NO2 Emissions from AgriculturalBurning in Sao Paulo, BrazilC L I V E O P P E N H E I M E R , * , †

V I T C H K O I . T S A N E V , †

A N D R E W G . A L L E N , ‡

A N D R E W J . S . M C G O N I G L E , †

A R N A L D O A . C A R D O S O , §

A N T O N Y W I A T R , ‡

W I L L I A N P A T E R L I N I , § A N DC R I S T I N E D E M E L L O D I A S §

Department of Geography, University of Cambridge,Downing Place, Cambridge, CB2 3EN, U.K.,School of Geography, Earth and Environmental Sciences,University of Birmingham, Edgbaston,Birmingham, B15 2TT, U.K., and Departamento de QuımicaAnalıtica, Instituto de Quımica, Universidade EstadualPaulista (UNESP), CP 355, CEP 14800-900,Araraquara, Sao Paulo, Brazil

We report here on the application of a compact ultravioletspectrometer to measurement of NO2 emissions fromsugar cane field burns in Sao Paulo, Brazil. The time-resolved NO2 emission from a 10 ha plot peaked at about240 g (NO2) s-1, and amounted to a total yield ofapproximately 50 kg of N, or about 0.5 g (N) m-2. Emissionof N as NOx (i.e., NO + NO2) was estimated at 2.5 g (N)m-2, equivalent to 30% of applied fertilizer nitrogen. Thecorresponding annual emission of NOx nitrogen from SaoPaulo State sugar cane burning was >45 Gg N. Incontrast to mechanized harvesting, which does not requireprior burning of the crop, manual harvesting with burningacts to recycle nitrogen into surface soils and ecosystems.

IntroductionNitrogen oxides (NOx ) NO + NO2) play crucial roles intropospheric chemistry, terrestrial ecosystems, and theEarth’s radiation budget. They are key species regulatingozone and hydroxyl radical production, and, in many regions,deposition of nitrogen compounds affects surface acidityand nutrient status (1-4). From the global climate perspec-tive, nitrate-containing aerosols act as cloud condensationnuclei, inducing direct and indirect negative radiative forcing(5).

NO2 is the main component of NOx in the planetaryboundary layer. Biomass burning is an important source ofNOx, accounting for some 15% of the total global annual NOx

source; recent estimates for global NOx output from biomassburning are in the region of 6-8 Tg (N) yr-1 (e.g., 6). Thissource is characterized by seasonal increases in NOx columnsover several regions including South America (between Juneand October), which provides a major fraction of the globaltotal NOx (estimated NOx emission from all sources of 36-38Tg (N) yr-1; 6). Current tropospheric chemistry models forglobal NOx distribution show an encouraging correspondence

with satellite observations, but the models are limited whereinformation on emission factors and quantity of biomassburned are poorly constrained (e.g., 7). The aim of this workis to demonstrate a simple approach to measurement of NO2

fluxes from biomass burning based on open-path ultraviolet(UV) differential optical absorption spectroscopy (DOAS).Long-path DOAS measurements of NO2 concentrations havepreviously been made using artificial light sources (e.g., 8)but we believe our observations are the first to constrainfluxes using skylight as UV source. This approach permitsestimation of emission rates and represents a more straight-forward experimental arrangement. Our field tests werecarried out during agricultural burns in Sao Paulo State, Brazil,where there is much debate concerning the relative meritsof sugar cane harvesting using either manual cutting, whichnecessarily requires prior burning of the crop, or mecha-nization, which does not.

The burning of agricultural residues is a dominant sourceof atmospheric pollution in rural regions of Sao Paulo duringthe dry season (typically May to October). Sugar cane is themain agricultural crop affected; pre-harvest burning of thecane reduces trash biomass by 80-90%, and increases theefficiency of labor by 30% (9). Burning is applied widely andthis greatly increases tropospheric concentrations of par-ticulate matter, CO, O3, and other trace gases, though touncertain levels (10, 11). Brazil is the largest sugar caneexporter, followed by India and Australia, and responsiblefor 25% of the global 19.5 × 106 ha harvest. About half (52%)of Brazil’s harvest comes from Sao Paulo, making the statea globally significant agricultural biomass burning emissionssource. The measurements reported here were made onSeptember 4 and 5, 2003 (Figure 1) near the city of Araraquarain an intensive sugar cane growing region. Further measure-ments had been anticipated but were canceled due to heavyrain.

