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We describe a miniaturized MEMS particulate matter (PM) monitor that employs the deposition of particulates from a sample stream onto a 1.6 GHz piezoelectric thin-film bulk acoustic wave resonator (FBAR) by means of thermophoresis, and determination of the mass deposited by measuring the resonant frequency shift of a Pierce oscillator. Real-time measurements made in an environmental chamber over several weeks and during a week-long field study in a residence showed excellent correlation with the responses of other commercial aerosol instruments. An added mass of 1 pg could be resolved with the sensor, and the level of detection was 18 mug / m3. The monitor weighs 114 g, has a volume of approximately 245 cm3, consumes less than 100 mW, and would cost less than $100 USD in small quantities. Efforts to further miniaturize the sensor and integrate it with a cell-phone are described.