John T. Merrill

Asian dust events of April 1998

On April 15 and 19, 1998, two intense dust storms were generated over the Gobi desert by springtime low-pressure systems descending from the northwest. The windblown dust was detected and its evolution followed by its yellow color on SeaWiFS satellite images, routine surface-based monitoring, and through serendipitous observations. The April 15 dust cloud was recirculating, and it was removed by a precipitating weather system over east Asia. The April 19 dust cloud crossed the Pacific Ocean in 5 days, subsided to the surface along the mountain ranges between British Columbia and California, and impacted severely the optical and the concentration environments of the region. In east Asia the dust clouds increased the albedo over the cloudless ocean and land by up to 10–20%, but it reduced the near-UV cloud reflectance, causing a yellow coloration of all surfaces. The yellow colored backscattering by the dust eludes a plausible explanation using simple Mie theory with constant refractive index. Over the West Coast the dust layer has increased the spectrally uniform optical depth to about 0.4, reduced the direct solar radiation by 30–40%, doubled the diffuse radiation, and caused a whitish discoloration of the blue sky. On April 29 the average excess surface-level dust aerosol concentration over the valleys of the West Coast was about 20–50 μg/m3 with local peaks >100 μg/m3. The dust mass mean diameter was 2–3 μm, and the dust chemical fingerprints were evident throughout the West Coast and extended to Minnesota. The April 1998 dust event has impacted the surface aerosol concentration 2–4 times more than any other dust event since 1988. The dust events were observed and interpreted by an ad hoc international web-based virtual community. It would be useful to set up a community-supported web-based infrastructure to monitor the global aerosol pattern for such extreme aerosol events, to alert and to inform the interested communities, and to facilitate collaborative analysis for improved air quality and disaster management.

Vertical distribution of dimethylsulfide, sulfur dioxide, aerosol ions, and radon over the Northeast Pacific Ocean

Dimethylsulfide (DMS), sulfur dioxide (SO2), methanesulfonate (MSA), nonsea-salt sulfate (nss-SO42−), sodium (Na+), ammonium (NH4+), and nitrate (NO3−) were determined in samples collected by aircraft over the open ocean in postfrontal maritime air masses off the northwest coast of the United States (3–12 May 1985). Measurements of radon daughter concentrations and isentropic trajectory calculations suggested that these air masses had been over the Pacific for 4–8 days since leaving the Asian continent. The DMS and MSA profiles showed very similar structures, with typical concentrations of 0.3–1.2 and 0.25–0.31 nmol m−3 (STP) respectively in the mixed layer, decreasing to 0.01–0.12 and 0.03–0.13 nmol m−3 (STP) at 3.6 km. These low atmospheric DMS concentrations are consistent with low levels of DMS measured in the surface waters of the northeastern Pacific during the study period.The atmospheric SO2 concentrations always increased with altitude from <0.16–0.25 to 0.44–1.31 nmol m−3 (STP). The nonsea-salt sulfate (ns-SO42−) concentrations decreased with altitude in the boundary layer and increased again in the free troposphere. These data suggest that, at least under the conditions prevailing during our flights, the production of SO2 and nss-SO42− from DMS oxidation was significant only within the boundary layer and that transport from Asia dominated the sulfur cycle in the free troposphere. The existence of a ‘sea-salt inversion layer’ was reflected in the profiles of those aerosol components, e.g., Na+ and NO3−, which were predominantly present as coarse particles. Our results show that long-range transport at mid-tropospheric levels plays an important role in determining the chemical composition of the atmosphere even in apparently ‘remote’ northern hemispheric regions.

Trace elements in the atmosphere over the North Atlantic

The concentrations of trace elements in aerosol particles from the atmosphere over the North Atlantic Ocean were determined as part of a program designed to characterize the chemical climatology of the region. For these studies, which were part of the Atmosphere- Ocean Chemistry Experiment (AEROCE), 2 years of samples were collected at Tudor Hill, Bermuda (BTT), and at Ragged Point, Barbados (BAT); and 1 year of samples was collected at Mace Head, Ireland (MHT) and at the Izafia Observatory, Tenerife, Canary Islands (IZT). One major component of the aerosol was atmospheric dust, and the ranking for the median mineral dust concentrations as represented by aluminum was BAT > IZT > BTT > MHT. The A1 concentrations at BAT, IZT, and BTT ranged over 4 orders of magnitude, i.e., from 0.001 to 10 gg m -3. At MHT the maximum dust concentrations were about a factor of 10 lower than at the other sites, but the lower end of the range in dust concentrations was similar at all sites. The mineral dust concentrations generally were highest in summer, and the flux of atmospheric dust was dominated by sources in North Africa. The elements showing clear enrichments over the concentrations expected from sea salt or crustal sources were I, Sb, Se, V, and Zn. At Izafia, which is in the free troposphere (elevation -2360 m), the concentrations of Se and I were much lower than at the boundary layer sites; this difference between sites most likely results from the marine emissions of these elements. The impact of pollution sources on trace element concentrations was evident at all sites but varied with season and location. The concentrations of elements originating from pollution sources generally were low at Barbados. Analyses of trace element ratios indicate that there are large-scale differences in the pollution emissions from North America versus those from Europe and Africa. Emissions from pyrometallurgical industries, steel and iron manufacturing, and possibly biomass burning are more evident in the atmospheric samples influenced by transport from Europe and Africa.

