Introduction

The oxidising capacity of the troposphere reflects the ability of the atmosphere to cleanse itself of man-made and natural compounds. It is primarily determined by the concentration of hydroxyl radicals (OH) which are formed mainly from the photodissociation of ozone by UV radiation. The emission of trace gases containing nitrogen and halogen (Cl, Br, I) atoms from the biosphere into the atmosphere affects the oxidising capacity, both as a source of reactive radicals such as NO₃, Cl and BrO, and as a result of their influence on the concentration of ozone. The alkyl nitrates are a reservoir species for NO_x (=NO₂ + NO). Photolysis of NO₂ is the only known way of producing ozone in the troposphere (Wayne, 1991), therefore the photochemical processes occurring in the lower atmosphere are critically dependent on the level of nitrogen oxides. As the alkyl nitrates are relatively long lived in the troposphere, they can act as a source of NO_x in remote environments away from continental sources and so influence ozone concentrations on regional levels.

In general, the alkyl nitrates have a predominantly anthropogenic source, but during the 1990s, several authors invoked an oceanic source of the light alkyl nitrates (C₁-C₃) to explain the distributions seen over the remote oceanic regions (Atlas, 1993)(Blake, 1999).

My research

As part of my Ph.D research, I have measured methyl and ethyl nitrate in seawater and air samples (along with some biogenic halocarbons) along two north-south Atlantic Ocean transects (see Figure 1 for cruise tracks) and as part of the fourth in situ iron enrichment experiment in the Southern Ocean (EisenEx).

The results have provided the first direct evidence that the ocean can be a source of the light alkyl nitrates. Calculations of the % saturation of methyl and ethyl nitrate in seawater shows that in particular, the equatorial region of the Atlantic Ocean appears to be a strong source region (see figure 2) and a simple calculation of the atmospheric residence time of MeONO₂, using our mean equatorial sea-to-air flux and atmospheric concentration, results in a lifetime comparable to that calculated using the known atmospheric destruction processes. This implies that, for the equatorial region at least, the oceanic flux of MeONO₂ is a significant source component of the MeONO₂ budget. Similar results are found for EtONO₂.

figure 1 - percentage saturation of MeONO₂

Measurements of methyl nitrate during EisenEx showed increasing concentrations over time, although as yet, it is difficult to say whether this was a result of iron-mediated effects on production.

Future work

The confirmation of an oceanic source for these compounds raises a multitude of questions: What is the global oceanic distribution? How does it vary with season and how important are the fluxes to the atmospheric odd nitrogen reservoir budget globally? Are higher alkyl nitrates also produced? What are the production processes?

I am currently investigating some of these questions in a project sponsored by the NERC's Antarctic Funding Initiative. The influence of the Southern Ocean in the sea-air flux of these compounds is likely to be significant, based on the few, current observations of alkyl nitates in the Antarctic troposphere. Laboratory studies using phytoplankton cultures and also photochemical experiments will hopefully help to elucidate the production mechanisms of the alkyl nitrates in seawater, and a summer season and weekly sampling strategy set up over a year will investigate the seasonality of production of these gases. Funding: UK Natural Environment Research Council (NER/G/S/2003/00024) October 2003-September 2006.