Sources of metals and bromine-containing particles in Milwaukee

• Se metal in individual aerosol particles is highly correlated with Cd, Sb, and Mo.
• Se in Milwaukee is emitted largely from local point sources.
• Local point sources are important contributors to metals but much less so to PM.
• Summertime levels of particulate Br have increased in Milwaukee in recent years.
• Submicron Br-containing particles are emitted by local point sources.

An Aerosol Time-of-Flight Mass Spectrometer (ATOFMS) was deployed in summer 2010 in central Milwaukee as part of a study to understand the sources of primary and secondary aerosol in the non-attainment area of Southeast Wisconsin. Measurements were made continually from mid-July to mid-August, collecting time, size, and chemical composition data on aerosol particles. Trace metals including Se, Cd, Mo, and Sb were detected in the particles. These metals were found to be related and provided information on source types in Milwaukee, generally located southwest of the sampling site, from plumes that appear to originate from point sources. Additionally, Br was detected in individual particle mass spectra during this study, the first such observation at an inland site. Particles that contained Br were found in two different size modes, each of which had a different representative chemical composition. In combination with an analysis of wind direction, the data suggest that the two different size modes of Br-containing particles originate from chemically distinct sources.