<p><strong>Abstract.</strong> During the 1998, 2000, 2001, 2008, and 2009 summer intensives of the Program for Research on Oxidants: PHotochemistry, Emissions and Transport (PROPHET), ambient measurement of nitrogen oxides (NO + NO₂ = NO_x) were conducted. NO and NO_x mole fractions displayed a diurnal pattern with NO_x frequently highest in early morning. This pattern has often been observed in other rural areas. In this paper, we discuss the potential

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... ources and contributing factors of the frequently observed morning pulse of NO_x. Of the possible potential contributing factors to the observed morning pulse of NO and NO_x, we find that surface-layer transport and slow upward mixing from soil emissions, related to the thermodynamic stability in the nocturnal boundary layer (NBL) before its morning breakup are the largest contributors. The morning NO_x peak can significantly impact boundary layer chemistry, e.g. through production of HONO on surfaces, and by increasing the importance of NO₃ chemistry in the morning boundary layer.</p>