Formulation and Evaluation of IMS, an Interactive Three-Dimensional Tropospheric Chemical Transport Model 2. Model Chemistry and Comparison of Modelled CH4, CO, and O3 with Surface Measurements

In part two of this series of papers on the IMS model, we present the chemistry reaction mechanism usedand compare modelled CH₄, CO, and O₃ witha dataset of annual surface measurements. The modelled monthly and 24-hour mean tropospheric OH concentrationsrange between 5–22 × 10⁵ moleculescm^–3, indicating an annualaveraged OH concentration of about 10 × 10⁵ moleculescm^–3. This valueis close to the estimated 9.7 ± 0.6 × 10⁵ moleculescm^–3 calculated fromthe reaction of CH₃CCl₃ with OH radicals.Comparison with CH₄ generally shows good agreementbetween model and measurements, except for the site at Barrow where modelledwetland emission in the summer could be a factor 3 too high.For CO, the pronounced seasonality shown in the measurements is generally reproduced by the model; however, the modelled concentrations are lower thanthe measurements. This discrepancy may due to lower the CO emission,especially from biomass burning,used in the model compared with other studies.For O₃, good agreement between the model and measurements is seenat locations which are away from industrial regions. The maximum discrepancies between modelled results and measurementsat tropical and remote marine sites is about 5–10 ppbv,while the discrepancies canexceed 30 ppbv in the industrial regions.Comparisons in rural areas at European and American continental sites arehighly influenced by the local photochemicalproduction, which is difficult to model with a coarse global CTM.The very large variations of O₃ at these locations vary from about15–25 ppbv in Januaryto 55–65 ppbv in July–August. The observed annual O₃amplitude isabout 40 ppbv compared with about 20 ppbv in the model. An overall comparison of modelled O₃ with measurements shows thatthe O₃seasonal surface cycle is generally governed bythe relative importance of two key mechanisms that drivea springtime ozone maximum and asummertime ozone maximum.     

Airborne measurements of savanna fire emissions and the regional distributio of pyrogenic pollutants over Western Africa

We measured CO₂, CO, CH₄, H₂, and NO₂ in air masses polluted by savanna fires over Côte d'Ivoire, western Africa. Elevated concentrations of these trace gases were found in fire plumes and also in extensive haze layers. Trace gas mixing ratios ranged as high as 605 ppmv for CO₂, 14.8 ppmv for CO, 2.7 ppmv for CH₄, 4.2 ppmv for H₂, and 25 ppbv for NO₂. We compare our emission ratios to those obtained in previous field and laboratory studies. The emission ratios, expressed as an average and as a range or as an average only, were: dCO/dCO₂ 5.3×10^–2 (3–18×10^–2); dCH₄/dCO 5.3×10^–2; dH₂/dCO 2.4×10^–1 and dNO₂/dCO₂ 1.8×10^–4 (1.5–2.2×10^–4). The values found match those found during similar measurements, though our results point to rather vigorous burning in the savanna of western Africa.     

Is the Arctic Surface Layer a Source and Sink of NOx in Winter/Spring?

Measurements of NO_x (NO +NO₂) and the sum of reactive nitrogenconstituents, NO_y, were made near the surface atAlert (82.5°N), Canada during March and April1998. In early March when solar insolation was absentor very low, NO_x mixing ratios were frequentlynear zero. After polar sunrise when the sun was abovethe horizon for much or all of the day a diurnalvariation in NO_x and NO_y was observed withamplitudes as large as 30–40 pptv. The source ofactive nitrogen is attributed to release from the snowsurface by a process that is apparently sensitized bysunlight. If the source from the snowpack is a largescale feature of the Arctic then the diurnal trendsalso require a competing process for removal to thesurface. From the diurnal change in the NO/NO₂ratio, mid-April mixing ratios for the sum of peroxyand halogen oxide radicals of 10 pptv werederived for periods when ozone mixing ratios were inthe normal range of 30–50 ppbv. Mid-day ozoneproduction and loss rates with the active nitrogensource were estimated to be 1–2 ppbv/day and in nearbalance. NO_y mixing ratios which averaged only295±66 pptv do not support a large accumulation inthe high Arctic surface layer in the winter and springof 1998. The small abundance of NO_y relative tothe elevated mixing ratios of other long-livedanthropogenic constituents requires that reactivenitrogen be removed to the surface during transport toor during residence within the high Arctic.     

