Surface exchange of nitric oxide, nitrogen dioxide, and ozone at a cattle pasture in Rondônia, Brazil

Abstract

[1] Measurements of NO-NO 2 -O 3 trace gas exchange were performed for two transition season periods during the La Niña year 1999 (30 April to 17 May, ''wet-dry,'' and 24 September to 27 October, ''dry-wet'') over a cattle pasture in Rondônia. A dynamic chamber system (applied during the dry-wet season) was used to directly measure emission fluxes of nitric oxide (NO) and surface resistances for nitrogen dioxide (NO 2 ) and ozone (O 3 ) deposition. A companion study was simultaneously performed in an oldgrowth forest. In order to determine ecosystem-representative NO 2 and O 3 deposition fluxes for both measurement periods, an inferential method (multiresistance model) was applied to measure ambient NO 2 and O 3 concentrations using observed quantities of turbulent transport. Supplementary measurements included soil NO diffusivity and soil nutrient analysis. The observed NO soil emission fluxes were nine times lower than oldgrowth rain forest emissions under similar soil moisture and temperature conditions and were attributed to the combination of a reduced soil N cycle and lower effective soil NO diffusion at the pasture. Canopy resistances (R c ) of both gases controlled the deposition processes during the day for both measurement periods. Day and night NO 2 canopy resistances were significantly similar (a = 0.05) during the dry-wet period. Ozone canopy resistances revealed significantly higher daytime resistances of 106 s m À1 versus 65 s m À1at night because of plant, soil, and wet skin uptake processes, enhanced by stomatal activity at night and aqueous phase chemistry on vegetative and soil surfaces. The surface of the pasture was a net NO x sink during 1999, removing seven times more NO 2 from the atmosphere than was emitted as NO.

Seasonal variation of ozone deposition to a tropical rain forest in southwest Amazonia

“…This might particularly be the case for the Brazilian state of Rondônia, where large areas of primary rain forest are continuously deforested and subsequent development to cattle pastures takes place (cf. Kirkman et al, 2002). The average deforestation rate in central Rondônia increased between 1990 and 1999 from ∼1.2% per year to ∼3.4% per year (Roberts et al, 2002).…”

“…The average deforestation rate in central Rondônia increased between 1990 and 1999 from ∼1.2% per year to ∼3.4% per year (Roberts et al, 2002). The LBA-EUSTACH 1 and 2 experiments offered the unique possibility to establish a direct and comprehensive comparison of O 3 uptake by pasture and rain forest, since Kirkman et al (2002) measured O 3 deposition at a cattle pasture site simultaneously to the forest work during both campaigns, using a combination of soil chamber and micrometeorological profile techniques. As shown in Fig.…”

“…The mean R c of the forest canopy is about 55 s m −1 during daytime (see Sect. 4.3), only half of the mean R c of the pasture canopy of 100 s m −1 (Kirkman et al, 2002).…”

“…We will report on ozone fluxes measured by eddy covariance during campaigns at the end of the wet season (May 1999) and at the end of the dry season (September/October 1999) at a forest tower site in Rondônia, Brazil. Simultaneously with our rain forest experiment, O 3 flux measurements were performed by Kirkman et al (2002) on a 60 km distant pasture site that was established by deforestation more than 20 years before. By comparing the forest and pasture fluxes under similar environmental conditions, we will assess the impact of deforestation and land-use change on ozone deposition.…”

Estudo da concentração do CO2 atmosférico em área de pastagem na região amazônica

“…Em contraste ao CO 2 , o O 3é geralmente esgotado na camada limite noturna. De meia-noite até o nascer do sol, normalmente as concentrações de O 3 são menores de 3ppb (Figura 9) devidoà deposição superficial e reações químicas do O 3 com oóxido nítrico emitido pelo solo (Kirkman et al, 2002). No dia 14 de maio das 01:30às 03:30 horas a concentração de O 3 e menor do que 1 ppb.…”