Reply to Liu et al.: On the importance of US deposition of nitrogen dioxide, coarse particle nitrate, and organic nitrogen

In PNAS (1), we highlight changes in the balance of oxidized and reduced nitrogen contributions to US reactive nitrogen (N) deposition. Successful reductions in nitrogen oxides (NO_x) emissions combined with modest growth in ammonia emissions have now tipped the balance in most regions toward deposition of reduced N species, ammonia and ammonium.

Liu et al. (2) respond, making two main points. First, they indicate that the trend in China is opposite to that in the United States with oxidized N deposition growing in importance. Second, they highlight our omission of some N deposition pathways, including dry deposition of nitrogen dioxide (NO₂) and coarse particle nitrate and wet and dry deposition of organic nitrogen (ON), questioning whether our recommendation to focus future attention on ammonia emissions reduction is justified. We appreciate the first point, that the situation in China differs from that in the United States, but are compelled to respond to the second.

In the original paper (1), we acknowledge omitting some forms of nitrogen deposition, due to a lack of necessary measurements. As illustrated below, most pathways were omitted with the understanding of their minor N deposition contributions.

Benedict et al. (3), whom Liu et al. (2) cite, report NO₂ dry deposition at Rocky Mountain National Park (RMNP), but its contribution to the N deposition budget is <1%. More broadly, Dennis et al. (4) used the Community Multiscale Air Quality (CMAQ) model to estimate N deposition fluxes across North America and found that NO_x comprised only ∼3% of the budget with a downward trend.

Our estimates (1) of particulate nitrate deposition make use of Clean Air Status and Trends Network (CASTNET) measurements that, lacking a particle size cut, capture some coarse particle nitrate. The deposition velocities provided by CASTNET are more representative for fine particles, permitting a low bias in nitrate dry deposition estimates. CMAQ estimates (4), which include a dynamically interactive coarse mode for nitrate, however, suggest a North American particulate nitrate contribution of 2% or less, also decreasing over time.

ON is a potentially important, but poorly understood, component of N deposition. In RMNP, wet ON deposition contributed 18% of the total N deposition budget (3); dry deposition of particulate ON was very small. In general, the relative fractions of reduced versus oxidized ON are not well constrained and reflect a mixture of species derived from emission of biological material and NO_x and ammonia that have reacted with atmospheric volatile organic compounds. Jickells et al. (5) conclude in a 2013 review paper: “There is some emerging evidence that suggests that the soluble ON may be dominated by reduced nitrogen, and hence that a major route for the formation of this material may involve reactions of organic matter with ammonia/ammonium or low molecular-weight gaseous reduced nitrogen compounds.”

The bottom line is that adding NO_x and coarse nitrate does not shift the relative contribution of oxidized and reduced nitrogen deposition significantly and those fluxes will continue to decrease with declining NO_x emissions. ON is important and needs to be better characterized, but its inclusion would not necessarily tip the scale away from reduced N.