Development and testing of a chemical mechanism for atmospheric photochemical transformations of 1,3-butadiene

Abstract

1,3-Butadiene (BD) in the atmosphere is a highly reactive hazardous air pollutant, which has a short lifetime and is quickly transformed to reaction products, some of which are also toxic. The ability to predict exposure to BD and its’ products requires models with chemical mechanisms which can simulate these transformations. The atmospheric photochemical reactions of BD have been studied in the University of North Carolina Outdoor smog chamber, which has been used for over 30 years to test photochemical mechanisms for air quality simulation models for ozone. Experiments have been conducted under conditions of real sunlight and realistic temperature and humidity to study the transformations of BD and to develop and test chemical mechanisms for the simulation of these processes. Experimental observation of time–concentration data of BD decay and the formation of many products is compared to simulation results. This chemical mechanism can be incorporated into air quality simulation models which can be used to estimate ambient concentrations needed for exposure estimates.

Introduction

1,3-Butadiene (BD) in the atmosphere is a highly reactive hazardous air pollutant, and is quickly transformed to reaction products, some of which are also toxic (products include acrolein and formaldehyde). Many of these products are also reactive, reducing their concentrations and producing other products, some of which may also be toxic. Toxicological studies aimed at understanding potential health effects of BD and similar compounds in urban air have shown that the adverse effects of the mixture of products of the photochemically transformed BD can be as great or greater than the effects of BD itself as measured by in vitro measures of cytotoxicity and inflammation [1], [2]. The mechanism development and testing presented here is a continuing work in progress to connect eventually the toxicity studies conducted by our group to chemical models needed for risk assessment. The ability to predict concentrations for exposure to BD and its’ products is necessary for determining a more realistic exposure and risk assessment, and requires air quality models with chemical mechanisms which can simulate these transformations. Since many other organic air toxics commonly found in urban air mixtures are also reactive and are transformed into other or similar photooxidation products, air quality models with more accurate chemical mechanisms will be needed to make relative risk calculations when considering alternative control scenarios. Also air quality models can be used to consider and investigate the potential effects of co-exposures to multiple toxic compounds (complex mixture effect) or pre-exposures to other air toxics including ozone.

The atmospheric photochemical reactions of BD and acrolein have been studied in the University of North Carolina outdoor “smog chamber” (environmental chamber) [3], [4], [5], which has been used for over 30 years to test photochemical mechanisms for air quality simulation models for ozone from organics and nitrogen oxides (NO_x). Experiments have been conducted under conditions of real sunlight and realistic temperature and humidity to study the transformations of BD and acrolein, and to develop and test an explicit chemical mechanism for the simulation of these processes. Analytical techniques including a derivative method previously enhanced to improve abilities for carbonyl-containing compounds were used to detect and measure many products [3], [4], [6], [7], [8]. Experimental observation of time–concentration data of BD and acrolein decay and the formation of many products is presented and where possible, compared to simulation results. This chemical mechanism can be incorporated into air quality simulation models which can be used to estimate ambient concentrations needed for exposure estimates. Usually explicit mechanisms are too long, computationally too costly to incorporate and be used directly with full air quality simulation models. Therefore “lumped” abbreviated mechanisms are used, which are generalized mechanisms which hopefully capture the essential nature of the chemical processes, written and tested against the original explicit mechanism.