Vertical Variations in Biogenic Organic Nitrates at Two Temperate Forest Sites

Forest canopies emit biogenic volatile organic compounds (BVOC), which contribute to the formation of regional ozone and secondary organic aerosol (SOA) formation. In the presence of NO_x, atmospheric oxidation of BVOC can produce organic nitrates that are an important component of SOA and act as a reservoir for NO_x, regulating local and regional atmospheric chemistry. Prior field observations of organic nitrates have focused on the above-canopy observations, yet there is little understanding of the vertical profiles of SOA and organic nitrates relative to the forest canopy emission sources. Here we contrast organic nitrate formation at two mixed deciduous forested ecosystems (the University of Michigan Biological Station (UMBS) and the Southern Aerosol Oxidant Study (SOAS) site in Alabama) with distinct differences in BVOC and NO_x to understand the formation of organic nitrates in and above a forest canopy. Gas-phase BVOC and NO_x concentrations are higher at SOAS than at UMBS, but the isoprene/monoterpene ratio is similar. We use a one-dimensional canopy model (Forest Canopy-Atmosphere Transfer Model version 2.0) to investigate the vertical profiles of isoprene- and monoterpene-derived secondary organic aerosol formation (iSOA and MT-SOA, respectively), with a focus on the aerosol organic nitrate. Simulated total iSOA shows similar temporal patterns as above-canopy observations at both sites, with the dominant fraction (50–80%) formed by isoprene epoxydiol (IEPOX). Within the canopy, the organic nitrate component of iSOA (iNIT-SOA) remains relatively small (10–20%) with slightly higher concentrations simulated at SOAS. The simulated iSOA composition at the two sites is similar, with four carbon (C4) nitrates dominating in the canopy and tetrafunctional nitrates dominating in the boundary layer. MT-SOA has different diurnal cycles at the two sites and is about 2–4 times greater at SOAS than UMBS, although the model underpredicts the observed top-of-canopy MT-SOA at both sites. The simulated organic nitrate component of the total MT-SOA (mNIT-SOA) is about 40% of the MT-SOA at SOAS and 20% at UMBS. Sensitivity analyses suggest that iNIT-SOA is highly sensitive to NO levels, while mNIT-SOA is more sensitive to dry deposition of preexisting organic particles. Overall, the 1D modeling suggests that organic nitrate aerosols are greater in and above the canopy near BVOC sources with a decrease in fractional contribution to total biogenic SOA with height in the boundary layer. Finally, organic nitrate aerosol composition can vary between similar ecosystems depending on the regional speciated BVOC emissions.