Nitrogen oxides in the lower troposphere catalyze the photochemical production of ozone (O₃) pollution during the day but react to form nitric acid, oxidize hydrocarbons, and remove O₃ at night. A ...key nocturnal reaction is the heterogeneous hydrolysis of dinitrogen pentoxide, N₂O₅. We report aircraft measurements of NO₃ and N₂O₅, which show that the N₂O₅ uptake coefficient, g(N₂O₅), on aerosol particles is highly variable and depends strongly on aerosol composition, particularly sulfate content. The results have implications for the quantification of regional-scale O₃ production and suggest a stronger interaction between anthropogenic sulfur and nitrogen oxide emissions than previously recognized.
A laser photoacoustic spectroscopy (LPAS) instrument was developed and used for aircraft measurements of ethene from industrial sources near Houston, Texas. The instrument provided 20 s measurements ...with a detection limit of less than 0.7 ppbv. Data from this instrument and from the GC-FID analysis of air samples collected in flight agreed within 15% on average. Ethene fluxes from the Mt. Belvieu chemical complex to the northeast of Houston were quantified during 10 different flights. The average flux was 520 ± 140 kg h−1 in agreement with independent results from solar occultation flux (SOF) measurements, and roughly an order of magnitude higher than regulatory emission inventories indicate. This study shows that ethene emissions are routinely at levels that qualify as emission upsets, which need to be reported to regional air quality managers.
We have developed a genetics-based phytoremediation strategy for arsenic in which the oxyanion arsenate is transported aboveground, reduced to arsenite, and sequestered in thiol-peptide complexes. ...The Escherichia coli arsC gene encodes arsenate reductase (ArsC), which catalyzes the glutathione (GSH)-coupled electrochemical reduction of arsenate to the more toxic arsenite. Arabidopsis thaliana plants transformed with the arsC gene expressed from a light-induced soybean rubisco promoter (SRS1p) strongly express ArsC protein in leaves, but not roots, and were consequently hypersensitive to arsenate. Arabidopsis plants expressing the E. coli gene encoding gamma-glutamylcysteine synthetase (gamma-ECS) from a strong constitutive actin promoter (ACT2p) were moderately tolerant to arsenic compared with wild type. However, plants expressing SRS1p/ArsC and ACT2p/gamma-ECS together showed substantially greater arsenic tolerance than gamma-ECS or wild-type plants. When grown on arsenic, these plants accumulated 4- to 17-fold greater fresh shoot weight and accumulated 2- to 3-fold more arsenic per gram of tissue than wild type or plants expressing gamma-ECS or ArsC alone. This arsenic remediation strategy should be applicable to a wide variety of plant species.
Data taken in aircraft transects of emissions plumes from rural U.S. coal-fired power plants were used to confirm and quantify the nonlinear dependence of tropospheric ozone formation on plume NOx(NO ...plus NO2) concentration, which is determined by plant NOxemission rate and atmospheric dispersion. The ambient availability of reactive volatile organic compounds, principally biogenic isoprene, was also found to modulate ozone production rate and yield in these rural plumes. Differences of a factor of 2 or greater in plume ozone formation rates and yields as a function of NOxand volatile organic compound concentrations were consistently observed. These large differences suggest that consideration of power plant NOxemission rates and geographic locations in current and future U.S. ozone control strategies could substantially enhance the efficacy of NOxreductions from these sources.
Nitrogen oxides are important to the regulation of ozone throughout the Earth's atmosphere. Of particular interest for regional air quality is photochemical production and nocturnal destruction of O3 ...in the lower troposphere in high NOx(=NO + NO2) environments. Nocturnal tropospheric odd oxygen (Ox), defined as O3 + NO2 + 2NO3 + 3N2O5, is used to assess the impact of NOx emissions on nocturnal O3 loss. Recent aircraft measurements of the components of Ox and HNO3 yield a detailed accounting of the nocturnal Ox budget in a regionally polluted environment. The analysis demonstrates the role of NO3 and N2O5 reactions in nocturnal O3 destruction and shows that multiple factors, including timing of NOx emissions, hydrocarbon and aerosol loading, seasonality and atmospheric mixing, govern the impact of NOx emissions on regional‐scale air quality.