Experimental SectionNO2 column measurements were made using two small UVspectrometers (Ocean Optics S2000 and USB2000), eachcoupled across a 50-µm entrance slit by quartz fiber opticbundle to a simple telescope, constructed with two quartzlenses, viewing the zenith sky (20 mrad field of view; 12). Thepurpose of using the two instruments was primarily tocompare their performance. Both use UV holographicgratings (2400 grooves mm-1) housed in a folded Czerny-Turner optical bench, and provide a nominal spectralresolution of 0.5 nm fwhm. The S2000 operated across thespectral range 253-404 nm, and the USB2000 operated across228-379 nm. The S2000 instrument suffered from temper-ature-drift of the electronic offset noise, and so was housedin a sealed box and held at 10 ( 0.3 °C using a Peltier cooler.The spectrometers were powered via USB connection tolaptop PCs that ran software for data collection (Ocean OpticsOOIbase32 for the S2000, and DOASIS version 2.7.1.9, http://crusoe.iup.uni-heidelberg.de/urmel/doasis/download/, forthe USB2000). Typically, two zenith sky spectra were co-added. The exposure times for the USB2000 and S2000 were575 and 1500-3000 ms, respectively. The overall time-stepfor the USB2000 data collection was approximately 2 s,allowing for on-line retrieval of gas column amounts. TheS2000 time-step was about 5 s. (A much higher samplingrate, of order 100 ms, would be readily achievable with theUSB2000 spectrometer toward solar noon.)

Transverse profiles of the plume NO2 column wereobtained by driving along tracks at the perimeter of the fieldsbeing burnt (Figure 2), with the telescope viewing the zenith

* Corresponding author phone: +44 1223 339 382; fax: +44 1223333 392; e-mail: co200@cam.ac.uk.

† University of Cambridge.‡ University of Birmingham.§ Universidade Estadual Paulista (UNESP).

Environ. Sci. Technol. 2004, 38, 4557-4561

10.1021/es0496219 CCC: $27.50 2004 American Chemical Society VOL. 38, NO. 17, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 4557Published on Web 07/30/2004

sky. Background (without plume) spectra were collected atthe limits of each traverse, along with “dark” spectra (nolight admitted to the spectrometer). The position of each UVspectrum was determined from the log of a continuouslyrecording GPS unit (1 Hz data rate).

NO2 columns were obtained using scripts running underDOASIS and following the procedures outlined in refs 12-15, as follows. (i) Subtraction of the dark spectrum from allmeasured spectra, including the background (out-of-plume)spectrum. (ii) Normalization of all spectra, including thoserecorded within the plume, by the background spectrum.This corrects for the influence of the sky spectrum, e.g.,Fraunhofer lines and their modification by the Ring effect,any background NO2, and instrumental noise. (iii) Removalof the low-frequency component of the spectrum by abinomial high-pass filter. (iv) Calculation of the logarithm ofeach ratiosthis is the “pure” absorbance of the plume. (v)Smoothing of the resulting absorbance spectrum by abinomial low-pass filter. (vi) Fitting the resulting spectrumby the reference spectrum using a nonlinear least-squaresprocedure and evaluating the NO2 column amount. Theoptimal fitting windows, found by obtaining a near randomfit residual with minimal standard deviation, were 342-389nm (700 detector pixels) for the USB2000 spectrometer and327-364 nm (550 detector pixels) for the S2000 spectrometer.The deviation between results obtained by the optimalwindow and those by other windows is about 15% in theabsence of plume and varies between 10 and 25% within theplume. The reference spectrum for each spectrometer wasobtained by convolving a 0.01 nm resolution NO2 spectrum

from Vandaele et al. (16) with the instrument line shape andthen removing the low-frequency component. (vii) The NO2

column cross-sections were projected onto the plane per-pendicular to the plume axis and then integrated across theplume. This integrated column cross-section was multipliedby horizontal plume speed to yield mass flux of the gas. Wealso attempted to measure SO2 emissions but there wasinsufficient source UV at the shorter wavelengths of itsabsorption fingerprint by late afternoon (around 17:00 h localtime) when the burns were underway.

