Anthropogenic emissions and land use changes have modified atmospheric aerosol concentrations and size distributions over time. Understanding preindustrial conditions and changes in organic aerosol due to anthropogenic activities is important because these features (1) influence estimates of aerosol radiative forcing and (2) can confound estimates of the historical response of climate to increases in greenhouse gases. Secondary organic aerosol (SOA), formed in the atmosphere by oxidation of organic gases, represents a major fraction of global submicron‐sized atmospheric organic aerosol. Over the past decade, significant advances in understanding SOA properties and formation mechanisms have occurred through measurements, yet current climate models typically do not comprehensively include all important processes. This review summarizes some of the important developments during the past decade in understanding SOA formation. We highlight the importance of some processes that influence the growth of SOA particles to sizes relevant for clouds and radiative forcing, including formation of extremely low volatility organics in the gas phase, acid‐catalyzed multiphase chemistry of isoprene epoxydiols, particle‐phase oligomerization, and physical properties such as volatility and viscosity. Several SOA processes highlighted in this review are complex and interdependent and have nonlinear effects on the properties, formation, and evolution of SOA. Current global models neglect this complexity and nonlinearity and thus are less likely to accurately predict the climate forcing of SOA and project future climate sensitivity to greenhouse gases. Efforts are also needed to rank the most influential processes and nonlinear process‐related interactions, so that these processes can be accurately represented in atmospheric chemistry‐climate models.

[1] Correlations between concentrations of newly formed particles and sulfuric acid vapor were analyzed for twenty one nucleation events measured in diverse continental and marine atmospheric environments. A simple power law model for formation rates of 1 nm particles, J 1 = K Á [H 2 SO 4 ] P , where P and K are least squares parameters, was tested for each environment. We found that, to within experimental uncertainty, P = 2. Constraining P to 2, the prefactor K kinetic ranges from 10 À14 to 10 À11 cm 3 s À1 . According to the nucleation theorem, an exponent value of 2 indicates that the critical cluster contains two sulfuric acid molecules. Existing nucleation rate expressions based on classical nucleation theory predict significantly larger values of P. The prefactor values vary with environment and are 1 to 4 orders of magnitude below the hard-sphere collision limit. These results provide a simple parameterization for atmospheric new particle formation that could be used in global climate models.

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.amines | atmospheric aerosol | climate forcing | nanoparticle | chamber study N ucleation of atmospheric trace gases occurs regularly throughout the continental boundary layer (1). Nucleated particles grow at typical rates of 1-10 nm/h, and can be a significant source of condensation nuclei (2) and cloud condensation nuclei (CCN) (3). The cloud albedo effect is a major source of uncertainty in estimates of climate radiative forcing (4). Because nucleation may affect CCN concentrations, there is a need for microphysical models that reliably predict atmospheric nucleation rates. Fig. 1 summarizes results for the dependence of boundary layer nucleation rates on the number concentration of sulfuric acid vapor, "[H 2 SO 4 ]," measured by the University of Minnesota-National Center for Atmospheric Research (NCAR) research team over the past two decades (5). Also included are data from the University of Helsinki group (6, 7). The considerable scatter in the measurements of the nucleation rate J at a given value of [H 2 SO 4 ] may be due to factors including dependencies on other nucleation precursor gases, temperature, and relative humidity (RH), as well as uncertainties introduced when J is deduced from measurements. Significantly, Fig. 1 shows that for all of these studies nucleation rates range from 1 × 10 −2 to 5 × 10 −6 times the sulfuric acid vapor collision rate, 0.5k 11 [H 2 SO 4 ] 2 , where k 11 is the hard-sphere collision rate constant for sulfuric acid vapor (8).The literature includes a lively debate about the relative importance of ion-induced and neutral nucleation (9, 10). Our work aims to explain nucleation rates observed in the polluted boundary layer atmospheres of Atlanta and Mexico City, where estimated nucleation rates (∼1-10 3 cm −3 ·s −1 ) were often much greater than typical ion production rates (∼2-30 cm −3 ·s −1 ). Therefore, although ion-induced nucleation could contribute to particle production in these locations it is not the dominant ...

[1] A semi-analytical expression has been developed that accurately models the population dynamics of an aerosol growing from the detection limit (3 nm) to a characteristic CCN size (100 nm), quantifying the contributions of size and time-dependent source and sink terms such as coagulation of smaller particles and scavenging by the pre-existing aerosol. These model inputs were calculated from measured aerosol size distributions and growth rates acquired during intensive measurement campaigns in Boulder, CO, Atlanta, GA, and Tecamac, Mexico. Twenty CCN formation events from these campaigns were used to test the validity of this model. Measured growth rates ranged from 3 -22 nm/h. The modeled and measured CCN production probabilities agreed well with each other, ranging from 1 -20%. The pre-existing CCN number concentration increased on average by a factor of 3.8 as a result of new particle formation. Citation: Kuang, C., P. H.McMurry, and A. V. McCormick (2009), Determination of cloud condensation nuclei production from measured new particle formation events, Geophys. Res. Lett., 36, L09822,

[1] 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 10 4 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.Citation: Zhao, J., F. L. Eisele, M. Titcombe, C. Kuang, and P. H. McMurry (2010), Chemical ionization mass spectrometric measurements of atmospheric neutral clusters using the cluster-CIMS,