Mohan Gupta

Relationship of loss, mean age of air and the distribution of CFCs to stratospheric circulation and implications for atmospheric lifetimes

[1] Projections of the recovery of the ozone layer are made with global atmospheric models using a specified time series of mixing ratios of ozone depleting substances (ODSs) at the lower boundary. This time series is calculated using atmospheric mixing ratio observations, emission rates, and an estimate for the atmospheric lifetime. ODS destruction and simulated atmospheric-lifetime vary among models because they depend on the simulated stratospheric transport and mixing. We investigate the balance between the annual change in ODS burden, its atmospheric loss, and the annual ODS input to the atmosphere using several models. Some models produce realistic distributions for the mean age of air and some do not. Back trajectory calculations relate the fractional release (one minus the amount of ODS at a location relative to its stratospheric entry value) to the mean age through the age spectrum, showing that, for the individual spectrum elements, the maximum altitude and loss increase with age. Models with faster circulations produce ‘‘young’’ distributions for the age of air and fail to reproduce the observed relationship between the mean age of air and the fractional release. Models with realistic mean age of air reproduce the observed relationship. These models yield a lifetime for CFCl3 of � 56 years, longer than the 45 year lifetime currently used to project future mixing ratios. Use of flux boundary conditions in assessment models would have several advantages, including consistency between the ODS evolution and simulated loss even if the simulated residual circulation changes due to climate change.

Stable carbon isotopic composition of atmospheric methane: A comparison of surface level and free tropospheric air

We report CH4 mixing ratios and δ13C of CH4 values for remote air at two ground-based atmospheric sampling sites for the period December 1994 to August 1998 and similar data from aircraft sampling of air masses from near sea level to near tropopause in September and October of 1996 during the Global Tropospheric Experiment Pacific Exploratory Mission (PEM)-Tropics A. Surface values of δ13C-CH4 ranged from −47.02 to −47.52‰ at Niwot Ridge, Colorado (40°N, 105°W), and from −46.81 to −47.64‰ at Montana de Oro, California (35°N, 121°W). Samples for isotopic analysis were taken from 2° to 27°S latitude and 81° to 158°W longitude and from sea level to 11.3 km in altitude during the PEM-Tropics A mission. They represent the first study of 13CH4 in the tropical free troposphere. At ∼11 km, δ13C-CH4 was ∼1‰ greater than surface level values. Methane was generally enriched in 13C as altitude increased and as latitude increased (toward the South Pole). Using criteria to filter out stratospheric subsidence and convective events on the basis of other trace gases present in the samples, we find evidence of a vertical gradient in δ13C-CH4 in the tropical troposphere. The magnitude of the isotopic shifts in atmospheric CH4 with altitude are examined with a two-dimensional tropospheric photochemical model and experimentally determined values for carbon kinetic isotope effects in chemical loss processes of CH4 Model-calculated values for δ13C-CH4 in both the troposphere and lower stratosphere significantly underpredict the enrichment in 13CH4 with altitude observed in our measurement data and data of other research groups.

Global atmospheric distributions and source strengths of light hydrocarbons and tetrachloroethene

The atmospheric distributions of CH4, C2H6, C3H8, C2H2, and C2Cl4 and their annual chemical removal rates in steady state are determined versus latitude using a modified version of the Oslo two-dimensional global tropospheric photochemical model. A photochemically calculated hydroxyl radical distribution, which has been validated with methylchloroform data, and seasonally varying surface measurements of the title species are used to compute their respective global annual surface source strengths and steady state lifetimes. Computed annual surface source strengths of CH4, C2H6, C3H8, C2H2, and C2Cl4 are 490, 10.4, 8.4, 3.1 Tg (1 Tg = 1012 g), and 432 kT (1 kT = 109 g), respectively. The calculated annual chemical removal rates of these compounds show distinct latitudinal distributions. Because their steady state global lifetimes are less than the model interhemispheric exchange time (about 1 year), the calculated north to south ratios of the deduced surface emission strengths of C2H6, C3H8, C2H2, and C2Cl4 probably reflect the locations of their sources. Within the limits of previously estimated industrial emissions of C2Cl4 (3–4 kT) for the southern hemisphere, our calculations indicate that about 47 kT of additional southern hemispheric source of C2Cl4 is required for 1989–1990 to attain steady state mass balance in this region. There are two possibilities for this needed source: either other industrial sources are missing, or there are unidentified natural sources of C2Cl4. So far, oceans have been suggested as a natural source. Normalization of monthly varying ratios of hemispherically averaged calculated surface mixing ratios of C2H6, C3H8, and C2H2 and their respective observed mixing ratios with respect to those for C2Cl4 indicates that the sources of these hydrocarbons are seasonal in nature. It is also shown that convective transport effectively redistributes these short-lived species but their calculated surface source strengths are relatively independent of this transport process.

