Rosa Fitzgerald

Rayleigh and Wood anomalies in the diffraction of light from a perfectly conducting reflection grating

By means of a modal method we have calculated the angular dependence of the reflectivity and the efficiencies of several other diffracted orders of a perfectly conducting lamellar reflection grating illuminated by p-polarized light. These dependencies display the signatures of Rayleigh and Wood anomalies, usually associated with diffraction from a metallic grating. The Wood anomalies here are caused by the excitation of the surface electromagnetic waves supported by a periodically corrugated perfectly conducting surface, whose dispersion curves in both the nonradiative and radiative regions of the frequency-wavenumber plane are calculated.

An inverse reconstruction model to retrieve aerosol size distribution from optical depth data

A novel in situ methodology for the inverse reconstruction of aerosol size distribution from optical depth data is presented and successfully validated against SEM data for the city of El Paso. Additional results using MFRSR optical depth data for La Jornada city are also shown. The retrieval methodology and corresponding codes are applicable to irregularly shaped aerosol particles in any region.

Projected ozone trends and changes in the ozone-precursor relationship in the South Coast Air Basin in response to varying reductions of precursor emissions

ABSTRACT This study examined the effects of varying future reductions in emissions of oxides of nitrogen (NOx) and volatile organic compounds (VOC) on the location and magnitude of peak ozone levels within California’s South Coast Air Basin (SoCAB or Basin). As ozone formation is currently VOC-limited in the Basin, model simulations with 2030 baseline emissions (−61% for NOx and −32% for VOC from 2008) predict 10–20% higher peak ozone levels (i.e., NOx disbenefit) in the western and central SoCAB compared with the 2008 base simulation. With additional NOx reductions of 50% beyond the 2030 baseline emissions (−81% from 2008), the predicted ozone levels are reduced by about 15% in the eastern SoCAB but remain comparable to 2008 levels in the western and central Basin. The Basin maximum ozone site shifts westward to more populated areas of the Basin and will result potentially in greater population-weighted exposure to ozone with even a relatively small shortfall in the required NOx reductions unless accompanied by additional VOC reductions beyond 2030 baseline levels. Once committed to a NOx-focused control strategy, NOx reductions exceeding 90% from 2008 levels will be necessary to attain the ozone National Ambient Air Quality Standards (NAAQS). The findings from this study and other recent work that the current VOC emission estimates are underestimated by about 50% suggest that greater future VOC reductions will be necessary to reach the projected 2030 baseline emissions. Increasing the base year VOC emissions by a factor of 1.5 result in higher 2008 baseline ozone predictions, lower relative response factors, and about 20% lower projected design values. If correct, these findings have important implications for the total and optimum mix of VOC and NOx emission reductions that will be required to attain the ozone NAAQS in the SoCAB. Implications: Results of this study indicate that ozone levels in the western and central SoCAB would remain the same or increase with even a relatively small shortfall in the projected NOx reductions under planned NOx-focused controls. This possibility, therefore, warrants a rigorous analysis of the costs and effects of varying reductions of VOC and NOx on the formation and combined health impacts of ozone and secondary particles. Given the nonlinearity of ozone formation, such analyses should include the implications of gradually increasing global background ozone concentrations and the Basin’s topography and meteorology on the practical limits of alternative emission control strategies.

Sensitivity modeling study for an ozone occurrence during the 1996 Paso Del Norte Ozone Campaign.

Surface ozone pollution has been a persistent environmental problem in the US and Europe as well as the developing countries. A key prerequisite to find effective alternatives to meeting an ozone air quality standard is to understand the importance of local anthropogenic emissions, the significance of biogenic emissions, and the contribution of long-range transport. In this study, an air quality modeling system that includes chemistry and transport, CMAQ, an emission processing model, SMOKE, and a mesoscale numerical meteorological model, WRF, has been applied to investigate an ozone event occurring during the period of the 1996 Paso del Norte Ozone Campaign. The results show that the modeling system exhibits the capability to simulate this high ozone occurrence by providing a comparable temporal variation of surface ozone concentration at one station and to capture the spatial evolution of the event. Several sensitivity tests were also conducted to identify the contributions to high surface ozone concentration from eight VOC subspecies, biogenic VOCs, anthropogenic VOCs and long-range transportation of ozone and its precursors. It is found that the reductions of ETH, ISOP, PAR, OLE and FORM help to mitigate the surface ozone concentration, and like anthropogenic VOCs, biogenic VOC plays a nonnegligible role in ozone formation. But for this case, long-range transport of ozone and its precursors appears to produce an insignificant contribution.

Numerical simulation for a wind dust event in the US/Mexico border region

Soil-derived dust represents one of the major components of the natural atmospheric aerosols. Arid and semiarid areas with unpaved and non-vegetated land cover are particularly vulnerable to windblown dust, which results in high particulate matter pollution. To understand, predict, and mitigate the impact of dust aerosol on air quality and climate, it is necessary to parameterize the emission rate of dust particles from the wind erosion processes accurately. However, windblown dust emission is poorly represented in existing air quality models. In this paper, a windblown dust emission model has been developed based on a parameterization of threshold wind friction velocity depending on the roughness of surface, vegetation type, soil type, soil moisture content, and on the size distribution of aerosols. The proposed dust model incorporates into a region air quality modeling system to simulate a North American dust storm episode occurring near the border of southwestern USA and northwestern region of Mexico on 23 February 2007. It is shown that the implementation of a windblown dust model in an air quality model can significantly improve the model capability for capturing the dust episode. The simulation of the model is in good agreement with the evolution of dust distribution. The modeled dust spatial patterns matched dust cloud patterns appearing on satellite images. Implementation of the windblown dust model successfully captured the time of peak particulate matter (PM) concentrations for both PM10 and PM2.5, as well as the peak value of the PM2.5 concentration. The modeled results clearly demonstrate an improved ability to predict PM events by applying the windblown dust emission scheme.

