Carbon monoxide short term measurements at Amsterdam island: estimations of biomass burning emission rates

Chemosphere: Global Change Science 1 (1999) 163±172

Carbon monoxide short term measurements at Amsterdam island: estimations of biomass burning emission rates V. Gros a

a,1

, B. Bonsang

a * ,

D. Martin a, P.C. Novelli b, V. Kazan

a

LSCE Orme des Merisiers, B^ at.709, CE Saclay, 91 191 Gif sur Yvette Cedex, France b NOAA/CMDL, 325 Broadway, Boulder, CO 80303, USA Received 15 April 1998 ; accepted 15 September 1998

Importance of this Paper: Biomass burning is a major source of carbon monoxide in the southern hemisphere. We show that, for some speci®c short periods, the direct in¯uence from biomass burning on the CO level is observed at the remote marine station of Amsterdam Island. For the events observed in 1996 and 1997, we estimate the CO emission rates at the source from the CO and Radon-222 correlations. However, these events don't contribute signi®cantly to the CO monthly mean and it appears that biomass burning has a global hemispheric impact and therefore contributes to the background CO level observed in the southern hemisphere. Abstract A fully automated gas chromatograph instrument for atmospheric carbon monoxide (CO) measurements based upon mercuric oxide reduction/UV absorbance has been operating at Amsterdam Island (37°S, 77°E) in the Indian ocean since 1996. This technique allows measurements every 10 min and enables the survey of short term variations of CO. In parallel, Radon-222, a well known tracer of continental air masses, was simultaneously monitored and used for the understanding of the CO variations. The measurements undertaken since March 1996 allow to draw a seasonal variation of CO with a typical summer minimum of about 35 ppbv, and a spring maximum of the order of 60 ppbv. This seasonal cycle is coherent with the observations in other stations of the Southern Hemisphere, particularly Crozet Islands (46°S, 51°E), and Cape Grim (41°S, 145°E, Tasmania). It is consistent with both the seasonal variation of the main CO sink, (i.e. reaction with OH radicals) and of one main CO source since the monthly CO maximum is phased with the seasonality of biomass burning in Africa which mainly occurs from June to November. A statistical analysis of the short term variations of CO is made for the 1996 and 1997 data. Episodes of elevated CO are observed between June and October, and most (65%) are associated with events of high Radon-222. A careful examination of the relative amplitudes of CO and Radon-222 peaks backed up with results of the back trajectories

* 1

Corresponding author. E-mail. [email protected] (V. Gros). Present address: Max Planck Institute for Chemistry, Air Chemistry Division, Mainz, Germany.

1465-9972/99/$ ± see front matter Ó 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 4 6 5 - 9 9 7 2 ( 9 9 ) 0 0 0 0 9 - 4

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analysis is made for 22 particular events. The largest variations of CO correspond to rapid advection of continental air across the Indian Ocean from Southern Africa. On the basis of the ratio CO/Rn observed at Amsterdam Island for these 22 events, we estimate the CO ¯ux from biomass burning in the range of 0.4±3.3 mg cmÿ2 hÿ1 . Furthermore, it is pointed out that these events do not signi®cantly in¯uence the monthly means, and thus our measurements at Amsterdam Island can be considered as representative of the Southern Hemisphere CO background. Ó 1999 Elsevier Science Ltd. All rights reserved. Keywords: Carbon monoxide; Marine boundary layer; Biomass burning; Radon-222

1. Introduction The key role of carbon monoxide (CO) in the tropospheric chemistry has been recognized since 1970s, and its main sources and sinks are now identi®ed. Its main sources are known as fossils fuel combustion, oxidation of methane, biomass burning, oxidation of non-methane hydrocarbons (NMHCs) (Khalil and Rasmussen, 1990) and, to a lesser extent, oceanic emissions (Bates et al., 1995). The main identi®ed sink is the removal by OH radicals. Because of its average 2 months lifetime, (Khalil and Rasmussen, 1990), the distribution of CO is strongly dependent on both source distributions and transport processes. While the global distribution of CO is really better known than 10 yrs ago (Novelli et al., 1998), several uncertainties remain. The WMO (1994) meeting of experts on global CO measurements stated the major questions and identi®ed four priorities for better evaluation and integration of the global CO budget. Among them, the important role of the surface network allowing the measurement of the interannual and decade trends has been outlined. We took the opportunity of the privileged location of

Fig. 1. Amsterdam island, Crozet and Cape Grim locations in the Southern Hemisphere.