Airborne measurements of dimethylsulfide, sulfur dioxide, and aerosol ions over the southern Ocean South of Australia

Vertical distributions of dimethylsulfide (DMS), sulfur dioxide (SO2), aerosol methane-sulfonate (MSA), non-sea-salt sulfate (nss-SO42-), and other aerosol ions were measured in maritime air west of Tasmania (Australia) during December 1986. A few cloudwater and rainwater samples were also collected and analyzed for major anions and cations. DMS concentrations in the mixed layer (ML) were typically between 15–60 ppt (parts per trillion, 10−12; 24 ppt=1 nmol m−3 (20°C, 1013 hPa)) and decreased in the free troposphere (FT) to about <1–2.4 ppt at 3 km. One profile study showed elevated DMS concentrations at cloud level consistent with turbulent transport (‘cloud pumping’) of air below convective cloud cells. In another case, a diel variation of DMS was observed in the ML. Our data suggest that meteorological rather than photochemical processes were responsible for this behavior. Based on model calculations we estimate a DMS lifetime in the ML of 0.9 days and a DMS sea-to-air flux of 2–3 μmol m−2 d−1. These estimates pertain to early austral summer conditions and southern mid-ocean latitudes. Typical MSA concentrations were 11 ppt in the ML and 4.7–6.8 ppt in the FT. Sulfur-dioxide values were almost constant in the ML and the lower FT within a range of 4–22 ppt between individual flight days. A strong increase of the SO2 concentration in the middle FT (5.3 km) was observed. We estimate the residence time of SO2 in the ML to be about 1 day. Aqueous-phase oxidation in clouds is probably the major removal process for SO2. The corresponding removal rate is estimated to be a factor of 3 larger than the rate of homogeneous oxidation of SO2 by OH. Model calculations suggest that roughly two-thirds of DMS in the ML are converted to SO2 and one-third to MSA. On the other hand, MSA/nss-SO42- mole ratios were significantly higher compared to values previously reported for other ocean areas suggesting a relatively higher production of MSA from DMS oxidation over the Southern Ocean. Nss-SO42- profiles were mostly parallel to those of MSA, except when air was advected partially from continental areas (Africa, Australia). In contrast to SO2, nss-SO42- values decreased significantly in the middle FT. NH4+/nss-SO42- mole ratios indicate that most non-sea-salt sulfate particles in the ML were neutralized by ammonium.

Assessment of ozone photochemistry in the western North Pacific as inferred from PEM-West A observations during the fall 1991

This study examines the influence of photochemical processes on ozone distributions in the western North Pacific. The analysis is based on data generated during NASAs western Pacific Exploratory Mission (PEM-West A) during the fall of 1991. Ozone trends were best described in terms of two geographical domains: the western North Pacific rim (WNPR) and the western tropical North Pacific (WTNP). For both geographical regions, ozone photochemical destruction, D(O3), decreased more rapidly with altitude than did photochemical formation, F(O3). Thus the ozone tendency, P(O3), was typically found to be negative for z 6–8 km. For nearly all altitudes and latitudes, observed nonmethane hydrocarbon (NMHC) levels were shown to be of minor importance as ozone precursor species. Air parcel types producing the largest positive values of P(O3) included fresh continental boundary layer (BL) air and high-altitude (z > 7 km) parcels influenced by deep convection/lightning. Significant negative P(O3) values were found when encountering clean marine BL air or relatively clean lower free-tropospheric air. Photochemical destruction and formation fluxes for the Pacific rim region were found to exceed average values cited for marine dry deposition and stratospheric injection in the northern hemisphere by nearly a factor of 6. This region was also found to be in near balance with respect to column-integrated O3 photochemical production and destruction. By contrast, for the tropical regime column-integrated O3 showed photochemical destruction exceeding production by nearly 80%. Both transport of O3 rich midlatitude air into the tropics as well as very high-altitude (10–17 km) photochemical O3 production were proposed as possible additional sources that might explain this estimated deficit. Results from this study further suggest that during the fall time period, deep convection over Asia and Malaysia/Indonesia provided a significant source of high-altitude NOx to the western Pacific. Given that the high-altitude NOx lifetime is estimated at between 3 and 9 days, one would predict that this source added significantly to high altitude photochemical O3 formation over large areas of the western Pacific. When viewed in terms of strong seasonal westerly flow, its influence would potentially span a large part of the Pacific.

