Nitrogen oxides (NOy) and nitric acid in particular (HNO₃) can be important contributors to nitrogen loadings in terrestrial and aquatic environments.  This presentation will describe one year of continuous, ground level measurements of NOy and HNO₃ at the Coweeta Hydrological Laboratory in rural southwestern North Carolina.   The site is located at an elevation of approximately 685 m (amsl) in an area of complex terrain, with nearby ridgetops approaching 1000 m within 1 km to the north, west and south.  Elevation decreases to the east in the direction of local highways and population centers.  NOy was measured via reduction to NO on hot (350 C) molybdenum with detection via O₃-NO chemiluminescence.  HNO₃ was measured inn parallel by removing it from the airstream with a KCl-impregnated annular denuder and calculating the difference between undenuded and denuded NOy values.  We will also present and compare HNO₃ data from the summer of 2015 and spring of 2016 that was collected with a MARGA system located on a nearby flux tower and with an inlet height of 44 m agl.

Results of ground-level measurements show that 12-month average concentrations of NOy and HNO₃ are relatively low (1.04 ppb and 0.16 ppb, respectively) and exhibit interesting temporal patterns.  Mean NOy concentrations are highest in first quarter (1.47 ppb) and lowest in third quarter (0.61 ppb), while HNO₃ concentrations are highest in first and second quarters (0.20 ppb) and lowest in fourth quarter (0.09 ppb).  Hourly data for NOy consistently show minimum values in the early morning, maximum values midday and a rapid decline through the evening hours.   This pattern differs markedly from that observed at sites in flat or rolling terrain which typically show overnight maxima and midday minima.  During first quarter, the diurnally averaged minimum and maximum are 1.14 ppb and 1.77 ppb, respectively.  During third quarter early morning concentrations are much lower (sometimes below 0.1 ppb) and diurnally averages minimum and maximum are 0.24 ppb and 1.20 ppb, respectively.  HNO3 concentrations exhibit a more familiar “photochemical” pattern, with minimum concentrations in the early morning (<0.05-0.1 ppb) and peak concentrations between 1200 and 1500 LST (0.2-0.4 ppb).