Aged Biomass Burning Research Articles

Soot particles in the free troposphere over Australia

Abstract Aircraft observations were carried out in the middle troposphere at 5–6 km altitude over Australia in the PACE-II (Pacific Atmospheric Chemistry Experiment) campaign in October 1994. Concentrations of particles with radii larger than 4 nm and 0.15 μm were measured with a condensation nucleus (CN) counter and an optical counter, respectively. Individual aerosol particles were collected on electron microscopic grids using a jet impactor and subsequently examined using a transmission electron microscope coupled with an energy-dispersive X-ray (EDX) analyzer. High concentrations of CN (⩾4 nm) and large particles (0.15–0.5 μm radius) were found over the areas in northern Australia between Alice Springs and Darwin. Samples collected in this region had a large fraction of submicrometer soot-containing particles that had both K and S with a median S/(S+K) weight ratio of 0.4. The hygroscopic properties of particles with radii between 0.03 and 0.8 μm were examined with the dialysis (extraction) of water-soluble material. The sample collected in the area with high particle concentrations showed that the abundance of internally mixed soot particles was 88% of particles in the radius range of 0.1–0.35 μm. Approximately 50% of these mixed particles with radii ⩾0.2 μm were covered with semi-transparent material (probably organic material). Present research suggested that these particles were aged biomass burning particles originating from northern Australia and had been modified by chemical reactions and probably by coagulation during long-range transport. These soot particles were found to be present as internally mixed particles, with a soluble fraction that would make them efficient cloud condensation nuclei (CCN).

Oxygenated compounds in aged biomass burning plumes over the Eastern Mediterranean: evidence for strong secondary production of methanol and acetone

Abstract. Airborne measurements of acetone, methanol, PAN, acetonitrile (by Proton Transfer Reaction Mass Spectrometry), and CO (by Tunable Diode Laser Absorption Spectroscopy) have been performed during the Mediterranean Intensive Oxidants Study (MINOS August 2001). We have identified ten biomass burning plumes from strongly elevated acetonitrile mixing ratios. The characteristic biomass burning signatures obtained from these plumes reveal secondary production of acetone and methanol, while CO photochemically declines in the plumes. Mean excess mixing ratios - normalized to CO - of 1.8%, 0.20%, 3.8%, and 0.65% for acetone, acetonitrile, methanol, and PAN, respectively, were found. By scaling to an assumed global annual source of 663-807Tg CO, biomass burning emissions of 25-31 and 29-35 Tg/yr for acetone and methanol are estimated, respectively. Our measurements suggest that the present biomass burning contributions of acetone and methanol are significantly underestimated due to the neglect of secondary formation within the plume. Median acetonitrile mixing ratios throughout the troposphere were around 150pmol/mol, in accord with current biomass burning inventories and an atmospheric lifetime of ~6 months.

Physical properties of the sub-micrometer aerosol over the Amazon rain forest during the wet-to-dry season transition - comparison of modeled and measured CCN concentrations

Abstract. Sub-micrometer atmospheric aerosol particles were studied in the Amazon region, 125 km northeast of Manaus, Brazil (-1°55.2'S, 59°28.1'W). The measurements were performed during the wet-to-dry transition period, 4-28 July 2001 as part of the LBA (Large-Scale Biosphere Atmosphere Experiment in Amazonia) CLAIRE-2001 (Cooperative LBA Airborne Regional Experiment) experiment. The number size distribution was measured with two parallel differential mobility analyzers, the hygroscopic growth at 90% RH with a Hygroscopic Tandem Mobility Analyzer (H-TDMA) and the concentrations of cloud condensation nuclei (CCN) with a cloud condensation nuclei counter. A model was developed that uses the H-TDMA data to predict the number of soluble molecules or ions in the individual particles and the corresponding minimum particle diameter for activation into a cloud droplet at a certain supersaturation. Integrating the number size distribution above this diameter, CCN concentrations were predicted with a time resolution of 10 min and compared to the measured concentrations. During the study period, three different air masses were identified and compared: clean background, air influenced by aged biomass burning, and moderately polluted air from recent local biomass burning. For the clean period 2001, similar number size distributions and hygroscopic behavior were observed as during the wet season at the same site in 1998, with mostly internally mixed particles of low diameter growth factor (~1.3 taken from dry to 90% RH). During the periods influenced by biomass burning the hygroscopic growth changed slightly, but the largest difference was seen in the number size distribution. The CCN model was found to be successful in predicting the measured CCN concentrations, typically within 25%. A sensitivity study showed relatively small dependence on the assumption of which model salt that was used to predict CCN concentrations from H-TDMA data. One strength of using H-TDMA data to predict CCN concentrations is that the model can also take into account soluble organic compounds, insofar as they go into solution at 90% RH. Another advantage is the higher time resolution compared to using size-resolved chemical composition data.

