B. W. Blomquist

Dynamics and chemistry of marine stratocumulus - DYCOMS II

The second Dynamics and Chemistry of Marine Stratocumulus (DYCOMS-II) field study is described. The field program consisted of nine flights in marine stratocumulus west-southwest of San Diego, California. The objective of the program was to better understand the physics a n d dynamics of marine stratocumulus. Toward this end special flight strategies, including predominantly nocturnal flights, were employed to optimize estimates of entrainment velocities at cloud-top, large-scale divergence within the boundary layer, drizzle processes in the cloud, cloud microstructure, and aerosol–cloud interactions. Cloud conditions during DYCOMS-II were excellent with almost every flight having uniformly overcast clouds topping a well-mixed boundary layer. Although the emphasis of the manuscript is on the goals and methodologies of DYCOMS-II, some preliminary findings are also presented—the most significant being that the cloud layers appear to entrain less and drizzle more than previous theoretical work led investigat...

The cycling of sulfur in surface seawater of the northeast Pacific

Oceanic dimethylsulfide (DMS) emissions to the atmosphere are potentially important to the Earths radiative balance. Since these emissions are driven by the surface seawater concentration of DMS, it is important to understand the processes controlling the cycling of sulfur in surface seawater. During the third Pacific Sulfur/Stratus Investigation (PSI-3, April 1991) we measured the major sulfur reservoirs (total organic sulfur, total low molecular weight organic sulfur, ester sulfate, protein sulfur, dimethylsulfoniopropionate (DMSP), DMS, dimethylsulfoxide) and quantified many of the processes that cycle sulfur through the upper water column (sulfate assimilation, DMSP consumption, DMS production and consumption, air-sea exchange of DMS, loss of organic sulfur by particulate sinking). Under conditions of low plankton biomass ( 8 μM nitrate), 250 km off the Washington State coast, DMSP and DMS were 22% and 0.9%, respectively, of the total particulate organic sulfur pool. DMS production from the enzymatic cleavage of DMSP accounted for 29% of the total sulfate assimilation. However, only 0.3% of sulfate-S assimilated was released to the atmosphere. From these data it is evident that air-sea exchange is currently only a minor sink in the seawater sulfur cycle and thus there is the potential for much higher DMS emissions under different climatic conditions.

Observations of Entrainment in Eastern Pacific Marine Stratocumulus Using Three Conserved Scalars

Abstract Fast measurements of three scalars, ozone, dimethyl sulfide (DMS), and total water, are used to investigate the entrainment process in the stratocumulus-topped boundary layer (STBL) observed over the eastern subtropical Pacific during the second Dynamics and Chemistry of Marine Stratocumulus Experiment (DYCOMS-II). Direct measurement of the flux profiles by eddy covariance is used to estimate the entrainment velocity, the average rate at which the boundary layer grows diabatically via incorporation of overlying free tropospheric air. The entrainment velocities observed over the course of the mission, which took place during July 2001, ranged from 0.12 to 0.72 cm s−1, and appear to outpace the estimated large-scale subsidence as the boundary layer advects over warmer sea surface temperatures. Observed entrainment velocities display only a weak correlation with the buoyancy Richardson number defined at the inversion, which suggests that processes other than inversion strength, such as wind shear, m...

An intercomparison of lidar-derived aerosol optical properties with airborne measurements near Tokyo during ACE-Asia

