Molecular characterization of water soluble organic nitrogen in marine rainwater by ultra-high resolution electrospray ionization mass spectrometry

K E Altieri,M G Hastings,D M Sigman,A J Peters

doi:10.5194/acp-12-3557-2012

Abstract

Abstract. Atmospheric water soluble organic nitrogen (WSON) is a subset of the complex organic matter in aerosols and rainwater, which impacts cloud condensation processes and aerosol chemical and optical properties and may play a significant role in the biogeochemical cycle of N. However, its sources, composition, connections to inorganic N, and variability are largely unknown. Rainwater samples were collected on the island of Bermuda (32.27° N, 64.87° W), which experiences both anthropogenic and marine influenced air masses. Samples were analyzed by ultra-high resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry to chemically characterize the WSON. Elemental compositions of 2281 N containing compounds were determined over the mass range m/z+ 50 to 500. The five compound classes with the largest number of elemental formulas identified, in order from the highest number of formulas to the lowest, contained carbon, hydrogen, oxygen, and nitrogen (CHON+), CHON compounds that contained sulfur (CHONS+), CHON compounds that contained phosphorus (CHONP+), CHON compounds that contained both sulfur and phosphorus (CHONSP+), and compounds that contained only carbon, hydrogen, and nitrogen (CHN+). Compared to rainwater collected in the continental USA, average O:C ratios of all N containing compound classes were lower in the marine samples whereas double bond equivalent values were higher, suggesting a reduced role of secondary formation mechanisms. Despite their prevalence in continental rainwater, no organonitrates or nitrooxy-organosulfates were detected, but there was an increased presence of organic S and organic P containing compounds in the marine rainwater. Cluster analysis showed a clear chemical distinction between samples collected during the cold season (October to March) which have anthropogenic air mass origins and samples collected during the warm season (April to September) with remote marine air mass origins. This, in conjunction with patterns identified in van Krevelen diagrams, suggests that the cold season WSON is a mixture of organic matter with both marine and anthropogenic sources while in the warm season the WSON appears to be dominated by marine sources. These findings indicate that, although the concentrations and percent contribution of WSON to total N is fairly consistent across diverse geographic regions, the chemical composition of WSON varies strongly as a function of source region and atmospheric environment.

Highlights

The global nitrogen (N) cycle has been greatly perturbed by anthropogenic activities since the industrial revolution
Changes in elemental ratios can be indicative of atmospheric reactions, for example, O:C ratios increase throughout the day from secondary organic aerosol (SOA) formation associated with photochemistry (Volkamer et al, 2006; Aiken et al, 2008)
Amine salts and Methanesulfonic acid (MSA) are the most abundant organic species in marine organic aerosols (Facchini et al, 2008a), which is consistent with the presence of amino acids, the dominance of reduced N species, and the prevalence of S containing compounds that we see in the rainwater organic matter

Summary

Introduction

The global nitrogen (N) cycle has been greatly perturbed by anthropogenic activities since the industrial revolution. The emission (and subsequent deposition) of anthropogenic reactive N (e.g., nitrate, ammonium, organic N) has increased by a factor of ten since preindustrial times (Galloway et al, 2004). During summer in the East China Sea, patches of water with elevated nitrate concentrations were ascribed to rainfall events based on detection of low salinity, low phosphorus and high N concentrations (Kodama et al, 2011). Even stronger evidence of the impact of N deposition on surface ocean biogeochemistry was reported in the Northwestern Pacific Ocean where changes in N* (N*=N-(RN:P )*P; a measure of excess nitrogen relative to phosphorus in the ocean (Gruber and Sarmiento, 1997)) were highly correlated with changes in anthropogenic N deposition downwind of East Asia where nitrogen oxide (NOx) emissions have increased over 250 % from the late 1980’s until today (Kim et al, 2011)

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