Vertical distribution of oxides of nitrogen in the semi-urban planetary boundary layer: mixing ratios, sources and transport

Measurements of the mixing ratios of tropospheric NO and NO_Y (defined as nitric oxide (NO) + nitrogen dioxide (NO₂) + peroxyacetyl nitrate (PAN) + nitric acid (HNO₃) + particulate nitrate (NO₃⁻) + ⋯) were made over a semi-urban area of central North Carolina at the surface (10 m) and on a tower at heights of 250 m (820 ft) and 433 m (1420 ft) above ground level (AGL) from December 1994 to February 1995. These measurements were compared with synoptic weather data and regional and local upper air soundings in an effort to characterize NO and NO_Y in the planetary boundary layer in terms of their vertical distributions, diurnal profile, and related transport mechanisms. A pronounced decreasing vertical gradient in both NO and NO_Y mixing ratios was observed, with a distinct diurnal cycle and nocturnal minimum. Furthermore, the results suggest that NO and NO_Y were mixed upward from the surface during passage of synoptic meteorological features (and their associated vertical motions). Most importantly, the data reveals that mixing ratios of NO and NO_Y at the elevated heights did not exist in sufficient concentrations above the inversion layer in the nocturnal boundary layer to be mixed downward upon breakup of the nocturnal inversion and affect surface measurements. Instead, concentrations of NO and NO_Y were apparently mixed upward during the morning and midday hours by vertical boundary layer processes. Thus, the association of observed increases in surface NO and NO_Y mixing ratios based solely on downward mixing processes is not justified in all cases, and other sources and processes for these increases must be considered, particularly over rural areas.