Emissions from Agricultural Burning in São Paulo, Brazil - American

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

NO2 Emissions from Agricultural Burning in Sa˜o Paulo, Brazil C L I V E O P P E N H E I M E R , * ,† VITCHKO I. TSANEV,† ANDREW G. ALLEN,‡ ANDREW J. S. MCGONIGLE,† ARNALDO A. CARDOSO,§ ANTONY WIATR,‡ WILLIAN PATERLINI,§ AND CRISTINE DE MELLO DIAS§ 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 Quı´mica Analı´tica, Instituto de Quı´mica, Universidade Estadual Paulista (UNESP), CP 355, CEP 14800-900, Araraquara, Sa˜o Paulo, Brazil

We report here on the application of a compact ultraviolet spectrometer to measurement of NO2 emissions from sugar cane field burns in Sa˜o Paulo, Brazil. The timeresolved NO2 emission from a 10 ha plot peaked at about 240 g (NO2) s-1, and amounted to a total yield of approximately 50 kg of N, or about 0.5 g (N) m-2. Emission of N as NOx (i.e., NO + NO2) was estimated at 2.5 g (N) m-2, equivalent to 30% of applied fertilizer nitrogen. The corresponding annual emission of NOx nitrogen from Sa˜o Paulo State sugar cane burning was >45 Gg N. In contrast to mechanized harvesting, which does not require prior burning of the crop, manual harvesting with burning acts to recycle nitrogen into surface soils and ecosystems.

Introduction Nitrogen oxides (NOx ) NO + NO2) play crucial roles in tropospheric chemistry, terrestrial ecosystems, and the Earth’s radiation budget. They are key species regulating ozone and hydroxyl radical production, and, in many regions, deposition of nitrogen compounds affects surface acidity and nutrient status (1-4). From the global climate perspective, nitrate-containing aerosols act as cloud condensation nuclei, inducing direct and indirect negative radiative forcing (5). NO2 is the main component of NOx in the planetary boundary layer. Biomass burning is an important source of NOx, accounting for some 15% of the total global annual NOx source; recent estimates for global NOx output from biomass burning are in the region of 6-8 Tg (N) yr-1 (e.g., 6). This source is characterized by seasonal increases in NOx columns over several regions including South America (between June and October), which provides a major fraction of the global total NOx (estimated NOx emission from all sources of 36-38 Tg (N) yr-1; 6). Current tropospheric chemistry models for global NOx distribution show an encouraging correspondence * Corresponding author phone: +44 1223 339 382; fax: +44 1223 333 392; e-mail: [email protected]. † University of Cambridge. ‡ University of Birmingham. § Universidade Estadual Paulista (UNESP). 10.1021/es0496219 CCC: $27.50 Published on Web 07/30/2004

 2004 American Chemical Society

with satellite observations, but the models are limited where information on emission factors and quantity of biomass burned are poorly constrained (e.g., 7). The aim of this work is 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 have previously been made using artificial light sources (e.g., 8) but we believe our observations are the first to constrain fluxes using skylight as UV source. This approach permits estimation of emission rates and represents a more straightforward experimental arrangement. Our field tests were carried out during agricultural burns in Sa˜o Paulo State, Brazil, where there is much debate concerning the relative merits of sugar cane harvesting using either manual cutting, which necessarily requires prior burning of the crop, or mechanization, which does not. The burning of agricultural residues is a dominant source of atmospheric pollution in rural regions of Sa˜o Paulo during the dry season (typically May to October). Sugar cane is the main agricultural crop affected; pre-harvest burning of the cane reduces trash biomass by 80-90%, and increases the efficiency of labor by 30% (9). Burning is applied widely and this greatly increases tropospheric concentrations of particulate matter, CO, O3, and other trace gases, though to uncertain levels (10, 11). Brazil is the largest sugar cane exporter, followed by India and Australia, and responsible for 25% of the global 19.5 × 106 ha harvest. About half (52%) of Brazil’s harvest comes from Sa˜o Paulo, making the state a globally significant agricultural biomass burning emissions source. The measurements reported here were made on September 4 and 5, 2003 (Figure 1) near the city of Araraquara in an intensive sugar cane growing region. Further measurements had been anticipated but were canceled due to heavy rain.

