Showing papers by "Kevin J. Noone published in 1996"

Sulfate, nitrate, methanesulfonate, chloride, ammonium, and sodium measurements from ship, island, and aircraft during the Atlantic Stratocumulus Transition Experiment/Marine Aerosol Gas Exchange

Abstract: We measured aerosol methanesulfonic acid (MSA), Cl−, NO3−, SO4=, Na+, and NH4+ concentrations and size distributions and gaseous ammonia and nitric acid concentrations from Santa Maria, Azores during the Atlantic Stratocumulus Transition Experiment/Marine Aerosol and Gas Exchange (ASTEX/MAGE) experiment in June 1992. We also sampled some of these species in the free troposphere and marine boundary layer (MBL) from the National Center for Atmospheric Research's (NCAR's) Electra aircraft and at the surface from the R/V Oceanus. In relatively clean Atlantic marine air masses the mean (and standard error of the mean) observed MSA concentrations (27 ± 2 parts per trillion molar mixing ratio, ppt) were smaller than in continental polluted air masses (36 ± 6 ppt), reflecting either longer aerosol lifetimes in air with higher cloud condensation nuclei (CCN) concentrations or more active photochemical production in polluted air. Mean NH4+/non-sea-salt (nss) SO4= molar ratios in marine and continental air masses were 0.65 ± 0.05 and 1.3 ± 0.2, respectively, indicating that continental sulfate was neutralized by ammonia to a greater extent than marine sulfate was. Most of the NH4+ and nss SO4= was contained in submicrometer modes; Na+, Cl−, and NO3− were in supermicron modes; and MSA showed bimodal behavior, with major and minor peaks at 0.3–0.4 μm and 1.5 μm, respectively. The molar ratio NH4+/nss SO4= relative to size confirms that most ammonia vapor condensed on submicron nss SO4=. The average dry deposition fluxes for aerosol nss SO4=, MSA, NH4+ and NO3− during ASTEX/MAGE were 1.49, 0.09, 0.18, and 3.25 μmol m−2 d−1, respectively.

The effect of hygroscopicity on cloud droplet formation

Abstract: The effects of particle hygroscopicity and the availability of condensable material (other than water) in the gas phase on cloud droplet formation and the radiative properties of clouds have been studied using an adiabatic air parcel model with detailed multicomponent condensation. The pre-existing log-normal particle distribution used is bimodal in size and bimodal in hygroscopicity. To simulate this, four log-normal distributions were used and in each mode particles were assumed to be internally mixed, i.e. they are composed partly of salt and partly of an insoluble substance. The mean diameters, standard deviations, total number of pre-existing particles, the mass fraction of the soluble salt and initial concentration of condensable vapour were varied in the simulations. There is a clear effect of hygroscopicity on the activated fraction of aerosol particles in our simulations. Thus hygroscopicity of pre-existing aerosol particles and concentrations of condensable gases can also influence the optical thickness and reflectance of clouds. The change in optical thickness varies as a function of the number concentration of pre-existing particles, having a maximum (Δ/τ = 0.2) near a concentration of 1000/cc. DOI: 10.1034/j.1600-0889.1996.t01-2-00002.x

Ozone in the marine atmosphere observed during the Atlantic Stratocumulus Transition Experiment/Marine Aerosol and Gas Exchange

Abstract: Measurements of vertical profiles and level averages of ozone over the north central Atlantic near the Azores are presented. Lagrangian experiments were carried out in clean air and polluted air, and differences between the cases are discussed. Significant vertical structure was found in the profiles from both cases. Trajectory analysis indicates that the source regions for ozone in the marine boundary layer and in the air above the inversion capping the boundary layer were different. No significant production or destruction of ozone in the boundary layer was observed in the clean case. No substantial differences in ozone concentration between cloudy and clear air were observed, although cloud processes in the polluted case may have been partially responsible for an observed overall reduction in boundary layer ozone concentration.

The size distribution and chemical composition of cloud droplet residual particles in marine stratocumulus clouds observed during the mast experiment

Abstract: Publisher Summary The chapter discusses the size distribution and chemical composition of cloud droplet residual particles in marine stratocumulus clouds observed during the mast experiment. Measurements of cloud droplet residual particle microphysics and chemistry were made in marine stratocumulus clouds off the central California coast during the Monterey area ship tracks (MAST) experiment in June, 1994. Typically, 80% or more of the residual particles were below 0.1 μm radius. While the mode size of residual particles was slightly higher in polluted clouds than in clean ones, the droplet population in the two cases was determined by the submicrometer aerosol. One explanation for the shift towards larger sizes in the residual particle size distributions for polluted versus clean clouds is a reduction of the peak supersaturation in the polluted clouds because of an increase in the rate at which water vapor is taken up by the larger number of particles present in the polluted clouds. Single particle analysis for samples taken in a clean marine boundary layer showed the residual particles larger than 0.1 μm radius to have been composed of sea salt and a combination of sea salt and sulfate. This composition is typical of the background marine boundary layer aerosol. Residual particles containing exclusively organic material, and also organic material plus CI were observed in a sample from a moderately polluted cloud. These clusters accounted for 182 of the 500 particles analyzed, indicating that organic particles were a substantial fraction of the cloud condensation nuclei in these particular clouds.