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Ted Shepherd
,
R. Alan Plumb
, and
Steven C. Wofsy
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Michael B. McElroy
,
Steven C. Wofsy
,
Joyce E. Penner
, and
John C. McConnell

Abstract

Models for stratosphere temperature and ozone are developed and shown to give good agreement with observational data. The atmosphere is in local radiative equilibrium at heights above about 35 km, and concentrations of ozone above 28 km can be satisfactorily estimated by models assuming photochemical equilibrium. Nitric oxide, formed by photochemical decomposition of nitrous oxide and ammonia, is an important catalyst for recombination of odd oxygen below 50 km, and is responsible for a reduction, by about a factor of 2, in the computed column density of ozone. Possible consequences of nitric oxide and water vapor, exhausted by stratosphere aircraft, are discussed. It is argued that there should be a significant reduction in the concentration of stratospheric ozone, with a related decrease in stratospheric temperature, if the globally averaged aircraft source of nitric oxide exceeds 2 × 107 molecules cm−2 sec−1, approximately half the natural source of stratospheric nitric oxide. An increase in stratospheric water vapor causes a small increase in the concentration of ozone but cannot compensate for the much larger effects associated with nitric oxide. The more detailed analysis reported here confirms conclusions drawn earlier by Crutzen and Johnston regarding the possible impact of large numbers of supersonic transports. A fleet of 320 Concordes operating for 7 hr a day at 17 km is predicted to lead to a decrease of 1% in the column density of ozone, and similar conclusions presumably apply to the Soviet TU144. Aircraft flying at higher altitudes, with similar exhaust characteristics, would be expected to induce relatively more serious changes in atmospheric ozone.

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Steven C. Wofsy
,
Gian Paolo Gobbi
,
Ross J. Salawitch
, and
Michael B. McElroy

Abstract

Growth of nitric acid trihydrate (NAT) particles on background stratospheric aerosols is examined for an isolated air parcel cooled at a uniform rate. The free energy barrier against nucleation and rates of cooling are varied over a range of probable values. During the process of nucleation, the saturation ratio of HNO3 vapor reaches a maximum value between 2 and 15, corresponding to supercooling by 1–4 K. Significant supersaturation may be maintained after nucleation due to the small surface area of NAT available for vapor deposition.

If cooling rates exceed 0.5–1 K day−1, small (<2 μm radius) particles of NAT are produced. A major fraction of the available condensation nuclei is activated and removal of HNO3 by gravitational settling is slow. If cooling rates are less than 0.5–1 K day−1, the number of aerosols that nucleate is reduced, leading to differential growth of large (>2 μm radius) NAT particles. Gravitational settling of NAT particles could result in removal of HNO3 on time scales close to one week.

Observations of 5 μm radius particles in clouds at temperatures above the water frost point may reflect condensation of NAT on ice particles that fall through a column of air as it is cooled. Rapid condensation of HNO3 on ice particles is promoted by the high supersaturation attained during nucleation and maintained during subsequent cooling. This process provides a mechanism for irreversible removal of HNO3.

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Kathleen E. Moore
,
David R. Fitzjarrald
,
Ricardo K. Sakai
,
Michael L. Goulden
,
J. William Munger
, and
Steven C. Wofsy

Abstract

Temperate deciduous forests exhibit dramatic seasonal changes in surface exchange properties following on the seasonal changes in leaf area index. Nearly continuous measurements of turbulent and radiative fluxes above and below the canopy of a red oak forest in central Massachusetts have been ongoing since the summer of 1991. Several seasonal trends are obvious. Global solar albedo and photosynthetically active radiation (PAR) albedo both are good indicators of the spring leaf emergence and autumnal defoliation of the canopy. The solar albedo decreases throughout the summer, a change attributed to decreasing near-infrared reflectance since the PAR reflectance remains the same. Biweekly satellite composite images in visible and near-infrared wavelengths confirm these trends. The thermal emissions from the canopy relative to the net radiation follow a separate trend with a maximum in the midsummer and minima in spring and fall. The thermal response number computed from the change in radiation temperature relative to the net radiation is directly related to the Bowen ratio or energy partition. The subcanopy space follows a different pattern dictated by the presence of the canopy; there the midday sensible heat flux is a maximum in spring and fall when the canopy is leafless, while subcanopy CO2 flux is maximum in midsummer. Subcanopy evapotranspiration did not have a distinct seaasonal peak in spring, summer, or fall. The temperature dependence of the respiration rate estimated from the eddy correlation subcanopy CO2 flux is comparable to that found using nocturnal flux measurements.