During the summer of 1995, especially between June and mid July, extensive wildfires occurred throughout Canada, primarily north of 55°N latitude. A previous report used aircraft and surface ...observations and tracer simulations to show these fires strongly influenced CO concentrations as far south as 35°N in the central and eastern United States Wotawa and Trainer, 2000. This study extends those results by incorporating wildfire emissions estimates for CO, NOx, and nonmethane hydrocarbons into a three‐dimensional photochemical transport model specifically designed to simulate ozone photochemistry in the continental United States. The results of the model are compared to observations from four measurement platforms deployed during the time period of interest: National Oceanic and Atmospheric Administration WP‐3 aircraft observations collected during the 1995 Southern Oxidants Study (SOS‐95) field campaign; 12 eastern U.S. surface stations that measured ozone, CO, and NOy; rural ozone measurements from the Aerometric Information Retrieval System network collected by the U.S. Environmental Protection Agency; and daily ozonesondes obtained near Nashville, Tennessee, during SOS‐95. Model performance, as determined by correlation and bias with observations from these four platforms, is significantly improved for both O3 and CO when the Canadian fires are considered. Both observations and model results show enhanced O3 from air transported from the Northwest Territory. The model results imply that during the period of strongest fire influence 10 to 30 ppbv enhancement of O3 throughout a large region of the central and eastern United States was due to these fires. Modeled O3 increases are sensitive to the NOx/CO emission ratio assumed for the fires, which is highly uncertain and variable. A molar NOx/CO ratio of 0.007 yields model comparisons that are most consistent for O3 and ΔO3/ΔCO observations within aged fire plumes during SOS‐95, and is also consistent with previously observed NOx/CO ratios from boreal fires. For this NOx/CO emission ratio, and considering the entire eastern United States, most of the O3 increase is associated with the NOx emitted directly by the fires and the photochemical O3 formation that occurs before the plumes actually reach the United States. However, the in situ oxidation of CO from the Canadian fires with NOx emitted locally leads to significantly higher O3 increases for high‐NOx‐emitting regions that are limited by hydrocarbon availability. Thus O3 in urban areas, or any other region modified by nearby NOx sources, is more sensitive to long‐range fires compared to less populated or polluted regions.
We examined the ability of yellow poplar (Liriodendron tulipifera) tissue cultures and plantlets to express modified mercuric reductase (merA) gene constructs. Mercury-resistant bacteria express merA ...to convert highly toxic, ionic mercury, Hg(II), to much less toxic, elemental mercury, Hg(O). Expression of merA in transgenic plants might provide an ecologically compatible approach for the remediation of mercury pollution. Because the alteration of the bacterial merA gene sequence is necessary for high-level expression in Arabidopsis thaliana, yellow poplar proembryogenic masses (PEMs) were transformed with three modified merA constructs via microprojectile bombardment. Each construct was synthesized to have altered flanking regions with increasing amounts of modified coding sequence. All merA constructs conferred resistance to toxic, ionic mercury in independently transformed PEM colonies. Stability of merA transgene expression increased in parallel with the extent of gene coding sequence modification. Regenerated plantlets containing the most modified merA gene (merA18) germinated and grew vigorously in media containing normally toxic levels of ionic mercury. The merA18 plantlets released elemental mercury at approximately 10 times the rate of untransformed plantlets. These results indicate that plants expressing modified merA constructs may provide a means for the phytoremediation of mercury pollution.
