Understanding the formation of secondary organic aerosol and the fates of organic nitrates formed from monoterpene oxidation

Secondary organic aerosol (SOA) contributes to a substantial fraction of fine particular matter (PM2.5) that imposes adverse health effects, large uncertainty in radiative forcing, and poor visibility. Monoterpenes represent about 15% of the annual emissions of biogenic volatile organic compounds (VOC), and their oxidation is considered to contribute to global SOA budget substantially. In the presence of NOx, oxidation of monoterpenes also produces organic nitrates. The formation of organic nitrates sequesters NOx from the atmosphere, directly suppressing the ozone production near source regions. To what extent organic nitrates affect atmospheric oxidation capacity largely depends on their fates; whether NOx is released back or permanently removed. Chemistry about the monoterpene SOA formation and the fates of monoterpene organic nitrates are not well understood, calling for the great need of fundamental laboratory studies. This dissertation presents the original studies investigating hydroxyl (OH) and nitrate (NO3) radical oxidation of monoterpenes (i.e., α-pinene, β-pinene, and limonene). First, the formation and properties of SOA formed from NO3 radical oxidation of α-pinene and limonene were studied. Oxidizing mixtures of monoterpenes simultaneously led to different formation and properties of SOA compared to oxidizing each monoterpene separately. Approximately 50% more α-pinene SOA was formed while limonene SOA formation was reduced by about 20%. New oxidation products were also observed, suggesting potentially important interactions of VOC and their oxidation products. Second, the SOA formation mechanism of OH radical oxidation of α-pinene in the presence of NOx was investigated. By integrating the laboratory and simulation studies, multi-generational autoxidation chemistry was found to be important to explain a substantial fraction of SOA formed in laboratory chamber experiments. Third, chemical composition and hydrolysis of particulate organic nitrates formed from OH and NO3 radical oxidation of α-pinene and β-pinene were investigated. Hydrolysis lifetime was found to be short (<30 min) for all the systems explored, significantly shorter than previous chamber studies (i.e., 3–6 h) but consistent with bulk solution measurement studies (i.e., 0.02–8.8 h). The discrepancy stemmed from the choice of proxy used to estimate the hydrolysis lifetime. The measured hydrolyzable fractions (FH) were approximately 30% and 10% for OH and NO3 radical oxidation, respectively. Fourth, photolysis of gaseous organic nitrates formed from OH and NO3 radical oxidation of α-pinene and β-pinene was investigated. While not all gaseous organic nitrates underwent photolysis, some showed a rapid photolysis decay (~10-4 s-1) possibly due to the conjugation of carbonyl and nitrooxy groups as well as long carbon backbone. Gas-phase photolysis may be comparably important to particle-phase hydrolysis unlike previously assumed, pointing out the role of monoterpene organic nitrates as a NOx sink at a lesser degree but as a temporary NOx reservoir (or NOx source in regions away from the actual emission sources) at a higher extent. Lastly, the response of an aerosol mass spectrometer (AMS) towards particulate organic nitrates was calibrated and evaluated using synthetic standards of monoterpene and isoprene organic nitrates. The nitrogen-containing moiety mass concentrations measured by the AMS corresponded to the molecular weight of 46 g mol-1 (NO2) rather than commonly used 62 g mol-1 (NO3), consistent with the expected thermal decomposition of organic nitrates (R-O-NO2) on the vaporizer predominantly breaking the O-N bond rather than the R-O bond. This suggests that actual mass concentrations of particulate organic nitrates as well as their contribution to organic aerosol could be higher than previously reported in literature by 35%.

Level thesis:degree_level

Doctoral

Department dc:contributor.department

Civil and Environmental Engineering

Grantor dc:publisher

Georgia Institute of Technology

Year dc:date.issued

2023

Author dc:creator

• Takeuchi, Masayuki

Advisor dc:contributor.advisor

• Ng, Nga Lee

Committee members dc:contributor.committeemember

• Russell, Armistead
• Huey, Gregory
• Weber, Rodney
• Liu, Pengfei

• Atmospheric chemistry
• Aerosol chemistry
• Air quality
• Secondary organic aerosol
• Organic nitrate
• Monoterpene

Language dc:language.iso

en_US

Handle dc:identifier.uri

https://hdl.handle.net/1853/75594

OAI identifier oai:identifier

oai:repository.gatech.edu:1853/75594