Aerosol scattering properties in northern China

The aerosol scattering properties were investigated at two continental sites in northern China in 2004. Aerosol light scattering coefficient (σ_sp) at 525 nm, PM₁₀, and aerosol mass scattering efficiencies (α) at Dunhuang had a mean value of 165.1±148.8 M m⁻¹, 157.6±270.0 μg m⁻³, and 2.30±3.41 m² g⁻¹, respectively, while these values at Dongsheng were, respectively, 180.2±151.9 M m⁻¹, 119.0±112.9 μg m⁻³, and 1.87±1.41 m² g⁻¹. There existed a seasonal variability of aerosol scattering properties. In spring, at Dunhuang PM₁₀, σ_sp, and α were 184.1±211.548 μg m⁻³, 126.3±89.6 M m⁻¹, and 1.05±0.97 m² g⁻¹, respectively, and these values at Dongsheng were 146.4±142.1 μg m⁻³, 183.4±81.7 M m⁻¹, and 1.98±1.52 m² g⁻¹, respectively. However, in winter at Dunhuang PM₁₀, σ_sp, and α were 158.1±261.4 μg m⁻³, 303.3±165.2 M m⁻¹, and 3.17±1.93 m² g⁻¹, respectively, and these values at Dongsheng were 155.7±170.1 μg m⁻³, 304.4±158.1 M m⁻¹, and 2.90±1.72 m² g⁻¹, respectively. σ_sp and α in winter were higher than that in spring at both the sites, which coincides with the characteristics of dust aerosol and pollution aerosol. Overall, the dominant aerosol types in spring and winter at both sites in northern China are dust aerosol and pollution aerosol, respectively.     

Kinetics of the oxidation of hydrogen sulfite by hydrogen peroxide in aqueous solution:: ionic strength effects and temperature dependence

Conductometry was used to study the kinetics of the oxidation of hydrogen sulfite, HSO⁻₃, by hydrogen peroxide in aqueous non-buffered solution at the low concentration level of 10⁻⁵–10⁻⁶ M, typically found in cloud water. The kinetic data confirm that the rate law reported for the pH range 3–6 at higher concentration levels, rate=k_H·[H⁺]·[HSO⁻₃]·[H₂O₂], is valid at the low concentration level and at low ionic strength I_c. At 298 K and I_c=1.5×10⁻⁴ M, third-order rate constant k_H was found to be k_H=(9.1±0.5)×10⁷ M⁻² s⁻¹. The temperature dependence of k_H led to an activation energy of E_a=29.7±0.9 kJ mol⁻¹. The effect of the ionic strength (adjusted with NaCl) on rate constant k_H was studied in the range I_c=2×10⁻⁴–5.0 M at pH=4.5–5.2 by conductometry and stopped-flow spectrophotometry. The dependence of k_H on I_c can be described with a semi-empirical relationship, which is useful for the purpose of comparison and extrapolation. The kinetic data obtained are critically compared with those reported earlier.     

Photooxidation of methylhydroperoxide and ethylhydroperoxide in the aqueous phase under simulated cloud droplet conditions

The photooxidation of methylhydroperoxide (MHP) and ethylhydroperoxide (EHP) was studied in the aqueous phase under simulated cloud droplet conditions. The kinetics and the reaction products of direct photolysis and OH-oxidation were studied for both compounds. The photolysis frequencies obtained were J_MHP=4.5 (±1.0)×10⁻⁵ s⁻¹ and J_EHP=3.8 (±1.0)×10⁻⁵ s⁻¹ for MHP and EHP respectively at 6 °C. The rate constants of OH-oxidation of MHP at 6 °C were 6.3 (±2.6)×10⁸ M⁻¹ s⁻¹ and 5.8 (±1.9)×10⁸ M⁻¹ s⁻¹ relative to ethanol and 2-propanol respectively, and the rate constant of OH-oxidation of EHP was 2.1 (±0.6)×10⁹ M⁻¹ s⁻¹ relative to 2-propanol at 6 °C. The reaction products obtained were not only the corresponding aldehydes, but also the corresponding acids, and hydroxyhydroperoxides as primary reaction products. The yields for these products were sensitive to the pH value. The carbon balance was higher than 85% for all experiments, showing that most reaction products were detected. A chemical mechanism was proposed for each reaction, and the atmospheric implications were discussed.     

