Tracking long-range transported upper-tropospheric pollution layers with a newly developed airborne Hyperspectral Sun/Sky spectrometer (4STAR): Results from the TCAP 2012 campaign

TCAP, the Two Column Aerosol Project, was aimed at providing a detailed set of observations to investigate topics related to radiation and aerosol-cloud interactions, and to learn about aging and transport of atmospheric aerosols and gaseous constituents that are related to tropospheric pollution events. During the year-long campaign, an intensive airborne deployment was held in the summer of 2012 based at the Hyannis airport, Cape-Cod, MA. In the course of the campaign, the newly developed Spectrometer for Sky-Scanning, Sun-Tracking Atmospheric Research (4STAR) flew onboard the DOE Gulfstream 1 (G-1) aircraft, together with a suite of in-situ instruments to measure atmospheric state parameters and aerosol and cloud characteristics. One of the unique features of the 4STAR instrument, stemming from its design using grating spectrometers that cover the UV-VIS-SWIR spectral range (i.e. 350-1700nm), is its capability to measure atmospheric trace gases such as water vapor, O₃ and NO₂ concurrently with spectrally resolved aerosol optical depth (AOD). Here, we utilize the 4STAR measurements above the planetary boundary layer (PBL) (i.e. above 3000 meters) to investigate atmospheric composition of elevated pollution layers transported from the continental US and Canada during the TCAP summer phase. The 4STAR-retrieved values of AOD at 500 nm, Ångstrom exponent (AE) at 500 nm, columnar water vapor (CWV), and NO₂ are used as variables in a k-means clustering algorithm to determine the atmospheric composition characteristics of the observed elevated polluted layers during the July flights. We found that, compared to AOD, NO₂ displays less variability in plumes that are related to biomass-burning (BB) emissions over the course of several days. HYSPLIT back-trajectory analysis has confirmed our clustering results of two major air-mass sources: a relatively dry and clean upper tropospheric source and a humid, polluted one. Our clustering analysis, resulting in different ocean and land columns of different atmospheric constituents was found to be in agreement with in-situ measurements by the SPLAT (Single Particle Laser Ablation Time-of-Flight mass spectrometer) instrument aboard the G-1 during the July 17 flight, which identified high nitrate and BB amounts, and with the aerosol type product from the HSRL-2 (High Spectral Resolution Lidar-2) aboard the NASA B-200 aircraft, which identified fresh-smoke and urban pollution at specific instances corresponding to our analysis results.