1,249 research outputs found
Direct observation of changing NO_x lifetime in North American cities
NO_x lifetime relates nonlinearly to its own concentration; therefore, by observing how NO_x lifetime changes with changes in its concentration, inferences can be made about the dominant chemistry occurring in an urban plume. We used satellite observations of NOâ‚‚ from a new high-resolution product to show that NOx lifetime in approximately 30 North American cities has changed between 2005 and 2014 in a manner consistent with our understanding of NO_x chemistry
Trends in Elasticity and Electronic Structure of Transition-Metal Nitrides and Carbides from First Principles
The elastic properties of the -structured transition-metal nitrides and
their carbide counterparts are studied using the {\it ab initio\} density
functional perturbation theory. The linear response results of elastic
constants are in excellent agreement with those obtained from numerical
derivative methods, and are also consistent with measured data. We find the
following trends: (1) Bulk moduli and tetragonal shear moduli
, increase and lattice constants decrease
rightward or downward on the Periodic Table for the metal component or if C is
replaced by N; (2) The inequality holds for
; (3) depends strongly on the number of valence electrons per
unit cell (). From the fitted curve of as a function of , we
can predict that MoN is unstable in structure, and transition-metal
carbonitrides ( ZrCN) and di-transition-metal carbides
( HfTaC) have maximum at .Comment: 4 pages, 2 figures, submitted to PRL. 2 typos in ref. 15 were
correcte
Lightning NOâ‚‚ simulation over the contiguous US and its effects on satellite NOâ‚‚ retrievals
Lightning is an important NO_x source representing ∼10 % of the global source of odd N and a much larger percentage in the upper troposphere. The poor understanding of spatial and temporal patterns of lightning contributes to a large uncertainty in understanding upper tropospheric chemistry. We implement a lightning parameterization using the product of convective available potential energy (CAPE) and convective precipitation rate (PR) coupled with the Kain–Fritsch convective scheme (KF/CAPE-PR) into the Weather Research and Forecasting-Chemistry (WRF-Chem) model. Compared to the cloud-top height (CTH) lightning parameterization combined with the Grell 3-D convective scheme (G3/CTH), we show that the switch of convective scheme improves the correlation of lightning flash density in the southeastern US from 0.30 to 0.67 when comparing against the Earth Networks Total Lightning Network; the switch of lightning parameterization contributes to the improvement of the correlation from 0.48 to 0.62 elsewhere in the US. The simulated NO₂ profiles using the KF/CAPE-PR parameterization exhibit better agreement with aircraft observations in the middle and upper troposphere. Using a lightning NO_x production rate of 500 mol NO flash−1, the a priori NO₂ profile generated by the simulation with the KF/CAPE-PR parameterization reduces the air mass factor for NO₂ retrievals by 16 % on average in the southeastern US in the late spring and early summer compared to simulations using the G3/CTH parameterization. This causes an average change in NO₂ vertical column density 4 times higher than the average uncertainty
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