South American Biomass Burning Research Articles

Long-term in situ observations of biomass burning aerosol at a high altitude station in Venezuela – sources, impacts and interannual variability

Abstract. First long-term observations of South American biomass burning aerosol within the tropical lower free troposphere are presented. The observations were conducted between 2007 and 2009 at a high altitude station (4765 m a.s.l.) on the Pico Espejo, Venezuela. Sub-micron particle volume, number concentrations of primary particles and particle absorption were observed. Orographic lifting and shallow convection leads to a distinct diurnal cycle at the station. It enables measurements within the lower free troposphere during night-time and observations of boundary layer air masses during daytime and at their transitional regions. The seasonal cycle is defined by a wet rainy season and a dry biomass burning season. The particle load of biomass burning aerosol is dominated by fires in the Venezuelan savannah. Increases of aerosol concentrations could not be linked to long-range transport of biomass burning plumes from the Amazon basin or Africa due to effective wet scavenging of particles. Highest particle concentrations were observed within boundary layer air masses during the dry season. Ambient sub-micron particle volume reached 1.4±1.3 μm3 cm−3, refractory particle number concentrations (at 300 °C) 510±420 cm−3 and the absorption coefficient 0.91±1.2 Mm−1. The respective concentrations were lowest within the lower free troposphere during the wet season and averaged at 0.19±0.25 μm3 cm−3, 150±94 cm−3 and 0.15±0.26 Mm−1. A decrease of particle concentrations during the dry seasons from 2007–2009 could be connected to a decrease in fire activity in the wider region of Venezuela using MODIS satellite observations. The variability of biomass burning is most likely linked to the El Niño–Southern Oscillation (ENSO). Low biomass burning activity in the Venezuelan savannah was observed to follow La Niña conditions, high biomass burning activity followed El Niño conditions.

Development of a cavity-enhanced absorption spectrometer for airborne measurements of CH<sub>4</sub> and CO<sub>2</sub>

Abstract. High-resolution CH4 and CO2 measurements were made on board the FAAM BAe-146 UK (Facility for Airborne Atmospheric Measurements, British Aerospace-146) atmospheric research aircraft during a number of field campaigns. The system was based on an infrared spectrometer using the cavity-enhanced absorption spectroscopy technique. Correction functions to convert the mole fractions retrieved from the spectroscopy to dry-air mole fractions were derived using laboratory experiments and over a 3 month period showed good stability. Long-term performance of the system was monitored using WMO (World Meteorological Office) traceable calibration gases. During the first year of operation (29 flights) analysis of the system's in-flight calibrations suggest that its measurements are accurate to 1.28 ppb (1σ repeatability at 1 Hz = 2.48 ppb) for CH4 and 0.17 ppm (1σ repeatability at 1 Hz = 0.66 ppm) for CO2. The system was found to be robust, no major motion or altitude dependency could be detected in the measurements. An inter-comparison between whole-air samples that were analysed post-flight for CH4 and CO2 by cavity ring-down spectroscopy showed a mean difference between the two techniques of −2.4 ppb (1σ = 2.3 ppb) for CH4 and −0.22 ppm (1σ = 0.45 ppm) for CO2. In September 2012, the system was used to sample biomass-burning plumes in Brazil as part of the SAMBBA project (South AMerican Biomass Burning Analysis). From these and simultaneous CO measurements, emission factors for savannah fires were calculated. These were found to be 2.2 ± 0.2 g (kg dry matter)−1 for CH4 and 1710 ± 171 g (kg dry matter)−1 for CO2, which are in excellent agreement with previous estimates in the literature.