Transmission of light through a metal film with two corrugated surfaces

We study theoretically the transmission of p- and s-polarized light through a thin, supported, silver or gold film, both of whose surfaces are one-dimensional, periodically corrugated surfaces, as a function of the wavelength of the incident light. The calculations are carried out by the use of reduced Rayleigh equations in the form of a pair of coupled inhomogeneous matrix equations for the amplitudes of the scattered and transmitted Bragg beams. The results show that enhanced transmission occurs for both p- and s-polarized incident light for large-amplitude periodic corrugations of the two surfaces. In p polarization an additional enhancement occurs at the wavelengths of the surface plasmon polaritons supported by the film. Thus, slits that pierce the film are not necessary for enhanced transmission of light through it.

Reconstruction of the surface power spectrum from reflectivity data

An analytical approach is used to determine the power spectrum of the surface roughness from experimental data for the reflectivity of randomly rough surfaces. A one-dimensional, randomly rough, perfectly conducting surface that is illuminated by s-polarized light whose plane of incidence is perpendicular to the generators of the surface is considered, and the power spectrum is obtained within the framework of phase perturbation theory. Good agreement with numerically generated experimental data is obtained.

S-polarized guided electromagnetic waves at a planar interface between vacuum and a graded-index dielectric

Abstract Analytic expressions for the dispersion relation and electric field profiles of guided waves supported by an asymmetric graded-index dielectric waveguide are derived. The system studied consists of vacuum in the region , and a dielectric medium in the region whose dielectric permittivity decreases continuously with increasing distance into it from the interface according to . Here is the (real) index of refraction of the medium at , and it is assumed that . It is found that for frequencies below a certain critical value given by , where is the speed of light in vacuum, the dispersion curve consists of a single branch that exists in a narrow spectral range. Its electric field decays exponentially with increasing distance into each medium. For frequencies above this critical value, the dispersion curve possesses several branches. The corresponding electric field decays exponentially with increasing distance into the vacuum, and decays in an oscillatory fashion with increasing distance into the graded-index dielectric medium. The number of nodes in the latter field equals the branch number, starting with zero for the lowest frequency branch.

Fitting normal modes to HF radial and total surface current vector data over enclosed bays and estuaries

A technique referred to as Normal Mode Analysis (NMA) has recently been developed for representing total vector CODAR HF radar data in Monterey Bay. These modes satisfy the coastal boundary constraint of no flow normal to the shore, and inherently represent both divergent and rotational flow as two sets of ortho-normal basis functions. In prior investigations by others, the domain had a large open boundary at which additional information from a numerical model was needed to completely represent the surface flows in the Bay. The modes were fitted to data only in the two-site overlap region where total vectors were calculated. We apply NMA for completely enclosed bays, using two-dimensional finite element methods to derive these modes where the shoreline is the mathematical boundary for the problem. This is an improvement over prior studies with open boundaries where additional information was needed to represent the flow within. We also extended this methodology by fitting to radial velocities from each radar by itself - as well as simultaneous radial data from multiple coastal radars viewing the same bay. It was applied to Corpus Christi Bay where two SeaSondes have been operated by Texas A&M University for. two years. First, we employed simulations, where we resolved arbitrary current flow patterns into two sets of radial data. Noise was added to the vectors, and the extraction accuracy was studied. Ability to derive meaningful total velocity patterns depends on the noise level; the percent coverage of the bay by the radial measurements, and the availability of simultaneous radial data from different sites/angles. Surprisingly good extraction is often obtained with only single radar coverage. Finally, this method is now tested here with actual, hourly SeaSonde HF radar data over this Bay, both at the single-site radial level and by employing both sites. Comparisons are made with the real-time total-vector maps produced by the radar software over the common coverage area. Our bay-conforming natural mode-pattern resonances will be used in ongoing studies that relate their strengths to wind stress across the bay surface.

Linking Air Quality and Human Health Effects Models: An Application to the Los Angeles Air Basin:

Proposed emission control strategies for reducing ozone and particulate matter are evaluated better when air quality and health effects models are used together. The Community Multiscale Air Quality (CMAQ) model is the US Environmental Protection Agency’s model for determining public policy and forecasting air quality. CMAQ was used to forecast air quality changes due to several emission control strategies that could be implemented between 2008 and 2030 for the South Coast Air Basin that includes Los Angeles. The Environmental Benefits Mapping and Analysis Program—Community Edition (BenMAP-CE) was used to estimate health and economic impacts of the different emission control strategies based on CMAQ simulations. BenMAP-CE is a computer program based on epidemiologic studies that link human health and air quality. This modeling approach is better for determining optimum public policy than approaches that only examine concentration changes.