Seasonal Emission Patterns Research Articles

Outdoor airborne allergens: Characterization, behavior and monitoring in Europe

Aeroallergens or inhalant allergens, are proteins dispersed through the air and have the potential to induce allergic conditions such as rhinitis, conjunctivitis, and asthma. Outdoor aeroallergens are found predominantly in pollen grains and fungal spores, which are allergen carriers. Aeroallergens from pollen and fungi have seasonal emission patterns that correlate with plant pollination and fungal sporulation and are strongly associated with atmospheric weather conditions. They are released when allergen carriers come in contact with the respiratory system, e.g. the nasal mucosa. In addition, due to the rupture of allergen carriers, airborne allergen molecules may be released directly into the air in the form of micronic and submicronic particles (cytoplasmic debris, cell wall fragments, droplets etc.) or adhered onto other airborne particulate matter. Therefore, aeroallergen detection strategies must consider, in addition to the allergen carriers, the allergen molecules themselves. This review article aims to present the current knowledge on inhalant allergens in the outdoor environment, their structure, localization, and factors affecting their production, transformation, release or degradation. In addition, methods for collecting and quantifying aeroallergens are listed and thoroughly discussed. Finally, the knowledge gaps, challenges and implications associated with aeroallergen analysis are described.

Occurrence, removal, emission and environment risk of 32 antibiotics and metabolites in wastewater treatment plants in Wuhu, China

Wastewater treatment plants (WWTPs) are considered important sources of antibiotics and metabolites in aquatic environments and pose a serious threat to the safety of aquatic organisms. In this study, we investigated the seasonal occurrence, removal, emission, and environmental risk assessment (ERA) of 32 antibiotics and metabolites at four WWTPs located in Wuhu, China. The main findings of this study are as follows: Ofloxacin concentrations dominated all WWTPs, and large quantities of sulfachinoxalin were only detected in WWTP 2 treating mixed sewage. The average apparent removal of individual parent antibiotics or metabolites ranged from −94.7 to 100 %. There was a noticeable seasonal emission pattern (independent t-test, t = 9.89, p < 0.001), with lower emissions observed during summer. WWTPs discharged 85.2 ± 43.8 g of antibiotics and metabolites each day. Approximately 87 % of emissions were discharged into the mainstream of the Yangtze River, while the remainder were discharged into its tributary, the Zhanghe River. The total emissions of 21 parent antibiotics were approximately 18 % of the prescription data, indicating that a considerable and alarming amount of prototype drugs entered the receiving water body. Based on the risk quotient (RQ) of the ERA, the Zhanghe River has a moderate risk of ofloxacin (RQ = 0.111–0.583), a low or insignificant risk of sulfamethoxazole (RQ = 0.003–0.048), and an insignificant risk of other antibiotics or metabolites. However, the risk of antibiotics or metabolites in the mainstream of Yangtze River is insignificant. This study could help understand the seasonal emission patterns of antibiotics and metabolites, as well as more antibiotics sensitive of environmental risks in tributary than that in mainstream.

Estimating methane emissions in the Arctic nations using surface observations from 2008 to 2019