High-Latitude Springtime Photochemistry. Part I: NOx, PAN and Ozone Relationships

Springtime measurements of NO_x, ozone, PAN,J(NO₂), and other compounds were made near Ny-Ålesund,Svalbard (78°54N, 11°53E), in 1994 and Poker Flat,Alaska (65°08N, 147°29W), in 1995. At Svalbard medianmixing ratios for PAN and NO_x of 237 and 23.7 pptv,respectively, were observed. The median mixing ratios at Poker Flat for PANand NO_x were 79.5 and 85.9 pptv, respectively. These data areused to estimate thermal PAN decomposition using several differentapproaches. At Svalbard PAN decomposition was very small, while at PokerFlat up to 30 pptv/h PAN decomposed. At both sites the NO_x/PANratio increased with temperature between –10 and 20°C implyingthat PAN decomposition is an important NO_x source. In-situozone production was calculated from the measured NO, NO₂,O₃, J(NO₂), and temperature data, using thesteady state assumption Median ozone production was 605 pptv/h at PokerFlat, and one order of magnitude smaller at Svalbard during the daytime.Only at Poker Flat could a direct influence on the diurnal ozone cycle beobserved from in-situ production. These results imply that PAN decompositionis a major source of NO_x in the high latitude troposphere, andthat this contributes to the observed spring maximum in surface ozone.     

Stratospheric N2O5, CH4, and N2O profiles from IR solar occultation spectra

Stratospheric volume mixing ratio profiles of N₂O₅, CH₄, and N₂O have been retrieved from a set of 0.052 cm^–1 resolution (FWHM) solar occultation spectra recorded at sunrise during a balloon flight from Aire sur l'Adour, France (44° N latitude) on 12 October 1990. The N₂O₅ results have been derived from measurements of the integrated absorption by the 1246 cm^–1 band. Assuming a total intensity of 4.32×10^–17 cm^–1/molecule cm^–2 independent of temperature, the retrieved N₂O₅ volume mixing ratios in ppbv (parts per billion by volume, 10^–9), interpolated to 2 km height spacings, are 1.64±0.49 at 37.5 km, 1.92±0.56 at 35.5 km, 2.06±0.47 at 33.5 km, 1.95±0.42 at 31.5 km, 1.60±0.33 at 29.5 km, 1.26±0.28 at 27.5 km, and 0.85±0.20 at 25.5 km. Error bars indicate the estimated 1- uncertainty including the error in the total band intensity (±20% has been assumed). The retrieved profiles are compared with previous measurements and photochemical model results.Laboratoire associé aux Universités Pierre et Marie Curie et Paris Sud.     

Gaseous formic and acetic acids in the atmosphere of Yokohama,Japan

Gaseous formic acid (HCOOH_g) and acetic acid (CH₃COOH_g) were measured every 30 minutes during a 10 hour daylight period in August, and a 24 hour period in October, 1990 in the urban atmosphere of Yokohama, Japan. An aqueous nebulizer sampler and ion-chromatography exclusion (ICE) were used for the measurements. In the August experiment (0800–1800 local time) the mean HCOOH_g concentration was found to be 7.3±2.5 ppbv. The mean CH₃COOH_g concentration was 3.8±1.2 ppbv. In the 24 hour experiment in October, concentrations of both acids were lower between 0800–1800 than during the same time-period in August (mean HCOOH_g=4.4±2.7 ppbv, mean CH₃COOH_g=1.4±0.5 ppbv). In October, concentrations of both acids were higher in daylight hours than at night; sporadic high HCOOH_g concentrations were observed. In both experiments the ratio HCOOH_g/CH₃COOH_g of individual samples was usually 2.0 (mean ratio of 2.0 in August, 3.1 in October).     

