Analysis of the UHSAS Particle-by-Particle data in BBOP: An indicator for new aerosol particle formation

Description

An aerial UHSAS (Ultra High Sensitivity Aerosol Spectrometer) was deployed on the G-1 Research Aircraft in the BBOP (Biomass Burning Observation Project) in 2013. The UHSAS characterizes aerosol particles by optical scattering providing a measure of atmospheric aerosol size and number density. The UHSAS also provides a Particle-by-Particle (PbP) datastream, which measures the time of arrival for each aerosol and its size. This PbP data is then size binned for a specified time period to generate the conventional UHSAS data. The aerosol time of arrival data can be used to calculate the spatial distribution of aerosols in the atmosphere that is sampled by the UHSAS probe. This is generally not of interest since almost all atmospheric aerosol spatial distributions follow a random Poisson distribution. A convenient parameter to test this is the Cluster Index (CI), defined as CI =[ < var(t)2>/< t>] -1 in which t is the inter-arrival times of aerosol particles, that is, the time between the arrival of aerosol particle and the next aerosol particle, is the mean inter-arrival time and the standard deviation over some measurement period. For most atmospheric aerosols, the spatial distribution is a random Poisson distribution and therefore the Cluster Index is zero. A Cluster Index greater than 0, indicates that the particles are beginning to "cluster". BBOP studied aerosols that were produced in flames in forest fires, which is new aerosol particle formation. It is expected that these new aerosol particles will be initially clustered when they formed from the fire processes. As these aerosol particles age they will be thermally equilibrated into a random poissionian distribution. The Cluster Index analysis is applied to the BBOP PbP datastreams and indeed there are regions of clustered aerosols. This analysis will useful in finding smoke plumes which are actively forming new aerosol particles by combustion versus aged smoke plumes with no combustion forming no new aerosol particles.