A12C-01
Formation of atmospheric nanoparticles
While fresh aerosol particle formation has been observed to take place almost everywhere in the atmosphere, several gaps in our knowledge regarding this phenomenon still exist. These gaps range from the basic process-level understanding of atmospheric aerosol formation to its various impacts on atmospheric chemistry, climate, human health and environment. Until recent years nucleation pathways have been poorly understood even though several different mechanisms have been suggested. Main reason for that has been the instrumental inability to detect particles below 3 nm in diameter. Direct measurements of both the nucleation process itself and the initial growth of the clusters are crucial in order to resolve the detailed pathways of the particle formation. Only very recently observations of atmospheric neutral particles and clusters below 3 nm have shed light on the first steps of particle formation. Those observations were made using newly developed instruments designed for maximal detection efficiency of small clusters, like UF- 02proto swirling flow condensation particle counter pair and an Air Ion Spectrometer equipped with an aerosol charger i.e. Neutral Cluster and Air Ion Spectrometer (NAIS).
A12C-02
From Neutral Clusters to Nano-particles: Recent Progress in Laboratory and Field Measurements
Nano-particles produced by nucleation are believed to contribute significantly to the formation of aerosol particles and cloud condensation nuclei (CCN) and hence affect the cloud formation and the atmospheric radiation budget. Nano-particles have also been found to have more adverse human health effect than larger particles. Molecular clusters bridge the gap between molecules and macroscopic nano-particles in the atmosphere. Information on the composition and concentrations of molecular clusters will help to elucidate the nucleation process and reduce the uncertainty of the nucleation rates used in global climate models. We have recently developed a novel chemical ionization mass spectrometer, the cluster chemical ionization mass spectrometer (the Cluster-CIMS), which is capable of measuring the pre-nucleation clusters in the laboratory and in the atmosphere. Together with the recently-implemented particle instruments, we measured the size distribution of the nano particles (~ 1-3 nm) simultaneously with the cluster-CIMS. Here we present the results from recent laboratory studies and field measurements at two distinct sites (Boulder and Manitou Forest). We show an experimental approach to distinguish neutral clusters from the secondary ion clusters under the current configuration of the cluster-CIMS. For the laboratory measurements, we added additional in-situ generated sulfuric acid (~108-109 molecule cm-3) and ammonia (0.1-10ppb) to the ambient air to simulate the cluster formation during the winter time when the ambient sulfuric acid concentration is low and obtained the sulfuric acid/ammonia cluster (up to 7-mers) distribution. For ambient measurements in Boulder, where we occasionally sampled in plumes from power plants, we observed high correlations of sulfur dioxide, sulfuric acid and its dimer with 1.2-1.8 nm nano-particles. We also observed that nano-particle concentrations were well correlated with certain meteorological parameters (e.g. UV radiation, RH etc) during the nucleation events. We also show measurement results from Southern Rocky Mountains Summer 2008 Field Campaign (SRM 08) at the biogenic-dominant site (Minitou Forest) and compare them to those at anthropogenic-dominant site (Boulder). Implications from this study will be discussed.
A12C-03
Experimental Investigation of the Role of Ions in Aerosol Nucleation
The role of ions in producing aerosols in Earth's atmosphere is an area of very active research. Atmospheric (Clarke et al. 1998) and experimental (Berndt et al. 2005) observations have shown that the nucleation of aerosol particles can occur under conditions that cannot be explained by classical nucleation theory. Several ideas have been put forward to solve this nucleation problem, e.g. Ion-Induced Nucleation and Ternary Nucleation. Experimental investigations exploring the role of ions in particle production are scarce, and often at conditions far removed from those relevant for the lower part of the atmosphere (Bricard et al. 1968). Recent experimental work (Svensmark et al. 2007) demonstrated that ions, produced by cosmic rays in the atmosphere, are likely to play an important role in the production of new aerosol particles. The mechanism whereby energetic cosmic rays can promote the production of cloud condensation nuclei at low altitudes constitutes a link between cosmic rays and Earth's climate and there is thus a need to corroborate the results in a different experiment. The present results are obtained in the same laboratory, but using a new setup The experiments were conducted in a 50 L cylindrical reaction chamber made of electropolished stainless steel. Aerosols were grown using photochemically produced sulphuric acid and ionization levels were controlled with a Cs-137 gamma-source. An increase in nucleation was observed when the chamber was exposed to the radioactive source. The results were analyzed using a model based on the General Dynamic Equation and the analysis revealed that Ion Induced Nucleation is the most likely mechanism for the observed nucleation increases and thus confirm the previous results. Berndt, T, Böge, O., Stratmann, F., Heintzenberg, J. & Kulmala, M. (2005), Science, 307, 698--700 Bricard, J., Billard, F. & Madelaine, G. (1968), J. Geophys. Res. 73, 4487--4496 Clarke, A.D., Davis, D., Kapustin, V. N. Eisele, F. Chen, G. Paluch, I., Lenschow, D., Bandy, A.R., Thornton, D., Moore, K., Mauldin, L., Tanner, D., Litchy, M., Carroll, M.A. Collins, J. & Albercook, G. (1998), Science 282, 89--92 Svensmark, H, Pedersen, J.O.P, Marsh, N.D., Enghoff, M.B. & Uggerhøj, U.I. (2007), Proc. R. Soc. A 463, 385--396.
