The relation between NOx emissions and precipitation NO3− in the eastern USA

Changes in total NOx emissions in the eastern USA have been relatively small through the 1990s, even with implementation of the Clean Air Act Amendments of 1990. The decreases in NOx emissions from the non-vehicle sectors have been significantly offset by increases from the vehicle sector. To quantify the impact of NOx emissions on precipitation NO3−, we used a random coefficient model. Regional NOx emission densities were regressed on NO3− concentrations in precipitation from 24 NADP/NTN and NADP/AIRMoN sites for precipitation chemistry in the northeastern and mid-Atlantic regions of the USA. These were areas where changes in NOx emissions have been the greatest. Total NOx, and non-vehicle NOx emissions have declined from 7% to 12% and 19% to 25%, respectively, from 1991 to 2000 in these regions. Different NOx source regions impacting particular sites were evaluated based on 12-, 24- and 36-h air mass back trajectories. A simple random coefficient model based on precipitation NO3− (dependent variable) and total NOx emissions (independent variable) showed a highly significant (P-value<0.0001) relation for the source areas based on 12-h back trajectories. Using this model, a 50% decline in total NOx emissions from the appropriate source regions predicted an overall decline in precipitation NO3− concentration of 38%. A model based on non-vehicle NOx emissions as the independent variable predicted that a 50% decline in non-vehicle emissions, which is a 23% decline in total emissions impacting the area, results in a 19% decline in precipitation NO3− concentration. Similar results are found when source areas based on 24- and 36-h back trajectories are used. The several models evaluated, using the different-sized source regions and either total or non-vehicle NOx emissions, predict that reductions in NOx emissions should reduce NO3− concentrations (and deposition) with an efficiency ranging between 75% and 95%. It can be inferred that equivalent reductions (in terms of μeq l−1) in precipitation acidity will also occur.