The surface energy balance follows a seasonal pattern in which the ratio of turbulent sensible heat flux to the net radiation (QH/Q* ) is a maximum in the spring and fall (0.5–0.6), while the latent heat flux (QE ) peaks in midsummer (QH/Q* = 0.5). This pattern gives rise to a parabolic growing season shape to the Bowen ratio with a minimum in early August. Growing season changes in the canopy resistance (Rc ), related to the trends in the Bowen ratio, are more likely to be predicted using the thermal channels of remote sensing instruments than the shorter-wavelength bands.

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Gretchen Keppel-Aleks
,
James T. Randerson
,
Keith Lindsay
,
Britton B. Stephens
,
J. Keith Moore
,
Scott C. Doney
,
Peter E. Thornton
,
Natalie M. Mahowald
,
Forrest M. Hoffman
,
Colm Sweeney
,
Pieter P. Tans
,
Paul O. Wennberg
, and
Steven C. Wofsy

Abstract

Changes in atmospheric CO2 variability during the twenty-first century may provide insight about ecosystem responses to climate change and have implications for the design of carbon monitoring programs. This paper describes changes in the three-dimensional structure of atmospheric CO2 for several representative concentration pathways (RCPs 4.5 and 8.5) using the Community Earth System Model–Biogeochemistry (CESM1-BGC). CO2 simulated for the historical period was first compared to surface, aircraft, and column observations. In a second step, the evolution of spatial and temporal gradients during the twenty-first century was examined. The mean annual cycle in atmospheric CO2 was underestimated for the historical period throughout the Northern Hemisphere, suggesting that the growing season net flux in the Community Land Model (the land component of CESM) was too weak. Consistent with weak summer drawdown in Northern Hemisphere high latitudes, simulated CO2 showed correspondingly weak north–south and vertical gradients during the summer. In the simulations of the twenty-first century, CESM predicted increases in the mean annual cycle of atmospheric CO2 and larger horizontal gradients. Not only did the mean north–south gradient increase due to fossil fuel emissions, but east–west contrasts in CO2 also strengthened because of changing patterns in fossil fuel emissions and terrestrial carbon exchange. In the RCP8.5 simulation, where CO2 increased to 1150 ppm by 2100, the CESM predicted increases in interannual variability in the Northern Hemisphere midlatitudes of up to 60% relative to present variability for time series filtered with a 2–10-yr bandpass. Such an increase in variability may impact detection of changing surface fluxes from atmospheric observations.

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Eric J. Jensen
,
Leonhard Pfister
,
David E. Jordan
,
Thaopaul V. Bui
,
Rei Ueyama
,
Hanwant B. Singh
,
Troy D. Thornberry
,
Andrew W. Rollins
,
Ru-Shan Gao
,
David W. Fahey
,
Karen H. Rosenlof
,
James W. Elkins
,
Glenn S. Diskin
,
Joshua P. DiGangi
,
R. Paul Lawson
,
Sarah Woods
,
Elliot L. Atlas
,
Maria A. Navarro Rodriguez
,
Steven C. Wofsy
,
Jasna Pittman
,
Charles G. Bardeen
,
Owen B. Toon
,
Bruce C. Kindel
,
Paul A. Newman
,
Matthew J. McGill
,
Dennis L. Hlavka
,
Leslie R. Lait
,
Mark R. Schoeberl
,
John W. Bergman
,
Henry B. Selkirk
,
M. Joan Alexander
,
Ji-Eun Kim
,
Boon H. Lim
,
Jochen Stutz
, and
Klaus Pfeilsticker

Abstract

The February–March 2014 deployment of the National Aeronautics and Space Administration (NASA) Airborne Tropical Tropopause Experiment (ATTREX) provided unique in situ measurements in the western Pacific tropical tropopause layer (TTL). Six flights were conducted from Guam with the long-range, high-altitude, unmanned Global Hawk aircraft. The ATTREX Global Hawk payload provided measurements of water vapor, meteorological conditions, cloud properties, tracer and chemical radical concentrations, and radiative fluxes. The campaign was partially coincident with the Convective Transport of Active Species in the Tropics (CONTRAST) and the Coordinated Airborne Studies in the Tropics (CAST) airborne campaigns based in Guam using lower-altitude aircraft (see companion articles in this issue). The ATTREX dataset is being used for investigations of TTL cloud, transport, dynamical, and chemical processes, as well as for evaluation and improvement of global-model representations of TTL processes. The ATTREX data are publicly available online (at https://espoarchive.nasa.gov/).

