Influence Of Biomass Burning Research Articles

Influence of biomass burning during recent fluctuations in the slow growth of global tropospheric methane

During the past 15 years the annual growth rate of tropospheric methane (CH4) has shown striking changes over 2–3 year periods, varying from +1% yr−1 to slightly negative values (−0.2% yr−1). These fluctuations are superimposed on an overall slowdown of the CH4 growth rate since the 1980s. Here we use our complementary measurement of other compounds (ethane, tetrachloroethene) to confirm the influence of biomass burning on large global CH4 pulses in 1998 and 2002–2003. Methane growth rate fluctuations also track ENSO indices, most likely via the influence of ENSO activity on large‐scale biomass burning. We also report the seventh year of near‐zero growth of global CH4 levels (Dec. 1998–Dec. 2005). The global CH4 mixing ratio was 1772 ± 1 ppbv in 2005, and CH4 increases of 118–376 ppbv between 2000–2020 (ten scenarios in the 2001 IPCC report, to levels around 1900+ ppbv by 2020, now appear quite unlikely.

Seasonal variation of carbon monoxide in northern Japan: Fourier transform IR measurements and source‐labeled model calculations

Tropospheric carbon monoxide (CO) was measured throughout 2001 using ground‐based Fourier transform IR (FTIR) spectrometers at Moshiri (44.4°N) and Rikubetsu (43.5°N) observatories in northern Japan, which are separated by 150 km. Seasonal and day‐to‐day variations of CO are studied using these data, and contributions from various CO sources are evaluated using three‐dimensional global chemistry transport model (GEOS‐CHEM) calculations. Seasonal maximum and minimum FTIR‐derived tropospheric CO amounts occurred in April and September, respectively. The ratio of partial column amounts between the 0–4 and 0–12 km altitude ranges is found to be slightly greater in early spring. The GEOS‐CHEM model calculations generally reproduce these observed features. Source‐labeled CO model calculations suggest that the observed seasonal variation is caused by seasonal contributions from various sources, in addition to a seasonal change in chemical CO loss by OH. Changes in meteorological fields largely control the relative importance of various source contributions. The contributions from fossil fuel (FF) combustion in Asia and photochemical CO production have the greatest yearly averaged contribution at 1 km among the CO sources (31% each). The Asian FF contribution increases from winter to summer, because weak southwesterly wind in summer brings more Asian pollutants to the observation sites. The seasonal variation from photochemical CO production is small (±17% at 1 km), likely because of concurrent increases (decreases) of photochemical production and loss rates in summer (winter), with the largest contribution between August and December. The contribution from intercontinental transport of European FF combustion CO is found to be comparable to that of Asian FF sources in winter. Northwesterly wind around the Siberian high in this season brings pollutants from Europe directly to Japan, in addition to southward transport of accumulated pollution from higher latitudes. The influences are generally greater at lower altitudes, resulting in a vertical gradient in the CO profile during winter. The model underestimates total CO by 12–14% between March and June. Satellite‐derived fire‐count data and the relationship between FTIR‐derived HCN and CO amounts are generally consistent with biomass burning influences, which could have been underestimated by the model calculations.

Carbon monoxide monitoring in Northeast Asia using MOPITT: Effects of biomass burning and regional pollution in April 2000

To assess the influence of biomass burning and regional pollution on CO levels in Northeast Asia, trajectory analysis and satellite observations from the Measurement of Pollution in the Troposphere (MOPITT) instrument were applied. As a case study, data for April 2000 were used. Ground measurement data at remote sites in Korea showed high CO levels and did not have typical seasonal variations due to regional pollution. Therefore, MOPITT data over the East/Japan Sea was recommended for identification of long-range transport of CO. The locations of biomass burning, distribution of MOPITT CO, and backward trajectories clearly indicated that Siberian fires and industrial activities in East China affected CO levels in Korea and Japan. CO levels over East China for the first two weeks were enhanced more than 35 ppb by biomass burning in Myanmar and Indo-China, and high CO levels over the East/Japan Sea for the last two weeks were affected by both anthropogenic emissions and biomass burning. The average difference in CO concentrations over the East/Japan Sea between fire days (217±18 ppb) and non-fire days (186±15 ppb) was 31 ppb ( p<0.05). These results suggest again that regional pollution as well as biomass burning plays an important role for CO levels in Northeast Asia and that MOPITT is a promising tool for the comprehensive understanding of CO emissions and transport.

