10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Environmental Factors Affecting Humic-like Substance Production in Photoreactions of Polycyclic Aromatic Hydrocarbons
JOHN HAYNES, Keith Miller, Brian Majestic, University of Denver
Abstract Number: 396 Working Group: Aerosol Chemistry
Abstract Humic-like substances (HULIS) are a class of light absorbing compounds and are present in the atmosphere as a collective of organic species, each containing significant aromatic and carboxylic acid character. They are found to be a major component of brown carbon and are connected with radiative forcing in the atmosphere. Aerosolized HULIS are consistently correlated with reactive oxygen species and soluble metals, and as such, they are implicated with respiratory problems, oxidative stress, and the production of phytoplankton in marine systems. Polycyclic aromatic hydrocarbons (PAH) and HULIS are commonly observed in particles sourced from pyrogenic processes, e.g. biomass burning and vehicle exhaust. PAH are produced by the inefficient burning of organic fuels and display extended lifetimes in the atmosphere. These aromatics are toxic to human health and many structures are considered highly carcinogenic. PAH have many atmospheric reaction pathways - one recently discovered endpoint is the formation of HULIS.
This bench study evaluates the impact of environmental changes on HULIS production by oxidation of suspensions of PAH with exposure to sunlight using several different matrix environments. The long range transport of PAH lend themselves to extended exposures to sunlight and reactive environments potentially producing branched and oxidized PAH products (oxPAH) which then act as intermediates for more complex structures, e.g. HULIS. This extended reactive exposure may be affected by changing cloud water environments, including buffer systems, dust entrainment, and soluble metals. Environmental PAH and HULIS are correlated with reactive oxygen species in cloud water and aerosolized particles. Atmospheric aromatics may incorporate ultraviolet radiation with redox reactions of buffered systems or bioavailable metals as photo-Fenton catalysts in an electron transfer interaction with elemental oxygen or water molecules to produce radicals, such as superoxide or hydroxyl radicals. These oxyradicals are highly reactive agents and may initiate several oxidizing pathways, resulting in the further production of HULIS via oxPAH. The introduction of environmental deviations may enhance or inhibit this production by favoring specific or new reaction pathways.
Reaction conditions use simulated sunlight introduced to varying PAH reagent and matrix composition, oxidizing agents, and temperature over 16 hours. Analysis of bulk products are performed using UV-vis, HPLC, and GCMS - used to define the spectroscopic properties and structure of these products. Solid phase extractions of HULIS material are analyzed by GCMS and ATR-FTIR. Initial reaction data include significant differences between dissolved and saturated PAH solutions, as well as changes in the production of oxPAH between soluble ferrous and ferric ion interactions with PAH. Comparison between systems are based on UV-vis analysis and the growth of new HPLC peaks during and following light exposure periods, which monitor the character and production of several new species. Continuous decays within the UV-vis chromatographic spectra and arrays of several products are observed for samples with soil and with dissolved ferric ions and are emblematic of HULIS materials. Comparing these products with strong differences in product data for reactions using dissolved PAH or spikes with ferrous ion are observed and are of particular interest. Evaluating the formation of products from dissolved and suspended PAH solutions with photo-reactions conducted with soluble metals, cloud buffer, and suspended soil allows for the determination of specific reaction mechanisms. A higher understanding of the creation of HULIS in atmospheric conditions will greatly aid in predictive models for air quality and human health downwind of pollution sources.