Reactive nitrogen in Mexico City and its relation to ozone-precursor sensitivity: results from photochemical models

We use results of a 3-D photochemistry/transport model for ozone formation in Mexico City during events in 1997 to investigate ambient concentrations of reactive nitrogen in relation to ozone-precursor sensitivity. Previous results from other locations suggest that ratios such as O<sub>3</sub>/NO<sub>y</sub> and H<sub>2</sub>O<sub>2</sub>/HNO<sub>3</sub> might provide measurement-based indicators for NO<sub>x</sub>-sensitive or VOC-sensitive conditions. Mexico City presents a different environment due to its high concentrations of VOC and high level of pollutants in general. The model predicts a correlation between PAN and O<sub>3</sub> with relatively high PAN/O<sub>3</sub> (0.07), which is still lower than measured values. The model PAN is comparable with results from a model for Paris but much higher than were found in Nashville in both models and measurements. The difference is due in part to the lower temperature in Mexico City relative to Nashville. Model HNO<sub>3</sub> in Mexico City is unusually low for an urban area and PAN/HNO<sub>3</sub> is very high, probably due to the high ratio of reactivity-weighted VOC to NO<sub>x</sub>. The model predicts that VOC-sensitive chemistry in Mexico is associated with high NO<sub>x</sub>, NO<sub>y</sub> and NO<sub>x</sub>/NO<sub>y</sub> and with low O<sub>3</sub>/NO<sub>y</sub> and H<sub>2</sub>O<sub>2</sub>/HNO<sub>3</sub>, suggesting that these indicators work well for Mexico City. The relation between ozone-precursor sensitivity and either O<sub>3</sub>/NO<sub>z</sub> or O<sub>3</sub>/HNO<sub>3</sub> is more ambiguous. VOC-sensitive conditions are associated with higher O<sub>3</sub>/HNO<sub>3</sub> than would be found in NO<sub>x</sub>-sensitive conditions, but model O<sub>3</sub>/HNO<sub>3</sub> associated with both NO<sub>x</sub>-sensitive and VOC-sensitive chemistry is higher in Mexico than in other cities. The model predicts a mixed pattern of ozone-precursor sensitivity in Mexico City, with VOC-sensitive conditions in the morning and NO<sub>x</sub>-sensitive in the afternoon, in contrast to results from other models for more recent events that predicted strongly VOC- sensitive conditions throughout the day. The difference in predicted ozone-precursor sensitivity is most likely due to different emission rates and to changes in emissions over time. The model with mixed sensitivity predicts much lower ambient NO<sub>x</sub> and NO<sub>x</sub>/NO<sub>y</sub> than the strongly VOC-sensitive model

Development of a secondary organic aerosol formation mechanism: comparison with smog chamber experiments and atmospheric measurements

International audienceA new mechanism to simulate the formation of secondary organic aerosols (SOA) from reactive primary hydrocarbons is presented, together with comparisons with experimental smog chamber results and ambient measurements found in the literature. The SOA formation mechanism is based on an approach using calculated vapor pressures and a selection of species that can partition to the aerosol phase from a gas phase photochemical mechanism. The mechanism has been validated against smog chamber measurements using ?-pinene, xylene and toluene as SOA precursors, and has an average error of 17%. Qualitative comparisons with smog chamber measurements using isoprene were also performed. A comparison against SOA production in the TORCH 2003 experiment (atmospheric measurements) had an average error of only 12%. This contrasts with previous efforts, in which it was necessary to increase partition coefficients by a factor of 500 in order to match the observed values. Calculations for rural and urban-influenced regions in the eastern U.S. suggest that most of the SOA is biogenic in origin, mainly originated from isoprene. A 0-dimensional calculation based on the New England Air Quality Study also showed good agreement with measured SOA, with about 40% of the total SOA from anthropogenic precursors. This mechanism can be implemented in a general circulation model (GCM) to estimate global SOA formation under ambient NOx and HOx levels

Naturally occurring highly pathogenic avian influenza virus H5N1 clade 2.3.4.4b infection in three domestic cats in North America during 2023

