphysical coupling of natural and anthropogenic aerosol cycles by the inclusion / omission of emissions of specific aerosol sources. Unlike the almost linear relationship of emissions and aerosol load in the bulk modelling approach, our results show that the microphysical coupling induces non-linearity in the system. In addition, the size-distribution and therefore the aerosol number concentrations are coupled non-linearly to the emissions with consequences for the direct and indirect aerosol radiative effects.
A44B-07 17:30h
The Feldberg Aerosol Characterisation Experiment 2004
First results from the Feldberg Aerosol Characterisation Experiment (FACE) 2004 campaign will be presented. The objectives of the experiment were: 1) Physical and chemical characterisation of the aerosol in central Europe by state-of-the-art aerosol instrumentation; 2) Test and comparison of new and established instrumentation; and 3) Investigation of links between aerosol properties in closure and model studies. The measurements were made at the Taunus Observatory (Kleiner Feldberg, 825 m asl) in central Germany, July - August 2004. The experiment included size-resolved real time chemical characterisation of volatile and semi-volatile molecular species with a standard (Quadrupole) Aerosol Mass Spectrometer (Q-AMS) and a Time-of-Flight Aerosol Mass Spectrometer (ToF-AMS), as well as impactor and filter sampling for analysis of size resolved chemical composition (inorganic ions and organic components), and aerosol light absorption. Furthermore we performed measurements of aerosol number concentrations using several Condensation Nuclei (CN) counters, and aerosol size distribution measurements at dry and ambient conditions using Differential Mobility Analysers (SMPS systems), Optical Particle Counters (OPC) and an Electrical Low Pressure Impactor (ELPI). In addition, size resolved measurements of Cloud Condensation Nuclei (CCN) concentrations and efficiencies as a function of supersaturation were conducted. Optical properties were investigated by measuring scattering at dry, humidified and ambient conditions using Nephelometers, and aerosol absorption using a Photo Acoustic Instrument. Total aerosol mass (PM 1) was measured using TEOM instruments. Interpretation of the aerosol measurements is supported by CO and CO$_{2}$ concentration measurements, continuous meteorological observations of the German Weather Service (DWD), as well as back trajectory calculations. The modelling tools include CCN closure studies, using size-resolved aerosol chemical and cloud microphysical information, including aerosol composition, water uptake, CCN activation, as well as optical and cloud nucleating properties. The aerosol modelling employs a size-resolving aerosol dynamical model that is coupled to a novel thermodynamic aerosol equilibrium model, which consistently account, besides the widely modelled inorganic aerosol compounds, for mineral and organic aerosol species. Preliminary results show that the aerosol was dominated by organic species. The aerosol number concentration varied between 1000 and 40000 cm$^{-3}$, with an average around 4500 cm$^{-3}$.
A44B-08 17:45h
Combined use of an Aerosol Radiation, Microphysics, and Transport Model with Satellite, Airborne, and Ground Based Observations to Infer the Aging of Southern African Biomass Burning Aerosols
We use an offline aerosol microphysics and transport model driven by assimilated meteorology to study the evolution of biomass burning aerosols during the Southern African Regional Science Initiative experiment (SAFARI 2000). We focus here on the extensive dataset of aerosol radiation measurements made during SAFARI 2000, which includes spaceborne observations from MODIS and TOMS, groundbased columnar measurements of spectral aerosol optical thickness from AERONET sun/sky photometers, and vertical profiles of aerosol extinction from groundbased MPLNET lidar, the airborne Cloud Physics Lidar, and the AATS-14 airborne sun photometer. Spatially explicit biomass burning sources for southern Africa derived from the MODIS satellite-based fire detection and burned area products are used. We summarize the model validation near source regions, presented in more detail elsewhere by Matichuk et al. We address whether the aging of the smoke aerosols during long-range transport can be detected from the MODIS and TOMS satellite radiance measurements and subsequent retrievals. We calculate the expected satellite radiance measurements from the modeled aerosol fields using a multiple-scattering radiative transfer model. Our goal is to determine an appropriate set of aerosol microphysical parameters and processes to include in the model in order to have a description of the aerosol lifecycle consistent with the near-source (sun photometer and lidar) and long-range (satellite) observations.