Abstract. 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.
Transport-driven aerosol differences above and below the canopy of a mixed deciduous forest