Measurements of CO in an aircraft experiment and their correlation with biomass burning and air mass origin in South America

Abstract Carbon monoxide (CO) measurements are obtained in an aircraft experiment during 1–7 September 2000, conducted over Central Brazil in a special region of anticyclonic circulation. This is a typical transport regime during the dry season (July–September), when intense biomass burning occurs, and which gives origin to the transport of burning poluents from the source to distant regions. This aircraft experiment included in situ measurements of CO concentrations in three different scenarios: (1) areas of fresh biomass burning air masses, or source areas; (2) areas of aged biomass burning air masses; and (3) areas of clean air or pristine air masses. The largest CO concentrations were of the order of 450 ppbv in the source region near Conceicao do Araguaia (PA), and the smallest value near 100 ppbv, was found in pristine air masses, for example, near the northeast coastline (clean air, or background region). The observed concentrations were compared to the number of fire pixels seen by the AVHRR satellite instrument. Backward isentropic trajectories were used to determine the origin of the air masses at each sampling point. From the association of the observed CO mixing ratios, fire pixels and air mass trajectories, the previous scenarios may be subdivided as follows: (1a) source regions of biomass burning with large CO concentrations; (1b) regions with few local fire pixels and absence of contributions by transport. Areas with these characteristics include the northeast region of Brazil; (1c) regions close to the source region and strongly affected by transport (region of Para and Amazonas); (2) regions that have a consistent convergence of air masses, that have traveled over biomass burning areas during a few days (western part of the Cerrado region); (3a) Pristine air masses with origin from the ocean; (3b) regions with convergent transport that has passed over areas of no biomass burning, such as frontal weather systems in the southern regions.

Characterization of carbonaceous aerosols during the MINOS campaign in Crete, July–August 2001: a multi-analytical approach

Abstract. During the major part of the Mediterranean Intensive Oxidant Study (MINOS) campaign (summer 2001, Crete Isl.), the Marine Boundary Layer (MBL) air was influenced by long range transport of biomass burning from the northern and western part of the Black Sea. During this campaign, carbonaceous aerosols were collected on quartz filters at a Free Tropospheric (FT) site, and at a MBL site together with size-resolved distribution of aerosols. Three Evolution Gas Analysis (EGA) protocols have been tested in order to better characterize the collected aged biomass burning smoke: A 2-step thermal method (Cachier et al., 1989) and a thermo-optical technique using two different temperature programs. The later temperature programs are those used for IMPROVE (Interagency Monitoring of Protected Visual Environments) and NIOSH 5040 (National Institute of Occupational Safety and Health). Artifacts were observed using the NIOSH temperature program and identified as interactions between carbon and dust deposited on the filter matrix at high temperature (T>550ºC) under the pure helium step of the analysis. During the MINOS campaign, Black Carbon (BC) and Organic Carbon (OC) mass concentrations were on average respectively 1.19±0.56 and 3.62±1.08 mgC/m3 for the IMPROVE temperature program, and 1.09±0.36 and 3.75±1.24 mgC/m3 for the thermal method. Though these values compare well on average and the agreement between the Total Carbon (TC) measurements sample to sample was excellent (slope=1.00, r2=0.93, n=56), important discrepancies were observed in determining BC concentrations from these two methods (average error of 33±22%). BC from the IMPROVE temperature program compared well with non-sea-salt potassium (nss-K) pointing out an optical sensitivity to biomass burning. On the other hand, BC from the thermal method showed a better agreement with non-sea-salt sulfate (nss-SO4), considered as a tracer for fossil fuel combustion during the MINOS campaign. The coupling between these two methods for determining BC brings here new insights on the origin of carbonaceous aerosols in a complex mixture of different sources. It brings also to our attention that important deviations in BC levels are observed using three widely used EGA's techniques and most probably none of the EGA tested here are well adapted to fully characterize this aerosol mixture. Spherical, smooth and silico-aluminated fly-ash observed by an Analytical Scanning Electron Microscope (ASEM) confirm the influence of coal combustion on the carbonaceous aerosol load throughout the campaign. A rough calculation based on a BC/nss-SO4 mass ratio suggests that biomass burning could be responsible for half of the BC concentration recorded during the MINOS campaign. From the plot of BC as a function of TC, two linear correlations were observed corresponding to 2 times series (before and after 12 August). Such good correlations suggest, from a first look, that both BC and OC have similar origin and atmospheric transport. On the other hand, the plot of BC as a function of TC obtained from the 2-step thermal method applied to DEKATI Low Pressure Cascade Impactor samples does not show a similar correlation and points out a non conservative distribution of this ratio with 2 super micron modes enriched in OC, correlated with sea salt aerosols and probably originating from gas-to-particle conversion.

