Engineering topics

Bradshaw, J. D.

Publications and source records attributed to Bradshaw, J. D..

At least 19 records

NASA NTRS1997

Large-Scale Distributions of Tropospheric Nitric, Formic, and Acetic acids Over the Westerm Pacific Basin During Wintertime

We report here measurements of the acidic gases nitric (HNO3), formic (HCOOH), and acetic (CH3COOH) over the western Pacific basin during the February-March 1994 Pacific Exploratory Mission-West (PEM-West B). These data were obtained aboard the NASA DC-8 research aircraft as it flew missions in the altitude range of 0.3 - 12.5 km over equatorial regions near Guam and then further westward encompassing the entire Pacific Rim arc. Aged marine air over the equatorial Pacific generally exhibited mixing ratios of acidic gases less than 100 parts per trillion by volume (pptv). Near the Asian continent, discrete plumes encountered below 6 km altitude contained up to 8 parts per billion by volume (ppbv) HNO3 and 10 ppbv HCOOH and CH3COOH. Overall there was a general correlation between mixing ratios of acidic gases with those of CO, C2H2, and C2Cl4, indicative of emissions from combustion and industrial sources. The latitudinal distributions of HNO3 and CO showed that the largest mixing ratios were centered around 15 deg N, while HCOOH, CH3COOH, and C2Cl4 peaked at 25 deg N. The mixing ratios of HCOOH and CH3COOH were highly correlated (r(sup 2) = 0.87) below 6 km altitude, with a slope (0.89) characteristic of the nongrowing season at midlatitudes in the northern hemisphere. Above 6 km altitude, HCOOH and CH3COOH were marginally correlated (r(sup 2) = 0.50), and plumes well defined by CO, C2H2, and C2Cl4 were depleted in acidic gases, most likely due to scavenging during vertical transport of air masses through convective cloud systems over the Asian continent. In stratospheric air masses, HNO, mixing ratios were several parts per billion by volume (ppbv), yielding relationships with 03 and N2O consistent with those previously reported for NO(y).

Talbot, R. W.↗

NASA NTRS1997

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 (greater than 20 deg N) and continental south (less than 20 deg 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 less than 20 deg 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 Pb-210 concentrations reaching 38 fCi (10(exp -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 (r(sup 2) = 0.70 - 0.97) in these air parcels and much weaker ones between C2H2 and H2O2 or CH3OOH (r(sup 2) = 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 03 (approximately equals 20 parts per billion by volume) and PAN (less than or equal to 10 parts per trillion by volume) were nearly identical at less than 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.

Talbot, R. W.↗

NASA NTRS1997

Distributions of Beryllium 7 and Lead 210, and Soluble Aerosol-Associated Ionic Species Over the Western Pacific: PEM West B, February - March 1994

Aerosol sampling for the determination of the concentrations of soluble ionic species and the natural radionuclides Be-7 and Pb-210 was conducted from the NASA DC-8 over the western Pacific as part of GTE/PEM-West B during February - March 1994. Concentrations of most soluble ionic species in the free troposphere were higher in samples collected on flights originating from Hong Kong and Japan than those collected further east over the open ocean. In both regions the measured concentrations were higher than those found during PEM-West A (fall 1991). Activities of Pb-210, a tracer of air masses influenced by sources on the Asian continent, showed the same patterns. These data indicate the effect of stronger continental outflow from Asia over the western Pacific during the spring compared to fall season. For readily scavenged aerosol-associated species and soluble acidic gases the strongest indications of Asian outflow were restricted to altitudes below 6 km. The distribution of the continental tracer Pb-210 was also compared to those of a large number of gas phase species measured on the DC-8. Relatively strong correlations were found with O3, and peroxyacetylnitrate (PAN), but only during the flights over the remote Pacific. During PEM-West A, similar correlations were seen, but they were stronger near Asia. We believe that correlations are a signature of continental air that has been processed by deep wet convection over land before being advected over the ocean. One flight over the Sea of Japan provided the opportunity to sample upper troposphere/lower stratosphere air in and around a tropopause fold. Concentrations of Be-7 reached 7 pCi/cu m STP, and peak O3, mixing ratios of 480 ppb were encountered at 10.7 km. The Be-7 data are used to estimate the fraction of stratospheric air mixed down into the troposphere by circulation in the fold.