Characteristics of regional nucleation events in urban East St. Louis

Continuous measurements of aerosol size distributions (3 nm–2 μm) were carried out over a 26 month period (1 April 2001–31 May 2003; 650 days with valid data) in urban East St. Louis, IL, as a part of the US Environmental Protection Agency's Supersite program. This paper analyzes data for the 155 days on which “regional nucleation events” were observed during this study. Such events were observed during every month of the study except January 2003. We observed some differences, however, between events in the summer (defined here as April–September) and winter (December–February). Regional nucleation events were observed more frequently in summer months (36±13% of days) than in winter (8±7%), and nucleated particles grew faster in the summer (6.7±4.8 nm h⁻¹) than in winter (1.8±1.9 nm h⁻¹). The daily maximum in the number concentration of nanoparticles formed by nucleation (4.8±3.5×10⁴ cm⁻³) was highly variable and showed no clear seasonal dependence. Particle formation increased particle concentrations by an average factor of 3.1±2.8. Maximum daily rates of 3 nm particle production (17±20 cm⁻³ s⁻¹) were also highly variable and without a clear seasonal dependence. During these events, particle formation rates were typically near their maxima at 08:00–09:00 a.m., but particle production sometimes persisted at diminishing rates until late in the afternoon (15:00–16:00 p.m.).     

Wet deposition of low molecular weight mono- and di-carboxylic acids,aldehydes and inorganic species in Los Angeles

About 60 rainwater samples were collected at west Los Angeles, California in 1981–1984 and were analyzed for C₁–C₉ monocarboxylic acids (0.33–79 μM, average (av.) 13±15 μM), C₂–C₁₀ dicarboxylic acids (2.9–51 μM, av. 7.5±14 μM) and C₁–C₄ aldehydes (0.85–28 μM, av. 9.2±11 μM). Distributions of monocarboxylic acids show a predominance of formic (average concentration: 6.5 μM) and acetic (av. 5.6 μM) acids followed by propionic acid (av. 0.44 μM). Oxalic acid is the dominant diacid (av. 3.9 μM) followed by succinic acid (av. 1.0 μM). Formaldehyde (av. 6.9 μM) is the dominant aldehyde, with the next most abundant, acetaldehyde, being minor (av. 0.65 μM). For select rain samples described in this paper, were found to comprise monocarboxylic acids 0.9–12.3% (av. 4.4±3.4%), diacids comprise 1.2–9.5% (av. 4.2±3.3%) and aldehydes comprise 0.2–6.2% (av. 2.1±2.2%) of total organic carbon (TOC, 2.0–18.6 mg C l⁻¹; av. 9.8±5.4 mg C l⁻¹). Annual rain fluxes of monocarboxylic acids and aldehydes during 1982–1983 were calculated to be 0.24 and 0.11 g m⁻² yr⁻¹, respectively, with an annual estimated wet deposition in the Los Angeles Basin of 3120 and 1430 tons, respectively. These fluxes are equivalent to 2500 times of the acids and 2.5 times of the aldehydes emitted from automobile exhausts in the Los Angeles air basin. This comparison suggests that major portions of the carboxylic acids detected in the rain are not directly emitted from auto-exhausts, but are most likely produced in the atmosphere by gaseous and/or aqueous phase photo-induced reactions.     