The Eurasian strain of highly pathogenic avian influenza (HPAI) H5N1 is a devastating pathogen for birds that also has the capacity to infect mammals. This report describes the presentation, clinical case findings (including haemogram and serum biochemistry), gross and microscopic lesions and virus detection in three HPAI H5N1-infected domestic cats from the USA in 2023. All three cats presented with neurological abnormalities and were euthanized due to a poor prognosis within 2 days (two cats) or 10 days (one cat) of known clinical disease onset. Necropsy consistently revealed pulmonary congestion and oedema, and cerebrocortical malacia with haemorrhage was also seen in the cat that survived for 10 days. On histology, all cats had necrotizing encephalitis and interstitial pneumonia with pulmonary congestion, oedema, vasculitis and vascular thrombosis. One cat also had microscopic multifocal necrosis in the liver, pancreas and an adrenal gland. To our knowledge, this report is the first to detail pathological findings in HPAI H5N1 naturally-infected cats during the widespread outbreak in North America beginning in 2021, and that describes a cat surviving for 10 days after onset of HPAI H5N1 encephalitis

Photochemical production and loss rates of ozone at Sable Island, Nova Scotia during the North Atlantic Regional Experiment (NARE) 1993 summer intensive

Three weeks of summertime surface‐based chemical and meteorological observations at Sable Island, Nova Scotia during the North Atlantic Regional Experiment (NARE) 1993 summer intensive are used to study instantaneous photochemical production and loss rates of ozone by means of a numerical photochemical model. Results are most sensitive to the averaging scheme of data used to constrain the model and the ambient variability of the measurements. Model simulations driven by a time series of 5 min averaged data, most representative of the chemistry at the site, yield an average net photochemical ozone production of 3.6 ppbv/d. Estimates of net ozone production designed to filter out local sources, by using 1000–1400 LT median values of observations to drive the model and by excluding short‐lived hydrocarbons, give values ranging from 1 to 4 ppbv/d. These positive values of net ozone production within the marine boundary layer over Sable Island demonstrate the impact of polluted continental plumes on the background photochemistry of the region during the intensive. The dominant ambient variables controlling photochemical production and loss rates of ozone at the site during the measurement campaign appear to be levels of nitrogen oxides, ozone, nonmethane hydrocarbons, and solar intensity determined by cloud cover. The model partitioning of nitrogen oxides agrees for the most part with measurements, lending credence to calculated photochemical production and loss rates of ozone as well as inferred levels of peroxy radicals not measured at the site. Discrepancies, however, often occur during episodes of intermittent cloud cover, fog, and rain, suggesting the influence of cloud processes on air masses reaching the site

Power-Based Droop Control in DC Microgrids Enabling Seamless Disconnection From Upstream Grids

This paper proposes a local power-based droop controller for distributed energy resource converters in dc microgrids that are connected to upstream grids by grid-interface converters. During normal operation, the grid-interface converter imposes the microgrid bus voltage, and the proposed controller allows power flow regulation at distributed energy resource converters\u2019 output. On the other hand, during abnormal operation of the grid-interface converter (e.g., due to faults in the upstream grid), the proposed controller allows bus voltage regulation by droop control. Notably, the controller can autonomously convert from power flow control to droop control, without any need of bus voltage variation detection schemes or communication with other microgrid components, which enables seamless transitions between these two modes of operation. Considering distributed energy resource converters employing the power-based droop control, the operation modes of a single converter and of the whole microgrid are defined and investigated herein. The controller design is also introduced. Furthermore, the power sharing performance of this control approach is analyzed and compared with that of classical droop control. The experimental results from a laboratory-scale dc microgrid prototype are reported to show the final performances of the proposed power-based droop control

Least independent variables method for simulation of tropospheric ozone

We describe a method for simulating photochemical production of O in a continental‐scale tropospheric model with only six independent chemical variables representing tracers transported in the model. The tracers are two primary hydrocarbon families, CO, NO, peroxyacylnitrates, and odd oxygen (O). A chemical module is developed to compute the production and loss rates of tracers over a model time step of 4 hours, using only information on the tracer concentrations input to the module at the beginning of the time step. Test simulations for summertime conditions at mid‐latitudes indicate little loss in accuracy compared to detailed model simulations of chemistry with high time resolution. Minimization of the number of tracers and use of a long time step reduces computer time and storage requirements. In addition, parameterization of the chemical computation is facilitated, allowing further savings in computer time.Engineering and Applied Science

