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[1] In this study, we examine the atmospheric lifetimes and Global Warming Potentials (GWPs) for six short-lived potential ozone-depleting substances and high-GWP hydrofluorocarbons (HFCs) replacement compounds and evaluate the potential reduced effects on climate forcing from adopting such compounds. We also examine the potential effects on climate from widespread use of these chemicals. The atmospheric lifetimes of these compounds are each less than 1 month and their 100 year GWPs range from 0.9 to 4.6. The derived GWPs are substantially lower than those in prior published studies, which used simplified estimation techniques that overestimate the GWPs for these short-lived compounds by 61% to 237%. If long-lived HFCs are completely replaced with these substitute compounds, radiative forcing of these short-lived compounds in 2050 will be 0.26 to 0.80 mW m−2, a factor of as much as 1000 smaller than would be the case with the potential growth in the use of the HFCs.

The existing solvents trichloroethylene (TCE) and perchloroethylene (PCE) and proposed solvent n-propyl bromide (nPB) have atmospheric lifetimes from days to a few months, but contain chlorine or bromine that could affect stratospheric ozone. Several previous studies estimated the Ozone Depletion Potentials (ODPs) for various assumptions of nPB emissions location, but these studies used simplified modeling treatments. The primary purpose of this study is to reevaluate the ODP for n-propyl bromide (nPB) using a current-generation chemistry-transport model of the troposphere and stratosphere. For the first time, ODPs for TCE and PCE are also evaluated in a three-dimensional, global atmospheric chemistry-transport model. Emissions representing industrial use of each compound are incorporated on land surfaces from 30° N to 60° N. The atmospheric chemical lifetime obtained for nPB is 24.7 days, similar to past literature, but the ODP is 0.0049, lower than in our past study of nPB. The derived atmospheric lifetime for TCE is 13.0 days and for PCE is 111 days. The corresponding ODPs are 0.00037 and 0.0050, respectively.

The concept of Ozone Depletion Potentials (ODPs) is extensively used in policy considerations related to concerns about the effects of various halocarbons and other gases on stratospheric ozone. Many of the recent candidate replacement compounds have atmospheric lifetimes shorter than one year in order to limit their environmental effects, especially on stratospheric ozone. Using a three-dimensional global chemistry-transport model (CTM) of the troposphere and the stratosphere, the purpose of this study is to evaluate the potential effects of several very short-lived iodinated substances, namely iodotrifluoromethane (CF3I) and methyl iodide (CH3I), on atmospheric ozone. Like other chemicals with extremely short lifetimes, the stratospheric halogen loading and resulting ozone effects from these compounds are strongly dependent on the location of emissions. For CF3I, a possible replacement candidate for bromotrifluoromethane (CF3Br), ODPs derived by the three-dimensional model are 0.008 with chemical lifetime of 5.03 days and 0.016 with a lifetime of 1.13 days for emissions assumed to be evenly distributed over land surfaces at mid-latitudes and the tropics, respectively. While this is the first time the ODPs have been evaluated with a three-dimensional model, these values are in good agreement with those derived previously. The model calculations suggest that tropical convection could deliver a larger portion of the gas and their breakdown products to the upper troposphere and lower stratosphere if emission source is located in the tropics. The resulting ODP for CH3I, emitted from mid-latitudes, is 0.017 with lifetime of 13.59 days. Valid simulations of convective transport, vertical mixing and degradation chemistry of CH3I are shown that have good qualitative agreement between the model derived distribution of background CH3I, based on global source emission fluxes from previous studies, and available observations especially in vertical profiles.

