Atmospheric Emission of Reactive Nitrogen during Biofuel Ethanol

Nov 30, 2007 - frequency, indicating that biomass burning is a major emission source, with mean ... biogenic) predominant during the wet season. Estim...
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Environ. Sci. Technol. 2008, 42, 381–385

Atmospheric Emission of Reactive Nitrogen during Biofuel Ethanol Production CRISTINE M. D. MACHADO, ARNALDO A. CARDOSO,* AND ANDREW G. ALLEN Analytical Chemistry Department, Chemistry Institute, São Paulo State University, CP 355, CEP 14800-900, Brazil

Received February 14, 2007. Revised manuscript received October 23, 2007. Accepted October 29, 2007.

This paper evaluates emissions to the atmosphere of biologically available nitrogen compounds in a region characterized by intensive sugar cane biofuel ethanol production. Large emissions of NH3 and NOx, as well as particulate nitrate and ammonium, occur at the harvest when the crop is burned, with the amount of nitrogen released equivalent to ∼35% of annual fertilizer-N application. Nitrogen oxides concentrations show a positive association with fire frequency, indicating that biomass burning is a major emission source, with mean concentrations of NOx doubling in the dry season relative to the wet season. During the dry season biomass burning is a source of NH3, with other sources (wastes, soil, biogenic) predominant during the wet season. Estimated NO2-N, NH3-N, NO3--N and NH4+-N emission fluxes from sugar cane burning in a planted area of ca. 2.2 × 106 ha are 11.0, 1.1, 0.2, and 1.2 Gg N yr-1, respectively.

1. Introduction The nitrogen oxides NO and NO2 (together denoted NOx), and ammonia (NH3), are considered to be the major reactive atmospheric nitrogen compounds (1), influencing photochemistry and secondary particulates formation (2). Nitrogen is an essential component of biological materials, and loss or gain of biologically available nitrogen can alter natural ecosystems, affecting species diversity, eutrophication, and changes in pH of surface waters (3–5). Anthropogenic combustion and soil emission are the largest sources of NOx, mainly as NO (6–8). Global NOx emissions from fossil fuel and biomass combustion are estimated at around 26 and 6 Tg N yr-1, respectively (9). Biomass burning can be a major source of NOx in tropical regions during some periods of the year (6, 9, 10). NOx emissions from soils are not well constrained, with estimates ranging from 9 to 21 Tg NO-N yr-1 (9, 11, 12), and agricultural practices perhaps responsible for around 40% of total soil NO emissions (9). In Brazil, the location of the present study, anthropogenic NOx emissions have almost doubled between 1995 and 2002, largely due to biological fixation in agricultural systems (13, 14), but with large geographical variations in soil NO fluxes (15, 16). Ammonia is emitted to the atmosphere from a variety of sources including soils, natural decomposition, agricultural practices (use of fertilizers, animal wastes) (17, 18), as well * Corresponding author tel: 5516 33016612; fax: 5516 33016692; e-mail: [email protected]. 10.1021/es070384u CCC: $40.75

Published on Web 11/30/2007

 2008 American Chemical Society

as during biomass burning (6, 19) and fossil fuel combustion (20–22). Emissions from biomass burning in the tropics have been estimated to account for ∼10% of the global total NH3 emission (19). The present work was undertaken in the ethanol fuel production agro-industrial region of São Paulo State, which accounts for ∼60% of total Brazilian sugar cane production (422 Mt yr-1 in 2005) (23). The sugar cane harvest occurs during the dry season, starting in April and ending in November, with manual (nonmechanized) cutting currently used on 75% of the planted area. For safety and to improve the efficiency of the manual process it is necessary to burn the excess foliage prior to cutting, which results in emissions to the atmosphere of very large quantities of gases and particulate matter (24–28). In the long term these emissions are likely to diminish, since current São Paulo State legislation requires reductions of 50% (by 2011) and 100% (by 2021) in open field burning for properties consisting of planted areas >150 ha and slope gradients e12%, with deadlines extended to 2026 and 2031 for properties where the burned area is 12% (29). Previous work by our research group (24–26) identified large seasonal differences in atmospheric concentrations of a number of trace gases and aerosol species, which we attributed to biomass burning and mechanical resuspension of particulates during the sugar cane harvest. Here we investigate the possibility that release of nitrogen in oxidized and reduced forms may also be increased, consequently influencing the amount of nitrogen biologically available in the environment.

