Investigating types and sources of organic aerosol in Rocky Mountain National Park using aerosol mass spectrometry

Abstract

The environmental impacts of atmospheric particles are highlighted in remote areas where visibility and ecosystem health can be degraded by even relatively low particle concentrations. Submicron particle size, composition, and source apportionment were explored at Rocky Mountain National Park using a High-Resolution Time-of-Flight Aerosol Mass Spectrometer. This summer campaign found low average, but variable, particulate mass (PM) concentrations (max = 93.1 µg m −3 , avg. = 5.13 ± 2.72 µg m −3 ) of which 75.2 ± 11.1 % is organic. Low-volatility oxidized organic aerosol (LV-OOA, 39.3 % of PM 1 on average) identified using Positive Matrix Factorization appears to be mixed with ammonium sulfate (3.9 % and 16.6 % of mass, respectively), while semi-volatile OOA (27.6 %) is correlated with ammonium nitrate (nitrate: 4.3 %); concentrations of these mixtures are enhanced with upslope (SE) surface winds from the densely populated Front Range area, indicating the importance of transport. A local biomass burning organic aerosol (BBOA, 8.4 %) source is suggested by mass spectral cellulose combustion markers (m/z 60 and 73) limited to brief, high-concentration, polydisperse events (suggesting fresh combustion), a diurnal maximum at 22:00 local standard time when campfires were set at adjacent summer camps, and association with surface winds consistent with local campfire locations. The particle characteristics determined here represent typical summertime conditions at the Rocky Mountain site based on comparison to ∼10 years of meteorological, particle composition, and fire data.

Free tropospheric aerosols at the Mt. Bachelor Observatory: more oxidized and higher sulfate content compared to boundary layer aerosols

“…OA during the clean periods at MBO was oxidized with an average (±1σ ) O/C of 0.85 (±0.36) and OM / OC of 2.26 (±0.46). The degree of oxidation was in agreement with regional background OA observed at other mountain sites such as Whistler Mountain in western Canada (Sun et al, 2009), Rocky Mountains in Colorado in the US (Schurman et al, 2015), and Mt. Cimone in Italy (Rinaldi et al, 2015).…”

“…A composition-dependent CE (ranging from 0.5 to 1; average = 0.66) was applied based on the algorithm by Middlebrook et al (2012) to account for possible CE changes induced by changes in particle phase in the AMS. A time-dependent gas-phase CO + 2 subtraction (Collier and Zhang, 2013) was performed to improve the quantification of organic aerosol (OA), which is critical for low-aerosolloading conditions (Setyan et al, 2012). Elemental analysis of high-resolution mass spectra (HRMS) utilized both the Aiken-ambient (AA) method (Aiken et al, 2008) and the improved-ambient (IA) method (Canagaratna et al, 2015).…”

Sources and composition of PM2.5 in the Colorado Front Range during the DISCOVER‐AQ study

“…Wind rose analyses showed no enhancements in either the OM or the sulfate concentration as a function of wind direction (not shown), consistent with a regional source for both components. This is also supported by the similarities in the secondary OC fractions between our measurements in the Denver metropolitan area and those in rural background areas [ Hallar et al ., ; Schurman et al ., ]. As discussed above, the EC/TC ratios suggest a large contribution of biogenic emissions to the SOA measured during DISCOVER‐AQ [ Schichtel et al ., ], which also supports a major regional source contributing to PM 2.5 levels during the summertime.…”

“…[] observed an organic PM 2.5 contribution of ~65% during the summer at a site that is representative of the regional background. Similarly, organics constituted ~75% of PM 1 composition during measurements at Rocky Mountain National Park carried out during the summer [ Schurman et al ., ].…”

“…Although the EC tracer method has limitations in estimating SOA [ Wu and Yu , ], the observations during DISCOVER‐AQ are also consistent with prior studies in Denver that find a high contribution of SOA to OC levels during summer [ Dutton et al ., ; Xie et al ., ]. The organic aerosol during summer at background locations in Colorado is comprised predominantly of SOA, as well, suggesting that this is a regional phenomenon in the western U.S. [ Hallar et al ., ; Schurman et al ., ]. More generally, this observation is consistent with the high contribution of SOA observed in both urban and rural locations globally [ Zhang et al ., ].…”