Biomass burning aerosol absorption measurements with MODIS using the critical reflectance method

[1] This research uses the critical reflectance technique, a space-based remote sensing method, to measure the spatial distribution of aerosol absorption properties over land. Choosing two regions dominated by biomass burning aerosols, a series of sensitivity studies were undertaken to analyze the potential limitations of this method for the type of aerosol to be encountered in the selected study areas, and to show that the retrieved results are relatively insensitive to uncertainties in the assumptions used in the retrieval of smoke aerosol. The critical reflectance technique is then applied to Moderate Resolution Imaging Spectrometer (MODIS) data to retrieve the spectral aerosol single-scattering albedo (SSA) in South African and South American biomass burning events. The retrieved results were validated with collocated Aerosol Robotic Network (AERONET) retrievals. Approximately 67% of the comparisons show a difference between MODIS and AERONET smaller than 0.03, the magnitude of the AERONET uncertainty. The overlap of the two retrievals increases to 88%, allowing for measurement variance in the MODIS retrievals, as well. The ensemble average of MODIS-derived SSA for the Amazon forest station is 0.92 at 670 nm, and 0.84–0.89 for the southern African savanna stations. The critical reflectance technique allows evaluation of the spatial variability of SSA and shows that SSA in South America exhibits higher spatial variation than in South Africa. The accuracy of the retrieved aerosol SSA from MODIS data indicates that this product can help to better understand how aerosols affect the regional and global climate.

Modeling the transport and optical properties of smoke plumes from South American biomass burning

This study investigates the transport and optical properties of smoke plumes from South American biomass burning by using an aerosol transport and microphysical model. In general, the model can reproduce the smoke aerosol optical properties observed by satellite and ground‐based instruments during the Large‐Scale Biosphere‐Atmosphere Experiment in Amazonia ‐ Smoke Aerosols, Clouds, Rainfall and Climate (LBA‐SMOCC) campaign. The simulated spatial distribution of smoke aerosol loading over South America also compares well to satellite observations, suggesting that the transport processes in the model are adequate over this region. The results further suggest that the emissions provided by version 2 of the Global Fire Emissions Database are too low over South America. However, wet deposition may be too aggressive in the model. Since the model wet deposition does not include the feedback smoke may have on cloud formation and precipitation suppression, too many aerosols may be removed. Surprisingly, given the different vegetation types, the model simulations also suggest that similar initial particle size distributions and aerosol optical properties can be used to simulate smoke from both African savanna and South American forest fires. However, to reproduce the observed smoke aerosol optical properties over South America, humidification of smoke aerosols needs to be considered. Model results and observations both suggest that the typical single scattering albedo of smoke over South America and Africa differ because of relative humidity, not vegetation type. Overall, this work suggests strategies for improved treatment of South American smoke plumes in climate and microphysical models.

First ground‐based FTIR observations of the seasonal variation of carbon monoxide in the tropics

We present the first ground‐based solar absorption Fourier Transform Infrared (FTIR) spectrometric measurements in the inner tropics over several years. The FTIR observations agree well with satellite data from the MOPITT instrument. MATCH‐MPIC model simulations reproduce the mean vertical structure of the FTIR observations. The model is generally not able to reproduce the extreme enhancements seen during the specific biomass burning events by both observation instruments. Nevertheless, the model indicates that beyond the background source of CO from methane oxidation, the main contributions to the CO mixing ratios are the episodic convective injection of NMHCs and CO from South American biomass burning into the upper troposphere, along with long range transport of African biomass burning CO, particularly during spring. In future studies with more extensive observed time series, observations such as these will be valuable for evaluating ongoing improvements in global chemistry transport models.

Chemistry, transport and dry deposition of trace gases in the boundary layer over the tropical Atlantic Ocean and the Guyanas during the GABRIEL field campaign

