A13A-0872
Observation of ultrafine particle events in urban Gwangju, Korea
Three types of ultrafine particle events (10-100 nm traffic event, 50-100 nm residential heating event, and 10-30 nm photochemical event) in the Gwangju (Korea) ambient atmosphere were reported. We occasionally observed particle growth after formation of particles larger than 10 nm in the 10-30 nm photochemical event with growth rates of 2.2 nm/hr - 4.7 nm/hr. We showed distinct seasonal and diurnal patterns of ultrafine particle concentration. In winter, the N (50-80 nm), probably due to traffics and residential heating, led to the increased ultrafine particles, while in summer N (10-30 nm) formed by photochemical activity led to the enhanced concentration of ultrafine particles. We found that ultrafine particle concentrations observed in the morning (6:00- 11:00) and evening (17:00-23:00) was correlated with NOx. Although we observed enhanced SO2 concentration in the photochemical event, in general there was not a clear association between N (3-100 nm) and SO2. TEM and EDS analyses for mobility size-selected particles during ultrafine particle events showed that sulfate mixtures with metals and carbonaceous particles were included in the photochemical event, and non-spherical (agglomerated) carbonaceous particles were often observed in the traffic event.
A13A-0873
A computationally efficient aerosol nucleation/condensation method: Pseudo-steady state gas phase sulfuric acid
Aerosol nucleation and condensation are two processes that compete for gas phase sulfuric acid when it is formed in the atmosphere. Without approximations, accurate numerical integration of aerosol microphysics requires time steps on the order of seconds or less when nucleation is occurring, significantly shorter than the time steps required by other processes governing aerosol microphysics. This computational burden makes the explicit numerical simulation burdensome in 3-dimensional atmospheric models. We have developed an efficient method for simulating nucleation/condensation by assuming that gas phase sulfuric acid is at a pseudo- steady state (PSS) concentration determined by chemical generation and its loss by nucleation and condensation. The time step for nucleation/condensation is then governed by 1) the change in the condensation sink, 2) the change in the generation rate of sulfuric acid, 3) the coagulation timescale, or 4) a master time step in the model. The PSS assumption fails only when the time for sulfuric acid to reach its PSS state concentration is not significantly shorter than the four timescales above. This may occur when the following conditions are met 1) the condensation sink is less than 10-3-10-4 s-1, 2) nucleation is not occurring, and 3) the gas phase production rate is changing. These conditions are not frequently met for a long period of time in the atmosphere. The PSS assumption allows for time step increases of two to three orders of magnitude during nucleation events in typical atmospheric conditions and has been shown to reduce the nucleation/condensation computational time by a factor of 10-100.
A13A-0874
A Study of Small Ions and Aerosol Nucleation in the Troposphere
Ions represent only a small fraction of the number of molecules in Earth's atmosphere and are most abundant in the upper troposphere and lower stratosphere. However recent model studies and observations of atmospheric aerosols have suggested that ions may play an important role in aerosol nucleation even in the lower troposphere due to enhanced growth and stability of ion clusters resulting from electrostatic interactions. The contribution of ions to aerosol nucleation in the whole atmosphere is not easy to assess because of the complex processes and calculations required. We use a 3D global chemical transport model to calculate ion densities in the atmosphere from the ion balance equation for small ion pairs, which are positive/negative molecular ion clusters. Our model includes modified packages for calculating the production of ions from galactic cosmic rays (GCRs) in troposphere and lower stratosphere and from the radioactive decay of radon-222 (222Rn) emitted from the surface. Our model also includes new packages for computing the loss of ions by ion-ion recombination and aerosol-ion attachment processes. In the model simulations, ion densities are very significant in the upper troposphere, where most of the production and loss occurs from GCRs and ion-ion recombination, respectively. The ions in the lower troposphere are produced by a combination of GCRs and 222Rn and are mostly balanced by loss through aerosol-ion attachment. In spite of the large aerosol-ion attachment sink, substantial amounts of ions are still present in the lower troposphere. We will assess the contribution of small ions to new aerosols formed in the atmosphere by comparing our model simulations to observations.
