Browsing by Author "Dr. S. Pal Arya, Committee Co-Chair"

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An Experimental Study of the Vertical Eddy Diffusivity and Dry Deposition of Ammonia on a Natural Grassy Surface
(2003-12-08) Peterson, Barry Todd; Dr. S. Pal Arya, Committee Co-Chair
Until recently ammonia has been relatively ignored as a primary pollutant in the United States. Due to the rapid growth of animal (hog) farms, eastern North Carolina experiences higher levels of ambient ammonia and ammonium concentrations. The primary focus of this work was on the eddy diffusivity and the dry deposition velocity of ammonia over a natural grassy surfaces downwind of some typical natural/anthropogenic sources in eastern North Carolina. All Data was collected on a 7m aluminum walk-up tower. Temperature, wind speed, wind direction and ammonia/ammonium concentrations were collected at two different heights (2m and 6m). Citric acid coated annular denuders with filter packs were used to measure the ammonia and ammonium concentrations. The tower was located at the NADP site NC41 in Raleigh, NC. The tower is located 300-400m from a small waste lagoon used by the NCSU Educational Swine Unit. The fetch is undisturbed in all quadrants except for the northeast. The northeast quadrant is disturbed by a group of small trailers, greenhouses and swine houses. The remaining quadrants are grassy fields used for grazing approximately 100 head of cattle Hourly-averaged measurements of temperatures, wind speeds and concentrations of ammonia and ammonium are made. We assume a horizontally-homogeneuous atmospheric surface layer. Surface-layer similarity relations were used for estimating the vertical fluxes of momentum and heat. The modified Bowen ratio and gradient method were used for estimating the vertical flux and the deposition velocity of ammonia.
Measurements, Modeling, and Analysis of Fluxes of Nitrogen Compounds
(2003-07-25) Phillips, Sharon Baker; Major Paul A. Roelle, Ph.D., Committee Member; Dr. Rohit Mathur, Committee Member; Dr. Viney P. Aneja, Committee Co-Chair; Dr. S. Pal Arya, Committee Co-Chair
Nitrogen compounds play a significant role in atmospheric chemistry, contributing to acute effects on human health and the environment. Oxidized and reduced forms of nitrogen are attributed to increasing concentration levels of tropospheric ozone, photochemical smog, acid rain, and eutrophication of sensitive ecosystems. Nationally and internationally, increasing interest has been given to reduced forms of nitrogen, ammonia (NH3, most abundant alkaline component in the atmosphere) and ammonium (NH4+, the primary atmospheric reaction product of NH3), with their combination represented by NHX = NH3 + NH4+. Emission and deposition of oxidized and reduced forms of nitrogen depend on several meteorological parameters: near-surface winds (wind speed at 10m), friction velocity, turbulence, atmospheric stability, air temperature, surface heat flux and relative humidity, as well as the spatial distribution of sources. This research initiative has been prepared in response to the expressed need for a better understanding of the nitrogen budget and related ammonia flux and dry deposition velocity in North Carolina. The primary focus of this research will be on the vertical fluxes of ammonia and implied dry deposition velocities over natural surfaces downwind of some typical natural/anthropogenic sources in eastern North Carolina. This particular area of emphasis is chosen because of the lack of data and knowledge of ammonia deposition in eastern North Carolina, where its sources (e.g. swine production facilities) have increased very rapidly in recent years. An experimental study was conducted on the emission and dry deposition fluxes of ammonia under different meteorological conditions, using a micrometeorological technique (micrometeorological gradient and modified Bowen-ratio methods in conjunction with the Monin-Obukhov similarity theory) over natural surfaces in North Carolina where intensively managed agriculture/animal farms are located. Ammonia concentrations were measured simultaneously with mean wind speeds, wind directions and temperatures during Fall 2001, Winter, Spring and Summer 2002 at two heights (2 and 6m) employing a technologically advanced mobile laboratory. Diurnal and seasonal variations of ammonia flux and dry deposition velocity were investigated under a wide range of wind and atmospheric stability conditions yielding hourly variation of NH3 flux and deposition velocity during each seasonal campaign. Greater NH3 concentrations were measured during the fall measurement campaign, which were directly related to spray-effluent irrigation practices; whereas the winter season had the lowest overall concentrations, collected during each seasonal campaign (effect of colder temperatures). The largest average NH3 deposition velocities were estimated during the summer measurement campaign, whereas the winter season estimated the lowest daytime velocities. This evaluation of nitrogen species was extended to addressing the total nitrogen budget for North Carolina during summer season. The portion of atmospherically deposited nitrogen, which reaches either land or water bodies is highly variable depending upon meteorological and seasonal conditions. Modeled dry deposition rates of NO (nitric oxide), NO2 , HNO3 and NH3, using a third generation Eulerian grid model (the United States Environmental Protection Agency's Models-3/Community Multiscale Air Quality (CMAQ) modeling system) in conjunction with measured wet deposition rates of nitrate (NO3-) and ammonium (NH4+), were evaluated in order to characterize the factors controlling the total nitrogen budget. In addition, model assessments were made of atmospheric inputs (loading) into the Neuse River Estuary in North Carolina. In North Carolina, approximately 50% of NHX or NO3- flux occurs in the form of dry and wet deposition during the summer season. The Neuse River watershed's largest contributor to dry deposition flux of nitrogen (nitrogen loading) was determined to be NH3, making up approximately 47% of the total atmospheric deposition.