Abstract. We present a comparison of different Lagrangian and chemical box model calculations with measurement data obtained during the GABRIEL campaign over the tropical Atlantic Ocean and the Amazon rainforest in the Guyanas, October 2005. Lagrangian modelling of boundary layer (BL) air constrained by measurements is used to derive a horizontal gradient (≈5.6 pmol/mol km−1) of CO from the ocean to the rainforest (east to west). This is significantly smaller than that derived from the measurements (16–48 pmol/mol km−1), indicating that photochemical production from organic precursors alone cannot explain the observed strong gradient. It appears that HCHO is overestimated by the Lagrangian and chemical box models, which include dry deposition but not exchange with the free troposphere (FT). The relatively short lifetime of HCHO implies substantial BL-FT exchange. The mixing-in of FT air affected by African and South American biomass burning at an estimated rate of 0.12 h−1 increases the CO and decreases the HCHO mixing ratios, improving agreement with measurements. A mean deposition velocity of 1.35 cm/s for H2O2 over the ocean as well as over the rainforest is deduced assuming BL-FT exchange adequate to the results for CO. The measured increase of the organic peroxides from the ocean to the rainforest (≈0.66 nmol/mol d−1) is significantly overestimated by the Lagrangian model, even when using high values for the deposition velocity and the entrainment rate. Our results point at either heterogeneous loss of organic peroxides and/or their radical precursors, underestimated photodissociation or missing reaction paths of peroxy radicals not forming peroxides in isoprene chemistry. We calculate a mean integrated daytime net ozone production (NOP) in the BL of (0.2±5.9) nmol/mol (ocean) and (2.4±2.1) nmol/mol (rainforest). The NOP strongly correlates with NO and has a positive tendency in the boundary layer over the rainforest.

Sources and Transport of Urban and Biomass Burning Aerosol Black Carbon at the South–West Atlantic Coast

The total extent of the atmospheric impacts associated to the aerosol black carbon (BC) emissions from South America is not completed described. This work presents results of BC monitored during three scientific expeditions (2002, 2003 and 2004) on board of a Brazilian oceanographic vessel Ary Rongel that covered the South–West Atlantic coast between 22–62°S. This latitudinal band encloses major urban regions of South America and the outflow region of the SACZ (South Atlantic Convergent Zone), which is an important mechanism of advective transport of heat, moisture, minor gases and aerosols from the South America continental land to the Southern Atlantic Ocean. Our results showed that aerosol BC enhanced concentrations from urban/industrial origin can be transported to the South–West Atlantic Ocean due to the migration of sub-polar fronts that frequently reach tropical/subtropical regions. Despite the decrease of aerosol BC concentrations southwards (from ∼1,200 ng m−3 at latitude 22°S to ∼10 ng m−3 at latitude 62°S), several observed peak events were attributed to regional urban activities. Most of such events could be explained by the use of air mass back trajectories analysis. In addition, a global model simulation is presented (Goddard Institute for Space Studies – GISS GCM BC simulation) to explore the origins of aerosol BC in the South–West Atlantic. The model allowed isolating the biomass emissions from South America and Africa and industrial (non-biomass) pollution from other regions of the globe. This model suggests that the apportionment of about half of the aerosol BC at the South–West Atlantic may derive from South American biomass burning.

Transport of biomass burning emissions from southern Africa

The transport of biomass burning emissions from southern Africa to the neighboring Atlantic and Indian Oceans during the dry season (May–October) of 2000 is characterized using ground, ozonesonde, and aircraft measurements of carbon monoxide (CO) and ozone (O3) in and around southern Africa, together with the GEOS‐CHEM global model of tropospheric chemistry. The model shows a positive bias of ∼20% for CO and a negative bias of ∼10–25% for O3 at oceanic sites downwind of fire emissions. Near areas of active fire emissions the model shows a negative bias of ∼60% and ∼30% for CO and O3, respectively, likely due to the coarse spatial (2° × 2.5°) and temporal (monthly) resolution of the model compared to that of active fires. On average, from 1994 to 2000, ∼60 Tg of carbon monoxide (CO) from biomass burning in southern Africa was transported eastward to the Indian Ocean across the latitude band 0°–60°S during the 6 months of the dry season. Over the same time period, ∼40 Tg of CO from southern African biomass burning was transported westward to the Atlantic Ocean over the latitudes 0°–20°S during the 6‐month dry season, but most of that amount was transported back eastward over higher latitudes to the south (21°–60°S). Eastward transport of biomass burning emissions from southern Africa enhances CO concentrations by ∼4–13 ppbv per month over the southern subtropical Indian Ocean during the dry season, with peak enhancements in September. Carbon monoxide from southern African and South American biomass burning is seen in the model simulations as far away as Australia, contributing ∼8 ppbv and ∼12–15 ppbv CO, respectively, and thus explaining the ∼20–25 ppbv observed enhancement of CO over Melbourne in mid‐September 2000.