Emission factors of PAHs,methoxyphenols, levoglucosan,elemental carbon and organic carbon from simulated wheat and Kentucky bluegrass stubble burns

Emission factors (EFs) of pollutants from post-harvest agricultural burning are required for predicting downwind impacts of smoke and inventorying emissions. EFs of polycyclic aromatic hydrocarbons (PAH), methoxyphenols (MP), levoglucosan (LG), elemental carbon (EC) and organic carbon (OC) from wheat and Kentucky bluegrass (KBG) stubble burning were quantified in a US EPA test burn facility. The PAH and MP EFs for combined solid+gas phases are 17±8.2 mg kg⁻¹ and 79±36 mg kg⁻¹, respectively, for wheat and 21±15 mg kg⁻¹ and 35±24 mg kg⁻¹, respectively, for KBG. LG, particulate EC and artifact-corrected OC EFs are 150±130 mg kg⁻¹, 0.35±0.16 g kg⁻¹ and 1.9±1.1 g kg⁻¹, respectively, for wheat and 350±510 mg kg⁻¹, 0.63±0.056 g kg⁻¹ and 6.9±0.85 g kg⁻¹, respectively, for KBG. Positive artifacts associated with OC sampling were evaluated and remedied with a two-filter system. EC and OC accounted for almost two-thirds of PM_2.5 mass, while LG accounted for just under 3% of the PM_2.5 mass. Since EFs of these pollutants generally decreased with increasing combustion efficiency (CE), identifying and implementing methods of increasing the CEs of burns would help reduce their emissions from agricultural field burning. PAH, OC and EC EFs are comparable to other similar studies reported in literature. MP EFs appear dependent on the stubble type and are lower than the EFs for hard and softwoods reported in literature, possibly due to the lower lignin content in wheat and KBG.     

Isoprene and monoterpene emission from the coniferous species Abies Borisii-regis—implications for regional air chemistry in Greece

The emission of isoprene has been studied from a forest of Abies Borisii-regis, a Mediterranean fir species previously thought to emit only monoterpenes. Emission studies from two independent enclosure experiments indicated a standardised isoprene emission rate of (18.4±3.8) μg g_dry-weight⁻¹ h⁻¹, similar in magnitude to species such as eucalyptus and oak which are considered to be strong isoprene emitters. Isoprene emission depended strongly on both leaf temperature (2°C–34°C) and photosynthetically active radiation (PAR) below 250 μmol m⁻² s⁻¹, becoming saturated with respect to PAR above this value. The annual isoprene emission rate was estimated to be (132±29) kT yr⁻¹ for those trees growing within Greece, comparable to current estimates of the total isoprene budget of Greece as a whole, and contributing significantly to regional ozone and carbon monoxide budgets. Monoterpene emission exhibited exponential temperature dependence, with 1,8-cineole, α-pinene, β-pinene and limonene forming the primary emissions. A standardised total monoterpene emission rate of (2.7±1.1) μg g_dry-weight⁻¹ h⁻¹ was calculated, corresponding to an annual monoterpene emission rate of (24±12) kT yr⁻¹. Research was conducted as part of the AEROBIC’97 (AEROsol formation from BIogenic organic Carbon) series of field campaigns.     

Biogenic nitric oxide emissions from cropland soils

Emissions of nitric oxide (NO) were determined during late spring and summer 1995 and the spring of 1996 from four agricultural soils on which four different crops were grown. These agricultural soils were located at four different sites throughout North Carolina. Emission rates were calculated using a dynamic flow-through chamber system coupled to a mobile laboratory for in-situ analysis. Average NO fluxes during late spring 1995 were: 50.9±47.7 ng N m⁻² s⁻¹ from soil planted with corn in the lower coastal plain. Average NO fluxes during summer 1995 were: 6.4±4.6 and 20.2±19.0 ng N m⁻² s⁻¹, respectively, from soils planted with corn and soybean in the coastal region; 4.2±1.7 ng N m⁻² s⁻¹ from soils planted with tobacco in the piedmont region; and 8.5±4.9 ng N m⁻² s⁻¹ from soils planted with corn in the upper piedmont region. Average NO fluxes for spring 1996 were: 66.7±60.7 ng N m⁻² s⁻¹ from soils planted with wheat in the lower coastal plain; 9.5±2.9 ng N m⁻² s⁻¹ from soils planted with wheat in the coastal plain; 2.7±3.4 ng N m⁻² s⁻¹ from soils planted with wheat in the piedmont region; and 56.1±53.7 ng N m⁻² s⁻¹ from soils planted with corn in the upper piedmont region. An apparent increase in NO flux with soil temperature was present at all of the locations. The composite data from all the research sites revealed a general positive trend of increasing NO flux with soil water content. In general, increases in total extractable nitrogen (TEN) appeared to be related to increased NO emissions within each site, however a consistent trend was not evident across all sites.     