Carbonyl sulfide emissions from biomass burning in the tropics

Carbonyl sulfide emissions from biomass burning have been studied during field experiments conducted both in an African savanna area (Ivory Coast) and rice fields, central highland pine forest and savanna areas in Viet-Nam. During these experiments CO₂, CO and C₂H₂ or CH₄ have also been also monitored. COS values range from 0.6 ppbv outside the fires to 73 ppbv in the plumes. Significant correlations have been observed between concentrations of COS and CO (R ²=0.92,n=25) and COS and C₂H₂ (R ²=0.79,n=26) indicating a COS production during the smoldering combustion. COS/CO₂ emission factors (COS/CO₂) during field experiments ranged from 1.2 to 61×10^–6 (11.4×10^–6 mean value). COS emission by biomass burning was estimated to be up to 0.05 Tg S/yr in tropics and up to 0.07 Tg S/yr on a global basis, contributing thus about 10% to the global COS flux. Based on the S/C ratio measured in the dry plant biomass and the COS/CO₂ emission factor, COS can account for only about 7% of the sulfur emitted in the atmosphere by biomass burning.     

Annual variation of atmospheric carbonyl sulfide in the marine atmosphere in the Southern Indian Ocean

Measurements of the concentrations of carbonyl sulfide (COS) in the marine atmosphere were made over a period of two years in the southern Indian Ocean (Amsterdam Island, 37°50 S–77°31 E; March 1987–February 1988 and April 1989–February 1990). The mean atmospheric COS concentration for the whole period was 475±48 pptv (n=544). Atmospheric COS concentrations show no significant seasonal variation with a summer to winter ratio of 1.05. Taking into account the observed variability of the atmospheric COS concentration (10%), a value of 1.4 yr is estimated as a lower limit for the atmospheric COS lifetime. A comparison of the COS data at Amsterdam Island with those obtained in the Southern Hemisphere in the past 12 yr does not reveal any significant trend in the tropospheric background COS mixing ratio.     

The Henry's Law Constants of the Haloacetic Acids

Henry's law constants K_H (mol kg^-1 atm^-1) have been measured between 278.15 K and 308.15 K for the following organic acids: CH₂FCOOH (ln(K_H[298.15 K]) = 11.3 ± 0.2), CH₂ClCOOH (11.59 ± 0.14), CH₂BrCOOH (11.94 ± 0.21), CHF₂COOH (10.32 ± 0.10), CHCl₂COOH (11.69 ± 0.11), CHBr₂COOH (12.33 ± 0.29), CBr₃COOH (12.61 ± 0.21), and CClF₂COOH (10.11 ± 0.12). The variation of K_H with temperature was determined for all acids except CH₂FCOOH and CBr₃COOH, with _r H° for the dissolution reaction ranging from –85.2 ± 2.6 to –57.1 ± 2.5 kJ mol^-1, meaning that their solubility is generally more sensitive to temperature than is the case for the simple carboxylic acids. The Henry's law constants show consistent trends with halogen substitution and, together with their high solubility compared to the parent (acetic) acid (ln(K_H[298.15 K]) = 8.61), present a severe test of current predictive models based upon molecular structure. The solubility of haloacetic acids and strong dissociation at normal pH mean that they will partition almost entirely into cloud and fog in the atmosphere (0.05–1.0 g H₂O m^-3), but can reside in both phases for the liquid water contents typical of aerosols (10^-5-10^-4 g H₂O m^-3).     