The Second Texas Air Quality Study (TexAQS II) was conducted in eastern Texas during 2005 and 2006. This 2‐year study included an intensive field campaign, TexAQS 2006/Gulf of Mexico Atmospheric ...Composition and Climate Study (GoMACCS), conducted in August–October 2006. The results reported in this special journal section are based on observations collected on four aircraft, one research vessel, networks of ground‐based air quality and meteorological (surface and radar wind profiler) sites in eastern Texas, a balloon‐borne ozonesonde‐radiosonde network (part of Intercontinental Transport Experiment Ozonesonde Network Study (IONS‐06)), and satellites. This overview paper provides operational and logistical information for those platforms and sites, summarizes the principal findings and conclusions that have thus far been drawn from the results, and directs readers to appropriate papers for the full analysis. Two of these findings deserve particular emphasis. First, despite decreases in actual emissions of highly reactive volatile organic compounds (HRVOC) and some improvements in inventory estimates since the TexAQS 2000 study, the current Houston area emission inventories still underestimate HRVOC emissions by approximately 1 order of magnitude. Second, the background ozone in eastern Texas, which represents the minimum ozone concentration that is likely achievable through only local controls, can approach or exceed the current National Ambient Air Quality Standard of 75 ppbv for an 8‐h average. These findings have broad implications for air quality control strategies in eastern Texas.
Volatile organic compounds (VOCs) and some of their oxidants (O3, NO3) were measured on board the National Oceanic and Atmospheric Administration research ship R/V Ronald H. Brown along the coast of ...New England, downwind of New York, Boston, and Portsmouth and large forested areas in New Hampshire, Maine, and Massachusetts in July and August 2002. The diurnal variations of isoprene, monoterpenes, and aromatics were mainly dependent on their emissions and the abundance of the oxidants OH and NO3. Elevated mixing ratios of short‐lived VOCs were only encountered at the ship, which was about 1–6 hours downwind of the sources, when the concentrations of the oxidants were low. For the biogenic compounds this was generally the case during morning and evening hours, when the lifetime of the biogenics was long because of low OH and NO3 concentrations. Most anthropogenic VOCs do not react with NO3, and therefore their mixing ratios remained elevated during the night. The products of isoprene oxidation, methyl vinyl ketone, methacrolein, and peroxymethacrylic nitric anhydride (MPAN) were, on average, more abundant than isoprene itself. Only during the transition periods from day to night, when oxidation rates were at a minimum, could isoprene exceed its products. The loss of the biogenic VOCs was dominated by reactions with NO3, whereas the loss of anthropogenics came mostly from reactions with OH. The oxygenated VOCs are the major contributor to the OH loss, except in close vicinity of emission sources. The total loss of biogenic compounds during the night was so effective that after one night of transport they were in most cases completely reacted away, whereas the mixing ratios of the anthropogenic compounds remained high during the night. The pool of reactive hydrocarbons at sunrise was thus typically dominated by anthropogenic VOCs.
Accurate emission inventories and their temporal trends must be incorporated into pollutant inventories to allow for reliable modeling of the country's past, present, and future air quality. Measured ...carbon monoxide (CO) and nitrogen oxide (NOx) concentrations from two urban areas show that the CO/NOx vehicular emission ratio has decreased at an average rate of 7–9% per year from 1987 to 1999. This amounts to a factor of nearly 3 over the 12 years. The current U.S. Environmental Protection Agency tabulations of estimated pollutant emission trends indicate a rate of decrease smaller by a factor of 2–3. The trend in maximum ambient CO levels in U.S. cities suggests a 5.2 ± 0.8% per year average annual decrease in CO vehicular emissions, which implies a 2–3% annual increase in NOx emissions from vehicles. Thus over the decade of the 1990s, annual U.S. CO emissions from vehicles have decreased from ∼65 to ∼38 Tg, representing approximate decreases of 6 and 3% in the annual global fuel‐use CO emissions and in total global anthropogenic CO emissions, respectively. It is expected that the volatile organic compound (VOC)/NOx vehicular exhaust emission ratio has decreased similarly, implying that the character of atmospheric photochemistry in U.S. urban areas has changed significantly over the decade.
PDF