Distribution of marine boundary layer ammonia over the Atlantic and Indian Oceans during the Aerosols99 cruise

Over the Atlantic Ocean and southern Indian Ocean an overall large variation in gas phase ammonia was encountered with peak values occurring in regions heavily influenced by the smoke plume from biomass combustion and continental sources on the African continent. Concentrations were typically in the range 7 to 22 nmol m−3. Over the remote clean South Atlantic and Indian Oceans, median gas phase ammonia concentrations ranged between 1.1 and 3.2 nmol m−3, but were occasionally as high as 8.1 nmol m−3 or as low as 0.3 nmol m−3. It was reasonable to assume that the ocean was a net emitter of ammonia to the atmosphere and thus responsible for the ammonia levels measured. An average residence time of the order of a few hours was estimated. One implication of such a rapid removal of ammonia is that it prevented attainment of equilibrium between the gas phase and particulate phase ammonium. In areas under the influence of African biomass burning or dust, however, the particulate phase ammonium was concluded to be in equilibrium with the gas phase ammonia. The removal of atmospheric ammonia during the time of travel from the African continent to the position of the ship was estimated using a simplified Lagrangian approach. A response or residence time of 20 to 130 hours resulted. Thus, in order to explain the observed atmospheric ammonia levels at the ship, it seemed necessary to allow for an ammonia residence time of the order of several days within the plume which differs widely from previous reported estimates.

Influence of Agricultural Biomass Burning on Aerosol Size Distribution and Dry Deposition in Southeastern Brazil

The size distributed composition of ambient aerosols is used to explore seasonal differences in particle chemistry and to show that dry deposition fluxes of soluble species, including important plant nutrients, increase during periods of biomass (sugar cane trash) burning in São Paulo State, Brazil. Measurements were made at a single site centrally located in the State's sugar cane growing region but away from the immediate vicinity of burns, so that the airsampled was representative of the regional background. Calculation of ion equivalent balances showed that during burning periods smaller particles (Aitken and accumulation modes) were more acidic, containing higher concentrations of SO4(2-), oxalate, NO3-, HCOO-, CH3COO-, and CI-, but insufficient NH4+ and K+ to achieve neutrality. Larger particles showed an anion deficit due to the presence of unmeasured ions and comprised resuspended dusts modified by accumulation of nitrate, chloride, and organic anions. Increases of resuspended particles during the burning season were attributed to release of earlier deposits from the surfaces of burning vegetation as well as increased vehicle movement on unsurfaced roads. During winter months the relative contribution of combined emissions from road transport and industry diminished due to increased emissions from biomass combustion and other activities specifically associated with the harvest period. Positive increments in annual particulate dry deposition fluxes due to higher fluxes during the sugar cane harvest were 44.3% (NH4+), 42.1% (K+), 31.8% (Mg2+), 30.4% (HCOO-), 12.8% (CI-), 6.6% (CH3COO-), 5.2% (Ca2+), 3.8% (SO4(2-)), and 2.3% (NO3-). Na+ and oxalate fluxes were seasonally invariant. Annual aerosol dry deposition fluxes (kg ha(-1)) were 0.5 (Na+), 0.25 (NH4+), 0.39 (K+), 0.51 (Mg2+), 3.19 (Ca2+), 1.34 (Cl-), 4.47 (NO3-), 3.59 (SO4(2-)), 0.58 (oxalate), 0.71 (HCOO-), and 1.38 (CH3COO-). Contributions of this mechanism to combined aerosol dry deposition and precipitation scavenging (inorganic species, excluding gaseous dry deposition) were 31% (Na+), 8% (NH4+), 26% (K+), 63% (Mg2+), 66% (Ca2+), 32% (Cl-), 33% (NO3-), and 36% (SO4(2-)).

