Estimating the aerosol scattering coefficient in near-pristine locations with a low-cost sensor

Current climate modeling requires a robust set of models and atmospheric data for predicting anthropogenic aerosol radiative forcing. Currently, there are uncertainties in the modeling results due in part to the sparseness of ground-based data used to evaluate and refine the models. These uncertainties can be reduced by the use of a distributed set of low-cost sensors which can provide scattering and absorption coefficient estimates at a higher geographical and temporal resolution. This presentation describes the evaluation of one such device, the PurpleAir monitor. These are optical particle counters that provide particle counts in several size ranges. The resulting size distributions can be used to calculate the aerosol scattering coefficient, making the PurpleAir monitors potential "surrogate nephelometers". In this work, PurpleAir instruments were deployed at the Mauna Loa Observatory at 3,400 m elevation, the White Mountain Research Center's Barcroft Observatory at 3,893 m elevation, and the Great Basin National Park at 2,063 m elevation. These monitors are collocated with nephelometers at Mauna Loa and Great Basin, and with a portable ozone monitor at Barcroft Observatory. All three sites are characterized by low aerosol concentrations, but Barcroft Observatory and Great Basin can be impacted by wildfire smoke resulting in high concentrations. Preliminary results at Mauna Loa show good correlation between the PurpleAir estimated and measured scattering coefficients, down to 1 Mm^-1. As a result, we are evaluating how these may be useful in estimating the scattering coefficient at other locations without nephelometers. The results to date at Barcroft Observatory indicate that, similar to Mauna Loa, ultra-low aerosol concentrations can occur at night when the observatory is likely in the free troposphere. The data also show that there is greater diurnal variability in aerosol concentrations at Barcroft than for ozone.