10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
A Laboratory and Modeling Investigation on the Effects of Ammonia Uptake on SOA Composition and Its Potential Impacts on Air Quality
JULIA MONTOYA-AGUILERA, Mallory Hinks, Jeremy Horne, Shupeng Zhu, Donald Dabdub, Sergey Nizkorodov, University of California, Irvine
Abstract Number: 1077 Working Group: Aerosol Chemistry
Abstract Particulate matter (PM) consists of suspended particles in the atmosphere large enough to diminish visibility, affect global temperatures by absorbing or scattering light, and contribute to cloud formation by acting as cloud condensation nuclei (CCN). The importance of inorganic nitrogen contribution to PM is well recognized and included in air quality models. Less understood is the contribution of nitrogen-containing organic compounds (NOC) to secondary organic aerosols (SOA), a major component of PM. Currently, the reactive uptake of ammonia (NH3) leading to the formation of NOC in SOA remains unaccounted for in air quality models. Ammonia is ubiquitous in the atmosphere and is emitted largely from agricultural processes. Intensifying use of NH3-based fertilizers suggest NH3 levels will continue to rise in the future. It is important to better characterize this NH3 uptake process, not only because of its potential effects on SOA composition, but also because, if the uptake is efficient, NH3 may be unavailable to contribute to inorganic PM formation.
This study investigates the effects of NH3 on SOA formation, optical properties, and chemical composition. Anthropogenic (toluene, n-hexadecane) and biogenic (isoprene, limonene) volatile organic compounds (VOCs) are oxidized in a smog chamber at various relative humidity and NOx levels. After SOA formation, a pulse of gas-phase NH3 is introduced into the chamber. Particle growth is monitored with a scanning mobility particle sizer (SMPS). A proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) is used to track VOCs. An NH3 analyzer is used to measure the decay of ammonia onto particles (and chamber walls) and a time-of-flight aerosol mass spectrometer (ToF-AMS) is employed to monitor the increase in N:C ratios in the organic components as well as increases in ammonium ion in the aerosol particles. Subsequently, samples are collected and analyzed via direct analysis in real time mass spectrometry (DART-MS). These techniques are used to determine to what extent NH3 reacts with SOA to form NOC (as opposed to being neutralized to ammonium ions by organic and inorganic acid vapors). Additionally, wavelength dependent mass absorption coefficients of SOA extracts are measured. Results indicate that the rate and maximal extent of the NH3+SOA→NOC reactions vary depending on the SOA system. Of the SOA studied, limonene SOA was the most reactive when exposed to NH3. DART mass spectra revealed that exposure of limonene SOA to NH3 converted up to 20% of SOA compounds to NOC. The experimental results were incorporated into the University of California, Irvine - California Institute of Technology (UCI-CIT) model, a state-of-the-art airshed model used to evaluate air quality in the South Coast Air Basin of California (SoCAB). A surface reaction of NH3 with SOA was implemented into the model to estimate the impact on NH3 and PM2.5 concentrations in the SoCAB region. Results indicate the chemical uptake of NH3 by SOA can potentially deplete gaseous NH3 concentrations, leading to indirect reductions in the amount of ammonium nitrate and ammonium sulfate in PM. Moreover, the impact of this chemical uptake was investigated on a larger scale by incorporating the results into the Community Multiscale Air Quality Modeling System (CMAQ) model, with a domain covering the continental United States. The CMAQ model simulations show that inclusion of the NH3+SOA→NOC chemistry decreases the concentration of inorganic components of PM2.5 but increases biogenic SOA, especially in the southeastern United States.