Recent decreases in nitrogen oxide (NOx = NO + NO2) emissions from eastern U.S. power plants and their effects on regional ozone are studied. Using the EPA 1999 National Emission Inventory as a ...reference emission data set, NOx and sulfur dioxide (SO2) emission rates at selected power plants are updated to their summer 2003 levels using Continuous Emission Monitoring System (CEMS) measurements. The validity of the CEMS data is established by comparison to observations made on the NOAA WP‐3 aircraft as part of the 2004 New England Air Quality Study. The impacts of power plant NOx emission decreases on O3 are investigated using the WRF‐Chem regional chemical forecast model. Summertime NOx emission rates decreased by approximately 50% between 1999 and 2003 at the subset of power plants studied. The impact of NOx emission reductions on ozone was moderate during summer 2004 because of relatively cool temperatures and frequent synoptic disturbances. Effects in individual plant plumes vary depending on the plant's NOx emission strength, the proximity of other NOx sources, and the availability of volatile organic compounds (VOCs) and sunlight. This study provides insight into the ozone changes that can be anticipated as power plant NOx emission reductions continue to be implemented throughout the United States.
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.
Petrochemical industrial facilities can emit large amounts of highly reactive hydrocarbons and NOx to the atmosphere; in the summertime, such colocated emissions are shown to consistently result in ...rapid and efficient ozone (O3) formation downwind. Airborne measurements show initial hydrocarbon reactivity in petrochemical source plumes in the Houston, TX, metropolitan area is primarily due to routine emissions of the alkenes propene and ethene. Reported emissions of these highly reactive compounds are substantially lower than emissions inferred from measurements in the plumes from these sources. Net O3 formation rates and yields per NOx molecule oxidized in these petrochemical industrial source plumes are substantially higher than rates and yields observed in urban or rural power plant plumes. These observations suggest that reductions in reactive alkene emissions from petrochemical industrial sources are required to effectively address the most extreme O3 exceedences in the Houston metropolitan area.
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.
Environmental samples typically contain hundreds or thousands of unique organic compounds, and even minor components may provide valuable insight
into their sources and transformations. To understand ...atmospheric processes, individual components are frequently identified and quantified using
gas chromatography–mass spectrometry. However, due to the complexity and frequently variable nature of such data, data reduction is a significant
bottleneck in analysis. Consequently, only a subset of known analytes is often reported for a dataset, and large amounts of potentially useful data
are discarded. We present an automated approach of cataloging and potentially identifying all analytes in a large chromatographic dataset and
demonstrate the utility of our approach in an analysis of ambient aerosols. We use a coupled factor analysis–decision tree approach to deconvolute
peaks and comprehensively catalog nearly all analytes in a dataset. Positive matrix factorization (PMF) of small subsections of multiple
chromatograms is applied to extract factors that represent chromatographic profiles and mass spectra of potential analytes, in which peaks are
detected. A decision tree based on peak parameters (e.g., location, width, and height), relative ratios of those parameters, peak shape, noise,
retention time, and mass spectrum is applied to discard erroneous peaks and combine peaks determined to represent the same analyte. With our
approach, all analytes within the small section of the chromatogram are cataloged, and the process is repeated for overlapping sections across the
chromatogram, generating a complete list of the retention times and estimated mass spectra of all peaks in a dataset. We validate this approach
using samples of known compounds and demonstrate the separation of poorly resolved peaks with similar mass spectra and the resolution of peaks that
appear in only a fraction of chromatograms. As a case study, this method is applied to a complex real-world dataset of the composition of
atmospheric particles, in which more than 1100 unique chromatographic peaks are resolved, and the corresponding peak information along with mass spectra are cataloged.
