  In this paper we statistically characterise the bio-aerosol background in an
urban environment. To do this we measure concentration levels of naturally
occurring microbiological material in the atmosphere over a two month period.
Naturally occurring bioaerosols can be considered as noise, as they mask the
presence of signals coming from biological material of interest (such as an
intentionally released biological agent). Analysis of this 'biobackground' was
undertaken in the 1-10 um size range and a 3-9% contribution was found to be
biological in origin - values which are in good agreement with other studies
reported in the literature. A model based on the physics of turbulent mixing
and dispersion was developed and validated against this analysis. The Gamma
distribution (the basis of our model) is shown to comply with the scaling laws
of the concentration moments of our data, which enables us to universally
characterise both biological and non-biological material in the atmosphere. An
application of this model is proposed to build a framework for the development
of novel algorithms for bio-aerosol detection and rapid characterisation.
