Role of atmospheric ammonia in particulate matter formation in Houston during summertime

Simultaneous high-time-resolution measurements of atmospheric NH3 ,H NO3, soluble gas-phase chloride, and aerosol species were made in Houston, TX, from August 5, 2010 to August 9, 2010. Gaseous NH3 was measured using a 10.4-mm external cavity quantum cascade laser-based sensor employing conventional photo-acoustic spectroscopy, while gaseous HNO3 and HCl were sampled using a mist chambereion chromatograph (IC) system. Particle chemical composition was determined using a particle-into-liquidsamplereIC system. There was a large amount of variability in the gas phase mixing ratios of NH3 (3.0 � 2.5 ppb), HNO3 (287.4 � 291.6 ppt), and HCl (221.3 � 260.7 ppt). Elevated NH3 levels occurred around mid-day when NH4 þ (0.5 � 1.0 mgm � 3 ) andSO4 2� (4.5 � 4.3mgm � 3 ) alsoincreasedconsiderably,indicating that NH3 likely influenced aerosol particle mass. By contrast, the formation of NH4NO3 and NH4Cl was not observed during the measurements. Point sources (e.g., power plant and chemical plant) might be potential contributors to the enhancements in NH3 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 NH3 emissions (based on these measurements) were included, highlighting the potential importance of NH3 with respect to particle number concentration. Separate measurements also indicate the role of NH3 in new particle formation in Houston under low-sulfur conditions.