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No description available.

Cloud reflectivity is a function of cloud liquid water content and droplet number concentration. Since cloud droplets form around pre-existing aerosol particles, cloud droplet number concentration depends on the availability of particles that can serve as cloud condensation nuclei. Given constant liquid water amount, increased availability of cloud condensation nuclei leads to clouds with a greater droplet number concentration, greater total droplet surface area and consequently, greater reflectivity. The change in cloud reflectivity resulting from the increased availability of condensation nuclei is known as the aerosol indirect effect. The aerosol indirect effect ranks as one of the largest sources of uncertainty in current estimates of global climate change, largely due to difficulties in measurement. Changes in cloud reflectivity resulting from the aerosol indirect effect are typically much smaller than the natural background variability observed in clouds. As a result, the modification signal is very difficult to detect against the background noise. Additionally, since atmospheric aerosols are ubiquitous, it is difficult to find polluted and nonpolluted clouds that are sufficiently alike for reasonable comparison. However, ship tracks seen in satellite images present one opportunity to study the aerosol indirect effect in relative isolation. Ship tracks are regions of enhanced reflectivity in marine stratus, resulting from the addition of aerosols from ship exhaust plumes to preexisting clouds. Ship tracks are a common feature of satellite images of the North Pacific. Since the marine atmosphere has comparatively low background aerosol concentrations, the addition of ship exhaust particles can lead to distinct increases in cloud reflectivity. Ship tracks allow for sampling of polluted and nonpolluted clouds from adjacent regions with similar solar and viewing geometry, cloud temperatures and surface properties, and consequently provide a unique opportunity to study the effects of aerosol modification of cloud reflectivity. Using satellite images of the North Pacific in July 1999, over 1000 ship tracks were identified, logged and analyzed, yielding 504 sets of radiance data matching polluted clouds with nearby nonpolluted clouds. It was expected that increasing the size of the region for selection of nonpolluted clouds would increase the variability in observed reflectivity, and make detection of the modification signal more difficult. In order to study this potential effect of domain size for selection of nonpolluted clouds on measurements of the aerosol indirect effect, three data sets were collected, using domain sizes for selection of nonpolluted clouds of 15, 50 and 100 km. Analysis of retrieved optical depth and droplet effective radius for modified and control pixels shows evidence of a 1-5% increase in visible optical depth of marine stratus following modification by addition of ship exhaust particles, but unexpectedly, shows only slight increases in uncertainty with increasing domain size. A subsequent study revealed that autocorrelation lengths of radiances and retrieved cloud properties were only 8-15 km. This indicates that even the 15 km control domain captured much of the background variability present. Domain sizes smaller than 15 km are difficult to sample automatically while avoiding the inclusion of polluted clouds in the nonpolluted cloud sample. As a result, it remains necessary to analyze large numbers of ship tracks to separate the aerosol modification signal from the background variability. / Graduation date: 2003

Ambient high-volume (hi-vol) air samples were collected between March 15th and May 30th 2002, at Cheeka Peak Observatory (CPO), located on the tip of the Olympic Peninsula, Washington State. This sampling campaign was in conjunction with the 2002 Inter-Continental Transport and Chemical Transformation (ITCT 2K2) Campaign and the Photochemical Ozone Budget of the Eastern North Pacific Atmosphere (PHOEBA2) experiment, both of which studied the effect of Trans-Pacific transport on the U.S. West Coast. The anthropogenic semi-volatile organic compounds (SOCs) measured during this time period included polycyclic aromatic hydrocarbons (PAHs) and various U.S. current-use and historical-use pesticides. The total PAH concentration ranged from 0.480-4.49 ng/m³, which is comparable to other remote sites throughout the globe. Ten pesticides (hexachlorobenzene, dacthal, chlorothalonil, heptachlor, trans-nonachlor, cis-nonachlor, endosulfan I, triallate, trifluralin, and mirex) were also measured and their concentrations (0.104-57.0 pg/m³) were comparable to other remote sites and less than agricultural areas. Gas-phase/particle-phase partitioning of SOCs was explored, with a significant correlation with temperature found for endosulfan I and retene. A possible relationship at CPO of low total suspended particulate (TSP) concentration with the concentration of non-exchangeable SOCs in the particle phase was found. Principal Component Analysis, as well as a t-test, showed there were elevated concentrations and a unique pattern of anthropogenic SOCs measured during possible Trans-Pacific events on March 15th-16th March 27th-28th and April 22nd-23rd, 2002. These Trans-Pacific events were identified using the GEOS-CHEM model and 10-day back air trajectories. The potential sources of these compounds at CPO were determined using diagnostic ratios of their concentrations, back trajectories calculated using HYSPLIT4, local meteorological conditions, and U.S. pesticide use data. / Graduation date: 2004

University of Macau / Faculty of Science and Technology / Department of Computer and Information Science