One of the greatest sources of uncertainty in simulations of climate and climate change is the influence of aerosols on the optical properties of clouds. The root of this influence is the droplet nucleation process, which involves the spontaneous growth of aerosol into cloud droplets at cloud edges, during the early stages of cloud formation, and in some cases within the interior of mature clouds. Numerical models of droplet nucleation represent much of the complexity of the process, but at a computational cost that limits their application to simulations of hours or days. Physically-based parameterizations of droplet nucleation are designed to quickly estimate the number nucleated as a function of the primary controlling parameters: the aerosol number size distribution, hygroscopicity and cooling rate. Here we compare and contrast the key assumptions used in developing each of the most popular parameterizations and compare their performances under a variety of conditions. We find that the more complex parameterizations perform well under a wider variety of nucleation conditions, but all parameterizations perform well under the most common conditions. We then discuss the various applications of the parameterizations to cloudresolving, regional and global models to study aerosol effects on clouds at a wide range of spatial and temporal scales. We compare estimates of anthropogenic aerosol indirect effects using two different parameterizations applied to the same global climate model, and find that the estimates of indirect effects differ by only 10%. We conclude with a summary of the outstanding challenges remaining for further development and application.

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Marine organic aerosol emissions have been implemented and evaluated within the National Center of Atmospheric Research (NCAR)'s Community Atmosphere Model (CAM5) with the Pacific Northwest National Laboratory's 7-mode Modal Aerosol Module (MAM-7). Emissions of marine primary organic aerosols (POA), phytoplankton-produced isoprene- and monoterpenes-derived secondary organic aerosols (SOA) and methane sulfonate (MS^&minus;) are shown to affect surface concentrations of organic aerosols in remote marine regions. Global emissions of submicron marine POA is estimated to be 7.9 and 9.4 Tg yr⁻¹, for the Gantt et al. (2011) and Vignati et al. (2010) emission parameterizations, respectively. Marine sources of SOA and particulate MS^&minus; (containing both sulfur and carbon atoms) contribute an additional 0.2 and 5.1 Tg yr⁻¹, respectively. Widespread areas over productive waters of the Northern Atlantic, Northern Pacific, and the Southern Ocean show marine-source submicron organic aerosol surface concentrations of 100 ng m⁻³, with values up to 400 ng m⁻³ over biologically productive areas. Comparison of long-term surface observations of water insoluble organic matter (WIOM) with POA concentrations from the two emission parameterizations shows that despite revealed discrepancies (often more than a factor of 2), both Gantt et al. (2011) and Vignati et al. (2010) formulations are able to capture the magnitude of marine organic aerosol concentrations, with the Gantt et al. (2011) parameterization attaining better seasonality. Model simulations show that the mixing state of the marine POA can impact the surface number concentration of cloud condensation nuclei (CCN). The largest increases (up to 20%) in CCN (at a supersaturation (<i>S</i>) of 0.2%) number concentration are obtained over biologically productive ocean waters when marine organic aerosol is assumed to be externally mixed with sea-salt. Assuming marine organics are internally-mixed with sea-salt provides diverse results with increases and decreases in the concentration of CCN over different parts of the ocean. The sign of the CCN change due to the addition of marine organics to sea-salt aerosol is determined by the relative significance of the increase in mean modal diameter due to addition of mass, and the decrease in particle hygroscopicity due to compositional changes in marine aerosol. Based on emerging evidence for increased CCN concentration over biologically active surface ocean areas/periods, our study suggests that treatment of sea spray in global climate models (GCMs) as an internal mixture of marine organic aerosols and sea-salt will likely lead to an underestimation in CCN number concentration

Lignocellulose-to-ethanol conversion is a promising technology to supplement corn-based ethanol production. However, the recalcitrant structure of lignocellulosic material is a major obstacle to the efficient conversion. To improve the enzymatic digestibility of switchgrass for the fermentable sugar production in hydrolysis, sodium hydroxide pretreatment of the biomass feedstock was investigated. At 121, 50, and 21 °C, raw switchgrass biomass at a solid/liquid ratio of 0.1 g/mL was pretreated, respectively, for 0.25−1, 1−48, and 1−96 h at different NaOH concentrations (0.5, 1.0, and 2.0%, w/v). Pretreatments were evaluated based on the yields of lignocellulose-derived sugars in the subsequent enzymatic hydrolysis. At the best pretreatment conditions (50 °C, 12 h, and 1.0% NaOH), the yield of total reducing sugars was 453.4 mg/g raw biomass, which was 3.78 times that of untreated biomass, and the glucan and xylan conversions reached 74.4 and 62.8%, respectively. Lignin reduction was closely related to the degree of pretreatment. The maximum lignin reductions were 85.8% at 121 °C, 77.8% at 50 °C, and 62.9% at 21 °C, all of which were obtained at the combinations of the longest residence times and the greatest NaOH concentration. Cellulase and cellobiase loadings of 15 FPU/g dry biomass and 20 CBU/g dry biomass were sufficient to maximize sugar production.

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The temperature-dependent phase diagrams, from 40 to −50 °C, of the binary (NH4)2SO4/H2O system are derived from data collected in a low-temperature, single-levitated-particle apparatus. The conditions of vapor pressure, temperature, and composition, at which particle phase transitions occur along the path of increasing relative humidity, define the equilibrium phase diagram, whereas the set of conditions that induce transitions in the reverse direction defines the metastability phase diagram. A new, stable crystalline phase of (NH4)2SO4·4H2O is identified. Below −19.35 ± 0.05 °C, the anhydrous (NH4)2SO4−ice eutectic point, ammonium sulfate does not deliquesce as the RH is increased but instead undergoes a solid−solid phase transition to form a tetrahydrate phase. (NH4)2SO4·4H2O incongruently melts at −19.35 °C. Equilibrium phase transition, as well as metastable to stable phase transitions, for this system are mapped out, and the derived phase diagrams allow for prediction of the composition and phase of (NH4)2SO4 aerosols under the full range of atmospheric conditions and paths. Present observations are in excellent agreement with previous data.

Though ammonium bisulfate is one of the most common of atmospheric hygroscopic aerosols, knowledge of its interaction with water has, until now, been extremely limited. This paper presents our observations on single isolated ammonium bisulfate aerosol particles, as they interact with water vapor at temperatures ranging from -40 to 30 °C. The complete phase diagram in the temperature/composition as well as the pressure/ temperature domains has been derived here for the first time, mapping out equilibrium and metastable to stable phase transitions. A new low-temperature crystalline hydrate phase that might have an important role in processes occurring in the upper troposphere has been discovered.