Short-Term Variability of Atmospheric DMS and Its Oxidation Products at Amsterdam Island during Summer Time

A one-month experiment was performed at Amsterdam Island in January 1998, to investigate the factors controlling the short-term variations of atmospheric dimethylsulfide (DMS) and its oxidation products in the mid-latitudes remote marine atmosphere. High mixing ratios of DMS, sulfur dioxide (SO₂) and dimethylsulfoxide (DMSO) have been observed during this experiment, with mean concentrations of 395 parts per trillion by volume (pptv) (standard deviation, = 285, n = 500), 114 pptv ( = 125, n = 12) and 3 pptv ( = 1.2, n = 167), respectively. Wind speed and direction were identified as the major factors controlling atmospheric DMS levels. Changes in air temperature/air masses origin were found to strongly influence the dimethylsulfoxide (DMSO)/DMS and SO₂/DMS molar ratios, in line with recent laboratory data. Methanesulfonic acid (MSA) and non-sea-salt sulfate (nss-SO₄ ^2–) mean concentrations in aerosols during this experiment were 12.2± 6.5 pptv (1, n=47) and 59 ± 33 pptv (1, n=47), respectively. Evidence of vertical entrainment was reported following frontal passages, with injection of moisture-poor, ozone-rich air. High MSA/ nss-SO₄ ^2– molar ratios (mean 0.44) were calculated during these events. Finally following frontal passages, few spots in condensation nuclei (CN) concentration were also observed.     

European sources of halocarbons and nitrous oxide: Update 1986

Semi-continuous measurements of CFCl₃, CF₂Cl₂, CCl₄, CH₃CCl₃ and N₂O were made at Adrigole, Ireland as part of the Atmospheric Lifetime Experiment (ALE). Clean, baseline air from the Atlantic Ocean was measured approximately 70% of the time; pollution events from Europe, for the remainder. The two final years of ALE data from Adrogole give a five-year record from July 1978 to June 1983. This paper extends previous work on the relative enhancements of trace gases during pollution episodes and presents (1) unambiguous identification of elevated levels of N₂O concurrent with halocarbon pollution events, (2) detection of trends in emission of CH₃CCl₃, (3) discovery of seasonal variations in emission of CF₂Cl₂, CCl₄ and CH₃CCl₃, (4) characterization of typical summer and winter pollution episodes, and (5) identification of weather patterns over Europe that are associated with high concentrations of CFCs at Adrigole. Some of these results assume that CFCl₃ represents a uniform, well buffered source from the continent. The latter two results are particularly useful in the testing and calibration of three-dimensional chemical transport models. Observed enhancements are marginally consistent with estimates of halocarbon use by the chemical industry. The source of nitrous oxide correlated with halocarbons is 0.8 Tg(N)/yr from Europe alone and represents approximately 10% of the global stratospheric loss.     

Formation and occurrence of organic hydroperoxides in the troposphere: Laboratory and field observations

The formation and occurrence of hydroperoxides in the troposphere have been studied by laboratory experiments and by preliminary field measurements. Nine alkenes were reacted individually with ozone in a reaction chamber in the presence of excess water, and the amounts of hydrogen peroxide and of nine organic hydroperoxides produced in the gas and aerosol phases and deposited on the chamber walls determined by HPLC. The reactions of ethene, propene, 1-butene and isoprene gave hydroxymethyl hydroperoxide as the major product with no hydrogen peroxide observed. In the case of - and -pinene, 2-carene and limonene the major product was hydrogen peroxide. Cis-2-butene produced hydrogen peroxide and methyl hydroperoxide. Preliminary measurements of hydrogen peroxide and five organic hydroperoxides in ambient air were made at Niwot Ridge, Colorado from 24 July–4 August 1989. The gas-phase species were preconcentrated by cryotrapping with subsequent HPLC separation. The gas-phase concentrations of H₂O₂ ranged from 0.5–2 ppbv with the lowest concentrations being measured at night and the highest under conditions of strong photochemical activity. The maximum concentrations of hydroxymethyl hydroperoxide approximated those of H₂O₂. Methyl hydroperoxide concentrations ranged from <50 to 800 pptv and three other organic hydroperoxides were detected at concentrations below 200 pptv. High volume aerosol samples yielded H₂O₂ and methyl hydroperoxide concentrations <10 ng m^-3 while H₂O₂ and six organic species were detected in rainwater at concentrations in the range <0.01–50 M.     

