Ambient Observations of Sub-1.0 Hygroscopic Growth Factor and f(RH) Values: Case Studies from Surface and Airborne Measurements

Abstract:

Hygroscopic growth occurs when particles take up water vapor and grow when exposed to elevated relative humidity (RH), and is controlled largely by chemical composition. Previous laboratory studies of biomass burning and combustion particles observed particle size shrinkage as soot aerosols, especially those with coatings, were exposed to increasing RH levels, which resulted in sub-1.0 hygroscopicity parameter values (i.e., ratio of humidified-to-dry diameter g(RH) and ratio of humidified-to-dry scattering coefficients f(RH)). To investigate the potential for sub-1.0 hygroscopicity in ambient aerosol, we utilized data from (i) a ship-board HTDMA during E-PEACE 2011, (ii) multiple instruments on the DC8 during SEAC4RS-2013, as well as (iii) the DASH-SP during measurement intensives in Summer 2014 and Winter 2015 in Tucson, Arizona. Suppressed hygroscopicity, including sub-1.0 g(RH), was observed during smoke-influenced periods in SEAC4RS, episodic events in the winter season in Arizona, and smoke-influenced air during E-PEACE. Across the range of RH investigated (75-95%), sub-1.0 g(RH) was lowest at the highest RH values probed (~95%). These sub-1.0 g(RH) observations are consistent with elevated black carbon and organic aerosol concentration in both E-PEACE and SEAC4RS. Collocated measurements during SEAC4RS indicate elevated spikes in black carbon concentrations are coincident with both sub-1.0 f(RH) and g(RH) observations, as well as elevated organic aerosol- and gas-phase fire tracers such as AMS f60 and PTR-MS acetonitrile concentration. This is the first set of ambient observations of sub-1.0 hygroscopicity factors g(RH) and f(RH), with consistency across different instruments, regions, and platforms. Although particle restructuring has been demonstrated in laboratory experiments, field observations are complex as soot coating, secondary chemistry, and heterogeneous processing can occur on the same time scale as measurements. This work motivates continued field observations examining combustion aerosol-water interactions, and instrument modifications to investigate the mechanisms responsible for biomass burning and fresh combustion particle shrinkage upon hydration.