Alan R. Bandy

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...

Rain in shallow cumulus over the ocean: the RICO Campaign

Shallow, maritime cumuli are ubiquitous over much of the tropical oceans, and characterizing their properties is important to understanding weather and climate. The Rain in Cumulus over the Ocean (RICO) field campaign, which took place during November 2004–January 2005 in the trades over the western Atlantic, emphasized measurements of processes related to the formation of rain in shallow cumuli, and how rain subsequently modifies the structure and ensemble statistics of trade wind clouds. Eight weeks of nearly continuous S-band polarimetric radar sampling, 57 flights from three heavily instrumented research aircraft, and a suite of ground- and ship-based instrumentation provided data on trade wind clouds with unprecedented resolution. Observational strategies employed during RICO capitalized on the advances in remote sensing and other instrumentation to provide insight into processes that span a range of scales and that lie at the heart of questions relating to the cause and effects of rain from shallow ...

Potential impact of iodine on tropospheric levels of ozone and other critical oxidants

A new analysis of tropospheric iodine chemistry suggests that under certain conditions this chemistry could have a significant impact on the rate of destruction of tropospheric ozone. In addition, it suggests that modest shifts could result in the critical radical ratio HO2/OH. This analysis is based on the first ever observations of CH3I in the middle and upper free troposphere as recorded during the NASA Pacific Exploratory Mission in the western Pacific. Improved evaluations of several critical gas kinetic and photochemical rate coefficients have also been used. Three iodine source scenarios were explored in arriving at the above conclusions. These include: (1) the assumption that the release of CH3I from the marine environment was the only iodine source with boundary layer levels reflecting a low-productivity source region, (2) same as scenario 1 but with an additional marine iodine source in the form of higher molecular weight iodocarbons, and (3) source scenario 2 but with the release of all iodocarbons occurring in a region of high biological productivity. Based on one-dimensional model simulations, these three source scenarios resulted in estimated Ix (Ix = I + IO + HI + HOI + 2I2O2 + INOx) yields for the upper troposphere of 0.5, 1.5, and 7 parts per trillion by volume (pptv), respectively. Of these, only at the 1.5 and 7 pptv level were meaningful enhancements in O3 destruction estimated. Total column O3 destruction for these cases averaged 6 and 30%, respectively. At present we believe the 1.5 pptv Ix source scenario to be more typical of the tropical marine environment; however, for specific regions of the Pacific (i.e., marine upwelling regions) and for specific seasons of the year, much higher levels might be experienced. Even so, significant uncertainties still remain in the proposed iodine chemistry. In particular, much uncertainty remains in the magnitude of the marine iodine source. In addition, several rate coefficients for gas phase processes need further investigating, as does the efficiency for removal of iodine due to aerosol scavenging processes.

Export efficiency of black carbon aerosol in continental outflow: Global implications

[1] We use aircraft observations of Asian outflow from the NASA Transport and Chemical Evolution over the Pacific (TRACE-P) mission over the NW Pacific in March-April 2001 to estimate the export efficiency of black carbon (BC) aerosol during lifting to the free troposphere, as limited by scavenging from the wet processes (warm conveyor belts and convection) associated with this lifting. Our estimate is based on the enhancement ratio of BC relative to CO in Asian outflow observed at different altitudes and is normalized to the enhancement ratio observed in boundary layer outflow (0-1 km). We similarly estimate export efficiencies of sulfur oxides (SO x = SO 2 (g) + fine SO 2- 4 ) and total inorganic nitrate (HNO T 3 = HNO 3 (g) + fine NO - 3 ) for comparison to BC. Normalized export efficiencies for BC are 0.63-0.74 at 2-4 km altitude and 0.27-0.38 at 4-6 km. Values at 2-4 km altitude are higher than for SO x (0.48-0.66) and HNO T 3 (0.29-0.62), implying that BC is scavenged in wet updrafts but not as efficiently as sulfate or nitrate. Simulation of the TRACE-P period with a global three-dimensional model (GEOS-CHEM) indicates that a model timescale of 1 ± 1 days for conversion of fresh hydrophobic to hydrophilic BC provides a successful fit to the export efficiencies observed in TRACE-P. The resulting mean atmospheric lifetime of BC is 5.8 ± 1.8 days, the global burden is 0.11 ± 0.03 Tg C, and the decrease in Arctic snow albedo due to BC deposition is 3.1 ± 2.5%.

