In order to improve the current understanding of the dynamics of ammonia (NH3) in the Greater Houston and Dallas-Fort Worth (DFW) areas and to examine the effects of NH3 on local and regional air quality with respect to particulate matter formation, intensive field investigations were made. A 10.4-μm external cavity quantum cascade laser based-sensor employing conventional photo-acoustic spectroscopy was used to conduct real-time and continuous measurements of atmospheric NH3 in this work. Results from the Houston campaign indicate that the mixing ratio of NH3 ranged from 0.1 to 8.7 ppb with a mean of 2.4±1.2 (1σ) ppb in winter and ranged from 0.2 to 27.1 ppb with a mean of 3.1±2.9 ppb in summer. The larger levels in summer probably are due to higher ambient temperature. A notable morning increase and a mid-day decrease were observed in the diurnal profile of NH3 mixing ratios. Motor vehicles were found to be major contributors to the elevated levels during morning rush hours in winter. However, changes in vehicular catalytic converter performance and other local or regional emission sources from different wind directions governed the behavior of NH3 during morning rush hours in summer. There was a large amount of variability, particularly in summer, with several episodes of elevated NH3 mixing ratios that could be linked to industrial facilities. A considerable discrepancy in NH3 mixing ratios existed between weekdays and weekends. During the simultaneous high-time-resolution measurements of gaseous and aerosol species in summer, elevated NH3 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 NH3 likely influenced aerosol particle mass. NH4+ mainly existed in the form of (NH4)2SO4 and NH4HSO4; by contrast, the formation of NH4NO3 and NH4Cl was suppressed. Power plant plumes were found to be potential contributors to the enhancements in NH3 at the urban sampling 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. Measurements also show the role of NH3 in new particle formation in Houston under low-sulfur conditions. Results from the DFW campaign indicate that the mixing ratio of NH3 ranged from 0.1 to 10.1 ppb, with a mean of 2.7 ± 1.7 ppb. The diurnal profile of NH3 exhibited a daytime increase, likely due to increasing temperatures affecting temperature-dependent sources in the study region. Automobiles might be potential sources of NH3 on Sundays based on the Pearson’s correlation coefficient between NH3 and carbon monoxide, but the relationship did not exist on weekdays and Saturdays, probably due to decreased traffic volume and different traffic composition. According to the results from the EPA PMF 3.0 model, biogenic (primarily vegetation and soil) emissions were major contributors to gas-phase NH3 levels measured at the suburban site during the campaign. In addition, agriculture (especially livestock-related activities) also was expected to be a potentially significant source of NH3 based on the nature of the region. Inorganic aerosol components of submicron particles (PM1) (4.41 ± 2.13 μg m-3) were dominated by SO42- (1.25 ± 0.66 μg m-3), followed by NH4+ (0.44 ± 0.24 μg m-3) and NO3- (0.12 ± 0.11 μg m-3). Pearson’s correlation coefficients between NH4+, SO42-, and NO3- imply that particulate NH4+ mainly existed as (NH4)2SO4 and that NH4NO3 was not formed during most of the study period, likely due to high temperatures (30.15 ± 4.12 oC) over the entire campaign. Ambient aerosols tended to be nearly neutral. Theoretical calculations of thermodynamic equilibrium were performed to consider the formation of NH4NO3 and NH4Cl. When relative humidity (RH) was lower than deliquescence relative humidity (DRH), the partial pressure products of PNH3PHNO3 and PNH3PHCl were smaller than the associated equilibrium constants, indicating the lack of NH4NO3 and NH4Cl formation. When RH was above DRH, higher levels of NO3- often were observed. A strong relationship between NO3- and SO42- at higher RH suggests that NH4NO3 might be formed on the moist surface of pre-existing sulfate aerosols. In the particle mixture, (NH4)2SO4 reduces the equilibrium constant, making the aqueous system a more favorable medium for NH4NO3 formation. In addition, measured particle number size distributions showed that an aerosol nucleation and growth event was coincident with humid periods characterized by substantially increased concentrations of particulate NH4+, NO3-, and SO42-. Excess NH4+ also was found to be correlated closely with NO3- during this episode when elevated PM1 levels imply aqueous NH4NO3 formation.