Abstract. The Arctic is a critical region in terms of global warming. Environmental changes are already progressing steadily in high northern latitudes, whereby, among other effects, a high potential for enhanced methane (CH4) emissions is induced. With CH4 being a potent greenhouse gas, additional emissions from Arctic regions may intensify global warming in the future through positive feedback. Various natural and anthropogenic sources are currently contributing to the Arctic's CH4 budget; however, the quantification of those emissions remains challenging. Assessing the amount of CH4 emissions in the Arctic and their contribution to the global budget still remains challenging. On the one hand, this is due to the difficulties in carrying out accurate measurements in such remote areas. Besides, large variations in the spatial distribution of methane sources and a poor understanding of the effects of ongoing changes in carbon decomposition, vegetation and hydrology also complicate the assessment. Therefore, the aim of this work is to reduce uncertainties in current bottom-up estimates of CH4 emissions as well as soil oxidation by implementing an inverse modelling approach in order to better quantify CH4 sources and sinks for the most recent years (2008 to 2019). More precisely, the objective is to detect occurring trends in the CH4 emissions and potential changes in seasonal emission patterns. The implementation of the inversion included footprint simulations obtained with the atmospheric transport model FLEXPART (FLEXible PARTicle dispersion model), various emission estimates from inventories and land surface models, and data on atmospheric CH4 concentrations from 41 surface observation sites in the Arctic nations. The results of the inversion showed that the majority of the CH4 sources currently present in high northern latitudes are poorly constrained by the existing observation network. Therefore, conclusions on trends and changes in the seasonal cycle could not be obtained for the corresponding CH4 sectors. Only CH4 fluxes from wetlands are adequately constrained, predominantly in North America. Within the period under study, wetland emissions show a slight negative trend in North America and a slight positive trend in East Eurasia. Overall, the estimated CH4 emissions are lower compared to the bottom-up estimates but higher than similar results from global inversions.

Linking seasonal N2O emissions and nitrification failures to microbial dynamics in a SBR wastewater treatment plant

Nitrous oxide (N2O) is a strong greenhouse gas and causal for stratospheric ozone depletion. During biological nitrogen removal in wastewater treatment plants (WWTP), high N2O fluxes to the atmosphere can occur, typically exhibiting a seasonal emission pattern. Attempts to explain the peak emission phases in winter and spring using physico-chemical process data from WWTP were so far unsuccessful and new approaches are required. The complex and diverse microbial community of activated sludge used in biological treatment systems also exhibit substantial seasonal patterns. However, a potentially causal link between the seasonal patterns of microbial diversity and N2O emissions has not yet been investigated. Here we show that in a full-scale WWTP nitrification failure and N2O peak emissions, bad settleability of the activated sludge and a turbid effluent strongly correlate with a significant reduction in the microbial community diversity and shifts in community composition. During episodes of impaired performance, we observed a significant reduction in abundance for filamentous and nitrite oxidizing bacteria in all affected reactors. In some reactors that did not exhibit nitrification and settling failures, we observed a stable microbial community and no drastic loss of species. Standard engineering approaches to stabilize nitrification, such as increasing the aerobic sludge age and oxygen availability failed to improve the plant performance on this particular WWTP and replacing the activated sludge was the only measure applied by the operators to recover treatment performance in affected reactors. Our results demonstrate that disturbances of the sludge microbiome affect key structural and functional microbial groups, which lead to seasonal N2O emission patterns. To reduce N2O emissions from WWTP, it is therefore crucial to understand the drivers that lead to the microbial population dynamics in the activated sludge.

