Charles C. Van Valin

The relationship between dimethyl sulfide and particulate sulfate in the mid-atlantic ocean atmosphere

Abstract Dimethyl sulfide (DMS) and atmospheric aerosols were sampled simultaneously over the Atlantic Ocean in the vicinity of Bermuda using the NOAA King Air research aircraft. Total and fine (50% cutoff at 2 μm diameter) aerosol fractions were sampled using two independent systems. The average nonsea-salt (nss)SO 4 2− concentrations were 1.9 and 1.0 μg m −3 (as SO 4 2− ) for the total and the fine fractions in the boundary layer (BL) and 0.53 and 0.27 μg m −3 in the free troposphere (FT). Non-sea-salt SO 4 2− in the two aerosol fractions were highly correlated ( r = 0.90), however a smaller percentage (55%) was found in the fine aerosol near Bermuda relative to that (90%) near the North American continent. The BL SO 4 2− concentrations measured in this study were higher than those measured by others at remote marine locations despite the fact that the 7-day air mass back trajectories indicated little or no continental contact at altitudes of 700 mb and below; the trajectories were over subtropical oceanic areas that are expected to be rich in DMS. DMS concentrations were higher near the ocean surface and decreased with increasing altitude within the BL; the average DMS concentration was 0.13 μg m −3 . Trace levels of DMS were also measured in the FT (0.01 μg m −3 ). Computer simultation of the oxidation and removal of DMS in the marine atmosphere suggests that 4 2− observed could be related to the natural S cycle.

O3, CO, Hydrocarbons and dimethyl sulfide over the Western Atlantic Ocean

Abstract The concentrations of O 3 , CO, dimethyl sulfide (DMS) and light hydrocarbons (C 2 –C 4 ) were measured from an instrumented aircraft during February–April 1985, near the U.S. East Coast and in the vicinity of Bermuda as part of the Western Atlantic Ocean Experiment (WATOX). Sampling flights were performed within the boundary layer (BL) and in the free troposphere (FT) at both locations. Photochemical generation of O 3 in polluted air parcels transported from the continent within the BL was identified as the probable source of excess O 3 (up to 50 ppbv above background). Convective lifting of boundary layer air carried pollutants into the free troposphere. The concentrations of HC compounds in air sampled near Bermuda had a significant inverse relation to air mass transport time from the continent. The BL concentrations of the more reactive HCs (ethylene, propane, propylene, normal- and isobutane) declined faster than the less reactive HCs (acetylene and ethane), and were found to be proportional to air mass transport time over the ocean. DMS was detected, with few exceptions, only within the BL at both sampling locations. The average concentrations in the BL samples collected near the U.S. East Coast and in the vicinity of Bermuda were 27 and 54 pptv. In all samples taken in the BL the DMS concentration decreased sharply as a function of altitude.

Atmospheric sulfur dioxide at Mauna Loa, Hawaii

Measurements of sulfur dioxide (S02) were made at the National Oceanic and Atmospheric Administrations Mauna Loa Observatory in Hawaii, during a 12-month period beginning in December 1988. SO2 concentrations varied from background levels of less than 0.05 ppbv to a maximum of 50 ppbv, during episodes that lasted from 2 to 24 hours. Emissions from the Kilauea crater, approximately 35 km southeast of the observatory at an elevation of about 1000 m above sea level (asl), and the current eruption at Puu O′o 50 km east-southeast, are the most likely sources for the higher concentrations. These episodes occurred 10–25 times each month, mostly during the day; peak concentrations were usually recorded at mid-day. The SO2 concentrations can be grouped into three periods; low (June–September), high (October–January) and intermediate (February–May). A clear diurnal cycle of SO2 concentration exists throughout the year, although day-night changes were greatest during October–January and were barely detectable during the June–September period. The highest SO2 concentrations were recorded when the predominant wind direction was northerly to northwesterly, even though the apparent sources are in the southeastern sector. Nighttime concentrations were usually at background levels; however, many exceptions were observed. A few cases of higher than background SO2 were observed when free tropospheric (FT) conditions were identified. The possibility that long-range transport was the cause for elevated SO2 concentrations under FT conditions was examined using air mass back trajectories analyses. The highest nighttime SO2 concentrations, under FT conditions, were observed during periods with slow easterly trajectories, and the lowest concentrations were found during westerly flows. Twenty-four nighttime free tropospheric events were recorded when the SO2 concentration exceeded 0.2 ppbv. During 18 of these episodes, unusually high CO2 concentrations were observed.

Sulfur dioxide flux measurements over the western atlantic ocean

Aircraft measurements of SO2 were made along the U.S. East Coast and in the vicinity of the Bermuda Islands during the period 2 March–11 April, 1985. SO2 was detected in all samples taken inside the boundary layer 100 km off shore. The maximum 1.0-min average concentration observed was 9.9 ppb (on the East Coast), and the average for the duration of the study was 2.1 ppb. The SO2 concentration in the free troposphere at the same location ranged from < 0.1 ppb to a maximum of 4.2 ppb. The air parcel sampled during the maximum event was back-tracked across the Ohio Valley region. Sulfur dioxide concentrations in the vicinity of the Bermuda Islands, inside and above the boundary layer, were less than the detection limit (0.1 ppb) during most of the time. On one event elevated SO2 levels were recorded, however they could not be traced to a source in N America. On the basis of the concentration and wind speed data, an altitude profile of SO2 flux was constructed for a portion of the U.S. East Coast. Integration of the analytical function describing the profile provided an estimate of SO2 flux eastbound of ~ 1 Tg (S)a−1.

