D. L. Wellman

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.

Aerosol size distribution and aerosol water content measurements during Atlantic Stratocumulus Transition Experiment/Marine Aerosol and Gas Exchange

Aerosol size distribution data measured during the June 1992 Marine Aerosol and Gas Exchange experiment are analyzed to investigate the characteristics of fine marine aerosol particles measured over the North Atlantic near the Azores Islands. Measured aerosol size distribution data were corrected using the corrected size calibration data based on the optical properties of particles being measured. The corrected size distribution data were then approximated with either one or two lognormal size distributions, depending on air mass conditions. Under clean air mass conditions <3 μm diameter aerosol size distributions typically exhibited two modes, consisting of an accumulation mode and the small end of the sea-salt particle mode. However, under the influence of continental polluted air masses, the aerosol size distribution was dominated by <1 μm diameter particles in a single mode with an increased aerosol concentration. Aerosol water content of accumulation mode marine aerosols was estimated from differences between several series of ambient and dried aerosol size distributions. The average aerosol water fraction was 0.31, which is in good agreement with an empirical aerosol growth model estimate. The average rate of SO4= production in the accumulation mode aerosol water by H2O2 oxidation was estimated to be <7×10−10 mol L−1 s−1, which is an insignificant contributor to the observed non-sea-salt SO4= in the accumulation mode.

Contribution of gulf area natural sulfur to the North American sulfur budget

To evaluate the contribution of natural sulfur compounds from the Gulf of Mexico to the overall North American sulfur budget two series of air sampling flights were performed over the gulf area. Total aerosol mass load and sulfate concentration data indicate, in agreement with our previous findings on gas-phase products, that these observations can be divided into two categories. One group of measurements was taken under offshore airflow and the other under onshore flow conditions. From the measurements performed under clean (onshore) flow, average inside boundary layer SO/sub 4//sup 2 -/ concentrations were evaluated. Using these data, together with our previously reported dimethyl sulfide levels, a simple model was developed to estimate the sulfur flux transported northward from the gulf area. Upper and lower limits of this contribution are estimated at 0.25 and 0.04 Tg (S) year/sup -1/, respectively. Although this quantity is relatively low compared with the national US anthropogenic emission, it has significance for the global sulfur cycle, and it can cause a significant acidification of cloud water. 24 references, 5 figures, 3 tables.

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.

Chemical characteristics of oil refinery plumes in Los Angeles

Abstract Airborne in situ measurements of gases (NO, NO2, SO2, O3) and aerosol size distributions have been made in the vicinity of oil refineries in southern Los Angeles. Filter and impactor samples have been collected for post-flight chemical particle analyses using the BaCl2 and Nitron reaction spot techniques, and X-ray energy spectrometry. It was found that the refinery operations increase the number concentration of aerosols in the size range between 0.05 μm and 23.5 μm particle radius. Sulfate and nitrate particles have been identified as oxidation productions of SO2 and NOx, respectively. Sulfate particles have been found with a mode of less than 0.1 μm diameter, whereas inorganic nitrates dominate the mode between 1 and 10 μm. The strong affinity for water of the nitrates strongly affects visibility, cloud formation and the cloud drop size spectrum.

A comparison of airborne and surface trace gas measurements during the Southern Oxidants Study (SOS)

The NOAA Twin Otter conducted more than a dozen overflights of ground-level air quality monitoring stations during the 1995 Southern Oxidants Study (SOS) Nashville/Middle Tennessee Ozone Project Field Intensive. Surface and aircraft observations of ozone and ozone precursors were examined to identify systematic sampling errors, and to assess the degree to which surface measurements may be considered representative of the larger planetary boundary layer (PBL). Overall agreement between surface and aircraft trace gas measurements was excellent in the well developed mixed layer, especially in rural-regional background air and under stagnant conditions, where surface concentrations change only slowly. On July 2, surface level measurements were representative of the larger mixed layer over distances as far as 70 km in background air, and 30 km in the weakly advected urban plume. Vertical variations in trace gas concentrations were often minimal in the well-mixed PBL, and measurements at the surface always agreed well with aircraft observations up to 460 m above ground level. Under conditions of rapidly varying surface concentrations (e.g., during episodes of power plant plume fumigation and early morning boundary layer development), agreement between surface and aloft is dependent upon the spatial (aircraft) and temporal (ground) averaging intervals used in the comparison. Under these conditions, surface sites may be representative of the PBL only to within a few kilometers. Under clear skies in the well-mixed PBL, regression of aircraft trace gas data collected within 5 km of the ground sites against 15–20 min average surface concentrations centered on the times of the overflights yielded the following relationships: [O3]aircraft = ([O3]surface × 0.9374) + 4.86 ppbv (r2 = 0.9642); [CO]aircraft = ([CO]surface × 0.8914) + 16.4 ppbv (r2=0.9673); [SO2]aircraft = ([SO2]surface × 0.9414) − 0.069 ppbv (r2 = 0.9945). Although O3, CO, and SO2 measurements at the surface and aloft generally agreed well, isolated examples of unexplained measurement discrepancies emerged, illustrating the need for side-by-side instrument comparisons. NOY measurements agreed poorly between surface and aircraft: [NOY]aircraft = ([NOY]surfacex × 0.9184) + 4.56 ppbv (r2 = 0.9188).