川西平原小流域不同水体N2O排放特征及驱动因素

氧化亚氮（N₂O）是一种潜在的、强大的温室气体，应该根据京都议定书规定开展监测和削减。河流、水库、鱼塘和沟渠等受人类影响的小流域水生生态系统是氮素生物地球化学循环的活跃区域，更是N₂O重要的源和汇。然而，同一流域不同水体N₂O的排放特征差异及其驱动因素尚不清楚。因此，选择川西平原西河流域作为研究区，于2016年6月到2017年5月连续监测不同水体水气界面的N₂O排放强度，并结合聚类分析解析N₂O排放特征的驱动因素。结果显示，不同水体的N₂O年排放通量差异显著，沟渠的N₂O年排放通量最高（（52.68±36.09）μg m^-2 h^-1），城市段河流和鱼塘次之（（34.16±23.97）μg m^-2 h^-1和（29.03±31.41）μg m^-2 h^-1），乡镇段和农区段河流再次（（8.32±28.60）μg m^-2 h^-1和（8.52±9.43）μg m^-2 h^-1），水库最低（（-16.45±29.76）μg m^-2 h^-1）。除水库表现为N₂O的汇，其他水体均表现为N₂O的排放源。另外，不同水体N₂O排放的季节特征差异显著，农区段河流和农业沟渠表现为夏天最高，冬春最低（P<0.05），而其他水体均表现为冬春显著高于夏秋（P<0.05）。根据N₂O排放季节特征及其驱动因素可将西河流域水体分为四类：第一类农业类水体的N₂O排放季节特征受气象因素和农业活动的联合驱动；第二类城乡类河流和第三类鱼塘分别受控于人类活动和养殖活动，与降雨温度等气象指标关系较弱；第四类水库主要受控于气象因素。并且，第一类农业类水体已成为大气N₂O排放的重要源，农业氮素管控是区域控制N₂O排放的重点。;Nitrous oxide (N₂O) is a powerful, long-lived greenhouse gas that should be monitored and reduced under the Kyoto Protocol. Aquatic ecosystem in small and human-impacted watershed, including river, reservoir, fishpond and ditch, is the vital area for biogeochemical cycle of nitrogen as well as the important source and sink for N₂O. However, flux, seasonal patterns and driving factors of N₂O emission from different aquatic ecosystems are still unknown. To help address this knowledge gap, monthly flux of N₂O emission from different aquatic ecosystems were measured from June 2016 to May 2017, and driving factors of N₂O seasonal emission pattern were researched by Cluster analysis and Pearson correlation analysis in Xihe watershed, west of Sichuan Province, China. The results show that annual flux of N₂O emission varied from different aquatic ecosystems, which were ranged as follow:ditch ((52.68±36.09) μg m^-2 h^-1) > urban river ((34.16±23.97) μg m^-2 h^-1) > fish pond ((29.03±31.41) μg m^-2 h^-1) > rural and agricultural river ((8.32±28.60) and (8.52±9.43) μg m^-2 h^-1) > reservoir ((-16.45±29.76) μg m^-2 h^-1). Ditch, river and fish pond, which were influenced by human activities, were sources of N₂O emissions. While, reservoir, which was hardly affected by human beings, was the sink of N₂O. In addition, seasonal pattern of N₂O emission varied from different aquatic ecosystems. Mean N₂O emissions during summer were higher than winter and spring in agricultural river and ditches significantly (P<0.05). While, mean N₂O emissions during winter and spring were higher than summer and fall in other rivers, fish pond and reservoir significantly (P<0.05). Finally, the aquatic ecosystems were divided into four types in terms of seasonal patterns and drivers of N₂O emission. Agricultural river and ditches were the typeⅠaquatic ecosystems, and their seasonal patterns of N₂O emission were driven by meteorological indicators and agricultural activities. Rural-urban rivers and fishpond were the typeⅡand type Ⅲ aquatic ecosystems, and their seasonal patterns of N₂O emission showed weak relationship with precipitation and temperature, but were driven by rural-urban activities and aquaculture management, respectively. Reservoir was the type Ⅳ aquatic ecosystems, and its seasonal patterns of N₂O emission had negative relationship with temperature significantly (P<0.05), suggesting that it was driven by meteorological indicators. In this study, the typeⅠaquatic ecosystems (agricultural river and ditches) has become the most important source of N₂O emission, and N loss control during agricultural activities was the most important measure for N₂O emission control.

Effects of contrasting catch crops on nitrogen availability and nitrous oxide emissions in an organic cropping system