Urban Land-surface Impacts on Convective Thunderstorm and Precipitation Characteristics
(2006-11-01) Pyle, Patrick Chase; Dr. Dev Niyogi, Committee Co-Chair; Dr. S. Pal Arya, Committee Co-Chair; Dr. Viney Aneja, Committee Member
A six year storm climatology in the Indianapolis, Indiana region is investigated using base reflectivity radar data. Severe storm reports provided by the Storm Prediction Center (SPC) are used to select relevant thunderstorm cases to study. Storm composition change was noted in urban and rural environments and further statistical analysis is performed to view the overall effect of the urban region on storm characteristics. A specific thunderstorm case on 13 June 2005 is further examined using the fifth-generation NCAR/Penn State Mesoscale Model (MM5) V3.7.2. The Noah Land Surface Model (LSM) is used to represent the urban environment with a sensitivity simulation removing the urban region and replacing it with the dominant rural land use category. Results show a drastic change in surface energy balance characteristics as well as model derived radar reflectivity patterns when the urban region is removed in the NOURBAN simulation. The same 13 June 2005 case is also investigated using the Weather Research and Forecasting (WRF) V.2.1.1 model. Identical control and nourban simulations are performed similar to the MM5. Results between the two mesoscale models are difficult to compare due to the domain structure as well as the different physics options and data initialization techniques used within the two models; however, few comparisons can be made. Several land use sensitivity simulations are also performed in an effort to better understand the underlying effects of the urban region on the case of interest. Simulations include a nourban case where the urban region is completely removed and replaced by the dominate surrounding rural land use, similar to the MM5 study. Other simulations performed are used to set up a statistical factor separation experiment. Land use variables that are manipulated include albedo, surface roughness length (z0), and urban sprawl. All variables are manipulated to increase the overall effectiveness of a larger, more 'robust' city (Indianapolis) to resemble the current and projected increase in urbanization. Model comparisons within these simulations were compared using only the nest 3 (4 km) resolution due to the poor representation of the event on the finest 1.33 km grid spacing nest. Surface energy balance parameters were not altered a great deal in the simulations; however, the overall precipitation patterns and storm characteristics show a wide range of variability. When the urban region is removed, the storm of interest does not propagate through the urban region. Accumulated precipitation totals through the time of the event are dramatically less around and downwind (northeast) of the urban region. The factorial experiment suggested that a more robust city acted to place the maximum precipitation amounts on the northern lateral edge of the city that may be caused by the urban 'building barrier effect.' Observations from the Joint Urban Project 2003 held in Oklahoma City are used to compare to the Coupled Ocean-Atmosphere Mesoscale (COAMPS) model sensitivity simulations of a Mesoscale Convective System (MCS) event that occurred on 30 July 2003. Two distinct simulations were used to compare the control simulation. The OKC urban region was enhanced by using a coupled urban canopy model (UCM) approach to attempt to resolve the finer scale heating and drag features of the urban environment (e.g., roof, wall, street, and anthropogenic heating contributions) to the urban heat island (UHI). Also, the Gas Exchange Photosynthesis Based model (GEM) was coupled to the Noah Land Surface Model (LSM) to enhance the overall heterogeneity of the urban-rural interface. Results indicate that the simulation with urban canopy parameterizations (UCP) enhanced the overall intensity of the event, while the GEM proved to show more heterogeneity by placing the convection away from the downtown area, similar to the observed.