Ammonia and ammonium measurements from the southeastern United States

Twenty-four-hour integrated gaseous ${\mathrm{NH}}_3$ and fine particulate $({\mathrm{PM}}_{2.5}){\mathrm{NH}}_4^{+}$ were measured during 2004 at eight sites in the southeastern U.S. Mean ${\mathrm{NH}}_3$ concentrations for 2004 ranged from 2.44 ppbv at an urban-industrial site in North Birmingham, AL, to 0.23 ppbv at a rural-forested site near Centreville, AL. ${\mathrm{NH}}_3$ mixing ratios were found to be higher at urban sites than at nearby rural (or suburban sites) except for sites which were directly influenced by local sources. Only weak correlations with temperature were observed for ${\mathrm{NH}}_3$ at the sites; slightly greater correlations were observed for total ammonia $({\mathrm{NH}}_x={\mathrm{NH}}_3+{\mathrm{NH}}_4^{+})$ vs. temperature. A weak seasonal variation was observed for ${\mathrm{NH}}_3$ mixing ratios at all sites, with all but one site exhibiting biannual maxima in spring and late summer/fall. Mean ${\mathrm{PM}}_{2.5}$ ${\mathrm{NH}}_4^{+}$ concentrations ranged from $1.78\mathrm{\mu }\mathrm{g}{\mathrm{m}}^{-3}$ in Atlanta, GA, to $1.02\mathrm{\mu }\mathrm{g}{\mathrm{m}}^{-3}$ at Oak Grove, MS, and were more uniform across the network than ${\mathrm{NH}}_3$ mixing ratios, with only slightly larger values at urban sites as compared to nearby rural (or suburban) sites. All sites exhibited highest ${\mathrm{NH}}_x$ between July and September and lowest ${\mathrm{NH}}_x$ in November and December. The gaseous ${\mathrm{NH}}_3$ fraction $({\mathrm{NH}}_3/({\mathrm{NH}}_3+{\mathrm{NH}}_4^{+}))$ was observed to decrease with increasing values of ${\mathrm{PM}}_{2.5}$ ${\mathrm{SO}}_4^{2-}$ at all sites. At two rural-forested sites and two sites near the Gulf of Mexico, the ${\mathrm{NH}}_3$ gaseous fraction approached a relatively constant value of 5–10% as ${\mathrm{PM}}_{2.5}{\mathrm{SO}}_4^{2-}$ increased beyond 5–$7\mathrm{\mu }\mathrm{g}{\mathrm{m}}^{-3}$, suggesting that ${\mathrm{NH}}_3$ availability at these locations limits aerosol neutralization.