Diffusion and emission of smoke from agricultural burning in Hawaii

Abstract

Agricultural burning has been common practice for many years in Hawaii. Cane fields are burnt before harvesting and the crowns of pineapple are cut-off during harvest, left in the field to dry and subsequently burned. While this practice has many advantages, it produces air pollution levels which at least one study finds affect public health. In 1974 open burns of cane and pineapple were simulated in a burn tower where samples were burned while monitoring weight loss and smoke density. From these measurements emission values were calculated. However, because questions were raised as to whether the vigorous field fires can be simulated. in a burn tower, a field survey was undertaken and a diffusion model, basically a limited Gaussian line source model, was developed and run using the data collected during the burns. The calculated emission rate for particulates is orders of magnitudes larger than the burn tower rate. This is attributed to dust drawn into the fire. When emission values from a survey of the open burn literature and from the present study were plotted against wind speed, low intensity fires such as grass, straw and pineapple residue fires showed decreased emissions with increasing wind speed while cane fires showed increased emission. This suggests that emission characteristics are different for low and high intensity fires. For low intensity fires increased air flow acts mainly to increase the combustion efficiency producing less particulates. For high intensity fires, such as cane fires, increased air flow results in hotter fires lofting more dust, which is the major pollutant from such fires. Revised emission values are recommended.