Legume-based catch crops (LBCCs) may act as an important source of nitrogen (N) in organic crop rotations because of biological N fixation. However, the potential risk of high nitrous oxide (N2O) emissions needs to be taken into account when including LBCCs in crop rotations. Here, we report the results from a one-year field experiment, which investigated N availability and N2O emissions as affected by three LBCCs, i.e., red clover (CL), red clover–ryegrass mixture (GC) and winter vetch (WV), two non-LBCCs, i.e., perennial ryegrass (GR) and fodder radish (FR), and a control (CO) without catch crops. The effect of two catch crop management strategies was also tested: autumn harvest of the catch crop versus incorporation of whole-crop residues by spring ploughing. LBCCs accumulated 59–67kgNha−1 in their tops, significantly more than those of the non-LBCC, 32–40kgNha−1. Macro-roots accounted for >33% of total N in the catch crops. In accordance with this, LBCCs enhanced the performance of the succeeding unfertilised spring barley, thus obtaining a grain yield of 3.3–4.5Mgha−1 compared to 2.6–3.3Mgha−1 grain yield from non-LBCC and the fallow control treatments. Autumn harvest of catch crops, especially LBCCs, tended to reduce crop yield. The annual N2O emissions were comparable across treatments except for fodder radish, which had the highest N2O emission, and also the highest average yield-scaled N2O emission, at 499gN2O-NMg−1 grain. Although the sampling strategy employed in this study introduces uncertainty about the spatial and temporal variability, differences in seasonal emission patterns among catch crops were captured and harvest of catch crops in late autumn induced significantly higher emissions during winter, but lower emissions after residue incorporation in spring. In comparison with non-LBCC, LBCCs have the potential to partly replace the effect of manure application in organic cropping systems with greater crop production and less environmental footprint with respect to N2O emissions. However, harvest of the catch crops may reduce crop yield unless the harvested N is recycled as fertiliser to the crops in the rotation.

Isoprene emissions track the seasonal cycle of canopy temperature, not primary production: evidence from remote sensing

Abstract. Isoprene is important in atmospheric chemistry, but its seasonal emission pattern – especially in the tropics, where most isoprene is emitted – is incompletely understood. We set out to discover generalized relationships applicable across many biomes between large-scale isoprene emission and a series of potential predictor variables, including both observed and model-estimated variables related to gross primary production (GPP) and canopy temperature. We used remotely sensed atmospheric concentrations of formaldehyde, an intermediate oxidation product of isoprene, as a proxy for isoprene emission in 22 regions selected to span high to low latitudes, to sample major biomes, and to minimize interference from pyrogenic sources of volatile organic compounds that could interfere with the isoprene signal. Formaldehyde concentrations showed the highest average seasonal correlations with remotely sensed (r = 0.85) and model-estimated (r = 0.80) canopy temperatures. Both variables predicted formaldehyde concentrations better than air temperature (r= 0.56) and a "reference" isoprene model that combines GPP and an exponential function of temperature (r = 0.49), and far better than either remotely sensed green vegetation cover, fPAR (r = 0.25) or model-estimated GPP (r = 0.14). Gross primary production in tropical regions was anti-correlated with formaldehyde concentration (r = −0.30), which peaks during the dry season. Our results were most reliable in the tropics, where formaldehyde observational errors were the least. The tropics are of particular interest because they are the greatest source of isoprene emission as well as the region where previous modelling attempts have been least successful. We conjecture that positive correlations of isoprene emission with GPP and air temperature (as found in temperate forests) may arise simply because both covary with canopy temperature, peaking during the relatively short growing season. The lack of a general correlation between GPP and formaldehyde concentration in the seasonal cycle is consistent with experimental evidence that isoprene emission rates are largely decoupled from photosynthetic rates, and with the likely adaptive significance of isoprene emission in protecting leaves against heat damage and oxidative stress.

Constraining carbonaceous aerosol sources in a receptor model by including 14C data with redox species, organic tracers, and elemental/organic carbon measurements

Sources of carbonaceous PM2.5 were quantified in downtown Cleveland, OH and Chippewa Lake, OH located ∼40 miles southwest of Cleveland during the Cleveland Multiple Air Pollutant Study (CMAPS). PM2.5 filter samples were collected daily during July–August 2009 and February 2010 to establish the seasonal emission patterns from local and regional sources. Radiocarbon (14C), redox species (NOx, SO2 and ozone), 28 primary and secondary organic aerosol tracers, elemental carbon (EC) and organic carbon (OC) measurements were analyzed using the EPA Positive Matrix Factorization (PMF) model to apportion carbonaceous aerosol sources. Five sources were identified at each site: mobile sources, fossil fuel combustion from fuels containing sulfur, local biomass combustion, other combustion (regional biomass, waste, meat, coal), and secondary organic aerosol (SOA). 14C data were incorporated in the PMF analysis as a novel method to obtain the modern carbon fraction (fmod) of each source individually which aided all factor interpretations. SOA was the principal carbon source during summer as shown by the PMF analysis and a separate tracer based mass fraction method while biomass burning and other combustion sources were dominant in winter. Elevated levels of EC and fossilized carbon in downtown Cleveland are primarily attributed to increased mobile source and coal combustion emissions.

