130 research outputs found
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Influence of street setbacks on solar reflection and air cooling by reflective streets in urban canyons
The ability of a climate model to accurately simulate the urban cooling effect of raising street albedo may be hampered by unrealistic representations of street geometry in the urban canyon. Even if the climate model is coupled to an urban canyon model (UCM), it is hard to define detailed urban geometries in UCMs. In this study, we relate simulated surface air temperature change to canyon albedo change. Using this relationship, we calculate scaling factors to adjust previously obtained surface air temperature changes that were simulated using generic canyon geometries. The adjusted temperature changes are obtained using a proposed multi-reflection urban canyon albedo model (UCAM), avoiding the need to rerun computationally expensive climate models. The adjusted temperature changes represent those that would be obtained from simulating with city-specific (local) geometries. Local urban geometries are estimated from details of the city's building stock and the city's street design guidelines. As a case study, we calculated average citywide seasonal scaling factors for realistic canyon geometries in Sacramento, California based on street design guidelines and building stock. The average scaling factors are multipliers used to adjust air temperature changes previously simulated by a Weather Research and Forecasting model coupled to an urban canyon model in which streets extended from wall to wall (omitting setbacks, such as sidewalks and yards). Sacramento's scaling factors ranged from 2.70 (summer) to 3.89 (winter), demonstrating the need to consider the actual urban geometry in urban climate studies
The impact of heat mitigation strategies on the energy balance of a neighborhood in Los Angeles
Heat mitigation strategies can reduce excess heat in urban environments. These strategies, including solar reflective cool roofs and pavements, green vegetative roofs, and street vegetation, alter the surface energy balance to reduce absorption of sunlight at the surface and subsequent transfer to the urban atmosphere. The impacts of heat mitigation strategies on meteorology have been investigated in past work at the mesoscale and global scale. For the first time, we focus on the effect of heat mitigation strategies on the surface energy balance at the neighborhood scale. The neighborhood under investigation is El Monte, located in the eastern Los Angeles basin in Southern California. Using a computational fluid dynamics model to simulate micrometeorology at high spatial resolution, we compare the surface energy balance of the neighborhood assuming current land cover to that with neighborhoodâwide deployment of green roof, cool roof, additional trees, and cool pavement as the four heat mitigation strategies. Of the four strategies, adoption of cool pavements led to the largest reductions in net radiation (downward positive) due to the direct impact of increasing pavement albedo on ground level solar absorption. Comparing the effect of each heat mitigation strategy shows that adoption of additional trees and cool pavements led to the largest spatialâmaximum air temperature reductions at 14:00h (1.0 and 2.0 °C, respectively). We also investigate how varying the spatial coverage area of heat mitigation strategies affects the neighborhoodâscale impacts on meteorology. Air temperature reductions appear linearly related to the spatial extent of heat mitigation strategy adoption at the spatial scales and baseline meteorology investigated here
Effects of urbanization on regional meteorology and air quality in Southern California
Urbanization has a profound influence on regional meteorology and air quality
in megapolitan Southern California. The influence of urbanization on
meteorology is driven by changes in land surface physical properties and land
surface processes. These changes in meteorology in turn influence air quality
by changing temperature-dependent chemical reactions and emissions,
gasâparticle phase partitioning, and ventilation of pollutants. In this study
we characterize the influence of land surface changes via historical
urbanization from before human settlement to the present day on meteorology and
air quality in Southern California using the Weather Research and Forecasting
Model coupled to chemistry and the single-layer urban canopy model
(WRFâUCMâChem). We assume identical anthropogenic emissions for the
simulations carried out and thus focus on the effect of changes in land
surface physical properties and land surface processes on air quality.
Historical urbanization has led to daytime air temperature decreases of up to
1.4 K and evening temperature increases of up to 1.7 K. Ventilation of air
in the LA basin has decreased up to 36.6 % during daytime and increased
up to 27.0 % during nighttime. These changes in meteorology are mainly
attributable to higher evaporative fluxes and thermal inertia of soil from
irrigation and increased surface roughness and thermal inertia from
buildings. Changes in ventilation drive changes in hourly
NOx concentrations with increases of up to 2.7 ppb during
daytime and decreases of up to 4.7 ppb at night. Hourly O3
concentrations decrease by up to 0.94 ppb in the morning and increase by up
to 5.6 ppb at other times of day. Changes in O3 concentrations are
driven by the competing effects of changes in ventilation and precursor
NOx concentrations. PM2.5 concentrations show slight
increases during the day and decreases of up to 2.5 ”g mâ3
at night. Process drivers for changes in PM2.5 include modifications
to atmospheric ventilation and temperature, which impact gasâparticle phase
partitioning for semi-volatile compounds and chemical reactions.