Dibb, J. E.↗

NASA NTRS1997

Upper Tropospheric/Lower Stratospheric Distribution of Reactive Nitrogen and Ozone

In-situ measurements of a large number of trace chemicals from upper troposphere/lower stratosphere (UT/LS) were performed with the NASA DC-8 aircraft during February/March 1994 over the Pacific Ocean (10 S to 60 N). Mixing ratios in the UT were relatively low in the warm tropical and subtropical air south of the polar jetstream (approx. =28 N) but increased sharply with latitude in the cold polar air north of the jetstream. At about 45 N, high concentrations of PAN (300 ppt) coexisted with extremely low (approx. = 20 ppt) concentrations of NOx. Elevated NOx levels in the UT did not always correspond to continental outflow conditions. Deepest penetrations into the stratosphere (550 ppb O3, 279 ppb NOx, and 350 K potential temperature) corresponded to a region that has been defined as the 'lowermost stratosphere' (LS) by Holton et al. Analysis of data shows that the mixing ratios of long-lived tracer species (e.g., CH4, HNO3, NOy, CFCs, HCFCs) are linearly correlated with those of O3 and N2O. A delta-NOY/delta-O3 of 0.0054 ppb/ppb and delta-NOy/delta-N2O of -0.081 ppb/ppb is in good agreement with other reported measurements from the DC-8. These slopes are however, somewhat steeper than those reported from the ER-2 studies. We find that the reactive nitrogen budget in the UT/LS is largely balanced with shortfalls that are no greater than 15%. A number of oxygenated species (e.g., acetone, H2O2) are present and may provide an important in-situ source of HOx in the UT/LS region.

Singh, H. S.↗

NASA NTRS1996

Chemical Characteristics of Continental Outflow Over the Tropical South Atlantic Ocean from Brazil and Africa

The chemical characteristics of air parcels over the tropical South Atlantic during September - October 1992 are summarized by analysis of aged marine and continental outflow classifications. Positive correlations between CO and CH3CL and minimal enhancements of C2CL40, and various ChloroFluoroCarbon (CFC) species in air parcels recently advected over the South Atlantic basin strongly suggest an impact on tropospheric chemistry from biomass burning on adjacent continental areas of Brazil and Africa. Comparison of the composition of aged Pacific air with aged marine air over the South Atlantic basin from 0.3 to 12.5 km altitude indicates potential accumulation of long-lived species during the local dry season. This may amount to enhancements of up to two-fold for C2H6, 30% for CO, and 10% for CH3Cl. Nitric oxide and NO(x) were significantly enhanced (up to approx. 1 part per billion by volume (ppbv)) above 10 km altitude and poorly correlated with CO and CH3Cl. In addition, median mixing ratios of NO and NO(x) were essentially identical in aged marine and continental outflow air masses. It appears that in addition to biomass burning, lightning or recycled reactive nitrogen may be an important source of NO(x) to the upper troposphere. Methane exhibited a monotonic increase with altitude from approx. 1690 to 1720 ppbv in both aged marine and continental outflow air masses. The largest mixing ratios in the upper troposphere were often anticorrelated with CO, CH3Cl, and CO2, suggesting CH, contributions from natural sources. We also argue, based on CH4/CO ratios and relationships with various hydrocarbon and CFC species, that inputs from biomass burning and the northern hemisphere are unlikely to be the dominant sources of CO, CH4 and C2H6 in aged marine air. Emissions from urban areas would seem to be necessary to account for the distribution of at least CH4 and C2H6. Over the African and South American continents an efficient mechanism of convective vertical transport coupled with large-scale circulations conveys biomass burning, urban, and natural emissions to the upper troposphere over the South Atlantic basin. Slow subsidence over the eastern South Atlantic basin may play an important role in establishing and maintaining the rather uniform vertical distribution of long-lived species over this region. The common occurrence of values greater than 1 for the ratio CH3OOH/H2O2 in the upper troposphere suggests that precipitation scavenging effectively removed highly water soluble gases (H2O2, HNO3, HCOOH, and CH3COOH) and aerosols during vertical convective transport over the continents. However, horizontal injection of biomass burning products over the South Atlantic, particularly water soluble species and aerosol particles, was frequent below 6 km altitude.

