Ozone and particulate matter enhancements from regional wildfires observed at Mount Bachelor during 2004–2011

Abstract

We report observations of normalized enhancement ratios (NER) for 32 wildfires measured at Mount Bachelor Observatory in central Oregon during June–September 2004–2011. All 32 plumes resulted from wildfires originating in the western United States and Canada. The observed NER of PM1 (particulate matter < 1 micron) to carbon monoxide (ΔPM1/ΔCO) ranged from 0.06 to 0.42 μg m−3 ppbv−1. The NER of ozone to CO (ΔO3/ΔCO) ranged from 0.01 to 0.51 ppbv ppbv−1 for the 13 observed plumes with a significant ΔO3/ΔCO NER (p ≤ 0.01, R2 ≥ 0.30). For wildfire plumes transported <540 km, or approximately <2 days, the ΔPM1/ΔCO NER is found to increase with increasing distance, suggesting that there is significant secondary organic aerosol (SOA) production in these plumes. However, two plumes transported over greater time periods have relatively low ΔPM1/ΔCO NER, indicating that PM1 loss is greater than SOA production in these plumes. Of the three plumes transported the longest distance to MBO, only two have significant O3 production. These two plumes were transported in boundary layer air masses, while the third was transported in a free tropospheric air mass, suggesting that conversion of nitrogen oxides (NOX) to peroxyacetyl nitrate (PAN) may be a factor affecting O3 production in these plumes. Two wildfire plumes are mixed with urban emissions from the Seattle/Tacoma metropolitan area, and have relatively higher ΔO3/ΔCO NER than other wildfire plumes transported over similar distances.