11/28/2018

Molecular and Physical Characteristics of Aerosol at Pico Mountain Observatory in the Azores

Getting above the boundary layer to understand aerosol aging.

The Science

Oxidation plays an important role in governing the interactions of organic aerosol with clouds and atmospheric heterogeneous chemical reactions. To learn more about the chemistry of long-range-transported free tropospheric aerosol, the researchers studied the molecular composition of organic aerosol collected at the Pico Mountain Observatory in the Azores.

The Impact

Aerosol transported in the free troposphere was found to be less oxidized than aerosol transported in the boundary layer. This has implications for the lifetime of aerosol transported in the free troposphere, where the relative humidity and temperature are lower compared to the marine boundary layer.  This is especially relevant for pyro-convected wildfire emissions, including brown carbon, and therefore may be important for an improved understanding of the role of aerosol on Earth’s radiative balance.

Summary

Three aerosol samples collected at the Pico Mountain Observatory in the Azores were analyzed using ultra-high-resolution mass spectrometry to determine their molecular compositions. Two samples exhibiting an overall lower extent of oxidation were transported in the free troposphere and had been aloft for approximately one week, as demonstrated by back trajectory simulations using the FLEXible PARTicle dispersion model (FLEXPART). The ambient relative humidity and temperature were retrieved from the Global Forecast System data for the air masses corresponding to the FLEXPART back trajectory plumes and were used to estimate the relative humidity-dependent glass transition temperatures of the identified components. The relative humidity-dependent glass transition temperatures for the less-oxidized samples indicated a relatively higher organic aerosol viscosity, implying a decreased susceptibility to oxidative processes. One sample, in particular, was heavily influenced by wildfire emissions and showed evidence of brown carbon after long-range transport. Previously, a majority of the brown carbon associated with wildfire emissions was expected to have a lifetime on the order of one day. In contrast, a sample with anthropogenic influence and transported in the boundary layer was much more oxidized despite a shorter transport time (~three days). These findings indicate the importance of the transport path and ambient conditions on the lifetime of organic aerosol.

Principal Investigator(s)

Lynn Mazzoleni
Michigan Technological University
lrmazzol@mtu.edu

Funding

This project was supported with funding from NSF (AGS-1110059) and DOE (DE-SC0006941).
Logistical support for the operation of the Pico Mountain Observatory was provided by the Regional Government of the Azores through the Regional Secretary for Science and the Pico Island Natural Park. Major equipment cost share and graduate student support associated with this project was provided by the Earth, Planetary, and Space Sciences Institute at Michigan Technological University.   Melissa Soule and Elizabeth Kujawinski of the Woods Hole Oceanographic Institution (WHOI) Mass Spectrometry Facility provided FT-ICR MS instrument time and assistance with data acquisition (NSF OCE-0619608 and Gordon and Betty Moore Foundation).

References

Schum, S., B. Zhang, K. Džepina, P. Fialho, C. Mazzoleni, and L. Mazzoleni. “Molecular and physical characteristics of aerosol at a remote free troposphere site: implications for atmospheric aging.” Atmospheric Chemistry and Physics 18(19), 14017-14036 (2018). [DOI:10.5194/acp-18-14017-2018]