2. Experimental Section To investigate seasonal differences in NOx and NH3 we made daily measurements, 1–5 times a month (Table 1) between July 2004 and May 2005, at a (mown) grassy field site on the outskirts of the municipality of Araraquara (21° 47′ 37′′ S, 48° 10′ 52′′ W, 646 m a.s.l.), in the center of the sugar cane growing region of São Paulo State, 270 km from the capital São Paulo. Around 40% of the surface area of the region is devoted to sugar cane production (30), and the nearest sugar cane plantations are ∼2 km distant. To collect NOx we used active samplers for consecutive periods of 3 h throughout the day, totalling 8 samples daily, with a timer, and at a flow rate of 0.8 L min-1. A shorter sampling period (90 min) was used for improved temporal resolution during a period of intense sugar cane burning, collecting 16 samples over a period of 24 h between 18:00 September 13, 2004 and 18:00 September 14. The sampling system comprised two C-18 silica cartridges impregnated with a solution containing 11% triethanolamine in combination with 3.6% ethyleneglycol and 25% acetone (31), and a converter containing CrO3 oxidant (32). The first C-18 cartridge collected any pre-existent NO2, with the sampled air then passing through the converter where NO was oxidized to NO2. This NO2 resulting from NO oxidation was collected on the second (downstream) C-18 cartridge. After sampling, the fixed compounds were solubilized and determined by spectrophotometry using the Griess-Saltzman reaction (33). The analytical method was calibrated using an NO2 permeation tube following the procedure previously described (31). Gaseous NH3 was collected using sequential filtration (34), where in the first stage particulate matter was removed by a 47 mm diameter PTFE filter (1 µm nominal pore size) and in the second stage gaseous NH3 was collected onto Whatman No. 41 cellulose filters impregnated with 5% oxalic acid. VOL. 42, NO. 2, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1.

Monthly Mean Values of NO, NO2, and NH3 Concentrations, SD in Parentheses, Total Monthly Precipitation, Maximum Temperature on Sampling Days, and Number of Fire Pixels Per Month in São Paulo State from June 2004 to May 2005 month June July August September October November December January February March April May a

sampling days 3 4 5 3 2 1 3 3 2 3 4 3

NO (ppbv)

NO2 (ppbv)

NH3 (ppbv)

fire pixels (no.)

precipitation (mm)

max. temp. (°C)

1.0 (0.4) 8.2 (15.3) 17.6 (19.9) 6.5(4.5) 3.2 (3.4) 1.1 (0.8) 2.5 (5.6) 0.4 (0.0) 2.0 (1.7) 1.4 (1.9) 1.1 (0.7) 1.7 (1.6)

29.8 (11.6) 33.5 (20.4) 50.1 (28.2) 36.2 (27.5) 15.8 (8.0) 11.3 (7.0) 14.5 (8.1) 8.3(3.6) 15.5 (9.0) 11.4 (6.2) 15.5 (8.4) 19.0 (10.1)

2.9 (1.1) 1.7 (0.7) 3.4 (2.4)a 2.9 (0.4)b 1.6 (0.4) 0.5 (0.0) 2.5 (0.8) 2.5 (0.9) 3.1 (0.9)a 2.1 (1.2) 1.9 (0.6)a 2.6 (0.7)

484 438 610 690 265 467 123 5 64 71 212 380

25 25 1 1 100 200 150 500 50 200 50 50

27 27.5 32.5 31.4 27.8 24.5 32.3 31.3 35.5 30 31 30

average concentration for 3 sampling days.

b

average concentration for 1 sampling day.