Talbot, R. W.↗

NASA NTRS1996

Origin of Ozone NO(x) in the Tropical Troposphere: A Photochemical Analysis of Aircraft Observations Over the South Atlantic Basin

The photochemistry of the troposphere over the South Atlantic basin is examined by modeling of aircraft observations up to 12-km altitude taken during the TRACE A expedition in September-October 1992. A close balance is found in the 0 to 12-km column between photochemical production and loss Of O3, with net production at high altitudes compensating for weak net loss at low altitudes. This balance implies that O3 concentrations in the 0-12 km column can be explained solely by in situ photochemistry; influx from the stratosphere is negligible. Simulation of H2O2, CH3OOH, and CH2O concentrations measured aboard the aircraft lends confidence in the computations of O3 production and loss rates, although there appears to be a major gap in current understanding of CH2O chemistry in the marine boundary layer. The primary sources of NO(x) over the South Atlantic Basin appear to be continental (biomass burning, lightning, soils). There is evidence that NO(x) throughout the 0 to 12-km column is recycled from its oxidation products rather than directly transported from its primary sources. There is also evidence for rapid conversion of HNO3 to NO(x) in the upper troposphere by a mechanism not included in current models. A general representation of the O3 budget in the tropical troposphere is proposed that couples the large scale Walker circulation and in situ photochemistry. Deep convection in the rising branches of the Walker circulation injects NO(x) from combustion, soils, and lightning to the upper troposphere, leading to O3 production; eventually, the air subsides and net O3 loss takes place in the lower troposphere, closing the O3 cycle. This scheme implies a great sensitivity of the oxidizing power of the atmosphere to NO(x) emissions in the tropics.

Jacob, D. J.↗

NASA NTRS1996

Asian Influence Over the Western North Pacific during the Fall Season: Inferences from Lead 210, Soluble Ionic Species and Ozone

Aerosol samples collected over the western Pacific during the NASA/Global Tropospheric Experiment Pacific Exploratory Mission (PEM-West A) expedition (September - October 1991) revealed mean Pb-210 concentrations in the free troposphere in the 5-10 fCi m(exp -3) STP range. Most soluble ionic aerosol-associated species were near detection limits [much less than 40 parts per trillion by volume (pptv)] in these same samples. The altitude distribution of O3 near Asia closely resembled that of Pb-210, while no relationship was found between the concentrations of O3 and Be-7. Free tropospheric air over the western Pacific was depleted in soluble aerosol-associated species but enriched in Pb-210 and O3, indicative of deep wet convection over the Asian continent. The influence of Asian air on the composition of the free troposphere over the western Pacific was evident on most of the PEM-West A flights. However, evidence of continental influence was largely restricted to those species that are relatively insoluble (or have insoluble precursors), hence escape scavenging during vertical transport from the boundary layer into the free troposphere by wet convective activity.

Dibb, J. E.↗

NASA NTRS1996

Chemical Characteristics of Continental Outflow from Asia to the Troposphere over the Western Pacific Ocean during September - October 1991: Results from PEM-West A