Experimental Section NO2 column measurements were made using two small UV spectrometers (Ocean Optics S2000 and USB2000), each coupled across a 50-µm entrance slit by quartz fiber optic bundle to a simple telescope, constructed with two quartz lenses, viewing the zenith sky (20 mrad field of view; 12). The purpose of using the two instruments was primarily to compare their performance. Both use UV holographic gratings (2400 grooves mm-1) housed in a folded CzernyTurner optical bench, and provide a nominal spectral resolution of 0.5 nm fwhm. The S2000 operated across the spectral range 253-404 nm, and the USB2000 operated across 228-379 nm. The S2000 instrument suffered from temperature-drift of the electronic offset noise, and so was housed in a sealed box and held at 10 ( 0.3 °C using a Peltier cooler. The spectrometers were powered via USB connection to laptop PCs that ran software for data collection (Ocean Optics OOIbase32 for the S2000, and DOASIS version 2.7.1.9, http:// crusoe.iup.uni-heidelberg.de/urmel/doasis/download/, for the USB2000). Typically, two zenith sky spectra were coadded. The exposure times for the USB2000 and S2000 were 575 and 1500-3000 ms, respectively. The overall time-step for the USB2000 data collection was approximately 2 s, allowing for on-line retrieval of gas column amounts. The S2000 time-step was about 5 s. (A much higher sampling rate, of order 100 ms, would be readily achievable with the USB2000 spectrometer toward solar noon.) Transverse profiles of the plume NO2 column were obtained by driving along tracks at the perimeter of the fields being burnt (Figure 2), with the telescope viewing the zenith VOL. 38, NO. 17, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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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 September 4 and 5, 2003. The first track performed at 17:04:15-17:08:27 (local time) on September 4, 2003 is highlighted. Altitudes above sea level for selected positions are shown in meters. The plot burned on September 5 is also shown. sky. Background (without plume) spectra were collected at the limits of each traverse, along with “dark” spectra (no light admitted to the spectrometer). The position of each UV spectrum was determined from the log of a continuously recording GPS unit (1 Hz data rate). NO2 columns were obtained using scripts running under DOASIS and following the procedures outlined in refs 1215, as follows. (i) Subtraction of the dark spectrum from all measured spectra, including the background (out-of-plume) spectrum. (ii) Normalization of all spectra, including those recorded 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) Removal of the low-frequency component of the spectrum by a binomial high-pass filter. (iv) Calculation of the logarithm of each ratiosthis is the “pure” absorbance of the plume. (v) Smoothing of the resulting absorbance spectrum by a binomial low-pass filter. (vi) Fitting the resulting spectrum by the reference spectrum using a nonlinear least-squares procedure and evaluating the NO2 column amount. The optimal fitting windows, found by obtaining a near random fit residual with minimal standard deviation, were 342-389 nm (700 detector pixels) for the USB2000 spectrometer and 327-364 nm (550 detector pixels) for the S2000 spectrometer. The deviation between results obtained by the optimal window and those by other windows is about 15% in the absence of plume and varies between 10 and 25% within the plume. The reference spectrum for each spectrometer was obtained by convolving a 0.01 nm resolution NO2 spectrum 4558

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from Vandaele et al. (16) with the instrument line shape and then removing the low-frequency component. (vii) The NO2 column cross-sections were projected onto the plane perpendicular to the plume axis and then integrated across the plume. This integrated column cross-section was multiplied by horizontal plume speed to yield mass flux of the gas. We also attempted to measure SO2 emissions but there was insufficient source UV at the shorter wavelengths of its absorption fingerprint by late afternoon (around 17:00 h local time) when the burns were underway. Retrieval precision can be gauged by analysis of fluctuations around the “zero” level outside the plume, which result primarily from variations in the background radiance. We collected 20-min sets of background (out-of-plume) spectra pointing the telescopes to the clear sky (Figure 3). On 4 September 2003, Cumulus mediocris were present near the horizon but the zenith sky was cloud-free. In this case the background approximates Gaussian noise with almost zero mean value and a standard deviation of 4.7 ppm m (Figure 3a). This value corresponds to the standard deviation of the fit residuals. On the following day, the sky was covered by 4/10 fair weather Cumulus humilis and the background signal reveals a weak increase from 9 to 14 ppm m for 20-min time interval. In this case the corresponding histogram (Figure 3b) is asymmetric with a positive skewness and the standard deviation does not coincide with that of the residuals. Fields were burned by igniting first the leeward edge of the plot, and then the upwind perimeter. This results in a linear fire-front that migrates rapidly downwind. The area of plots being burned was straightforwardly obtained from GPS survey of the field perimeter using the system of tracks around each field (Figure 2). Plot sizes ranged between about 10 and 25 ha. We focus on the best of the datasets obtained, which relates to a single 10 ha plot.

