Influence of background particulate matter (PM) on urban air quality in the Pacific Northwest

• Long-range transported pollution events were observed in the free troposphere (FT).
• Data provided new evidence about transport of aerosol from the FT to boundary layer.
• Contribution of Asian sources to PM2.5 in the background areas was between 50 and 80%.
• Due to local emissions contribution of Asian sources was less evident in urban areas.

Elevated particulate matter concentrations due to Asian long-range transport (LRT) are frequently observed in the free troposphere (FT) above the Pacific Northwest, U.S. Transport of this aerosol from the FT to the boundary layer (BL) and its effect to local air quality remain poorly constrained. We used data collected at the Mount Bachelor observatory (MBO, 2.8 km a.s.l) and from ground stations in the Pacific Northwest to study transport of fine particulate matter (PM) from the FT to the BL. During Asian LRT episodes PM concentrations were clearly elevated above the corresponding monthly averages at MBO as well as at low elevation sites across Washington and Oregon. Also, a clear correlation between MBO and low elevation sites was observed, indicating that LRT episodes are seen in both the FT and BL. In addition, drum impactor measurements show that the chemical composition of PM at MBO was similar to that measured at the BL sites. Using a simple regression model, we estimate that during springtime, when the transport from Asia is most effective, the contribution of Asian sources to PM2.5 in clean background areas of the Pacific Northwest was on average 1.7 μg m−3 (representing approximately 50–80% of PM). The influence of LRT PM was also seen in measurement stations situated in the urban and urban background areas. However, the fraction of LRT PM was less pronounced (36–50% of PM) due to larger local emissions in the urban areas.