RP team

The development team

Global air-sea ammonia exchange:

Past present and future scenarios and implications for biogeochemically important processes

Martin Johnson, UEA

This piece was written to inform the production of a review paper on the marine N cycle and global change for the SOLAS-INI nitrogen meeting at UEA in November 2006

See also Field observations of air-sea ammonia flux and my PhD thesis on air-sea ammonia flux

Models of the atmospheric transport and deposition of nitrogen separately parameterize the emission of gas phase NH₃ from the ocean and the dry deposition of NH₃ to the ocean surface (along with particulate NHx ) e.g. Dentener et al (2006), Galloway et al (2004). However, whilst too computationally expensive for global atmospheric chemistry models, it is more correct to consider the exchange of ammonia across the air-sea interface as a single process controlled by the relative concentrations of ammonia in the atmosphere and ocean, according to Henry’s law and the exchange velocity (under atmospheric control for soluble gases such as NH₃ – exchange velocity is a function of wind speed and molecular mass e.g. Duce et al 1991). Details of the method for calculating NH₃ exchanges in this manner can be found elsewhere (e.g. Johnson et al, 2007).

Johnson 2004 presents past (1860), present day, and future (doubled pCO2) scenarios of the net global air-sea exchange of ammonia. These budgets are based on a global database of measured seawater NHx and atmospheric NH₃ data, past emissions estimates and assumed values of the pH of the surface ocean. Temperature is found to be a key driver of air-sea ammonia flux magnitude and direction and as such, the equatorial region dominates the emission of ammonia to the atmosphere. Further, shelf-sea regions appear to be approximately of equal importance for global NH₃ emissions as the open ocean, highlighting the biogeochemical importance of these relatively small but highly productive regions. The present day net global air-sea ammonia exchange from the budget of Johnson (2004) is estimated to be in the range -3 to +4 Tg-N/yr (a negative value representing a net deposition), with gross emission from the ocean of between 1 and 6 Tg-N/yr (just encompassing the most recent estimates of ocean NH₃ emissions from Galloway et al (2004) of 5.6 Tg-N/yr).

The 1860 estimate, constructed using the emission scenario of Holland et al (1999) and reduced seawater ammonium concentrations in shelf regions due to lower eutrophication, suggests a net air-sea ammonia exchange in pre-industrial times of +1 to +4 Tg-N/yr (the ocean a net source for ammonia). The pH of the pre-industrial ocean was up to 1.5 pH units higher than today (Caldeira and Wickett, 2003). This pH change is not accounted for in the budget of Johnson 2004 and would further favor emission of NH₃ from the ocean. Even in 1860, the emissions scenario of Holland et al (1999) suggests that the anthropogenic source of NHx to the atmosphere (~9 Tg-N/yr) was sufficient to drive a net flux of reduced N from the continental to the marine atmosphere. However, in the absence of the anthropogenic signal (i.e. at some point in the distant past), it is likely that ocean emissions of NH₃ fertilized the terrestrial environment with reduced nitrogen.

The budget for the doubled-CO₂ scenario presented by Johnson (2004) makes two simple assumptions: (i) that the pH of the ocean surface will have decreased by 0.4 pH units (Caldeira and Wickett, 2003) and (ii) that the temperature of the ocean surface will have increased, on average, by 2 Celsius. The predicted net NH₃ flux under these conditions is found to be in the range -1.5 to -5 Tg-N/yr i.e. predicts that the ocean will be a net sink for ammonia, in spite of rising temperature. In this budget, Johnson (2004) assumes emissions identical to those used in the present day scenario. In fact, whilst some regions are projected to reduce their future NH₃ emissions, the net global trend is likely to be increasing emissions well into the next century (Dentener et al, 2006). Thus the future ocean is likely to be a stronger net sink than predicted by the budget of Johnson (2004).

Whilst the uncertainties in these budgets of net global air-sea NH₃ exchange are substantial (due to the paucity of measured data and uncertainties in emissions scenarios and surface ocean pH), they clearly indicate that the ocean system is undergoing a change from being a net source of NH₃ to the atmosphere (and in the distant past possibly a net source of NHx), to being a net sink for NH₃ in the future, with substantially reduced areas of the remote ocean emitting ammonia to the atmosphere. This has potentially important biogeochemical consequences. Firstly, there will be an increase of up to 10 Tg of N per year deposited to the ocean surface, with knock on consequences for anoxia and N₂O emissions, P-limitation and carbon draw down. Relative to total atmospheric deposition of N to the ocean (~30 Tg-N/yr), this effect is likely to be small, although much of this increased sink will be in the remote ocean where little terrestrial N reaches the ocean, potentially making a significant difference to nutrient limited ecosystems e.g. oligotrophic gyres.

Secondly, the reduced emission of ammonia from the ocean surface may lead to a reduction in new particle formation (and hence albedo and cloud formation) and an increase in atmospheric acidity in the remote marine boundary layer. Ammonia is thought to be a key species in nucleation processes in the atmosphere (e.g. Korhonen et al, 1999; Kulmala et al, 2004). Nucleation involving water and sulphuric acid is predicted to occur between 1 and 30 orders of magnitude faster in the presence of ammonia (e.g. Coffman and Hegg, 1995; Ball et al, 1999; Yu, 2003), although higher estimates are almost certainly unrealistic (Yu, 2006). Nucleation can occur at an appreciable rate at much lower H₂SO₄ concentrations in the presence of NH₃, due to the extremely low vapour pressure over ammonium sulphate salts. Whilst other species have been identified as important to irreversible particle growth from nucleated clusters e.g. condensible iodine vapours, these mechanisms still require a nucleation event, which is most likely to involve sulphuric acid and ammonia (O’Dowd et al, 2005). In studies of coastal and terrestrial particle ‘burst’ events, the presence of ammonia alone has not always been found to be sufficient to lead to the particle formation rates observed, according to aerosol formation models (e.g. O’Dowd et al, 2002). However, this does not preclude ammonia as a fundamentally important species in the remote marine ‘background’ nucleation rate (Kulmala et al, 2004). In the remote marine boundary layer, where particle concentrations are relatively low and rain events regularly strip particles from the atmosphere, a reduced marine source of ammonia to the atmosphere may have a significant effect on particle formation and growth and cloud formation and a consequent reduction of albedo. The magnitude of this effect is unknown at present and requires investigation.

References