James E. Johnson

A reevaluation of the open ocean source of methane to the atmosphere

Seawater and atmospheric methane (CH4) mixing ratios were measured on five cruises throughout the Pacific Ocean from 1987 to 1994 to assess the magnitude of the ocean-atmosphere flux. The results showed consistent regional and seasonal variations with surface seawater concentrations ranging from 1.6 to 3.6 nM and saturation ratios ranging from 0.95 to 1.17. The equatorial Pacific Ocean was supersaturated with respect to atmospheric CH4 partial pressures, while areas outside the tropics often were undersaturated during fall and winter. Although atmospheric CH4 mixing ratios over the North Pacific during April increased 3.4% from 1988 to 1993, the saturation ratios remained constant. Based on the concentration fields, the data were divided into two seasons and 10 latitude zones from 75°S to 75°N. Using monthly Comprehensive Ocean-Atmosphere Data Set (COADS) wind and surface seawater temperature data and the Wanninkhof [1992] wind speed/transfer velocity relationship, the calculated zonal average fluxes ranged from −0.1 to 0.4 μmol m−2 d−1. The combined seasonal and zonal fluxes result in a total global ocean-to-atmosphere flux of 25 Gmol yr−1 (0.4 Tg CH4 yr−1), which is an order of magnitude less than previous estimates [Intergovernmental Panel on Climate Change (IPCC), 1994]. The estimated uncertainty in this number is approximately a factor of 2.

Regional and seasonal variations in the flux of oceanic carbon monoxide to the atmosphere

Carbon monoxide (CO) is produced photochemically in the surface ocean and emitted to the atmosphere. To assess the magnitude of this ocean-atmosphere flux, seawater and atmospheric CO mole fractions were measured on six cruises throughout the Pacific Ocean from 1987 to 1994. The results showed consistent regional and seasonal variations in surface seawater CO concentrations with daily averaged concentrations ranging from 0.1 to 4.7 nM. Based on the concentration fields, the data were divided into four seasons and 10 latitude zones from 75°S to 75°N. Using monthly Comprehensive Ocean-Atmosphere Data Set wind and surface seawater temperature data and the Wanninkhof [1992] wind speed/transfer velocity relationship, the calculated zonal average fluxes ranged from 0.25 to 13 μmol/m2/d. The combined seasonal and zonal fluxes result in a total global flux of 0.46 Tmol CO/y with 2/3 of this flux in the southern hemisphere. The estimated uncertainty in this number is approximately a factor of 2.

Regional physical and chemical properties of the marine boundary layer aerosol across the Atlantic during Aerosols99: An overview

The Aerosols99 cruise crossed the Atlantic Ocean from Norfolk, Virginia, to Cape Town, South Africa, between January 14 and February 8, 1999. The goals of the cruise were to determine the chemical, physical, and optical properties of the marine boundary layer (MBL) aerosol, the vertical distribution of aerosols and ozone, the column-integrated aerosol optical depth, and the ozone, CO, and peroxy radical chemistry in the MBL. Sampling strategies were optimized to obtain data sets to evaluate satellite-derived ocean color (Sea-viewing Wide Field-of-view Sensor), aerosol optical depth (advanced very high resolution radiometer) and total column ozone (Total Ozone Mapping Spectrometer). The cruise track crossed through seven different meteorological/oceanographic regimes ranging from background marine air masses in the Northern and Southern Hemispheres to air masses containing mineral dust and the products of biomass burning. This overview discusses the seven regimes encountered enroute and the chemical and physical properties of the MBL aerosol in each regime.

Reevaluation of the open ocean source of carbonyl sulfide to the atmosphere

Carbonyl sulfide (COS) concentrations were measured in surface seawater samples and the overlying marine boundary layer of the Pacific Ocean by using gas chromatography (GC) and electron capture sulfur detection (ECD-S). A wide latitudinal range was covered (55°N–70°S) on two cruises 9 months apart. COS saturation ratios (SRs) in seawater were found to be less than 1 (undersaturated) across wide regions of the open ocean, especially in the subtropical gyres and wintertime subpolar waters. SRs were highest in coastal/shelf regions and in spring/summertime temperate waters. Extensive undersaturation is attributed to a low COS photoproduction potential of the water, limited sunlight, and/or a rapid hydrolysis rate constant. Decreasing COS concentrations during diurnal cycles in tropical waters were fitted to first-order exponentials, with resulting decay times agreeing with calculated hydrolysis lifetimes to within 15%. Air-sea fluxes of COS from the open ocean were calculated by using two different expressions for the transfer velocity and averaged into six latitude bands and three seasons. On the basis of these data we report a global open ocean sea-air flux of −0.032 (−0.010 to −0.054) Tg COS/yr, which is much lower than and of different sign from the current global estimate (0.14–0.58 Tg COS/yr). Atmospheric COS mixing ratios averaged 470 pptv on the first cruise and 442 pptv on the second cruise, with much of the difference possibly a result of a seasonal decrease in the northern hemisphere COS mixing ratio of up to 10%.