Sources and chemistry of nitrogen oxides over the tropical Pacific

We examine the sources and chemistry affecting nitrogen oxides (NOx = NO + NO2) over the tropical Pacific (30°S–20°N) using observations from the Pacific Exploratory Mission to the Tropics B (PEM‐Tropics B) aircraft mission conducted in March–April 1999. A global model of tropospheric chemistry driven by assimilated meteorological data is used to interpret the observations. Median concentrations observed over the South Pacific during PEM‐Tropics B were 7 pptv NO, 16 pptv peroxyacetyl nitrate (PAN), and 34 pptv nitric acid (HNO3); the model generally reproduces these observations but overestimates those over the North Pacific. Lightning was the largest source of these species in the equatorial and South Pacific tropospheric column and in the tropical North Pacific upper troposphere. The oceanic source of acetone implied by high observations of acetone concentrations (mean 431 pptv) allows an improved simulation of PAN/NOx chemistry. However, the high acetaldehyde concentrations (mean 78 pptv) measured throughout the troposphere are inconsistent with our understanding of acetaldehyde and PAN chemistry. Simulated concentrations of HNO3 and HNO3/NOx are highly sensitive to the model representation of deep convection and associated HNO3 scavenging. Chemical losses of NOx during PEM‐Tropics B exceed chemical sources by a factor of 2 in the South Pacific upper troposphere. The chemical imbalance, also apparent in the low observed HNO3/NOx ratio, is explained by NOx injection from lightning and by frequent convective overturning which depletes HNO3. The observed imbalance was less during the PEM‐Tropics A campaign in September 1996, when aged biomass burning effluents over the South Pacific pushed the NOx budget toward chemical steady state.

Aircraft measurements of the latitudinal, vertical, and seasonal variations of NMHCs, methyl nitrate, methyl halides, and DMS during the First Aerosol Characterization Experiment (ACE 1)

Canister sampling for the determination of atmospheric mixing ratios of nonmethane hydrocarbons (NMHCs), selected halocarbons, and methyl nitrate was conducted aboard the National Center for Atmospheric Research (NCAR) C‐130 aircraft over the Pacific and Southern Oceans as part of the First Aerosol Characterization Experiment (ACE 1) during November and December 1995. A latitudinal profile, flown from 76°N to 60°S, revealed latitudinal gradients for most trace gases. NMHC and halocarbon gases with predominantly anthropogenic sources, including ethane, ethyne, and tetrachloroethene, exhibited significantly higher mixing ratios in the northern hemisphere at all altitudes. Methyl chloride exhibited its lowest mixing ratios at the highest northern hemisphere latitudes, and the distributions of methyl nitrate and methyl iodide were consistent with tropical and subtropical oceanic sources. Layers containing continental air characteristic of aged biomass burning emissions were observed above about 3 km over the remote southern Pacific and near New Zealand between approximately 19°S and 43°S. These plumes originated from the west, possibly from fires in southern Africa. The month‐long intensive investigation of the clean marine southern midlatitude troposphere south of Australia revealed decreases in the mixing ratios of ethane, ethyne, propane, and tetrachloroethene, consistent with their seasonal mixing ratio cycle. By contrast, increases in the average marine boundary layer concentrations of methyl iodide, methyl nitrate, and dimethyl sulfide (DMS) were observed as the season progressed to summer conditions. These increases were most appreciable in the region south of 44°S over Southern Ocean waters characterized as subantarctic and polar, indicating a seasonal increase in oceanic productivity for these gases.