Shipboard measurements of atmospheric dimethylsulfide and hydrogen sulfide in the Caribbean and Gulf of Mexico

Simultaneous shipboard measurements of atmospheric dimethylsulfide and hydrogen sulfide were made on three cruises in the Gulf of Mexico and the Caribbean. The cruise tracks include both oligotrophic and coastal waters and the air masses sampled include both remote marine air and air masses heavily influenced by terrestrial or coastal inputs. Using samples from two north-south Caribbean transects which are thought to represent remote subtropical Atlantic air, mean concentrations of DMS and H₂S were found to be 57 pptv (74 ng S m^-3, =29 pptv, n=48) and 8.5 pptv (11 ng S m^-3, =5.3 pptv, n=36), respectively. The ranges of measured concentrations for all samples were 0–800 pptv DMS and 0–260 pptv H₂S. Elevated concentrations were found in coastal regions and over some shallow waters. Statistical analysis reveals slight nighttime maxima in the concentrations of both DMS and H₂S in the remote marine atmosphere. The diurnal nature of the H₂S data is only apparent after correcting the measurements for interference due to carbonyl sulfide. Calculations using the measured ratio of H₂S to DMS in remote marine air suggest that the oxidation of H₂S contributes only about 11% to the excess (non-seasalt) sulfate in the marine boundary layer.     

GREENHOUSE GASES FROM DEFORESTATION IN BRAZILIAN AMAZONIA: NET COMMITTED EMISSIONS

Deforestation in Brazilian Amazonia is a significant source of greenhouse gases today and, with almost 90% of the originally forested area still uncleared, is a very large potential source of future emissions. The 1990 rate of loss of forest (13.8 × 10³ km²/year) and cerrado savanna (approximately 5 × 10³ km²/year) was responsible for releasing approximately 261 × 10⁶ metric tons of carbon (10⁶ t C) in the form of CO2, or 274–285 × 10⁶ t of CO2-equivalent C considering IPCC 1994 global warming potentials for trace gases over a 100-year horizon. These calculations consider conversion to a landscape of agriculture, productive pasture, degraded pasture, secondary forest, and regenerated forest in the proportions corresponding to the equilibrium condition implied by current land-use patterns. Emissions are expressed as net committed emissions, or the gases released over a period of years as the carbon stock in each hectare deforested approaches a new equilibrium in the landscape that replaces the original forest. For low and high trace gas scenarios, respectively, 1990 clearing produced net committed emissions (in 10⁶ t of gas) of 957–958 for CO2, 1.10–1.42 for CH₄, 28–35 for CO, 0.06–0.16 for N₂O, 0.74–0.74 for NO_x and 0.58–1.16 for non-methane hydrocarbons.     

A determination of the CH4, NO x and CO2 emissions from the Prudhoe Bay,Alaska oil development

In this paper we quantify the CH₄, CO₂ and NO _x emissions during routine operations at a major oil and gas production facility, Prudhoe Bay, Alaska, using the concentrations of combustion by products measured at the NOAA-CMDL observatory at Barrow, Alaska and fuel consumption data from Prudhoe Bay. During the 1989 and 1990 measurement campaigns, 10 periods (called events) were unambiguously identified where surface winds carry the Prudhoe Bay emissions to Barrow (approximately 300 km). The events ranged in duration from 8–48 h and bring ambient air masses containing substantially elevated concentrations of CH₄, CO₂ and NO _y to Barrow. Using the slope of the observed CH₄ vs CO₂ concentrations during the events and the CO₂ emissions based on reported fuel consumption data, we calculate annual CH₄ emissions of (24+/–8)×10³ metric tons from the facility. In a similar manner, the annual NO _x emissions are calculated to be (12+/–4)×10³ metric tons, which is in agreement with an independently determined value. The calculated CH₄ emissions represent the amount released during routine operations including leakage. However this quantity would not include CH₄ released during non-routine operations, such as from venting or gas flaring.     

