Pre-harvest Burning

Sugar cane field burning is carried out before harvesting the cane to make the process easier and require less manual labor. It takes place during the harvest season, lasting from May to November (dry season) in the South and East, (Cannavam Rípoli, 2000) with the peak of the burning season being in August. ((1)Lara, 2005) In the burning process, the field is set fire to and the leaves are burned off of the stalks. About 80% of the “trash,” including straw, the tops, and green and dry leaves, are burned off. These components constitute about 25% of the entire sugar cane stalk. The burning kills microorganisms and burns the trash, both of which keep the soil rich when left in the fields. In place of burning the cane, the leaves could be removed and burned to create steam for electricity generation or be converted into fuel themselves. (Cannavam Rípoli, 2000)

The Amazon is blanketed with clouds for the majority of the year due to the large amount of water vapor released into the air by the thick canopy. The sugar cane industry is strongly rooted in the Amazon, where the soil and climate are well suited for sugar cane growth. Southern and Southeastern Brazil are heavily involved in the production of sugar cane in response to the increase in demand for bio-fuels such as Ethanol.

During the burning season, smoke covers huge areas of the Amazon, warming the cloud layer and reducing the updrafts which form clouds. Smoke has a lower albedo than the clouds, allowing more solar energy to enter the Earth’s atmosphere, affecting the climate. Concentrations of smoke over the Amazon were measured from 2000 to 2007, with the results showing an increase of 60% in concentration from 2000-2005, a drop in 2006, and then a large spike in 2007. In 2007, much of the land which was previously used for soy farms and cattle pastures were converted to sugar cane fields. Many of the farmers are phasing out burning over the next ten years, leaving the smoke emissions from mechanical harvest an issue. (Lindsey, 2008)

The aerosols emitted from sugar cane field burning act as cloud condensation nuclei (CCN), enabling the formation but decreasing the size of cloud droplets. Associated decrease in formation of larger water droplets and precipitation allows for increase in water and pollutant transport to the upper troposphere. Although smaller droplet size increases the reflectivity of the clouds, the increase in aerosol optical depth (AOD) resulting from smoke in the atmosphere counteracts the temperature decrease. As AOD increases from 0.1 to 1.0, cloud temperature increases by about 3K (Yu, 2007) (clean background atmosphere is considered to be AOD <0.2, and very hazy conditions are indicated by an AOD = 1 ). (Remer, 2009) Atmospheric residence time for the aerosols lasts from days to several weeks and they can be widespread from one hundred to thousands of kilometers. ((1)Lara, 2005)

Other resulting air pollutants from sugar cane burning include acidic fine particles, such as secondary nitrates and sulfates and carbon compounds (Allen, 2004) Carbon monoxide and methane react with atmospheric hydroxyl radicals, which decreases oxidation efficiency, nitric oxide and hydrocarbons produce high ozone concentrations during the dry season. (Crutzen, 1990) One study carried out showed that burning season produced significantly higher concentrations of HCOO-, CH3COO-, C2O2-4, SO2-4, NO-3, , K+, NH+4, Mg2+and Ca+, with the only tested species unaffected being Cl- and Na+. The aerosols are a mechanism for transport of species which affect soil nutrients and cause surface acidification, such as sulfates, nitrates, ammonium and organic acids. (Allen, 2004) Results of some research shows that sugar cane fires contribute to about 60% of fine particle mode mass, 64% of the black carbon mass, and 25% of the course particle mode mass present in urban areas adjacent to cane fields.. ((1)Lara, 2005)

Comparative to forest fires, which result in 200 to 300 tons of burned organic material per hectare, sugar cane fires produce only about a tenth of the carbon emissions. About 20 tons of sugar cane trash is burned per hectare, which releases about 0.48 Tg of carbon into the atmosphere annually,. Sugar cane burning occurs over a comparatively short period of time, after which the crop does not smolder, which is the main cause of carbon emissions in forest fires. ((2)Lara, 2005)

While removal of leaves may also be carried out by field workers, field burning costs much less money and makes the cane easier to harvest. The practice burns scorpions, snakes, and bees which would otherwise be a danger to the laborers harvesting “green,” or unburned, cane. Workers also collect on average only about a fifth of green cane as they can burned cane per day. Modern day mechanical cane harvesters are also inefficient, yet the difference between harvesting burned and green cane is, at maximum, 20 percent. (Cannavam Rípoli, 2000)

Works Cited

Allen, A., Cardoso, A., and Da Rocha, G. (2004). Influence of sugar cane burning on aerosol soluble ion composition in Southeastern Brazil. Atmospheric Environment ISSN 1352-2310. vol. 38, no30, pp. 5025-5038 [14 page(s) (article)] (1 p.1/4).

Cannavam Rípoli, T., Molina, W., and Cunali Rípoli, M. (December, 2000). Energy Potential of Sugarcane Biomass in Brazil. Scientia Agricola.

Crutzen, P. and Andreae, M. (December 21, 1990). Biomass Burning in the Tropics: Impact on Atmospheric Chemistry and Biogeochemical Cycles. Science, Vol. 250. no. 4988, pp. 1669 - 1678
DOI: 10.1126/science.250.4988.1669.

(1)Lara, L., Artaxo, P., Martinelli, L., Camargo, P., Victoria, R., and Ferraz, E. (April 11, 2005). Properties of aerosols from sugar-cane burning emissions in Southeastern Brazil.

Remer, L. (October, 2009). Global Maps: Fire/Aerosol Depths. NASA Earth Observatory.

Yu, H., Fu, R., Dickinso, R., Zhang, Y., Chen, M., and Wang, H. (March 26, 2007). Interannual variability of smoke and warm cloud relationships in the Amazon as inferred from MODIS retrievals.