Global terrestrial isoprene emission models: sensitivity to variability in climate and vegetation

Abstract. Due to its effects on the atmospheric lifetime of methane, the burdens of tropospheric ozone and growth of secondary organic aerosol, isoprene is central among the biogenic compounds that need to be taken into account for assessment of anthropogenic air pollution-climate change interactions. Lack of process-understanding regarding leaf isoprene production as well as of suitable observations to constrain and evaluate regional or global simulation results add large uncertainties to past, present and future emissions estimates. Focusing on contemporary climate conditions, we compare three global isoprene models that differ in their representation of vegetation and isoprene emission algorithm. We specifically aim to investigate the between- and within model variation that is introduced by varying some of the models' main features, and to determine which spatial and/or temporal features are robust between models and different experimental set-ups. In their individual standard configurations, the models broadly agree with respect to the chief isoprene sources and emission seasonality, with maximum monthly emission rates around 20–25 Tg C, when averaged by 30-degree latitudinal bands. They also indicate relatively small (approximately 5 to 10 % around the mean) interannual variability of total global emissions. The models are sensitive to changes in one or more of their main model components and drivers (e.g., underlying vegetation fields, climate input) which can yield increases or decreases in total annual emissions of cumulatively by more than 30 %. Varying drivers also strongly alters the seasonal emission pattern. The variable response needs to be interpreted in view of the vegetation emission capacities, as well as diverging absolute and regional distribution of light, radiation and temperature, but the direction of the simulated emission changes was not as uniform as anticipated. Our results highlight the need for modellers to evaluate their implementations of isoprene emission models carefully when performing simulations that use non-standard emission model configurations.

Greenhouse gas fluxes associated with soybean production under two tillage systems in southwestern Quebec

Agricultural soils are an important contributor to greenhouse gas emissions and the size of this contribution can be influenced by tillage practice and crop. The objective of this work was to study greenhouse gas (carbon dioxide—CO 2 and nitrous oxide—N 2O) emissions associated with N 2 fixing soybean ( Glycine max) grown under two tillage systems (conventional—CT and no-till—NT). The experiment was organized following a randomized complete block design with four blocks. The CO 2 and N 2O fluxes were evaluated throughout the growing seasons of 2002 and 2003. The seasonal emission patterns were different for CO 2 and N 2O. Soil CO 2 emissions during the season were associated with soil temperature while the N 2O fluxes were mainly associated with precipitation. The CT system generally had greater CO 2 fluxes than the NT system, particularly in 2002. In that year the maximum peak, which occurred in the summer, was about 160 g m −2 d −1 under CT and 68 g m −2 d −1 under NT. N 2O emissions were low in the first year but high in the second, and were greater for CT than NT with a maximum peak about 18.1 mg m −2 d −1 under CT and 7.4 mg m −2 d −1 under NT. Our findings suggests that use of NT in the production of N 2 fixing soybean may reduce both CO 2 and N 2O emissions, in comparison to CT. Soybean residue incorporation increased N 2O emissions, leading to greater emissions from the CT production system.

Carbon Dioxide and Nitrous Oxide Fluxes in Corn Grown under Two Tillage Systems in Southwestern Quebec

