Simultaneous high-time-resolution measurements of atmospheric NH 3 , HNO 3 , soluble gas-phase chloride, and aerosol species were made in Houston, TX, from August 5, 2010 to August 9, 2010. Gaseous NH 3 was measured using a 10.4-μm external cavity quantum cascade laser-based sensor employing conventional photo-acoustic spectroscopy, while gaseous HNO 3 and HCl were sampled using a mist chamber–ion chromatograph (IC) system. Particle chemical composition was determined using a particle-into-liquid-sampler–IC system. There was a large amount of variability in the gas phase mixing ratios of NH 3 (3.0 ± 2.5 ppb), HNO 3 (287.4 ± 291.6 ppt), and HCl (221.3 ± 260.7 ppt). Elevated NH 3 levels occurred around mid-day when NH4+ (0.5 ± 1.0 μg m −3 ) and SO42− (4.5 ± 4.3 μg m −3 ) also increased considerably, indicating that NH 3 likely influenced aerosol particle mass. By contrast, the formation of NH 4 NO 3 and NH 4 Cl was not observed during the measurements. Point sources (e.g., power plant and chemical plant) might be potential contributors to the enhancements in NH 3 at the measurement site under favorable meteorological conditions. Increased particle number concentrations were predicted by the SAM-TOMAS model downwind of a large coal-fired power plant when NH 3 emissions (based on these measurements) were included, highlighting the potential importance of NH 3 with respect to particle number concentration. Separate measurements also indicate the role of NH 3 in new particle formation in Houston under low-sulfur conditions.