J. Cozic

A large atomic chlorine source inferred from mid-continental reactive nitrogen chemistry.

Halogen atoms and oxides are highly reactive and can profoundly affect atmospheric composition. Chlorine atoms can decrease the lifetimes of gaseous elemental mercury and hydrocarbons such as the greenhouse gas methane. Chlorine atoms also influence cycles that catalytically destroy or produce tropospheric ozone, a greenhouse gas potentially toxic to plant and animal life. Conversion of inorganic chloride into gaseous chlorine atom precursors within the troposphere is generally considered a coastal or marine air phenomenon. Here we report mid-continental observations of the chlorine atom precursor nitryl chloride at a distance of 1,400 km from the nearest coastline. We observe persistent and significant nitryl chloride production relative to the consumption of its nitrogen oxide precursors. Comparison of these findings to model predictions based on aerosol and precipitation composition data from long-term monitoring networks suggests nitryl chloride production in the contiguous USA alone is at a level similar to previous global estimates for coastal and marine regions. We also suggest that a significant fraction of tropospheric chlorine atoms may arise directly from anthropogenic pollutants.

Counterflow Virtual Impactor Based Collection of Small Ice Particles in Mixed-Phase Clouds for the Physico-Chemical Characterization of Tropospheric Ice Nuclei: Sampler Description and First Case Study

A ground-based sampling system named Ice-CVI is introduced that is able to extract small ice particles with sizes between 5 and 20 μ m out of mixed-phase clouds. The instrument is based on a counterflow virtual impactor (CVI) removing interstitial particles and is supplemented by additional modules that pre-segregate other constituents of mixed-phase clouds. Ice particles of 20 μ m and smaller are expected to grow only by water vapor diffusion and there is a negligible probability that they scavenge aerosol particles by impaction and riming. Thus, their residuals which are released by the Ice-CVI can be interpreted as the original ice nuclei (IN). In a first field test within the Cloud and Aerosol Characterization Experiment (CLACE-3) at the high alpine research station Jungfraujoch, the collection behavior of the single components and the complete system was evaluated under atmospheric sampling conditions. By comparing parameters measured by the Ice-CVI with corresponding results obtained from other inlets or with in-situ instrumentation it is verified that the small ice particles are representatively collected whereas all other mixed phase cloud constituents are effectively suppressed. In a case study it is observed that super-micrometer particles preferentially serve as IN although in absolute terms the IN concentration is dominated by sub-micrometer particles. Mineral dust (Si), non-volatile organic matter and black carbon could be identified as IN components by means of different chemical analyses. The latter suggests an anthropogenic influence on the heterogeneous ice nucleation in supercooled, tropospheric clouds.

Aerosol partitioning between the interstitial and the condensed phase in mixed-phase clouds

{[}1] The partitioning of aerosol particles between the cloud and the interstitial phase (i.e., unactivated aerosol) has been investigated during several Cloud and Aerosol Characterization Experiments (CLACE-3, CLACE-3 1/2 and CLACE-4) conducted in winter and summer 2004 and winter 2005 at the high alpine research station Jungfraujoch ( 3580 m altitude, Switzerland). Ambient air was sampled using different inlets in order to determine the activated fraction of aerosol particles, F(N), defined as the fraction of the total aerosol number concentration ( with particle diameter d(p) > 100 nm) that has been incorporated into cloud particles. The liquid and ice water content of mixed-phase clouds were characterized by analyzing multiple cloud probes. The dependence of the activated fraction on several environmental factors is discussed on the basis of more than 900 h of in-cloud observations and parameterizations for key variables are given. FN is found to increase with increasing liquid water content and to decrease with increasing particle number concentration in liquid clouds. FN also decreases with increasing cloud ice mass fraction and with decreasing temperature from 0 to -25 degrees C. The Wegener-Bergeron-Findeisen process probably contributed to this trend, since the presence of ice crystals causes liquid droplets to evaporate, thus releasing the formerly activated particles back into the interstitial phase. Ice nucleation could also have prevented additional cloud condensation nuclei from activating. The observed activation behavior has significant implications for our understanding of the indirect effect of aerosols on climate.

Worldwide data sets constrain the water vapor uptake coefficient in cloud formation

Cloud droplet formation depends on the condensation of water vapor on ambient aerosols, the rate of which is strongly affected by the kinetics of water uptake as expressed by the condensation (or mass accommodation) coefficient, αc. Estimates of αc for droplet growth from activation of ambient particles vary considerably and represent a critical source of uncertainty in estimates of global cloud droplet distributions and the aerosol indirect forcing of climate. We present an analysis of 10 globally relevant data sets of cloud condensation nuclei to constrain the value of αc for ambient aerosol. We find that rapid activation kinetics (αc > 0.1) is uniformly prevalent. This finding resolves a long-standing issue in cloud physics, as the uncertainty in water vapor accommodation on droplets is considerably less than previously thought.

Influence of particle chemical composition on the phase of cold clouds at a high-alpine site in Switzerland

This paper studies the influence of particle chemical composition on the phase of cold clouds observed during two intensive measurement periods of the Cloud and Aerosol Characterization Experiments conducted at the Jungfraujoch site (Switzerland). Cloud droplets and particles were sampled simultaneously using a suite of optical, chemical, and microphysical instruments connected downstream of a total inlet and an interstitial inlet. Sulphate and organic matter were the most abundant semivolatile species observed in the particulate phase during both campaigns. Periods of relatively large loadings of organic and inorganic species were also accompanied by enhancement of light-absorbing aerosol concentrations. The cloud phase exhibited sharp transitions, alternating between highly glaciated and liquid phases over a few seconds within the same cloud event. It was also observed that conditions of elevated pollution were accompanied by an increase in occurrence of glaciated periods. The 24-hour cloud event investigated on the 8 March 2004 was in the mixed phase for approximately 260 minutes, in the glaciated phase for approximately 64 minutes and in the liquid phase for the remainder of the time. On the 23 March 2004, another 24-hour cloud event was captured in which the number of minutes as mixed-phase and glaciated cloud were 196 and 31, respectively. The loadings of BC as well as organic and inorganic species were larger during the first period. The investigation was extended for the whole data set, and a statistical analysis was performed across the chemical data measured off the total inlet. The amount of organic and inorganic material found in liquid and glaciated clouds was statistically different, with organic and inorganic material as well as BC being enriched in glaciated conditions. The case studies and the statistical analysis together suggest an influence of the particle chemical composition on the cloud phase, which may be important in perturbing cloud microphysics in polluted regions.

Constraining the water vapor uptake coefficient in ambient cloud droplet formation

Cloud droplet formation depends on the condensation of water vapor on ambient aerosols, the rate of which is strongly affected by the condensation (or mass accommodation) coefficient, αc. Estimates of αc for droplet growth from activation of ambient particles vary considerably and represent a critical source of uncertainty in estimates of global cloud droplet distributions and the aerosol indirect forcing of climate. An analysis of ten globally relevant data sets of cloud condensation nuclei is used to constrain αc, and find that rapid activation kinetics (αc > 0.1) is uniformly prevalent. This means that uncertainty in water vapor accommodation on droplets is less than previously thought and resolves a long-standing issue in cloud physics.