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.

Vertical distribution of atmospheric aerosol size distribution over south-central New Mexico

Abstract The vertical distribution of background atmospheric aerosols was measured over south-central New Mexico as a part of the Atmospheric Lidar Verification Experiment (ALIVE) during four research field periods in the summers and winters of 1989 and 1990. Aerosol size distribution was measured from the surface up to 4500 m above sea level (asl) over the particle size range 0.1∼32 μm, using two Particle Measuring Systems (PMS) probes mounted on the wings of the NOAA King Air research aircraft. Vertical profiles of aerosol number concentrations of both fine- (0.1–2.0 μm) and coarse- (>2.0 μm) particle modes show seasonal differences, with higher number concentrations and higher mixed layer heights during summers. The measured aerosol size distribution data of each ALIVE intensive were averaged for boundary layer and free troposphere regions. These data mostly exhibit bi-modal distributions, typical for the continental atmospheric aerosols. Exceptions were the free troposphere size distributions measured during December 1989 (ALIVE III) and June 1990 (ALIVE IV), which resemble Junges power-law distribution. Each of the averaged aerosol size distributions was approximated by the sum of log-normal distributions. Different characteristics of aerosol size distribution were observed between the two summer measurements of 1989 and 1990. Back-trajectory analysis revealed that decreased aerosol concentrations were observed during June 1990 when the air mass was transported from the southwestern U.S.A.

Continuous atmospheric sulfur gas measurements aboard an aircraft: A comparison between the flame photometric and fluorescence methods

Abstract Analyzers that use the flame photometric and pulsed fluorescence techniques measured trace concentrations of S gas aboard an instrumented aircraft. Concentrations in the range of 1–20 ppbv were found at various locations over the U.S. East Coast and near Bermuda at altitudes up to 4000 m (650 mb). The response of both instruments changes significantly with ambient air pressure. In the case of the fluorescence method, a simple correction is applied to both the zero and span values. For the flame photometric instrument, the correction is more complicated, less accurate and valid only for ambient air pressures above 750 mb. A comparison between the two methods, based on several thousand 1-min averages, shows that the flame photometer produced consistently larger concentrations (27%) than the fluorescence device. Additional comparisons between the continuous monitors aboard the NOAA King Air and similar instruments aboard other aircraft sampling in parallel produced reasonable agreement.The use of two different techniques for measuring S gas establishes a range in the S gas concentration. This range is meaningful, since it delineates the contributions of the various interferences.

Lake Michigan Ozone Study (LMOS): Measurements from an instrumented aircraft

Abstract As part of the Lake Michigan Ozone Study, the NOAA instrumented King Air research aircraft made a series of flights over Lake Michigan during the summers of 1990 and 1991 to characterize the atmospheric conditions prevailing during times when O 3 concentrations exceeded the air quality standard. Most of the time, O 3 concentrations were within the normal range (40–70 ppbv) for the location and season, but higher concentrations were measured during the afternoon flights at several isolated locations. During three afternoon flights, high O 3 concentrations (> 120 ppbv) were observed along portions of the flight path; the highest 1-min average exceeded 160 ppbv. In two flights the highest O 3 concentrations were observed in the lower boundary layer over the eastern portion of the flight track; in one case the high concentrations were found over the western side of the lake throughout the boundary layer. The increased O 3 was accompanied by moderately increased SO 2 and NO x (10–20 ppbv); outside the region of elevated O 3 , the SO 2 and NO x were less than 2–3 ppbv. The elevated zone concentrations were related to emissions from the urban region located near the southern and southwestern shores of Lake Michigan.