Sampling periods were of 6 h duration throughout the day, so that 4 samples were collected per day, using a timer, and with a flow rate of 12 L min-1. Deionized water was used to extract the filters, with analysis of extracts via spectrophotometry, using the Berthelot reaction, as soon as possible after extraction (35). NO2, NH3, and particulate nitrate (NO3-) and ammonium (NH4+) were collected on four occasions at three separate locations during the period September 3–5, 2003, ∼20 m downwind within the plumes of large sugar cane burns, during the early evening and under light wind conditions. Sampling periods were of a few minutes. The measurement and analytical techniques for the particulate phase species have been reported previously (25, 26). Solar radiation and wind field data were obtained using a micrometeorological station (Campbell Scientific Ltd., UK) installed at 25 m above ground level. Temperature and relative humidity were measured manually with an Extech Instruments sensor. Sugar cane burning frequency was obtained using data from the AVHRR (Advanced Very High Resolution Radiometer) sensor on board the NOAA-12 satellite, where detection is based on the emission of mid-infrared radiation from fires having dimension of >30 m2, with each 1 km pixel being identified as a fire pixel if it contained one or more fires at the time of the overpass (36). These satellite data were accessed via the Brazilian National Institute of Space Research website (37).

FIGURE 1. Relationships obtained between (a) NO2 concentrations and fire frequency, (b) NO concentrations and fire frequency, (c) NO and NO2 concentrations, and (d) NH3 concentrations and fire frequency during the dry season.

3. Results and Discussion

FIGURE 2. Nighttime and daytime concentrations of NO and NO2 during burning and nonburning periods.

3.1. Nitrogen Oxides. Monthly NO and NO2 concentrations, mean maximum temperature on sampling days, number of fire pixels and total monthly precipitation for the period June 2004 to May 2005 are presented in Table 1. Although the number of fires detected was reduced during the wet season, fires were detected at all times due to the widespread practices of trash and waste ground burning. During the rainy season, which began in November 2004 and ended in March 2005, mean concentrations were 12.2 ( 2.9 ppbv (NO2) and 1.4 ( 0.9 ppbv (NO), whereas during the dry season (the harvest period, from April to October) mean concentrations were 28.2 ( 13.2 and 5.3 ( 6.0 ppbv, respectively. Application of the unpaired Student’s t test showed that the mean concentrations of NO2 during the dry and wet seasons were significantly different (significance level of 0.05 and df ) 10), while mean concentrations of NO in each season were not significantly different. There was a positive correlation between monthly mean concentrations and fire frequency during the dry season (Figure 1), indicated by Pearson correlation coefficients (r) of 0.85 (NO2; n ) 7, p < 0.01) and 0.78 (NO; n ) 7, p < 0.05). A correlation coefficient of 0.88

(n ) 7, p < 0.005) for the relationship between NO and NO2 indicated the similar origin of these gases. Mean daytime (from 06:00 to 18:00) and nighttime (from 18:00 to 06:00) concentrations of NO and NO2 were also always higher during burning than nonburning periods (Figure 2). Mean concentrations were higher at night, except of NO during the nonburning period, although differences were not statistically significant (p > 0.1). Peak concentrations were observed during August 2004, when a large, dry air mass was stationary over southeastern Brazil (38), precipitation was negligible, and when 610 fires were detected across São Paulo State. During the rainy season NOx concentrations were not only lower but also more stable, suggesting a predominance of more constantly emitting sources (such as road transport) throughout this period. Despite large seasonal differences in precipitation amounts, there was negative correlation between NOx concentrations and relative humidity (RH) throughout the year, with values of the Pearson correlation coefficient of -0.72 (n ) 7, p < 0.1), -0.67 (n ) 7, p < 0.1), -0.49 (n ) 5, p > 0.1), and -0.81 (n ) 5, p < 0.1) for the

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FIGURE 3. NO2 concentrations and solar radiation intensity on a sunny day and on a cloudy day during June 2004.

