John R. Hannan

A case study of transport of tropical marine boundary layer and lower tropospheric air masses to the northern midlatitude upper troposphere

Low-ozone (<20 ppbv) air masses were observed in the upper troposphere in northern midlatitudes over the eastern United States and the North Atlantic Ocean on several occasions in October 1997 during the NASA Subsonic Assessment, Ozone and Nitrogen Oxide Experiment (SONEX) mission. Three cases of low-ozone air masses were shown to have originated in the tropical Pacific marine boundary layer or lower troposphere and advected poleward along a warm conveyor belt during a synoptic-scale disturbance. The tropopause was elevated in the region with the low-ozone air mass. Stratospheric intrusions accompanied the disturbances. On the basis of storm track and stratospheric intrusion climatologies, such events appear to be more frequent from September through March than the rest of the year.

A meteorological overview of the second Pacific Exploratory Mission in the Tropics

Meteorological conditions over the central Pacific Basin are summarized during NASAs second Pacific Exploratory Mission in the Tropics (PEM-B) which was conducted during February-April 1999. Mean flow patterns during PEM-B are described. Important features near the surface include subtropical anticyclones, the South Pacific Convergence Zone (SPCZ), and the Intertropical Convergence Zone (ITCZ). The ITCZ is found to exhibit a double structure, with branches at ∼5°N and ∼5°S. Both the ITCZ and SPCZ are areas of widespread cloudiness and convection. Extensive lightning occurs over the land masses surrounding the Pacific Basin and over the central South Pacific Ocean itself. PEM-B occurs during a La Nina period of relatively cold sea surface temperatures in the tropical Pacific. Compared to climatology, the PEM-B period exhibits deep convection located west of its typical position, stronger than normal easterly trade winds, a relatively strong (weak) northern (southern) hemispheric jet stream, the SPCZ located west of its normal position, and an upper tropospheric cyclonic wind couplet that straddles the equator. Circulation patterns during PEM-B are compared with those of PEM-A which occurred during August-September 1996. PEM-B is found to exhibit a less organized ITCZ, a comparatively weak jet stream in the Southern Hemisphere, a relatively strong jet stream in the Northern Hemisphere, and enhanced convection over the central Pacific. Finally, meteorological conditions for selected flights are discussed utilizing streamlines, 10-day backward trajectories, thermodynamic soundings, and satellite imagery. Air parcels sampled by the aircraft are found to originate or pass over diverse regions, including Asia, South America, southern Africa, and Australia. Some parcels remain over the Pacific Ocean during the preceding 10-day period.

Evolution and chemical consequences of lightning-produced NOxobserved in the north atlantic upper troposphere

Airborne observations of NO during the Subsonics Assessment Ozone and Nitrogen Oxides Experiment (SONEX) reveal episodes of high NOx in the upper troposphere believed to be associated with lightning. Linkage to specific periods of lightning activity is possible through back trajectories and data from the National Lightning Detection Network. Lagrangian model calculations are used to explore the evolution of these high NOx plumes over the 1–2 days between their introduction and subsequent sampling by NASAs DC-8 aircraft. Simulations include expected changes in HNO3, H2O2, CH3OOH, HO2, and OH. Depending on the time of injection and dilution rate, initial NOx concentrations are estimated to range from 1 to 7 ppbv. Similar to many previous studies, simulated HNO3 concentrations tend to be greater than observations. Several possible explanations for this difference are explored. H2O2 observations are shown to be consistent with removal in convective activity. While it is possible that upper tropospheric CH3OOH is enhanced by convection, simulations show such increases in CH3OOH can be short-lived (e.g., <12 hours) with no perceptible trace remaining at the time of sampling. High NO levels further prevent elevated levels of CH3OOH from propagating into increases in H2O2. HO2 is suppressed through reaction with NO in all cases. Simulated increases in OH exceeded a factor of 2 for some cases, but for the highest NOx levels, loss of OH via OH+NO2 offset production from HO2+NO. Additional increases in OH of 30–60% could result from convection of CH3OOH. A final point of discussion concerns how the chemistry within these plumes, their long-range transport, and their potential importance in sustaining background NOx far from source regions present a challenge to global and regional model simulations.