the French station of Amsterdam Island, which allows us to study a pure marine atmosphere during most of the year, and to observe some events of continental in¯uence from Africa during winter months (Miller et al., 1993; Ramonet and Monfray, 1996), to begin CO measurements. Fig. 1 presents the locations of Amsterdam Island, Crozet and Cape Grim (two another CO monitoring stations in the Southern Hemisphere). Amsterdam Island (37°S, 77°E) in the Indian Ocean, 4000 km far away from any continent, and the very low ship trac in this area (once every two months on average), provides a remote location for the in-situ measurement of CO and ozone. In this paper, we present the results of the two ®rst years of CO measurements that we analyze both in terms of short-term variations and seasonal cycle. Results obtained for ozone are described in Gros et al. (1998). 2. Experimental methods A GC instrument for atmospheric CO measurements has been adapted and set up at Amsterdam Island since March 1996. Measurements take place at ÔPointe BenedicteÕ (located 2.5 km windward of the local base) and are made halfway up on a 20 m high tower, 65 m above the sea level. Air is sampled through a 15 m Te¯on line which was previously ¯ushed with ambient air and conditioned during a few days. CO measurement is based on gas chromatography and detection by hot mercuric oxide reduction as described by Novelli et al. (1991). This system has been adapted in order to make automatic measurement at Amsterdam Island, where standard and air samples are alternately injected at ambient pressure with a

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full measurement cycle performed every 10 min. Details of the measurement procedure are in Gros et al. (this issue). 3. Results and discussion 3.1. Seasonal variations The measurements made at Amsterdam Island are presented in Fig. 2 in comparison with observations at Cape Grim (40.6°S, 144.68°E, Steele et al., 1996a), and Crozet (46°S, 51°E). Results for 1996 have been corrected of +25% and results for January and February 1997 have been corrected of +7%, this was due to the occurrence of a pressure drop in the sampling loop (see Gros et al., this issue). Therefore, they cannot be precisely compared with other measurements. In March 1997, this technical problem was solved and no further correction was applied. The seasonal variation of CO shows a typical minimum of 35 ppbv in February±April and a maximum of 60 ppbv in late winter-early spring (September±November). These results are in very good agreement with observations at Cape Grim and Crozet, where the amplitude of the seasonal cycle shows signi®cant interannual variations. The simultaneous maximum observed in the three locations in October is quite remarkable. Other measurements of CO in the Southern Hemisphere, mainly based on ¯ask

165

samples measurements, are sparse but all of them present a similar seasonal cycle (Khalil and Rasmussen, 1990; Brunke et al., 1990; Novelli et al., 1992, 1998). This cycle can be explained in terms of removal by OH and biomass burning source. Reaction with OH is the major sink for CO (Khalil and Rasmussen, 1990). OH radical concentrations expected to be lower in winter leads to an increase of CO during winter and early spring. OH reaches maximum concentrations in summer, and reaction of OH + CO is the main removal process for both molecules. The seasonal cycle of CO is also in¯uenced by its sources. In the Southern Hemisphere these are mainly marine and biomass burning emissions (Khalil and Rasmussen, 1990). Studies of air mass back trajectories ending at Amsterdam Island (Miller et al., 1993; Ramonet and Monfray, 1996) and Radon measurements at Amsterdam Island (Gros et al., 1999) showed that Southern Africa (including Madagascar) is the predominant non marine source of trace materials transported to Amsterdam Island. This long range transport mainly occurs between June and October, which corresponds to the period of intense biomass burning in Southern Africa (Cooke et al., 1996). CO is a major product of biomass burning (Crutzen et al., 1979), and its mixing ratio at Amsterdam Island could be in¯uenced by its emission over the African continent and Madagascar. One major concern is how does this continental in¯uence a€ect the CO measurements? Unfortunately, all measurements in the Southern Hemisphere, except those at Cape Point (Brunke et al., 1990) and Cape Grim (Steele et al., 1996b), are based on weekly ¯asks samples and do not allow the observation of the short term in¯uence of biomass burning on CO measurements. The recent measurements at Amsterdam Island allow the analysis of their short-term variations in combination with the Radon-222. 3.2. Short-term variations

Fig. 2. CO variations at Cape Grim, (after Steele et al., 1996a) Crozet and Amsterdam Island. The lines attached to the means are the within month variabilities (‹1 standard deviation).