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Chelsea R. Thompson
,
Steven C. Wofsy
,
Michael J. Prather
,
Paul A. Newman
,
Thomas F. Hanisco
,
Thomas B. Ryerson
,
David W. Fahey
,
Eric C. Apel
,
Charles A. Brock
,
William H. Brune
,
Karl Froyd
,
Joseph M. Katich
,
Julie M. Nicely
,
Jeff Peischl
,
Eric Ray
,
Patrick R. Veres
,
Siyuan Wang
,
Hannah M. Allen
,
Elizabeth Asher
,
Huisheng Bian
,
Donald Blake
,
Ilann Bourgeois
,
John Budney
,
T. Paul Bui
,
Amy Butler
,
Pedro Campuzano-Jost
,
Cecilia Chang
,
Mian Chin
,
Róisín Commane
,
Gus Correa
,
John D. Crounse
,
Bruce Daube
,
Jack E. Dibb
,
Joshua P. DiGangi
,
Glenn S. Diskin
,
Maximilian Dollner
,
James W. Elkins
,
Arlene M. Fiore
,
Clare M. Flynn
,
Hao Guo
,
Samuel R. Hall
,
Reem A. Hannun
,
Alan Hills
,
Eric J. Hintsa
,
Alma Hodzic
,
Rebecca S. Hornbrook
,
L. Greg Huey
,
Jose L. Jimenez
,
Ralph F. Keeling
,
Michelle J. Kim
,
Agnieszka Kupc
,
Forrest Lacey
,
Leslie R. Lait
,
Jean-Francois Lamarque
,
Junhua Liu
,
Kathryn McKain
,
Simone Meinardi
,
David O. Miller
,
Stephen A. Montzka
,
Fred L. Moore
,
Eric J. Morgan
,
Daniel M. Murphy
,
Lee T. Murray
,
Benjamin A. Nault
,
J. Andrew Neuman
,
Louis Nguyen
,
Yenny Gonzalez
,
Andrew Rollins
,
Karen Rosenlof
,
Maryann Sargent
,
Gregory Schill
,
Joshua P. Schwarz
,
Jason M. St. Clair
,
Stephen D. Steenrod
,
Britton B. Stephens
,
Susan E. Strahan
,
Sarah A. Strode
,
Colm Sweeney
,
Alexander B. Thames
,
Kirk Ullmann
,
Nicholas Wagner
,
Rodney Weber
,
Bernadett Weinzierl
,
Paul O. Wennberg
,
Christina J. Williamson
,
Glenn M. Wolfe
, and
Linghan Zeng

Abstract

This article provides an overview of the NASA Atmospheric Tomography (ATom) mission and a summary of selected scientific findings to date. ATom was an airborne measurements and modeling campaign aimed at characterizing the composition and chemistry of the troposphere over the most remote regions of the Pacific, Southern, Atlantic, and Arctic Oceans, and examining the impact of anthropogenic and natural emissions on a global scale. These remote regions dominate global chemical reactivity and are exceptionally important for global air quality and climate. ATom data provide the in situ measurements needed to understand the range of chemical species and their reactions, and to test satellite remote sensing observations and global models over large regions of the remote atmosphere. Lack of data in these regions, particularly over the oceans, has limited our understanding of how atmospheric composition is changing in response to shifting anthropogenic emissions and physical climate change. ATom was designed as a global-scale tomographic sampling mission with extensive geographic and seasonal coverage, tropospheric vertical profiling, and detailed speciation of reactive compounds and pollution tracers. ATom flew the NASA DC-8 research aircraft over four seasons to collect a comprehensive suite of measurements of gases, aerosols, and radical species from the remote troposphere and lower stratosphere on four global circuits from 2016 to 2018. Flights maintained near-continuous vertical profiling of 0.15–13-km altitudes on long meridional transects of the Pacific and Atlantic Ocean basins. Analysis and modeling of ATom data have led to the significant early findings highlighted here.

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