Causes of high O3 in the lower free troposphere over the Pacific Northwest as observed at the Mt. Bachelor Observatory

We measured O-3, CO, Hg, sub-micron particle scattering of mid-visible light (asp), and water vapor (WV) at the Mt. Bachelor Observatory (MBO) in central Oregon. USA since 2004. The data were used to identify sources of large ozone enhancements in the lower free troposphere (FT). A total of 25 high-ozone events, defined as time periods when the 8-h averaged O-3 mixing ratio was >70.0 ppbv, were recorded. The events occurred between early March and late September. For 18 of the 25 event days we were able to identify sources based on (1) relative enhancements of O-3, CO, Hg and WV compared with the corresponding monthly distributions and (2) supporting data from the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model, the Navy Aerosol Analysis and Prediction System (NAAPS) global aerosol model, and Moderate Resolution Imaging Spectroradiometer (MODIS) satellite-derived active fire counts from the Fire Information for Resource Management System (FIRMS). Our analysis suggests that enhanced O-3 levels during all the identified events were transported to MBO in the FT and were driven mostly by subsidence of O-3-rich air masses from the upper troposphere/lower stratosphere (UT/LS), Asian long range transport (ALRT) and mixed ALRT + UT/LS influences. The UT/LS events were depleted in CO and total atmospheric mercury (TAM) compared to monthly median values. Levels of O-3 and CO tended to be anti-correlated during UT/LS events, consistent with transport from clean regions in the UT. Conversely, the ALRT events were characterized by concomitant enhancements in CO and TAM with mean values during each event that were >70th percentile values for the months during which the events occurred. Unlike for UT/LS events, levels of O-3 and CO during ALRT events tended to be correlated, consistent with photochemical O-3 production in polluted air masses transported across the Pacific. The mixed ALRT + UT/LS events exhibited characteristics that were intermediate between those of the ALRT and UT/LS events. For the classifiable cases the fractional distribution of each event type was: UT/LS, similar to 44% (n = 8); ALRT + UT/LS, similar to 33% (n = 6); ALRT, similar to 22% (n = 4). In terms of the total number of classifiable high-ozone event hours UT/LS, ALRT + UT/LS and ALRT events accounted for similar to 52% (t = 85), similar to 36% (t = 59) and similar to 13% (t = 21), respectively. The results indicate that downward mixing of O-3-rich air masses from the UT/LS together with trans-Pacific transport of urban/industrial and biomass burning emissions from the Asian continent are the most important mechanisms for delivering high O-3 levels (i.e., 8-h averages >70.0 ppbv) to the lower FT in the Pacific Northwest. (C) 2011 Elsevier Ltd. All rights reserved.