Reply on RC1

Abstract

Atmospheric deposition of nitrogen (N) and sulfur (S) compounds from human activity has greatly declined in the United States (US) over the past several decades in response to emission controls set by the Clean Air Act. While many studies have investigated the spatial and temporal trends of atmospheric deposition, few assess dry deposition, incorporate a measurement-model fusion approach to improve wet deposition estimates, or focus on changes within specific US climate regions. In this analysis, we evaluate wet, dry, and total N and S deposition from multiyear simulations across climatologically consistent regions within the contiguous US (CONUS). Community Multiscale Air Quality (CMAQ) model estimates from 2002 to 2017 from the EPA’s Air QUAlity TimE Series (EQUATES) project contain important model updates to atmospheric deposition algorithms compared to previous model data, including the new Surface Tiled Aerosol and Gaseous Exchange (STAGE) bidirectional deposition model and improvements to organic N chemistry. The model generally underestimates wet deposition of SO4, NO3, and NH4 compared to National Atmospheric Deposition Program observations. Measurement-model fusion employing a precipitation- and bias- correction to wet deposition estimates is found to reduce model bias and improve correlations compared to the unadjusted model values. Comparisons to Clean Air Status and Trends network ambient concentrations show the model underestimates NH4 and SO4 and overestimates SO2 and TNO3. Model agreement is poor over parts of the West and Northern Rockies, due to errors in precipitation estimates caused by complex terrain and uncertainty in emissions at the relatively coarse 12 km grid resolution used in this study. Total deposition of N and S in the eastern US is larger than the western US with a steeper decreasing trend from 2002–2017, i.e., total N declined at a rate of approximately ─0.30 kg-N/ha/yr in the Northeast and Southeast and by ─0.06 kg-N/ha/yr in the Northwest and Southwest. Widespread increases in reduced N deposition across the Upper Midwest, Northern Rockies, and West indicate evolving atmospheric composition due to increased precipitation amounts over some areas, growing agricultural emissions, and regional NOx/SOx emission reductions; these increases in reduced N deposition are generally masked by the larger decreasing oxidized N trend. We find larger average declining trends of total N and S between 2002–2009 than 2010–2017, suggesting a slowdown of the rate of decline. The average total N deposition budget over the CONUS decreases from 7.8 kg-N/ha in 2002 to 6.3 kg-N/ha in 2017 due to declines in oxidized N deposition from NOx emission controls. Across the US during the 2002–2017 time period, the average contribution of dry deposition to the total N deposition budget drops from 60 % to 52 %, whereas wet deposition dominates the S budget rising from 45 % to 68 %. Our analysis extends upon the literature documenting the growing contribution of reduced N to the total deposition budget, particularly in the Upper Midwest and Northern Rockies, and documents a slowdown of the declining oxidized N deposition trend, which may have consequences on vegetation diversity and productivity. Future progress in decreasing the total N budget remains exceedingly difficult without new controls on ammonia emissions, including from agricultural sources as the demand for food grows and oxidized N emissions continue to decrease due to emission controls implemented to achieve the National Ambient Air Quality Standards.