Understanding process drivers related to how land surface changes effect
regional meteorology and air quality is crucial for decision-making on urban
planning in megapolitan Southern California to achieve regional climate
adaptation and air quality improvements.</p
A Numerical Investigation into the Anomalous Slight NOx Increase When Burning Biodiesel; A New (Old
Biodiesel is a notable alternative to petroleum derived diesel fuel because it comes from natural domestic sources and thus reduces dependence on diminishing petroleum fuel from foreign sources, it likely lowers lifecycle greenhouse gas emissions, and it lowers an engineâs emission of most pollutants as compared to petroleum derived diesel. However, the use of biodiesel often slightly increases a diesel engineâs emission of smog forming nitrogen oxides (NOx) relative to petroleum diesel. In this paper, previously proposed theories for this slight NOx increase are reviewed, including theories based on biodieselâs cetane number, which leads to differing amounts of charge preheating, and theories based on the fuelâs bulk modulus, which affects injection timing. This paper proposes an additional theory for the slight NOx increase of biodiesel. Biodiesel typically contains more double bonded molecules than petroleum derived diesel. These double bonded molecules have a slightly higher adiabatic flame temperature, which leads to the increase in NOx production for biodiesel. Our theory was verified using numerical simulations to show a NOx increase, due to the double bonded molecules, that is consistent with observation. Further, the details of these numerical simulations show that NOx is predominantly due to the Zeldovich mechanism
Pulse Control of Decoherence with Population Decay
The pulse control of decoherence in a qubit interacting with a quantum
environment is studied with focus on a general case where decoherence is
induced by both pure dephasing and population decay. To observe how the
decoherence is suppressed by periodic pi pulses, we present a simple method to
calculate the time evolution of a qubit under arbitrary pulse sequences
consisting of bit-flips and/or phase-flips. We examine the effectiveness of the
two typical sequences: bb sequence consisting of only bit-flips, and bp
sequence consisting of both bit- and phase-flips. It is shown that the
effectiveness of the pulse sequences depends on a relative strength of the two
decoherence processes especially when a pulse interval is slightly shorter than
qubit-environment correlation times. In the short-interval limit, however, the
bp sequence is always more effective than, or at least as effective as, the bb
sequence.Comment: 11 pages, 7 figure
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Trends in on-Road Vehicle Emissions of Ammonia
Motor vehicle emissions of ammonia have been measured at a California highway tunnel in the San Francisco Bay area. Between 1999 and 2006, light-duty vehicle ammonia emissions decreased by 38 {+-} 6%, from 640 {+-} 40 to 400 {+-} 20 mg kg{sup -1}. High time resolution measurements of ammonia made in summer 2001 at the same location indicate a minimum in ammonia emissions correlated with slower-speed driving conditions. Variations in ammonia emission rates track changes in carbon monoxide more closely than changes in nitrogen oxides, especially during later evening hours when traffic speeds are highest. Analysis of remote sensing data of Burgard et al. (Environ Sci. Technol. 2006, 40, 7018-7022) indicates relationships between ammonia and vehicle model year, nitrogen oxides, and carbon monoxide. Ammonia emission rates from diesel trucks were difficult to measure in the tunnel setting due to the large contribution to ammonia concentrations in a mixed-traffic bore that were assigned to light-duty vehicle emissions. Nevertheless, it is clear that heavy-duty diesel trucks are a minor source of ammonia emissions compared to light-duty gasoline vehicles
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Optical and Physical Properties from Primary On-Road Vehicle ParticleEmissions And Their Implications for Climate Change
During the summers of 2004 and 2006, extinction and scattering coefficients of particle emissions inside a San Francisco Bay Area roadway tunnel were measured using a combined cavity ring-down and nephelometer instrument. Particle size distributions and humidification were also measured, as well as several gas phase species. Vehicles in the tunnel traveled up a 4% grade at a speed of approximately 60 km h{sup -1}. The traffic situation in the tunnel allows the apportionment of emission factors between light duty gasoline vehicles and diesel trucks. Cross-section emission factors for optical properties were determined for the apportioned vehicles to be consistent with gas phase and particulate matter emission factors. The absorption emission factor (the absorption cross-section per mass of fuel burned) for diesel trucks (4.4 {+-} 0.79 m{sup 2} kg{sup -1}) was 22 times larger than for light-duty gasoline vehicles (0.20 {+-} 0.05 m{sup 2} kg{sup -1}). The single scattering albedo of particles - which represents the fraction of incident light that is scattered as opposed to absorbed - was 0.2 for diesel trucks and 0.3 for light duty gasoline vehicles. These facts indicate that particulate matter from motor vehicles exerts a positive (i.e., warming) radiative climate forcing. Average particulate mass absorption efficiencies for diesel trucks and light duty gasoline vehicles were 3.14 {+-} 0.88 m{sup 2} g{sub PM}{sup -1} and 2.9 {+-} 1.07 m{sup 2} g{sub PM}{sup -1}, respectively. Particle size distributions and optical properties were insensitive to increases in relative humidity to values in excess of 90%, reinforcing previous findings that freshly emitted motor vehicle particulate matter is hydrophobic
Evaluation Research and Institutional Pressures: Challenges in Public-Nonprofit Contracting
This article examines the connection between program evaluation research and decision-making by public managers. Drawing on neo-institutional theory, a framework is presented for diagnosing the pressures and conditions that lead alternatively toward or away the rational use of evaluation research. Three cases of public-nonprofit contracting for the delivery of major programs are presented to clarify the way coercive, mimetic, and normative pressures interfere with a sound connection being made between research and implementation. The article concludes by considering how public managers can respond to the isomorphic pressures in their environment that make it hard to act on data relating to program performance.This publication is Hauser Center Working Paper No. 23. The Hauser Center Working Paper Series was launched during the summer of 2000. The Series enables the Hauser Center to share with a broad audience important works-in-progress written by Hauser Center scholars and researchers
Sensible heat has significantly affected the global hydrological cycle over the historical period
Globally, latent heating associated with a change in precipitation is balanced by changes to atmospheric radiative cooling and sensible heat fluxes. Both components can be altered by climate forcing mechanisms and through climate feedbacks, but the impacts of climate forcing and feedbacks on sensible heat fluxes have received much less attention. Here we show, using a range of climate modelling results, that changes in sensible heat are the dominant contributor to the present global-mean precipitation change since preindustrial time, because the radiative impact of forcings and feedbacks approximately compensate. The model results show a dissimilar influence on sensible heat and precipitation from various drivers of climate change. Due to its strong atmospheric absorption, black carbon is found to influence the sensible heat very differently compared to other aerosols and greenhouse gases. Our results indicate that this is likely caused by differences in the impact on the lower tropospheric stability
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