Influence of Biomass Burning in Southeast Asia on the Lower Tropospheric Ozone Distribution Over South China

AbstractOzone enhancement was observed in the lower troposphere over Hong Kong and Kunming on 7 and 8 March 2001 using the electrochemical concentration cell ozonesondes. Based on the data of NCEP, total column ozone, numerical simulation of MM5, AI from TOMS, aerosol optical depths from MODIS, and CO concentration from MOPITT, the influence of biomass burning in Southeast Asia on the ozone distribution in the lower troposphere over Hong Kong and Kunming is analyzed. The transport of the emissions of biomass burning shows that the enhancement of the lower tropospheric ozone over Hong Kong and Kunming is from the region where large‐scale fires occurred in Southeast Asia. TOMS AI images and the atmospheric background circulations reveal that the biomass burning plumes in Southeast Asia can be transported to downwind South/Southwest China and lead to enhancement of the ozone concentration in the lower troposphere.

Spatial and temporal variation of MOPITT CO in Africa and South America: A comparison with SHADOZ ozone and MODIS aerosol

Carbon monoxide (CO) measurements from the Measurements of Pollution in the Troposphere (MOPITT) experiment are used to explore the correlation between biomass burning and ozone profiles at six tropical stations namely Reunion, Irene, Natal, Ascension, San Cristobal, and Paramaribo. Distinct seasonal patterns of CO at each station indicate the strong influence of African and South American biomass burning. All stations show enhanced CO columns during September–November (SON) corresponding to austral burning. Furthermore, the effects of Sahelian burning can be seen at Natal and Ascension. Similarly, the signature of northern Amazonian fires can be observed at San Cristobal. The CO variations are generally similar to the variations of aerosol optical depth (AOD) retrieved contemporaneously from Moderate Resolution Imaging Spectroradiometer (MODIS) at most stations, with notable differences at Irene, San Cristobal, and Paramaribo. Tropospheric ozone from Southern Hemisphere Additional Ozonesonde (SHADOZ) ozonesonde measurements at all stations show elevated levels, corresponding to the CO enhancements in SON months. However, there are several instances of ozone enhancements unaccompanied by any CO increase. This might indicate that sources other than biomass burning such as stratospheric tropospheric exchange (STE) or lightning related NOx may be operative. At San Cristobal, strong CO enhancements during March–April are not accompanied by any significant change in ozone.

Identification and estimate of biomass burning contribution to the urban aerosol organic carbon concentrations in Beijing

Abstract Daily particulate matter samples were collected during 1 year from 7 November 1997 to 31 October 1998 in Beijing area, at two monitoring sites, the Ming Tomb (a background site) situated northernmost of the basin and the Temple of Heaven (a residential site) inside the city. For 110 atmospheric aerosol samples organic carbon (OC) was determined with a two-step thermal procedure using a CHN elemental analyzer, and water-soluble potassium (K+) with flame atomic absorption spectrometry. The annual average of OC and K+ concentrations of the Ming Tomb site were 22.0 μg C m−3 and 1.21 μg m−3, respectively, while that of the Temple Heaven site were 41.5 μg C m−3 and 1.94 μg m−3. At the Ming Tomb site the monthly average of K+ concentration appeared the highest in June (3.07 μg m−3), three times as much as that in May (1.02 μg m−3). At the Temple Heaven site, the highest value was also in June (4.22 μg m−3 while 1.97 μg m−3 in May). Regression analysis results showed that at both sites the OC/K+ concentration ratio value (5) is what expected for biomass burning. Our results thus suggest that biomass burning influence is on a regional scale. It was calculated as an upper estimate, that at Ming Tomb site about 50%, 70%, and 46% of the OC concentrations in May, June, and July, respectively, were attributed to biomass burning while at the more urban site of Temple of Heaven, the biomass burning OC contribution is still high (32%, 43%, and 10% respectively). In the case of a severe pollution event in June 2000 in Beijing, it has been shown that temperature inversion and wind direction aggravated substantially air pollution caused by biomass burning. Our work suggests that biomass burning in the Beijing region is a significant repetitive pollution factor that cannot be neglected.