FIGURE 4. NO and NO2 concentrations for the 24-h period beginning 18:00 September 13, 2004. relationships of RH with NO during dry and rainy seasons, and of RH with NO2 during dry and rainy seasons, respectively. These correlations are considered significant (with the exception of that obtained between dry season RH and NO2), and suggest that faster losses of NO and NO2 occurred at higher humidity. During daylight hours NO2 reacts with the hydroxyl radical (OH) forming gas-phase nitric acid (HNO3) (39). NO2 + OH + M f HNO3 + M

(1)

Concentrations of HNO3 in this region have been previously found to be much higher during the dry season than the rainy season (24), an observation which could not be explained solely by direct emissions of HNO3 during biomass burning, supporting a contribution from NOx oxidation. This mechanism appears to be the main factor controlling the NO2 lifetime during the daytime, with concentrations frequently showing an inverse relationship with radiation intensity. This is illustrated in Figure 3, which shows two sets of measurements in June 2004, one set collected on a sunny day (June 29) and the other on a rainless cloudy day (June 2). On both occasions the NO2 concentration decreased during the previous night, however during the daytime NO2 increased on the cloudy day and decreased on the sunny day. Later nighttime peaks were observed for NO and NO2 on September 23 and 24, 2004, during a period of intense burning when 23 fire pixels were detected in São Paulo State (37). NO2 concentrations showed a steady increase after 22:00 (Figure 4), which was followed by an increase of NO after 02:30 suggesting that, by that time, the ozone reservoir was nearing complete depletion, inhibiting further NO2 formation and preserving NO. Both NO2 and NO reached peak concentrations (of 56 and 11 ppbv, respectively) around 03: 30, by which time emissions from the various fires had dispersed into the regional lower troposphere, and then gradually declined following further dispersion and reaction, with NO reaching its diurnal minimum by 07:00 and NO2 showing diminishing concentrations during the hours of daylight following a more rapid decline (than NO) in the early morning.