Radon-222, a continental tracer, has been measured through the alpha activity of its daughters on a 2 h basis since 1960 at Amsterdam Island (Polian et al., 1986). It is emitted mainly over continents and has a half-life due to its radioactive

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decay of 3.825 days. The Rn-220 (ÔThoronÕ) halflife is only 54 s but its daughter, Lead-212, has a 10.6 h half-life and therefore can be also transported from close continental sources and simultaneously measured also as an alpha emitter. Simultaneous measurements of Radon-222 and Lead-212 allow us to distinguish between remote and local origin of an air mass. Air with Radon222 activity higher than 2 pCi/m3 but with the absence of Rn-220 (Lead-212) activity, is considered to be signi®cantly in¯uenced by the continent (Polian, personal communication). Except for some gaps in the data set due to technical problems, 10 min CO measurements are available for the whole period March 1996±December 1997. CO measurements were aggregated into hourly means and the di€erence between each value and the monthly mean, labelled as dCO, has

been calculated to provide indication of background variability. Monthly standard deviations of the dCO measurements have been calculated and average is 3.7 ppbv. We examined dCO events higher than 10 ppbv, about 3 times of the background. About 65% of these events were associated with Radon-222 greater than 2 pCi/m3 , indicating a recent (less than one week) continental in¯uence. Note that during these events, there was no measurable Radon-220 activity, excluding therefore any local in¯uence. Fig. 3 presents the distributions of these events (dCO > 10 ppbv with Radon-222 > 2 pCi/m3 ) according to months. It is worth noting that, except two events in 1997 (one in May and one in November), all the events are observed between June and October. This re¯ects both the seasonality of transport (enhancement of air masses coming from the north-west sector in winter and early spring, as described by Miller et al., 1993) and the seasonality of the source (biomass burning in Southern Africa, Cooke et al., 1996). Therefore most of the high CO events observed at Amsterdam Island are due to transport from CO mainly produced in biomass burning over Southern Africa and Madagascar. 3.3. Estimations of CO emissions An example of a time series for one such high CO event, in June 1996, is presented in Fig. 4 with the corresponding Radon-222 observations. Two clear CO events can be seen in this ®gure: one on 25th June and the other on 26th June, both presenting a good correlation with Radon-222 varia-

Fig. 3. Seasonal distribution of the events de®ned by dCO > 10 ppbv and Rn-222 > 2 pCi mÿ3 for 1996 and 1997.

Fig. 4. CO and Radon-222 variations during the last week of June 1996 and presenting two clear events of high CO on 25th and 26th June.

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tions. 5-day backward trajectories were calculated with the TM2Z model using the procedure described in Ramonet et al. (1996). These trajectories showed that corresponding air masses originated from the sector of Southern Africa. An example of trajectory, calculated on 25 June 1996 0h00 UT and corresponding to the ®rst event, is given in Fig. 5. Another time series is depicted in Fig. 6 and presents the largest CO variation observed during the whole period of study. Indeed, on 15th September, CO increased in 10 h from 50 ppbv to 110 ppbv. The Radon-222 variations and the corresponding back trajectory (Fig. 7) suggest this huge peak can be attributed to a fast transport of CO produced in Madagascar. During the considered period (March 1996± December 1997), 22 such CO events were selected and studied in detail, as those described above. All

Fig. 5. Five-day backward trajectory ending on 25 June 1996 0h00 UT and corresponding to the ®rst event of the Fig. 4. Points correspond to 12 h intervals.

Fig. 6. CO and Radon-222 variations for the period 12±20 September 1996 and presenting a large event on 15th September.

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Fig. 7. Five-day backward trajectory ending on 15th September 1996 0h00 UT and corresponding to the event presented in Fig. 6. Points correspond to 12 h intervals.

Fig. 8. Example of correlation observed between CO and Radon-222 for the events observed on 9 June 1996, 15 August 1996 and 8 September 1997. Corresponding equations of regression lines and square correlation coecient are given.

CO events presented also some common features: they were all observed between June and October, characterized by a strong positive correlation with Radon-222 observations, and the corresponding backward trajectories of all these events showed an origin from the sector of Southern Africa and Madagascar. As example, Fig. 8 present the correlation observed between CO and Radon-222 for three events characterized by di€erent CO/Rn ratios. As continental emissions of Radon-222 are now quite well quanti®ed (Lambert et al., 1982; Liu et al., 1984; Polian, 1984) this will allow us to determine an estimation of the CO ¯uxes emitted over Southern Africa following a similar procedure than Gaudry et al. (1990) for CO2 ¯uxes.