The concept of Global Warming Potentials (GWPs) has been extensively used in policy consideration as a relative index for comparing the climate impact of an emitted greenhouse gas (GHG), relative to carbon dioxide with equal mass emissions. Ozone depletion due to emission of chlorinated or brominated halocarbons leads to cooling of the climate system in the opposite direction to the direct warming contribution by halocarbons as GHGs. This cooling is a key indirect effect of the halocarbons on climatic radiative forcing, which is accounted for by indirect GWPs. With respect to climate, it is critical to understand net influences considering direct warming and indirect cooling effects especially for Halons due to the greater ozone-depleting efficiency of bromine over chlorine. Until now, the indirect GWPs have been calculated using a parameterized approach based on the concept of Equivalent Effective Stratospheric Chlorine (EESC) and the observed ozone depletion over the last few decades. As a step towards obtaining indirect GWPs through a more robust approach, we use atmospheric models to explicitly calculate the indirect GWPs of Halon-1211 and Halon-1301 for a 100-year time horizon. State-of-the-art global chemistry-transport models (CTMs) were used as the computational tools to derive more realistic ozone depletion changes caused by an added pulse emission of the two major Halons at the surface. The radiative forcings on climate from the ozone changes have been calculated for indirect GWPs using an atmospheric radiative transfer model (RTM). The simulated temporal variations of global average total column Halons after a pulse perturbation follow an exponential decay with an e-folding time which is consistent with the expected chemical lifetimes of the Halons. Our calculated indirect GWPs for the two Halons are much smaller than those from past studies but are within a single standard deviation of WMO (2007) values and the direct GWP values derived agree with the published values. Our model-based assessment of the Halon indirect GWPs thus confirms the significant importance of indirect effects on climate.

Future projections of near-surface ozone concentrations depend on the climate/emissions scenario used to drive future simulations, the direct effects of the changing climate on the atmosphere, and the indirect effects of changing temperatures and CO2 levels on biogenic ozone precursor emissions. The authors investigate the influence of these factors on potential future changes in summertime daily 8-h maximum ozone over the United States and China by comparing Model for Ozone and Related Chemical Tracers, version 2.4, (MOZART-2.4) simulations for the period 1996–2000 with 2095–99, using climate projections from NCAR–Department of Energy Parallel Climate Model simulations driven by the Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios A1fi (higher) and B1 (lower) emission scenarios, with corresponding changes in biogenic emissions. The effect of projected climate changes alone on surface ozone is generally less than 3 ppb over most regions. Regional ozone increases and decreases are driven mainly by local warming and marine air dilution enhancement, respectively. Changes are approximately the same magnitude under both scenarios, although spatial patterns of responses differ. Projected increases in isoprene emissions (32%–94% over both countries), however, result in significantly greater changes in surface ozone. Increases of 1–15 ppb are found under A1fi and of 0–7 ppb are found under B1. These increases not only raise the frequency of “high ozone days,” but are also projected to occur nearly uniformly across the distribution of daily ozone maxima. Thus, projected future ozone changes appear to be more sensitive to changes in biogenic emissions than to direct climate changes, and the spatial patterns and magnitude of future ozone changes depend strongly on the future emissions scenarios used.

Iodotrifluoromethane (CF3I) has been considered to be a candidate replacement for bromotrifluoromethane (CF3Br), which is used in aircraft for fuel inerting and for fire fighting. In this study, the chemical effects of aircraft-released CF3I on atmospheric ozone were examined with the University of Illinois at Urbana-Champaign two-dimensional chemical-radiative-transport (UIUC 2-D CRT) model. Using an earlier estimate of the aircraft emission profile for tank inerting in military aircraft, the resulting equivalent Ozone Depletion Potentials (ODPs) for CF3I were in the range of 0.07 to 0.25. As a sensitivity study, we also analyzed CF3I emissions associated with fuel inerting if it were to occur at lower altitudes using an alternative estimate. The model calculations of resulting effects on ozone for this case gave ODPs≤0.05. Furthermore, through interactions with the National Institute of Standards and Technology (NIST), we analyzed the potential effects on ozone resulting from using CF3I in fire fighting connected with engine nacelle and auxiliary power unit applications. The scenarios evaluated using the NIST estimate suggested that the ODPs obtained by assuming aircraft flights occurring in several different latitude regions of the Northern Hemisphere are extremely low. According to the model calculation, the altitude where CF3I is released from aircraft is a dominant factor in its ozone depletion effects. On the assumption that the CF3I emission profile is representative of actual release characteristics, aircraft-released CF3I has much lower impacts than CF3Br.

Several studies in modeling atmospheric processes have suggested that heterogeneous chemistry on soot emitted from high altitude aircraft could affect stratospheric ozone depletion. However, these modeling studies were limited because they did not adequately consider the decrease in reaction probability with time as the surface of the soot becomes “poisoned” by its interactions with various gases. Here we extend UIUC's two-dimensional chemical-transport model to investigate possible effects of heterogeneous reactions of ozone on aircraft-generated carbon particles, including a treatment of soot poisoning in the model. We generally follow literature recommendations for ozone uptake probabilities and determine the available active sites on soot given partial pressures of the reactants, temperature, and time since soot emission in order to investigate ozone decrease. The regeneration of soot active sites is also taken into account in this study. We find that, even if active sites on soot surfaces are regenerated, upper troposphere and lower stratosphere ozone losses on aircraft emitted soot occurring through heterogeneous reactions are insignificant once poisoning effects are considered.