Evaluating the Impacts of Aviation on Climate Change

Aviation is an integral part of the global economic and transportation systems. In fact, aviation expansion outpaces the economic growth. Projections indicate that over the next 2 decades, the demand for aviation could grow to about 3 times its present level. This projected growth will likely result in higher aviation emissions and associated impacts on the environment and on human health and welfare, depending upon a variety of factors (such as the size and mix of the operational fleet necessary to meet the stated demand, as well as mitigation steps that could include new technological advances, more efficient operational procedures, market-based options, or regulatory intervention). Nonetheless, it is critical to balance the economic benefits of air travel with environmental concerns associated with this projected aviation growth.

Impact of aviation on climate: research priorities.

Though presently small in magnitude, aviations future impact on climate will likely increase with the absence of effective mitigation measures. With the exception of CO2 emissions, climate impacts of aviation emissions are quite uncertain, and there are scientific gaps that need to be addressed to guide decision making. An objective of the Next Generation Air Transportation System is to limit or reduce aviations impact on climate. Therefore, the Federal Aviation Administration has developed the Aviation Climate Change Research Initiative (ACCRI) to address key scientific gaps and reduce uncertainties while providing timely scientific input to advance and implement mitigation measures. This paper provides a brief overview of the priority-driven research areas that ACCRI has identified and that need to be pursued to better characterize aviations impact on climate change.

Impact of aviation on climate: FAA’s Aviation Climate Change Research Initiative (ACCRI) Phase II

AbstractUnder the Federal Aviation Administration’s (FAA) Aviation Climate Change Research Initiative (ACCRI), non-CO2 climatic impacts of commercial aviation are assessed for current (2006) and for future (2050) baseline and mitigation scenarios. The effects of the non-CO2 aircraft emissions are examined using a number of advanced climate and atmospheric chemistry transport models. Radiative forcing (RF) estimates for individual forcing effects are provided as a range for comparison against those published in the literature. Preliminary results for selected RF components for 2050 scenarios indicate that a 2% increase in fuel efficiency and a decrease in NOx emissions due to advanced aircraft technologies and operational procedures, as well as the introduction of renewable alternative fuels, will significantly decrease future aviation climate impacts. In particular, the use of renewable fuels will further decrease RF associated with sulfate aerosol and black carbon. While this focused ACCRI program effort...

On‐line simulations of passive chemical tracers in the University of California, Los Angeles, atmospheric general circulation model: 1. CFC‐11 and CFC‐12

Long-term simulations of the response of atmospheric CFC-11 and CFC-12 to standard emission scenarios have been carried out using the University of California, Los Angeles (UCLA) atmospheric general circulation model (AGCM) coupled on-line with the UCLA atmospheric chemistry model. For both compounds, photochemical loss rates are computed interactively over the entire model domain at each time step of the integration. Using industrial-based emission estimates, the simulations for CFC-12 closely track the long-term trends recorded in both hemispheres by the Atmospheric Lifetime Experiment/Global Atmospheric Gases Experiment/Advanced Global Atmospheric Gases Experiment (AGA) and Climate Monitoring and Diagnostics Laboratory monitoring networks. The agreement between simulations and observations is best when AGA-deduced emissions are employed. The predicted surface mixing ratios of CFC-11, on the other hand, are somewhat overestimated by the model. Because the transport and loss processes, as well as source distributions, are roughly similar for these halocarbons, the divergence in surface concentrations points to the possibility that emissions of CFC-11 may be overestimated for the period extending from the late 1980s through the early 1990s, and perhaps even at earlier times. As for CFC-12, the best agreement is achieved using AGA emissions. The simulated interhemispheric exchange time constant for these CFCs is about 0.6 year. In the annual cycle, maximum transport occurs from the Northern to Southern Hemisphere within the lowest atmospheric layers during northern winter. Our best estimates of the annually averaged mean global lifetimes of CFC-11 and CFC-12 are about 55 and 100 years, respectively. The simulations indicate that both the mean residence time and interhemispheric exchange rate depend on the assumed model vertical domain. For the mass balance analysis, when the upper boundary of the AGCM is artificially fixed below ∼35 km for CFC-11, or ∼43 km for CFC-12, there is a tendency for the timescales (lifetimes and interhemispheric exchange times) to be overestimated. Comparisons between CFC distributions and trends calculated using low and high spatial resolution show relatively small differences in the present case. These results, especially regarding CFC persistence and interhemispheric exchange, suggest that the present model accurately represents the global dispersion of long-lived chemical tracers.