Results A simultaneous record of the NO2 column amount obtained by both USB2000 and S2000 spectrometers on September 4, 2003 is presented in Figure 4. The data represent eight traverses beneath a young smoke plume, whose maximum width reached about 300 m. The plume speed, estimated from time-series photographs, was 5 ( 1 m s-1 and its azimuth was 270 ( 20° (the traverse route was almost perpendicular to the plume transport direction). Combined errors in flux measurements amount to around 25%. The first seven NO2 profiles are clear, but the fire had nearly burned out by the last traverse and there is a limited NO2 signal in the waning plume. The first two traverses probably reflect emissions from the initial burn along the leeward perimeter of the plot. The next five show the rapid development of a firestorm along the upwind edge of the field and its subsequent decay. NO2 emission peaked at around 240 g s-1 during traverse 3, though it may have exceeded this between traverses. Taking the peak NO2 column amounts observed (50-100 ppm m) and assuming 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 collection by the two instruments, and their differences in performance, the correspondence of NO2 column amounts is very close and provides confidence in the fitting procedure and the DOAS approach in general. The retrieved NO2 column amounts and their corresponding geographical coordinates can also be used to represent spatio-temporal variations of the plume (Figure 5). Figure 6 shows NO2 fluxes obtained using the data from the traverses. Equivalent N yields are also shown. As mentioned, the area of the plot being burned in this case was 10 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 3. Histograms of the background (out-of-plume zenith sky) NO2 retrieval for (a) September 4, 2003 and (b) September 5, 2003. The curves represent the best fit by a Gaussian function. Note that we report column amounts in ppm m units. These can be converted to molecules 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 obtained by the S2000 (thin line) and USB2000 (thick line) on September 4, 2003. Measurements were made along a track adjacent to the leeward 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 6. N flux (as NO2) vs time (squares), and cumulative N emission (circles) for the burn recorded in Figures 4 and 5. The error bars represent the standard deviations evaluated accounting for variations caused by wind azimuths 270 ( 20° and plume transport speed (horizontal component) 5 ( 1 m s-1. The abscissa values correspond to the time at the halfway point of each traverse.

Discussion

FIGURE 5. Time-distance plot of NO2 columns (cylindrical equidistant projection) obtained on September 4, 2003. Interpolation was performed using triangulation with linear approximation gridding. The thick lines indicate the traverses. To convert ppm m units to molecules cm-2 multiply by 2.34 × 1015. in Sa˜o Paulo state (about 2.4 × 106 ha) is burned, this would produce on the oder of 9 Gg of N as NO2. The total Brazilian emission of NO2-N from sugar cane burning is then ∼20 Gg N (partial mechanization of the harvest only occurs in Sa˜o Paulo state; manual harvest with burning is the norm throughout the other sugar cane producing states). Average dry 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.

Temperatures during biomass fires peak at around 1800 K during flaming combustion, which is lower than the temperatures required for any significant thermal formation of nitrogen oxides. During biomass burning NOx derives overwhelmingly from nitrogen in the fuel (17), as evidenced by strong correlation between emission ratios NOx/CO2 and fuel N/C ratios (18). From an economic perspective, nitrogen content of the cane can be considered to derive largely from fertilizer nitrogen, which is applied to fields at a rate of 7590 kg (N) ha-1 during the first year of growth, and at 70-80 kg (N) ha-1 during subsequent years (plants are typically renewed every five years). Around 6-7% of nitrogen applied is therefore released annually as NO2 during the burns. This is a small fraction of the total nitrogen emitted, since relative abundances of nitric oxide and nitrogen dioxide, NO:NO2, in young plumes from burning light vegetation have been found to be 3-5 (19, 20) or as high as 9 (21). Lobert and Warnatz (17) suggest an overall NO:NO2 ratio in emissions during biomass fires of 85:15. Assuming a (low) NO:NO2 molar ratio of 4, emission of N as NOx (i.e., NO + NO2) is around 2.5 g (N) m-2, or 30% of applied fertilizer nitrogen. This is likely to be a conservative estimate and further work will be needed to quantify the NO contribution more accurately. In addition to NO2 and NO, the other NOy component likely to be present in measurable quantities is nitric acid (HNO3). HNO3 formation appears to be favored during VOL. 38, NO. 17, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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flaming combustion (22), however HNO3 is a minor component relative to NOx, with ratios of N relative to fuel N of 12.7% (NOx) and 1% (HNO3) (23). Other N-containing plume constituents, including NH3, N2O, HCN, and CH3CN, may contribute further nitrogen to the plume but are not considered here. Peroxyacetyl nitrate (PAN) may be produced downwind in the plume but is thermally unstable at high combustion temperatures. Akeredolu and Isichei (24) reported molar emission ratios, relative to CO2, from burning savannah 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 emission of nitrogen as NOx from Sa˜o Paulo state sugar cane burning, at >45 Gg N (or >98 Gg N from all Brazilian sugar cane burning), is considerably higher than the annual total reported from Nigerian savannah burns (∼20 Gg N as NOx, assuming [NO]/[NO2] of 4; 25). Our estimates are in agreement with the total nitrogen loss during burning of 40 kg N ha-1 reported by Urquiaga et al. (26). Concentrations of NO2 in the lower troposphere depend on several reaction schemes controlling its production and loss. NO2 may react directly with the hydroxyl radical, producing nitric acid, giving an NO2 lifetime of ∼1 day (27).

