University of Houston

About: University of Houston is a education organization based out in Houston, Texas, United States. It is known for research contribution in the topics: Population & Poison control. The organization has 23074 authors who have published 53903 publications receiving 1641968 citations.

Gas-particle partitioning of atmospheric Hg(II) and its effect on global mercury deposition

Abstract: Atmospheric deposition of Hg(II) represents a major input of mercury to surface environments. The phase of Hg(II) (gas or particle) has important implications for deposition. We use long-term observations of reactive gaseous mercury (RGM, the gaseous component of Hg(II)), particle-bound mercury (PBM, the particulate component of Hg(II)), fine particulate matter (PM 2.5), and temperature (T ) at five sites in North America to derive an empirical gas-particle partitioning relationship log 10(K 1 ) = (10±1)- (2500±300)/T where K = (PBM/PM2.5)/RGM with PBM and RGM in common mixing ratio units, PM2.5 in µg m 3 , and T in K. This relationship is within the range of previ- ous work but is based on far more extensive data from mul- tiple sites. We implement this empirical relationship in the GEOS-Chem global 3-D Hg model to partition Hg(II) be- tween the gas and particle phases. The resulting gas-phase fraction of Hg(II) ranges from over 90 % in warm air with lit- tle aerosol to less than 10 % in cold air with high aerosol. Hg deposition to high latitudes increases because of more effi- cient scavenging of particulate Hg(II) by precipitating snow. Model comparison to Hg observations at the North Ameri- can surface sites suggests that subsidence from the free tro- posphere (warm air, low aerosol) is a major factor driving the seasonality of RGM, while elevated PBM is mostly associ- ated with high aerosol loads. Simulation of RGM and PBM at these sites is improved by including fast in-plume reduc- tion of Hg(II) emitted from coal combustion and by assum- ing that anthropogenic particulate Hg(p) behaves as semi- volatile Hg(II) rather than as a refractory particulate compo- nent. We improve the simulation of Hg wet deposition fluxes in the US relative to a previous version of GEOS-Chem; this largely reflects independent improvement of the washout al- gorithm. The observed wintertime minimum in wet depo- sition fluxes is attributed to inefficient snow scavenging of gas-phase Hg(II).

An Empirical Study of Bond Market Transactions

Abstract: Short histories of dealer-market and exchange-based bond transactions in machine-readable form have recently become available. They permitted us to provide for the first time direct estimation of the effective bid-ask spread for corporate bonds in the institutional and retail markets. Overall, we found effective spreads for NYSE-traded corporate bonds to be similar to effective spreads for dealer-market transactions. Evidence is that corporate bond spreads have declined over time and that dealers carry out U.S. government bond trades with major institutional clients as a nonprofit service, perhaps to support other (ostensibly) profitable activities. We demonstrate that bid-ask spreads and the magnitude of price discrepancies between data sources are reliably associated with proxies for risk and liquidity.