Influences of biomass burning during the Transport and Chemical Evolution Over the Pacific (TRACE‐P) experiment identified by the regional chemical transport model

Using a regional chemical transport model, STEM 2K1, and the emission inventory for the Transport and Chemical Evolution Over the Pacific (TRACE‐P) period [Woo et al., Streets et al., this issue], we successfully simulated important features of the biomass burning (BB) CO outflow. Simulated results agree well with the TRACE‐P aircraft measurements and Thailand surface observations. On the basis of sensitivity studies with and without biomass emissions, we identified nine flight segments that are affected by biomass plumes during the TRACE‐P period and compared the characteristics of the BB air masses with the other air masses. The BB air masses emitted from Southeast Asia contain relatively high HCN (ΔHCN/ΔCO ∼ 0.0015) and potassium (ΔK+/ΔCO ∼ 0.0038) but very low NOy (ΔNOy/ΔCO ∼ 0.005) mixing ratios, which may be associated with the special burning condition in this region. The biomass burning air masses have high ozone production efficiency. The observed ΔO3/ΔNOz values were ∼17 in biomass events and 1.7 in other events. The BB influence on the trace gas distributions can be divided into two categories: the influence through direct reactions and the influence caused by BB aerosols changing J values. These two influences are discussed for the BB‐affected TRACE‐P flights and for east Asia. The BB influences on chemical species are not only determined by the BB plume intensity but also by the ambient environment caused by other emissions. In Southeast Asia, where the biogenic emissions are very strong, the OH background concentration is low, and the BB gas‐phase compounds mainly contribute to OH production. Arranged in the sensitivity to the J value change caused by BB aerosols, we have OH &gt; HO2 &gt; HCHO &gt; O3 when evaluated on a regional average. Averaged over March, the biomass burning net influence is as high as 50% for OH, 40% for HO2, 60% for HCHO, and 10 ppbv for O3 for the layers below 1 km.

Transport pathways for Asian pollution outflow over the Pacific: Interannual and seasonal variations

The meteorological pathways contributing to Asian pollution outflow over the Pacific are examined with a global three‐dimensional model analysis of CO observations from the Transport and Chemical Evolution over the Pacific (TRACE‐P) aircraft mission (February–April 2001). The model is used also to place the TRACE‐P observations in an interannual (1994–2001) and seasonal context. The major process driving Asian pollution outflow in spring is frontal lifting ahead of southeastward‐moving cold fronts (the leading edge of cold surges) and transport in the boundary layer behind the cold fronts. Orographic lifting over central and eastern China combines with the cold fronts to promote the transport of Chinese pollution to the free troposphere. Outflow of seasonal biomass burning in Southeast Asia during spring takes place mostly by deep convection but also by northeastward transport and frontal lifting, mixing with the anthropogenic outflow. Boundary layer outflow over the western Pacific is largely devoid of biomass burning influence. European and African (biomass burning) plumes in Asian outflow during TRACE‐P were weak (&lt;60 ppbv and 20 ppbv CO, respectively) and were not detectable in the observations because of superposition of the much larger Asian pollution signal. Spring 2001 (La Niña) was characterized by unusually frequent cold surge events in the Asian Pacific rim and strong convection in Southeast Asia, leading to unusually strong boundary layer outflow of anthropogenic emissions and convective outflow of biomass burning emissions in the upper troposphere. The Asian outflow flux of CO to the Pacific is found to vary seasonally by a factor of 3–4 (maximum in March and minimum in summer). The March maximum results from frequent cold surge events and seasonal biomass burning emissions.

Chemical properties and outflow patterns of anthropogenic and dust particles on Rishiri Island during the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia)

Investigations of chemical properties and transport mechanisms of continental aerosols are necessary for estimating their influences on global radiative budget and on the global material cycle. Intensive measurements of atmospheric aerosols and the associated species on Rishiri Island, near the northern tip of Japan, were conducted from March to May 2001, in order to understand the chemical properties, source regions, transport pathways, and transport patterns of anthropogenic and mineral aerosols over the east Asian Pacific Rim region during the spring. Mean concentrations of nss‐SO42−, NO3−, NH4+, nss‐Ca2+ in aerosols were 2.48, 0.64, 0.72, and 0.17 μg m−3, respectively. Elemental carbon and organic carbon in fine particles (d &lt; 2.5 μm) yielded mean concentrations of 0.25 and 0.80 μg m−3, respectively. The concentrations of these species frequently increased to higher values because of outbreaks of continental polluted air masses, whereas under background conditions, they decreased to lower values similar to those observed over the remote ocean. Our results demonstrate that nss‐SO42− and NH4+ coexist in fine particles, that NO3− and nss‐Ca2+ coexist in coarse particles, and that each set is transported in an alternate manner. Continentally derived NO3− is transported as coarse particle to the east Asian Pacific Rim region. Anthropogenic pollutants and dust particles are not necessarily transported together. It was often found that anthropogenic fine particles containing abundant nss‐SO42− appeared first and were then followed by large mineral particles that had absorbed NO3−. Short‐term intrusion of the air masses containing abundant particulate carbonaceous compounds, probably due to the influence of biomass burning, also often occurred during the outflow events of continental air masses. Atmospheric behaviors of sulfate, nitrate, and carbonaceous species are different from one another, although they are all derived mainly from combustion processes.