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For each event, CO and Radon-222 concentrations measured at Amsterdam Island [CO]ams and [Rn]ams , respectively, can be explained by the sum of their background concentrations ([CO]bkg and [Rn]bkg ) at these latitudes and a continental component according to the equation: ‰COŠams ˆ ‰COŠbkg ‡ ‰COŠc  exp …ÿkCO  t†  f …t†; ‰RnŠams ˆ ‰RnŠbkg ‡ ‰RnŠc  exp …ÿkRn  t†  f …t†;

‰RnŠams ˆ ‰RnŠbkg ‡ ‰RnŠc  exp …ÿkRn  t†  f …t†

‰COŠc  ‰RnŠbkg ‰RnŠc  exp …ÿkRn  t† ‰COŠc  ‰RnŠams : ‡ ‰RnŠc  exp …ÿkRn  t†

‰COŠams ˆ ‰COŠbkg ÿ

…1†

…5†

…2†

Experimental slope of the curve [CO]ams as a function of [Rn]ams has been determined for each event and will be named as ‰DCOŠams =‰DRnŠams

where [CO]c and [Rn]c are, respectively, the concentrations of CO and Radon-222 produced over the continent. kCO is the chemical destruction rate of CO ( ˆ kOH [OH]) where kOH ˆ 1.5 ´ 10ÿ13 (1+0.6 P/bar) cm3 moleculeÿ1 sÿ1 (Atkinson et al., 1992) and winter average [OH] ˆ 7 ´ 105 rad cmÿ3 (Poisson, personal communication) then kCO ˆ 1.5 ´ 10ÿ2 dayÿ1 . kRn is the radioactive decay constant of Radon-222 (0.18 dayÿ1 ), t is the transport time between the continent and the sampling site, f(t) is a function of time representing the dilution processes during the transport, assumed to be identical for CO and Radon. In this equation, the small oceanic source of CO does not appear explicitly but is included in the background concentrations. Radon-222 emission rates from the sea surface is about 100 times lower than the emission rate from continental surfaces (Lambert et al., 1982; Heimann et al., 1990), and therefore, as for CO, the oceanic component of Radon-222 will be neglected. The transport from Africa to Amsterdam Island occurs on average in 3 days, (Polian et al., 1986) and the term exp (ÿkCO t) is close to unity. On the contrary, the radioactive decay of Radon-222 is signi®cant during the transport and must be taken into account in the term exp(ÿkRn t) where kRn ˆ 0.18 dayÿ1 ˆ (ln2/ T1=2 ) with T1=2 representing Radon-222 half-life. We obtain therefore : ‰COŠams ˆ ‰COŠbkg ‡ ‰COŠc  f …t†;

and ®nally by combining these two equations in removing f(t):

…3†

…4†

‰COŠc ‰DCOŠams 1 ˆ  ‰RnŠc ‰DRnŠams exp …kRn  t†

…6†

assuming that the ratio ‰COŠc =‰RnŠc near the source is equal to the ratio of their ¯uxes /CO =/Rn , we have /CO ˆ

‰DCOŠams /Rn ;  ‰DRnŠams exp …kRn  t†

…7†

where the slope ‰DCOŠams =‰DRnŠams is measured at Amsterdam Island for each event and converted into gC/atom Rn by CO…gC† ˆ CO…ppbv†  12  10ÿ6 =22:4; Rn…atom mÿ3 † ˆ Rn…pCi mÿ3 †  3:7  10ÿ2  3:825  86400=ln2:

…8† …9†

For Radon-222, we assume a uniform mean continental ¯ux of 1 atom cmÿ2 sÿ1 (Gaudry et al., 1990; Ramonet et al., 1996). For transit time between Africa and Amsterdam Island, Polian et al. (1986) calculated an average of 3 days in the conditions of Ôradonic stormÕ (characterized by sharp increase of Radon-222, as those observed during the CO events). Calculations have also been made with a transit time of 4 days to test the sensibility of the calculated CO ¯ux to this parameter. Table 1 summarizes the 22 events studied with their date, their total duration, the slope [DCO]ams / [DRn]ams with the corresponding square correlation coecient as well as the estimations for the CO ¯uxes (calculated with a transit time of 3 or 4 days). Range of the estimated CO ¯uxes is typically 0.4±3.3 mg cm2 hÿ1 with an average of

