• Introduction
• Methods
• Results
• Conclusions
• References

FIGURES

• Figure 1
• Figure 2
• Figure 3

Direct sun measurements using a Brewer spectrophotometer have been made at five wavelengths in the UV-B range: 306.3, 310.1, 313.5, 316.8, and 320.1 nm. During August 1999, a field campaign was organized in Campo Grande, MS (20.4^O S, 54.7^O W, 532 m a.s.l.), a Brazilian savanna region subject to seasonal biomass burning. The instrument was especially transported to the observation site, located outside of the city limits. It was determined that the SO₂ amount was negligible. The aerosol optical thickness was deduced in order to assess the impact of biomass burning on this important parameter of the lower atmosphere. It was determined that the technique could resolve time variations of about 10 m duration. The optical thickness was deduced using the Langley plot technique for the direct sun UV-B measurements giving values in the range 0.1 to 3.5 at 306.3 nm.

Introduction

Biomass burning is one of the most important sources of aerosols in the atmosphere. From late winter to middle spring about two thirds of the Brazilian territory is subjected to biomass burning (Kirchhoff et al, 2000). High solar irradiance, dry winter season, and agricultural activity yield an elevated number of fires in the savanna with large amounts of aerosols released to the atmosphere.

A Brewer spectrophotometer MKII was used to measure direct sun (ds) UV-B radiation at wavelengths 306.3, 310.1, 313.5, 316.8, and 320.1 nm during a field campaign in August, 1999 in Campo Grande, which is a rural site in Brazilian savanna region prone to biomass burning. Measurements were done for about one hour and a half both in the morning and in the afternoon. The campaign extended from the 209^th to the 236^th julian day, and cloudless conditions occurred during almost all of the days. Aerosol optical thickness can be obtained from the Langley method applied to the sample of ds measurements and the knowledge of Rayleigh, ozone, and sulfur dioxide optical thicknesses.

Methods

A special observation schedule was used for the Brewer spectrophotometer operation during the Campo Grande campaign . The same solar zenith angles from 60^o to 41^o were chosen as values for ds measurements both in the morning and in the afternoon. A total of 20 measurements were made per period, each lasting about 2.5 minutes.

The Langley method was applied to the 20 ds measurements to obtain the atmospheric optical thickness. Atmospheric optical thickness is equal to

t _at = t _R + t _O3 + t _SO2 + t _ae

where Rayleigh, ozone, sulfur dioxide, and aerosol optical thicknesses are considered, respectively. t _R can be obtained from Hansen and Travis (Teillet, 1990)

t _R = 0.008569l ^-4(1 + 0.0113l ^-2 + 0.00013l ^-4)p/1013

with l in m m as the wavelength, and p as the atmospheric pressure in mbar at the site. t _O3, and t _SO2 can be obtained by the general formula

t = s N2.69E16

where s is the absorption cross section (Molina and Molina, 1986; McGee and Burris Jr, 1987; Davidson et al, 1988), and N the gaseous column integral in the atmosphere. This quantity for ozone and sulfur dioxide was obtained by the Brewer.

Results

Figures 1 and 2 show the aerosol optical thicknesses in the morning and in the afternoon during the campaign at the 5 wavelengths. t _ae ranges from 0.1 to 3.75 with larger values in the afternoon. Uncertainty ranges up to 0.8 and larger aerosol optical thickness seems to carry larger uncertainty values. Large variability can be seen both in the morning and in the afternoon.

Figure 3 shows typical optical thicknesses as a function of wavelength for the afternoon of the julian day 215 (Aug 3), 1999 in Campo Grande. t _R , and t _O3 are the largest components of the atmospheric optical thicknesses, but for wavelengths larger than about 314 nm. There is a weak dependence of aerosol optical thickness on wavelength, and t _SO2 is the smallest optical thickness.

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Conclusions

The Brewer MKII measurements have been made in the narrow band of the UV-B radiation, and the aerosol optical thicknesses have been obtained from the Langley method for a biomass burning site. Values have shown large variability along the campaign, varying from 0.1 to 3.5 at 306.3 nm. Large uncertainties seem to be often associated to large aerosol optical thickness, or a strongly polluted day. The sulfur dioxide optical thickness was much smaller than that of ozone at that rural site.

References

Kirchhoff, V. W. J. H.; Silva, A. A.; Costa, C. A.; Leme, N. P.; Pav“o, H. G.; Zaratti, F. UV-B optical thickness observations of the atmosphere. Journal of Geophysical Research, in press, 2000.

McGee, T. J.; Burris Jr, J. SO₂ absorption cross sections in the near uv. J. Quant. Spectrosc. Radiat. Transfer, v. 37, n. 2, 165-182, 1987.

Molina, T. J.; Molina, M. J. Absolute absorption cross section of ozone in the 185 to 350 nm wavelength range. Journal of Geophysical Research, v. 91, n. D13, 14501-14508, Dec 1986.

Teillet, P. M. Rayleigh optical depth comparisons from various sources. Applied Optics, v. 29, n. 13, 1897-1900, 1990.