Ozone and aerosol distributions and air mass characteristics over the South Pacific during the burning season

In situ and laser remote measurements of gases and aerosols were made with airborne instrumentation to establish a baseline chemical signature of the atmosphere above the South Pacific Ocean during the NASA Global Tropospheric Experiment (GTE)/Pacific Exploratory Mission-Tropics A (PEM-Tropics A) conducted in August-October 1996. This paper discusses general characteristics of the air masses encountered during this experiment using an airborne lidar system for measurements of the large-scale variations in ozone (O3) and aerosol distributions across the troposphere, calculated potential vorticity (PV) from the European Centre for Medium-Range Weather Forecasting (ECMWF), and in situ measurements for comprehensive air mass composition. Between 8°S and 52°S, biomass burning plumes containing elevated levels of O3, over 100 ppbv, were frequently encountered by the aircraft at altitudes ranging from 2 to 9 km. Air with elevated O3 was also observed remotely up to the tropopause, and these air masses were observed to have no enhanced aerosol loading. Frequently, these air masses had some enhanced PV associated with them, but not enough to explain the observed O3 levels. A relationship between PV and O3 was developed from cases of clearly defined O3 from stratospheric origin, and this relationship was used to estimate the stratospheric contribution to the air masses containing elevated O3 in the troposphere. The frequency of observation of the different air mass types and their average chemical composition is discussed in this paper.

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.

Atmospheric sulfur cycle simulated in the global model GOCART: Comparison with field observations and regional budgets

We present a detailed evaluation of the atmospheric sulfur cycle simulated in the Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model. The model simulations of SO2, sulfate, dimethylsulfide (DMS), and methanesulfonic acid (MSA) are compared with observations from different regions on various timescales. The model agrees within 30% with the regionally averaged sulfate concentrations measured over North America and Europe but overestimates the SO2 concentrations by more than a factor of 2 there. This suggests that either the emission rates are too high, or an additional loss of SO2 which does not lead to a significant sulfate production is needed. The average wintertime sulfate concentrations over Europe in the model are nearly a factor of 2 lower than measured values, a discrepancy which may be attributed largely to the sea-salt sulfate collected in the data. The model reproduces the sulfur distributions observed over the oceans in both long-term surface measurements and short-term aircraft campaigns. Regional budget analyses show that sulfate production from SO2 oxidation is 2 to 3 times more efficient and the lifetimes of SO2 and sulfate are nearly a factor of 2 longer over the ocean than over the land. This is due to a larger free tropospheric fraction of SO2 column over the ocean than over the land, hence less loss to the surface. The North Atlantic and northwestern Pacific regions are heavily influenced by anthropogenic activities, with more than 60% of the total SO2 originating from anthropogenic sources. The average production efficiency of SO2 from DMS oxidation is estimated at 0.87 to 0.91 in most oceanic regions.

Long‐range transport of SO x and dust in East Asia during the PEM B Experiment

The transport of SO2 and sulfate in East Asia (including eastern China, Korea, and Japan) during the period of March 1 through March 14, 1994, is studied using a three-dimensional regional-scale atmospheric chemistry model. This period corresponds to that in which the Pacific Exploratory Mission in the Western Pacific Ocean (PEM-West B) was being conducted around Japan. During this period, characterized by the passage of cold fronts and relatively dry conditions, the anthropogenic sulfur emitted from the source regions in East Asia is transported out into the central Pacific Ocean. The sulfur transport is largely limited to the lower 4 km of the atmosphere, with the maximum flux occurring in the 30° to 40°N latitude band containing the bulk of the anthropogenic emissions. The interactions between the sulfur cycle and mineral aerosol are also included in the analysis. It is found that the chemical conversion of SO2 to sulfate in the presence of mineral aerosol may be a significant process during this time period, and may contribute from 20% to 40% of the total sulfate production. Sulfur dioxide arising from volcanic sources in Japan is also discussed.