Ozone over southern Africa during SAFARI‐92/TRACE A

Characteristics of total O3 in southern Africa and over the adjacent Atlantic during the IGAC/STARE/SAFARI‐92/TRACE A (International Global Atmospheric Chemistry/South Tropical Atlantic Regional Experiment/Southern African Fire Atmospheric Research Initiative/Transport and Atmospheric Chemistry near the Equator‐Atlantic) field experiments are described. Most of the analysis is based on data from the Nimbus 7/total ozone mapping spectrometer (TOMS) gridded O3 data archive (version 6.0), which is used to examine O3 in terms of seasonal and interannual variability. Total O3 column variability is compared to the tropospheric O3 column derived from balloonborne ozonesondes at four fixed SAFARI‐92/TRACE A sites (Ascension Island, Brazzaville, Okaukuejo, and Irene) from September 1 to October 23, 1992. All of these sites except Okaukuejo had regular ozonesonde launches from 1990 to 1992. Total O3 and integrated tropospheric O3 at the sounding sites showed the expected September–October maxima over southern Africa and the adjacent Atlantic Ocean. Statistical analysis of the TOMS record for 1979–1992 allows disaggregation of components contributing to total O3 variability: Signals due to semiannual and annual cycles and the quasi‐biennial oscillation are identified at the sounding sites. The tropospheric O3 column estimated from integrated sondes (to ∼16 km) at the four sites ranged from 24 to 62 Dobson units (DU) (mean, 45 DU) and averaged 15% of total O3 at Irene (14 launches) and 19% of total O3 at Ascension (20 launches). Tropospheric O3 was higher at Ascension and Brazzaville than at the sites south of 15°S because transport from biomass burning regions was more direct at these sites. This transport is seen in Hovmöller (time‐longitude) plots of total O3. A comparison of 1990–1992 integrated tropospheric O3 amounts with the annual total ozone cycle shows that tropospheric ozone variations may account for all of the annual signal at Ascension (8°S) and Brazzaville (4°S) but only 30–40% of the seasonal total O3 variation at Irene (26°S). Hovmöller plots of daily TOMS O3 over southern Africa and the Atlantic show easterly flow of local O3 maxima at 0°–10°S and westerly movement from 30°–40°S. At 0°–10°S the continent‐ocean total O3 gradient and Ascension and Brazzaville O3 soundings are used to estimate a photochemical O3 formation rate of 1–2 ppbv O3/d over the Atlantic. This agrees with model calculations of moderately aged biomass burning emissions from SAFARI‐92/TRACE A [Jacob et al., this issue; Thompson et al., 1996, this issue].

An ice-core-based record of biomass burning in the Arctic and Subarctic, 1750–1980

Ammonium records from 3 ice cores, 20D and GISP2 (Greenland) and Mt. Logan (Yukon), covering the period from 1750 to the 1980s are analyzed. For each data set, samples with NH 4 + concentrations greater than one standard deviation above the mean value also tend to be enriched in NO 3 - and K + , similar to the chemical composition of aerosols from aged biomass burning plumes. We believe the NH 4 + spikes originate from biomass burning events. There is not a one to one correspondence between documented large fires and NH 4 + spikes, nor are specific annual layers with elevated NH 4 + concentrations often found in more than one core. However, frequency of NH 4 + spikes increase during periods of more extensive and intensive biomass burning in the NH 4 + source areas for the ice core sites. The 20D and GISP2 records are characterized by increased spike frequency from 1790 to 1810 and from 1830 to 1910. This latter time coincides with a period of increased biomass burning documented in the historical fire records for northern North America. In contrast to both Greenland ice core records, the Mt. Logan NH 4 + record shows periods of increased spike frequency from 1770–1790, 1810–1830, 1850–1870 and 1930–1980. The poor agreement between the Mt. Logan record and the records from Greenland suggests that another source area, perhaps Siberia, may be the dominant summertime source area for NH 4 + spikes in Mt. Logan snow. DOI: 10.1034/j.1600-0889.1994.t01-2-00006.x

The distribution of light nonmethane hydrocarbons over the mid-Atlantic: Results of the Polarstern cruise ANT VII/1

During the cruise ANT VII/1 (September/October 1988) of the German research vessel Polarstern the latitudinal distributions of several nonmethane hydrocarbons were measured over the Atlantic between 45°N and 30°S by in-situ gas chromatography.On the average, the highest mixing ratios of ethane, propane, i- and n-butane, ethene and acetylene were observed in the Northern Hemisphere around 40° N and just north of the intertropical convergence zone, respectively. South of the equator, a bulge in the mixing ratios of ethane and acetylene was observed indicating aged biomass burning emissions. This observation coincided with enhanced tropospheric ozone found in this region at this season. On the average ethane and acetylene mixing ratios were around 500 and 100 ppt, respectively, whereas the levels of the other NMHC were in the range of some ppt up to 100 ppt.compared with the results of the cruise ANT V/5 (March/April, 1987), the ethane mixing ratios in September/October proved to be a factor of 3 lower in the Northern Hemisphere and a factor of 2 higher in the Southern Hemisphere, probably due to seasonal effects. Possible causes are the higher OH radical concentrations in summer, which result in a faster removal of ethane or stronger emission from biomass burning which also peaks in the dry season.The relative pattern of the hydrocarbons just north of the ITCZ was very similar for both measurement series. In this region, the NMHC were advected by long-range transport from the continent, whereas generally the ocean itself acts as a major NMHC source. This is supported by the results of a balance calculation between oceanic emissions and atmospheric removal rates.