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Table 1 Estimations of CO ¯uxes from Southern Africa and based on the study of 22 particular eventsa Date

Total duration

CO/Rn

R2

CO ¯ux (mgC/m2 /h)

CO0 ¯ux (mgC/m2 /h)

9 June 96 25 June 96 26 June 96 7 July 96 2 Aug 96 4 Aug 96 15 Aug 96 20 Aug 96 24 Aug 96 30 Aug 96 10 Sep 96 15 Sep 96 22 May 97 23 May 97 19 June 97 21 June 97 26 June 97 12 July 97 14 Aug 97 24 Aug 97 8 Sep 97 5 Oct 97

16 20 30 16 34 44 44 46 22 14 8 20 24 24 36 36 54 36 42 30 48 38

0.67 0.98 1.71 1.70 1.68 1.98 2.01 1.73 2.88 1.90 4.12 4.09 1.65 2.50 2.50 2.55 2.29 1.09 2.32 4.95 5.17 4.19

0.99 0.85 0.85 0.79 0.79 0.9 0.96 0.79 0.83 0.92 0.85 0.72 0.99 0.64 0.64 0.84 0.94 0.85 0.77 0.86 0.8 0.91

0.43 0.62 1.09 1.08 1.07 1.26 1.28 1.10 1.83 1.21 2.61 2.60 1.05 1.59 1.59 1.62 1.45 0.69 1.47 3.14 3.28 2.66

0.35 0.52 0.91 0.9 0.89 1.05 1.06 0.92 1.52 1.01 2.18 2.17 0.87 1.32 1.32 1.23 1.21 0.58 1.35 2.62 2.74 2.22

Mean Standard deviation

31.00 12.58

2.48 1.26

0.84 0.10

1.58 0.80

1.32 0.67

a

First column gives the date of the event, second column its total duration (h), third column the slope with its corresponding square correlation coecient (R2 ) in the fourth column and the two last columns present the estimations for the CO ¯uxes, CO and CO0 calculated with a transit time of 3 or 4 days, respectively.

1.58 ‹ 0.80 mg cm2 hÿ1 and 1.32 ‹ 0.67 mg cm2 hÿ1 for transit times of 3 and 4 days, respectively. Note that the standard deviation takes into account the variability of the source. If we assume that the transit time is 3 or 4 days, we obtain an uncertainty of 20% on the estimated ¯ux due to uncertainty on the transport time. Furthermore, Radon-222 ¯ux over continent is known with an uncertainty of about 20% and precision on the slope is estimated of about 10%. Therefore, the overall uncertainty is estimated to be about 50%. Fig. 9 presents the estimated CO ¯uxes for 1996 and 1997, calculated with a 3-day transit time. The CO emission by biomass burning seems to increase between May and November on the one hand and between 1996 and 1997 on the other hand. As the transit time has been chosen equivalent for all the events, this seasonal variation observed for the CO

¯ux takes into account variations of both transport and sources. Indeed, when the transit time between the source and the receptor is fast, Ra-

Fig. 9. Seasonal variation of the estimated CO ¯ux (mg cmÿ2 hÿ1 ) for 1996 and 1997.

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don-222 observed at Amsterdam Island is overestimated in comparison with Radon-222 observed when the transport is slower. However, Miller et al. (1993) showed that fast transport from Southern Africa or Madagascar was enhanced during winter and spring months but do not present a signi®cant seasonality between June and October. This shows that the seasonal and interannual variation observed for the calculated CO ¯uxes re¯ects mainly the variations of the source. This is in agreement with the seasonal variation of biomass burning in Africa previously published (Hao and Liu, 1994; Cooke et al., 1996) where intensity can vary from year to year but which maximum generally occurs in September±October. Moreover, CO ¯uxes calculated at Amsterdam Island seem to indicate that biomass burning has been quite more intensive in 1997 than in 1996. An independent estimation of the ¯ux of CO produced by biomass burning in Africa can be made on the basis of the estimations of the CO2 produced by burnings and of the average emission factor of CO/CO2 . We used the spatial distributions of CO2 emissions by savannas and forest burning published by Crutzen et al. (1989) and Hao and Liu (1994), where the amount of dry matter burned or the amount of C±CO2 produced per month is reported in each grid of 5° latitude by 5° longitude. The CO/CO2 emission factor is di€erent for savannas and forest burning emissions and mainly dependent on the combustion eciency. We used the emissons factors previously reported by various authors (Greenberg et al., 1984; Bonsang et al., 1995; Hao et al., 1996; Andraea et al., 1996). For savannas ®res a general agreement is obtain for a CO/CO2 ratio of 6±7%, we used an average ®gure of 6.3% reported by Bonsang et al. (1995). For forest ®res we used the ®gure of 11.3% reported by Greenberg et al. (1984). Our calculations are made in each grid of Southern Africa for months June± September and averaged. We obtain ®nally an average ®gure of 1.48 ‹ 0.75 mg C mÿ2 hÿ1 with a minimum and a maximum values of, respectively, 0.1 mg cmÿ2 hÿ1 and 2.95 mg cmÿ2 hÿ1 . As CO ¯ux determined with Amsterdam Island measurements gave a mean value of 1.58 ‹ 0.80 mg cm2 hÿ1 for a 3-day transit time, the agreement between the two independent calculations of CO

