Aircraft measurements of the impacts of urban plume on cloud activation properties during GoAmazon – preliminary results

Abstract:

Currently, the indirect effects of atmospheric aerosols remain the most uncertain components in forcing of climate change over the industrial period (IPCC, 2007). This large uncertainty is partially a result of our incomplete understanding of the ability of particles to form cloud droplets under atmospherically relevant supersaturations. One of the objectives of the US Department of Energy (DOE) Green Ocean Amazon Project (GoAmazon) is to understand the influence of the emission from Manaus, a tropical megacity, on aerosol size, concentration, and chemical composition, and their impact on aerosol cloud condensation nuclei (CCN) spectrum.

During the GoAmazon study, size distributions, CCN spectra and chemical composition of aerosols both under pristine conditions and inside Manaus plume were measured in-situ from the DOE Gulfstream 1 (G-1) research aircraft during two Intensive Operations Periods, one conducted in the wet season (Feb 22- March 24, 2014) and the other in dry season (Sep 1 – Oct 10, 2014). Aerosol size distributions were measured by a Fast Integrated Mobility Spectrometer (FIMS) and compared with the merged size distribution from two other instruments, an Ultra High Sensitivity Aerosol Spectrometer – Airborne (UHSAS-A, DMT), and a Passive Cavity Aerosol Spectrometer Probe (PCASP-200, DMT). Optical measurements of light scattering by nephelometer and absorption by a particle soot absorption photometer (PSAP) were combined with number/size distributions data in a iterative method, which retrieves the effective imaginary refractive index of the particles at a wavelength of 545 nm. Aerosol chemical composition was characterized using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS, Aerodyne Inc.). CCN number concentration was measured by a DMT dual column CCN counter at two supersaturations 0.25% and 0.5%. Based on the aerosol properties mentioned above, CCN closure is carried out. In addition, the sensitivity of calculated CCN spectrum to organic aerosol hygroscopicity is examined. The differences in aerosol/CCN properties between two seasons will be discussed.