Tracers of wood smoke
M.A.K. Khalila,*, R.A. Rasmussenb
Department of Physics, Portland State University, P.O. Box 751, Portland, OR 97207, USA Department of Environmental Science and Engineering, Oregon Graduate Institute, 20000 N.W. Walker Rd, Beaverton, OR 97501, USA Received 8 August 2002; accepted 12 November 2002
Abstract Smokefrom wood burning is a signiﬁcant source of air pollution in many parts of the world. When several sources simultaneously cause air pollution, it is often difﬁcult to determine how much comes from wood burning. Wood smoke has unique chemical characteristics that can be used as indicators, including elemental composition, particularly potassium and chlorine, the ratios of organic and elementalcarbon, and gases such as methyl chloride. This paper deals with formulating and applying the chemical mass balance approach incorporating both gas- and aerosol-phase tracers to a study carried out at Olympia-Lacey in Washington. In this study, three types of tracers were measured simultaneously and used to estimate wood smoke pollution during Winter, namely elemental composition and organic carbon inthe particle phase and methyl chloride in the gas phase. The results from the different tracers are found to be in agreement. The air pollution at the study site was dominated by wood smoke mostly from low-temperature combustion (about 80%), with additional but smaller contributions from oil furnaces (15%), automobiles (4%) and occasionally from other nearby sources. r 2003 Elsevier Science Ltd.All rights reserved.
Keywords: Methyl chloride; CH3Cl; Receptor model; CMB; Regional air pollution
1. Introduction In many parts of the United States, particularly in the Paciﬁc Northwest, wood burning is a substantial source of air pollution during Winter. When many sources of air pollution are present, it is often difﬁcult to determine how much comes from wood burning. Tracers of wood smokeare elements, compounds or gases that come from wood burning in characteristic amounts or ratios. Measurement of these tracers in the environment can be used to quantitatively estimate the amount of air pollution from wood burning. Here we will discuss the major tracers that indicate the presence of wood smoke and how these are used to apportion the observed air pollution among its sources.*Corresponding author. Tel.: +1-503-725-8396; fax: +1503-725-8550. E-mail address: firstname.lastname@example.org (M.A.K. Khalil).
Of particular interest is the use of methyl chloride as a novel gaseous tracer of wood smoke. Methyl chloride is a trace gas that comes mostly from natural processes and except for wood burning, it has no known wintertime sources in the Paciﬁc Northwest where we conducted our study.This circumstance makes it a unique indicator of wood smoke providing advantages over other tracers that also come from various local sources such as automobiles and small industry. Carbon is a major constituent of air pollution from combustion processes. The ratios of elemental to organic carbon (EC/OC) distinguish sources such as wood burning, which has a low elemental to organic carbon ratio,and automobiles, which have a high ratio. Carbon analysis also can be used to distinguish between the amount of smoke from hot- and cool-wood burning. The proportion of cool burning, which produces most of the air pollution, is determined by local practices of using wood stoves and ﬁreplaces. The usual tracers of wood smoke are elemental chlorine and potassium. We will compare
1352-2310/03/$ -see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1352-2310(02)01014-2
M.A.K. Khalil, R.A. Rasmussen / Atmospheric Environment 37 (2003) 1211–1222
the results based on the use of these various and disparate tracers. We will discuss the general theory and models to represent the sources. We will then apply these ideas to pollution measurements taken at...