Size-resolved,real-time measurement of water-insoluble aerosols in metropolitan Atlanta during the summer of 2004

During the month of August 2004, the size-resolved number concentration of water-insoluble aerosols (WIA) from 0.25 to 2.0 μm was measured in real-time in the urban center of Atlanta, GA. Simultaneous measurements were performed for the total aerosol size distribution from 0.1 to 2.0 μm, the elemental and organic carbon mass concentration, the aerosol absorption coefficient, and the aerosol scattering coefficient at a dry (RH=30%) humidity. The mean aerosol number concentration in the size range 0.1–2.0 μm was found to be 360±175 cm⁻³, but this quantity fluctuated significantly on time scales of less than one hour and ranged from 25 to 1400 cm⁻³ during the sample period. The mean WIA concentration (0.25–2.0 μm) was 13±7 cm⁻³ and ranged from 1 to 60 cm⁻³. The average insoluble fraction in the size range 0.25–2.0 μm was found to be 4±2.5% with a range of 0.3–38%. The WIA population was found to follow a consistent diurnal pattern throughout the month with concentration maxima concurring with peaks in vehicular traffic flow. WIA concentration also responded to changes in meteorological conditions such as boundary layer depth and precipitation events. The temporal variability of the absorption coefficient followed an identical pattern to that of WIA and ranged from below the detection limit to 55 Mm⁻¹ with a mean of 8±6 Mm⁻¹. The WIA concentration was highly correlated with both the absorption coefficient and the elemental carbon mass concentration, suggesting that WIA measurements are dominated by fresh emissions of elemental carbon. For both the total aerosol and the WIA size distributions, the maximum number concentration was observed at the smallest sizes; however the WIA size distribution also exhibited a peak at 0.45 μm which was not observed in the total population. Over 60% of the particles greater than 1.0 μm were observed to be insoluble in the water sampling stream used by this instrumentation. Due to the refractive properties of black carbon, it is highly unlikely that these particles could be composed of elemental carbon, suggesting a crustal source for super-micron WIA.     

Partitioning of formaldehyde between air and ice at −35°C to −5°C

The equilibrium partitioning of formaldehyde (HCHO) between air and snow was studied in a series of laboratory experiments conducted at −5°C, −15°C, and −35°C, in order to understand how partitioning of HCHO between air and polar snow varies with temperature, and thus seasonally on the ice sheet. Measured partitioning coefficients were 56, 93, and 245 mol l⁻¹ atm⁻¹ for −5°C, −15°C and −35°C, respectively, showing a similar trend as the values previously estimated from field observations. Estimates of the pseudo-first-order rate coefficient for air–snow exchange for the same three temperatures were 4.1×10⁻⁴, 1.1×10⁻⁴, and 1.1×10⁻⁵ s⁻¹, respectively. This implies a time scale for air–snow equilibration of the order of hours to days for HCHO accumulated at or near the ice–air interface on snow grains. Comparing the current laboratory partitioning coefficients with those estimated from measurements of air and freshly fallen snow in Greenland during summer demonstrates that the snow is supersaturated and should degas HCHO to the surrounding air. During this degassing, polar snow should be a significant source of HCHO to the lower troposphere.     

