Juno Hsu

Reactive greenhouse gas scenarios: Systematic exploration of uncertainties and the role of atmospheric chemistry

Knowledge of the atmospheric chemistry of reactive greenhouse gases is needed to accurately quantify the relationship between human activities and climate, and to incorporate uncertainty in our projections of greenhouse gas abundances. We present a method for estimating the fraction of greenhouse gases attributable to human activities, both currently and for future scenarios. Key variables used to calculate the atmospheric chemistry and budgets of major non-CO2greenhouse gases are codified along with their uncertainties, and then used to project budgets and abundances under the new climate-change scenarios. This new approach uses our knowledge of changing abundances and lifetimes to estimate current total anthropogenic emissions, independently and possibly more accurately than inventory-based scenarios. We derive a present-day atmospheric lifetime for methane (CH4) of 9.1 ± 0.9 y and anthropogenic emissions of 352 ± 45 Tg/y (64% of total emissions). For N2O, corresponding values are 131 ± 10 y and 6.5 ± 1.3 TgN/y (41% of total); and for HFC-134a, the lifetime is 14.2 ± 1.5 y.

Stratospheric variability and tropospheric ozone

Changes in the stratosphere-troposphere exchange (STE) of ozone over the last few decades have altered the tropospheric ozone abundance and are likely to continue doing so in the coming century as climate changes. Combining an updated linearized stratospheric ozone chemistry (Linoz v2) with parameterized polar stratospheric clouds (PSCs) chemistry, a 5-year (2001–2005) sequence of the European Centre for Medium-Range Weather Forecasts (ECMWF) meteorology data, and the University of California, Irvine (UCI) chemistry transport model (CTM), we examined variations in STE O3 flux and how it perturbs tropospheric O3. Our estimate for the current STE ozone flux is 290 Tg/a in the Northern Hemisphere (NH) and 225 Tg/a in the Southern Hemisphere (SH). The 2001–2005 interannual root-mean-square (RMS) variability is 25 Tg/a for the NH and 30 Tg/a for the SH. STE drives a seasonal peak-to-peak NH variability in tropospheric ozone of about 7–8 Dobson unit (DU). Of the interannual STE variance, 20% and 45% can be explained by the quasi-biennial oscillation (QBO) in the NH and SH, respectively. The CTM matches the observed QBO variations in total column ozone, and the STE O3 flux shows negative anomalies over the midlatitudes during the easterly phases of the QBO. When the observed column ozone depletion from 1979 to 2004 is modeled with Linoz v2, we predicted STE reductions of at most 10% in the NH, corresponding to a mean decrease of 1 ppb in tropospheric O3.

Coupling of Nitrous Oxide and Methane by Global Atmospheric Chemistry

Lingering Atmospheric Perturbations Nitrous oxide and methane are chemically active greenhouse gases whose atmospheric abundances are greatly influenced by anthropogenic emissions. Prather and Hsu (p. 952) used an atmospheric chemistry model to show how nitrous oxide emissions lower the concentration of tropospheric methane through a chain of chemical reactions that include stratospheric ozone depletion, changes in solar ultraviolet radiation fluxes, altered fluxes of ozone transport from the stratosphere to the troposphere, and increases in the amount of tropospheric hydroxyl radicals. This mechanism acts on a 108-year-long time scale—10 times longer than the atmospheric residence time of methane. Nitrous oxide emissions affect atmospheric methane concentrations on a time scale that is 10 times longer than the methane lifetime. Nitrous oxide (N2O) and methane (CH4) are chemically reactive greenhouse gases with well-documented atmospheric concentration increases that are attributable to anthropogenic activities. We quantified the link between N2O and CH4 emissions through the coupled chemistries of the stratosphere and troposphere. Specifically, we simulated the coupled perturbations of increased N2O abundance, leading to stratospheric ozone (O3) depletion, altered solar ultraviolet radiation, altered stratosphere-to-troposphere O3 flux, increased tropospheric hydroxyl radical concentration, and finally lower concentrations of CH4. The ratio of CH4 per N2O change, –36% by mole fraction, offsets a fraction of the greenhouse effect attributable to N2O emissions. These CH4 decreases are tied to the 108-year chemical mode of N2O, which is nine times longer than the residence time of direct CH4 emissions.