¯uxes from biomass burning over Southern Africa is quite noteworthy. 4. Summary and conclusions Study of the CO short term variations show that all the CO events (CO increase of 10±60 ppbv) were observed between June and October. Most of these events can be attributed to rapid advection from continents (Southern Africa, Madagascar) where the main CO source, at this time of the year, is biomass burning. Examination of the ratio CO/ Rn for 22 selected events allowed us to estimate the CO ¯ux emitted by biomass burning in the range 0.4±3.3 mg cmÿ2 hÿ1 , in very good agreement with CO ¯uxes calculated from the CO2 ¯uxes for forest and savanna burning given by Crutzen et al. (1989) and Hao and Liu (1994). However, it is important to note that all the CO events observed during the two years do not signi®cantly a€ect the CO monthly mean. Indeed for each month, medians have been calculated and they all di€er from the arithmetical means by less than 1 ppbv which is less than the precision of the measurement. Therefore the observed CO episodes are not contributing signi®cantly to the monthly mean and thus our data could be considered as representative of CO background in the Southern Hemisphere. Concerning the seasonal cycle, this means that the maximum of CO observed in October and attributed to biomass burning is not the e€ect of enhanced CO events but re¯ects the increase of CO background. As CO observations at Crozet and Cape Grim present the same CO seasonal cycle, it seems that biomass burning a€ects at least all the middle latitude belt of the Southern Hemisphere and therefore has a hemispheric impact contributing to the background CO level observed in this hemisphere. This result con®rms the assumption of Fishman et al. (1991) of biomass burning as a widespread pollution. Importance of the paper: Biomass burning is a major source of carbon monoxide in the southern hemisphere. We show that, for some speci®c short periods, the direct in¯uence from biomass burning on the CO level is observed at the remote marine station of Amsterdam Island. For the events ob-

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served in 1996 and 1997, we estimate the CO emission rates at the source from the CO and Radon-222 correlations. However, these events donÕt contribute signi®cantly to the CO monthly mean and it appears that biomass burning has a global hemispheric impact and therefore contributes to the background CO level observed in the southern hemisphere. Acknowledgements We express our gratitude to the Institut Francßais pour la Recherche et la Technologie Polaires (IFRTP), the Territoire des Terres Australes et Antarctiques Francßaises (TAAF) and the Institut Francßais de Recherche pour lÕExploitation de la Mer (IFREMER) for ®nancial and logistical support of the monitoring station at Amsterdam Island (grant RACEA 146). We are very grateful to Georges Polian and Benedicte Ardouin for providing Radon-222 results and to P. Nadeau and B. Pelczar for technical assistance in measuring CO at Amsterdam Island. We thank the Centre National de la Recherche Scienti®que (CNRS) and the Commisariat a lÕEnergie Atomique (CEA). This is LSCE contribution No.149. References Andraea, M.O., Atlas, E., Cachier, H., Co€erIII, W.R., Harris, G.W., Helas, G., Koppmann, R., Lacaux, J.P., Ward, D.E., 1996. Trace gas and aerosol emissions from savanna ®res. In: Levine, J.S. (Ed.), Biomass Burning and Global Change, vol. 1. MIT Press, Cambridge, MA, pp. 278±295. Atkinson, R., Baulch, D.L., Cox, R.A., Hampson Jr., R.F., Kerr, J.A., Troe, J., 1992. Evaluated kinetic and photochemical data for atmospheric chemistry : supplement IV. Atmos. Environ. 26A (7), 1187±1230. Bates, T.S., Kelly, K.C., Johnson, J.E., Gammon, R.H., 1995. Regional and seasonal variations in the ¯ux of oceanic carbon monoxide to the atmosphere. J. Geophys. Res. 100, 23093±23101. Bonsang, B., Boissard, C., LeCloarec, M.F., Rudolph, J., Lacaux, J.P., 1995. Methane, carbon monoxide and light non-methane hydrocarbon emissions from african savanna burnings during the FOS/DECAFE experiment. J. Atmos. Chem. 22, 149±162. Brunke, E.G, Scheel, H.E., Seiler, W., 1990. Trends of tropospheric CO, N2 O and CH4 as observed at Cape Point, South Africa. Atmos. Environ. 24A, 585±595.

