10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
New Methods for the Study of the Effects of NOx on SOA Formation
WEIHAN PENG, David R. Cocker III, University of California, Riverside
Abstract Number: 1329 Working Group: Carbonaceous Aerosol
Abstract NOx effects are important on SOA formation in atmosphere since both low vapor pressure species formed in peroxide rich (low-NO) conditions and organic nitrate formation in higher NOx environments affect aerosol formation. Current regional air quality models, when considering NOx effects, are guided by results from classic environmental chamber experiments, most of which explore the influences of initial NOx concentration on final SOA formation. During most of these experiments, significant SOA formation does not start until the NO in the system is depleted through oxidation to NO2 and such experiments might have multiple NOx regimes regardless of the absolute amount of NOx injected, so they may miss important SOA formation pathways leading to errors to these models. Therefore, it is important to design new methodologies to study NOx effects at different regimes in environment chamber to provide a more accurate guidance to air quality models. In this study, instead of completing the precursor VOCs and NOx injections before the start of SOA formation like it is in traditional chamber experiments, the timing and method of VOC precursors and NOx are altered in the current work to enhance and simulate the conditions for peroxide and organic nitrate formation. Varied amounts of NOx were injected after the SOA formation commences to explore its effects in peroxide rich environments. Different injection methods including continuous and step-wise injection were conducted to study the instantaneous effects. Additional experiments were performed with precursors VOCs injected after depletion of NO to study NOx effects in low NO conditions.
This study presents experimental observations from the photo-oxidation systems of major anthropogenic VOCs (e.g., aromatics), biogenic monoterpene and poly-aromatic hydrocarbons. Experiments were conducted in the dual 90m3 environment chamber at UCR/CECERT and modelling of gas-phase chemical reactions were guided by the SAPRC model gas-phase chemical mechanism along with measurements of gas species including ozone, NOx, VOCs, aerosol number and volume time traces as well as aerosol density, volatility and bulk chemical composition. For the same total NOx injected, the timing of the NOx injections significantly influenced the SOA formation in the system by altering the gas-phase chemical reactivity of the system that the VOC precursor reacted in. Changes in SOA formation along with chemical and physical characteristics of the resulting aerosol will be discussed.