Continuous ammonia (NH
3) measurements with a temporal resolution of 5 min were implemented at selected SEARCH sites in the southeastern U. S. during 2007. The SEARCH continuous NH
3 instrument uses a citric acid denuder difference technique employing a dual-channel nitric oxide-ozone chemiluminescence analyzer. Data from two SEARCH sites are presented, Jefferson Street, Atlanta (JST) (urban), and Yorkville, Georgia (YRK) (rural), for the period July–December, 2007. Highest NH
x (total ammonia = gaseous NH
3 + PM
2.5 NH
4+) values were observed in August and September at both JST and YRK. Highest NH
3 values occurred in August and September at JST, but in August through October at YRK. Lowest NH
3 and NH
x values occurred in December at both sites. YRK is significantly impacted by nearby poultry sources, routinely experiencing hourly average NH
3 mixing ratios above 20 ppbv. Wind sector analysis clearly implicates the nearby poultry operations as the source of the high NH
3 values. Weekday versus weekend differences in composite hourly mean diurnal profiles of NH
3 at JST indicate that mobile sources have a measurable but relatively small impact on NH
3 observed at that site, and little or no impact on NH
3 observed at YRK. A distinctive composite mean hourly diurnal variation was observed at both JST and YRK, exhibiting maxima in the morning and evening with a broad minimum during midday. Analysis of observed NH
3 diurnal variations from the literature suggests a hypothesized mechanism for the observed behavior based on interaction of local emissions and dry deposition with the formation and collapse of the dynamically mixed atmospheric boundary layer during the day and shallow nocturnal layer at night. Simple mixed layer concentration box model simulations confirm the plausibility of the suggested mechanism.
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