Physico-chemical modeling of the First Aerosol Characterization Experiment (ACE 1) Lagrangian B: 1. A moving column approach

During Lagrangian experiment B (LB in the following) of the First Aerosol Characterization Experiment (ACE 1), a clean maritime air mass was followed over a period of 28 hours. During that time span, the vertical distribution of aerosols and their gas phase precursors were characterized by a total of nine aircraft soundings which were performed during three research flights that followed the trajectory of a set of marked tetroons. The objective of this paper is to study the time evolution of gas phase photochemistry in this Lagrangian framework. A box model approach to the wind shear driven and vertically stratified boundary layer is questionable, since its basic assumption of instantaneous turbulent mixing of the entire air column is not satisfied here. To overcome this obstacle, a one-dimensional Lagrangian boundary layer meteorological model with coupled gas phase photochemistry is used. To our knowledge, this is the first time that such a model is applied to a Lagrangian experiment and that enough measurements are available to fully constrain the simulations. A major part of this paper is devoted to the question of to what degree our model is able to reproduce the time evolution and the vertical distribution of the observed species. Comparison with observations of O3, OH, H2O2, CH3OOH, DMS, and CH3I, made on the nine Lagrangian aircraft soundings shows that this is in general the case, although the dynamical simulation started to deviate from the observations on the last Lagrangian flight. In agreement with experimental findings reported by Q. Wang et al. (unpublished manuscript, 1998b), generation of turbulence in the model appears to be most sensitive to the imposed sea surface temperature. Concerning the different modeled and observed chemical species, a number of conclusions are drawn: (1) Ozone, having a relatively long photochemical lifetime in the clean marine boundary layer, is found to be controlled by vertical transport processes, in particular synoptic-scale subsidence or ascent. (2) Starting with initally constant vertical profiles, the model is able to “create” qualitatively the vertical structure of the observed peroxides. (3) OH concentrations are in agreement with observations, both on cloudy and noncloudy days. On the first flight, a layer of dry ozone rich air topped the boundary layer. The model predicts a minimum in OH and peroxides at that altitude consistent with observations. (4) Atmospheric DMS concentrations are modeled correctly only when using the Liss and Merlivat [1986] flux parameterization, the Wanninkhof [1992] flux parameterization giving values twice those observed. To arrive at this conclusion, OH is assumed to be the major DMS oxidant, but no assumptions about mixing heights or entrainment rates are necessary in this type of model. DMS seawater concentrations are constrained by observations.

Concentrations and fluxes of dissolved biogenic gases (DMS, CH4, CO, CO2) in the equatorial Pacific during the SAGA 3 experiment

The equatorial Pacific Ocean is a source of both sulfur and carbon to the atmosphere. In February and March 1990, as part of the Soviet-American Gases and Aerosols (SAGA 3) expedition, dimethysulfide (DMS), methane (CH4), carbon monoxide (CO), and carbon dioxide (CO2) partial pressures were determined in both surface seawater and the overlying atmosphere of the central equatorial Pacific Ocean (15°N to 10°S, 145°W to 165°W). The partial pressures were used to calculate the net flux of these gases from the ocean to the atmosphere. The average regional DMS and CO fluxes were similar, 7.1 and 4.2 μmol/m2/d, respectively. The mixing ratio of CH4 in surface seawater was close to equilibrium with the overlying atmosphere and hence the average flux was only 0.39 μmol/m2/d. The flux of CO2 clearly dominated the air-sea carbon exchange with an average regional flux of 5.4 mmol/m2/d.

Calibration of the TOPEX altimeter using a GPS buoy

The use of a spar buoy equipped with a Global Positioning System (GPS) antenna to calibrate the height measurement of the TOPEX radar altimeter is described. In order to determine the height of the GPS antenna phase center above the ocean surface, the buoy was also equipped with instrumentation to measure the instantaneous location of the waterline, and tilt of the buoy from vertical. The experiment was conducted off the California coast near the Texaco offshore oil platform, Harvest, during cycle 34 of the TOPEX/POSEIDON observational period. GPS solutions were computed for the buoy position using two different software packages, K&RS and GIPSY-OASIS II. These solutions were combined with estimates of the waterline location on the buoy to yield the height of the ocean surface. The ocean surface height in an absolute coordinate system combined with knowledge of the spacecraft height from tracking data provides a computed altimeter range measurement. By comparing this computed value to the actual altimeter measurement, the altimeter bias can be calibrated. The altimeter height bias obtained with the buoy using K&RS was −14.6±4 cm, while with GIPSY-OASIS II it was −13.1±4 cm. These are 0.1 cm and 1.6 cm different from the −14.7±4 cm result obtained for this overflight with the tide gauge instruments located on Platform Harvest.

