Anthropogenic aerosol drives uncertainty in future climate mitigation efforts.

The 2016 Paris agreement set a global mean surface temperature (GMST) goal of not more than 2 degrees Celsius above preindustrial. This is an ambitious goal that will require substantial decreases in emission rates of long-lived greenhouse gasses (GHG). This work provides a mathematical framework, based on current state of the art climate models, to calculate the GHG emissions consistent with prescribed GMST pathways that meet the Paris agreement goal. The unique capability of this framework, to start from a GMST timeseries and efficiently calculate the emissions required to meet that temperature pathway, makes it a powerful resource for policymakers. Our results indicate that aerosol emissions play a large role in determining the near-term allowable greenhouse gas emissions that will limit future warming to 2 °C, however in the long term, drastic GHG emissions reductions are required under any reasonable aerosol scenario. With large future aerosol emissions, similar to present day amounts, GHG emissions need to be reduced 8% by 2040 and 74% by 2100 to limit warming to 2 °C. Under a more likely low aerosol scenario, GHG emissions need to be reduced 36% and 80% by 2040 and 2100, respectively. The Paris agreement Intended Nationally Determined Contributions are insufficient to meet this goal.

Efficacy of Climate Forcings in PDRMIP Models.

Quantifying the efficacy of different climate forcings is important for understanding the real-world climate sensitivity. This study presents a systematic multimodel analysis of different climate driver efficacies using simulations from the Precipitation Driver and Response Model Intercomparison Project (PDRMIP). Efficacies calculated from instantaneous radiative forcing deviate considerably from unity across forcing agents and models. Effective radiative forcing (ERF) is a better predictor of global mean near-surface air temperature (GSAT) change. Efficacies are closest to one when ERF is computed using fixed sea surface temperature experiments and adjusted for land surface temperature changes using radiative kernels. Multimodel mean efficacies based on ERF are close to one for global perturbations of methane, sulfate, black carbon, and insolation, but there is notable intermodel spread. We do not find robust evidence that the geographic location of sulfate aerosol affects its efficacy. GSAT is found to respond more slowly to aerosol forcing than CO2 in the early stages of simulations. Despite these differences, we find that there is no evidence for an efficacy effect on historical GSAT trend estimates based on simulations with an impulse response model, nor on the resulting estimates of climate sensitivity derived from the historical period. However, the considerable intermodel spread in the computed efficacies means that we cannot rule out an efficacy-induced bias of ±0.4 K in equilibrium climate sensitivity to CO2 doubling when estimated using the historical GSAT trend.

Transport of ice into the stratosphere and the humidification of the stratosphere over the 21st century.

Climate models predict that tropical lower-stratospheric humidity will increase as the climate warms. We examine this trend in two state-of-the-art chemistry-climate models. Under high greenhouse gas emissions scenarios, the stratospheric entry value of water vapor increases by ~1 part per million by volume (ppmv) over this century in both models. We show with trajectory runs driven by model meteorological fields that the warming tropical tropopause layer (TTL) explains 50-80% of this increase. The remainder is a consequence of trends in evaporation of ice convectively lofted into the TTL and lower stratosphere. Our results further show that, within the models we examined, ice lofting is primarily important on long time scales - on interannual time scales, TTL temperature variations explain most of the variations in lower stratospheric humidity. Assessing the ability of models to realistically represent ice-lofting processes should be a high priority in the modeling community.

Stratospheric ozone depletion due to nitrous oxide: influences of other gases.

The effects of anthropogenic emissions of nitrous oxide (N(2)O), carbon dioxide (CO(2)), methane (CH(4)) and the halocarbons on stratospheric ozone (O(3)) over the twentieth and twenty-first centuries are isolated using a chemical model of the stratosphere. The future evolution of ozone will depend on each of these gases, with N(2)O and CO(2) probably playing the dominant roles as halocarbons return towards pre-industrial levels. There are nonlinear interactions between these gases that preclude unambiguously separating their effect on ozone. For example, the CH(4) increase during the twentieth century reduced the ozone losses owing to halocarbon increases, and the N(2)O chemical destruction of O(3) is buffered by CO(2) thermal effects in the middle stratosphere (by approx. 20% for the IPCC A1B/WMO A1 scenario over the time period 1900-2100). Nonetheless, N(2)O is expected to continue to be the largest anthropogenic emission of an O(3)-destroying compound in the foreseeable future. Reductions in anthropogenic N(2)O emissions provide a larger opportunity for reduction in future O(3) depletion than any of the remaining uncontrolled halocarbon emissions. It is also shown that 1980 levels of O(3) were affected by halocarbons, N(2)O, CO(2) and CH(4), and thus may not be a good choice of a benchmark of O(3) recovery.