A12C-04
Contribution of Organic Vapors to the Growth of Secondary Aerosols
Processes governing the growth of atmospheric aerosols represent an important aspect of anthropogenic climate forcing but remain poorly understood. Condensation of organic vapors onto the pre-existing atmospheric aerosols, potentially followed by chemical reactions within the particles medium, is believed to be one of the major pathways that contribute to particle growth. Recent research has focused on the total mass increase on pre-existing seed particles, but the chemistry that determines the efficiency of organic uptake remains to be elucidated. In this study, attenuated total reflection- Fourier transform infrared spectroscopy (ATR-FT-IR) was used to study the formation of new chemical bonds in the sub-micron sulfuric acid droplets deposited on ATR crystal and subjected to exposure to organic vapors (2,4-hexadienal and glyoxal). The observation of new functional groups, together with the dependence of the absorption intensity on the relative humidity, indicates that the uptake of 2,4-hexadienal is through an aldol condensation reaction and uptake of glyoxal is an acid-catalyzed hydration followed by self-reaction of hydrated and carbonyl forms to form cyclic acetal structures. The evolvement of infrared absorption features also suggests that the uptake of both compounds is at least partly reversible.
A12C-05
Acid-catalyzed Reactions in Model Secondary Organic Aerosol (SOA): Insights using Desorption-electrospray Ionization (DESI) Tandem Mass Spectrometry
Atmospheric aerosols are presently little understood in terms of their sources, formation, and effect on climate forcing, despite their significant impacts on climate change and respiratory health. Secondary organic aerosols (SOA), which were thought to arise entirely from simple gas-particle partitioning, have recently been found to contain oligomeric species which result from the condensed-phase reactions of volatile organic compounds (VOCs). The non-methane VOC with the greatest emission flux, isoprene, is known to produce aerosols through chemistry involving its oxidation products. We selected one of its major oxidation product, methacrolein, to assess its role in oligomeric SOA formation in response to the acidic conditions found in cloud water. Since it has been found that acidified aerosol produces oligomeric species with greater molecular weight and yield, acid-catalyzed oligomerization is likely a significant process in the formation of SOA. Aqueous solutions of methacrolein were acidified with sulfuric acid, and studied using linear ion trap mass spectrometry (LIT-MS) with a home-built desorption-electrospray ionization (DESI) source. An extremely heterogeneous mixture of products was produced in this system, resulting from hydrolysis, acid- catalyzed oxidation, reduction, and organosulfate formation. Evidence for disproportionation and heterocycle formation are proposed as reaction mechanisms hitherto unrecognized in the production of SOA. The proposed structure and formation mechanism for several species, based upon their MS/MS spectra, will also be presented.
A12C-06
Particle Nucleation and Growth During the NIFTy Experiment
The measurements presented herein are being collected in order to assess the frequency and characteristics of nucleation events and high ultra-fine particle concentrations in this rural (but regionally polluted) setting, including the chemical composition of the ultra-fine particles, the principal mechanisms of nucleation, limitations on nucleation and growth and the ultimate fate of the resulting ultra-fine particles. We present a year of data from continuous measurements of particle size distributions (6 to 400 nm) at three- levels above and within a deciduous forest in southern Indiana (in the Ohio River Valley). These data are collected using two SMPS and one FMPS, and are supplemented with data concerning the spatial extent and the composition of ultra-fine particles using samples collected during May 2008 as part of the Nucleation In ForesTs (NIFTY) campaign. The long-term measurements indicate evidence of class A nucleation events on approximately 1 day in 5, and lesser magnitude or less well defined events on another 15% of days. These data indicate nucleation is most frequently observed in spring, but occurs in all seasons and subsequent growth is more rapid during leaf-on. During the NIFTY field experiment we enhanced the ongoing data collection at the forest site to include sulfuric acid, ammonia and VOC concentrations, and particle composition using two MOUDI-nano-MOUDI combinations. Preliminary analyses of these data indicate that the sub-32 nm diameter particles are almost completely ammonium-sulfate or ammonium-bisulfate and associated water but there is evidence that continued growth is aided by the addition of condensable organics. We further instrumented two additional sites – one directly south of the forest in a relatively rural setting, and one to the north in the city of Indianapolis. Results from these sites indicate strong evidence that the events are regional but the sites differ in terms of the magnitude of the events and the relative growth behavior.