A13A-0875
Laboratory-Measured Sulfuric Acid and Water Homogeneous Nucleation Rates from the SO2 + OH Reaction
Sulfuric acid and water binary homogeneous nucleation is the most important atmospheric nucleation system, but the nucleation mechanisms are poorly understood. Here, we report laboratory-measured sulfuric acid and water binary homogeneous nucleation rates at the atmospheric pressure, 288 K and 10 - 55 % relative humidity. The SO2 + OH reaction was used to produce sulfuric acid vapor. Residual sulfuric acid concentrations measured with a chemical ionization mass spectrometer (CIMS) ranged from 3e6 to 2e9 per cc, but the wall loss factors of sulfuric acid were substantial, ranging from 10 to 1,000 depending on nucleation time. Therefore, actual sulfuric acid concentrations in the nucleation region were one or three orders of magnitude higher with wall loss factors taken into account. The measured aerosol nucleation rates were proportional to sulfuric acid concentration with the second to tenth powers and proportional to RH with the eleventh to fifteenth powers. These power dependences became weaker at higher RH or higher sulfuric acid concentrations, indicating that the sizes of critical clusters are smaller at higher supersaturation ratios of aerosol precursors. The measured aerosol sizes were larger at higher sulfuric acid concentrations and higher RH, consistent with the predictions of homogeneous nucleation theories. The measured aerosol sizes were proportional to log RH, showing a weak dependence of aerosol growth on RH. However, the RH effects were more pronounced at higher sulfuric acid concentrations, indicating that RH effects on aerosol growth are more important in the polluted atmosphere. Furthermore, sulfuric acid concentrations required to produce the unit nucleation rate were on the order of e9 to e10 per cc, inconsistent with Berndt et al. [Science, 2005; GRL, 2006] where only e7 per cc sulfuric acid concentrations are required to produce the unit nucleation rates, although both our and Berndt et al. studies used the SO2 + OH reaction to form sulfuric acid vapor. Because wall loss factor is a sensitive function of nucleation time, considering Berndt et al. used extraordinary long nucleation times (290 s), it is likely that their sulfuric acid concentrations were strongly underestimated. Different power dependences of nucleation rate on sulfuric acid concentration measured from atmospheric observations (linear) and from laboratory binary homogeneous nucleation studies [This study, Ball et al., JGR, 1999; Berndt et al., 2005, Science; Zhang et al., Science, 2004; Berndt et al., GRL, 2006] (often fifth to eighth powers) also suggest that ternary species (such as ammonia and organics) play important roles in aerosol nucleation in the atmosphere.
A13A-0876
When Does New Particle Formation Not Occur in the Upper Troposphere?
Recent aircraft studies showed that new particle formation is very active in the free troposphere and lower stratosphere. And, these observations lead to a new question: when does new particle formation not occur? Here, we provide case studies to show how convection and surface area affect new particle formation in the upper troposphere, using the measured aerosol size distributions during the NSF/NCAR GV Progressive Science Missions in December 2005. There were ten research flights, including three days of nighttime experiments, at latitudes from 18 °N to 52 °N and altitudes up to 14 km. About 78 % of the total samples showed the new particle formation feature with number concentrations of particles with the diameter from 4 to 9 nm, 670 ± 1270 per cc, and the total particle number concentrations in the diameter range from 4 to 2000 nm, 920 ± 1470 per cc. Our case studies show that new particle formation was closely associated with convection and low surface areas of preexisting aerosol particles. On the other hand, for the cases where no new particle formation events were observed, air masses did not experience a vertical motion and there were also high surface area densities. These observations were consistent other observations during the Progressive Science Missions. Our results indicate that low temperatures, low surface areas, and convection together are responsible for frequent new particle formation events observed in this region. Latitude dependence of new particles also shows higher particle concentrations in the midlatitude tropopause region than in the subtropics, consistent with Hermann et al. [2003].