Agriculture has an important potential role in mitigating greenhouse gas emissions (GHG). However, practices that reduce CO2 emissions from soils and increase the soil organic C level may stimulate N2O emissions. This is particularly critical in Quebec where heavy soils and a humid climate may limit the adoption of agricultural practices designed to mitigate GHG. The objective of this work was to study the effects of two tillage and N fertilization regimes on CO2 and N2O fluxes and the seasonal variability in emissions of these gases, associated with corn (Zea mays L.) grown in southwestern Quebec. Different seasonal emission patterns of CO2 and N2O were observed. Higher N2O fluxes occurred during the spring and were associated with precipitation events, while higher CO2 fluxes occurred in mid‐season and were related to temperature. Conventional tillage (CT) had greater peaks of CO2 emissions than no‐till (NT) only after disking in the spring. Once corn was established, differences between tillage systems were small. Peaks of N2O emission occurred in both systems (NT and CT) following N application. Plots receiving 180 kg N ha−1 in both tillage systems had large peak of N2O emission rates during the wettest parts of the season. The CT and NT systems generally had similar cumulative CO2 emissions but NT had higher cumulative N2O emissions than CT. Our findings suggests that changing from CT to NT under the heavy soil conditions of Quebec may increase GHG, mainly as result of the increase in N2O emission. This negative effect of NT could be reduced by avoiding fertilizing when precipitation is more intense.

Emissions of nitrous oxide after application of dairy slurry on bare soil and perennial grass in a maritime climate

Over half the slurry manure produced on dairy farms in the high-rainfall, coastal region of British Columbia (BC), Washington State and northern Oregon is applied from mid-February to early May. This study was conducted to compare the emissions of nitrous oxide (N2O) after manure application during this period on perennial forage grass or winter fallow land. The experimental site soil was moderately well- to well-drained medium-textured river deposit of the Monroe series. Treatments consisted of liquid dairy manure applied either at 270 (Early) or 450 (Late) Tsum (accumulation of average air temperatures above 0ºC from Jan. 01) on bare land or a perennial stand of tall fescue (Festuca arundinacea Schreb.) at a rate of 55.5 m3 ha-1 giving a total ammoniacal N (TAN) loading of 100 and 111 kg ha-1 in 2001 and 2002, respectively. An additional grass treatment consisted of split applications (Split) of manure at half the rate on each of the two application dates. Untreated (Control) bare and grass treatments were also included. Emissions were monitored for 105 d (28 Feb. to 12 Jun.) in 2001 and 121 d (26 Feb. to 26 Jun.) in 2002. Cumulative N2O emissions, during the measurement period (averaged over manure application times), from manured bare soil were 2.19 and 2.74 kg N ha-1 for 2001 and 2002, respectively and those from manured grass treatment were 0.21 and 0.58 kg N ha-1 for 2001 and 2002, respectively. Time of application altered seasonal emission pattern, but effect on total emission was inconsistent, probably due to conflicting effects of temperature and moisture. Significant differences in soil NO3-N levels between the grass and the bare soil treatments may explain the differences in N2O emission. Key words: Mineral N, herbage, water-filled pore space, winter fallow, Festuca arundinacea Schreb

Seasonal trends in vegetation and atmospheric concentrations of PAHs and PBDEs near a sanitary landfill

Spruce needle and atmospheric (gaseous and particulate-bound) concentrations were surveyed near a sanitary landfill from February 2004 to June 2005. The influence of several parameters such as temperature, relative humidity, wind speed and direction, as well as increased domestic heating during the winter was assessed. In general, polybrominated diphenyl ethers (PBDE) and polycyclic aromatic hydrocarbons (PAH) concentrations in spruce needles increased over time and decreased following snowmelt with a minimum coinciding with bud burst of deciduous trees. Atmospheric concentrations of PBDE and PAH, both particulate-bound and gaseous phase, were linked to daily weather events and thus showed more variability than those in spruce needles. Highest PAH concentrations were encountered during the winter, likely reflecting increased emission from heating homes. Pseudo Clausius-Clapeyron plots revealed higher PBDE gaseous concentrations with increasing temperature, but showed no correlation between PAH gaseous concentrations and temperature as this effect was obscured by seasonal emission patterns. Finally, air mass back trajectories and local wind directions revealed that particulate-bound PBDEs, along with both gaseous and particulate-bound PAHs were from local sources, whereas gaseous PBDEs were likely from distant sources.