These observations demonstrate that daytime photochemistry and ozone reactions at night contributed to the eventual removal of NOx, with HNO3 probably being the main oxidative product. Our earlier work has shown that HNO3 is readily scavenged onto coarse aerosols (25, 26). Since both HNO3 and large particles possess high dry deposition velocities, and hence have fairly short atmospheric lifetimes, this should limit long-range transport of biologically available nitrogen in these oxidized forms. 3.2. Ammonia. Atmospheric NH3 demonstrated a seasonal pattern different from that of NOx (Table 1) with overall average concentrations only slightly lower during the rainy season (2.1 ( 1.0 ppbv) than the dry season (2.4 ( 0.7 ppbv). The seasonal difference was not statistically significant (t test at 0.05 significance level). The measured concentrations are comparatively modest in global terms (18, 40). During the dry season NH3 concentrations were weakly correlated with fire frequency (r ) 0.53, p > 0.1) (Figure 1), indicating that sugar cane burning was a source of NH3 to the atmosphere. There are two possible explanations for the observation of similar seasonal concentrations. Emissions from nonburning sources (organic wastes/biogenic/soil) could be higher during the wet season, with the increase being roughly equivalent to the magnitude of the dry season biomass burning source. Emissions from nonburning sources are influenced by moisture levels and temperature, which are more variable during the wet season than the dry season. Although chemical fertilization of soils during the rainy season (usually January-March) augments available nitrogen, and under certain circumstances has been found to lead to increased emissions of NH3-N (1, 18, 40–43), soils in this region are naturally acidic which should limit ammonia emissions, with maximum soil pH of ∼5.5–6.0 only being obtained after applications of lime (CaCO3/MgCO3) at the time of planting. A more plausible explanation for the lack of seasonality is that ammonia emitted during burning reacts rapidly with acidic species within the plume, forming particulate phase ammonium (NH4+). Although NH3 concentrations showed little seasonality, previous work has shown that, in contrast to the gas phase, NH4+ concentrations in autumn and winter during the dry season are around twice those measured during the nonburning period (25, 26). Due to rapid removal by reaction, it was not possible to discern any significant diurnal trends in mean NH3 concentrations, during either season. Furthermore some of the particulate forms (ammonium nitrates, chlorides, and salts of organic acids) may be unstable, dissociating during the daytime due to higher temperature and lower humidity, releasing NH3 which had been formed during the nighttime burning. Reaction of NH3 with acidic sulfate aerosols (which are also present at elevated levels in the plumes) forms stable ammonium sulfates, which, together with dry and wet deposition, are final sinks for atmospheric ammonia. Such fine ammonium containing aerosols possess a long atmospheric lifetime (up to several days) and may be deposited far from the source, in contrast to gas phase NH3, of which a large fraction (20-40%) may be rapidly redeposited (17). As previously noted, the primary species required for production of fine aerosols are coemitted during biomass burning, and the presence of NH4+ within these particles capable of long-range transport suggests that biologically available reduced nitrogen is likely to be exported from the region. 3.3. Emission Fluxes from Sugar Cane Burning. Previous work by our research group (44) showed that, in 2003, ∼9 Gg N yr-1 of active nitrogen was emitted in the form of NO2 due to sugar cane burning in a planted area equivalent to 1.8 × 106 ha. The estimated total annual NOx (NO2 + NO) emission was >45 Gg N yr-1 (NH3 was not considered). Between 2003 and 2007 the planted area has increased by ∼22%, so that VOL. 42, NO. 2, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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the current estimated total annual emission fluxes from a burned area of 2.2 × 106 ha are ∼11 Gg N yr-1 of NO2-N and ∼55 Gg N yr-1 of NOx-N. Mean concentrations (n ) 4) obtained during simultaneous measurements of NO2, NH3, and the particulate forms NO3- and NH4+ in sugar cane burning plumes immediately downwind of fires were 624 µg NO2-N m-3, 63 µg NH3-N m-3, 11 µg NO3--N m-3, and 70 µg NH4+-N m-3. Annual sugar cane burning emissions of NH3, NO3-, and NH4+ may be estimated from the concentration ratios of these species with NO2, since the NO2 flux has been measured directly (44). The corresponding emission fluxes are 1.1 Gg NH3-N yr-1, 0.2 Gg NO3--N yr-1, and 1.2 Gg NH4+-N yr-1. The total current combined emission of the measured gas phase and particulate species is then ∼57 Gg N yr-1. These calculations assume that ratios of the different species to NO2 in emissions are spatially and temporally invariant. Further work would be required to confirm this, however we believe that our data are representative of typical conditions, since similar sugar cane production and burn techniques are used across São Paulo State. These emissions can be considered in the context of fertilizer usage for sugar cane production. Currently, at the time of first planting approximately 75–90 kg N ha-1 is used in the fertilizer mix, and during the subsequent five years (after which the plants are renewed) applications are in the region of 70–80 kg N ha-1. Around 2.25 × 105 ton N yr-1 (225 Gg N yr-1) is applied as fertilizer to a total planted area of ∼3.0 × 106 ha. Considering the burned area (∼2.2 × 106 ha), the quantity of nitrogen released to the atmosphere as [NOx-N + NH3-N + NO3--N + NH4+-N] during burning is equivalent to ∼35% of the annual fertilizer-N application to the plantations.

4. Acknowledgements We gratefully acknowledge financial support provided by the Brazilian National Council for Scientific and Technological Development (CNPq, processes 140931/2003-1 and 150547/2007-2), and the State of São Paulo Research Foundation (FAPESP, process 05/53001-8).

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