Nien Dak Sze

Global simulation of atmospheric mercury concentrations and deposition fluxes

Results from a numerical model of the global emissions, transport, chemistry, and deposition of mercury (Hg) in the atmosphere are presented. Hg (in the form of Hg(0) and Hg(II)) is emitted into the atmosphere from natural and anthropogenic sources (estimated to be 4000 and 2100 Mg yr−1, respectively). It is distributed between gaseous, aqueous and particulate phases. Removal of Hg from the atmosphere occurs via dry deposition and wet deposition, which are calculated by the model to be 3300 and 2800 Mg yr−1, respectively. Deposition on land surfaces accounts for 47% of total global deposition. The simulated Hg ambient surface concentrations and deposition fluxes to the Earths surface are consistent with available observations. Observed spatial and seasonal trends are reproduced by the model, although larger spatial variations are observed in Hg(0) surface concentrations than are predicted by the model. The calculated atmospheric residence time of Hg is ∼1.7 years. Chemical transformations between Hg(0) and Hg(II) have a strong influence on Hg deposition patterns because Hg(II) is removed faster than Hg(0). Oxidation of Hg(0) to Hg(II) occurs primarily in the gas phase, whereas Hg(II) reduction to Hg(0) occurs solely in the aqueous phase. Our model results indicated that in the absence of the aqueous reactions the atmospheric residence time of Hg is reduced to 1.2 from 1.7 years and the Hg surface concentration is ∼25% lower because of the absence of the Hg(II) reduction pathway. This result suggests that aqueous chemistry is an essential component of the atmospheric cycling of Hg.

Atmospheric sulfur hexafluoride: Sources, sinks and greenhouse warming

Model calculations using estimated reaction rates of sulfur hexafluoride (SF6) with OH and O(1D) indicate that the atmospheric lifetime due to these processes may be very long (25,000 years). An upper limit for the UV cross section would suggest a photolysis lifetime much longer than 1000 years. The possibility of other removal mechanisms are discussed. The estimated lifetimes are consistent with other estimated values based on recent laboratory measurements. There appears to be no known natural source of SF6. An estimate of the current production rate of SF6 is about 5 kt/yr. Based on historical emission rates, we calculated a present-day atmospheric concentrations for SF6 of about 2.5 parts per trillion by volume (pptv) and compared the results with available atmospheric measurements. It is difficult to estimate the atmospheric lifetime of SF6 based on mass balance of the emission rate and observed abundance. There are large uncertainties concerning what portion of the SF6 is released to the atmosphere. Even if the emission rate were precisely known, it would be difficult to distinguish among lifetimes longer than 100 years since the current abundance of SF6 is due to emission in the past three decades. More information on the measured trends over the past decade and observed vertical and latitudinal distributions of SF6 in the lower stratosphere will help to narrow the uncertainty in the lifetime. Based on laboratory-measured IR absorption cross section for SF6, we showed that SF6 is about 3 times more effective as a greenhouse gas compared to CFC 11 on a per molecule basis. However, its effect on atmospheric warming will be minimal because of its very small concentration. We estimated the future concentration of SF6 at 2010 to be 8 and 10 pptv based on two projected emission scenarios. The corresponding equilibrium warming of 0.0035°C and 0.0043°C is to be compared with the estimated warming due to CO2 increase of about 0.8°C in the same period.

Photochemistry of COS, CS2, CH3SCH3 and H2S: Implications for the atmospheric sulfur cycle

The chemistry of reduced sulfur compounds (COS, CS2, H2S, CH3SCH3), SO2 and excess sulfates are discussed in the light of recent laboratory data. Attention is directed to the role of OH radicals in atmospheric sulfur chemistry. Models are presented for the altitude distribution of various sulfur compounds and compared with available observations reported over the remote troposphere. A global sulfur cycle involving reduced sulfur compounds as sole source for atmospheric sulfur is investigated. It is found that a relatively low flux of reduced sulfur compounds about 28 Tg(S) y−1 may suffice to account for much of the observed global burdens of SO2 and SO2−4. The tropospheric burden of COS is estimated to be 2.2 Tg(S) and it may represent the major form of atmospheric sulfur on a global scale. Several features of the model results characteristic to the low sulfur budget are highlighted. Measurements that may ascertain the role of reduced sulfur compounds in the global sulfur cycle are suggested.

Infrared measurements of HF and HCl total column abundances above Kitt Peak, 1977-1990: seasonal cycles, long-term increases, and comparisons with model calculations.

