At the Rocky Mountain Biogenic Aerosol Study (BEACHON-RoMBAS) field campaign in the Colorado front range, July–August 2011, measurements of gas- and aerosol-phase organic nitrates enabled a study of the role of NO_x (NO_x = NO + NO₂) in oxidation of forest-emitted volatile organic compounds (VOCs) and subsequent aerosol formation. Substantial formation of peroxy- and alkyl-nitrates is observed every morning, with an apparent 2.9% yield of alkyl nitrates from daytime RO₂ + NO reactions. Aerosol-phase organic nitrates, however, peak in concentration during the night, with concentrations up to 140 ppt as measured by both optical spectroscopic and mass spectrometric instruments. The diurnal cycle in aerosol fraction of organic nitrates shows an equilibrium-like response to the diurnal temperature cycle, suggesting some reversible absorptive partitioning, but the full dynamic range cannot be reproduced by thermodynamic repartitioning alone. Nighttime aerosol organic nitrate is observed to be positively correlated with [NO₂] × [O₃] but not with [O₃]. These observations support the role of nighttime NO₃-initiated oxidation of monoterpenes as a significant source of nighttime aerosol. Nighttime production of organic nitrates is comparable in magnitude to daytime photochemical production at this site, which we postulate to be representative of the Colorado front range forests.

2-Methyl-3-buten-2-ol (MBO) is an important biogenic volatile organic compound (BVOC) emitted by pine trees and a potential precursor of atmospheric secondary organic aerosol (SOA) in forested regions. In the present study, hydroxyl radical (OH)-initiated oxidation of MBO was examined in smog chambers under varied initial nitric oxide (NO) and aerosol acidity levels. Results indicate measurable SOA from MBO under low-NO conditions. Moreover, increasing aerosol acidity was found to enhance MBO SOA. Chemical characterization of laboratory-generated MBO SOA reveals that an organosulfate species (C₅H₁₂O₆S, MW 200) formed and was substantially enhanced with elevated aerosol acidity. Ambient fine aerosol (PM_2.5) samples collected from the BEARPEX campaign during 2007 and 2009, as well as from the BEACHON-RoMBAS campaign during 2011, were also analyzed. The MBO-derived organosulfate characterized from laboratory-generated aerosol was observed in PM_2.5 collected from these campaigns, demonstrating that it is a molecular tracer for MBO-initiated SOA in the atmosphere. Furthermore, mass concentrations of the MBO-derived organosulfate are well correlated with MBO mixing ratio, temperature, and acidity in the field campaigns. Importantly, this compound accounted for an average of 0.25% and as high as 1% of the total organic aerosol mass during BEARPEX 2009. An epoxide intermediate generated under low-NO conditions is tentatively proposed to produce MBO SOA.