Quantifying the Oceanic Contribution to Atmospheric Organic Bromine
Introduction to Science Problem:
The role of halogens in the catalytic destruction of
tropospheric and
stratospheric ozone (O3) is well established. Ocean fluxes of
organohalogens represent a significant global source of halogens to
the atmosphere, however the magnitude and spatial and temporal
distributions of these compounds is not well understood. Relatively
little attention has focused on reactive organohalogens (atmospheric
lifetimes much less than 6 months), despite growing recognition in the climate
community that these compounds represent an important halogen source
to the troposphere, and to the lower stratosphere via rapid convective
processes. Current model calculations estimate that short-lived
biogenic bromine compounds from the troposphere can contribute 20-30%
of stratospheric O3 depletion. Bromoform (CHBr3) and
dibromomethane (CH2Br2) represent the largest natural
contributions to atmospheric organic bromine after methyl bromide. Natural
ocean sources of these reactive compounds include macroalgae and
phytoplankton, and their main atmospheric sinks include photolysis and
oxidation by OH, leading to atmospheric lifetimes typically less than a few
months. Ship data, collected over a decade, provide the largest source of
information on the spatial and temporal flux variability of CHBr3
and CH2Br2 over the Atlantic, Pacific, and Southern
Oceans.
Project Definition:
The principal outcome of this project will be to develop, using these ship
data, a better quantitative understanding of ocean fluxes of CHBr3
and CH2Br2 and their subsequent impact on oxidant
chemistry in the atmosphere. This is an
internationally recognized goal in the climate
community, which is relevant to chemistry-climate modelling and
Earth system
modelling. The studentship has three specific objectives:
- Develop Model Parameterizations to Accurately Describe Observed Flux
Variability of CHBr3 and CH2Br2.
The underlying processes responsible for the variability in observed fluxes of
CHBr3 and CH2Br2 are related to commonly
measured variables (e.g., sea-surface temperature). Once these relationships
are understood they can be used as a proxy for predicting CHBr3 and
CH2Br2 fluxes. First, the student will develop and
evaluate proxies that accurately describe the observed spatial and temporal
variability of air-sea fluxes of CHBr3 and
CH2Br2 for different ocean regions and seasons. The
proxy relationships will be used to construct 2-D maps of ocean flux.
- Analyze Uncertainties of Individual Ocean Production
and Loss Rates of CHBr3 and CH2Br2.
The student will develop an established ocean-atmosphere box model to evaluate
the role of ocean cycling of CHBr3 and CH2Br2
has on the distribution of their air-sea fluxes. Published empirical
relationships between ocean parameters and production rates of CHBr3
and CH2Br2, estimated chemical and microbial loss rates,
flux measurements, and depth profiles of these compounds (that can be used
to estimate local lifetimes) provide statistical constraints to the
chemical mass balance of these compounds, and quantify uncertainties
of resultant model ocean fluxes.
- Quantify the Oceanic Contribution of CHBr3 and
CH2Br2 to Atmospheric Composition. 2-D ocean fluxes
of CHBr3 and CH2Br2 developed in (2) will be
implemented into the
TOMCAT
atmospheric chemistry transport model
to improve the assessment of the oceanic contribution of organic
bromine to atmospheric O3 destruction.