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Cooke, W.F., Ko, B., Gregoire, J.M., 1996. Seasonality, of vegetation ®res in Africa from remote sensing data and application to a global chemistry model. J. Geophys. Res. 101 (D15), 21051±21065. Crutzen, P.J., Heidt, L.E., Krasnec, J.P., Pollock, W.H., Seiler, W., 1979. Biomass burning as a source of atmospheric gases CO, H2 , N2 O, NO, CH3 Cl and COS. Nature 282, 253±256. Crutzen, P.J., Hao, W.M., Liu, M.H., Lobert, J.M., Schar€e, D., 1989. Emission of CO2 and other trace gases to the atmosphere from ®res in the tropics. In: Crutzen, P.J., Gerard, J.C., Zander, R. (Eds.), Our changing atmosphere, Proceedings of the 28th Liege International Astrophysical Colloquium, 26±30 June, 1989, pp. 449±471. Fishman, J., Fukhruzjasman, K., Cros, B., Nganga, D., 1991. Identi®cation of widespread pollution in the southern hemisphere deduced from satellites analyses. Science 252, 1693±1696. Greenberg, J.P., Zimmerman, P.R., Heidt, L., Pollock, W., 1984. Hydrocarbon and Carbon monoxide emissions from biomass burning in Brazil. J. Geophys. Res. 89, 1350±1354. Gaudry, A., Polian, G., Ardouin, B., Lambert, G., 1990. Radon-calibrated emissions of CO2 from South Africa. Tellus 42B, 9±19. Gros, V., Bonsang, B., Sarda Esteve, R., 1999. Atmospheric carbon monoxide ``in situ'' monitoring by automatic gas chromatography. Chemosphere: Global Change Sci. 1, 153± 161. Gros, V., Martin, D., Poisson, N., Kanakidou, M., Bonsang, B., 1998. Observations and modeling of the seasonal variations of surface ozone at Amsterdam Island: 1994± 1996, J. Geophys. Res. 103, 28103±28109. Hao, W.M., Liu, M.H., 1994. Spatial and temporal distribution of tropical biomass burning. Global Biogeochim. Cycles 8, 495±503. Hao, W.M., Ward, D.E., Olbu, G., Baker, S.P., 1996. Emissions of CO2 , CO and hydrocarbons from ®res in diverse African savanna ecosystemes. J. Geophys. Res. 101, 23577±23584. Heimann, M., Monfray, P., Polian, G., 1990. Modeling the long-range transport of 222 Rn to subantarctic and antarctic areas. Tellus 42B, 83±99. Khalil, M.A.K, Rasmussen, R.A., 1990. The global cycle of carbon monoxide: trends and mass balance. Chemosphere 20 (1/2), 227±242. Lambert, G., Polian, G., Sanak, J., Ardouin, B., Buisson, A., Jegou A, , LeRoulley, J.C., 1982. Cycle du radon et de ses descendants: application  a lÕetude des echanges tropospherestratosphere; (translated as: radon and radon-daughters cycle: application to the study of troposphere-stratosphere exchange). Annales de Geophysiques 38, 497±531. Liu, S.C., McAfee, J.R., Cicerone, R.J., 1984. Radon-222 and tropospheric vertical transport. J. Geophys. Res 89, 7291± 7297. Miller, J.M., Moody, J.L., Harris, J.M., Gaudry, A., 1993. A 10 yr trajectory ¯ow climatology for Amsterdam Island, 19801989. Atmos. Environ. 27A (12), 1909±1916. Novelli, P.C., Elkins, J.W., Steele, L.P., 1991. The development and evaluation of a gravimetic reference scale for measure-