Relationships between the dust concentrations over eastern Asia and the remote North Pacific

Dust storm reports as well as chemical data indicate that 1988 and 1989 were high-dust and low-dust years in eastern Asia, respectively. This pattern was not reflected in data from Midway, a remote island in the North Pacific, where the mineral aerosol concentrations during 1989 were as high as or higher than any year for which data are available. Analyses of meteorological data showed that the amount of precipitation, the strength of the westerly winds, and the position of large-scale meteorological features were related to the dust concentrations over eastern Asia. The observed disparity in the interannual trend in dust concentrations over Beijing and Midway was explained by differences in the source regions for the dust. Dust over eastern Asia evidently originates from source regions to the northeast of the Tibetan Plateau, while dust at Midway is ascribed to dust materials transported from the desert lands in western China. The Asian dust flux to the open ocean is influenced not only by conditions in or near the source regions but also by the large-scale atmospheric circulation patterns.

Reactive nitrogen and ozone over the western Pacific: Distribution, partitioning, and sources

Measurements of important reactive nitrogen species (NO, NO2, HNO3, PAN, PPN, NO3−, NOy), C1 to C6 hydrocarbons, O3, chemical tracers (C2Cl4, CO), and meteorological parameters were made in the troposphere (0 to 12 km) over the western Pacific (0°–50°N) during the Pacific Exploratory Mission-West A campaign (September–October 1991). Under clean conditions, mixing ratios of NO, NO2, NOy, and O3 increased with altitude and showed a distinct latitudinal gradient. PAN showed a midtropospheric maximum, while nitric acid mixing ratios were generally highest near the surface. Measured NOy concentrations were significantly greater than the sum of individually measured nitrogen species (mainly NOx, PAN, and HNO3), suggesting that a large fraction of reactive nitrogen present in the atmosphere is made up of hitherto unknown species. This shortfall was larger in the tropics (≈65%) compared to midlatitudes (≈40%) and was minimal in air masses with high HNO3 mixing ratios (>100 ppt). A global three-dimensional photochemical model has been used to compare observations with predictions and to assess the significance of major sources. It is possible that the tropical lightning source is much greater than commonly assumed, and both lightning source and its distribution remain a major area of uncertainty in the budgets of NOy and NOx. A large disagreement between measurement and theory exists in the atmospheric distribution of HNO3. It appears that surface-based anthropogenic emissions provide nearly 65% of the global atmospheric NOy reservoir. Relatively constant NOx/NOy ratios imply that NOy and NOx are in chemical equilibrium and the NOy reservoir may be an important in situ source of atmospheric NOx. Data are interpreted to suggest that only about 20% of the upper tropospheric (7–12 km) NOx is directly attributable to its surface NOx source, and free tropospheric sources are dominant. In situ release of NOx from the NOy reservoir, lightning, direct transport of surface NOx, aircraft emissions, and small stratospheric input collectively maintain the NOx balance in the atmosphere. It is shown that atmospheric ratios of reactive nitrogen and sulfur species, along with trajectory analysis, can be used to pinpoint the source of Asian continental outflow. Compared to rural atmospheres over North America, air masses over the Pacific are highly efficient in net O3 production. Sources of tropospheric NOx cannot yet be accurately defined due to shortcomings in measurements and theory.