An important objective of the Pacific Exploratory Mission-West A (PEM-West A) was the chemical characterization of the outflow of tropospheric trace gases and aerosol particles from the Asian continent over the western Pacific Ocean. This paper summarizes the chemistry of this outflow during the period September - October 1991. The vertical distributions of CO, C2H6, and NO(x), showed regions of outflow at altitudes below 2 km and from 8 to 12 km. Mixing ratios of CO were approx. equals 130 parts per billion by volume (ppbv), approx. equals 1OOO parts per trillion by volume (pptv) for C2H6, and approx. equals 100 pptv for NO(x) in both of these regions. Direct outflow of Asian industrial materials was clearly evident at altitudes below 2 km, where halocarbon tracer compounds such as CH3CCl3 and C2Cl4 were enhanced about threefold compared to aged Pacific air. The source attribution of species outflowing from Asia to the Pacific at 8-12 km altitude was not straightforward. Above 10 km altitude there were substantial enhancements of NO(y), O3, CO, CH4, SO2, C2H6, C3H8, C2H2, and aerosol Pb-210 but not halocarbon industrial tracers. These air masses were rich in nitrogen relative to sulfur and contained ratios of C2H2/CO and C3H8/C2H6 (approx. equals l.5 and 0.1 respectively) indicative of several- day-old combustion emissions. It is unclear if these emissions were of Asian origin, or if they were rapidly transported to this region from Europe by the high wind speeds in this tropospheric region (60 - 70 m/s). The significant cyclonic activity over Asia at this time could have transported to the upper troposphere emissions from biomass burning in Southeast Asia or emissions from the extensive use of various biomass materials for cooking and space heating. Apparently, the emissions in the upper troposphere were brought there by wet convective systems since water-soluble gases and aerosols were depleted in these air masses. Near 9 km altitude there was a distinct regional outflow that appeared to be heavily influenced by biogenic processes on the Asian continent, especially from the southeastern area. These air masses contained CH4 in excess of 1800 ppbv, while CO2 and OCS were significantly depleted (349 - 352 ppmv and 450 - 500 pptv, respectively). This signature seemingly reflected CH4 emissions from wetlands and rice paddies with coincident biospheric uptake of tropospheric CO2 and OCS.

Talbot, R. W.↗

NASA NTRS1994

Origin of tropospheric NO(x) over subarctic eastern Canada in summer

The original of NO(X) in the summertime troposphere over subarctic eastern Canada is investigated by photochemical modeling of aircraft and ground-based measurements from the Arctic Boundary Layer Expedition (ABLE 3B). It is found that decomposition of peroxyacetyl nitrate (PAN) can account for most of the NO(X) observed between the surface and 6.2 km altitude (aircraft ceiling). Forest fires represent the principal source of PAN in the region, implying the same origin for NO(X). There is, however, evidence for an unidentified source of NO(X) in occasional air masses subsiding from the upper troposphere. Isoprene emissions from boreal forests maintain high NO(X) concentrations in the continental boundary layer over eastern Canada by scavenging OH and NO3, thus slowing down conversion of NO(X) to HNO3, both in the daytime and at night. This effect is partly compensated by the production of CH3CO3 radicals during isoprene oxidation, which slows down the decomposition of PAN subsiding from the free troposphere. The peroxy radical concentrations estimated from concurrent measurements of NO and NO2 concentrations during ABLE 3B are consistent with values computed from our photochemical model below 4 km, but model values are low at higher altitudes. The discrepancy may reflect either a missing radical source in the model or interferences in the NO2 measurement.

Fan, S.-M↗

NASA NTRS1994

Influence of plumes from biomass burning on atmospheric chemistry over the equatorial and tropical South Atlantic during CITE 3

During all eight flights conducted over the equatorial and tropical South Atlantic in the course of the Chemical Instrumentation Test and Evaluation (CITE 3) experiment, we observed haze layers with elevated concentrations of aerosols, O3, CO, and other trace gases related to biomass burning emissions. They occurred at altitudes between 1000 and 5200 m and were usually only some 100-300 m thick. These layers extended horizontally over several 100 km and were marked by the presence of visible brownish haze. Air mass trajectories indicate that these layers originate in the biomass burning regions of Africa and South America and typically have aged at least 10 days since the time of emission. In the haze layers, O3 and CO concentrations up to 90 and 210 ppb were observed, respectively. The two species were highly correlated. The ratio concentrations in plume minus background concentrations of O3/CO is typically in the range 0.2-0.7, much higher than the ratios in the less aged plumes investigated previously in Amazonia. In most cases, aerosol (0.12-3 micrometer diameter) number concentrations were also elevated by up to 400/cu cm in the layers; aerosol enrichments were also strongly correlated with elevated CO levels. Clear correlations between CO and NO(x) enrichments were not apparent due to the age of the plumes, in which most NO(x) would have already reacted away within 1-2 days. Only in some of the plumes could clear correlations between NO(y) and CO be identified; the absence of a general correlation between NO(y) and CO may be due to instrumental limitations and to variable sinks for NO(y). The average enrichment of the ratio concentrations in plume minus background concentrations of NO(y)/CO was quite high, consistent with the efficient production of ozone observed in the plumes. The chemical characteristics of the haze layers, together with remote sensing information and trajectory calculations, suggest that fire emissions (in Africa and/or South America) are the primary source of the haze layer components.

