Ubiquitous presence of particle-phase organic nitrates in the southeastern United States

Abstract:

Organic nitrates (ON) are important species in the atmosphere as their fate could affect the nitrogen cycle and ozone production. However, the amount of particle-phase ON (pON) is uncertain due to difficulty in measurement. In this study, we applied and evaluated three different methods to estimate the concentration of pON in the southeastern US based on the High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) measurements. The HR-ToF-AMS was deployed in both rural and urban sites in the greater Atlanta area, Georgia, and Centreville, Alabama for approximately one year, as part of Southeastern Center for Air Pollution and Epidemiology study (SCAPE) and Southern Oxidant and Aerosol Study (SOAS). The year-long measurements at multiple sites provide a unique opportunity to investigate the seasonal and spatial variation of pON in the SE US. We found that organic nitrates contributed 5-20% of total OA depending on season, site, and estimation method, suggesting that organic nitrates are important components in the ambient aerosol in the southeastern US. The uncertainties of each method will be discussed in detail. In addition to the ubiquitous presence of organic nitrates, we also found that the time series of less-oxidized oxygenated organic aerosol (LO-OOA), which was resolved from Positive Matrix Factorization (PMF) analysis, correlated well with organic nitrate. Both LO-OOA and pON reach a daily maximum at night, suggesting that they are related to nighttime nitrate radical chemistry. LO-OOA is found to be the second largest source of organic aerosol absorption, which implies potential climate impact of organic nitrates. In terms of health effect, LO-OOA only shows weak capability to generated reactive oxygen species as measured by dithiothreitol (DTT) assay.