The vertical distribution of atmospheric H2O2: A case study

Vertical profiles of H2O2 mixing ratios were obtained for each season from a site in central Arkansas during 1988. Aircraft-based measurements indicated that H2O2 mixing ratios followed an annual cycle, peaking during the summer at >6 parts per billion by volume (ppbv). The minimum occurred in winter when mixing ratios for H2O2 averaged about 0.2 ppbv. The H2O2 mixing ratio generally peaked at an altitude of about 800 mbar (2 km), although there may have been some seasonal dependence. The annual cycle followed variations in solar intensity, water mixing ratio, and temperature. Within a season, strong variations could be related to meteorological events. A daily cycle was inferred in which the H2O2 mixing ratio varied by a factor of 2 to 3; the peak observed values were at night. H2O2 mixing ratios at altitudes higher than 0.7 km were generally greater than local SO2 values above 0.7 km during all but the winter season.

Eastward sulfur flux from the Northeastern United States

During January and February 1986 the concentrations of sulfur dioxide (SO2) and particulate sulfate (SO42−) were measured from an instrumented aircraft 80–120 km east of the New England coast. The average concentration of SO2 in the boundary layer (BL) was 10 μg m−3; the maximum 30-min average was 26 μg m−3. The average and maximum values in the free troposphere (FT) were 3.9 and 31 μg m−3, respectively. The concentrations of non-sea-salt SO42− averaged 2.0 and 0.7 μg m−3 in the BL and FT, and the maximum concentrations were 7.7 and 3.2 μg m−3. Continuous wind speed records from the aircraft LORAN system were used to estimate altitude profiles of the offshore fluxes of SO2 and SO42− for the duration of the study. The estimated advection flux, is (3.5 ± 0.4) × 10−3 Tg(S)day−1 from the coastal segment between 41 and 43°N latitudes. Most (89%) of the S flux was found to be in the form of SO2; the remainder corresponded to particulate SO42−. The ratio of aerosol to gas-phase S in the BL was found to be similar to that in the FT, despite the fact that removal of SO2 from the BL is expected to be much faster than that from the FT.

The compatibility between aircraft and ground-based air quality measurements

Trace gas concentrations and atmospheric state parameters were measured aboard the NOAA King Air research aircraft during flights on August 16, 1988, along the Appalachian Mountains from central Pennsylvania to northern Georgia. Stepwise profiles were flown over five surface sites where measurements of certain atmospheric parameters were being made. A stationary cold front lying across southern Virginia effectively divided the area into two weather regimes; to the north of the frontal zone the air was slightly cooler and much drier than that to the south. Considerable convective activity developed from early to midafternoon along and south of the front. The comparison between the aircraft and ground sites included measurements of the primary pollutants SO2 and several hydrocarbons, NOy, the secondary pollutants H2O2 and O3, and meteorological parameters. Continuity between the aircraft and surface meteorological and trace gas measurements was consistent at the northernmost site, which is situated in a relatively level valley. The agreement was poorer at the other four ground sites, which are located on or near mountaintops. Most of the meteorological and trace gas measurements, other than those made at Scotia, were found to differ by substantial margins, often by more than 10 times the resolution of the instruments. However, within a few hours after the flights, the surface measurements, particularly those of H2O2 and O3, achieved values comparable to those measured with the aircraft, thus suggesting that air sampled at the canopy level did not mix readily with the bulk of the boundary layer.

A comparison of surface and airborne trace gas measurements at a rural Pennsylvania site

A comparison of trace gas concentrations between a ground sampling site and an instrumented aircraft was performed at the Scotia field site in central Pennsylvania. The comparison included four research flights in August 1988. The ground site was operated by the National Oceanic and Atmospheric Administration (NOAA) Aeronomy Laboratory, and the NOAA King Air instrumented aircraft was operated by the NOAA Air Resources Laboratory. The study demonstrates continuity in the values of the state parameters (wind speed and direction, temperature, and dew point) from the surface to the aircraft observations at the times of the four flights. Considered within the context of the differing time-space scales, continuity was also demonstrated in the trace gas measurements. The aircraft and surface measurements of the trace gas concentrations were in better agreement when the aircraft measurements demonstrated uniform concentrations over the length of the flight path. The mean O3 and H2O2 values of the lowest-altitude aircraft readings were usually greater than those measured at the surface, thus indicating higher-altitude sources and removal processes at lower altitudes. The aircraft-measured values for SO2, NOy, and hydrocarbons were usually lower than those measured at the surface, as expected for primary pollutants emitted near ground level. Closer agreement was obtained in the comparisons of secondary (O3, H2O2) than of primary (SO2, NOy, hydrocarbons) pollutants.