Retrieval precision can be gauged by analysis of fluctua-tions around the “zero” level outside the plume, which resultprimarily from variations in the background radiance. Wecollected 20-min sets of background (out-of-plume) spectrapointing the telescopes to the clear sky (Figure 3). On 4September 2003, Cumulus mediocris were present near thehorizon but the zenith sky was cloud-free. In this case thebackground approximates Gaussian noise with almost zeromean value and a standard deviation of 4.7 ppm m (Figure3a). This value corresponds to the standard deviation of thefit residuals. On the following day, the sky was covered by4/10 fair weather Cumulus humilis and the background signalreveals a weak increase from 9 to 14 ppm m for 20-min timeinterval. In this case the corresponding histogram (Figure3b) is asymmetric with a positive skewness and the standarddeviation does not coincide with that of the residuals.

Fields were burned by igniting first the leeward edge ofthe plot, and then the upwind perimeter. This results in alinear fire-front that migrates rapidly downwind. The areaof plots being burned was straightforwardly obtained fromGPS survey of the field perimeter using the system of tracksaround each field (Figure 2). Plot sizes ranged between about10 and 25 ha. We focus on the best of the datasets obtained,which relates to a single 10 ha plot.

ResultsA simultaneous record of the NO2 column amount obtainedby both USB2000 and S2000 spectrometers on September 4,2003 is presented in Figure 4. The data represent eighttraverses beneath a young smoke plume, whose maximumwidth reached about 300 m. The plume speed, estimatedfrom time-series photographs, was 5 ( 1 m s-1 and its azimuthwas 270 ( 20° (the traverse route was almost perpendicularto the plume transport direction). Combined errors in fluxmeasurements amount to around 25%. The first seven NO2

profiles are clear, but the fire had nearly burned out by thelast traverse and there is a limited NO2 signal in the waningplume. The first two traverses probably reflect emissions fromthe initial burn along the leeward perimeter of the plot. Thenext five show the rapid development of a firestorm alongthe upwind edge of the field and its subsequent decay. NO2

emission peaked at around 240 g s-1 during traverse 3, thoughit may have exceeded this between traverses. Taking the peakNO2 column amounts observed (50-100 ppm m) andassuming a vertical thickness of the plume of order 50 m,mean in-plume NO2 concentrations were up to 1-2 ppmv.

Despite the different temporal resolution of data collectionby the two instruments, and their differences in performance,the correspondence of NO2 column amounts is very closeand provides confidence in the fitting procedure and theDOAS approach in general. The retrieved NO2 columnamounts and their corresponding geographical coordinatescan also be used to represent spatio-temporal variations ofthe plume (Figure 5).

Figure 6 shows NO2 fluxes obtained using the data fromthe traverses. Equivalent N yields are also shown. Asmentioned, the area of the plot being burned in this case was10 ha. This field produced approximately 50 kg of N as NO2,equivalent to 0.5 g (N) m-2. Based on this emission rate,given that 75% (1.8 × 106 ha) of the total area of sugar cane

FIGURE 1. Photograph of 10 ha plot being burned on September 4,2003. The dashed line indicates the track used for the traverse.

FIGURE 2. Map of the spectroscopic traverses made on September4 and 5, 2003. The first track performed at 17:04:15-17:08:27 (localtime) on September 4, 2003 is highlighted. Altitudes above sea levelfor selected positions are shown in meters. The plot burned onSeptember 5 is also shown.

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in Sao Paulo state (about 2.4 × 106 ha) is burned, this wouldproduce on the oder of 9 Gg of N as NO2. The total Brazilianemission of NO2-N from sugar cane burning is then ∼20 GgN (partial mechanization of the harvest only occurs in SaoPaulo state; manual harvest with burning is the normthroughout the other sugar cane producing states). Averagedry weight of trash leaves burned was 1.3 kg m-2, so that∼0.4 g of NO2-N was released per kg of dry fuel biomass.

