Browsing by Author "Erickson, Matt H."

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    Differences in BVOC oxidation and SOA formation above and below the forest canopy
    (Copernicus Publications on behalf of the European Geosciences Union, 2017) Schulze, Benjamin C.; Wallace, Henry W.; Flynn, James H.; Lefer, Barry L.; Erickson, Matt H.; Jobson, B. Tom; Dusanter, Sebastien; Griffith, Stephen M.; Hansen, Robert F.; Stevens, Philip S.; VanReken, Timothy; Griffin, Robert J.
    Gas-phase biogenic volatile organic compounds (BVOCs) are oxidized in the troposphere to produce secondary pollutants such as ozone (O3), organic nitrates (RONO2), and secondary organic aerosol (SOA). Two coupled zero-dimensional models have been used to investigate differences in oxidation and SOA production from isoprene and α-pinene, especially with respect to the nitrate radical (NO3), above and below a forest canopy in rural Michigan. In both modeled environments (above and below the canopy), NO3 mixing ratios are relatively small (< 0.5 pptv); however, daytime (08:00–20:00 LT) mixing ratios below the canopy are 2 to 3 times larger than those above. As a result of this difference, NO3 contributes 12 % of total daytime α-pinene oxidation below the canopy while only contributing 4 % above. Increasing background pollutant levels to simulate a more polluted suburban or peri-urban forest environment increases the average contribution of NO3 to daytime below-canopy α-pinene oxidation to 32 %. Gas-phase RONO2 produced through NO3 oxidation undergoes net transport upward from the below-canopy environment during the day, and this transport contributes up to 30 % of total NO3-derived RONO2 production above the canopy in the morning (∼ 07:00). Modeled SOA mass loadings above and below the canopy ultimately differ by less than 0.5 µg m−3, and extremely low-volatility organic compounds dominate SOA composition. Lower temperatures below the canopy cause increased partitioning of semi-volatile gas-phase products to the particle phase and up to 35 % larger SOA mass loadings of these products relative to above the canopy in the model. Including transport between above- and below-canopy environments increases above-canopy NO3-derived α-pinene RONO2 SOA mass by as much as 45 %, suggesting that below-canopy chemical processes substantially influence above-canopy SOA mass loadings, especially with regard to monoterpene-derived RONO2.
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    The impacts of regional shipping emissions on the chemical characteristics of coastal submicron aerosols near Houston, TX
    (Copernicus Publications, 2018) Schulze, Benjamin C.; Wallace, Henry W.; Bui, Alexander T.; Flynn, James H.; Erickson, Matt H.; Alvarez, Sergio; Dai, Qili; Usenko, Sascha; Sheesley, Rebecca J.; Griffin, Robert J.
    The air quality of the Texas Gulf Coast region historically has been influenced heavily by regional shipping emissions. However, the effects of the recently established North American Emissions Control Area on aerosol concentrations and properties in this region are presently unknown. In order to better understand the current sources and processing mechanisms influencing coastal aerosol near Houston, a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) was deployed for 3 weeks at a coastal location during May–June 2016. Total mass loadings of organic and inorganic non-refractory aerosol components during onshore flow periods were similar to those published before establishment of the regulations. Based on estimated methanesulfonic acid (MSA) mass loadings and published biogenic MSA/non-sea-salt sulfate (nss-SO4) ratios, an average of over 75% of the observed nss-SO4 was from anthropogenic sources, predominantly shipping emissions. Mass spectral analysis indicated that for periods with similar backward-trajectory-averaged meteorological conditions, air masses influenced by shipping emissions had an increased mass fraction of ions related to carboxylic acids and larger oxygen-to-carbon ratios than those that avoided shipping lanes, suggesting that shipping emissions increase marine organic aerosol (OA) oxidation state. Amine fragment mass loadings were correlated positively with anthropogenic nss-SO4 during onshore flow, implying anthropogenic–biogenic interaction in marine OA production. Model calculations also suggest that advection of shipping-derived aerosol may enhance inland aqueous-phase secondary OA production. These results imply a continuing role of shipping emissions on aerosol properties over the Gulf of Mexico and suggest that further regulation of shipping fuel sulfur content will reduce coastal submicron aerosol mass loadings near Houston.
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    Transport-driven aerosol differences above and below the canopy of a mixed deciduous forest
    (Copernicus Publications, 2021) Bui, Alexander A.T.; Wallace, Henry W.; Kavassalis, Sarah; Alwe, Hariprasad D.; Flynn, James H.; Erickson, Matt H.; Alvarez, Sergio; Millet, Dylan B.; Steiner, Allison L.; Griffin, Robert J.
    Exchanges of energy and mass between the surrounding air and plant surfaces occur below, within, and above a forest's vegetative canopy. The canopy also can lead to vertical gradients in light, trace gases, oxidant availability, turbulent mixing, and properties and concentrations of organic aerosol (OA). In this study, a high-resolution time-of-flight aerosol mass spectrometer was used to measure non-refractory submicron aerosol composition and concentration above (30 m) and below (6 m) a forest canopy in a mixed deciduous forest at the Program for Research on Oxidants: PHotochemistry, Emissions, and Transport tower in northern Michigan during the summer of 2016. Three OA factors are resolved using positive matrix factorization: more-oxidized oxygenated organic aerosol (MO-OOA), isoprene-epoxydiol-derived organic aerosol (IEPOX-OA), and 91Fac (a factor characterized with a distinct fragment ion at m/z 91) from both the above- and the below-canopy inlets. MO-OOA was most strongly associated with long-range transport from more polluted regions to the south, while IEPOX-OA and 91Fac were associated with shorter-range transport and local oxidation chemistry. Overall vertical similarity in aerosol composition, degrees of oxidation, and diurnal profiles between the two inlets was observed throughout the campaign, which implies that rapid in-canopy transport of aerosols is efficient enough to cause relatively consistent vertical distributions of aerosols at this scale. However, four distinct vertical gradient episodes are identified for OA, with vertical concentration differences (above-canopy minus below-canopy concentrations) in total OA of up to 0.8 µg m−3, a value that is 42 % of the campaign average OA concentration of 1.9 µg m−3. The magnitude of these differences correlated with concurrent vertical differences in either sulfate aerosol or ozone. These differences are likely driven by a combination of long-range transport mechanisms, canopy-scale mixing, and local chemistry. These results emphasize the importance of including vertical and horizontal transport mechanisms when interpreting trace gas and aerosol data in forested environments.