Reply on RC1

TL;DR: In this paper , the authors evaluated wet, dry, and total nitrogen and sulfur (S) deposition from multi-year simulations within the contiguous US (CONUS) and found that the estimates of wet deposition and ambient concentrations were 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.

Abstract: <strong class="journal-contentHeaderColor">Abstract.</strong> 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 observational studies have investigated spatial and temporal trends of atmospheric deposition, modeling assessments can provide useful information over areas with sparse measurements, although they usually have larger horizontal resolutions and are limited by input data availability. In this analysis, we evaluate wet, dry, and total N and S deposition from multiyear simulations within the contiguous US (CONUS). Community Multiscale Air Quality (CMAQ) model estimates from the EPA's (Environmental Protection Agency) 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 which contains land-use-specific resistance parameterization and land-use-specific deposition estimates needed to estimate the differential impacts of N deposition to different land use types. First, we evaluate model estimates of wet deposition and ambient concentrations, finding underestimates of SO<span class="inline-formula">₄</span>, NO<span class="inline-formula">₃</span>, and NH<span class="inline-formula">₄</span> wet deposition compared to National Atmospheric Deposition Program observations and underestimates of NH<span class="inline-formula">₄</span> and SO<span class="inline-formula">₄</span> and overestimates of SO<span class="inline-formula">₂</span> and TNO<span class="inline-formula">₃</span> (HNO<span class="inline-formula">₃+</span>NO<span class="inline-formula">₃</span>) compared to the Clean Air Status and Trends Network (CASTNET) ambient concentrations. Second, a measurement–model fusion approach 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. Model agreement of wet deposition 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. Next, we assess modeled N and S deposition trends across climatologically consistent regions in the CONUS. 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 <span class="inline-formula">−0.30</span> kg N ha<span class="inline-formula">^−1</span> yr<span class="inline-formula">^−1</span> in the Northeast and Southeast and by <span class="inline-formula">−0.02</span> kg N ha<span class="inline-formula">^−1</span> yr<span class="inline-formula">^−1</span> 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 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">NO</mi><mi>x</mi></msub><mo>/</mo><msub><mi mathvariant="normal">SO</mi><mi>x</mi></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="50pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="34b644bcdd8aef8dbb3b4c623908cb86"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-12749-2022-ie00001.svg" width="50pt" height="14pt" src="acp-22-12749-2022-ie00001.png"/></svg:svg></span></span> emission reductions shifting gas–aerosol partitioning; 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 deposition between 2002–2009 than 2010–2017, suggesting a slowdown of the rate of decline likely in response to smaller emission reductions. Finally, we document changes in the modeled total N and S deposition budgets. The average annual total N deposition budget over the CONUS decreases from 7.8 in 2002 to 6.3 kg N ha<span class="inline-formula">^−1</span> yr<span class="inline-formula">^−1</span> in 2017 due to declines in oxidized N deposition from NO<span class="inline-formula">_<i>x</i></span> emission controls. Across the CONUS 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.