[1] During the ACE-Asia intensive observation period (IOP), an intercomparison experiment with ground-based lidars and aircraft observations was conducted near Tokyo. On 23 April 2001, four Mie backscatter lidars were simultaneously operated in the Tokyo region, while the National Center for Atmospheric Research C-130 aircraft flew a steppedascent profile between the surface and 6 km over Sagami Bay southwest of Tokyo. The C-130 observation package included a tracking Sun photometer and in situ packages measuring aerosol optical properties, aerosol size distribution, aerosol ionic composition, and SO2 concentration. The three polarization lidars suggested that the observed modest concentrations of Asian dust in the free troposphere extended up to an altitude of 8 km. We found a good agreement in the backscattering coefficient at 532 nm among lidars and in situ 180� backscatter nephelometer observations. The intercomparison indicated that the aerosol layer between 1.6 and 3.5 km was a remarkably stable and homogenous in mesoscale. We also found reasonable agreement between the aerosol extinction coefficients (sa � 0.03 km � 1 ) derived from the airborne tracking Sun photometer, in situ optical instruments, and those estimated from the lidars above the planetary boundary layer (PBL). We also found considerable vertical variation of the aerosol depolarization ratio (da) and a negative correlation between da and the backscattering coefficient (da) below 3.5 km. Airborne measurements of size-dependent optical parameters (e.g., the fine mode fraction of scattering) and of aerosol ionic compositions suggests that the mixing ratio of the accumulation-mode and coarse-mode (dust) aerosols was primarily responsible for the observed variation of da. Aerosol observations during the intercomparison period captured the following three types of layers in the atmosphere: a PBL (surface to 1.2–1.5 km) where fine (mainly sulfate) particles with a low da (<10%) dominated; an intermediate layer (between the top of the PBL and 3.5 km) where fine particles and dust particles were moderately externally mixed, giving moderate da; and an upper layer (above � 3.5 km) where dust dominated, giving a high da (30%). A substantial dust layer between 4.5 and 6.5 km was observed just west of Japan by the airborne instruments and found to have a lidar ratio of 50.4 ± 9.4 sr. This agrees well with nighttime Raman lidar measurements made later on this same dust layer as it passed over Tokyo, which found a lidar ratio of 46.5 ± 10.5 sr. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles

Low yields of SO2 from dimethyl sulfide oxidation in the marine boundary layer

Sulfur dioxide and dimethyl sulfide were determined in the marine boundary layer of the northeast Pacific Ocean west of Seattle, Washington. The mean DMS and SO2 concentrations were 75 and 28 pptv, respectively. During periods of high DMS levels (180 pptv) we found that SO2 levels remained low (25 pptv) and statistically the same as periods of low DMS. Sulfur dioxide showed no observable diurnal variation indicating that nonphotochemically driven losses to aerosol and other surfaces cannot explain the low SO2 levels observed. We conclude that a low efficiency of conversion of DMS to SO2 is the most likely explanation for the low SO2 levels. Implications of the low yield of SO2 in terms of the kinetics of oxidation of DMS are discussed.

The MATERHORN: Unraveling the Intricacies of Mountain Weather

AbstractEmerging application areas such as air pollution in megacities, wind energy, urban security, and operation of unmanned aerial vehicles have intensified scientific and societal interest in mountain meteorology. To address scientific needs and help improve the prediction of mountain weather, the U.S. Department of Defense has funded a research effort—the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program—that draws the expertise of a multidisciplinary, multi-institutional, and multinational group of researchers. The program has four principal thrusts, encompassing modeling, experimental, technology, and parameterization components, directed at diagnosing model deficiencies and critical knowledge gaps, conducting experimental studies, and developing tools for model improvements. The access to the Granite Mountain Atmospheric Sciences Testbed of the U.S. Army Dugway Proving Ground, as well as to a suite of conventional and novel high-end airborne and surface measurement platfor...

