ARM aerosol and trace gas measurements during the MOSAiC expedition

Abstract:

U.S. Dept. of Energy's Atmospheric Radiation Measurement (ARM) program deployed its second mobile facility (AMF 2), including an ARM Aerosol Observing System (AOS) as part of the MOSAiC expedition. The AOS has a complement of instruments for measuring the properties of atmospheric aerosol particles and trace gases.

Time series of ARM aerosol data analyzed here span over a time range from mid-October 2019 to mid-May 2020. Average ambient temperature during this period remains below -15 C with a minimum of -30 C. The entire period is characterized by generally low trace gas and aerosol number concentrations. CO and O₃ concentrations are below 0.15 ppmv and 35 ppbv respectively, without strong seasonal variation. CO data shows occasional small spikes correlated with similarly small increases in particle number concentration, likely originating from snowmobile and helicopter activity near the ship. Particle number concentrations are below 200 cm^-3. Average particle diameter is around 120 nm, increasing to about 150 in January-February and displaying less variability during this period before decreasing to previous levels. Aerosol particles are moderately hygroscopic with a growth factor of 1.6-1.7 and kappa of 0.6-0.7. CCN activation ratios are above 0.4 for all supersaturation set points over 0.1%. Average activation ratios for all supersaturations are higher in January and February by about 15-25% compared to all other months and display lower variability during this time.

From general baseline conditions, short periods with high particle number concentration stand out. During these periods the average particle concentration increases to above 10⁴ cm^-3 and the average particle diameter drops down to < 50 nm. There is also a significant decrease in CCN activation with activation ratios remaining below 0.3 for all supersaturations set points. These periods are strongly correlated with winds from the direction of the ship, indicating local contamination. However, the average CO concentrations do not change during these periods, suggesting that the contamination does not originate from the ship's stack, but instead from one or more numerous air vents scattered around the ship.

Further data analysis will include particle chemical composition while excluding samples associated with local contamination.