DiscussionTemperatures during biomass fires peak at around 1800 Kduring flaming combustion, which is lower than the tem-peratures required for any significant thermal formation ofnitrogen oxides. During biomass burning NOx derivesoverwhelmingly from nitrogen in the fuel (17), as evidencedby strong correlation between emission ratios NOx/CO2 andfuel N/C ratios (18). From an economic perspective, nitrogencontent of the cane can be considered to derive largely fromfertilizer nitrogen, which is applied to fields at a rate of 75-90 kg (N) ha-1 during the first year of growth, and at 70-80kg (N) ha-1 during subsequent years (plants are typicallyrenewed every five years). Around 6-7% of nitrogen appliedis therefore released annually as NO2 during the burns. Thisis a small fraction of the total nitrogen emitted, since relativeabundances of nitric oxide and nitrogen dioxide, NO:NO2,in young plumes from burning light vegetation have beenfound to be 3-5 (19, 20) or as high as 9 (21). Lobert andWarnatz (17) suggest an overall NO:NO2 ratio in emissionsduring biomass fires of 85:15. Assuming a (low) NO:NO2 molarratio of 4, emission of N as NOx (i.e., NO + NO2) is around2.5 g (N) m-2, or 30% of applied fertilizer nitrogen. This islikely to be a conservative estimate and further work will beneeded to quantify the NO contribution more accurately.

In addition to NO2 and NO, the other NOy componentlikely to be present in measurable quantities is nitric acid(HNO3). HNO3 formation appears to be favored during

FIGURE 3. Histograms of the background (out-of-plume zenith sky) NO2 retrieval for (a) September 4, 2003 and (b) September 5, 2003. Thecurves represent the best fit by a Gaussian function. Note that we report column amounts in ppm m units. These can be converted tomolecules cm-2 for the experimental conditions (air temperature 25 °C and pressure 960 mbar) by multiplying by 2.34 × 1015 (27).

FIGURE 4. Simultaneous record of the NO2 column amount obtainedby the S2000 (thin line) and USB2000 (thick line) on September 4,2003. Measurements were made along a track adjacent to theleeward perimeter of the plot (Figure 2). The 7 traverses are indicated.To convert ppm m units to molecules cm-2 multiply by 2.34 × 1015.

FIGURE 5. Time-distance plot of NO2 columns (cylindricalequidistant projection) obtained on September 4, 2003. Interpolationwas performed using triangulation with linear approximationgridding. The thick lines indicate the traverses. To convert ppm munits to molecules cm-2 multiply by 2.34 × 1015.

FIGURE 6. N flux (as NO2) vs time (squares), and cumulative Nemission (circles) for the burn recorded in Figures 4 and 5. Theerror bars represent the standard deviations evaluated accountingfor variations caused by wind azimuths 270 ( 20° and plume transportspeed (horizontal component) 5 ( 1 m s-1. The abscissa valuescorrespond to the time at the halfway point of each traverse.

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flaming combustion (22), however HNO3 is a minor com-ponent relative to NOx, with ratios of N relative to fuel N of12.7% (NOx) and 1% (HNO3) (23). Other N-containing plumeconstituents, including NH3, N2O, HCN, and CH3CN, maycontribute further nitrogen to the plume but are notconsidered here. Peroxyacetyl nitrate (PAN) may be produceddownwind in the plume but is thermally unstable at highcombustion temperatures. Akeredolu and Isichei (24) re-ported molar emission ratios, relative to CO2, from burningsavannah vegetation as follows: (NOx) 1.6 × 10-3, (NH3) 1.0× 10-3, (HCN and CH3CN) 6.0 × 10-4, and (N2O) 5.0 × 10-5.To provide further perspective, our estimated annual emis-sion of nitrogen as NOx from Sao Paulo state sugar caneburning, at >45 Gg N (or >98 Gg N from all Brazilian sugarcane burning), is considerably higher than the annual totalreported from Nigerian savannah burns (∼20 Gg N as NOx,assuming [NO]/[NO2] of 4; 25). Our estimates are in agree-ment with the total nitrogen loss during burning of 40 kg Nha-1 reported by Urquiaga et al. (26).

