Henry E. Fuelberg

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.

Pacific Exploratory Mission in the tropical Pacific: PEM‐Tropics A, August‐September 1996

The NASA Pacific Exploratory Mission to the Pacific tropics (PEM-Tropics) is the third major field campaign of NASAs Global Tropospheric Experiment (GTE) to study the impact of human and natural processes on the chemistry of the troposphere over the Pacific basin. The first two campaigns, PEM-West A and B were conducted over the northwestern regions of the Pacific and focused on the impact of emissions from the Asian continent. The broad objectives of PEM-Tropics included improving our understanding of the oxidizing power of the tropical atmosphere as well as investigating oceanic sulfur compounds and their conversion to aerosols. Phase A of the PEM-Tropics program, conducted between August-September 1996, involved the NASA DC-8 and P-3B aircraft. Phase B of this program is scheduled for March/April 1999. During PEM-Tropics A, the flight tracks of the two aircraft extended zonally across the entire Pacific Basin and meridionally from Hawaii to south of New Zealand. Both aircraft were instrumented for airborne measurements of trace gases and aerosols and meteorological parameters. The DC-8, given its long-range and high-altitude capabilities coupled with the lidar instrument in its payload, focused on transport issues and ozone photochemistry, while the P-3B, with its sulfur-oriented instrument payload and more limited range, focused on detailed sulfur process studies. Among its accomplishments, the PEM-Tropics A field campaign has provided a unique set of atmospheric measurements in a heretofore data sparse region; demonstrated the capability of several new or improved instruments for measuring OH, H2SO4, NO, NO2, and actinic fluxes; and conducted experiments which tested our understanding of HOx and NOx photochemistry, as well as sulfur oxidation and aerosol formation processes. In addition, PEM-Tropics A documented for the first time the considerable and widespread influence of biomass burning pollution over the South Pacific, and identified the South Pacific Convergence Zone as a major barrier for atmospheric transport in the southern hemisphere.

Eastern Asian emissions of anthropogenic halocarbons deduced from aircraft concentration data

Asian carbon tetrachloride (CCl4) source of 21.5 Gg yr � 1 , several-fold larger than previous estimates and amounting to ’30% of the global budget for this gas. Our emission estimate for CFC-11 from eastern Asia is 50% higher than inventories derived from manufacturing records. Our emission estimates for methyl chloroform (CH3CCl3) and CFC-12 are in agreement with existing inventories. For halon 1211 we find only a strong local source originating from the Shanghai area. Our emission estimates for the above gases result in a ’40% increase in the ozone depletion potential (ODP) of Asian emissions relative to previous estimates, corresponding to a ’10% global increase in ODP. INDEX TERMS: 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; KEYWORDS: anthropogenic, halocarbon emissions, troposphere, TRACE-P

Ozone, hydroperoxides, oxides of nitrogen, and hydrocarbon budgets in the marine boundary layer over the South Atlantic

Author(s): Heikes, B; Lee, M; Jacob, D; Talbot, R; Bradshaw, J; Singh, H; Blake, D; Anderson, B; Fuelberg, H; Thompson, AM | Abstract: The NASA GTE TRACE A mission sampled air over the South Atlantic and western Indian Oceans. Thirteen flight legs were flown within the marine boundary layer (MBL). The MBL was typically the cleanest air sampled (e.g., CH4 l 1680 ppb, CO l 70 ppb, C2H6 l 400 ppt, C3H8 l 40 ppt, NOx l 15 ppt, and midday NO l 5 ppt) but was overlain by polluted air. The photochemistry of the MBL was influenced by oceanic emissions, surface deposition, and entrainment of pollutants from aloft. Chemical budgets were constructed for several species in the MBL in order to investigate these effects and are presented for ethane, ethylene, propane, propylene, n-butane, formic acid (HFo), methylhydroperoxide (CH3OOH), oxides of nitrogen (i.e., NO, NO2, PAN, HNO3), hydrogen peroxide (H2O2), and ozone (O3). A photochemical point model was used to evaluate local chemical production and loss. An entrainment model was used to assess material exchange between the lower free troposphere (FT) and the MBL and a resistance deposition model was used to evaluate material exchange across the air-sea interface. The results suggested the ocean to be the source of measured alkenes in the MBL and to be the most likely source of the shorter-lived alkanes: propane and n-butane. Ethane was the only hydrocarbon for which input from aloft may have exceeded its photochemical destruction. The estimated hydrocarbon sources from the ocean were in agreement with prior analyses. Transport from the lower FT together with surface loss could not account for measured concentrations of CH2O, HFo, and HNO3. The transport of peroxyacetylnitrate (PAN) from the FT to the MBL exceeded the rate of HNO3 production and was more than sufficient to maintain observed NOx levels without having to invoke an oceanic source for NO. The flux of NOx, PAN, and HNO3 was in balance with the surface deposition flux of HNO3. However, the predicted rates of HNO3 formation from the oxidation of NO2 and HNO3 entrainment from aloft were inadequate to maintain observed levels of HNO3 unless HNO3 was partitioned between the gas phase and a more slowly depositing aerosol phase. The estimated dry deposition flux of HNO3 to the South Atlantic during TRACE A, 2-4 × 109 molecules cm-2 s-1, was about 10 times the annual average estimate for this region. The destruction of O3 within the MBL was found to be exceeded by transport into the MBL from aloft, 6 ±2 × 1010 compared to 11 ± 10 × 1010 molecules cm-2 s-1. The principal O3 destruction process was mediated by the formation and surface deposition of H2O2 and CH3OOH, 4 ± 4 × 1010 and 1.1 ± 0.5 × 1010 molecules cm-2 s-1. The direct loss of O3 to the sea surface was estimated to be 1.7 ± 0.2 × 1010 molecules cm-2 s-1. CH3OOH was lost to the sea and transported into the FT from the MBL. Its first-order loss rate was estimated to be 7 × 10-6 s-1 for a mean MBL height of 700 m. H2O2 and CH2O losses from the MBL were estimated at rates of 1.3 × 10-5 s-1 for both species. The inclusion of surface deposition improved the agreement between predicted and measured concentrations of HNO3, CH3OOH, H2O2, and CH2O. However, model CH2O remained significantly greater than that measured in the MBL.