Relative rate measurements of reactions of unsaturated alcohols with atomic chlorine as a function of temperature

The kinetics of two structurally similar unsaturated alcohols, 3-butene-2-ol and 2-methyl-3-butene-2-ol (MBO232), with Cl atoms have been investigated for the first time, as a function of temperature using a relative method. As far as we know, the present work also provides the first value for 3-buten-2-ol. The coefficient at room temperature was also obtained for 2-propene-1-ol (allyl alcohol). The reactions were investigated using a 400 L Teflon reaction chamber coupled with gas chromatograph-coupled with flame-ionization detection (GC-FID) detection. The experiments were performed at atmospheric pressure and at temperatures between 256 and 298 K in air or nitrogen as the bath gas. The obtained kinetic data were used to derive the Arrhenius expressions, k_MBO232=(2.83±2.50)×10⁻¹⁴ exp (2670±249)/T, k_3-buten-2-ol=(0.65±1.60)×10⁻¹⁵ exp (3656±695)/T (in units of cm³ molecule⁻¹ s⁻¹). Finally, results and atmospheric implications are discussed and compared with the reactivity with OH and NO₃ radicals.     

FT-IR kinetic and product study of the Br-radical initiated oxidation of α, β-unsaturated organic carbonyl compounds

Using the relative kinetic technique the kinetics of the gas-phase reactions of Br radicals with acrolein, methacrolein and methylvinyl ketone have been investigated at (301±3) K in 1013 mbar of (N₂+O₂) bath gas at varying proportions. In 1013 mbar of synthetic air the following rate coefficients have been obtained (in units of cm³ molecule⁻¹ s⁻¹): acrolein (3.21±0.11)×10⁻¹²; methacrolein (2.33±0.08)×10⁻¹¹; methyl vinyl ketone (1.87±0.06)×10⁻¹¹. This study represents the first determination of the rate coefficients for these compounds. As for other unsaturated hydrocarbons the rate coefficient with Br was found to increase with increasing partial pressure of O₂. From the product studies of the reactions it has been established that addition of Br radicals to the terminal C-atom is the major pathway in all three cases. However, for acrolein H atom abstraction from the -CO–H group is also significant. Mechanisms are proposed to explain the observed products, mainly β-brominated carbonyl compounds.     

Measurements of GEM fluxes and atmospheric mercury concentrations (GEM,RGM and Hgp) from an agricultural field amended with biosolids in Southern Ont., Canada (October 2004–November 2004)

Five weeks of gaseous elemental mercury (GEM), reactive gaseous mercury (RGM) and particle bound mercury (Hg^p) concentrations as well as fluxes of GEM were measured at Maryhill, Ontario, Canada above a biosolids amended field. The study occurred during the autumn of 2004 (October–November) to capture the effects of cool weather conditions on the behaviour of mercury in the atmosphere. The initial concentration of total mercury (Hg) in the amended soil was relatively low (0.4 μg g⁻¹±10%).A micrometeorological approach was used to infer the flux of GEM using a continuous two-level sampling system with inlets at 0.40 and 1.25 m above the soil surface to measure the GEM concentration gradient. The required turbulent transfer coefficients were derived from meteorological parameters measured on site. The average GEM flux over the study was 0.1±0.2 ng m⁻² h⁻¹(±one standard deviation). The highest averaged hourly GEM fluxes occurred when the averaged net radiation was highest, although the slight diurnal patterns observed were not statistically significant for the complete flux data series. GEM emission fluxes responded to various local events including the passage of a cold front when the flux increased to 2 ng m⁻² h⁻¹ and during a biosolids application event at an adjacent field when depositional fluxes peaked at −3 ng m⁻² h⁻¹. Three substantial rain events during the study kept the surface soil moisture near field capacity and only slightly increased the GEM flux. Average concentrations of RGM (2.3±3.0 pg m⁻³), Hg^p (3.0±6.2 pg m⁻³) and GEM (1.8±0.2 ng m⁻³) remained relatively constant throughout the study except when specific local events resulted in elevated concentrations. The application of biosolids to an adjacent field produced large increases in Hg^p (25.8 pg m⁻³) and RGM (21.7 pg m⁻³) concentrations only when the wind aligned to impact the experimental equipment. Harvest events (corn) in adjacent fields also corresponded to higher concentrations of GEM and Hg^p but with no elevated peaks in RGM concentrations. Diurnal patterns were not statistically significant for RGM and Hg^p at Maryhill.     