Concentrations of NO2 in the lower troposphere dependon several reaction schemes controlling its production andloss. NO2 may react directly with the hydroxyl radical,producing nitric acid, giving an NO2 lifetime of ∼1 day (27).

However, this reaction is expected to be insignificantwithin the near-source plume, due to rapid OH depletion byreaction with many plume components. Furthermore OH israrely formed at night, when most burning normally takesplace. Reaction of NO and NO2 with ozone leads, respectively,to production of NO2 and NO3.

NO3 net production is negligible during daytime, due torapid photodissociation (of NO3), however it leads to forma-tion of N2O5 and nitric acid at night.

Reaction of NO with ozone entrained in the plume fromthe surrounding air mass (reaction 2) allows continuedformation of NO2 during downwind transport, and thereforerepresents a potential secondary source of NO2, additionalto primary emissions, contributing to our measured con-centrations. Direct ground level measurements of NO2 duringthe burns, using triethanolamine-treated active samplers,showed that NO2 concentrations were in the range 500-1000 ppbv, consistent with the spectroscopic results, andindicating that expected NO levels would be of the order ofseveral ppmv. NO2 production via reaction [2] was thereforelimited by ozone availability. Ozone concentrations acrossSao Paulo state during September were 20-80 ppbv (28).Assuming an upper limit ozone concentration of 80 ppbv,and complete depletion of ozone following mixing with theplume, reaction 2 could account for 8-16% of the NO2

measured.The environmental and economic implications of these

findings are considerable. The oxidized nitrogen releasedduring burning of the cane crop will be re-deposited to thesurface via combined wet and dry deposition in variouschemical forms (as HNO3 and NH3 in the gas phase, ordissolved in precipitation water, and particulate nitrate andammonium) during regional scale transport. Takegawa et al.(29) measured a photochemical lifetime for NOx in tropo-

spheric biomass burning plumes of 0.1-0.3 days, with themain sink being deposition of nitric acid to the surface. Re-deposited nitrogen may therefore be rapidly available as plantnutrient to sugar cane or other crops, as well as contributeto acidification of sensitive natural ecosystems or eutrophi-cation of rivers and other water bodies (for this reason, currentgoals for reduction of anthropogenic nitrogen inputs tofreshwaters may not be achieved). This contrasts with thesituation during mechanized harvest (which is used for ∼20-30% of the total area under cane), where burning isunnecessary and nitrogen is therefore not “recycled” intothe local and/or regional surface environments (except wherecane bagasse is used as power generation fuel, with con-comitant emissions of nitrogen oxides, or where mulchingis used in land management).

The methodology we have reported here could beextrapolated to look at larger-scale burns by airborne survey.Such measurements could be of use for comparison withsatellite-based retrievals of NO2 emissions (e.g., 30) and invalidation of numerical models for young biomass burningplumes (e.g., 31-33). It is feasible to detect and measureadditional species, including SO2, O3, OH, and HCHO,depending on the characteristics of the ultraviolet source.

In future work we hope to evaluate the conversion rateof NO to NO2 by making NO2 measurements at variabledistance from source. The NO2 column cross-sections oughtto be seen to increase as long as there is substantial NOremaining in the plume, depending on the extent ofhorizontal dispersion. The emission rates of other environ-mentally or economically relevant species, including sulfurand phosphorus-containing compounds as well as traceelements, will also be measured.

AcknowledgmentsWe gratefully acknowledge FUNDUNESP and PROPP UNESPfor support of the fieldwork. W.P. is supported by FAPESPand C.M.D is supported by CNPq. V.I.T. is supported by theEC Framework 5 project DORSIVA, and A.J.S.M is supportedby the U.K. Natural Environment Research Council. We thankthe manager, Narciso Zanin, and staff of Fazenda Alabamafor approving and coordinating the pyrotechnics. We arevery grateful to the anonymous referees and Associate EditorRussell for their beneficial comments on the originalmanuscript.

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Received for review March 10, 2004. Revised manuscriptreceived June 20, 2004. Accepted June 22, 2004.

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