A meteorological overview of the Subsonic Assessment Ozone and Nitrogen Oxide Experiment (SONEX) period

Meteorological conditions are described during NASAs Subsonic Assessment (SASS) Ozone and Nitrogen Oxide Experiment (SONEX) that was conducted over the North Atlantic Flight Corridor (NAFC) during October and November 1997 to study the impact of aircraft emissions on atmospheric concentrations of NO x and ozone. The SONEX period exhibited frequent closed cyclones and anticyclones, as well as high-amplitude troughs and ridges. These flow patterns often caused aircraft exhaust from the NAFC to follow broad looping north-south trajectories, instead of more easterly routes that would have occurred if the flow had been more zonal. Mean meteorological conditions during SONEX include a pronounced long wave trough over the East Coast of the United States, as well as weaker low pressure over middle-latitude portions of the Atlantic Ocean. Conversely, a well-developed ridge was apparent over the North Atlantic near Iceland. Cloudiness exceeded climatology off the East Coast and the subtropical North Atlantic, with abundant lightning in these regions. There was less than average cloud cover over the middle latitudes between Newfoundland and central Europe. The tropopause was higher than climatology over much of the SONEX region, and the jet stream was located north of its typical position. These circulation features during SONEX are consistent with typical year-to-year variations. Meteorological conditions during individual SONEX flights also are described. Upper tropospheric flow patterns, 5-day backward trajectories from the flight tracks, tropopause heights, lightning data, and differential absorption lidar ozone imagery are employed. Effects of aircraft were observed on numerous flights. Stratospheric conditions were encountered during many flights, sometimes because the DC-8 passed through a tropopause fold. SONEX flight tracks frequently were downwind of regions of lightning, especially during flights from Bangor and the Azores. Finally, trajectories indicated that continental pollution signatures observed during some flights had originated over the United States.

Atmospheric chemical transport based on high‐resolution model‐derived winds: A case study

Flight 10 of NASAs Subsonic Assessment (SASS) Ozone and Nitrogen Oxide Experiment (SONEX) extended southwest of Lajes, Azores. A variety of chemical signatures was encountered. These signatures are examined in detail, relating them to meteorological data from a high-resolution numerical model having a horizontal grid spacing of 30 and 90 km with 26 vertical levels. The meteorological output at hourly intervals is used to create backward trajectories from the locations of the chemical signatures. Four major categories of chemical signatures are discussed: stratospheric, lightning, continental pollution, and a mixed chemical layer. The strong stratospheric signal is encountered just south of the Azores in a region of depressed tropopause height. Three chemical signatures at different altitudes in the upper troposphere are attributed to lightning. Backward trajectories from these signatures extend to locations of cloud-to-ground lightning. Specifically, results show that the trajectories pass over regions of lightning 1–2 days earlier over the eastern Gulf of Mexico and off the southeast coast of the United States. The lowest leg of the flight exhibits a chemical signature consistent with continental pollution. Trajectories from this signature are found to pass over the highly populated Northeast Corridor of the United States. Surface-based pollution apparently is lofted to the altitudes of the trajectories by convective clouds along the East Coast that did not contain lightning. Finally, a mixed layer is described. Its chemical signature is intermediate to those of lightning and continental pollution. Backward trajectories from this layer pass between the trajectories of the lightning and pollution signatures. Thus they likely are impacted by both sources.

Effects of aircraft on aerosol abundance in the upper troposphere

A significant increase in sulfuric acid aerosol concentration was detected above 10 km pressure altitude during a cross-corridor flight out of Shannon on October 23, 1997. The source of this aerosol is ascribed to commercial aircraft operations in flight corridors above 10 km, because (1) a stable atmosphere prevented vertical air mass exchanges and thus eliminated surface sources, (2) air mass back trajectories documented the absence of remote continental sources, and (3) temperature profiler data showed the tropopause at least one kilometers above flight altitude throughout the flight. Particle volatility identified 70% H2SO4, 20% (NH4)2SO4 and 10% nonvolatile aerosol in the proximity of flight corridors, and (10-30)% H2SO4, up to 50% (NH4)2SO4, and (40-60)% nonvolatile aerosols in air that was not affected by aircraft operations below 10 km. Only a very small fraction of the nonvolatile particles (determined with a condensation nucleus counter) could be morphologically identified as soot aerosol (validated by scanning electron microscopy of wire impactor samples). The newly formed H2SO4 particles did not measurably affect surface area and volume of the background aerosol due to their small size, hence did not affect radiative transfer directly.

