Chemistry and processes of aerosols at Mt. Bachelor, a high elevation site in the Pacific Northwest U.S.: influences from regional transport and wildfire plumes

The Mt. Bachelor Observatory (MBO; 43.9794° N, 121.6885° W, altitude 2,763 m asl)) has been used for 10 years to study wildfire impacts on CO, O3, aerosols and other pollutants in the free troposphere. In the summer of 2013, we deployed an Aerodyne High Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) at the summit of MBO to obtain high resolution data on the chemical composition of aerosols, with an emphasis on biomass burning aerosols, as part of the DOE sponsored Biomass Burning Observation Project (BBOP). A main goal of BBOP is to study the downwind time evolution of the microphysical, morphological, chemical, hygroscopic, and optical properties of carbonaceous aerosol generated by biomass burning (BB). MBO is an ideal location for studying remote and high elevation aerosol and the location allows for free tropospheric air masses to be sampled during the night and air coming from the boundary layer during daytime. Our ground-based measurements are also complimentary to simultaneous aircraft BB plume measurements. Our observations indicate a dynamic variation in the chemical composition and physical properties of aerosols with repeatable diurnal patterns. Periods of low particulate matter (PM) loading show distinctly oxidized organic aerosol (OA) with oxygen-to-carbon atomic ratios (O/C) reaching above 1 as well as containing an ammonium sulfate fraction of up to 50% of submicron aerosol (PM1) mass. Methanesulfonic acid (MSA) is also present during low loading periods, which, together with an aerosol size distribution characteristic of a droplet accumulation mode centered at 500-600 nm in vacuum aerodynamic diameter (Dva), suggests that aqueous-phase processing plays an important role in the regional aerosol sampled at this site. During these same measurements, contrasting periods of higher loading and markedly different characteristics have been observed due to effects from injection of wildfire plumes into air masses transported to MBO. Observations during the affected periods show elevated organic PM1 loading of up to 60 μg/m3 and an overall organic mass fraction of 90% while correlating with elevated aerosol light scattering and gas-phase CO concentration. OA from these events have shown an enhancement in the BB characteristic ion (C2H4O2+) at m/z = 60 and with O/C ranging from 0.4-0.6, suggesting these BB aerosols are intermediately oxidized. Inorganic nitrates and amines also appear to be important components of these detected BB events. More detailed analyses, such as back-trajectory analysis and factor analysis of the HR-ToF-AMS spectra, will be performed to unravel the significance of these sampled events and how the transport affects fresh BB plumes. These analyses may also shed light on the importance of BB emissions as precursors to secondary organic aerosol and their overall effect on regional air quality in the Pacific Northwest.