Modelling Seasonal Dynamics from Temporal Variation in Agricultural Practices in the UK Ammonia Emission Inventory

Most ammonia (NH3) emission inventories have been calculated on an annual basis and do not take into account the seasonal variability of emissions that occur as a consequence of climate and agricultural practices that change throughout the year. When used as input to atmospheric transport models to simulate concentration fields, these models therefore fail to capture seasonal variations in ammonia concentration and dry and wet deposition. In this study, seasonal NH3 emissions from agriculture were modelled on a monthly basis for the year 2000, by incorporating temporal aspects of farming practice. These monthly emissions were then spatially distributed using the AENEID model (Atmospheric Emissions for National Environmental Impacts Determination). The monthly model took the temporal variation in the magnitude of the ammonia emissions, as well as the fine scale (1-km) spatial variation of those temporal changes into account to provide improved outputs at 5-km resolution. The resulting NH3 emission maps showed a strong seasonal emission pattern, with the highest emissions during springtime (March and April) and the lowest emissions during summer (May to July). This emission pattern was mainly influenced by whether cattle were outside grazing or housed and by the application of manures and fertilizers to the land. When the modelled emissions were compared with measured NH3 concentrations, the comparison suggested that the modelled emission trend corresponds fairly well with the seasonal trend in the measurements. The remaining discrepancies point to the need to develop functional parametrisations of the interactions with climatic seasonal variation.

Nitrous oxide emission inventory of German forest soils

Annual fluxes of N2O trace gas emissions were assessed after stratifying German forest soils into Seasonal Emission Pattern (SEP) and Background Emission Pattern (BEP). Broad‐leaved forests with soil pH(KCl) ≤ 3.3 were assigned to have SEP, broad‐leaved forests with soil pH(KCl) &gt; 3.3 and all needle‐leaved forests to have BEP. BEPs were estimated by a relationship between annual N2O emissions and carbon content of the O‐horizon. SEPs were primarily controlled by temperature and moisture and simulated by the model Expert‐N after calibration to a 9‐year record of N2O measurements. Analysis with different climate and soil properties indicated that the model reacts highly sensitive to changes in soil temperature, soil moisture, and soil texture. A geographic information system (ARC/INFO) was used for a spatial resolution of 1 km × 1 km grid where land cover, dominant soil units, and hygro climate classes were combined. The mean annual N2O emission flux from German forest soils was estimated as 0.32 kg ha−1 yr−1. Broad‐leaved forests with SEP had the highest emissions (2.05 kg ha−1 yr−1) followed by mixed forests (0.38 kg ha−1 yr−1), broad‐leaved forests (0.37 kg ha−1 yr−1), and needle‐leaved forests with BEP (0.17 kg ha−1 yr−1). The annual N2O emission from German forest soils was calculated as 3.26 Gg N2O‐N yr−1. Although needle‐leaved trees cover about 57% of the entire forest area in Germany, their contribution is low (0.96 Gg N2O‐N yr−1). Broad‐leaved forests cover about 22% of the forest area but have 55% higher emissions (1.49 Gg N2O‐N yr−1) than needle‐leaved. Mixed forests cover 21% of the area and contribute 0.81 Gg N2O‐N yr−1. Compared to the total N2O emissions in Germany of 170 Gg N yr−1, forest soils contribute only 1.9%. However, there are some uncertainties in this emission inventory, which are intensely discussed.

A comparative study of the gas exchange potential between three wetland species using sulfur hexafluoride as a tracer

The gas-exchange potential of three wetland species (helophytes) was examined in an aquatic model vegetation facility (AMOVA) using sulfur hexafluoride (SF 6) as a tracer. Three beds containing gravel and vegetated with Phragmites australis, Typha latifolia and Schoenoplectus lacustris were compared to an unvegetated gravel bed as a reference. A mass balance of SF 6 emissions revealed a different seasonal emission pattern for the three species investigated. Helophytes capable of pressurized ventilation, e.g. Phragmites and Typha, showed the highest gas-exchange values. On an annual basis net SF 6 emission values of Phragmites were the highest in the autumn month of October (65.8%) and the lowest in the winter month of January (37.1%). P. australis represents a helophyte species with highly developed gas-exchange tissues facilitating diffusion and transport of the tracer gas SF 6 to the atmosphere via the intercellular lacunar system.