Andreae, M. O.↗

NASA NTRS1994

Enhancement of acidic gases in biomass burning impacted air masses over Canada

Biomass-burning impacted air masses sampled over central and eastern Canada during the summer of 1990 as part of ABLE 3B contained enhanced mixing ratios of gaseous HNO3, HCOOH, CH3COOH, and what appears to be (COOH)2. These aircraft-based samples were collected from a variety of fresh burning plumes and more aged haze layers from different source regions. Values of the enhancement factor, delta X/delta CO, where X represents an acidic gas, for combustion-impacted air masses sampled both near and farther away from the fires, were relatively uniform. However, comparison of carboxylic acid emission ratios measured in laboratory fires to field plume enhancement factors indicates significant in-plume production of HCOOH. Biomass-burning appears to be an important source of HNO3, HCOOH, and CH3COOH to the troposphere over subarctic Canada.

Lefer, B. L.↗

NASA NTRS1994

Acetone in the atmosphere: Distribution, sources, and sinks

Acetone (CH3COCH3) was found to be the dominant nonmethane organic species present in the atmosphere sampled primarily over eastern Canada (0-6 km, 35 deg-65 deg N) during ABLE3B (July to August 1990). A concentration range of 357 to 2310 ppt (= 10(exp -12) v/v) with a mean value of 1140 +/- 413 ppt was measured. Under extremely clean conditions, generally involving Arctic flows, lowest (background) mixing ratios of 550 +/- 100 ppt were present in much of the troposphere studied. Correlations between atmospheric mixing ratios of acetone and select species such as C2H2, CO, C3H8, C2Cl4 and isoprene provided important clues to its possible sources and to the causes of its atmospheric variability. Biomass burning as a source of acetone has been identified for the first time. By using atmospheric data and three-dimensional photochemical models, a global acetone source of 40-60 Tg (= 10(exp 12) g)/yr is estimated to be present. Secondary formation from the atmospheric oxidation of precursor hydrocarbons (principally propane, isobutane, and isobutene) provides the single largest source (51%). The remainder is attributable to biomass burning (26%), direct biogenic emissions (21%), and primary anthropogenic emissions (3%). Atmospheric removal of acetone is estimated to be due to photolysis (64%), reaction with OH radicals (24%), and deposition (12%). Model calculations also suggest that acetone photolysis contributed significantly to PAN formation (100-200 ppt) in the middle and upper troposphere of the sampled region and may be important globally. While the source-sink equation appears to be roughly balanced, much more atmospheric and source data, especially from the southern hemisphere, are needed to reliably quantify the atmospheric budget of acetone.

Singh, H. B.↗

NASA NTRS1994

Summertime distribution of PAN and other reactive nitrogen species in the northern high-latitude atmosphere of eastern Canada

Aircraft measurements of key reactive nitrogen species (NO, NO2, HNO3, PAN, PPN, NO3(-), NO(y)), C1 to C6 hydrocarbons, acetone, O3, chemical tracers (C2Cl4, CO), and important meteorological parameters were performed over eastern Canada during July to August 1990 at altitudes between 0 and 6 km as part of an Arctic Boundary Layer Expedition (ABLE3B). In the free troposphere, PAN was found to be the single most abundant reactive nitrogen species constituting a major fraction of NO(y) and was significantly more abundant than NO(x) and HNO3. PAN and O3 were well correlated both in their fine and gross structures. Compared to data previously collected in the Arctic/subarctic atmosphere over Alaska (ABLE3A), the lower troposphere (0-4 km) over eastern Canada was found to contain larger reactive nitrogen and anthropogenic tracer concentrations. At higher altitudes (4-6 km) the atmospheric composition was in many ways similar to what was seen over Alaska and supports the view that a large-scale reservoir of PAN (and NO(y)) is present in the upper troposphere over the entire Arctic/subarctic region. The reactive nitrogen budget based on missions conducted from the North Bay site (missions 2-10) showed a small shortfall, whereas the budget for data collected from the Goose Bay operation (missions 11-19) showed essential balance. It is calculated that 15-20 ppt of the observed NO(x) may find its source from the available PAN reservoir. Meteorological considerations as well as relationships between reactive nitrogen and tracer species suggest that the atmosphere over eastern Canada during summer is greatly influenced by forest fires and transported industrial pollution.