A number of the compounds proposed as replacements for substances controlled under the Montreal Protocol have extremely short atmospheric lifetimes, on the order of days to a few months. An important example is n-propyl bromide (also referred to as 1-bromopropane, CH2BrCH2CH3 or simplified as 1-C3H7Br or nPB). This compound, useful as a solvent, has an atmospheric lifetime of less than 20 days due to its reaction with hydroxyl. Because nPB contains bromine, any amount reaching the stratosphere has the potential to affect concentrations of stratospheric ozone. The definition of Ozone Depletion Potentials (ODP) needs to be modified for such short-lived compounds to account for the location and timing of emissions. It is not adequate to treat these chemicals as if they were uniformly emitted at all latitudes and longitudes as normally done for longer-lived gases. Thus, for short-lived compounds, policymakers will need a table of ODP values instead of the single value generally provided in past studies. This study uses the MOZART2 three-dimensional chemical-transport model in combination with studies with our less computationally expensive two-dimensional model to examine potential effects of nPB on stratospheric ozone. Multiple facets of this study examine key questions regarding the amount of bromine reaching the stratosphere following emission of nPB. Our most significant findings from this study for the purposes of short-lived replacement compound ozone effects are summarized as follows. The degradation of nPB produces a significant quantity of bromoacetone which increases the amount of bromine transported to the stratosphere due to nPB. However, much of that effect is not due to bromoacetone itself, but instead to inorganic bromine which is produced from tropospheric oxidation of nPB, bromoacetone, and other degradation products and is transported above the dry and wet deposition processes of the model. The MOZART2 nPB results indicate a minimal correction of the two-dimensional results in order to derive our final results: an nPB chemical lifetime of 19 days and an Ozone Depletion Potential range of 0.033 to 0.040 for assumed global emissions over landmasses, 19 days and 0.021 to 0.028, respectively, for assumed emissions in the industrialized regions of the Northern Hemisphere, and 9 days and 0.087 to 0.105, respectively, for assumed emission in tropical Southeast Asia.

Bromopentafluoropropane (C3H2BrF5) has come under consideration as a potential replacement compound for ozone-depleting chlorofluorocarbons and Halons. No rate constants for the reaction of the isomers of C3H2BrF5 with hydroxyl radical in the atmosphere have yet been measured. The existing ozone depletion potential recommendations, which are based on estimated reaction rate constants for the C3H2BrF5 isomers, are not in agreement. In this study, we seek to clarify the ozone depletion potential of the C3H2BrF5 isomers. First, we apply two hydroxyl reaction rate constant estimation methods to a series of bromofluorocarbons likely to have similar reactivity to C3H2BrF5 in order to select one estimation method to apply to C3H2BrF5. We then predict hydroxyl reaction rate constants using that estimation method for the C3H2BrF5 isomers and calculate both atmospheric lifetimes and ozone depletion potentials through semiempirical methods. This study obtains C3H2BrF5 atmospheric lifetimes ranging from 1.4 to 59 years and ozone depletion potentials ranging from 0.30 to 12, which reflects the strong dependence of hydroxyl reactivity on the isomeric structure of the potential replacement compound.

The period of low solar activity between 1645 and 1715, known as the Maunder Minimum, corresponds to a period in which the ultraviolet spectral irradiance was significantly smaller than found in the current atmosphere [Lean et al., 1995a]. In this study, we use our two-dimensional chemical-radiative-transport model of the global atmosphere to examine the effect of this reduced solar flux on the distribution of stratospheric ozone and temperature relative to that normally expected for the pre-industrial atmosphere. A decrease in total global ozone of about 3% was determined for the Maunder Minimum period. Lean et al. [1995b] have examined the implications of the direct solar irradiance effects on climate during that period. The changes in stratospheric ozone may further have influenced the radiative forcing on climate during that period. In addition, such events are lilely to happen again with resulting decreases in ozone.