Dust and pollution transport on global scales: Aerosol measurements and model predictions

Vertical profiles of aerosol and gas phase species were measured on flights near Hawaii on April 9 and 10, 1999, during NASAs Pacific Exploratory Mission (PEM) Tropics B program. These measurements characterized aerosol microphysics, inferred chemistry, optical properties, and gases in several extensive dust and pollution plumes, also detected by satellites, which had 10,000-km trajectories back to sources in Asia. Size-resolved measurements indicative of aerosol sulfate, black carbon, dust, light scattering, and absorption allowed determination of their concentrations and contributions to column aerosol optical depth. A new Chemical Transport Model (CTM) that includes aerosol, meteorological fields, dynamics, gas and particle source emissions, a chemistry component (MATCH), and assimilated satellite data was used to predict aerosol and gas concentrations and the aerosol optical effects along our flight path. Flight measurements confirmed the “river-like” plume structures predicted by the CTM and showed close agreement with the predicted contributions of dust and sulfate to aerosol concentrations and optical properties for this global-scale transport path. Consistency between satellite, model and in situ assessment of aerosol optical depth was found, with noted exceptions, within ∼25%. Both observations and model results confirmed that this aerosol was being entrained into the marine boundary layer between Hawaii and California where it can be expected to modify the type and concentration of cloud condensation nuclei in ways that may alter properties of low-level clouds. These observations document the significance and complexity of long-range aerosol transport and highlight the potential of emerging CTM models to extend observational data and address related issues on global scales.

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.

Pacific Exploratory Mission in the Tropical Pacific: PEM-Tropics B, March-April 1999

The Pacific Exploratory Mission - Tropics B (PEM-Tropics B) was conducted by the NASA Global Tropospheric Experiment (GTE) over the tropical Pacific Ocean in March-April 1999. It used the NASA DC-8 and P-3B aircraft equipped with extensive instrumentation for measuring numerous chemical compounds and gases. Its central objective was to improve knowledge of the factors controlling ozone, OH, aerosols, and related species over the tropical Pacific. Geographical coverage ranged from 38°N to 36°S and 148°W to 76°E. Major deployment sites included Hilo, Hawaii, Christmas Island, Tahiti, Fiji, and Easter Island. PEM-Tropics B was a sequel to PEM-Tropics A, which was conducted in September-October 1996 and encountered considerable biomass burning. PEM-Tropics B, conducted in the wet season of the southern tropics, observed an exceedingly clean atmosphere over the South Pacific but a variety of pollution influences over the tropical North Pacific. Photochemical ozone loss over both the North and the South Pacific exceeded local photochemical production by about a factor of 2, implying a major deficit in the tropospheric ozone budget. Dedicated flights investigated the sharp air mass transitions at the Intertropical Convergence Zone (ITCZ) and the South Pacific Convergence Zone (SPCZ). Extensive OH observations permitted the first large-scale comparisons with photochemical model predictions. High concentrations of oxygenated organics were observed ubiquitously in the tropical Pacific atmosphere and may have important implications for global HOx and NOx budgets. Extensive equatorial measurements of dimethyl sulfide and OH suggest that important aspects of marine sulfur chemistry are still poorly understood.

