Climate models show that particles formed by nucleation can affect cloud cover and, therefore, the earth's radiation budget. Measurements worldwide show that nucleation rates in the atmospheric boundary layer are positively correlated with concentrations of sulfuric acid vapor. However, current nucleation theories do not correctly predict either the observed nucleation rates or their functional dependence on sulfuric acid concentrations. This paper develops an alternative approach for modeling nucleation rates, based on a sequence of acid–base reactions. The model uses empirical estimates of sulfuric acid evaporation rates obtained from new measurements of neutral molecular clusters. The model predicts that nucleation rates equal the sulfuric acid vapor collision rate times a prefactor that is less than unity and that depends on the concentrations of basic gaseous compounds and preexisting particles. Predicted nucleation rates and their dependence on sulfuric acid vapor concentrations are in reasonable agreement with measurements from Mexico City and Atlanta.

Recent ab initio calculations showed that amines can enhance atmospheric sulfuric acid-water nucleation more effectively than ammonia, and this prediction has been substantiated in laboratory measurements. Laboratory studies have also shown that amines can effectively displace ammonia in several types of ammonium clusters. However, the roles of amines in cluster formation and growth at a microscopic molecular scale (from molecular sizes up to 2 nm) have not yet been well understood. Processes that must be understood include the incorporation of amines into sulfuric acid clusters and the formation of organic salts in freshly nucleated particles, which contributes significantly to particle growth rates. We report the first laboratory and ambient measurements of neutral sulfuric acid-amine clusters using the Cluster CIMS, a recently-developed mass spectrometer designed for measuring neutral clusters formed in the atmosphere during nucleation. An experimental technique, which we refer to as Semi-Ambient Signal Amplification (SASA), was employed. Sulfuric acid was added to ambient air, and the concentrations and composition of clusters in this mixture were analyzed by the Cluster CIMS. This experimental approach led to significantly higher cluster concentrations than are normally found in ambient air, thereby increasing signal-to-noise levels and allowing us to study reactions between gas phase species in ambient air and sulfuric acid containing clusters. Mass peaks corresponding to clusters containing four H2SO4 molecules and one amine molecule were clearly observed, with the most abundant sulfuric acid-amine clusters being those containing a C2- or C4-amine (i.e. amines with masses of 45 and 73 amu). Evidence for C3- and C5-amines (i.e. amines with masses of 59 and 87 amu) was also found, but their correlation with sulfuric acid tetramer was not as strong as was observed for the C2- and C4-amines. The formation mechanisms for those sulfuric acid-amine clusters were investigated by varying the residence time in the inlet. It was concluded that the amines react directly with neutral clusters and that ion-induced clustering of sulfuric acid cluster ions with amines was not a dominant process. Results from ambient measurements using the Cluster CIMS without addition of sulfuric acid have shown that the sulfuric acid-amine clusters were reasonably well correlated with sulfuric acid tetramer and consistent with the SASA experiments at the same Boulder sampling site. Also, clusters that contain C2- or C4-amines were more abundant and better correlated with sulfuric acid tetramer than other types of amine containing clusters. However, ambient measurements of sulfuric acid-amine clusters remain difficult and highly uncertain because their concentrations are only slightly above background levels, even during nucleation events.

Recent observations throughout the atmosphere have shown that nucleation occurs frequently (Kulmala et al. 2004). Modeling studies and observations have shown that nucleated particles contribute si...

New particle formation (NPF) in the atmospheric is a two-step process: Nucleation leads to the birth of stable nuclei that subsequently grow to sizes that can be detected and affect the atmosphere’s radiative properties. Our group is studying both of these processes. Our nucleation research is largely supported by NSF and involves measurements of neutral molecular clusters formed by nucleation with a new custom-designed mass spectrometer (the Cluster-CIMS) and measurements of nanoparticle size distributions as small as 1 nm with a new aerosol spectrometer (the DEG SMPS). These measurements are providing new insights into aspects of cluster behavior that affect nucleation rates. The U.S. DOE supports our research on nanoparticle growth rates. This research couples physical and chemical measurements of aerosol properties and behavior. The TDCIMS, which enables real-time measurements of composition for freshly nucleated particles as small as 8 nm and was developed with support from DOE, is the most important tool in this work. Our most important discoveries about processes that affect growth rates are summarized in a recent PNAS article (doi:10.1073/pnas.0912127107). In short, this work has shown that alkylammonium-carboxylate salts, formed, for example, by reactions between amines and carboxylic acids, account for 20–50% of the mass of more » freshly nucleated particles in locations that include Atlanta, Mexico City, Boulder, and Hyytiala, while sulfates account for only about 10%. These newly discovered compounds help to explain the high growth rates of freshly nucleated particles that have been observed around the globe and help to explain why nucleation is an important atmospheric process, not just a scientific curiosity. Our poster will provide an overview of this work. « less