Chlorobromomethane (CH,ClBr) and 1-bromo-propane (CH,BrCH,CH, or simplified as 1 - C,H,Br) are being considered for use as solvents and potentially in other applications. As with other chemicals that contain chlorine and/or bromine, it is important to determine the potential environmental effect from use and emissions of such compounds, including effects on stratospheric ozone. In this paper, the Ozone Depletion Potentials (ODPs), an important measure of the potential effects on ozone, are evaluated for these two compounds using our two-dimensional chemical-transport model of the troposphere and stratosphere. This is the first time these compounds have, to our knowledge, been evaluated with atmospheric models. Our model results show that the main removal process (cu. 99%) in the atmosphere for these compounds is the reaction with OH radicals. Photolysis has only a minor ( < 1%) effect on the atmospheric lifetimes of either compound. The atmospheric lifetimes of CH,ClBr and 1 - C3H,Br due to atmospheric reactions are evaluated to be 0.40 yr (146 d) and 0.03 y (11 d), respectively. However, oceanic losses are likely to be important for CH,ClBr. Because of limited data on solubility and degradation in sea water, the lifetime for ocean loss currently has a range of 0.43-0.65 yr. This results in a total lifetime for CH,ClBr of 0.21-0.25 yr. An ocean sink for 1 - C,H,Br is likely to have an insignificant effect on its atmospheric lifetime or ODP. The ODP for 1 - C,H,Br is evaluated to be 0.006, while the ODP for CH,ClBr including the effects of the ocean sink is 0.11-0.13. There are additional uncertainties in these values due to ambiguities in measured reaction rates and in the model treatment of atmospheric processes.

The concept of global warming potential was developed as a relative measure of the potential effects on climate of a greenhouse gas as compared to CO2. In this paper a series of sensitivity studies examines several uncertainties in determination of Global Warming Potentials (GWPs). For example, the original evaluation of GWPs for the Intergovernmental Panel on Climate Change (IPCC, 1990) did not attempt to account for the possible sinks of carbon dioxide (CO2) that could balance the carbon cycle and produce atmospheric concentrations of CO2 that match observations. In this study, a balanced carbon cycle model is applied in calculation of the radiative forcing from CO2. Use of the balanced model produces up to 21% enhancement of the GWPs for most trace gases compared with the IPCC (1990) values for time horizons up to 100 years, but a decreasing enhancement with longer time horizons. Uncertainty limits of the fertilization feedback parameter contribute a 20% range in GWP values. Another systematic uncertainty in GWPs is the assumption of an equilibrium atmosphere (one in which the concentration of trace gases remains constant) versus a disequilibrium atmosphere (one in which the concentration of trace gases varies with time). The latter gives GWPs that are 19 to 32% greater than the former for a 100 year time horizons, depending upon the carbon dioxide emission scenario chosen. Five scenarios are employed: constant-concentration, constant-emission past 1990 and the three IPCC (1992) emission scenarios. For the analysis of uncertainties in atmospheric lifetime (τ) the GWP changes in direct proportion toτ for short-lived gases, but to a lesser extent for gases withτ greater than the time horizontal for the GWP calculation.

[1] The upper troposphere and lower stratosphere (UTLS) plays an important role in climate and atmospheric chemistry. Despite its importance on the point of causing deep intrusions of tropics originated air into the midlatitudes, the quasi-horizontal transport process in the UTLS, represented by global chemistry-transport models (CTMs) or chemistry-climate models (CCMs), cannot easily be diagnosed with conventional analyses on isobaric surfaces. We use improved diagnostic tools to better evaluate CTMs and CCMs relative to satellite observations in the region of UTLS. Using the Hellinger distance, vertical profiles of probability density functions (PDFs) of chemical tracers simulated by the Model for OZone And Related chemical Tracers 3.1 (MOZART-3.1) are quantitatively compared with satellite data from the Microwave Limb Sounder (MLS) instrument in the tropopause relative altitude coordinate to characterize features of tracer distributions near the tropopause. Overall, the comparison of PDFs between MLS and MOZART-3.1 did not satisfy the same population assumption. Conditional PDFs are used to understand the meteorological differences between global climate models and the real atmosphere and the conditional PDFs between MOZART-3.1 and MLS showed better agreement compared to the original PDFs. The low static stability during high tropopause heights at midlatitudes suggests that the variation of tropopause height is related to transport processes from the tropics to midlatitudes. MOZART-3.1 with the GEOS4 GCM winds reproduces episodes of the tropical air intrusions. However, our diagnostic analyses show that the GEOS4 GCM did not properly reproduce the high tropopause cases at midlatitudes especially in spring.