Series of high-resolution (approximately 0.01 cm-1) solar absorption spectra recorded with the McMath Fourier transform spectrometer on Kitt Peak (altitude 2.09 km, 31.9 degrees N, 111.6 degrees W) have been analyzed to deduce total column amounts of HF on 93 different days and HCl on 35 different days between May 1977 and June 1990. The results are based on the analysis of the HF and H35Cl (1-0) vibration-rotation band R(1) lines which are located at 4038.9625 and 2925.8970 cm-1, respectively. All of the data were analyzed using a multilayer, nonlinear least squares spectral fitting procedure and a consistent set of spectroscopic line parameters. The results indicate a rapid increase in total HF and a more gradual increase in total HCl with both trends superimposed on short-term variability. In addition, the total columns of both gases undergo a seasonal cycle with an early spring maximum and an early fall minimum, with peak-to-peak amplitudes equal to 25% for HF and 13% for HCl. In the case of HF, the changes over the 13 years of measurement are sufficiently large to determine that a better fit is obtained assuming a linear rather than an exponential increase with time. For HCl, linear and exponential models fit the data equally well. Referenced to calendar year 1981.0 and assuming a sinusoidal seasonal cycle superimposed on a linear total column increase with time, HF and HCl increase rates of (10.9 +/- 1.1)% yr-1 and (5.1 +/- 0.7)% yr-1 and total columns of (3.17 +/- 0.11) x 10(14) and (1.92 +/- 0.06) x 10(15) molecules cm-2 (2 sigma) are derived, respectively; the corresponding best fit mean exponential increase rates are equal to (7.6 +/- 0.6)% yr-1 and (4.2 +/- 0.5)% yr-1 (2 sigma). Over the 13-year observing period, the HF and HCl total columns increased by factors of 3.2 and 1.8, respectively. Based on HF and HCl total columns deduced from measurements on the same day, the HF/HCl total columns ratio increased from 0.14 in May 1977 to 0.23 in June 1990. Short-term temporal variations in the HF and HCl total columns are highly correlated; these fluctuations are believed to be caused by dynamical variability in the lower stratosphere. The results of this investigation are compared with previously reported measurements and with time-dependent, two-dimensional model calculations of HF and HCl total columns based on emission histories and photo-oxidation rates for the source molecules.

Use of satellite data to constrain the model‐calculated atmospheric lifetime for N2O: Implications for other trace gases

The source gases, such as nitrous oxide (N2O) and chlorofluorocarbons (CFCs), are released into the atmosphere at the Earths surface and are removed mainly by photolysis in the stratosphere. The atmospheric abundance of a source gas is proportional to its emission rate and atmospheric lifetime. The lifetime is, in turn, determined by the local photochemical removal rate of the gas and the efficiency of transport that carries the gas from where it is emitted to the dominant photochemical removal region. The latter is particularly important for source gases for which the removal is restricted to the tropical stratosphere, a region where both the photolysis rates and concentrations are highest. We calculate that approximately 80% Of N2O is removed in the stratosphere between 30°N and 30°S. Using the data for N2O obtained from the stratospheric and mesospheric sounder (SAMS) instrument on the Nimbus 7 satellite to constrain the model-calculated distributions, we concluded that previous models may have underestimated the magnitude of vertical transport over the tropics and that the calculated lifetimes for N2O and CFC source gases could be 30% shorter than previously reported values. The calculated lifetime for N2O of 110 years would imply a source strength of 13×106 tons (N) yr−1, compared to a source strength of 9.2×106 tons (N) per year for a lifetime of 160 years. A shorter lifetime for the CFCs (47 years for CFC-11 and 95 years for CFC-12) would imply a more rapid decrease in the atmospheric chlorine content once the CFC emissions are stopped, making it possible to reach the pre-ozone hole value of 2 ppbv as early as 2045. Accurate determination of the lifetime of CFC-11 is particularly important, since the lifetime is used in the definitions of the ozone depletion potentials (ODP), chlorine loading potentials (CLP), and greenhouse warming potentials (GWP) of the replacement chemicals for the CFCs. A shorter lifetime for CFC-11 would elevate the magnitudes of ODP, CLP, and GWP for these chemicals.