Singh, H. B.↗

NASA NTRS1994

Summertime distribution and relations of reactive odd nitrogen species and NO(y) in the troposphere over Canada

We report here large-scale features of the distribution of NO(x), HNO3, PAN, particle NO3, and NO(y) in the troposphere from 0.15 to 6 km altitude over central Canada. These measurements were conducted in July - August 1990 from the NASA Wallops Electra aircraft as part of the joint United States-Canadian Arctic Boundary Layer Expedition (ABLE) 3B-Northern Wetlands Study. Our findings show that this region is generally NO(x) limited, with NO(x) mixing ratios typically 20-30 parts per trillion by volume (pptv). We found little direct evidence for anthropogenic enhancement of mixing ratios of reactive odd nitrogen species and NO(y) above those in 'background' air. Instead, it appears that enhancements in the mixing ratios of these species were primarily due to emissions from several day old or CO-rich-NO(x)-poor smoldering local biomass-burning fires. NO(x) mixing ratios in biomass-burning impacted air masses were usually less than 50 pptv, but those of HNO3 and PAN were typically 100-300 pptv representing a twofold-threefold enhancement over 'background' air. During our study period, inputs of what appeared to be aged tropical air were a major factor influencing the distribution of reactive odd nitrogen in the midtroposphere over northeastern North America. These air masses were quite depleted in NO(y) (generally less than 150 pptv), and a frequent summertime occurrence of such air masses over this region would imply a significant influence on the reactive odd nitrogen budget. Our findings show that the chemical composition of aged air masses over subarctic Canada and those documented in the Arctic during ABLE 3A have strikingly similar chemistries, suggesting large-scale connection between the air masses influencing these regions.

Talbot, R. W.↗

NASA NTRS1994

Factors influencing atmospheric composition over subarctic North America during summer

Elevated concentrations of hydrocarbons, CO, and nitrogen oxides were observed in extensive haze layers over northeastern Canada in the summer of 1990, during ABLE 3B. Halocarbon concentrations remained near background in most layers, indicating a source from biomass wildfires. Elevated concentrations of C2Cl4 provided a sensitive indicator for pollution from urban/industrial sources. Detailed analysis of regional budgets for CO and hydrocarbons indicates that biomass fires accounted for approximately equal to 70% of the input to the subarctic for most hydrocarbons and for acetone and more than 50% for CO. Regional sources for many species (including CO) exceeded chemical sinks during summer, and the boreal region provided a net source to midlatitudes. Interannual variations and long-term trends in atmospheric composition are sensitive to climatic change; a shift to warmer, drier conditions could increase the areas burned and thus the sources of many trace gases.