Transport climatology of tropospheric ozone: Bermuda, 1988–1991

We determined the major transport patterns for Bermuda and quantified the degree to which they influenced variability in ozone concentrations by applying cluster analysis to isentropic trajectories from September 1988, through September 1991. Concentration distributions of ozone associated with these transport patterns were significantly different. The highest concentrations of ozone in each season were associated with transport off the North American continent; the lowest concentrations were during low-level maritime transport around the Bermuda high. Using the vertical component of the isentropic trajectories, we also showed that the most extreme concentrations of ozone occurred with rapidly descending air from midtropospheric levels. This pattern was most pronounced in April and May when more than 50% of the O3 variability was related to transport differences. We conclude that this relatively remote marine site, which normally experienced low maritime ozone levels (∼30 parts per billion by volume (ppbv)), periodically entrained dry, ozone-rich (∼55 ppbv) midtropospheric air in association with strong subsidence in high pressure behind spring low-pressure systems. Although the ultimate source of these midtroposphere, midlatitude, elevated-ozone concentrations is still being investigated, the synoptic meteorology associated with these transport patterns supports a significant contribution from the upper troposphere and lower stratosphere.

X-ray spectrometry of individual Asian dust-storm particles over the Japanese islands and the North Pacific Ocean

Abstract Individual aerosol particles were collected during spring 1986 near the surface over the Japanese islands (Nagasaki and Nagoya) and the North Pacific Ocean near Hawaii. Asian dust-storm particles found in these samples were examined by use of an electron microscope equipped with an energy-dispersive X-ray analyzer (EDX). These dust-storm particles usually consisted of Mg, Al, Si, Ca, Ti and Fe, together with S and Cl. For the individual particles collected over Japan, changes in morphological features and in the amounts of elements before and after the dialysis (extraction) of water-soluble material were studied. The examination indicated that the dust particles were present as mixed particles (internal mixture of water-soluble and -insoluble material), wheras the the water-soluble material mainly contained Ca and S. Over the North Pacific Ocean, the dust-storm particles were present internally in sea-salt particles. It is suggested that the internal mixture of minerals and sea-salt is probably due to interaction within clouds. Formation of CaSO4 on the dust particles was also suggested on the basis of quantitative results obtained by the use of the EDX.

Chemical characteristics of continental outflow from Asia to the troposphere over the western Pacific Ocean during February-March 1994: Results from PEM-West B

We present here the chemical composition of outflow from the Asian continent to the atmosphere over the western Pacific basin during the Pacific Exploratory Mission-West (PEM-West B) in February–March 1994. Comprehensive measurements of important tropospheric trace gases and aerosol particulate matter were performed from the NASA DC-8 airborne laboratory. Backward 5 day isentropic trajectories were used to partition the outflow from two major source regions: continental north (>20°N) and continental south (<20°N). Air parcels that had not passed over continental areas for the previous 5 days were classified as originating from an aged marine source. The trajectories and the chemistry together indicated that there was extensive rapid outflow of air parcels at altitudes below 5 km, while aged marine air was rarely encountered and only at <20°N latitude. The outflow at low altitudes had enhancements in common industrial solvent vapors such as C2Cl4, CH3CCl3, and C6H6, intermixed with the combustion emission products C2H2, C2H6, CO, and NO. The mixing ratios of all species were up to tenfold greater in outflow from the continental north compared to the continental south source region, with 210Pb concentrations reaching 38 fCi (10−15 curies) per standard cubic meter. In the upper troposphere we again observed significant enhancements in combustion-derived species in the 8–10 km altitude range, but water-soluble trace gases and aerosol species were depleted. These observations suggest that ground level emissions were lofted to the upper troposphere by wet convective systems which stripped water-soluble components from these air parcels. There were good correlations between C2H2 and CO and C2H6 (r2=0.70–0.97) in these air parcels and much weaker ones between C2H2 and H2O2 or CH3OOH (r2 ≈0.50). These correlations were the strongest in the continental north outflow where combustion inputs appeared to be recent (1–2 days old). Ozone and PAN showed general correlation in these same air parcels but not with the combustion products. It thus appears that several source inputs were intermixed in these upper tropospheric air masses, with possible contributions from European or Middle Eastern source regions. In aged marine air mixing ratios of O3 (≈20 parts per billion by volume) and PAN (≤10 parts per trillion by volume) were nearly identical at <2 km and 10–12 km altitudes due to extensive convective uplifting of marine boundary layer air over the equatorial Pacific even in wintertime. Comparison of the Pacific Exploratory Mission-West A and PEM-West B data sets shows significantly larger mixing ratios of SO2 and H2O2 during PEM-West A. Emissions from eruption of Mount Pinatubo are a likely cause for the former, while suppressed photochemical activity in winter was probably responsible for the latter. This comparison also highlighted the twofold enhancement in C2H2, C2H6, and C3H8 in the continental north outflow during PEM-West B. Although this could be due to reduced OH oxidation rates of these species in wintertime, we argue that increased source emissions are primarily responsible.