[1] The rate constant for the gas phase reaction of OH radicals with BTP (2-bromo-3,3,3-trifluoropropene, CH2 = CBrCF3) was measured using a flash photolysis resonance-fluorescence technique over the temperature range 220 K to 370 K. The Arrhenius plot was found to exhibit noticeable curvature. The temperature dependence of the rate constant can be represented askBTP(220 − 370 K) = 4.85 × 10−13 × (T/298)0.92 × exp{+613/T} cm3 molecule−1 s−1. For atmospheric modeling purposes, kBTP(T) can be equally well represented by the standard Arrhenius expression kBTP(220 − 298 K) = 1.05 × 10−12 × exp{+381/T} cm3 molecule−1 s−1. The IR absorption cross-sections of BTP were also measured between 450 cm−1 and 1900 cm−1. BTP atmospheric lifetime, Ozone Depletion Potential (ODP), and Global Warming Potential (GWP) were evaluated in the Whole Atmosphere Community Climate Model for land emissions from 30 to 60°N and from 60°S to 60°N. The global, annual average atmospheric lifetime of BTP in the former scenario was 7.0 days, its ODP was 0.0028, and its GWP (100-yr time horizon) was 0.0050; in the latter scenario, the global, annual average BTP lifetime was 4.3 days, ODP was 0.0052, and 100-yr GWP was 0.0028. The short lifetime, low ODP, and low GWP indicate that BTP should have minimal effects on ozone and climate. Little BTP reaches the stratosphere in either emission scenario, but 27% of the ozone loss in the 30 to 60°N scenario and 46% of the ozone loss in the 60°S to 60°N scenario occurs above the tropopause due to Bry from BTP.

We follow the collisional deactivation of laser-excited nitrogen dioxide through its dispersed fluorescence. The energy acceptor gases are NO[sub 2] at four excitation energies ranging from 18828 to 24989 cm[sup [minus]1] and five monatomic gases, four diatomic gases, and three polyatomic gases with 18828-cm[sup [minus]1] excitation energy. The nominal products are the shapes of the internal energy distributions, which are obtained and plotted for several representative cases. From these distributions, the first three moments of the internal energy distributions are derived as a function of molecular collisions and tabulated as (i) the average internal energy, (ii) energy spread, and (iii) skewness. These quantities are plotted against c(M)t, the product of buffer gas concentration c(M) and delay time after laser excitation t(0.5-2 [mu]s), which is a quantity proportional to number of collisions. The negative slope of average energy vs c(M)t is the macroscopic energy-transfer rate constant, k[sub [epsilon]](M). Average energies (E) for all NO[sub 2]-buffered data taken at four excitation wavelengths are well represented by the single equation, fourth order in energy: d(E)/d(c[sub NO(2)]t) = [minus]k[sub 4](E)[sup 4], where k[sub 4] = 8.06 [times] 10[sup [minus]25] (cm[sup 3])[sub energy] molecule[sup [minus]1] cm[sup 3] s[sup [minus]1]. 32 refs., 16 figs., 7 tabs.

HCFC-123 (C2HCl2F3) is used in large refrigeration systems and as a fire suppression agent blend. Like other hydrochlorofluorocarbons, production and consumption of HCFC-123 is limited under the Montreal Protocol on Substances that Deplete the Ozone Layer. The purpose of this study is to update the understanding of the current and projected impacts of HCFC-123 on stratospheric ozone and on climate and to discuss the potential environmental effects from continued use of this chemical for specific applications. For the first time, the Ozone Depletion Potential (ODP) of a HCFC is determined using a three-dimensional model (MOZART-3) of atmospheric physics and chemistry. All previous studies have relied on results from two-dimensional models. The derived HCFC-123 ODP of 0.0098 is smaller than previous values. Analysis of the projected uses and emissions of HCFC-123, assuming reasonable levels of projected growth and use in centrifugal chiller and fire suppressant applications, suggests an extremely small impact on the environment due to its short atmospheric lifetime, low ODP, low Global Warming Potential (GWP), and the small production and emission of its limited applications. The current contribution of HCFC-123 to stratospheric reactive chlorine is too small to be measurable.

