A Network of Autonomous Particle Spectrometers to Measure Spatiotemporal Variability in Particle Size Distributions in Intraurban Environments

Ambient aerosol size distribution plays an important role in influencing the earth’s radiative budget, cloud formation and properties, human health and visibility. Ambient aerosol size distributions are governed by proximity to sources and type of source, atmospheric processes, meteorology, and land-use and, hence, are expected to vary by location and time within urban areas. Given the complexity, footprint, and cost of instrumentation, aerosol size distributions are not routinely measured, and data are generally restricted to short durations during intensive field campaigns. Hence, spatiotemporal heterogeneity in aerosol size distributions in urban environments remains poorly constrained.

In this work, we developed and deployed a network of autonomous aerosol size spectrometers (AS²) to measure ambient particle size distributions in Fort Collins and Denver, CO. The AS² system consists of a portable optical particle spectrometer (POPS; Handix Scientific), a solar-powered battery system, and an IoT (Internet of Things) module. The POPS uses a 405 nm laser to size individual particle between 0.14 and 3 µm at 1 Hz. The power system consists of a 100 W photovoltaic solar panel charging a 36 Ah battery pack, designed to provide sufficient power for continuous summertime operation in Colorado. The IoT module is a cellular based microcontroller that filters, averages, and transmits particle size distribution data for up to 20 size bins to the cloud every minute.

Pilot deployments of the AS² system were co-located with an EPA PM_2.5 monitor (GRIMM EDM108) in Fort Collins, CO for five days and Denver, CO for three weeks. PM_2.5 mass concentrations from the AS² agreed closely with those measured by the EPA monitors (N=703 and R²=0.770 for Fort Collins, CO for an assumed particle density of 1.8 g cm^-3). Ongoing work includes a multi-week deployment of five AS² systems in two different seasons in the metropolitan region of Denver, CO. The five locations are representative of environments dominated by traffic, construction, industrial emissions, private residences and rural land. Targeted deployment of a scanning mobility particle sizer alongside the AS² will be used to develop in-situ calibrations to convert optical diameters from the AS² to geometric diameters. The field data will be used to evaluate the spatial and temporal heterogeneity in particle size distributions in a representative urban environment.