Experimental and theoretical study of the atmospheric chemistry and global warming potential of SO2F2.

In this work, potential atmospheric loss processes for SO2F2, a commercially used biocide (fumigant), have been studied and its global warming potential calculated. Rate coefficients for the gas-phase reactions OH + SO2F2 --> products, k1, and Cl + SO2F2 --> products, k4, were determined using a relative rate technique to be k1 < 1 x 10(-16) cm3 molecule-1 s-1 at 296 and 333 K and k4(296 K) < 5 x 10(-17) cm3 molecule(-1) s(-1). UV absorption cross sections of SO2F2 were measured at 184.9, 193, and 213.9 nm, and its photolysis quantum yield at 193 nm was determined to be <0.02. The atmospheric lifetime of SO2F2 with respect to loss by OH, Cl, and O(1D) reaction and UV photodissociation is estimated to be >300, >10000, 700, and >4700 years, respectively. The stratospheric lifetime of SO2F2 is calculated using a two-dimensional model to be 630 years. The global warming potential (GWP) for SO2F2 was calculated to be 4780 for the 100 year time horizon using infrared absorption cross sections measured in this work and a SO2F2 globally averaged atmospheric lifetime of 36 years, which is determined primarily by ocean uptake, reported by Mühle et al. (Mühle, J.; Huang, J.; Weiss, R. F.; Prinn, R. G.; Miller, B. R.; Salameh, P. K.; Harth, C. M.; Fraser, P. J.; Porter, L. W.; Greally, B. R.; O'Doherty, S.; Simonds, P. G. J. Geophys. Res., submitted for publication, 2008). Reaction channels and the possible formation of stable adducts in reactions 1 and 4 were evaluated using ab initio, CCSD(T), and density functional theory, B3P86, quantum mechanical electronic structure calculations. The most likely reaction product channels were found to be highly endothermic, consistent with the upper limits of the rate coefficients measured in this work.

CF3CF=CH2 and (Z)-CF3CF=CHF: temperature dependent OH rate coefficients and global warming potentials.

Rate coefficients over the temperature range 206-380 K are reported for the gas-phase reaction of OH radicals with 2,3,3,3-tetrafluoropropene (CF(3)CF=CH(2)), k(1)(T), and 1,2,3,3,3-pentafluoropropene ((Z)-CF(3)CF=CHF), k(2)(T), which are major components in proposed substitutes for HFC-134a (CF(3)CFH(2)) in mobile air-conditioning units. Rate coefficients were measured under pseudo-first-order conditions in OH using pulsed-laser photolysis to produce OH and laser-induced fluorescence to detect it. Rate coefficients were found to be independent of pressure between 25 and 600 Torr (He, N(2)). For CF(3)CF=CH(2), the rate coefficients, within the measurement uncertainty, are given by the Arrhenius expression k(1)(T)=(1.26+/-0.11) x 10(-12) exp[(-35+/-10)/T] cm(3) molecule(-1) s(-1) where k(1)(296 K)=(1.12+/-0.09) x 10(-12) cm(3) molecule(-1) s(-1). For (Z)-CF(3)CF=CHF, the rate coefficients are given by the non-Arrhenius expression k(2)(T)=(1.6+/-0.2) x 10(-18)T(2) exp[(655+/-50)/T] cm(3) molecule(-1) s(-1) where k(2)(296 K)=(1.29+/-0.06) x 10(-12) cm(3) molecule(-1) s(-1). Over the temperature range most relevant to the atmosphere, 200-300 K, the Arrhenius expression k(2)(T)=(7.30+/-0.7) x 10(-13) exp[(165+/-20)/T] cm(3) molecule(-1) s(-1) reproduces the measured rate coefficients very well and can be used in atmospheric model calculations. The quoted uncertainties in the rate coefficients are 2sigma (95% confidence interval) and include estimated systematic errors. The global warming potentials for CF(3)CF=CH(2) and (Z)-CF(3)CF=CHF were calculated to be <4.4 and <3.6, respectively, for the 100 year time horizon using infrared absorption cross sections measured in this work, and atmospheric lifetimes of 12 and 10 days that are based solely on OH reactive loss.

Rate coefficients for the reactions of OH with CF3CH2CH3 (HFC-263fb), CF3CHFCH2F (HFC-245eb), and CHF2CHFCHF2 (HFC-245ea) between 238 and 375 K.