[1] The U.S. air quality is impacted by emissions both within and outside the United States. The latter impact is manifested as long-range transport (LRT) of pollutants across the U.S. borders, which can be simulated by lateral boundary conditions (LBC) into a regional modeling system. This system consists of a regional air quality model (RAQM) that integrates local-regional source emissions and chemical processes with remote forcing from the LBC predicted by a nesting global chemical transport model (model for ozone and related chemical tracers (MOZART)). The present-day simulations revealed important LRT effects, varying among the five major regions with ozone problems, i.e., northeast United States, midwest United States, Texas, California, and southeast United States. To determine the responses of the LRT impacts to projected global climate and emissions changes, the MOZART and RAQM simulations were repeated for future periods (2048–2052 and 2095–2099) under two emissions scenarios (IPCC A1Fi and B1). The future U.S. air quality projected by the MOZART is less sensitive to the emissions scenarios than that simulated by the RAQM with or without incorporating the LRT effects via the LBC from the MOZART. The result of RAQM with the LRT effects showed that the southeast United States has the largest sensitivity of surface ozone mixing ratio to the emissions changes in the 2095–2099 climate (−24% to +25%) followed by the northeast and midwest United States. The net increase due to the LRT effects in 2095–2099 ranges from +4% to +13% in daily mean surface ozone mixing ratio and +4% to +11% in mean daily maximum 8-h average ozone mixing ratios. Correspondingly, the LRT effects in 2095–2099 cause total column O3 mixing ratio increases, ranging from +7% to +16%, and also 2 to 3 more days with the surface ozone exceeding the national standard. The results indicate that future U.S. air quality changes will be substantially affected by global emissions.

At room temperature, the radiative lifetimes of nitrogen dioxide excited by pulsed, tunable, dye laser radiation are measured in a 50-L flask, for excitation wavelengths ranging from 400 to 750 nm, at total pressures between 0.25 and 2.0 mTorr, and for times between 0.7 and 60 {mu}s. The data are analyzed in terms of the Stern-Volmer mechanism to give zero-pressure radiative constants (k{sub 0} = 1/{tau}{sub 0}) and empirical collisional quenching rate constants k{sub q} by ground-state nitrogen dioxide. (a) The observed radiative coefficients generally increase linearly with increasing excitation energy (cm{sup {minus}1}): k{sub 0}/s{sup {minus}1} = 0.504(E-E{sub 0}) + 7.96 {times} 10{sup 3}, where E is the excitation energy and E{sub 0} is the energy of the origin of the {sup 2}B{sub 2} excited state. At wavelengths above about 540 nm there are strong state-to-state variations of radiative lifetime, producing large apparent scatter in the data. The zero-pressure radiative lifetimes {tau}{sub 0} are compared to previous measurements and are in rough agreement with a number of earlier studies. (b) On the other hand, different investigators typically obtain different numerical values for the Stern-Volmer collisional quenching rate constants. As observed here, the quenching rate constants k{sub q} are found tomore » vary by almost a factor of 6 as filters are changed in front of the photomultiplier tube with all other experimental conditions held constant. Some of the difficulties of observing meaningful quenching rate constants for nitrogen dioxide are discussed.« less

A program at the LLNL for the measurement and monitoring of ultraviolet radiation is briefly described, with an emphasis on seasonal variations, and aerosol effects.

Methane has indirect effects on climate due to chemical interactions as well as direct radiative forcing effects as a greenhouse gas. We have calculated the indirect, time-varying tropospheric radiative forcing and GWP of O{sub 3} and stratospheric H{sub 2}O due to an impulse of CH{sub 4}. This impulse, applied to the lowest layer of the atmosphere, is the increase of the atmospheric mass of CH{sub 4} resulting from a 25 percent steady state increase in the current emissions as a function of latitude. The direct CH{sub 4} radiative forcing and GWP are also calculated. The LLNL 2-D radiative-chemistry-transport model is used to evaluate the resulting changes in the O{sub 3}, H{sub 2}O and CH{sub 4} atmospheric profiles as a function of time. A correlated k-distribution radiative transfer model is used to calculate the radiative forcing at the tropopause of the globally-averaged atmosphere profiles. The O{sub 3} indirect GWPs vary from {approximately}27 after a 20 yr integration to {approximately}4 after 500 years, agreeing with the previous estimates to within about 10 percent. The H{sub 2}O indirect GWPs vary from {approximately}2 after a 20 yr integration to {approximately}0.3 after 500 years, and are in close agreement with other estimates. The CH{sub 4}more » GWPs vary from {approximately}53 at 20 yrs to {approximately}7 at 500 yrs. The 20 year CH{sub 4} GWP is {approximately}20% larger than previous estimates of the direct CH{sub 4} GWP due to a CH{sub 4} response time ({approximately}17 yrs) that is much longer than the overall lifetime (10 yrs). The increased CH{sub 4} response time results from changes in the OH abundances caused by the CH{sub 4} impulse. The CH{sub 4} radiative forcing results are consistent with IPCC values. Estimates are made of latitude effects in the radiative forcing calculations, and UV effects on the O{sub 3} radiative forcing calculations (10%).« less

