Patrick downloading met tower data at -20F. Currently we have low temperatures and a very stable atmosphere. We are not seeing N2O5 because there are high amounts of NO (nitric oxide). When NO is present in the atmosphere it reacts with NO3 (nitrate radical) in the following reaction:
NO + NO3 --> 2NO2
This previous reaction is fast and NO3 does not exist to even form N2O5(NO2+NO3<-->N2O5).