1.1 The O2/N2 Ratio and CO2 Airborne Southern Ocean (ORCAS) Study

The O2/N2 Ratio and CO2 Airborne Southern Ocean (ORCAS) Study was an NSF sponsored airborne field campaign conducted with the NCAR/NSF GV aircraft with research flights from Punta Arenas, Chile during January and February 2016.  A major goal of ORCAS is to gain a more quantitative understanding of the dominant role that the Southern Ocean plays in the uptake of anthropogenic carbon and to provide the basis for a better representation of this process in global atmospheric models. ACOM scientists provided key measurements of a wide range of VOCs (Apel, Hornbrook, and Hills) using the Trace Organic Gas Analyzer (TOGA).

Sample template image
Figure 1. ORCAS observations.

Figure 1 shows a subset of the ORCAS observations as binned altitude profiles of VOC mixing ratios observed by TOGA. The data in each altitude bin are also separated by the corresponding observed CO2 mixing ratio (high CO2 ≥ 399 ppm > low CO2) according to the CO2 averaged over the timescale of the respective VOC measurement. In Figure 1(a and b), the vertical profiles of acetone and acetonitrile show higher mixing ratios were observed in air with higher CO2. NMHC less susceptible to ocean uptake such as benzene (Figure 1c), indicate no correlation with CO2 in the lower troposphere and marine boundary layer, and a small correlation in the mid- and upper troposphere with CO2. The findings are consistent with continental emissions and interhemispheric transport influencing the mixing ratios of both longer- and shorter-lived gases in the middle and upper troposphere, and loss to the oceans impacting mixing ratios of more soluble gases in the middle and lower troposphere.

The ocean is a primary source for bromoform (CHBr3) and dibromomethane (CH2Br2) [e.g., Goodwin et al., 1997], species that have a large impact on natural stratospheric ozone destruction. Figure 1(d and e) shows that bromoform and dibromomethane were anticorrelated with CO2 in the ORCAS domain in the boundary layer and mid-troposphere to ≈ 6 km. This anticorrelation of ocean-emitted species with CO2 in the lower atmosphere is consistent with increased ocean uptake of CO2 in air that has had more interaction with the ocean surface. The ORCAS dataset provides a means to identify biological processes responsible for the emissions and uptake of reactive (VOCs) and non-reactive (CO2, O2) gases and explore the relationships and commonalities between the two.

The relationships shown between CO2 and reactive VOCs demonstrate that the air masses sampled in the ORCAS domain are not uniformly influenced by continental sources or air-sea exchange in marine environments, and provide a mechanism to constrain the relative influence of the ocean surface and long-range transport on the observations of longer-lived gases measured during ORCAS.