Rate coefficients for reaction of the hydroxyl radical (OH) with three hydrofluorocarbons (HFCs) CF3CH2CH3, HFC-263fb, (k1); CF3CHFCH2F, HFC-245eb, (k2); and CHF2CHFCHF2, HFC-245ea, (k3); which are suggested as potential substitutes to chlorofluorocarbons (CFCs), were measured using pulsed laser photolysis-laser-induced fluorescence (PLP-LIF) between 235 and 375 K. The Arrhenius expressions obtained are k1(T) = (4.36 +/- 0.72) x 10(-12) exp[-(1290 +/- 40)/T] cm3 molecule(-1) s(-1); k2(T) = (1.23 +/- 0.18) x 10(-12) exp[-(1250 +/- 40)/T] cm3 molecule(-1) s(-1); k3(T) = (1.91 +/- 0.42) x 10(-12) exp[-(1375 +/- 100)/T] cm3 molecule(-1) s(-1). The quoted uncertainties are 95% confidence limits and include estimated systematic errors. The present results are discussed and compared with rate coefficients available in the literature. Our results are also compared with those calculated using structure activity relationships (SAR) for fluorinated compounds. The IR absorption cross-sections at room temperature for these compounds were measured over the range of 500 to 4000 cm-1. The global warming potentials (GWPs) of CF3CH2CH3(HFC-263fb), CF3CHFCH2F(HFC-245eb), and CHF2CHFCHF2(HFC-245ea) were calculated to be 234, 962, and 723 for a 20-year horizon; 70, 286, and 215 for a 100-year horizon; and 22, 89, and 68 for a 500-year horizon; and the atmospheric lifetimes of these compounds are 0.8, 2.5, and 2.6 years, respectively. It is concluded that these compounds are acceptable substitutes for CFCs in terms of their impact on Earth's climate.

Rate coefficients for the OH + CFH2CH2OH reaction between 238 and 355 K.

The rate coefficient for the reaction OH + CFH2CH2OH --> products (k1) between 238 and 355 K was measured using the pulsed laser photolysis-laser induced fluorescence (PLP-LIF) technique to be (5.15 +/- 0.88)x 10(-12) exp[-(330 +/- 45)/T] cm3 molecule(-1) s(-1); k1(298 K)= 1.70 x 10(-12) cm3 molecule(-1) s(-1). The quoted uncertainties are 2sigma(95% confidence level) and include estimated systematic errors. The present results are discussed in relation to the measured rate coefficients for the reaction of OH with other fluorinated alcohols and those calculated using recently reported structure additivity relationships for fluorinated compounds (K. Tokuhashi, H. Nagai, A. Takahashi, M. Kaise, S. Kondo, A. Sekiya, M. Takahashi, Y. Gotoh and A. Suga, J. Phys. Chem. A, 1999, 103, 2664-2672, ). Infrared absorption cross sections for CFH2CH2OH are reported and they are used to calculate the global warming potentials (GWP) for CFH2CH2OH of approximately 8, approximately 2, and approximately 1, respectively, for the 20, 100 and 500 year horizons. A brief discussion of the atmospheric degradation of CFH2CH2OH is provided. It is concluded that CFH2CH2OH is an acceptable substitute for CFCs in terms of its impact on Earth's climate and the composition of the atmosphere. The room temperature rate coefficient for the reaction OH + CFH2CH2OH --> products (k10) was measured to be 3.26 x 10(-12) cm3 molecule(-1) s(-1), in good agreement with recent measurements from this laboratory.

Measuring reactive nitrogen emissions from point sources using visible spectroscopy from aircraft.

Accurate measurements of nitrogen dioxide (NO2), a key trace gas in the formation and destruction of tropospheric ozone, are important in studies of urban pollution. Nitrogen dioxide column abundances were measured during the Texas Air Quality Study 2000 using visible absorption spectroscopy from an aircraft. The method allows for quantification of the integrated total number of nitrogen dioxide molecules in the polluted atmosphere and is hence a useful tool for measuring plumes of this key trace gas. Further, we show how such remote-sensing observations can be used to obtain information on the fluxes of nitrogen dioxide into the atmosphere with unique flexibility in terms of aircraft altitude, and the height and extent of mixing of the boundary layer. Observations of nitrogen dioxide plumes downwind of power plants were used to estimate the flux of nitrogen oxide emitted from several power plants in the Houston and Dallas metropolitan areas and in North Carolina. Measurements taken over the city of Houston were also employed to infer the total flux from the city as a whole.