NO2 + OH + M f HNO3 + M

(1)

However, this reaction is expected to be insignificant within the near-source plume, due to rapid OH depletion by reaction with many plume components. Furthermore OH is rarely formed at night, when most burning normally takes place. Reaction of NO and NO2 with ozone leads, respectively, to production of NO2 and NO3.

NO + O3 f NO2 + O2

(2)

NO2 + O3 f NO3 + O2

(3)

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

NO3 + NO2 T N2O5

(4)

N2O5 + H2O f 2HNO3

(5)

Reaction of NO with ozone entrained in the plume from the surrounding air mass (reaction 2) allows continued formation of NO2 during downwind transport, and therefore represents a potential secondary source of NO2, additional to primary emissions, contributing to our measured concentrations. Direct ground level measurements of NO2 during the burns, using triethanolamine-treated active samplers, showed that NO2 concentrations were in the range 5001000 ppbv, consistent with the spectroscopic results, and indicating that expected NO levels would be of the order of several ppmv. NO2 production via reaction [2] was therefore limited by ozone availability. Ozone concentrations across Sa˜o 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 the plume, reaction 2 could account for 8-16% of the NO2 measured. The environmental and economic implications of these findings are considerable. The oxidized nitrogen released during burning of the cane crop will be re-deposited to the surface via combined wet and dry deposition in various chemical forms (as HNO3 and NH3 in the gas phase, or dissolved in precipitation water, and particulate nitrate and ammonium) during regional scale transport. Takegawa et al. (29) measured a photochemical lifetime for NOx in tropo4560

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spheric biomass burning plumes of 0.1-0.3 days, with the main sink being deposition of nitric acid to the surface. Redeposited nitrogen may therefore be rapidly available as plant nutrient to sugar cane or other crops, as well as contribute to acidification of sensitive natural ecosystems or eutrophication of rivers and other water bodies (for this reason, current goals for reduction of anthropogenic nitrogen inputs to freshwaters may not be achieved). This contrasts with the situation during mechanized harvest (which is used for ∼2030% of the total area under cane), where burning is unnecessary and nitrogen is therefore not “recycled” into the local and/or regional surface environments (except where cane bagasse is used as power generation fuel, with concomitant emissions of nitrogen oxides, or where mulching is used in land management). The methodology we have reported here could be extrapolated to look at larger-scale burns by airborne survey. Such measurements could be of use for comparison with satellite-based retrievals of NO2 emissions (e.g., 30) and in validation of numerical models for young biomass burning plumes (e.g., 31-33). It is feasible to detect and measure additional species, including SO2, O3, OH, and HCHO, depending on the characteristics of the ultraviolet source. In future work we hope to evaluate the conversion rate of NO to NO2 by making NO2 measurements at variable distance from source. The NO2 column cross-sections ought to be seen to increase as long as there is substantial NO remaining in the plume, depending on the extent of horizontal dispersion. The emission rates of other environmentally or economically relevant species, including sulfur and phosphorus-containing compounds as well as trace elements, will also be measured.

Acknowledgments We gratefully acknowledge FUNDUNESP and PROPP UNESP for support of the fieldwork. W.P. is supported by FAPESP and C.M.D is supported by CNPq. V.I.T. is supported by the EC Framework 5 project DORSIVA, and A.J.S.M is supported by the U.K. Natural Environment Research Council. We thank the manager, Narciso Zanin, and staff of Fazenda Alabama for approving and coordinating the pyrotechnics. We are very grateful to the anonymous referees and Associate Editor Russell for their beneficial comments on the original manuscript.

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Received for review March 10, 2004. Revised manuscript received June 20, 2004. Accepted June 22, 2004. ES0496219

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