Personal exposures to NO2 in the EXPOLIS-study: relation to residential indoor,outdoor and workplace concentrations in Basel,Helsinki and Prague

Personal exposures, residential indoor, outdoor and workplace levels of nitrogen dioxide (NO₂) were measured for 262 urban adult (25–55 years) participants in three EXPOLIS centres (Basel; Switzerland, Helsinki; Finland, and Prague; Czech Republic) using passive samplers for 48-h sampling periods during 1996–1997. The average residential outdoor and indoor NO₂ levels were lowest in Helsinki (24±12 and 18±11 μg m⁻³, respectively), highest in Prague (61±20 and 43±23 μg m⁻³), with Basel in between (36±13 and 27±13 μg m⁻³). Average workplace NO₂ levels, however, were highest in Basel (36±24 μg m⁻³), lowest in Helsinki (27±15 μg m⁻³), with Prague in between (30±18 μg m⁻³). A time-weighted microenvironmental exposure model explained 74% of the personal NO₂ exposure variation in all centres and in average 88% of the exposures. Log-linear regression models, using residential outdoor measurements (fixed site monitoring) combined with residential and work characteristics (i.e. work location, using gas appliances and keeping windows open), explained 48% (37%) of the personal NO₂ exposure variation. Regression models based on ambient fixed site concentrations alone explained only 11–19% of personal NO₂ exposure variation. Thus, ambient fixed site monitoring alone was a poor predictor for personal NO₂ exposure variation, but adding personal questionnaire information can significantly improve the predicting power.     

Characterization of emissions from portable household combustion devices: particle size distributions,emission rates and factors,and potential exposures

A series of source tests were conducted to characterize emissions of particulate matter (PM), carbon monoxide (CO), carbon dioxide (CO₂), methane (CH₄), and total hydrocarbon (THC ) from five types of portable combustion devices. Tested combustion devices included a kerosene lamp, an oil lamp, a kerosene space heater, a portable gas range, and four unscented candles. All tests were conducted either in a well-mixed chamber or a well-mixed room, which enables us to determine emission rates and emission factors using a single-compartment mass balance model. Particle mass concentrations and number concentrations were measured using a nephelometric particle monitor and an eight-channel optical particle counter, respectively. Real-time CO concentrations were measured with an electrochemical sensor CO monitor. CO₂, CH₄, and THC were measured using a GC-FID technique. The results indicate that all particles emitted during steady burning in each of the tested devices were smaller than 1.0 μm in diameter with the vast majority in the range between 0.1 and 0.3 μm. The PM mass emission rates and emission factors for the tested devices ranged from 5.6±0.1 to 142.3±40.8 mg h⁻¹ and from 0.35±0.06 to 9.04±4.0 mg g⁻¹, respectively. The CO emission rates and emission factors ranged from 4.7±3.0 to 226.7±100 mg h⁻¹ and from 0.25±0.12 to 1.56±0.7 mg g⁻¹, respectively. The CO₂ emission rates and emission factors ranged from 5500±700 to 210,000±90,000 mg h⁻¹ and from 387±45 to 1689±640 mg g⁻¹, respectively. The contributions of CH₄ and THC to emission inventories are expected to be insignificant due both to the small emission factors and to the relatively small quantity of fuel consumed by these portable devices. An exposure scenario analysis indicates that every-day use of the kerosene lamp in a village house can generate fine PM exposures easily exceeding the US promulgated NAAQS for PM_2.5.     

