We conclude that the stratospheric source is too small to account for background surface NOy concentrations observed in the remote (i.e., regions a few thousand kilometers from continental source regions) troposphere. Surface NOy mixing ratios seldom exceed 10 parts per trillion by volume (pptv) in the model northern hemisphere and are always below 20 pptv. Together, fossil fuel combustion emissions and stratospheric injection account for less than 10% of observed surface nitrate concentrations in the remote tropical Pacific. Inclusion of PAN chemistry has the effect of increasing model-calculated surface NOy mixing ratios in the northern hemisphere middle and high latitudes by factors of 1.5-3 during winter/spring and by factors of 2-4 during summer/fall. -from Authors