Observed aerosol properties and aerosol forecast evaluation in the Arctic region during cold air outbreaks

Arctic aerosols play an important role in modulating high-latitude weather and climate. However, spatiotemporal aerosol distributions in the Arctic remain poorly quantified. Here, we present aerosol physicochemical properties observed during the Cold Air Outbreak Experiment in the Sub-Arctic Region (CAESAR) campaign. The airborne measurements covered a wide vertical range, from the near-surface to the free troposphere, across remote regions between Scandinavia and the coast of Greenland. A merged particle distribution product, spanning from 10 nm to 3 μm in diameter, was developed by combining a suite of five aerosol sensors. Number closure was achieved between the different instruments. Estimated aerosol concentration from the merged size distribution varied between ~50 and ~ 700 scm −3 . Refractory black carbon mass concentrations (70–800 nm) varied between ~0.9 and ~ 80.4 ng sm −3 . Aerosol mass concentration observations were compared to the NASA Goddard Earth Observing System composition forecast (GEOS-CF). The model concentrations correlated with the observations exhibit agreement with the spatial and temporal distribution of localized black carbon aerosol plumes. During Research Flight (RF) 09, the black carbon mass concentration model-vs-observation Pearson correlation coefficient r is 0.84. Observed fine particulate matter (PM 2.5 ) were significantly larger than the model concentrations within the marine boundary layer (MBL). The high observed PM 2.5 concentrations were due to coarse mode aerosol, suggesting that the model did not accurately capture sea-spray production. Over the study domain, the median normalized mean bias (NMB) was −2454% and the normalized mean root square error (NMRSE) was 35% for PM 2.5 ; and 1372% (10,619%) and 90% (177%) for refractory black carbon within the marine boundary layer (free troposphere), respectively. The dataset and data/model intercomparison provide new constraints for understanding aerosol properties in the Arctic region.