Direct radiative forcing and atmospheric absorption by boundary layer aerosols in the southeastern US: model estimates on the basis of new observations

In an effort to reduce uncertainties in the quantification of aerosol direct radiative forcing (ADRF) in the southeastern United States (US), a field column experiment was conducted to measure aerosol radiative properties and effects at Mt. Mitchell, North Carolina, and at an adjacent valley site. The experimental period was from June 1995 to mid-December 1995. The aerosol optical properties (single scattering albedo and asymmetry factor) needed to compute ADRF were obtained on the basis of a procedure involving a Mie code and a radiative transfer code in conjunction with the retrieved aerosol size distribution, aerosol optical depth, and diffuse-to-direct solar irradiance ratio. The regional values of ADRF at the surface and top of atmosphere (TOA), and atmospheric aerosol absorption are derived using the obtained aerosol optical properties as inputs to the column radiation model (CRM) of the community climate model (CCM3). The cloud-free instantaneous TOA ADRFs for highly polluted (HP), marine (M) and continental (C) air masses range from 20.3 to −24.8, 1.3 to −10.4, and 1.9 to −13.4 W m⁻², respectively. The mean cloud-free 24-h ADRFs at the TOA (at the surface) for HP, M, and C air masses are estimated to be −8±4 (−33±16), −7±4 (−13±8), and −0.14±0.05 (−8±3) W m⁻², respectively. On the assumption that the fractional coverage of clouds is 0.61, the annual mean ADRFs at the TOA and the surface are −2±1, and −7±2 W m⁻², respectively. This also implies that aerosols currently heat the atmosphere over the southeastern US by 5±3 W m⁻² on annual timescales due to the aerosol absorption in the troposphere.     

Mercury deposition through litterfall in an Atlantic Forest at Ilha Grande,Southeast Brazil

Atmospheric Hg transfer to the forest soil through litterfall was investigated in a primary rainforest at Ilha Grande (Southeast Brazil) from January to December 1997. Litter mass deposition reached 10.0 t ha⁻¹ y⁻¹, with leaves composing 50–84% of the total litter mass. Concentrations of Hg in the total fallen litter varied from 20 to 244 ng g⁻¹, with higher concentrations during the dry season, between June and August (225 ± 17 ng g⁻¹), and lower concentrations during the rainy season (99 ± 54 ng g⁻¹). This seasonal variability was reflected in the Hg flux through litterfall, which corresponded to a Hg input to the forest floor of 122 μg m⁻² y⁻¹, with average Hg deposition of 16.5 ± 1.5 μg m⁻² month⁻¹ during and just after the dry season (June–September) and 7.0 ± 3.6 μg m⁻² month⁻¹ in the rest of the year. The variability in meteorological conditions (determining atmospheric Hg availability to foliar scavenging) may explain the pulsed pattern of Hg deposition, since litterfall temporal variability was generally unrelated with such deposition, except by a peak in litterfall production in September. Comparisons with regional data on Hg atmospheric deposition show that litterfall promotes Hg deposition at Ilha Grande two to three orders of magnitude higher than open rainfall deposition in non-industrialized areas and approximately two times higher than open rainfall deposition in industrialized areas in Rio de Janeiro State. The observed input suggests that atmospheric Hg transfer through litterfall may explain a larger fraction of the total Hg input to forest soils in Southeast Brazil than those recorded at higher latitudes.     

Simultaneous use of relaxed eddy accumulation and gradient flux techniques for the measurement of sea-to-air exchange of dimethyl sulphide

The sea-to-air flux of the biogenic volatile sulphur compound dimethyl sulphide was assessed with the relaxed eddy accumulation (REA) and the gradient flux (GF) techniques from a stationary platform in the coastal Atlantic Ocean. Fluxes varied between 2 and 16 μmol m⁻² d⁻¹. Fluxes derived from REA were on average 7.1±5.03 μmol m⁻² d⁻¹, not significantly different from the average flux of 5.3±2.3 μmol m⁻² d⁻¹ derived from GF measurements. Gas transfer velocities were calculated from the fluxes and seawater DMS concentrations. They were within the range of gas transfer rates derived from the commonly used parameterizations that relate gas transfer to wind speed.     

