Lessons from long-term measurements of ice nucleating particles at ARM sites

Description

Interactions between aerosols and clouds are some of the least understood atmospheric processes, especially those involving the airborne particles that facilitate cloud ice formation, the ice nucleating particles (INPs). By triggering primary ice above the homogenous freezing point of water (~-38°C), INPs modify cloud reflectivity, longevity, and the initiation of precipitation. Boundary layer INP concentrations range over orders of magnitude, depending on such factors as season and the surfaces over which the air masses have flowed. In 2020, ARM added long-term observations of INP measurements to its portfolio, with the goal of establishing base measures in representative locations and to improve the understanding of factors that drive INP emissions from sources within the INP “catchment”, the size of which may vary greatly.

We sample using 24 h filters every 3 or 6 days (in duplicate, the 2^nd second filter available to ARM-ASR researchers), and measure INPs with an ice nucleation spectrometer over the range 0 to ~-28°C. We also test a subset of samples after heating (to 95°C) and H₂O₂ digestion to estimate the relative abundance of, respectively, heat-labile (biological) organic INPs and heat stable organic INPs, with the residual being inorganic INPs (probably minerals). Campaigns in which we have collected samples include: 1) AMF3 at Oliktok Point (8/20−6/21), 2) SGP (10/20−ongoing), 3) SAIL (9/21−6/23), 4) TRACER (6/22−9/22, including event-driven measures in coordination with the NSF-funded ESCAPE flight campaign), and 5) EPCAPE (2/23−2/24). Future campaigns include BNF in Alabama, CAPE-K in Tasmania and a plan for INP measurements at the NSA site at Utqiaġvik, AK.

We will present highlights from SGP, SAIL and TRACER. The latter has provided unexpected added value since it included a period dominated by Saharan dust as well as potential insights from using paired sample sites, 76 km apart, that enable the investigation of relative influences of local vs. long-range sources. Additionally, we will report on the successful use of a miniature filter sampler installed on the DOE ARM tethered balloon during SAIL to obtain vertical profiles of INPs to 500 m. We have also learnt valuable lessons regarding quality control: Good communication, novel training methods (e.g., films), clean filter handling, and maintenance. INP data are currently publicly-available on the DOE Data Archive for both observational and modelling studies.