The Meteorological Environment of the Tropospheric Ozone Maximum Over the Tropical South Atlantic Ocean

Atmospheric flow patterns are examined over the South Atlantic Ocean where a maximum of tropospheric ozone has been observed just west of southern Africa. We investigate the flow climatology during October and perform a case study for 6 days during October 1989. Analyses from the European Center for Medium-Range Weather Forecasting are employed, and a high-resolution global spectral model is used to prepare forecasts during the period. Horizontal and vertical motions are examined and used to prepare three-dimensional backward trajectories from the region of greatest ozone. An initially zonally symmetric distribution of ozone is treated as a passive tracer and advected by three-dimensional flows forecast by the global model. Results from the passive tracer simulation indicate that three-dimensional advection alone can produce a maximum of tropospheric ozone in the observed location. In addition, the trajectories suggest that by-products of biomass burning could be transported to the area of maximum ozone. Low-level flow from commonly observed regions of burning in Africa streams westward to the area of interest. Over Brazil, if the burning by-products are carried into the upper troposphere by convective process, they then could be transported eastward to the ozone feature in approximately 5 days. There is considerable subsidence over the tropical southern Atlantic, such that stratospheric influences also are a factor in producing the ozone maximum. Both planetary-scale and transient synoptic-scale circulation features play major roles in the various transport processes that influence the region. In summary, the observed tropospheric ozone maximum appears to be caused by a complex set of horizontal and vertical advections, transport from regions of biomass burning, and stratospheric influences.

Summertime influence of Asian pollution in the free troposphere over North America

We analyze aircraft observations obtained during INTEX-A (1 July to 14 August 2004) to examine the summertime influence of Asian pollution in the free troposphere over North America. By applying correlation analysis and principal component analysis (PCA) to the observations between 6 and 12 km, we find dominant influences from recent convection and lightning (13% of observations), Asia (7%), the lower stratosphere (7%), and boreal forest fires (2%), with the remaining 71% assigned to background. Asian air masses are marked by high levels of CO, O_3, HCN, PAN, C_2H_2, C_6H_6, methanol, and SO_4^(2–). The partitioning of NO_y species in the Asian plumes is dominated by PAN (∼600 pptv), with varying NO_x/HNO_3 ratios in individual plumes, consistent with individual transit times of 3–9 days. Export of Asian pollution occurred in warm conveyor belts of midlatitude cyclones, deep convection, and in typhoons. Compared to Asian outflow measurements during spring, INTEX-A observations display lower levels of anthropogenic pollutants (CO, C_3H_8, C_2H_6, C_6H_6) due to shorter summer lifetimes; higher levels of biogenic tracers (methanol and acetone) because of a more active biosphere; and higher levels of PAN, NO_x, HNO_3, and O_3 reflecting active photochemistry, possibly enhanced by efficient NO_y export and lightning. The high ΔO_3/ΔCO ratio (0.76 mol/mol) in Asian plumes during INTEX-A is due to strong photochemical production and, in some cases, mixing with stratospheric air along isentropic surfaces. The GEOS-Chem global model captures the timing and location of the Asian plumes. However, it significantly underestimates the magnitude of observed enhancements in CO, O_3, PAN and NO_x.

TRACE A trajectory intercomparison: 2. Isentropic and kinematic methods

Kinematic and isentropic trajectories are compared quantitatively during a single 5-day period (October 13-18, 1992) when several flights for the Transport and Atmospheric Chemistry Near the Equator--Atlantic (TRACE A) experiment were conducted off the west coast of Africa. European Centre for Medium-Range Weather Forecasts (ECMWF) data are used to compute the 5-day backward trajectories arriving at locations over the South Atlantic Ocean and nearby parts of South America and southern Africa. Two versions of kinematic trajectories are examined. One version employs vertical motions supplied with the ECMWF data. These trajectories often differ greatly from those based on the isentropic assumption. The kinematic trajectories usually undergo considerably greater vertical displacements than their isentropic counterparts; however, most diabatic rates are consistent with those of synoptic-scale systems. Ratios of acetylene to carbon monoxide are related to backward trajectories at various locations along a TRACE A flight. A second version of kinematic trajectories employs vertical motions diagnosed from ECMWF horizontal wind components using the continuity equation. These vertical motions are stronger than those supplied with the ECMWF data, causing many of the trajectories to have larger vertical displacements and considerably different paths than the original kinematic versions. Many of these kinematic trajectories undergo diabatic rates that exceed generally accepted values on the synoptic scale. This occurs, in part, because the diagnosed vertical motions are inconsistent with the ECMWF data. The research indicates that the kinematic procedure yields realistic 5-day backward trajectories when the three-dimensional wind data are available from a numerical model or other dynamically consistent data set such as provided by ECMWF.