A13A-0877
Neutral and charged binary sulfate aerosol nucleation in the aerosol-climate modeling system ECHAM5-HAM
Aerosol particles play an important role in the Earth's atmosphere and in the climate system: Aerosols scatter and absorb solar radiation, facilitate heterogeneous and multiphase chemistry, and change cloud characteristics in many ways. Aerosol particles can be directly emitted from surface sources (primary aerosol) or form from the gas phase (secondary aerosol). Secondary aerosol formation can significantly increase concentrations of cloud condensation nuclei. Two important pathways of aerosol formation from the gas phase are neutral and charged binary nucleation of sulfuric acid and water. We have introduced laboratory data based representations of these pathways into the aerosol-climate modeling system ECHAM5-HAM, and investigate their relative importance and spatial distribution in the troposphere, and discuss ramifications for processes in the Earth's atmosphere.
A13A-0878
Does Ammonia Accelerate New Particle Formation in the Marine Sulfur Cycle?
Gaseous ammonia (NH3) enhances aerosol nucleation and is present in the remote marine atmosphere. These factors suggest that NH3 may accelerate new particle formation in the marine sulfur cycle that connects dimethyl sulfide (DMS) to cloud condensation nuclei. Besides aerosol nucleation, NH3 also couples to the marine sulfur cycle in other ways, some of which act to suppress new particle formation. Using an atmospheric transport model with coupled sulfur and ammonia chemistry, the role of NH3 in modulating new particle production in the marine sulfur cycle is tested. The following three effects are investigated: (1) enhancements to aerosol nucleation by NH3; (2) changes to cloudwater pH which reduce nucleation precursors; and (3) modifications to the sea air fluxes of DMS and NH3 by a marine bacteria ( Nitrosomonas oceanus).
A13A-0879
New particle formation and its relation to meteorological and chemical characteristics at Gosan and Anmyeon-do, Korea
The concentrations of trace gases, the submicron aerosol size distributions and the chemical composition of aerosols were measured at two coastal sites in Korea: Gosan in Jeju Island during March, as a part of the Atmospheric Brown Clouds-East Asian Regional Experiment 2005 (ABC-EAREX 2005) and Anmyeon-do, the Korea Global Atmospheric Watch Observatory (KGAWO), during October, 2005. New particle formation and growth events occurred around noon and lasted for 4 to 10 hours. For these events, relative humidity was less than 60% and the northwesterly wind was dominant. The air masses were transported from continent and over the Yellow Sea or East Sea. In general, daily mean of PM2.5 mass concentrations decreased. In addition, NOx concentrations remained low and OC/EC ratios were enhanced during the ABC-EAREX 2005. Detailed discussion will be presented in the meeting. http://atmos.korea.ac.kr
A13A-0880
Influence of SO2 on Nucleation, Growth Rates and Yield of Secondary Organic Aerosol (SOA)
A laboratory study was carried out to investigate the secondary organic aerosol formation of 1,3,5- trimethylbenzene (TMB) in the presence of SO2. Experiments were carried out using SO2 concentrations between 0.2 and 20 ppb for TMB concentrations of 150 to 1200 ppb, while keeping all the other parameters constant. Aerosol size distributions were measured using a twin SMPS system. The first is a nano SMPS composed of a short DMA column and a TSI UWCPC, while the second one consisted of a long DMA column and a TSI 3022CPC. This combination of instruments provided measurements of the size distributions over the size range from 4 to 700 nm diameter. VOC concentrations were measured using a Proton Transfer Reaction-Mass Spectrometer while organic acids and SO2 concentrations were measured using a wet effluent diffusion denuder/aerosol collector connected to IC-MS. The particle chemical composition was measured using an Aerosol Time Of Flight Mass Spectrometer (ATOFMS, TSI, USA). The empirical particle nucleation and growth rates were determined using only the aerosol size distribution as input, using the recently developed inverse modelling procedure PARGAN (Particle Growth and Nucleation). Growth rates were determined by regression analysis of the General Dynamic Equation. The empirical growth rates were then used to estimate the time of nucleation for particles in each size bin, defined as the time when their diameter surpassed 1 nm. Their number density at the time and size of nucleation were determined by integrating the particle losses that occurred in the time interval between nucleation and measurement. The nucleation rate was then given by the rate at which particles grow past the critical cluster size, assumed to be 1 nm. The observed nucleation and growth rates as well as the observed SOA yields will be discussed with regard to sulphuric acid and TMB concentrations.
