摘要:

Nitrogen (N) fertilization of agricultural systems is thought to be a major source of the increase in atmospheric N2O; NO emissions from soils have also been shown to increase due to N fertilization. While N fertilizer use is increasing rapidly in the developing world and in the tropics, nearly all of our information on gas emissions is derived from studies of temperate zone agriculture. Using chambers, we measured fluxes of N2O and NO following urea fertilization in tropical sugar cane systems growing on several soil types in the Hawaiian Islands, United States. On the island of Maui, where urea is applied in irrigation lines and soils are mollisols and inceptisols, N2O fluxes were elevated for a week or less after fertilization; maximum average fluxes were typically less than 30 ng cm−2h−1. NO fluxes were often an order of magnitude less than N2O. Together, N2O and NO represented from 0.03 to 0.5% of the applied N. In fields on the island of Hawaii, where urea is broadcast on the surface and soils are andisols, N2O fluxes were similar in magnitude to Maui but remained elevated for much longer periods after fertilization. NO emissions were 2–5 times higher than N2O through most of the sampling periods. Together the gas losses represented approximately 1.1–2.5% of the applied N. Laboratory studies indicate that denitrification is a critical source of N2O in Maui, but that nitrification is more important in Hawaii. Experimental studies suggest that differences in the pattern of N2O/NO and the processes producing them are a result of both carbon availability and placement of fertilizer and that the more information‐intensive fertilizer management practice results in lower

ISSN:0148-0227    

DOI:10.1029/96JD01536

年代:1996

数据来源: WILEY

摘要:

Methane (CH4) fluxes between soils and the atmosphere were measured in two tropical forest‐to‐pasture chronosequences in the state of Rondônia, Brazil. Forest soils always consumed atmospheric CH4with maximum uptake rates in the dry season. Pasture soils consumed atmospheric CH4during the dry season, but at lower rates than those in the forests. When soil moisture increased in the pasture soils, they became a source of CH4to the atmosphere. Integrated over the year, forest soils were a net sink of approximately 470 mg CH4‐C/m2, while pastures were a net source of about 270 mg CH4‐C/m2. Thus forest‐to‐pasture conversion resulted in a net source of CH4from the soil of about 1 g CH4/m2/yr. The total pasture‐related CH4release for the entire Brazilian Amazon increased from 0.8 Tg CH4in 1970 to about 2.5 Tg CH4in 1990, with a maximum of 3.1 Tg CH4/yr in 1988. Soils accounted for a small part (about 5%) of the total CH4release from the basin, while biomass burning and cattle emissions accounted for 95%. The average rate of increase in CH4emission from pastures was about 0.2 Tg CH4/yr between 1975 and 1988. This represents between 12% and 14% of the global average rate of change in tropospheric CH4content for th

ISSN:0148-0227    

DOI:10.1029/96JD01551

年代:1996

数据来源: WILEY

摘要:

Isoprene fluxes were estimated using eight different measurement techniques at a forested site near Oak Ridge, Tennessee, during July and August 1992. Fluxes from individual leaves and entire branches were estimated with four enclosure systems, including one system that controls leaf temperature and light. Variations in isoprene emission with changes in light, temperature, and canopy depth were investigated with leaf enclosure measurements. Representative emission rates for the dominant vegetation in the region were determined with branch enclosure measurements. Species from six tree genera had negligible isoprene emissions, while significant emissions were observed forQuercus,Liquidambar, andNyssaspecies. Above‐canopy isoprene fluxes were estimated with surface layer gradients and relaxed eddy accumulation measurements from a 44‐m tower. Midday net emission fluxes from the canopy were typically 3 to 5 mg C m−2h−1, although net isoprene deposition fluxes of −0.2 to −2 mg C m−2h−1were occasionally observed in early morning and late afternoon. Above‐canopy CO2fluxes estimated by eddy correlation using either an open path sensor or a closed path sensor agreed within ±5%. Relaxed eddy accumulation estimates of CO2fluxes were within 15% of the eddy correlation estimates. Daytime isoprene mixing ratios in the mixed layer were investigated with a tethered balloon sampling system and ranged from 0.2 to 5 ppbv, averaging 0.8 ppbv. The isoprene mixing ratios in the mixed layer above the forested landscape were used to estimate isoprene fluxes of 2 to 8 mg C m−2h−1with mixed layer gradient and mixed layer mass balance techniques. Total foliar density and dominant tree species composition for an approximately 8100 km2region were estimated using high‐resolution (30 m) satellite data with classifications supervised by ground measurements. A biogenic isoprene emission model used to compare flux measurements, ranging from leaf scale (10 cm2) to landscape scale (102km2), indicated agreement to within ±25%, the uncertainty associated with these measurement techniques. Existing biogenic emission models use isoprene emission rate capacities that range from 14.7 to 70 μg C g−1h−1(leaf temperature of 30°C and photosynthetically active radiation of 1000 μmol m−2s−1) for oak foliage. An isoprene emission rate capacity of 100 μg C g−1h−1for oaks in this region is more realistic and is

