An intense impact flux upon a planet having a CO 2 + N 2 atmosphere, such as Mars, provides energy to synthesize nitric oxide, NO, which is likely converted into nitrate minerals. The same impact flux can decompose nitrate minerals if present in the crust. We build a numerical model to study the effects of early impact processes on the evolution of nitrogen in a dominantly CO 2 atmosphere. We model the period of intense post-accretionary bombardment, the roughly 500 Myr period after crustal stabilization that locks in previously accreted volatiles. A best-guess, "fiducial" set of parameters is chosen, with a fixed "veneer" of post-accretionary impactors (ฮด R = 950 m thick), assumed to contain carbon at 1 wt% ( f g = 0.01), with a molar C/N ratio of 18, an initial atmospheric pressure of 1 bar (with CO 2 /N 2 = 36), and a power law impactor mass distribution slope b = 0.75. This model produces a nitrate reservoir R NO 3 0.5 ร 10 19 moles, equivalent to โผ30 mbars of N 2 , during the intense impact phase. Starting with 1 bar, the atmosphere grows to 2.75 bars. Results of models with variations of parameter values show that R NO 3 responds sluggishly to changes in parameter values. To significantly limit the size of this reservoir, one is required to limit the initial total atmospheric pressure be less than about 0.5 bars, and the impactor volatile content f g to be less than 0.003. The value of f g substantially determines whether the atmosphere grows or not; when f g = 0.01, the atmosphere gains about 1.7 bars, while for f g = 0.003, the atmosphere gains less than 200 mbars, and for f g = 0.001, it loses about 400 mbars.
Impact erosion is a minor sink of N, constituting generally less than 10% of the total supply. The loss of impactor volatile plumes can take almost 50% of incoming N and C under fiducial parameters, when atmospheric pressures are low. This nitrogen does not significantly interact with Mars, and hence is not properly delivered. When the initial N is greater than the delivered N, most of the nitrogen ends up as nitrates; when delivered N is larger, most nitrogen ends up in the atmosphere. The reason for this dichotomy seems to be that initial nitrogen is present during the whole bombardment, while delivered N, on average, only experiences half the bombardment. The operating caveat here is that the above results are all conditioned on the assumption that impact processes dominate this period of Mars atmospheric evolution.