A key step in having a global daily PM2.5 data product for health and other applications is estimating what fraction of the total Aerosol Optical Depth (AOD) seen routinely by satellites is in the atmospheric boundary layer. The atmospheric boundary layer is the layer of the atmosphere closest to the surface where we generally live and breath. We have for the first time automated a process by which we can estimate the fraction of the total AOD in the lowest km of the atmosphere on a daily basis. We do this by taking every single NASA CALIPSO vertical profile and flying forward and backward trajectories from each point on the profile using a trajectory model.This is a massive number crunching exercise that we perform on our local computational cluster. This allows us to estimate the global 3D aerosol backscatter distribution on a daily basis. We then split the planet up into a regular grid (in the above example it is a 1×1 degree grid). For each grid cell we calculate the fraction, f, of the total backscatter which is in the lowest km. In the movie above we have animated the subset of our global analysis over just the USA for first six weeks of CALIPSO data.

Following from our prototype studies we have identified that the determination of the fraction, f, of the total aerosol extinction due to aerosols in the boundary layer is an important missing parameter needed to improve accuracy in the estimate of the PM2.5 abundance.

#### Aerosol in the lowest km