ISSN:0148-0227    

DOI:10.1029/96JD00697

年代:1996

数据来源: WILEY

摘要:

Differences between measured concentrations of atmospheric trace gases at different locations across the globe contain information about the locations of the sources of these gases. We use an orthogonal function approach and experiments with a three‐dimensional transport model to determine what source distributions can, in principle, be identified from such information. The ability to detect and to locate localized sources is severely limited by the number of observational sites and by the error growth implicit in the inverse problem. The former limitation appears to be the most severe restriction in practice, in contrast to results obtained using two‐dimensional (zonally averaged) models. Differences between results from two and three‐dimensional models are especially severe for tropical so

ISSN:0148-0227    

DOI:10.1029/96JD00573

年代:1996

数据来源: WILEY

摘要:

The global distribution of NOxin the troposphere is calculated using a simple three‐dimensional chemical tracer model. This model includes a simplified chemistry scheme for the tracers NOx≡ NO + NO2and HNO3, which are redistributed by advection, dry and wet convection, and large‐scale diffusion. The sources of NOxconsidered are fossil fuel combustion, emissions from soil microbial activity, biomass burning, lightning discharges, emissions by aircraft, and downward transport from the stratosphere. Dry and wet deposition act as final sinks. At northern middle and high latitudes the calculated tropospheric NOxcontent is dominated by the surface sources, fossil fuel combustion in particular. In the tropical free troposphere, lightning discharges provide about 80% of the total NOxthroughout the year. The zonally averaged fractional contribution of aircraft emissions strongly depends on the season. The largest contribution of this source, over 60%, occurs during January in the upper troposphere between 45°N and 60°N. The NO mixing ratios determined by the model show good overall agreement with vertical profiles measured during the Stratospheric Ozone Experiment (STRATOZ) III aircraft c

ISSN:0148-0227    

DOI:10.1029/96JD01546

年代:1996

数据来源: WILEY

摘要:

The sunrise column abundance of NO3has been measured at Fritz Peak, Colorado (40°N, 105°W), using visible absorption spectroscopy with scattered sky light as the light source. Measurements were made in the “off‐axis” configuration, viewing the sky near the horizon. This is a convenient alternative to direct Moon measurements, and the tropospheric component of the total NO3column can be measured with comparable signal‐to‐noise levels to the direct Moon data. The average tropospheric NO3vertical column measured during August–October 1993 was about 4 × 1013molecule cm−2. Assuming a layer 1 km thick in the lowest part of the troposphere, this corresponds to 4 × 108molecule cm−3, or about 20 pptv. Under these conditions, the nighttime oxidation of isoprene by NO3is comparable to the dayt

ISSN:0148-0227    

DOI:10.1029/96JD01537

年代:1996

数据来源: WILEY

摘要:

A set of photoelectric detectors for airborne measurements of the photolysis frequency of NO2, i.e.,JNO2, was developed and integrated aboard the research aircraft Hercules C‐130 operated by the U.K. Meteorological Office. The instrument consists of two separate sensors, each of which provides an isotropic response over a solid angle of 2π steradian (sr). The sensors are mounted on top and below the aircraft, respectively, to obtain a field of view of 4π sr, and permit the discrimination of the upwelling and downwelling components of the actinic flux. From experimental tests and model calculations it is demonstrated that small differences between the spectral sensitivity of the sensors and the spectral response ofJNO2can lead to significant errors in the determination ofJNO2, especially under cloudy conditions. We present correction factors for clear sky conditions and suggest the use of a new filter combination in the sensors which requires only small corrections and provides acceptable accuracy, even under cloudy conditions. A climatology ofJNO2values is presented from a series of flights made in 1993 at latitudes of 36°–59°N. For clear sky conditions and solar zenith angles of 33°–35°,JNO2was 8.3 × 10−3s−1at sea level and increased with altitude to values of 13 × 10−3s−1at 7.5 km altitude. Above clouds,JNO2reached maximum values of 24 × 10−3s−1, and peak values of 29 × 10−3s−1were observed for very short periods in the uppermost layers of clouds. Enhancement of the actinic flux due to light scattered from clouds was also observed at altitudes below 0.5 km. Comparison of the clear sky data with predictions from different radiative transfer models reveals the best agreement for models of higher angular resolution. The Delta Eddington method underpredicts the measurements significantly, whereas theJNO2values predicted by the discrete ordinate method and multidirectional model are only about 5% smaller than our measurements, a difference that is within

