Transport of Asian ozone pollution into surface air over the western United States in spring

Many prior studies clearly document episodic Asian pollution in the western U. S. free troposphere. Here, we examine the mechanisms involved in the transport of Asian pollution plumes into western U. S. surface air through an integrated analysis of in situ and satellite measurements in May-June 2010 with a new global high-resolution (similar to 50 x 50 km(2)) chemistry-climate model (GFDL AM3). We find that AM3 with full stratosphere-troposphere chemistry nudged to reanalysis winds successfully reproduces observed sharp ozone gradients above California, including the interleaving and mixing of Asian pollution and stratospheric air associated with complex interactions of midlatitude cyclone air streams. Asian pollution descends isentropically behind cold fronts; at similar to 800 hPa a maximum enhancement to ozone occurs over the southwestern U. S., including the densely populated Los Angeles Basin. During strong episodes, Asian emissions can contribute 8-15 ppbv ozone in the model on days when observed daily maximum 8-h average ozone (MDA8 O-3) exceeds 60 ppbv. We find that in the absence of Asian anthropogenic emissions, 20% of MDA8 O-3 exceedances of 60 ppbv in the model would not have occurred in the southwestern USA. For a 75 ppbv threshold, that statistic increases to 53%. Our analysis indicates the potential for Asian emissions to contribute to high-O-3 episodes over the high-elevation western USA, with implications for attaining more stringent ozone standards in this region. We further demonstrate a proof-of-concept approach using satellite CO column measurements as a qualitative early warning indicator to forecast Asian ozone pollution events in the western U. S. with lead times of 1-3 days.