Atmospheric SO/sub 2/ measurements on Project GAMETAG

On the 1978 Global Atmospheric Measurement Experiment of Tropospheric Aerosols and Gases (GAMETAG) flights, 201 measurements of the tropospheric concentration of SO/sub 2/ were made over a latitude range 57 degrees S to 70 degrees N. The area sampled included the central and the southern Pacific Ocean and the western section of the United States and Canada. Sulfur dioxide levels averaged 89 +- 69 pptv in the boundary layer and 122 +- 85 pptv in the free troposphere in the northern hemisphere. In the southern hemisphere, SO/sub 2/ concentrations averaged 57 +- 18 pptv in the boundary layer and 90 +- 21 pptv in the free troposphere. The mean concentration of the continental data was 112 +- 79 pptv in the boundary layer and 160 +- 100 pptv in the free troposphere. The SO/sub 2/ marine values were 54 +- 19 pptv in the boundary layer and 85 +- 28 pptv in the free troposphere. From a simple chemical model we conclude that a significant amount of background SO/sub 2/ may originate from the oxidation of OCS.

Dimethyl sulfide oxidation in the equatorial Pacific: Comparison of model simulations with field observations for DMS, SO2, H2SO4(g), MSA(g), MS and NSS

Reported here are results from an airborne photochemical/sulfur field study in the equatorial Pacific. This study was part of NASAs Global Tropospheric Experiment (GTE) Pacific Exploratory Mission (PEM) Tropics A program. The focus of this paper is on data gathered during an airborne mission (P-3B flight 7) near the Pacific site of Christmas Island. Using a Lagrangian-type sampling configuration, this sortie was initiated under pre-sunrise conditions and terminated in early afternoon with both boundary layer (BL) as well as buffer layer (BuL) sampling being completed. Chemical species sampled included the gas phase sulfur species dimethyl sulfide (DMS), sulfur dioxide (SO2), methane sulfonic acid (MSA)g, and sulfuric acid (H2SO4)g. Bulk aerosol samples were collected and analyzed for methane sulfonate (MS), non-sea-salt sulfate (NSS), Na+,Cl−, and NH4+. Critical non-sulfur parameters included real-time sampling of the hydroxyl radical (OH) and particle size/number distributions. These data showed pre-sunrise minima in the mixing ratios for OH, SO2, and H2SO4 and post-sunrise maxima in the levels of DMS, OH, and H2SO4. Thus, unlike several previous studies involving coincidence DMS and SO2 measurements, the Christmas Island data revealed that DMS and SO2 were strongly anticorrelated. Our “best estimate” of the overall efficiency for the conversion of DMS to SO2 is 72±22%. These results clearly demonstrate that DMS was the dominant source of SO2 in the marine BL. Using as model input measured values for SO2 and OH, the level of agreement between observed and simulated BL H2SO4(g) profiles was shown to be excellent. This finding, together with supporting correlation analyses, suggests that the dominant sulfur precursor for formation of H2SO4 is SO2 rather than the more speculative sulfur species, SO3. Optimization of the fit between the calculated and observed H2SO4 values was achieved using a H2SO4 first-order loss rate of 1.3 × 10−3 s−1. On the basis of an estimated total “wet” aerosol surface area of 75 µm2/cm3, a H2SO4 sticking coefficient of 0.6 was evaluated at a relative humidity of ≃95%, in excellent agreement with recent laboratory measurements. The Christmas Island data suggest that over half of the photochemically generated SO2 forms NSS, but that both BL NSS and MS levels are predominantly controlled by heterogeneous processes involving aerosols. In the case of MS, the precursors species most likely responsible are the unmeasured oxidation products dimethyl sulfoxide (DMSO) and methane sulfinic acid (MSIA). Gas phase production of MSA was shown to account for only 1% of the observed MS; whereas gas phase produced H2SO4 accounted for ∼20% of the NSS. These results are of particular significance in that BL-measured values of the ratio MS/NSS have often been used to estimate the fraction of NSS derived from biogenic DMS and to infer the temperature environment where DMS oxidation occurred. If our conclusions are correct and both products are predominantly formed from complex and still poorly characterized heterogeneous processes, it would suggest that for some environmental settings a simple interpretation of this ratio might be subject to considerable error.