Global long‐lived chemical modes excited in a 3‐D chemistry transport model: Stratospheric N2O, NOy, O3 and CH4 chemistry

The two longest-lived, major chemical response patterns (eigenmodes) of the atmosphere, coupling N2O and CH4, are identified with the UCI chemistry-transport model using a linearized (N2O, NO y , O3, CH4, H2O)-system for stratospheric chemistry and specified tropospheric losses. As in previous 1D and 2D studies, these century-long 3D simulations show that the e-folding decay time of a N2O perturbation (mode-1: 108.4 y) caused by a pulse emission of N2O is 10-years shorter than the N2O atmospheric lifetime (118.2 y). This mode-1 can also be excited by CH4emissions due to CH4-O3 stratospheric chemistry: a pulse emission of 100 Tg CH4 creates a +0.1 Tg N2O perturbation in mode-1 with a 108-yr e-folding decay time, thus increasing the CH4 global warming potential by 1.2%. Almost all of the 100 Tg CH4 appears in mode-2 (10.1 y).

Measuring and modeling the lifetime of nitrous oxide including its variability

Abstract The lifetime of nitrous oxide, the third‐most‐important human‐emitted greenhouse gas, is based to date primarily on model studies or scaling to other gases. This work calculates a semiempirical lifetime based on Microwave Limb Sounder satellite measurements of stratospheric profiles of nitrous oxide, ozone, and temperature; laboratory cross‐section data for ozone and molecular oxygen plus kinetics for O(1D); the observed solar spectrum; and a simple radiative transfer model. The result is 116 ± 9 years. The observed monthly‐to‐biennial variations in lifetime and tropical abundance are well matched by four independent chemistry‐transport models driven by reanalysis meteorological fields for the period of observation (2005–2010), but all these models overestimate the lifetime due to lower abundances in the critical loss region near 32 km in the tropics. These models plus a chemistry‐climate model agree on the nitrous oxide feedback factor on its own lifetime of 0.94 ± 0.01, giving N2O perturbations an effective residence time of 109 years. Combining this new empirical lifetime with model estimates of residence time and preindustrial lifetime (123 years) adjusts our best estimates of the human‐natural balance of emissions today and improves the accuracy of projected nitrous oxide increases over this century.

Are the TRACE-P measurements representative of the western Pacific during March 2001?

[1] Observations of CO and O-3 from the Transport and Chemical Evolution over the Pacific (TRACE-P) campaign are compared with modeled distributions from the FRSGC/ UCI CTM driven by the Oslo T63L40 ECMWF forecast meteorology. The model-measurement comparison is made within the context of how well the TRACE-P observations represent the springtime chemistry and ozone distributions over eastern Asia and the western Pacific in March 2001 and uses the four-dimensional (4-D) extended domain from the model to provide unbiased statistics. A key question is whether the limited sampling density or mission strategy led to a statistically biased sample. To address this question, we examine a diverse range of statistical analyses of the observations of CO and O-3. The middle percentiles of the cumulative probability functions for CO in the free troposphere are representative ( and reproduced by the CTM), but those in the boundary layer are not. The frequency of low-CO, stratospheric influence is well matched along flight tracks but is atypical of the extended domain. The percentiles of the latitude-by-height distribution of lidar O-3 show how the CTM reproduces the nonrepresentative clumpy nature of the observations but has too low a tropopause about the jet region (30-35N). Adaptive kernel estimation of the 2-D probability density of O-3-CO correlations shows a very good simulation of two different chemical regimes ( stratospheric and polluted) that is quite different from the extended domain but also highlights the failure to predict CO > 400 ppb. Empirical orthogonal function analysis of the O-3 vertical profiles shows how six EOFs can effectively describe the 4-D structures of O-3 over this entire domain. The latitude-by-longitude maps of the principal components provide an excellent test of the CTM simulation along flight tracks and clearly show the unique sampling of O-3 events by the TRACE-P flights. In many cases the ability of the model to simulate the nonrepresentative observations implies a clear skill in matching the unique meteorological and chemical features of the region.

Is the residual vertical velocity a good proxy for stratosphere-troposphere exchange of ozone?

Stratosphere-troposphere exchange (STE) of ozone (O3) is key in the budget of tropospheric O3, in turn affecting climate forcing and global air quality. We compare three commonly used diagnostics meant to quantify cross-tropopause O3 fluxes with a Chemistry-Transport Model driven by two distinct European Centre forecast fields. Our reference case calculates accurate, geographically resolved net transport across an isosurface in artificial tracer e90 representing the tropopause. Hemispheric fluxes derived from the ozone mass budget of the lowermost stratosphere yield similar results. Use of the Brewer-Dobson residual vertical velocity as a scaled proxy for ozone flux, however, fails to capture the interannual variability. Thus, the common notion that the strength of stratospheric overturning circulation is a good measure for global STE does not apply to O3. Climatic variability in the modeled O3 flux needs to be diagnosed directly rather than indirectly through the overturning circulation.