ISSN:0148-0227    

DOI:10.1029/96JD01375

年代:1996

数据来源: WILEY

摘要:

Major chemical species (Cl−, NO−3, SO2−4, Na+, K+, Mg2+, Ca2+) from 24 snowpits (sampled at a resolution of 3 cm, total 2995 samples) collected from northern, central, and southern Greenland were used for this investigation. The annual and seasonal (winter and summer) concentration of each chemical species was calculated and used to study the spatial distribution of chemical species over the central portion of the Greenland Ice Sheet. A two‐sidedt‐distribution test (α = 0.05) suggests that concentrations of major chemical species in snow do not vary significantly over this portion of central Greenland. The relationship between chemical concentration and snow accumulation rate was investigated using annual data from two groups of snowpits: those from coastal sites (northern and southern Greenland); and those from high‐altitude inland sites (central Greenland). The snowpit data from a single group, when examined independently of the other group, show that chemical concentrations do not vary with snow accumulation rate. However, when data from the two groups are integrated into a single data set, pseudorelationships appear, with NO−3concentration decreasing and Na+, K+, Mg2+, and Cl−increasing as snow accumulation rate increases. Therefore we suggest that it is improper to study the relationship between chemical concentration and snow accumulation rate by using data collected from different geographic sites. The relationship between elevation and chemical concentration was investigated using the same suite of annual data sets. We find that Cl−, Na+, and Mg2+concentrations decrease, while NO−3concentration increases, with increasing elevation on the

ISSN:0148-0227    

DOI:10.1029/96JD01061

年代:1996

数据来源: WILEY

摘要:

A two‐dimensional dynamic model with spectral microphysics and a spectral treatment of aerosol particle and gas scavenging (DESCAM) was used to estimate the transport of gases from the marine boundary layer to the free troposphere by a medium‐sized warm precipitating convective cloud. In the simulation, three gases were considered, covering a range of Henry's law constants: an inert tracer, SO2, and H2O2. SO2was also used as the inert tracer by artificially suppressing any interaction with the cloud drops. The horizontal and vertical fluxes, their vertical means and the transport across the cloud boundaries were studied. It was calculated that for SO2as an inert tracer 37 kg, for SO2as a scavenged gas 34 kg, and for H2O212 kg were transported from the marine boundary layer across cloud base to the free troposphere for an estimated three‐dimensional cloud. This represents a depletion of the marine boundary layer in the vicinity of the cloud by about 60%. After about half an hour of cloud life time, however, only 75% of the SO2and only 30% of the H2O2transported aloft still existed in the cloudy air. These residual gases could eventually participate in a long range transport if the cloud would dissipate. The rest had been scavenged by the

ISSN:0148-0227    

DOI:10.1029/96JD01581

年代:1996

数据来源: WILEY

摘要:

We use the natural radionuclides7Be and210Pb as aerosol tracers in a three‐dimensional chemical tracer model (based on the Goddard Institute for Space Studies general circulation model (GCM) 2) in order to study aerosol transport and removal in the troposphere. Beryllium 7, produced in the upper troposphere and stratosphere by cosmic rays, and210Pb, a decay product of soil‐derived222Rn, are tracers of upper and lower tropospheric aerosols, respectively. Their source regions make them particularly suitable for the study of vertical transport processes. Both tracers are removed from the troposphere primarily by precipitation and are useful for testing scavenging parameterizations. In particular, model convection must properly transport and scavenge both ascending210Pb and descending7Be. The ratio7Be/210Pb cancels most model errors associated with precipitation and serves as an indicator of vertical transport. We show that over land the annual average7Be/210Pb ratio for surface concentrations and deposition fluxes vary little globally. In contrast, the seasonal variability of the7Be/210Pb concentration ratio over continents is quite large; the ratio peaks in summer when convective activity is maximum. The model overestimates7Be in the tropics, a problem which we relate to flaws in the GCM parameterization of wet convection (excessive convective mass fluxes and no allowance for entrainment). The residence time of tropospheric7Be calculated by the model is 23 days, in contrast with a value of about 9 days calculated for210Pb, reflecting the high‐altitude versus low‐altitude source regions of these two

ISSN:0148-0227    

DOI:10.1029/96JD01176

年代:1996

数据来源: WILEY