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Determining the magnitude and origins of nitrogen (N) deposition in the open ocean is vital for understanding how anthropogenic activities influence oceanic biogeochemical cycles. Excess N in the North Pacific Ocean(NPO) is suggested to reflect recent anthropogenic atmospheric deposition from the Asian continent, changes in nutrient dynamics due to marine N-fixation, and/or lateral transport of nutrients. We investigate the impact of anthropogenic and marine sources on reactive N deposition in the NPO, with a focus on ammonium (NH4+), an important bioavailable nutrient, using aerosol samples (n =108) collected off the coast of China (Changdao Island). This study site is used as a proxy for continental emissions that can be exported and subsequently deposited to the ocean. The NH4+concentration of aerosol samples varied seasonally (p < 0.05), with a higher average value in winter (2.8 ±1.1 μg/m3) and spring (1.9 ±0.8 μg/m3) compared to autumn (0.7 ±0.6 μg/m3) and summer (1.4 ±0.4 μg/m3). The isotopic composition of aerosol NH4+ varied seasonally, with higher averages in spring (13.3 ±7.9‰) and summer (15.6 ±6.2‰) compared to autumn (3.2 ±2.5 ‰) and winter (3.8 ±11.4‰). These seasonal patterns in the isotopic composition of NH4+ are investigated based on correlations of aerosol chemical species, seasonal shifts in transport patterns, partitioning of ammonia/ammonium between the gas and particle phase, and continental versus marine sources of ammonia. We find that anthropogenic activities, mainly agricultural practices (e.g., volatilization, fertilizer, animal husbandry), are the primary sources of NH4+ deposited to the NPO. 

Abstract. Despite significant precursor emission reductions in theUS over recent decades, atmospheric nitrate deposition remains an importantterrestrial stressor. Here, we utilized statistical air mass back trajectoryanalysis and nitrogen stable isotope deltas (δ(15N)) toinvestigate atmospheric nitrate spatiotemporal trends in the northeastern USfrom samples collected at three US EPA Clean Air Status and Trends Network(CASTNET) sites from December 2016–2018. For the considered sites, similarseasonal patterns in nitric acid (HNO3) and particulate nitrate(pNO3) concentrations were observed with spatial differences attributedto nitrogen oxide (NOx) emission densities in source contributingregions that were typically ≤ 1000 km. Significant spatiotemporalδ(15N) variabilities in HNO3 and pNO3 were observedwith higher values during winter relative to summer, like previous reportsfrom CASTNET samples collected in the early 2000s for our study region. Inthe early 2000s, δ(15N) of atmospheric nitrate in the northeastUS had been suggested to be driven by NOx emissions; however, we didnot find significant spatiotemporal changes in the modeled NOxemissions by sector and fuel type or δ(15N, NOx) for thesource regions of the CASTNET sites. Instead, the seasonal and spatialdifferences in the observed δ(15N) of atmospheric nitrate weredriven by nitrate formation pathways (i.e., homogeneous reactions ofNO2 oxidation via hydroxyl radical or heterogeneous reactions ofdinitrogen pentoxide on wetted aerosol surfaces) and their associatedδ(15N) fractionation. Under the field conditions of lowNOx relative to O3 concentrations and when δ(15N,NOx) emission sources do not have significant variability, wedemonstrate that δ(15N) of atmospheric nitrate can be a robusttracer for diagnosing nitrate formation. 

Abstract. The northeastern US represents a mostly urban corridorimpacted by high population and fossil fuel combustion emission density.This has led to historically degraded air quality and acid rain that hasbeen a focus of regulatory-driven emissions reductions. Detailing thechemistry of atmospheric nitrate formation is critical for improving themodel representation of atmospheric chemistry and air quality. The oxygenisotopic compositions of atmospheric nitrate are useful indicators intracking nitrate formation pathways. Here, we measured oxygen isotope deltas(Δ(17O) and δ(18O)) for nitric acid (HNO3)and particulate nitrate (pNO3) from three US EPA Clean AirStatus and Trends Network (CASTNET) sites in the northeastern US fromDecember 2016 to 2018. The Δ(17O, HNO3) and δ(18O, HNO3) values ranged from 12.9 ‰ to 30.9 ‰ and from 46.9 ‰ to 82.1 ‰, and the Δ(17O, pNO3) and δ(18O, pNO3) ranged from 16.6 ‰ to 33.7 ‰ and from 43.6 ‰ to 85.3 ‰, respectively. There was distinct seasonality ofδ(18O) and Δ(17O), with higher values observedduring winter compared to during summer, suggesting a shift in O3 to HOxradical chemistry, as expected. Unexpectedly, there was a statisticaldifference in Δ(17O) between HNO3 and pNO3, withhigher values observed for pNO3 (27.1 ± 3.8) ‰relative to HNO3 (22.7 ± 3.6) ‰, andsignificant differences in the relationship between δ(18O) andΔ(17O). This difference suggests atmospheric nitratephase-dependent oxidation chemistry that is not predicted in models. Basedon the output from GEOS-Chem and both the δ(18O) and Δ(17O) observations, we quantify the production pathways of atmosphericnitrate. The model significantly overestimated the heterogeneousN2O5 hydrolysis production for both HNO3 and pNO3, afinding consistent with observed seasonal changes in δ(18O) andΔ(17O) of HNO3 and pNO3, though large uncertaintiesremain in the quantitative transfer of δ(18O) from majoratmospheric oxidants. This comparison provides important insight into therole of oxidation chemistry in reconciling a commonly observed positive biasfor modeled atmospheric nitrate concentrations in the northeastern US.