Measurements of atmospheric carbonyl sulfide during the NASA Chemical Instrumentation Test and Evaluation project: Implications for the global COS budget

Atmospheric COS concentrations were measured by three analytical systems during the Chemical Instrumentation Test and Evaluation (CITE 3) project. The three systems all used cryogenic sample preconcentration and gas chromatographic (GC) separation but differed in the method of detection. The FPD system used a flame photometric detector, the MS system used a mass selective detector, and the ECD-S system used a fluorinating catalyst followed by an electron capture detector. With the FPD system, we found a mean COS concentration of 510 ppt over the North Atlantic and 442 ppt over the Tropical Atlantic. With the ECD-S system, we found a mean COS concentration of 489 ppt over the North Atlantic and 419 ppt over the Tropical Atlantic. All three systems registered a latitudinal gradient in atmospheric COS of between 1.6 and 2.0 ppt per degree of latitude, with increasing COS concentrations northward which was similar to the gradient measured by Bingemer et al. (1990). It is difficult to reconcile the measured latitudinal concentration gradient with present theories of the global COS budget since the largest sink of COS is thought to be a flux to land plants, most of which are in the northern hemisphere.

Physico‐chemical modeling of the First Aerosol Characterization Experiment (ACE 1) Lagrangian B: 2. DMS emission, transport and oxidation at the mesoscale

A three-dimensional mesoscale meteorological model was used to study the interplay between the dynamical (turbulent mixing and advection) and physico-chemical (sea-air flux and photochemical sink by OH) processes that control dimethylsulfide DMS concentrations and their distribution in the marine boundary layer (MBL) during the First Aerosol Characterization Experiment ACE 1. Atmospheric DMS concentrations were constrained using observed seawater DMS concentrations and box model derived OH concentrations. Lateral boundary values of dynamical parameters were derived from the 6-hourly meteorological analysis of the European Centre for Medium-Range Weather Forecasts. Calculated DMS concentrations, wind speed and direction, and cloud cover were compared with measurements made aboard the R/V Discoverer and on the three NCAR/C130 aircraft flights during the LagB experiment. Model-generated atmospheric DMS concentrations agreed with the DMS observations from the NCAR/C130 aircraft flights during the LagB experiment (R 2 = 0.69) assuming OH is the only oxidant and DMS flux parameterization based on Liss and Merlivat [1986]. Comparison with Eulerian measurements made aboard the R/V Discoverer showed that the model simulated the range of observed values but not the hour-to-hour variation observed in the atmospheric DMS concentrations. Part of the discrepancy was attributed to uncertainties in DMS sea-to-air transfer velocity, small scale features of seawater DMS that are beyond the model resolution, and uncertainties in the venting of the boundary layer by shallow clouds. A quantitative budget at the ship location revealed a strong impact of advection processes in determining DMS levels and temporal evolution. The three-dimensional mesoscale meteorological model was also used to estimate the effect of the low spatial resolution used in global models on seawater DMS concentrations and atmospheric OH concentrations.

Carbon disulfide measurements in the atmosphere of the western North Atlantic and the northwestern south Atlantic oceans

Carbon disulfide (CS2) measurements were made over the western and equatorial North Atlantic Ocean and the northwestern and equatorial South Atlantic Ocean. Carbon disulfide was in the range 0.4–50 pptrv in the atmosphere of the western North Atlantic Ocean. Emissions from anthropogenic sources and wet lands were found to be important although anthropogenic sources were 4–6 times larger than biogenic sources. The flux of CS2 from eastern North America between 30 and 39° latitude was estimated to be 2 × 108g yr−1 of sulfur. The anthropogenic contribution was 1.8 × 108g yr−1 of sulfur whereas the contribution of marshes was 0.2 × 108g yr−1 of sulfur. Sources of CS2 at high latitudes in the northern hemisphere were comparatively weak. Carbon disulfide levels in the western South Atlantic Ocean between −5 and 1° latitude were in the range 0.2–6 pptrv. Most of the CS2 appeared to come from biomass burning in Africa. Carbon disulfide was much higher close to shore suggesting that the South American continent was a significant source although too few data were available to quantify it. On ferry flights from Wallops, Virginia to Natal, Brazil, CS2 levels at the ferry altitude of about 6 km averaged 1.2 pptrv. This background CS2 was adequate to account for all the OCS in the atmosphere.