Sulfur dioxide distribution over the Pacific Ocean 1991–1996

In this study we combined the sulfur dioxide (SO2) data from the NASA Pacific Exploratory Missions (PEM) and the First Aerosol Characterization Experiment (ACE 1) to create a data set containing 4679 observations of SO2 in the troposphere of the Pacific Ocean during the period 1991–1996. These data have exceptionally high precision due to the use of isotopically labeled SO2 as an internal standard in each sample. The lower limit of detection was less than 2 pptv. The spatial extent of the data ranged from 60°N to 72°S, 110°E to 80°W, and from 50 m to 12 km above the ocean surface. A significant zonal gradient was observed between the northern and southern hemispheres. The western North Pacific was particularly well characterized during the NASA PEM-West A and B missions that focused on that region. Our data show that anthropogenic sources in eastern Asia dominated the sulfur chemistry in the lower troposphere of the western North Pacific eastward from the Asian continent for more than 1500 km and substantially farther in the mid and upper troposphere. The impact of Asian sources far from the continent was due primarily to transported SO2 with a substantially smaller impact from transported sulfate. Dimethyl sulfide was a significant source of SO2 only in the tropical boundary layer. In the southern hemisphere, anthropogenic sources had much less impact with very little SO2 detected in biomass burning plumes. Sulfur dioxide in the middle and upper troposphere of both hemispheres was strongly influenced by volcanic sources. Sulfur dioxide from the eruption of Mount Pinatubo dominated the SO2 distribution in the upper troposphere in the northern hemisphere in the second half of 1991. A significant fraction of the SO2 in the upper free troposphere in the northern hemisphere was attributed to SO2 transported from the stratosphere to the upper troposphere. Evidence for the transport of SO2 from the stratosphere to troposphere existed as far south as 30°N, but it was most intense at high latitudes. In the absence of major volcanic activity, such as the cataclysmic eruption of Mount Pinatubo, volcanic sources in East Asia contribute significant amounts of SO2 in the mid and upper troposphere of the northern hemisphere. In the southern hemisphere where anthropogenic sources are much weaker, volcanoes may contribute most of the SO2 found in the mid and upper troposphere. Deep convection by tropical and extratropical storms appeared to be a significant process contributing to long-range transport of volcanic SO2 for the southern hemisphere.

The ASTEX/MAGE Experiment

The Atlantic Stratocumulus Transition Experiment/Marine Aerosol and Gas Exchange (ASTEX/MAGE) was a multinational experiment designed to study the relationship between marine chemistry, aerosols, clouds, and air/sea exchange. Several aircraft, ships, and island sites supported measurements of marine boundary layer chemistry in both pristine and polluted North Atlantic air masses in the vicinity of the Azores in June of 1992. The International Global Atmospheric Chemistry (IGAC) Programs MAGE activity organized the chemical experiments in ASTEX because such experiments are beyond the capabilities of any one platform, discipline, or nation working alone. One highlight of ASTEX/MAGE was the development of a Lagrangian experimental strategy. It offered a unique way of constraining fluxes and reaction rates by observing the effect those processes have on a tagged parcel of air.

Sulfur dioxide as a source of condensation nuclei in the upper troposphere of the Pacific Ocean

The NASA Pacific Exploratory Mission-West A (PEM-West A) experiment in 1991 covered large portions of the northwestern Pacific Ocean troposphere and a transect of the subtropical troposphere of the North Pacific Ocean. Sulfur dioxide consistently increased with altitude from the surface to 11 km except for a few cases of anthropogenic inputs confined to the planetary boundary layer. Using the PEM West A data set for SO2, sulfate, and calculated OH over the central Pacific Ocean, we have estimated the impacts of the SO2 on the formation of condensation nuclei and the lifetimes of SO2 and sulfate for this region.

A study of DMS oxidation in the tropics : Comparison of Christmas Island field observations of DMS, SO2, and DMSO with model simulations

This study reports comparisonsbetween model simulations, based on current sulfurmechanisms, with the DMS, SO2 and DMSOobservational data reported by Bandy et al.(1996) in their 1994 Christmas Island field study. For both DMS and SO2, the model results werefound to be in excellent agreement with theobservations when the observations were filtered so asto establish a common meteorological environment. Thisfiltered DMS and SO2 data encompassedapproximately half of the total sampled days. Basedon these composite profiles, it was shown thatoxidation of DMS via OH was the dominant pathway withno more than 5 to 15% proceeding through Cl atoms andless than 3% through NO3. This analysis wasbased on an estimated DMS sea-to-air flux of 3.4 ×109 molecs cm-2 s-1. The dominant sourceof BL SO2 was oxidation of DMS, the overallconversion efficiency being evaluated at 0.65 ± 0.15. The major loss of SO2 was deposition to theoceans surface and scavenging by aerosol. Theresulting combined first order k value was estimated at 1.6 × 10-5 s-1. In contrast to the DMSand SO2 simulations, the model under-predictedthe observed DMSO levels by nearly a factor of 50. Although DMSO instrument measurement problems can notbe totally ruled out, the possibility of DMSO sourcesother than gas phase oxidation of DMS must beseriously considered and should be explored in futurestudies.