172

V. Gros et al. / Chemosphere: Global Change Science 1 (1999) 163±172

ments of atmospheric carbon monoxide. J. Geophys. Res. 96, 13109±13121. Novelli, P.C, Steele, L.P., Tans, P.P., 1992. Mixing ratios of carbon monoxide in the troposphere. J. Geophys. Res. 97, 20731±20750. Novelli, P.C, Masarie, K.A., Lang, P.M., 1998. Distributions and recent changes in tropospheric CO. J. Geophys. Res. 103, 19015±19033. Polian, G, 1984. Atmospheric transport in the Southern Hemisphere and the global balance of 222 Rn, thesis, Universite P. and M. Curie, Paris 6, France. Polian, G., Lambert, G., Ardouin, B., Jegou, A., 1986. Long range transport of continental radon in subantarctic and antarctic areas. Tellus 38B, 178±189. Ramonet, M., LeRoulley, J.C., Bousquet, P., Monfray, P., 1996. Radon-222 measurements during the Tropoz II campaign and comparison with a global atmospheric transport model. J. Atmos. Chem. 23, 107±136. Ramonet, M., Monfray, P., 1996. CO2 baseline concept in 3-D atmospheric transport models. Tellus 48B, 502±520. Steele, L.P., Langenfelds, R.L., Lucarelli, M.P., Fraser, P.J., Cooper, L.N., Spencer, D.A., Chea, S., Broadhurst, K., 1996a. Atmospheric methane, carbon dioxide, carbon monoxide, hydrogen, and nitrous oxide from Cape Grim ¯asks air samples analysed by gas chromatography. In: Francey, R.J., Dick, A.L., Derek, N. (Eds.), Baseline Atmospheric Program Australia, 1994±1995, Bureau of Meteorology and CSIRO Division of Atmospheric Research, Melbourne, Australia, pp. 107±111. Steele, L.P., Lucarelli, M.P., Fraser, P.J., Derek, N., Porter, L.P., 1996b. Halocarbons, nitrous oxide, methane, carbon monoxide and hydrogen, In: Francey, R.J., Dick, A.L., Derek, N. (Eds.), The AGAGE Program, 1993±1995, Baseline Atmospheric Program, Australia, 1994±1995, Bureau of Meteorology and CSIRO Division of Atmospheric Research, Melbourne, Australia, pp. 134±140. World Meteorological Organization (WMO), 1994. Report of the WMO meeting of experts. In: Novelli, P.C., Rosson, R.M. (Eds.), global carbon monoxide measurements, Boulder, No. 98, pp. 14±18.

Valerie Gros obtained her Ph.D. from the university of Paris 7 in December 1998. She worked on the study of ozone (O3 ) and CO variabilities in the marine boundary layer of the southern hemisphere. She particularly studied the marine atmosphere of Amsterdam Island, Indian ocean, with pointing out the key processes involved in the O3 and CO budgets over di€erent time scales. She has several publications in international referred journals and conference proceedings. Bernard Bonsang has worked in the ®eld of atmospheric sciences since 1972. He is Directeur de Recherche (CNRS) since 1993. He studied the oceanic source of reduced sulphur gases and hydrocarbons. One part of his activities concerned also the evaluation of the production of CO, methane and NMHC's by biomass burning in the tropics, with particulate emphasis on the tropospheric ozone budget. His main activities are focused on the oxidizing capacity of the troposphere in the frame of national and European projects and on a global scale on long range transport of ozone and its precursors over the oceans. He has been coordinator of EC projects dealing with oxidation processes of VOC's. He is author of more than 60 publications in international referred journals and conference proceedings. Daniel Martin is a research physicist at METEO-FRANCE since 1973. He developed techniques and probes for measuring aerological parameters, aerosols, sulfur and chloride compounds using remotely piloted aircraft in atmospheric soundings. He moved to the LSCE in 1992 for studying the oxidant capacity of the marine boundary layer with a particular emphasis on ozone, hydrogen peroxide and CO. He is responsible for the aircraft operations in the Etude et Simulation de la QUalite de l'air en Ile de France (ESQUIF) project dealing with the air pollution around Paris. Paul Novelli is a research chemist at the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory in Boulder, Colorado. Working within the NOAA/CMDL carbon cycle group he is studying the global distributions and budgets of CO and molecular hydrogen. Dr. Novelli has worked closely with NASA in validation of the Measurement of Air Pollution from Satellite (MAPS) instrument, and will play a role in the validation of Measurement of Pollution In TheTroposphere (MOPITT) a joint NASA and Canadian Space Agency instrument on the Mission To Planet Earth AM-1 satellite.