Pergamon Ammpheric Enuironmenf Vol. 31, No. 7, pp. iii-vi, 1997

Elsevier Science Ltd Printed in Great Britain

CHEMOSPHERE ABSTRACTS

ESTIMATION OF GLOBAL BIOGEOCHEMICAL CONTROLS IN SOIL METHANE CONSUMPTION

C. S. Potter’, E. A. Davidson* and L. V. Verchot*

AND SEASONALITY

‘Johnson Controls World Services, Inc., NASA Ames Research Center, Mail Stop 242-4, Moffett Field, CA 94035, U.S.A. 2\Voods Hole Research Center, P.O. Box 296, Woods Hole, MA 02543, U.S.A.

ABSTRACT Uptake by soils is a relatively small flux in the global budget of atmospheric methane, but CH4 consumption rates in soils could be susceptible to changes in land use and climate. Global estimates of the soil sink for atmospheric CH4 are usually made by multiplying iaverages of small chamber measurements for various ecosystem types (or other strata) by estimates of the area covered by each stratum. Process-level models driven by gridded databases can also be used to make global flux estimates, to evaluate potential effects of changes in climate and land use, and to identify weaknesses in both data and mechanistic understanding. Methane uptake by soils is an appropriate process to model globally because the probable controls are simple relative to many other microbially mediated soil processes of trace gas production and consumption. Field experience suggests that diffusion of atmospheric CHI into the soil is the primary factor limiting rates of CH4 oxidation in many soils. We have applied a modified version of Fick’s first law based on theoretical computations for diffusivity in aggregated media, together with a soil water balance model run on a 1” global grid, to make independent estimates of CHI uptake by soils worldwide. Uptake rates were assumed to be zero in very dry desert soils that are mostly devoid of microbial activity, in frozen soils, and in wetlands that are usually CH4 sources. Our mechanistically-based model supports a reference case for global net consumption of CHd in soils of 17-23 Tg yr-‘, which is near the middle of previously reported ranges, and is close to our own mean estimate from extrapolation of flux means across ecosystem strata (21 Tg CHI yr-I). A new inference of our modeling approach is that over 40% of the soil sink for CHI occurs in warm and relatively dry ecosystems, such as semi-arid steppe, tropical savanna, tropical seasonal forest, and chaparral. This model prediction results from a favorable climate regime, high porosity in coarse-to-medium textured soils, and low moisture content that permits rapid gaseous diffusion in these semi-arid and seasonally dry tropical ecosystems. Very few data on CHI fluxes exist from these areas that can be used to compare with model predictions. Because of this paucity of data where uptake rates may be relatively high, and because humans have altered these landscapes extensively, our results suggest that more study is needed in seasonally dry ecosystems in order to understand the impacts of land-use change on soil sinks for methane.

THE DISTRIBUTION OF SOLAR RADIATION IN THE EARTH’S ATMOSPHERE: THE EFFECTS OF OZONE, AEROSOLS AND CLOUDS

Yu Lu’ and M. A. K. KhaliP ‘TBCR/EERD/AREAL, MS-75, USEPA, Research Triangle Park, NC 27711, U.S.A.

rDept of Physics, Portland State University, P.O. Box 751, Portland, OR 97207-0751, U.S.A.

ABSTRACT We have developed a detailed model of solar radiation in the atmosphere as it is affected by atmospheric constituents, aerosols, clouds and the surface characteristics of the Earth. Such a model is the foundation for studying global change and atmospheric chemistry under natural and disturbed conditions. The model includes radiative transfer processes for solar ultraviolet and visible wavelengths (29&700 nm) under different environmental conditions. It calculates the optical properties of aerosols and cloud droplets as well as the direct, diffuse, net, and actinic fluxes for different wavelengths, altitudes, and zenith iangles at a relatively high computational speed and accuracy. It only takes about three and a half minutes to calculate all the optical properties and radiative fluxes in a cloudy air (including all the properties and fluxes in 100 sub-layers inside a cloud), and about 20 seconds in a clear sky and clean air condition at a SUN SPARC Station lo/50 (with single SPARC CPU running at about 50 MHz). We show that local environmental conditions, particularly in the lower atmosphere, can greatly alter the actinic flux throughout the atmosphere. This feature is especially apparent in the wavelengths ~1 th weak or no 0, absorption, as multiple scatiering dominates the atmospheric radiative transfer. Compared to the actinic flux under clear sky and clean air conditions, for example, the actinic flux around 400 nm at zero zenith angle decreases by a factor of 5 at the Earth’s surface while increasing by more than 100% at the top of the atmosphere when a one-km altostratus cloud is added to the middle troposphere. According to our calculations, the radiation field outside a cloud is mainly controlled by the total liquid water content of the cloud; however, the actinic fl’ux inside a cloud is very sensitive to the macro structure of the cloud. Readers may acquire the computer model from the authors.

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