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.

Ozone production and transports in the tropical Atlantic region during the biomass burning season

Tropospheric O3 distributions over the Atlantic Ocean have been calculated with a coupled chemistry‐general circulation model. Photochemically produced O3, mainly from biomass burning emissions, dominates O3 abundances over the tropical Atlantic Ocean in the southern hemisphere (SH) during September/October. On the other hand, O3 of stratospheric origin strongly contributes to the tropospheric O3 column at latitudes poleward of 30°S. Simulated tropospheric flow patterns are in good agreement with European Centre for Medium‐Range Weather Forecasts analyses and trajectory studies. Tracer transports over the tropical South Atlantic Ocean are strongly influenced by wind shear between the boundary layer and the free troposphere, leading to a stratification of O3. At the surface relatively O3‐poor air is transported from SH middle latitudes. At higher altitudes, relatively O3‐rich air is transported from the African biomass burning regions between 2 and 5 km and from South American biomass burning regions in the mid‐to‐upper troposphere. The model simulates O3 production rates of 10–50 parts per billion by volume (ppbv) O3 d−1 in the lower troposphere over the biomass burning regions and 2–6 ppbv O3 d−1 in the middle and upper troposphere. Photochemical destruction of O3 prevails in the lower troposphere over the ocean, maximizing in the African and South American outflow regions. In the northern hemisphere, in situ photochemically produced O3 dominates throughout most of the troposphere. Calculated O3 volume mixing ratios are compared with a latitude‐altitude O3 distribution measured during an Atlantic ship cruise along 30°W, between 55°N and 30°S, in September/October 1988, and with ozone sonde measurements from Southern African Fire Atmospheric Research Initiative/Transport and Atmospheric Chemistry Near the Equator—Atlantic (SAFARI/TRACE A) in September/October 1992. Calculated O3 levels agree reasonably well with the ship cruise data, except for the tropical SH lower troposphere where the model significantly underestimates O3. However, modeled vertical O3 distributions are underpredicted compared to TRACE A sonde measurements. Simulated O3 columns over the ocean are somewhat lower compared to values retrieved from satellite data, in particular over the tropical Atlantic. The underestimation is probably due to the neglect of higher hydrocarbon chemistry in the model.

Trends in South American biomass burning detected with the GOES visible infrared spin scan radiometer atmospheric sounder from 1983 to 1991

In an effort to get a better understanding of the extent and patterns of burning in South America, geostationary satellite data have been used to monitor active fires. Previous work demonstrated the ability to manually detect subpixel fire activity in selected areas of the selva and cerrado regions in South America with shortwave and longwave infrared data available from the Geostationary Operational Environmental Satellite (GOES) visible infrared spin scan radiometer atmospheric sounder (VAS) This paper presents the GOES VAS automated biomass‐burning algorithm (ABBA) which was developed to determine basin‐wide trends in fire activity in South America utilizing the GOES VAS archive. Comparisons between the manual and the automated techniques revealed that overall, the ABBA proved to be more consistent in identifying fires and better suited for trend analysis. The automated algorithm was applied daily to a study area extending from 5°S to 15°S and from 45°W to 70°W for 2 weeks at the peak of the burning seasons in South America in 1983, 1988, 1989, and 1991 in an effort to measure the areal extent of burning in South America during the past decade and to provide additional insight into the diurnal signature in satellite detection of biomass‐burning activities. The expansion of the regions of burning are readily detected in a comparison of these 4 years. From 1983 to 1991 the amount of burning detected by the GOES VAS ABBA during these 2‐week periods nearly doubled in the selva and mixed regions and tripled in the cerrado. Diurnal analyses confirmed earlier results indicating that the optimum time to monitor biomass burning is around 1530 UTC.