Global distribution and sources of volatile and nonvolatile aerosol in the remote troposphere

densities of highly volatile CN (10 4 10 5 cm 3 ) are present in the upper troposphere and particularly over the tropical/subtropical region. CN number densities in all regions are largest when the atmosphere is devoid of nonvolatile particles. Through marine convection and long-distance horizontal transport, volatile CN originating from the tropical/ subtropical regions can frequently impact the abundance of aerosol in the middle and upper troposphere at mid to high latitudes. Nonvolatile aerosols behave in a manner similar to tracers of combustion (CO) and photochemical pollution (peroxyacetylnitrate (PAN)), implying a continental pollution source from industrial emissions or biomass burning. In the upper troposphere we find that volatile and nonvolatile aerosol number densities are inversely correlated. Results from an aerosol microphysical model suggest that the coagulation of fine volatile particles with fewer but larger nonvolatile particles, of principally anthropogenic origin, is one possible explanation for this relationship. In some instances the larger nonvolatile particles may also directly remove precursors (e.g., H2SO4) and effectively stop nucleation. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0345 Atmospheric Composition and Structure: Pollution— urban and regional (0305); 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry; 4801 Oceanography: Biological and Chemical: Aerosols (0305); KEYWORDS: aerosol, particles, upper troposphere, pollution, condensation nuclei, PAN

Mesoscale numerical investigations of air traffic emissions over the North Atlantic during SONEX flight 8: A case study

Chemical data from flight 8 of NASAs Subsonic Assessment (SASS) Ozone and Nitrogen Oxide Experiment (SONEX) exhibited signatures consistent with aircraft emissions, stratospheric air, and surface-based pollution. These signatures are examined in detail, focusing on the broad aircraft emission signatures that are several hundred kilometers in length. A mesoscale meteorological model provides high-resolution wind data that are used to calculate backward trajectories arriving at locations along the flight track. These trajectories are compared to aircraft locations in the North Atlantic Flight Corridor (NAFC) over a 27–33 hour period. Time series of flight level NO and the number of trajectory/aircraft encounters within the NAFC show excellent agreement. Trajectories arriving within the stratospheric and surface-based pollution regions are found to experience very few aircraft encounters. Conversely, there are many trajectory/aircraft encounters within the two chemical signatures corresponding to aircraft emissions. Even many detailed fluctuations of NO within the two aircraft signature regions correspond to similar fluctuations in aircraft encountered. These NO spikes are due to the superposition of 14 to 25 aircraft plumes transported to the DC-8 flight track during the previous 33 hours. Results confirm that aircraft emissions were responsible for two chemical signatures observed during SONEX flight 8. They also indicate that high-resolution meteorological modeling, when coupled with detailed aircraft location data, is useful for understanding chemical signatures from aircraft emissions at scales of several hundred kilometers.

Contrasting the Use of Single-Realization versus Ensemble-Average Atmospheric Dispersion Solutions for Chemical and Biological Defense Analyses

AbstractChemical and biological (CB) defense systems require significant testing and evaluation before they are deployed for real-time use. Because it is not feasible to evaluate these systems with open-air testing alone, researchers rely on numerical models to supplement the defense-system analysis process. These numerical models traditionally describe the statistical properties of CB-agent atmospheric transport and dispersion (AT&D). While the statistical representation of AT&D is appropriate to use in some CB defense analyses, it is not appropriate to use this class of dispersion model for all such analyses. Many of these defense-system analyses require AT&D models that are capable of simulating dispersion properties with very short time-averaging periods that more closely emulate a “single realization” of a contaminant or CB agent dispersing in a turbulent atmosphere. The latter class of AT&D models is superior to the former for performing CB-system analyses when one or more of the following factors a...