Skies Over America: Measuring Aerosols with High-Flying Instruments

In October and November 2017, NASA and the Netherlands Institute for Space Research (SRON) teamed up for the ACEPOL campaign. This event saw six advanced instruments, including four multi-angle polarimeters (MAPs) and two lidars, fly over the western United States on NASA's ER-2 high-altitude aircraft. The goal? To study aerosols, tiny particles in the air, and how they affect our environment. The four MAPs – SPEX airborne, AirHARP, AirMSPI, and RSP – measured aerosol optical depth (AOD), which tells us how much sunlight is blocked by these particles. They also looked at the size of these tiny particles, known as the fine-mode effective radius. The SRON aerosol retrieval algorithm was used to analyze the data from SPEX, RSP, and AirMSPI. How did they do? Well, when compared to AERONET measurements, the MAPs were pretty accurate. The mean absolute error (MAE) for AOD at 440 nanometers ranged from 0. 014 to 0. 024. For the fine-mode effective radius, the MAE was between 0. 021 and 0. 028 microns. But what about when the aerosol load was low? On October 26, over California, Arizona, and Nevada, the MAPs showed they could still provide accurate AOD retrievals, with MAEs of 0. 014 and 0. 022 at 532 nanometers for SPEX and RSP, respectively. Even in a high AOD situation, like over a forest fire in Arizona on November 9, the MAPs and HSRL-2 (another instrument) agreed reasonably well, with MAEs of 0. 088 and 0. 079. The aerosol depolarization ratio, a measure of how round these particles are, also showed good agreement between the MAPs and HSRL-2, with MAEs of 0. 023 and 0. 016. So, what does all this mean? It shows that these high-flying instruments are pretty good at measuring aerosols, even in challenging conditions.