The impact of changing nitrogen oxide emissions on wet and dry nitrogen deposition in the northeastern USA

摘要

This study is an attempt to quantify the relation between changes in NO_x emissions and nitric acid (HNO_3) in the northeastern USA. From this relation, and previous work relating NO_x emission changes and wet NO_3~- deposition, we can estimate how changing NO_x emissions may impact total (wet + dry) measured nitrogen (N) deposition. Electric utility emissions account for 1/4, and vehicle emissions account for over 1/2 of the total NO_x emissions in the eastern USA. Canadian NO_x emissions from the seven easternmost provinces (Manitoba and east) represent less than 10% (1.2 teragrams (Tg) NO_x) of the NO_x emissions compared with those from the eastern USA. Emissions from eastern Canada are dominated by vehicle NO_x emissions, which account for f of the total NO_x emissions from eastern Canada. Data from the EPA National Emissions Inventory show, for the period 1991-2001, that nitrogen oxide (NO_x) emissions in the eastern USA have declined by 17% to from 16.1 to 13.1 Tg. Large declines in vehicle emissions in 2001 may be questionable. If 2001 data are excluded the decline in total NO_x is only 7%. A recent assessment of EPA's emissions estimates suggest that vehicle NO_x emissions may be underestimated, and total NO_x emissions reductions may be less than what is reported by the EPA. The CASTNet (Clean Air Status and Trends Network) measurements of N dry deposition include HNO_3, particulate NO_3~- and NH_4~+. The dominant N dry deposition product measured is HNO_3, which represents 80% of measured N dry deposition for the sites used in this study. Amounts of NH_3, NO_2, organic nitrate and PAN dry deposition are not measured by CASTNet. The NH_3 and NO_2 deposition are probably significant, and may be major N dry deposition components in some areas. Random coefficient models with total NO_x emissions as the independent variable, and HNO_3 concentrations as the dependent variable, show that reducing total NO_x emissions by 50% should reduce HNO_3 concentrations by 36%. The average efficiency (the ratio of % change in HNO_3 to % change in NO_x emissions) is 72%. Random coefficient models with non-vehicle NO_x emissions as the independent variable, and HNO_3 concentrations as the dependent variable, show a 50% decline in non-vehicle NO_x emissions (which is a 23% decline in total NO_x emissions) should reduce HNO_3 by 17-20%. The average efficiency in this case is 81%. Because non-vehicle NO_x emissions data are more reliable than vehicle NO_x emissions, non-vehicle NO_x models are likely more accurate than the total NO_x models. Combining the results of this study with previous work, which examined the relation between NO_x emissions and wet NO_3 concentrations, show that reducing total NO_x emissions by 50% should reduce total NO_3~- deposition by 37% (wet + dry combined efficiency is 74%), and total N deposition (as measured by CASTNet sites in the northeastern USA) by 25%. A decline in total NO_x. emissions of 23%, from a 50% reduction in non-vehicle NO_x emissions should, on average, reduce total NO_3~- deposition by 20% (wet + dry combined efficiency is 87%), and total N deposition, as measured by CASTNet sites, by 15%.