Particulate Matter, Ozone, and Nitrogen Species in Aged Wildfire Plumes Observed at the Mount Bachelor Observatory

Abstract

During the summer of 2012 and 2013, we measured carbon monoxide (CO), carbon dioxide (CO 2 ), ozone (O 3 ), nitrogen oxides (NO x ), reactive nitrogen (NO y ), peroxyacetyl nitrate (PAN), aerosol scattering (σ sp ) and absorption, elemental and organic carbon (EC and OC), and aerosol chemistry at the Mount Bachelor Observatory (2.8 km above sea level, Oregon, US). Here we analyze 23 of the individual plumes from regional wildfires to better understand production and loss of aerosols and gaseous species. We also developed a new method to calculate enhancement ratios and Modified Combustion Efficiency (MCE), which takes into account possible changes in background concentrations during transport. We compared this new method to existing methods for calculating enhancement ratios. The MCE values ranged from 0.79-0.98, ΔO 3 /ΔCO ranged from 0.01-0.07 ppbv ppbv -1 , Δσ sp /ΔCO ranged from 0.23-1.32 Mm -1 (at STP) ppbv -1 , ΔNO y /ΔCO ranged from 2.89-12.82 pptv ppbv -1 , and ΔPAN/ΔCO ranged from 1.46-6.25 pptv ppbv -1 . A comparison of three different methods to calculate enhancement ratios (ER) showed that the methods generally resulted in similar Δσ sp /ΔCO, ΔNO y /ΔCO, and ΔPAN/ΔCO; however, there was a significant bias between the methods when calculating ΔO 3 /ΔCO due to the small absolute enhancement of O 3 in the plumes. The ΔO 3 /ΔCO ERs calculated using two common methods were biased low (~20-30%) when compared to the new proposed method. Two pieces of evidence suggest moderate secondary particulate formation in many of the plumes studied: 1) mean observed ΔOC/ΔCO 2 was 0.028 g particulate-C gC -1 (as CO 2 )-27% higher than the midpoint of the biomass burning emission ratio range reported by a recent review-and 2) single scattering albedo (ω) was relatively constant at all MCE values, in contrast with results for fresh plumes. The observed NO x , PAN, and aerosol nitrate represented 6-48%, 25-57%, and 20-69% of the observed NO y in the aged plumes, respectively, and other species represented on average 11% of the observed NO y .

Influence of Long-Range Transport of Siberian Biomass Burning at the Mt. Bachelor Observatory during the Spring of 2015

“…From the scatter plots of multiple measurements of Y plotted against X, we can calculate the enhancement ratio (ΔY/ΔX) by taking the slope of a Reduced Major Axis (RMA) linear regression. A comparison of different approaches to get the enhancement ratio is shown in the paper by Briggs et al (2016). These enhancement ratios, along with results from similarly aged Siberian plumes observed in North America, are summarized in Table 1.…”

“…However, if the plume warmed up (i.e., via descent), then PAN would decompose back to NO x and ozone formation can take place. For comparison, Briggs et al (2016) looked at regional fires observed at MBO during summer 2012 and 2013 and found that PAN comprised 25-57% of the total reactive nitrogen. Table S1 quantifies the reactive nitrogen speciation for AC plumes 1, 2, and 3.…”

Can d;PM2.5/d;CO and d;NOy/d;CO Enhancement Ratios Be Used to Characterize the Influence of Wildfire Smoke in Urban Areas?

“…Bachelor Observatory. Briggs et al (2016) found little difference between Δσ scat /ΔCO and ΔNO y /ΔCO NERs calculated using the three methods if the enhancement of the species in the plumes was large relative to the background concentrations (σ scat is the aerosol scattering coefficient, which is well correlated to PM 2.5 ). Large differences were found for ΔO 3 /ΔCO and ΔCO/ΔCO 2 , where the enhancement is small relative to the background.…”

“…Normalized enhancement ratios (NERs), also known as normalized excess mixing ratios, are a good way to help identify the source of a pollution plume observed at ambient monitoring sites (Andreae and Merlet, 2001;Briggs et al, 2016). During a pollution or smoke event in which concentrations of two species (X and Y) increase substantially above background levels, NERs relate the excess concentrations of a target species X with that of a reference species Y (NER = ΔX/ΔY, where Δ is the enhancement over background concentrations).…”

“…In addition, Yokelson et al (2013) has argued that the two primary methods for calculating NERs mentioned previously can be inaccurate due to changes in background concentrations during plume transport. Briggs et al (2016) used the two primary NER methods (absolute enhancement over background, and regression analysis), as well as a third method developed to address Yokelson et al (2013)'s concerns, while studying WF plumes at the Mt. Bachelor Observatory.…”