A recently developed chemical ionization mass spectrometer for detecting neutral molecular clusters in the atmosphere (the Cluster‐CIMS) is described. This instrument is unique in that it uses a highly sensitive atmospheric chemical ionization technique combined with two neutral cluster separation methods to measure the very low concentrations of clusters formed during nucleation events. This is apparently the first time that selected‐ion chemical ionization mass spectrometry has been used to identify nucleating clusters in the atmosphere. The Cluster‐CIMS was well calibrated by using an electrospray high‐resolution differential mobility analyzer technique, a novel approach to generating and classifying known ion clusters. Field measurements at a moderately polluted urban site and a relatively remote forested site show that the instrument is capable of detecting neutral sulfuric acid clusters (containing up to four sulfuric acid molecules) during relatively strong nucleation events, with a concentration on the order of 104 molecular clusters per cubic centimeter at both sites. These measurements also provide evidence that for a given sulfuric acid concentration, the forested site appears to be significantly more efficient at producing sulfate clusters than the urban site is. A comparison between the Cluster‐CIMS measurements and the size distribution measurements of nanoparticles demonstrates that the observed nucleation events at the two measurement sites are always associated with high concentrations of sulfuric acid and that, if the clusters are measurable, they are well correlated with the nanoparticles down to ∼2 nm; however, other nucleation events are either relatively small or may have occurred prior to reaching the measurement sites, and hence the concentrations of the sulfuric acid clusters are most likely under the detection limit of the Cluster‐CIMS. Limitations of the instrument and possible future directions for its development are discussed.

The sizing capability of an ultrafine particle condensation nucleus counter (which uses butanol as the condensing fluid) equipped with pulse height analysis was evaluated in terms of particle composition for sulfuric acid aerosol and sulfuric acid aerosol to which gas-phase ammonia had been added. The response of the counter depended on composition for a range of particle sizes when the water partial pressure was low. For water partial pressures <5 Torr and for particles >4 nm in diameter, the response (pulse heights) of the instrument to particles of a given size was substantially different for sulfuric acid particles and those that were neutralized with ammonia. For water partial pressures >5 Torr, however, neutralizing the particles with ammonia had little effect on pulse height distributions. For particles smaller than 4 nm diameter the pulse heights were insensitive to exposure to ammonia.

We report measurements of the atmospheric plume emitted by Erebus volcano, Antarctica, renowned for its persistent lava lake. The observations were made in December 2005 both at source, with an infrared spectrometer sited on the crater rim, and up to 56 km downwind, using a Twin Otter aircraft; with the two different measurement platforms, plume ages were sampled ranging from <1 min to as long as 9 h. Three species (CO, carbonyl sulfide (OCS), and SO2) were measured from both air and ground. While CO and OCS were conserved in the plume, consistent with their long atmospheric lifetimes, the downwind measurements indicate a SO2/CO ratio about 20% of that observed at the crater rim, suggesting rapid chemical conversion of SO2. The aircraft measurements also identify volcanogenic H2SO4, HNO3 and, recognized for the first time in a volcanic plume, HO2NO2. We did not find NOx in the downwind plume despite previous detection of NO2 above the crater. This suggests that near‐source NOx was quickly oxidized to HNO3 and HO2NO2, and probably NO32−(aq), possibly in tandem with the conversion of SO2 to sulfate. These fast processes may have been facilitated by “cloud processing” in the dense plume immediately downwind from the crater. A further striking observation was O3 depletion of up to ∼35% in parts of the downwind plume. This is likely to be due to the presence of reactive halogens (BrO and ClO) formed through heterogeneous processes in the young plume. Our analysis adds to the growing evidence for the tropospheric reactivity of volcanic plumes and shows that Erebus volcano has a significant impact on Antarctic atmospheric chemistry, at least locally in the Southern Ross Sea area.

Measurements of OH, H2SO4, and MSA at South Pole (SP) Antarctica were recorded as a part of the 2003 Antarctic Chemistry Investigation (ANTCI 2003). The time period 22 November, 2003-2 January, 2004 provided a unique opportunity to observe atmospheric chemistry at SP under both natural conditions as well as those uniquely defined by a solar eclipse event. Results under natural solar conditions generally confirmed those reported previously in the year 2000. In both years the major chemical driver leading to large scale fluctuations in OH was shifts in the concentration levels of NO. Like in 2000, however, the 2003 observational data were systematically lower than model predictions. This can be interpreted as indicating that the model mechanism is still missing a significant HOx sink reaction(s); or, alternatively, that the OH calibration source may have problems. Still a final possibility could involve the integrity of the OH sampling scheme which involved a fixed building site. As expected, during the peak in the solar eclipse both NO and OH showed large decreases in their respective concentrations. Interestingly, the observational OH profile could only be approximated by the model mechanism upon adding an additional HOx radical source in the form of snow emissions of CH2O and/or H2O2. This would lead one to think that either CH2O and/or H2O2 snow emissions represent a significant HOx radical source under summertime conditions at SP. Observations of H2SO4 and MSA revealed both species to be present at very low concentrations (e.g., 5 x 10(5) and 1 x 10(5) molec cm(-3), respectively), but similar to those reported in 2000. The first measurements of SO2 at SP demonstrated a close coupling with the oxidation product H2SO4. The observed low concentrations of MSA appear to be counter to the most recent thinking by glacio-chemists who have suggested that the plateau's lower atmosphere should have elevated levels of MSA. We speculate here that the absence of MSA may reflect efficient atmospheric removal mechanisms for this species involving either dynamical and/or chemical processes. (C) 2009 Elsevier Ltd. All rights reserved.

