The biologically effective ultraviolet irradiance at the earth's surface varies with the elevation of the sun, the atmospheric ozone amount, and with the abundance of scatterers and absorbers of natural and anthropogenic origin. Taken alone, the reported decrease in column ozone over the Northern Hemisphere between 1969 and 1986 implies an increase in erythemal irradiance at the ground of four percent or less during summer. However, an increase in tropospheric absorption, arising from polluting gases or particulates over localized areas, could more than offset the predicted enhancement in radiation. Any such extra absorption is likely to be highly regional in nature and does not imply that a decrease in erythemal radiation has occurred on a global basis. The Antarctic ‘ozone hole’ represents a special case in which a portion of the earth has experienced ultraviolet radiation levels during spring that are far in excess of those which prevailed prior to the present decade.

[1] As part 1 in a series of papers describing long-term simulations using the Community Multiscale Air Quality (CMAQ) modeling system and subsequent process analyses and sensitivity simulations, this paper presents a comprehensive model evaluation for the full year of 2001 over the continental U.S. using both ground-based and satellite measurements. CMAQ is assessed for its ability to reproduce concentrations and long-term trends of major criteria pollutants such as surface ozone (O 3 ) and fine particulate matter (PM 2.5 ) and related variables such as indicator species, wet deposition fluxes, and column mass abundances of carbon monoxide (CO), nitrogen oxides (NO 2 ), tropospheric ozone residuals (TORs), and aerosol optical depths (AODs). The domain-wide and site-specific evaluation of surface predictions shows an overall satisfactory performance in terms of normalized mean biases for annual mean maximum 1 h and 8 h average O 3 mixing ratios (À11.6 to 0.1% and À4.6 to 3.0%, respectively), 24 h average concentrations of PM 2.5 (4.2-35.3%), sulfate (À13.0 to 43.5%), and organic carbon (OC) (À37.6 to 24.8%), and wet deposition fluxes (À13.3 to 31.6%). Larger biases, however, occur in the concentrations and wet deposition fluxes of ammonium and nitrate domain-wide and in the concentrations of PM 2.5 , sulfate, black carbon, and OC at some urban/suburban sites. The reasons for such model biases may be errors in emissions, chemistry, aerosol processes, or meteorology. The evaluation of column mass predictions shows a good model performance in capturing the seasonal variations and magnitudes of column CO and NO 2 , but relatively poor performance in reproducing observed spatial distributions and magnitudes of TORs for winter and spring and those of AODs in all seasons. Possible reasons for the poor column predictions include the underestimates of emissions, inaccurate upper layer boundary conditions, lack of model treatments of sea salt and dust, and limitations and uncertainties in satellite data.Citation: Zhang, Y., K. Vijayaraghavan, X.-Y. Wen, H. E. Snell, and M. Z. Jacobson (2009), Probing into regional ozone and particulate matter pollution in the United States: 1. A 1 year CMAQ simulation and evaluation using surface and satellite data,

This paper describes a rapid and accurate technique for the numerical modeling of band transmittances and radiances in media with nonhomogeneous thermodynamic properties (i.e., temperature and pressure), containing a mixture of absorbing gases with variable concentrations. The optimal spectral sampling (OSS) method has been designed specifically for the modeling of radiances measured by sounding radiometers in the infrared and has been extended to the microwave; it is applicable also through the visible and ultraviolet spectrum. OSS is particularly well suited for remote sensing applications and for the assimilation of satellite observations in numerical weather prediction models. The novel OSS approach is an extension of the exponential sum fitting of transmittances technique in that channel-average radiative transfer is obtained from a weighted sum of monochromatic calculations. The fact that OSS is fundamentally a monochromatic method provides the ability to accurately treat surface reflectance and spectral variations of the Planck function and surface emissivity within the channel passband, given that the proper training is applied. In addition, the method is readily coupled to multiple scattering calculations, an important factor for treating cloudy radiances. The OSS method is directly applicable to nonpositive instrument line shapes such as unapodized or weakly apodized interferometric measurements. Among the advantages of the OSS method is that its numerical accuracy, with respect to a reference line-by-line model, is selectable, allowing the model to provide whatever balance of accuracy and computational speed is optimal for a particular application. Generally only a few monochromatic points are required to model channel radiances with a brightness temperature accuracy of 0.05 K, and computation of Jacobians in a monochromatic radiative transfer scheme is straightforward. These efficiencies yield execution speeds that compare favorably to those achieved with other existing, less accurate parameterizations.

The decrease in atmospheric ozone over Antarctica during spring implies enhanced levels of ultraviolet (UV) radiation received at the earth's surface. Model calculations show that UV irradiances encountered during the occurrence of an Antarctic "ozone hole" remain less than those typical of a summer solstice at low to middle latitudes. However, the low ozone amounts observed in October 1987 imply biologically effective irradiances for McMurdo Station, Antarctica, that are comparable to or greater than those for the same location at December solstice. Life indigenous to Antarctica thereby experiences a greatly extended period of summerlike UV radiation levels.

No abstract