FTIR studies of reactions between the nitrate radical and haloethenes

Products and mechanisms for the gas-phase reactions of NO₃ radicals with CH₂=CHCl, CH₂=CCl₂, CHCl=CCl₂,cis-CHCl=CHCl andtrans-CHCl=CHCl in air have been studied. The experiments were carried out at 295±2 K and 740±5 Torr in a 480-L Teflon-coated reaction chamber and at 295±2 K and 760±5 Torr in a 250-L stainless steel reactor. NO₃ was generated by the thermal dissociation of N₂O₅. Experiments with¹⁵NO₃ and CD₂CDCl have also been performed. The initially formed nitrate peroxynitrates decay into carbonyl compounds, nitrates, HCl and ClNO₂. In adidtion, there are indications of nitrooxy acid chlorides being produced. The reactions with CH₂=CCl₂ and CHCl=CCl₂ are more complex due to release of chlorine atoms which eventually lead to formation of chloroacid chlorides.A general reaction mechanism is proposed and the observed concentration-time profiles of reactants and products are simulated for each compound. The rate constants for the initial step of NO₃ addition to the chloroethenes are determined as: (2.6±0.5, 9.4±0.9, 2.0±0.4 and 1.4±0.4) × 10^–16 cm³ molecule^–1 s^–1 for CH₂=CHCl, CH₂=CCl₂, CHCl=CCl₂ andcis-CHCl=CHCl, respectively.     

Reactive Nitrogen Compounds at Spitsbergen in the Norwegian Arctic

Simultaneousindependent measurements of NO_y and NO_x(NO_x= NO + NO₂) by high-sensitivitychemiluminescence systems and of PAN (peroxyacetylnitrate) and PPN (peroxypropionyl nitrate) by GC-ECDwere made at Spitsbergen in the Norwegian Arcticduring the first half year of 1994. The average mixingratio of the sum of PAN and PPN (denoted PANs)increased from around 150 pptv in early winter to amaximum of around 500 pptv in late March, whereasepisodic peak values reached 800 pptv. This occurredsimultaneously with a maximum in ozone which increasedto 45–50 ppbv in March–April. The average NO_xmixing ratio was 27 pptv and did not show any cyclethrough the period. The NO_y mixing ratio showeda maximum in late March, while the difference betweenNO_y and PAN decreased during spring. This is anindication of the dominance of PAN in the NO_ybudget in the Arctic, but possible changes in theefficiency of the NO_y converter could alsocontribute to this. Although most PAN in theArctic is believed to be due to long range transport,the observations indicate local loss and formationrates of up to 1–2 pptv h^-1 in April–May.Measurements of carbonyl compounds suggest thatacetaldehyde was the dominant, local precursor ofPAN.Now at 1.     

Influence of Transport over a Mountain Ridge on the Chemical Composition of Marine Aerosols during the ACE-2 Hillcloud Experiment

Aerosol chemical composition and trace gas measurements were made at twolocations on the northeastern peninsula of Tenerife during the ACE-2HILLCLOUD experiment, between 28 June and 23 July 1997. Measurementswere made of coarse (#gt;2.5 m aerodynamic diameter) and fine (#lt; 2.5m) aerosol Cl^–, NO₃ ^–,SO₄ ^2–, non-sea saltSO₄ ^2– (NSSS),CH₃SO₃ ^– (MSA) andNH₄ ⁺, and gas phase dimethylsulphide (DMS), HCl,HNO₃, SO₂, CH₃COOH, HCOOH andNH₃. Size distributions were measured using a cascadeimpactor. Results show that in marine air masses NSSS and MSA wereformed via DMS oxidation, with additional NSSS present in air massescontaining a continental component. Using a Eulerian box model approachfor aerosols transported between upwind and downwind sites, a mean NSSSproduction rate of 4.36 × 10^–4 gm^–3 s^–1 was calculated for daytimeclear sky periods (highest insolation), with values for cloudy periodsduring daytime and nighttime of 3.55 × 10^–4 and2.40 × 10^–4 g m^–3s^–1, respectively. The corresponding rates for MSA were6.23 × 10^–6, 8.49 × 10^–6and 6.95 × 10^–6 g m^–3s^–1, respectively. Molar concentration ratios forMSA/NSSS were 8.7% (1.8–18.2%) and 1.9%(1.3–3.5%) in clean and polluted air masses, respectively.Reactions occurring within clouds appeared to have a greater influenceon rates of MSA production, than of NSSS, while conversely daytime gasphase reactions were more important for NSSS. For MSA, nighttimein-cloud oxidation rates exceeded rates of daytime gas phase productionvia OH oxidation of DMS. NSSS, MSA and ammonium had trimodal sizedistributions, with modes at 0.3, 4.0 and >10.0 m (NSSS andNH₄ ⁺), and 0.3, 1.5 and 4.0 m (MSA). Nosignificant production of other aerosol species was observed, with theexception of ammonium, which was formed at variable rates dependent onneutralisation of the aerosol with ammonia released from spatiallynon-uniform surface sources. Seasalt components were mainly present incoarse particles, although sub-micrometre chloride was also measured.Losses by deposition exceeded calculated expectations for all species,and were highest for the seasalt fraction and nitrate.     