A13A-0881
Experimental and Theoretical Investigation of Nucleation and Growth of Atmospheric Aerosols
Aerosol particles have profound impacts on human health, atmospheric radiation, and cloud microphysics and these impacts are strongly dependent on the particle sizes. Currently, formation and growth of atmospheric aerosol particles are not well understood. Our recent experimental study has shown that new particle formation in the H2SO4–H2O system is considerably enhanced in the presence of sub-ppb level of aromatic acids (e.g. benzoic acid, m-toluic acid, p-toluic acid), which represent the oxidation products of anthropogenic VOCs. In this presentation, we present experimental study of ternary nucleation of sulfuric acid, water and cis-pinonic acid to elucidate the role of biogenic organic acids in the new particle formation. In addition, we also discuss quantum chemical calculations, quantum theory of atoms in molecules (QTAIM) and molecular dynamic simulations to investigate the nature of the bonding interactions between the atmospheric nucleation precursors. Recent environmental chamber studies have suggested that acid-catalyzed particle-phase reactions of organic carbonyls lead to multifold increases in secondary organic aerosol (SOA) mass, contributing to the growth of the atmospheric aerosol particles. We present experimental measurements of heterogeneous uptake of several carbonyls on liquid H2SO4 in a wide range of acid concentrations and temperatures. The results indicate that the uptakes of small carbonyls and large carbonyls on liquid sulfuric acid correspond to different mechanisms, which may have important implication on the initial growth of newly nucleated nano-particles.
A13A-0882
Microphysical Modeling of New Particle Formation and Growth in Tropospheric Volcanic Plumes
Volcanic eruption is an important natural cause of regional and global climate changes. New aerosols formed inside volcanic plumes influence earth radiation budget by backscattering the solar radiation and serving as the cloud condensation nuclei (CCN). Previous volcanic aerosol studies focused more on aerosol dynamics and climate effects of stratospheric volcanic plumes with little attention paid to tropospheric ones. Recent global modeling studies of new particle formation indicate that volcano eruption can be an important source of new particles in the troposphere, at least in a regional scale. Due to the non-linear dependence of nucleation rates on precursor gas concentrations, it is important to understand the impact of sub-grid dilution and microphysics processes on the volcanic particle source strength. In this study, a size and composition resolved aerosol microphysics model is employed to study the formation and evolution of particles in volcanic plumes. We find that binary H2SO4-H2O homogeneous nucleation (BHN), while generally negligible in the ambient troposphere, can lead to significant new particle formation in tropospheric volcanic plumes. It should be noted that we use the recently improved kinetic BHN model (Yu, J. Chem. Phys., 127, 054301, 2007) which has been constrained by multiple laboratory measurements and has reduced uncertainties. Our simulations also indicate the newly formed particles can grow to CCN size within about one day because of relatively high precursor gas concentrations in the volcanic plume. The effects of various key parameters (volcanic injection altitude, initial SO2 concentration, dilution processes, ambient conditions, etc.) on number concentrations and size ranges of nucleated particles will be discussed as well.
A13A-0883
Ab Initio Investigation of the Structure, Stability and Atmospheric Distribution of Molecular Clusters Containing H2O, CO2 and N2O.
We present results from ab initio calculations for the structures, energetics and atmospheric abundances of neutral clusters containing water, carbon dioxide and nitrous oxide up to 45km altitude using the complete basis set CBS-Q and G3 multi-level procedures. Gas phase hydration energies, enthalpies and entropies for the stepwise attachment of water onto clusters according to X(H2O)n + H2O = X(H2O)n+1 (where X = H2O, CO2 and N2O) are reported for up to n=5. In particular, our results demonstrate that values for the incremental hydration enthalpies and entropies of all three gases H2O, CO2 and N2O asymptotically approach values characteristic of bulk liquid water (i.e. -44.0 kJ/mol for the enthalpy and -118.8 J/Kmol for the entropy of condensation) following attachment of around 3-4 water molecules. Interestingly, our calculated number densities for the water dimer at 292Kelvin are in excellent agreement with recent values obtained from IR measurements of atmospheric media (Pfeilsticker et al., 2003, Science). Our quantum chemical calculations indicate that water attachment onto H2O, CO2 and N2O is a thermodynamically favorable process, such that hydrated clusters would form a significant atmospheric repository of these species.