The Seasonal and Latitudinal Behavior of Trace Gases and O3 as Simulated by a Two-Dimensional Model of the Atmosphere

Abstract A two-dimensional zonal-mean model with parameterized dynamics and an advanced photochemical scheme is used to simulate the stratospheric distributions of atmospheric trace gases including ozone. The model calculates the distributions of 37 species that are photochemically coupled via 140 reactions with rate data from WMO/NASA. A full diurnal treatment is used to calculate the diurnal variations of the short-lived species and the diurnal mean of the production/loss rates for the long-lived species. The calculated concentrations are compared with a wide range of observations with emphasis on the seasonal and latitudinal features. In this work, no post hoc adjustment of the dynamical parameters has been attempted to improve agreement with observations. In general, the model results are in good agreement with observations, although several discrepancies are noted. Rather than focusing on any individual species, we look for systematic agreements and discrepancies between model and observations for a ...

H2SO4 photolysis: A source of sulfur dioxide in the upper stratosphere

Numerous absorption lines of stratospheric sulfur dioxide (SO{sub 2}) have been identified in solar occultation spectra recorded by the Atmospheric Trace Molecule Spectroscopy (ATMOS) Fourier transform spectrometer during the Atmospheric Laboratory for Applications and Science (ATLAS)-1 shuttle mission (March 24-April 2, 1992). Based on their analysis, a volume mixing ratio profile of SO{sub 2} increasing from (13 {plus_minus}4) p.p.t.v. (parts per 10{sup {minus}12} by volume) at 16 mbar ({approximately} 28 km) to 455 {plus_minus}90 p.p.t.v. at 0.63 mbar ({approximately} 52 km) has been measured with no significant profile differences between 20{degrees}N and 60{degrees}S latitude. The increase in the SO{sub 2} mixing ratios with altitude indicates the presence of a source of SO{sub 2} in the upper stratosphere. Profiles retrieved from ATMOS spectra recorded during shuttle flights in April-May 1985 and April 1993 show similar vertical distributions but lower concentrations. Two-dimensional model calculations with SO{sub 2} assumed as the end product of H{sub 2}SO{sub 4} photolysis produce SO{sub 2} profiles consistent with the ATMOS measurements to within about a factor of 2. 27 refs., 2 figs.

Increase in levels of stratospheric chlorine and fluorine loading between 1985 and 1992

Mixing ratios of 3.44 ppbv (parts per billion by volume) and 1.23 ppbv for HCl and HF above 50 km, surrogates for total chlorine and fluorine, have been measured by the Atmospheric Trace Molecule Spectroscopy (ATMOS) experiment on a March 1992 flight of the Space Shuttle. Compared to the measured values obtained on a 1985 flight, these correspond to a 37% and 62% increase for HCl and HF, respectively. The derived trend in HCl (approx. 0.13 ppbv per year) is in good agreement with the model-predicted increase in chlorine loading of 0.13 ppbv per year, and with the measured trends in HCl total column abundance from reported ground-based observations. The main source of this change can be attributed to the release of man-made chlorofluorocarbons (CFCs) and hydro-chloro-fluoro-carbons (HCFCs). This new value for HCl represents an upper limit to the inorganic chlorine concentration in the stratosphere available for participation in photochemical processes which destroy ozone.

Photochemistry of the Venus Atmosphere

Carbon monoxide, produced in the Venus atmosphere by photolysis of CO_2, is removed mainly by reaction with OH. The radical OH is formed in part by photolysis of H_2O_2, in part by reaction of O with HO_2. Photolysis of HCl provides a major source of H radicals near the visible clouds of Venus and plays a major role in the overall photochemistry. The mixing ratio of O_2 is estimated to be approximately 10^(−7), about a factor of 10 less than a recent observational upper limit reported by Traub and Carleton. A detailed model, which accounts for the photochemical stability of Venus CO_2, is presented and discussed.

Profiles of stratospheric chlorine nitrate (ClONO2) from atmospheric trace molecule spectroscopy/ATLAS 1 infrared solar occultation spectra

Stratospheric volume mixing ratio profiles of chlorine nitrate (ClONO{sub 2}) have been retrieved from 0.01-cm{sup {minus}1} resolution infrared solar occultation spectra recorded at latitudes between 14{degrees}N and 54{degrees}S by the atmospheric trace molecule spectroscopy Fourier transform spectrometer during the ATLAS 1 shuttle mission (March 24 to April 2, 1992). The results were obtained from nonlinear least squares fittings of the ClONO{sub 2} {nu}{sub 4} band Q branch at 780.21 cm{sup {minus}1} with improved spectroscopic parameters generated on the basis of recent laboratory work. The individual profiles, which have an accuracy of about {+-}20%, are compared with previous observations and model calculations. 25 refs., 3 figs., 2 tabs.