Ground-based PTR-MS measurements of reactive organic compounds during the MINOS campaign in Crete, July–August 2001

Abstract. This study presents measurements of acetonitrile, benzene, toluene, methanol and acetone made using the proton-transfer-reaction mass spectrometry (PTR-MS) technique at the Finokalia ground station in Crete during the Mediterranean INtensive Oxidant Study (MINOS) in July-August 2001. Three periods during the campaign with broadly consistent back trajectories are examined in detail. In the first, air was advected from Eastern Europe without significant biomass burning influence (mean acetonitrile mixing ratio 154 pmol/mol). In the second period, the sampled air masses originated in Western Europe, and were advected approximately east-south-east, before turning south-west over the Black Sea and north-western Turkey. The third well-defined period included air masses advected from Eastern Europe passing east and south of/over the Sea of Azov, and showed significant influence by biomass burning (mean acetonitrile mixing ratio 436 pmol/mol), confirmed by satellite pictures. The mean toluene:benzene ratios observed in the three campaign periods described were 0.35, 0.37 and 0.22, respectively; the use of this quantity to determine air mass age is discussed. Methanol and acetone were generally well-correlated both with each other and with carbon monoxide throughout the campaign. Comparison of the acetone and methanol measurements with the MATCH-MPIC model showed that the model underestimated both species by a factor of 4, on average. The correlations between acetone, methanol and CO implied that the relatively high levels of methanol observed during MINOS were largely due to direct biogenic emissions, and also that biogenic sources of acetone were highly significant during MINOS (~35%). This in turn suggests that the model deficit in both species may be due, at least in part, to missing biogenic emissions.

Impact of biomass burning on cloud properties in the Amazon Basin

We used a one‐dimensional (1‐D) cloud parcel model to assess the impact of biomass‐burning aerosol on cloud properties in the Amazon Basin and to identify the physical and chemical properties of the aerosol that influence droplet growth. Cloud condensation nuclei (CCN) measurements were performed between 0.15% and 1.5% supersaturation at ground‐based sites in the states of Amazonas and Rondônia, Brazil during several field campaigns in 1998 and 1999 as part of the Large‐Scale Biosphere–Atmosphere (LBA) Experiment in Amazonia. CCN concentrations measured during the wet season were low and resembled concentrations more typical of marine conditions than most continental sites. During the dry season, smoke aerosol from biomass burning dramatically increased CCN concentrations. The modification of cloud properties, such as cloud droplet effective radius and maximum supersaturation, is most sensitive at low CCN concentrations. Hence, we could expect larger interannual variation of cloud properties during the wet season that the dry season. We found that differences between CCN spectra from forested and deforested regions during the wet season are modest and result in modifications of cloud properties that are small compared to those between wet and dry seasons. Our study suggests that the differences in surface albedo, rather than cloud albedo, between forested and deforested regions may dominate the impact of deforestation on the hydrological cycle and convective activity during the wet season. During the dry season, on the other hand, cloud droplet concentrations may increase by up to 7 times, which leads to a model‐predicted decrease in cloud effective radius by a factor of 2. This could imply a maximum indirect radiative forcing due to aerosol as high as ca. −27 W m−2 for a nonabsorbing cloud. Light‐absorbing substances in smoke darken the Amazonian clouds and reduce the net radiative forcing, and a comparison of the Advanced Very High Resolution Radiometer (AVHRR) analysis and our modeling studies suggests that absorption of sunlight due to smoke aerosol may compensate for about half of the maximum aerosol effect. Sensitivity tests show that complete characterization of the aerosol is necessary when kinetic growth limitations become important. Subtle differences in the chemical and physical makeup are shown to be particularly influential in the activation and growth behavior of the aerosol. Knowledge of the CCN spectrum alone is not sufficient to fully capture the climatic influence of biomass burning.

