Trends and sources of ozone and sub-micron aerosols at the Mt. Bachelor Observatory (MBO) during 2004-2015

To better understand the causes for the seasonal and interannual trends at MBO, we identified four major categories of air masses that impact O3, carbon monoxide (CO) and aerosols: upper troposphere and lower stratosphere (UTLS) O3 intrusion, Asian long-range transport (ALRT), Arctic air pollution (AAP) and plumes from the Pacific Northwest region (PNW). ALRT and PNW plumes can be further divided into wildfires (WF), industrial pollution (IP) and mineral dust (MD). Over the 12 years of observations, 177 individual plume events have been identified. Enhancement ratios (ERs) and Ångström exponents (AEs) of aerosols were calculated for all events. The lowest slope of Δσsp/ΔO3 is a unique feature of UTLS events. PNW-WF events have the highest averages for Δσsp/ΔCO, Δσsp/ΔO3 and Δσsp/ΔNOy compared to other events. These ERs decrease during long-range transport due to the shorter residence time of aerosols compared to the other pollutants. ALRT-WF events have lower absorption AEs (Åap) than PNW-WF, implying that brown carbon (BrC) is generated from biomass burning but its fraction decreases during long-range transport. Signatures of ERs and AEs are useful tools to identify different plume categories. These results demonstrate the increasing impact of baseline O3 on US air quality due to both global sources and regional wildfire events.