Evolution of gases and particles from a savanna fire in South Africa

Peter V Hobbs,Song Gao,Robert J Yokelson,Tica Novakov,Ted J Christian,Parikhit Sinha,Peter Pilewskie,Donald R Blake,Thomas W Kirchstetter

doi:10.1029/2002jd002352

Abstract

Airborne measurements of particles and gases from a 1000‐ha savanna fire in South Africa are presented. These measurements represent the most extensive data set reported on the aging of biomass smoke. The measurements include total concentrations of particles (CN), particle sizes, particulate organic carbon and black carbon, light‐scattering coefficients, downwelling UV fluxes, and mixing ratios for 42 trace gases and 7 particulate species. The ratios of excess nitrate, ozone, and gaseous acetic acid to excess CO increased significantly as the smoke aged over ∼40–45 min, indicating that these species were formed by photochemistry in the plume. For 17 other species, the excess mixing ratio normalized by the excess mixing ratio of CO decreased significantly with smoke age. The relative rates of decrease for a number of chemical species imply that the average OH concentration in the plume was ∼1.7 × 107 molecules cm−3. Excess CN, normalized by excess CO, decreased rapidly during the first ∼5 min of aging, probably due to coagulation, and then increased, probably due to gas‐to‐particle conversion. The CO‐normalized concentrations of particles <1.5 μm in diameter decreased, and particles >1.5 μm diameter increased, with smoke age. The spectral depletion of solar radiation by the smoke is depicted. The downwelling UV flux near the vertical center of the plume was about two‐thirds of that near the top of the plume.

Highlights

Liousse et al [1995] used ground-based Sun photometer measurements, and measurements of carbonaceous particles collected on filters, to infer that smoke aerosol from a savanna fire on the Ivory Coast underwent rapid transformations with aging
As the smoke aged the excess nonmethane hydrocarbons (C < 11) ratioed to excess Carbon monoxide (CO) dropped by about one-third, and the aerosol mass to CO ratio increased by $20– 40% with one-third to one-half of the mass growth likely occurring in the first few hours of aging
In this paper we describe such measurements for a relatively large prescribed savanna fire that was ignited in the Timbavati game reserve in South Africa on 7 September 2000

Summary

Introduction

[2] Biomass burning is a major source of atmospheric trace gases and particles [Crutzen and Andreae, 1990; Fishman et al, 1991]. Measurements of particles and gases were made on smoke samples of known ages downwind of several savanna fires. A TSI 3025A ultrafine condensation particle counter was used to obtain continuous measurements in the smoke plume of the total concentrations of particles (CN), in the size range 0.003 – 3 mm diameter, with a precision of 10%. The AFTIR was used to grab smoke samples and to detain them for 2 –3 min of signal averaging This allowed measurements of the major reactive and stable trace gases present above 5 – 20 ppbv. [32] Shown in Figures 5 and 6 are the light-scattering coefficient due to particles and CN concentrations measured in horizontal tracks across the width of the smoke plume and at various distances downwind of the fire.

Results and Discussion

Transformations of Chemical Species in the Plume

Conclusions