Investigation of Isoprene Dynamics During the Day‐to‐Night Transition Period

At the University of Michigan Biological Station during the 2016 AMOS field campaign, isoprene concentrations typically peak in the early afternoon (around 15:00 local time, LT) under well‐mixed conditions. However, an end‐of‐day peak (around 21:00 LT) occurs on 23% of the campaign days, followed by a rapid removal (from 21:00–22:00 LT) at rate of 0.57 hr−1 during the day‐to‐night transition period. During the end‐of‐day peak, in‐canopy isoprene concentrations increase by 77% (from 3.5 to 6.2 ppbv) on average. Stratification and weak winds (<3.4 m s−1 at 46 m) significantly suppress turbulent exchanges between in‐ and above‐canopy, leading to accumulation of isoprene emitted at dusk. A critical standard deviation of the vertical velocity (σw) of 0.14, 0.2, and 0.29 m s−1 is identified to detect the end‐of‐day peak for the height of 13, 21, and 34 m, respectively. In 85% of the end‐of‐day cases, the wind speed increases above 2.5 m s−1 after the peak along with a shift in wind direction, and turbulence is reestablished. Therefore, the wind speed of 2.5 m s−1 is considered as the threshold point where turbulence switches from being independent of wind speed to dependent on wind speed. The reinstated turbulence accounts for 80% of the subsequent isoprene removal with the remaining 20% explained by chemical reactions with hydroxyl radicals, ozone, and nitrate radicals. Observed isoprene fluxes do not support the argument that the end‐of‐day peak is reduced by vertical turbulent mixing, and we hypothesize that horizontal advection may play a role.Key PointsLow turbulent mixing during clear and calm nights leads to accumulation of isoprene within the canopyTurbulent mixing accounts for 80% of the observed nighttime isoprene loss ratesIsoprene flux measurements did not capture the majority of the removal of the accumulated isoprenePeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/163406/2/jgrd56554.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/163406/1/jgrd56554_am.pd

Visual pigments, ocular filters and the evolution of snake vision

Much of what is known about the molecular evolution of vertebrate vision comes from studies of mammals, birds and fish. Reptiles (especially snakes) have barely been sampled in previous studies despite their exceptional diversity of retinal photoreceptor complements. Here we analyse opsin gene sequences and ocular media transmission for up to 69 species to investigate snake visual evolution. Most snakes express three visual opsin genes (rh1, sws1, lws). These opsin genes (especially rh1 and sws1) have undergone much evolutionary change, including modifications of amino acid residues at sites of known importance for spectral tuning, with several tuning site combinations unknown elsewhere among vertebrates. These changes are particularly common among dipsadine and colubrine ‘higher’ snakes. All three opsin genes are inferred to be under purifying selection, though dN/dS varies with respect to some lineages, ecologies, and retinal anatomy. Positive selection was inferred at multiple sites in all three opsins, these being concentrated in transmembrane domains and thus likely to have a substantial effect on spectral tuning and other aspects of opsin function. Snake lenses vary substantially in their spectral transmission. Snakes active at night and some of those active by day have very transmissive lenses, while some primarily diurnal species cut out shorter wavelengths (including UVA). In terms of retinal anatomy, lens transmission, visual pigment spectral tuning and opsin gene evolution the visual system of snakes is exceptionally diverse compared to all other extant tetrapod orders

Sequential LASER ART and CRISPR treatments eliminate HIV-1 in a subset of infected humanized mice

Elimination of HIV-1 requires clearance and removal of integrated proviral DNA from infected cells and tissues. Here, sequential long-acting slow-effective release antiviral therapy (LASER ART) and CRISPR-Cas9 demonstrate viral clearance in latent infectious reservoirs in HIV-1 infected humanized mice. HIV-1 subgenomic DNA fragments, spanning the long terminal repeats and the Gag gene, are excised in vivo, resulting in elimination of integrated proviral DNA; virus is not detected in blood, lymphoid tissue, bone marrow and brain by nested and digital-droplet PCR as well as RNAscope tests. No CRISPR-Cas9 mediated off-target effects are detected. Adoptive transfer of human immunocytes from dual treated, virus-free animals to uninfected humanized mice fails to produce infectious progeny virus. In contrast, HIV-1 is readily detected following sole LASER ART or CRISPR-Cas9 treatment. These data provide proof-of-concept that permanent viral elimination is possible