Field study of the emissions of methyl chloride and other halocarbons from biomass burning in Western Africa

A field study of trace gas emissions from biomass burning in Equatorial Africa gave methyl chloride emission ratios of 4.3×10^–5±0.8×10^–5 mol CH₃Cl/mol CO₂. Based on the global emission rates for CO₂ from biomass burning we estimate a range of 226–904×10⁹ g/y as global emission rate with a best estimate of 515×10⁹ g/y. This is somewhat lower than a previous estimate which has been based on laboratory studies. Nevertheless, our emission rate estimates correspond to 10–40% of the global turnover of methyl chloride and thus support the importance of biomass burning as methyl chloride source. The emission ratios for other halocarbons (CH₂Cl₂, CHCl₃, CCl₄, CH₃CCl₃, C₂HCl₃, C₂Cl₄, F-113) are lower. In general there seems to be a substantial decrease with increasing complexity of the compounds and number of halogen atoms. For dichloromethane biomass burning still contributes significantly to the total global budget and in the Southern Hemisphere biomass burning is probably the most important source for atmospheric dichloromethane. For the global budgets of other halocarbons biomass burning is of very limited relevance.     

Correlations among combustion effluent species at Barrow,Alaska: Aerosol black carbon,carbon dioxide,and methane

As part of the second Arctic Gas and Aerosol Sampling Program (AGASP II) continuous measurements of atmospheric aerosol black carbon (BC) were made at the NOAA/GMCC observatory at Barrow, Alaska (71°19N, 156°36W) during the period March 21–April 22, 1986. Black carbon is produced only by incomplete combustion of carbonaceous materials and so is a particularly useful atmospheric indicator of anthropogenic activities. The BC data have been analyzed together with the concurrent measurements of carbon dioxide (CO₂), methane (CH₄), and condensation nuclei (CN) that are routinely made at the observatory. All four species showed elevated and highly variable concentrations due to local human activities, principally in the township of Barrow, 7 km to the southwest, and at the DEW Line radar installation 1 km to the northwest. We distinguish between those periods of the record that are affected by local activities and those that are not, on the basis of the short-term (periods of up to 1 hour) variability of the continuous CO₂ and CN records, with large short-term variabilities indicating local sources. We identified seven periods of time (events) with durations ranging from 13 to 37 hours when the BC, CO₂, and CH₄ concentrations changed smoothly over time, were highly correlated with each other, and were not influenced by local activities. These events had BC/CO₂ ratios in the range (50–103)×10^–6. These ratios are dimensionless since we convert the CO₂ concentrations to units of ng m^–3 of carbon. Such values of BC/CO₂ are characteristic of the combustion effluent from large installations burning heavy fuel oil or coal, automobiles, and domestic-scale natural gas usage. We conclude that these events are indicative of air masses that have been polluted with combustion emissions in a distant location and then transported to the Arctic. In the absence of species-selective loss mechanisms, these air masses will maintain their combustion effluent signatures during the transport. The BC/CO₂ ratios found for the local combustion activities are consistent with those expected from known combustion processes.