A12C-07
What Controls Particle Formation in Boreal Forests?
Aerosol nucleation events observed worldwide may have significant climatic and health implications. Despite decades of intensive studies and impressive progress achieved in both theory and instrumentation within the last decade, a clear understanding of the mechanisms controlling the burst of new particles in the atmosphere remains to be achieved. Detailed analysis of nucleation events characterized in different part of world may shed light on key processes and parameters controlling the secondary particle formation in the atmosphere. Here, we report case studies of eight nucleation events observed during an intensive field campaign at a boreal forest site (Hyytiala, Finland) in spring 2005. The present analysis is based on comprehensive kinetic simulations using an up-to-date kinetically consistent ion-mediated nucleation model (IMN) incorporating recently available thermodynamic data and schemes (Yu, Atmos. Chem. Phys., 6, 5193, 2006). Out of the 22 days of the campaign on which nucleation events were observed, eight major events were selected for detailed analysis on the basis of indications that the observed air masses were relatively homogeneous (in term of well-defined and consistent diurnal variations in measured concentrations of sulfuric acid vapor and freshly nucleated particles between 3 and 6 nm). With the model constrained by observed time series of sulfuric acid vapor concentration, condensation sink, and meteorological conditions, we show that, in most of these cases, reasonable agreement is found between IMN predictions and field data for a range of variables, including critical nucleation sizes, size- dependent overcharging ratios, and the concentrations of 1.8-3 nm stable clusters and 3-6 nm particles, and their diurnal variations. The possible reasons leading to substantial differences between observation and theory in some cases are also explored. Statistically, roughly 80% of the nucleation events recorded during the Hyytiala campaign exhibited mean size-dependent particle overcharging ratios within the range of, or exceeding, those predicted by the IMN model, suggesting that ion nucleation processes were significant during these events. Sulfuric acid vapor appears to be able to account for the growth of sub-3 nm particles while condensable organic species dominate the growth of particles larger than around 3 nm in boreal forests. Sulfuric acid vapor concentration and temperature are two key parameters controlling nucleation rate variations in the atmosphere, although ionization rate, relative humidity, and condensation sink also affect nucleation rates. The nucleation rates calculated using other existing nucleation theories/models are also contrasted with measurements. We show that, for the well documented conditions of the Hyytiala project, the binary H2SO4-H2O and ternary H2SO4-H2O-NH3 homogeneous nucleation rates calculated using the most current theories would fall well below ~10-7 cm-3s- 1, and thus would be negligible. A new online interactive tool, which can be used by anyone to quickly calculate binary, ternary, and ion-mediated nucleation rates under the conditions specified by users, will be introduced.
A12C-08
Evaluating a sesquiterpene-ozone nucleation mechanism in a global model
Nucleation events have been observed widely around the world and contribute significantly to global particle number concentrations. However, the mechanisms responsible for these events are not fully understood, which limits our ability to quantify their relevance for climate. Particle formation rates generally correlate with gas-phase sulfuric acid concentrations to the power of one or two. However, the wide range of nucleation rates that have been observed suggest that other factors or chemical species are also important. A nucleation mechanism involving sulfuric acid and the reaction products of sesquiterpenes and ozone has recently been suggested. We have included this mechanism in the GLOMAP global chemical transport model which can calculate size-resolved aerosol distributions. We evaluate the mechanism using observations of total particle number from long-term surface sites around the world (from CREATE and WDCA) and vertical profiles of aerosol number over North America and the Pacific and Atlantic Oceans (from the NASA INTEX aircraft missions). This mechanism can explain the observed seasonal cycle of nucleation events, vertical profiles of aerosol number and the lack of formation events over oceanic areas. These characteristics are difficult to explain using a mechanism based solely on gas-phase sulfuric acid.