Global chemical model analysis of biomass burning and lightning influences over the South Pacific in austral spring

A global three‐dimensional model of tropospheric chemistry driven by reanalyzed European Centre for Medium‐Range Weather Forecasts meteorological data is used to examine the sources of O3, CO, and nitrogen oxides (NOx = NO + NO2) in the South Pacific troposphere during the NASA Pacific Exploratory Mission to the Tropics (PEM‐Tropics A) in September–October 1996. Aircraft observations up to 12 km during that mission revealed considerable biomass burning influence on O3 and CO in terms of elevated pollution layers and regional enhancements. The model reproduces the long‐range transport of biomass burning effluents from southern Africa and South America in the westerly subtropical flow over the South Pacific. Meteorological conditions in 1996 were particularly favorable for this transport. Africa and South America make comparable contributions to the biomass burning pollution over the South Pacific; the contribution from Australia and Indonesia is much less. Biomass burning dominates the supply of NOx in the lower troposphere over the South Pacific (through long‐range transport and decomposition of peroxyacetylnitrate), but lightning dominates in the upper troposphere. Observations in PEM‐Tropics A and elsewhere indicate low HNO3/NOx concentration ratios and an imbalance in the chemical budget of NOx in the upper troposphere. We reproduce these observations in our model and show that they reflect the subsidence of primary NOx injected by lightning into the uppermost troposphere, rather than any fast chemistry recycling HNO3 to NOx. We find that biomass burning and lightning made similar contributions to O3 production over the South Pacific during PEM‐Tropics A. Biomass burning plumes sampled in PEM‐Tropics A contained little NOx, and the O3 enhancements observed in these plumes originated from production over the source continents rather than over the South Pacific.

Tropospheric NO 2 from GOME measurements

Measurements from the Global Ozone Monitoring Experiment (GOME) have been analysed for tropospheric NO 2 using the Differential Optical Absorption method. The retrieval technique is described and a detailed error analysis is given. A case study of tropospheric NO 2 above Africa in fall 1997 is presented, showing the influence of both biomass burning and lightning. Comparison of clear and cloudy pixels reveals, that substantial amounts of NO 2 are present in the free troposphere over the African continent and in the outflow regions over the Southern Atlantic and the Indian Ocean. This has important implications for the formation of the tropospheric ozone maxima observed in these areas.

Organic trace gas composition of the marine boundary layer over the northwest Indian Ocean in April 2000

In April 2000 atmospheric trace gas measurements were performed on the western Indian Ocean on a cruise of the Dutch research vessel Pelagia from the Seychelles (5°S, 55°E) to Djibouti (12°N, 43°E). The measurements included analysis of dimethyl sulfide (DMS), acetone and acetonitrile every 40 s using PTR-MS (proton-transfer-reaction mass spectrometry) and gas chromatographic analyses of C 2–C 7 hydrocarbons in air samples taken during the cruise. The measurements took place at the end of the winter monsoon season and the sampled air masses came predominantly from the Southern Hemisphere, resulting in low concentrations of some long-lived hydrocarbons, halocarbons, acetone (350 pptv) and acetonitrile (120 pptv). On three consecutive days a diurnal cycle in DMS concentration was observed, which was used to estimate the emission of DMS (1.5±0.7×10 13 molecules m −2 s −1) and the 24 h averaged concentration of hydroxyl (OH) radicals (1.4±0.7×10 6 molecules cm −3). A strongly increased DMS concentration was found at a location where upwelling of deeper ocean waters took place, coinciding with a marked decrease in acetone and acetonitrile. In the northwestern Indian Ocean a slight increase of some trace gases was noticed showing a small influence of pollution from Asia and from northeast Africa as indicated with back trajectory calculations. The air masses from Asia had elevated acetonitrile concentrations showing some influence of biomass burning as was also found during the 1999 Indian Ocean Experiment, whereas the air masses from northeast Africa seemed to have other sources of pollution.