Nitric oxide emissions from soils amended with municipal waste biosolids

Land spreading nitrogen-rich municipal waste biosolids (NO₃⁻-N<256 mg N kg⁻¹ dry weight, NH₃-N∼23,080 mg N kg⁻¹ dry weight, Total Kjeldahl N∼41,700 mg N kg⁻¹ dry weight) to human food and non-food chain land is a practice followed throughout the US. This practice may lead to the recovery and utilization of the nitrogen by vegetation, but it may also lead to emissions of biogenic nitric oxide (NO), which may enhance ozone pollution in the lower levels of the troposphere. Recent global estimates of biogenic NO emissions from soils are cited in the literature, which are based on field measurements of NO emissions from various agricultural and non-agricultural fields. However, biogenic emissions of NO from soils amended with biosolids are lacking. Utilizing a state-of-the-art mobile laboratory and a dynamic flow-through chamber system, in-situ concentrations of nitric oxide (NO) were measured during the spring/summer of 1999 and winter/spring of 2000 from an agricultural soil which is routinely amended with municipal waste biosolids. The average NO flux for the late spring/summer time period (10 June 1999–5 August 1999) was 69.4±34.9 ng N m⁻² s⁻¹. Biosolids were applied during September 1999 and the field site was sampled again during winter/spring 2000 (28 February 2000–9 March 2000), during which the average flux was 3.6±1.7 ng N m⁻² s⁻¹. The same field site was sampled again in late spring (2–9 June 2000) and the average flux was 64.8±41.0 ng N m⁻² s⁻¹. An observationally based model, developed as part of this study, found that summer accounted for 60% of the yearly emission while fall, winter and spring accounted for 20%, 4% and 16% respectively. Field experiments were conducted which indicated that the application of biosolids increases the emissions of NO and that techniques to estimate biogenic NO emissions would, on a yearly average, underestimate the NO flux from this field by a factor of 26. Soil temperature and % water filled pore space (%WFPS) were observed to be significant variables for predicting NO emissions, however %WFPS was found to be most significant during high soil temperature conditions. In the range of pH values found at this site (5.8±0.3), pH was not observed to be a significant parameter in predicting NO emissions.     

Air–land exchanges of CO2, CH4 and N2O measured by FTIR spectrometry and micrometeorological techniques

Micrometeorological flux-gradient and nocturnal boundary layer methods were combined with Fourier transform infrared (FTIR) spectroscopy for high-precision trace gas analysis to measure fluxes of the trace gases CO₂, CH₄ and N₂O between agricultural fields and the atmosphere. The FTIR measurements were fully automated and routinely obtained a precision of 0.1–0.2% for several weeks during a measurement campaign in October 1995. In flux-gradient measurements, vertical profiles of the trace gases were measured every 30 min from the ground to 22 m. When combined with independent micrometeorological measurements of water vapour fluxes, trace gas fluxes from the underlying surface could be determined. In the nocturnal boundary layer method the rate of change in mass storage in the 0–22 m layer was combined with fluxes measured at 22 m to estimate surface fluxes. Daytime fluxes for CO₂ were −0.78±0.40 (1σ) mg CO₂ m⁻² s⁻¹. Daytime fluxes of N₂O and CH₄ were very small and difficult to measure reliably using the flux-gradient technique, despite the high precision of the concentration measurements. Mean daytime flux for N₂O was 17±48 ng N m⁻² s⁻¹, while the corresponding flux for CH₄ was 47±410 ng CH₄ m⁻² s⁻¹. The mean nighttime flux of CO₂ estimated using the nocturnal boundary layer method was +0.15±0.05 mg CO₂ m⁻² s⁻¹, in good agreement with chamber measurements of respiration rates. Nighttime fluxes of CH₄ and N₂O from the nocturnal boundary layer method were 109±69 ng CH₄ m⁻² s⁻¹ and 2±3.2 ng N m⁻² s⁻¹, respectively, in good agreement with chamber measurements and inventory estimates based on the sheep and cattle stocking rates in the region. The suitability of FTIR-based methods for long term monitoring of spatially and temporally averaged flux measurements is discussed.