Photochemical and transport processes involving reactive nitrogen compounds were studied in plumes of urban pollutants using measurements obtained from the NOAA WP‐3 aircraft during the ICARTT study ...in July and August 2004. Observations close to Boston and New York City were used to characterize urban emissions, and plume transport and transformation processes were studied in aged plumes located up to 1000 km downwind from the east coast of North America. Pollution was observed primarily below 1.5 km altitude in well‐defined layers that were decoupled from the marine boundary layer. In aged plumes located over the North Atlantic Ocean, the nitric acid (HNO3) mixing ratios were large (up to 50 ppbv), and HNO3 accounted for the majority of reactive nitrogen. Plume CO and reactive nitrogen enhancement ratios were nearly equivalent in fresh and aged plumes. Efficient transport of HNO3 explained the observed trace gas ratios and abundances. Without substantial HNO3 loss, the ratio of HNO3 to NOx was between 13 and 42 in most highly aged plumes and sometimes exceeded calculated photochemical steady state values. Box model calculations that include nighttime reactions that convert NOx to HNO3 reproduce the observations. Photolysis and OH oxidation of over 10 ppbv of HNO3 that was in the troposphere for days resulted in reformation of hundreds of pptv of NOx, which is sufficient to maintain photochemical ozone production. Efficient transport of HNO3 over the North Atlantic Ocean for days carried both HNO3 and NOx far from their continental sources and increased their photochemical influence.
Airborne formaldehyde (CH2O) measurements were made by tunable diode laser absorption spectroscopy (TDLAS) at high time resolution (1 and 10 s) and precision (±400 and ±120 parts per trillion by ...volume (pptv) (2σ), respectively) during the Texas Air Quality Study (TexAQS) 2000. Measurement accuracy was corroborated by in‐flight calibrations and zeros and by overflight comparison with a ground‐based differential optical absorption spectroscopy (DOAS) system. Throughout the campaign, the highest levels of CH2O precursors and volatile organic compound (VOC) reactivity were measured in petrochemical plumes. Correspondingly, CH2O and ozone production was greatly enhanced in petrochemical plumes compared with plumes dominated by power plant and mobile source emissions. The photochemistry of several isolated petrochemical facility plumes was accurately modeled using three nonmethane hydrocarbons (NMHCs) (ethene (C2H4), propene (C3H6) (both anthropogenic), and isoprene (C5H8) (biogenic)) and was in accord with standard hydroxyl radical (OH)‐initiated chemistry. Measurement‐inferred facility emissions of ethene and propene were far larger than reported by inventories. Substantial direct CH2O emissions were not detected from petrochemical facilities. The rapid production of CH2O and ozone observed in a highly polluted plume (30+ parts per billion by volume (ppbv) CH2O and 200+ ppbv ozone) originating over Houston was well replicated by a model employing only two NMHCs, ethene and propene.
The concept of ozone production efficiency (OPE) per unit NOx is based on photochemical models and provides a tool with which to assess potential regional tropospheric ozone control strategies ...involving NOx emissions reductions. An aircraft study provided data from which power plant emissions removal rates and measurement‐based estimates of OPE are estimated. This study was performed as part of the Southern Oxidants Study‐1995 Nashville intensive and focuses on the evolution of NOx, SO2, and ozone concentrations in power plant plumes during transport. Two approaches are examined. A mass balance approach accounts for mixing effects within the boundary layer and is used to calculate effective boundary layer removal rates for NOx and SO2 and to estimate net OPE. Net OPE is more directly comparable to photochemical model results than previous measurement‐based estimates. Derived net production efficiencies from mass balance range from 1 to 3 molecules of ozone produced per molecule of NOx emitted. A concentration ratio approach provides an estimate of removal rates of primary emissions relative to a tracer species. This approach can be combined with emissions ratio information to provide upper limit estimates of OPE that range from 2 to 7. Both approaches illustrate the dependence of ozone production on NOx source strength in these large point source plumes. The dependence of total ozone production, ozone production efficiency, and the rate of ozone production on NOx source strength is examined. These results are interpreted in light of potential ozone control strategies for the region.
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