Wofsy, Steven C.↗

NASA NTRS1994

Low Ozone in the Marine Boundary Layer of the Tropical Pacific Ocean

Aircraft measurements of ozone, its key precursors, and a variety of chemical tracers were made in the troposphere of the western and central Pacific in October 1991. These data are presented and analyzed to examine the occurrence of low ozone concentrations in the remote marine boundary layer of the tropical and equatorial Pacific Ocean. The data from these flights out of Guam, covering an area extending from the equator to 20 N and from south of the Philippines to Hawaii, show average O3 concentrations as low as 8-9 ppb (ppb=10(exp-9)v/v) at altitudes of 0.3-0.5 km in the boundary layer. Individual measurements as low as 2-5 ppb were recorded. Low O3 concentrations do not always persist in space and time. High O3, generally associated with the transport of upper tropospheric air, was also encountered in the boundary layer. In practically all cases, O3 increased to values as large as 25-30 ppb within 2 km above the boundary layer top. Steady state model computations are used to suggest that these low O3 concentrations are a result of net photochemical O3 destruction in a low NO environment, sea-surface deposition, and extremely low net entrainment rates (1-2 mm per second) from the free troposphere. Day/night measurements of ethane, propane, gaseous and aerosol Cl suggest that daytime (morning) Cl atom concentrations in the vicinity of 10(exp 5) molecules per cubic centimeter may be present in the marine boundary layer. This Cl atom abundance can be rationalized only if sea salt aerosols can release free chlorine (Cl2) to the gas phase in the presence of sun light (and possibly O3). These Cl atom concentrations, however, are still insufficient and Cl (or Br) chemistry is not likely to be an important cause of the observed low O3.

Singh, Hanwant B.↗

NASA NTRS1993

The impact of U.S. continental outflow on ozone and aerosol distributions over the western Atlantic

Aircraft measurements of selected trace gas species, aerosols, and meteorological parameters were performed in the lower troposphere off the U.S. east coast during August and September 1989 as part of the NASA Global Tropospheric Experiment (GTE) Chemical Instrumentation Test and Evaluation (CITE 3) expedition. In this paper, we examine these data to assess the impact of continental outflow on western Atlantic O3 and small aerosol budgets. Results show that mixed layer (ML) O3 concentrations and small aerosol number densities (Np) were enhanced by factors of 3 and 6, respectively, within air masses of predominantly continental origin compared with clean maritime background air. These enhancements exhibited a marked altitude dependence, declining rapidly above the ML to the point where only slight to moderate differences in O3 and Np, respectively, were notable above 2.4 km. Within continentally influenced ML's, both O3 and Np were correlated with CO, exhibiting linear regression slopes averaging 0.4 ppbv (O3)/ppbv(CO) for O3 and 7.7 (particles/cc)/ppbv(CO) for Np and indicating a primarily anthropogenic origin for the observed enhancement of these species. Comparisons between profiles in continental and background maritime air masses suggest that photochemical production below 1.4-km altitude adds over 10% to western Atlantic tropospheric column O3 abundance in continental outflow regimes. For aerosols, eastward advection of low-level continental air contributes an average net flux of 2.8 metric tons of submicron (accumulation mode) particles per kilometer of shoreline per day to the western Atlantic troposphere.

Anderson, B. E.↗

NASA NTRS1993

Factors influencing dry season ozone distributions over the tropical South Atlantic

Airborne measurements of trace gas and aerosol species were obtained in the lower troposphere (less than 5 km) over the western Atlantic Ocean between 13 deg S and 40 deg N during the August/September 1990 NASA Chemical Instrument Test and Evaluation (CITE 3) experiment. The largest background O3 mixing ratios, averaging 35 and 70 ppbv within the mixed layer (ML) and free troposphere (FT; altitudes greater than 2.4 km), respectively, were found over the tropical South Atlantic. Several competing processes were observed to regulate O3 budgets in this region. Within the ML, rapid photochemical destruction produced a diurnal O3 variation of 8 ppbv and an O3/altitude gradient between the surface and 5 km of almost 10 ppbv (O3)/km. ML O3 concentrations were replenished by atmospheric downwelling which occurred at rates of up to and exceeding 1 cm/s. Ozone values within the subsiding FT air were enriched both by long-range transport of O3 produced photochemically within biomass combustion plumes and the downward propagation of dry, upper tropospheric air masses. Overall, the tropospheric O3 column below 3.3 km averaged 13.5 Dobson units (DU) over the South Atlantic region, which is 8-9 DU higher than observed during CITE 3 ferry flights over the northern tropical Atlantic Ocean or measured by ozonesondes over coastal Brazil during the wet season. An examination of simultaneous dew point and combustion tracer (e.g., CO) measurements suggests that the dry subsiding layers and biomass burning layers make approximately equal contributions to the observed O3 enhancement.

Anderson, B. E.↗