Vertical profile measurements of ozone, water vapor, and meteorological conditions, as well as surface and tower measurements of these parameters during the 2003 Antarctic Tropospheric Chemistry Investigation (ANTCI) yielded their vertical (between the surface and 500m) and temporal distribution in the boundary layer at South Pole (SP) during December 13–30, 2003. Ozone in the surface and lower planetary boundary layer above SP was frequently enhanced over lower free tropospheric levels. During stable atmospheric conditions (which typically existed during low wind and fair sky conditions) ozone accumulated in the surface layer to reach up to twice its background concentration. These conditions were correlated with air transport from the N–SE sector, when air flowed downslope from the Antarctic plateau towards the SP. These data provide further insight into the vigorous photochemistry and ozone production that result from the highly elevated levels of nitrogen oxides (NOx) in the Antarctic surface layer.

One of the major goals of the 2005 Antarctic Tropospheric Chemistry Investigation (ANTCI) was to bridge the information gap between current knowledge of South Pole (SP) chemistry and that of the plateau. The former has been extensively studied, but its geographical position on the edge of the plateau makes extrapolating these findings across the plateau problematic. The airborne observations reported here demonstrate that, as at SP, elevated levels of nitric oxide (NO) are a common summertime feature of the plateau. As in earlier studies, planetary boundary layer (PBL) variations were a contributing factor leading to NO fluctuations. Thus, extensive use was made of in situ measurements and models to characterize PBL depths along each flight path and over broader areas of the plateau. Consistent with earlier SP studies that revealed photolysis of nitrate in surface snow as the source of NO x , large vertical gradients in NO were observed over most plateau areas sampled. Similar gradients were also found for the nitrogen species HNO 3 and HO 2 NO 2 and for O 3 . Thus, a common meteorological‐chemical feature found was shallow PBLs associated with nitrogen species concentrations that exceeded free tropospheric levels. Collectively, these new results greatly extend the geographical sampling footprint defined by earlier SP studies. In particular, they suggest that previous assessments of the plateau as simply a chemical depository need updating. Although the evidence supporting this position comes in many forms, the fact that net photochemical production of ozone occurs during summer months over extensive areas of the plateau is pivotal.

The vertical distribution of nitric oxide (NO) was investigated by profiling from a tethered balloon platform during the 2003 Antarctic Tropospheric Chemistry Investigation (ANTCI) at South Pole (SP), Antarctica. The lower atmosphere was probed between the surface and 120m height by pulling air from an inlet attached to the balloon through a thin-wall, 135m-long Teflon sampling line and by analyzing NO in this airflow with a ground-borne monitor. Losses and conversion of NO during the 2–4-min residence time in the sampling line were on average on the order of 6–16%, providing a feasible approach for the measurement of vertical NO profiles under SP conditions. NO was found to be highly variable within the lowest 100m of the atmosphere. Greatly enhanced NO mixing ratios were constrained to a shallow (20–50m height) air layer nearest to the surface, above which NO rapidly dropped to its mixed boundary layer background levels. Concurrent measurements of ozone and meteorological conditions provide insight into linkages between the ongoing snowpack and boundary layer nitrogen oxides (NOx=NO+NO2) and ozone chemistry. Since [OH] and [HO2] are non-linearly coupled to absolute levels of NOx, their concentrations and the rate of ozone production are expected to similarly show appreciable changes on small vertical scales during conditions with enhanced [NOx].

The Antarctic Tropospheric Chemistry Investigation (ANTCI) was carried out from late November to December 2003 with both extended ground-based and tethered balloon studies at Amundsen Scott Station, South Pole. ANTCI 2003 was the first of two Antarctic field studies with the primary goal of further exploring the active photochemistry of the South Pole region that was first identified in the previous Investigation of Sulfur Chemistry in the Antarctic Troposphere (ISCAT) program. Since ISCAT was fully ground-based, ANTCI 2003 goals included expanding chemical studies both vertically upward to investigate mixing and horizontally to better understand large-scale plateau NOx production and transport. Thus, in addition to ground-based experiments at South Pole, Twin Otter aircraft sampling took place out to hundreds of kilometers in several directions from the South Pole. These were designed to specifically address the issue of how representative past South Pole chemical measurements are of the surrounding high plateau region. The Twin Otter was also used to make transects along the coast both north and south of McMurdo Station. The present paper summarizes the overall setting and results of this investigation and highlights the many new findings that were obtained.