Observed below-Cloud and Cloud Interstitial Submicron Aerosol Chemical and Physical Properties at Whiteface Mountain, New York, during August 2017

Abstract

A pilot study took place at Whiteface Mountain (WFM) in the Adirondacks of upstate New York during the summer of 2017 to evaluate the chemical processing of aerosol within clouds. Below-cloud and cloud interstitial submicron aerosols were characterized in real-time using an Aerodyne High Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) and other instruments deployed via the Atmospheric Sciences Research Center (ASRC) sprinter van mobile laboratory. The primary observation is a dominance of organic aerosol mass, averaging 66%–78% of PM2.5 during urban-influenced periods and 83–93% during biogenic-influenced periods, in stark contrast to observations in the northeastern U.S. in past decades when sulfate dominated the aerosol mass. The observations also show chemical differences between the below-cloud layer, the transition layer (just below-cloud), and the in-cloud layer. Comparison between these layers revealed enhanced inorganic nitrate in interstitial aerosol sampled in the transition and in-cloud layers compared to the below-cloud aerosol. Enhanced particle nitrate mass concentrations were more significant during urban influenced cloud events than biogenic influenced ones. The below-cloud and transition layer aerosols were characterized by similar bimodal number and mass size distributions, and when entrained into the cloud, aerosols with number mean dry diameters of > ∼50 nm were incorporated into cloud droplets, suggesting that cloud supersaturations at WFM can be quite high (>0.6%). The organic component of the interstitial aerosols in the transition and in-cloud layers is slightly less oxidized than the below-cloud aerosol. The hygroscopicity parameter (κ), as determined from size-resolved cloud condensation nuclei (CCN) measurements at water vapor supersaturations ranging from 0.23% to 0.78%, was 0.13–0.19 throughout the majority of the pilot study. Exceptions were two of the five urban-influenced periods (with κ values of 0.27–0.33) and at the lowest supersaturation (0.23%) for two of the three biogenic-influenced periods (with κ values of 0.23–0.25), the latter of which may be attributed to greater accumulation of soluble aerosol mass during aqueous/cloud processing of larger particles. The hygroscopicity of the organic component was estimated to range from 0.08 to 0.22, and was well correlated with the organic oxidation state. The below-cloud and cloud interstitial aerosol observations shown here comprise an important part of the 2017 pilot study at WFM. These observations also provide a baseline for future intensive field campaigns focused on cloud processing at WFM, which will ultimately inform atmospheric models to better simulate cloud processing in the northeastern U.S in comparison to other biogenic-dominated locations.