Correction to “NF3, the greenhouse gas missing from Kyoto”

Click Here GEOPHYSICAL RESEARCH LETTERS, VOL. 37, L11807, doi:10.1029/2010GL043831, 2010 for Full Article Correction to “NF 3 , the greenhouse gas missing from Kyoto” Michael J. Prather 1 and Juno Hsu 1 Received 30 April 2010; published 10 June 2010. Citation: Prather, M. J., and J. Hsu (2010), Correction to “NF 3 , the greenhouse gas missing from Kyoto,” Geophys. Res. Lett., 37, L11807, doi:10.1029/2010GL043831. 1. Correction to CFC‐114 Lifetime and CFC‐115 Lifetime [ 1 ] In Prather and Hsu [2008] we have mistakenly used the overall quenching rate of O( 1 D) from JPL 06‐2 [Sander et al., 2006] to compute the lifetimes of CFC‐114 and CFC‐ 115. As a result, their lifetimes are underestimated. When removing 25% physical quenching for CFC‐114 and 70% for CFC‐115 from the total quenching rate (see notes A48 and A49 in JPL 06‐2), the lifetime is corrected to 190 years for CFC‐114 and to 1020 years for CFC‐115. Table 1 in Prather and Hsu [2008] is revised in this note with correct CFC‐114 and CFC‐115 lifetimes. [ 2 ] Acknowledgments. We thank Vladimir Orkin and Dr. Michael Kurylo for graciously pointing out our misreading in (O 1 D) reaction rate with CFC‐114 and ‐115. References Forster, P. V., P. Artaxo, T. Bernstsen, R. Betts, D. W. Fahey, J. Haywood, J. Lean, D. C. Lowe, G. Myhre, J. Nganga, R. Prinn, G. Raga, M. Schulz, and R. Van Dorland (2007), Changes in atmospheric constituents and in radiative forcing, in Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, edited by S. Solomon et. al., pp. 129–235, Cambridge Univ. Press, Cambridge, U. K. Prather, M. J., and J. Hsu (2008), NF 3 , the greenhouse gas missing from Kyoto, Geophys. Res. Lett., 35, L12810, doi:10.1029/2008GL034542. Sander, S. P., et al. (2006), Chemical kinetics and photochemical data for use in atmospheric studies, JPL Publ., 06‐2. Table 1. Atmospheric Budget Lifetimes (given in years) of Fluorianted Gases From UCI CTM With and Without O( 1 D) Reactions Compared to IPCC (Forster et al. [2007]). The lifetimes of CFC‐114 and CFC‐115 with O( 1 D) are corrected from those published in Prather and Hsu [2008] Gas UCI Without O( 1 D) UCI With O( 1 D) IPCC NF 3 CFC‐11(CFCl 3 ) CFC‐12(CF 2 Cl 2 ) CFC‐113(CF 2 ClCFCl 2 ) CFC‐114(CF 2 ClCF 2 Cl) CFC‐115(CF 3 CF 2 Cl) Earth System Science, University of California, Irvine, California, USA. Copyright 2010 by the American Geophysical Union. 0094‐8276/10/2010GL043831 L11807 1 of 1

Interactive Photochemistry in Earth System Models to Assess Uncertainty in Ozone and Greenhouse Gases. Final report

Atmospheric chemistry controls the abundances and hence climate forcing of important greenhouse gases including N2O, CH4, HFCs, CFCs, and O3. Attributing climate change to human activities requires, at a minimum, accurate models of the chemistry and circulation of the atmosphere that relate emissions to abundances. This DOE-funded research provided realistic, yet computationally optimized and affordable, photochemical modules to the Community Earth System Model (CESM) that augment the CESM capability to explore the uncertainty in future stratospheric-tropospheric ozone, stratospheric circulation, and thus the lifetimes of chemically controlled greenhouse gases from climate simulations. To this end, we have successfully implemented Fast-J (radiation algorithm determining key chemical photolysis rates) and Linoz v3.0 (linearized photochemistry for interactive O3, N2O, NOy and CH4) packages in LLNL-CESM and for the first time demonstrated how change in O2 photolysis rate within its uncertainty range can significantly impact on the stratospheric climate and ozone abundances. From the UCI side, this proposal also helped LLNL develop a CAM-Superfast Chemistry model that was implemented for the IPCC AR5 and contributed chemical-climate simulations to CMIP5.