Lower‐Tropospheric Ozone (LTO) derived from TOMS near mountainous regions

Using Total Ozone Mapping Spectrometer (TOMS) version‐7 level‐2 clear‐sky (reflectivity ≤ 20%) ozone measurements corrected for aerosol effects and sea‐glint errors, we derived Lower Tropospheric Ozone (LTO) west and east of the Andes, the Mexican and Rocky Mountains, the mountains in Africa and the Arabian Peninsula, New Guinea, and the Himalayan Mountains. The derived results agree reasonably well with the seasonality of LTO from ozonesonde observations at Boulder, Cristobal, Fiji, Java, and Tahiti. The LTO seasonality found in the biomass burning seasons characterized by the ATSR World Fire Atlas west and east of the Andes (23°S–2°N), east of the Mexican Mountains (15°–23°N), South Sudan (6°–14°N), South Africa (30°–28°S), and west of New Guinea is consistent with the influence of biomass burning on the formation of tropospheric ozone in these regions. The significant El Niño influence on LTO west of New Guinea is evident throughout several El Niño cycles. The spring maximum in ozone west of the Mexican Mountains, in western China, and west of the Andes (32°–23°S) is consistent with a stratospheric intrusion source. East of the Mexican Mountains (23°–30°N), both west and east of the Rocky Mountains, in north Sudan and Iraq, and in western China, high concentrations of ozone are found in these continental and coastal regions which are affected by anthropogenic sources. The maximum ozone in these regions usually occurs in the summer due to photochemical ozone production. A summer LTO minimum occurs in coastal regions west of the Andes and west of Mexico, due to ozone destruction in low NOx and high H2O marine environment. A summer minimum also occurs in south Sudan in the rainy season. The LTO in the northern tropics of South America (4°–10°N), Africa (1°S–2°N), and east of New Guinea (7°–3°S) experiences little seasonal variation.

Marine boundary layer peroxy radical chemistry during the AEROSOLS99 campaign: Measurements and analysis

During the AEROSOLS99 cruise over the Atlantic and Southern Indian Oceans, peroxy radicals (RO2* = HO2 + ΣRO2, where R is an organic group) were continuously measured. Regular diurnal variations were observed with maximum mixing ratios of 30 to 55 parts per trillion by volume around noon. The latitudinal distribution of RO2* over the ocean seems to remain uniform under similar conditions of solar insolation. The composition of the marine boundary layer reflects long‐range transport of air from the African continent. The influence of biomass burning over remote areas of the marine boundary layer is confirmed, although the atmosphere over the Southern Hemisphere was often pristine, with rapid destruction of ozone observed during the daylight hours. Observed RO2* concentrations are in reasonable agreement with box model calculations, but cannot account for the highest diurnal variations of ozone observed. Possible explanations include halogen chemistry and nocturnal entrainment of O3 from the layers above, but available data cannot readily determine their relative contributions.

Influence of southern hemispheric biomass burning on midtropospheric distributions of nonmethane hydrocarbons and selected halocarbons over the remote South Pacific.

Aircraft measurements of nonmethane hydrocarbons (NMHCs) and halocarbons were made over the remote South Pacific Ocean during late August‐early October 1996 for NASA's Global Tropospheric Experiment (GTE) Pacific Exploratory Mission‐Tropics A (PEM‐Tropics A). This paper discusses the large‐scale spatial distributions of selected trace gases encountered during PEM‐Tropics A. The PEM‐Tropics A observations are compared to measurements made over the southwestern pacific in early November 1995 as part of Aerosol Characterization Experiment (ACE 1). Continental pollution in the form of layers containing elevated levels of O3 was observed during a majority of PEM‐Tropics flights, as well as during several ACE 1 flights. The chemical composition of these air masses indicates that they were not fresh and were derived from nonurban combustion sources. The substantial impact of biomass burning on the vertical structure of the South Pacific troposphere is discussed.

99/02094 On the influence of biomass burning on the seasonal CO 2 signal as observed at monitoring stations : Wittenberg, U. et al. Global Biogeochem. Cycles, 1998, 12, (3), 531–544

99/02094 On the influence of biomass burning on the seasonal CO 2 signal as observed at monitoring stations : Wittenberg, U. et al. Global Biogeochem. Cycles, 1998, 12, (3), 531–544