Casper Labuschagne

Saturation of the Southern Ocean CO2 Sink Due to Recent Climate Change

Based on observed atmospheric carbon dioxide (CO2) concentration and an inverse method, we estimate that the Southern Ocean sink of CO2 has weakened between 1981 and 2004 by 0.08 petagrams of carbon per year per decade relative to the trend expected from the large increase in atmospheric CO2. We attribute this weakening to the observed increase in Southern Ocean winds resulting from human activities, which is projected to continue in the future. Consequences include a reduction of the efficiency of the Southern Ocean sink of CO2 in the short term (about 25 years) and possibly a higher level of stabilization of atmospheric CO2 on a multicentury time scale.

Atmospheric mercury concentrations observed at ground-based monitoring sites globally distributed in the framework of the GMOS network

Long-term monitoring of data of ambient mercury (Hg) on a global scale to assess its emission, transport, atmospheric chemistry, and deposition processes is vital to understanding the impact of Hg pollution on the environment. The Global Mercury Observation System (GMOS) project was funded by the European Commission (http://www.gmos.eu) and started in November 2010 with the overall goal to develop a coordinated global observing system to monitor Hg on a global scale, including a large network of ground-based monitoring stations, ad hoc periodic oceanographic cruises and measurement flights in the lower and upper troposphere as well as in the lower stratosphere. To date, more than 40 ground-based monitoring sites constitute the global network covering many regions where little to no observational data were available before GMOS. This work presents atmospheric Hg concentrations recorded worldwide in the framework of the GMOS project (2010-2015), analyzing Hg measurement results in terms of temporal trends, seasonality and comparability within the network. Major findings highlighted in this paper include a clear gradient of Hg concentrations between the Northern and Southern hemispheres, confirming that the gradient observed is mostly driven by local and regional sources, which can be anthropogenic, natural or a combination of both.

Response to Comments on "Saturation of the Southern Ocean CO2 Sink Due to Recent Climate Change"

We estimated a weakening of the Southern Ocean carbon dioxide (CO2) sink since 1981 relative to the trend expected from the large increase in atmospheric CO2. We agree with Law et al. that network choice increases the uncertainty of trend estimates but argue that their network of five locations is too small to be reliable. A future reversal of Southern Ocean CO2 saturation as suggested by Zickfeld et al. is possible, but only at high atmospheric CO2 concentrations, and the effect would be temporary.

Seasonal cycles of O3 in the marine boundary layer: Observation and model simulation comparisons

We present a two-step approach for quantitatively comparing modeled and measured seasonal cycles of O3: (1) fitting sine functions to monthly averaged measurements and model results (i.e., deriving a Fourier series expansion of these results) and (2) comparing the phase and amplitude of the statistically significant terms between the models and measurements. Two and only two sine terms are sufficient to quantify the O3 seasonal cycle in the marine boundary layer (MBL) in both the measurements and the model results. In addition to the expected fundamental (one sine cycle per year), a second harmonic term (i.e., two sine cycles per year) is identified as a ubiquitous feature of O3 in the MBL. Three chemistry climate models (Community Atmosphere Model with chemistry, GFDL-CM3, and GISS-E2-R) approximately reproduce many features of the measured seasonal cycles at MBL surface sites throughout the globe, with some notable quantitative disagreements, but give divergent results that do not agree with O3 sonde measurements above the MBL. This disagreement and divergence of results between models indicate that the treatment of the MBL dynamics in the chemistry-climate models is not adequate to reproduce the isolation of the MBL indicated by the observations. Within the MBL the models more accurately reproduce the second harmonic term than the fundamental term. We attribute the second harmonic term to the second harmonic of opposite phase in the photolysis rate of O3, while the fundamental term evidently has many influences. The parameters derived from the Fourier series expansion of the measurements are quantitative metrics that can serve as the basis for future model-measurement comparisons.

Chapter 20 - Ozone Concentrations and Their Potential Impacts on Vegetation in Southern Africa

Abstract The tropospheric ozone concentrations over southern Africa are relatively high due to large precursor emissions from biomass burning, vegetation and anthropogenic sources and the intensity of solar radiation. In this study, we summarize ozone concentration data from both literature and new measurements. Further, we present data on the accumulated vegetation exposure to ozone in terms of the AOT40 index, which is calculated for the sites with hourly ozone and radiation data, with an aim to assess the potential for adverse effects on vegetation. As the seasonal variation of ozone concentrations is large, we discuss the effect of growing season onset on the stomatal gas exchange and the related potential for vegetation impacts. Finally, future research needs are discussed.

Development of a Simplified, Cost Effective GC-ECD Methodology for the Sensitive Detection of Bromoform in the Troposphere

Wherever measurements have been made bromoform was found to be ubiquitous in the surface ocean in pmolar-nmolar concentrations. These measurements show concentrations in coastal regions orders of magnitude higher than in the pelagic oceans. Its atmospheric presence is primarily due to its release from algae and rapid transport to the marine boundary troposphere where it is known to participate in ozone chemistry via photochemical and catalytic pathways. Until quite recently, a limited number of studies existed (compared to other marine volatile organic compounds (VOCs)), mainly due to the analytical challenge(s) presented by the low environmental mixing ratios. In this work we detail the development of a simplified, cost effective method to detect and quantify bromoform in environmental air samples. Air samples (1.5 L) were preconcentrated onto a precooled adsorbent (Carbopack X/Carboxen 1016) trap. These samples were injected by means of rapid thermal desorption for separation and detection by GC-ECD. The system was calibrated by means of a custom-built permeation oven. A linear system response was achieved, having a detection limit of 0.73 ± 0.09 ppt. A range of environmental samples was analysed to demonstrate the ability of the technique to separate and identify bromoform from air samples. The results showed that bromoform concentrations typically averaged 24.7 ± 17.3 ppt in marine air samples, 68.5 ± 26.3 ppt in Cape Town urban air samples and 33.9 ± 40.5 ppt in simulated biomass burning plumes (SBBP).

A vegetation control on seasonal variations in global atmospheric mercury concentrations

Anthropogenic mercury emissions are transported through the atmosphere as gaseous elemental mercury (Hg(0)) before they are deposited to Earth’s surface. Strong seasonality in atmospheric Hg(0) concentrations in the Northern Hemisphere has been explained by two factors: anthropogenic Hg(0) emissions are thought to peak in winter due to higher energy consumption, and atmospheric oxidation rates of Hg(0) are faster in summer. Oxidation-driven Hg(0) seasonality should be equally pronounced in the Southern Hemisphere, which is inconsistent with observations of constant year-round Hg(0) levels. Here, we assess the role of Hg(0) uptake by vegetation as an alternative mechanism for driving Hg(0) seasonality. We find that at terrestrial sites in the Northern Hemisphere, Hg(0) co-varies with CO2, which is known to exhibit a minimum in summer when CO2 is assimilated by vegetation. The amplitude of seasonal oscillations in the atmospheric Hg(0) concentration increases with latitude and is larger at inland terrestrial sites than coastal sites. Using satellite data, we find that the photosynthetic activity of vegetation correlates with Hg(0) levels at individual sites and across continents. We suggest that terrestrial vegetation acts as a global Hg(0) pump, which can contribute to seasonal variations of atmospheric Hg(0), and that decreasing Hg(0) levels in the Northern Hemisphere over the past 20 years can be partly attributed to increased terrestrial net primary production.Terrestrial vegetation contributes to the seasonal variation of atmospheric mercury concentrations, according to analyses of atmospheric trace gas dynamics and satellite data. The data show that the photosynthetic activity of vegetation correlates with atmospheric mercury.