The purpose of this report is to give interim account of the progress being made at Lawrence Livermore National Laboratory (LLNL) in developing an improved capability for assessing the direct and indirect effects on Global Warming Potentials. Much of our current efforts are being devoted to improving the capability for modeling of global tropospheric processes in our state-of-the-art zonally-averaged chemical-radiative-transport model of the troposphere and stratosphere. These efforts are in preparation for an improved evaluation and better quantification of the indirect GWPs resulting from effects on tropospheric ozone from ethane and other gases with significant human-related emissions. There are three major findings that should result from this project that should have significant impacts on EPA and its programs. First, the current and ongoing studies of the direct and indirect GWPs should have a significant influence on the continuing national and international assessments of climate change. Second, the improved capability for modeling of chemical and physical processes should lead to enhanced understanding of the controlling factors influencing ozone, hydroxyl and other key tropospheric constituents. Third, the enhanced modeling capability should be important to future studies of human-related influences on tropospheric and stratospheric chemical processes.

A measurement of UV-B reaching the ground has been established at the Lawrence Livermore National Laboratory. The instrument is the same as those operated by the National Institute for Water and Atmospheric Research in their network in New Zealand. The wavelength response of the radiometer is similar to the response of human skin to UV-B. Intensity data are collected by averaging meter readings over 10 minutes from 6:00 am to 6:00 pm Pacific Standard Time, then converting to effective UV-B intensity normalized at 310 nm. This report checks the intensities obtained at LLNL from November 1992 to July 1993 against the expected results: Increased solar zenith angle, whether from the daily cycle or from the yearly cycle in solar position, should decrease UV-B intensity at the ground due to increased optical path; and, intervening cloud cover should decrease ground UV-B intensity. Three additional findings are reported here: Maximum UV-B intensity on cloudless days does not always follow a smooth curve, but instead varies either high or low to some extent; Morning UV-B intensities are less than those in the afternoon at comparable solar zenith angles at certain times of year; LLNL wintertime daily-averaged UV-B intensities are somewhat higher than thosemore » observed at Auckland, New Zealand in their winter of 1992.« less

Photolysis of vibrationally excited oxygen produced by ultraviolet photolysis of ozone in the upper stratosphere is incorporated into the Lawrence Livermore National Laboratory two-dimensional zonally averaged chemical-radiative-transport model of the troposphere and stratosphere. The importance of this potential contributor of odd oxygen to the concentration of ozone is evaluated based on recent information on vibrational distributions of excited oxygen and on preliminary studies of energy transfer from the excited oxygen. When energy transfer rate constants similar to those of Toumi et al. (1991) are assumed, increases in model ozone concentrations of up to 4.0% in the upper stratosphere are found, and the model ozone concentrations are found to agree slightly better with measurements, including recent data from the Upper Atmosphere Research Satellite. However, the ozone increase is only 0.3% when the larger energy transfer rate constants indicated by recent experimental work are applied to the model. An ozone increase of 1% at 50 km requires energy transfer rate constants one-twentieth those of the preliminary observations. As a result, vibrationally excited oxygen processes probably do not contribute enough ozone to be significant in models of the upper stratosphere.

FTIR matrix studies of ethylene–hydrogen fluoride complexes have shown that the base ν7 mode is perturbed slightly more by DF than by HF complexation, and that the νs (HF) mode is displaced more in C2D4‐‐HF than in C2H4‐‐HF. The νl (HF) mode perpendicular to the C=C bond increases while the νl (HF) mode parallel to the C=C bond decreases with deuteration of the ethylene base. These trends are explained by decreases in electrostatic repulsions in the deuterium species owing to reduced amplitudes of zero‐point motion.