AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
New Particle Formation Increases CCN Yield in Veldt Fire Plumes in Southern Africa
VILLE VAKKARI, Johan Paul Beukes, Petri Tiitta, Andrew D. Venter, Kerneels Jaars, Miroslav Josipovic, Pieter G. van Zyl, Veli-Matti Kerminen, Markku Kulmala, Lauri Laakso, University of Helsinki, Helsinki, Finland
Abstract Number: 245 Working Group: Carbonaceous Aerosols in the Atmosphere
Abstract Biomass burning is one of the largest sources of carbonaceous aerosol particles and several trace gases in the atmosphere. In southern Africa, barring the industrial hub around Johannesburg, wild fires and prescribed burning are the most important source of aerosol particles during the dry season, from June to September. In this study, combining remote sensing fire observations to ground-based long-time measurements of aerosol particle and trace gas properties at the Welgegund measurement station (www.welgegund.org), we have been able to follow the time evolution of veldt fire plumes up to several hours in the atmosphere in both day and night time conditions.
From May 20th 2010 to April 15th 2012 altogether 61 veldt fire plumes were observed at Welgegund. Approximately 50% of the time the plumes were transported in fully dark and 20% of the time in daylight conditions. The evolution of the aerosol size distribution was rapid for five hours after the emission: during this period the growth rate of the fire aerosol count mean diameter (size range 12 to 840 nm) was 24 nm h$^(-1) in daytime plumes and 8 nm h$^(-1) in nighttime plumes. The online chemical composition measurements show a factor of 2.5 higher organic aerosol emission ratio in daytime plumes compared to nighttime plumes.
New particle formation in daytime plumes was found to increase the number of particles larger than 100 nm (N100) so that after three hours the N100 emission ratio was two to three times as high as the initial emission, indicating that in daylight conditions most CCN-sized particles in a veldt fire plume originate in nucleation within the plume. In the nighttime plumes no new particle formation was observed, but during the first two hours the N100 emission ratio did increase, although substantially less than in the daytime plumes.