Hierarchical control on nitrous oxide emission in forest ecosystems

While much is known about process level control on N2O production by nitrification and denitrification, knowledge of the environmental controls responsible for site variation in annual N2O fluxes on ecosystem level is low. Our goal was to improve existing concepts of controls on N2O fluxes. We measured N2O emission weekly or biweekly during 1 year in 11 temperate forest ecosystems using closed chambers. We identified three types of forest with different temporal emission patterns: forest with seasonal, event‐based and background emission patterns. Comparison of annual data sets from literature showed that most temperate forests had low N2O emissions throughout the year (background emission pattern) with mean annual fluxes of 0.39 ±0.27 kg N ha−1 yr−1 (n = 21). Event‐based emission patterns were observed during frost/thaw periods and after rewetting. Highest fluxes up to 72 kg N ha−1 were emitted from a drained alder forest with organic soil in 46 weeks, followed by well drained tropical and temperate forests with seasonal emission patterns and fluxes between 2 − 6 (n = 3) and 1 − 5 kg N ha−1 yr−1 (n = 4), respectively. Seasonal emission patterns were explained by combined effect of high annual precipitations; broad leave trees; amount and structure of organic upper horizon; high mineral bulk densities; and plant community. These state variables reduce gas diffusivity so that oxygen demand by microorganism and roots exceeded oxygen supply during wet and warm periods (&gt;10° C). The resultant upper mean level was about 100 μg N2O−N m−2 h−1 in both temperate and tropical forests. Annual N2O losses of the seasonal emission type were controlled by both duration and upper mean level of the periods with high emissions. We conclude that “short‐term controls” of climate determine the duration of high emissions, whereas “long‐term controls” by state variables determine the difference between background and seasonal emission types.

Take the Shuttle--from Marine Algae to Atmospheric Chemistry

The dimethyl sulfide (DMS) molecule is a key element of the sulfur cycle. Emitted by marine organisms, it is responsible for the formation of sulfate particles which affect climate and transport sulfur from the sea to the terrestrial biosphere. Numerous cruises have measured the emission of DMS from the sea. In a paper in Biogeochemical Sciences , Kettle et al . summarize and integrate the data from 134 cruises, and establish monthly and seasonal emission patterns across the world oceans. There is rough correspondence between areas of high DMS emission and the blooming of marine phytoplankton species known to emit DMS, although correlations with parameters such as chlorophyll concentrations are too low to allow determination of DMS emissions from satellite measuremens. Progress is most likely to come from further studies into the biology of DMS emission and accurate measurements of DMS sea-air fluxes.

Seasonal course of isoprene emissions from a midlatitude deciduous forest

Continuous measurements of whole canopy isoprene emissions over an entire growing season are reported from Harvard Forest (42°32′N, 72°11′W). Emissions were calculated from the ratio of observed CO2 flux and gradient multiplied by the observed hydrocarbon gradients. In summer 1995, 24‐hour average emissions of isoprene from June 1 through October 31 were 32.7×1010 molecules cm−2 s−l (mg C m−2 h−l = 2.8 × 1011 molecules cm−2 s−1), and the mean midday mixing ratio was 4.4 ppbv at 24 m. Isoprene emissions were zero at night, increased through the morning with increasing air temperature and light, reached a peak in the afternoon between the peaks in air temperature and light, and then declined with light. Isoprene emissions were observed over a shorter seasonal period than photosynthetic carbon uptake. Isoprene emission was not detected from young leaves and reached a peak rate (normalized for response to measured light and temperature conditions) 4 weeks after leaf out and 2 weeks after emissions began. The normalized emission rate remained constant for approximately 65 days, then decreased steadily through September and into October. Total isoprene emissions over the growing season (42 kg C ha−1 yr−1) were equal to 2% of the annual net uptake of carbon by the forest. Measured isoprene emissions were higher than the Biogenic Emission Inventory System‐II model by at least 40% at midday and showed distinctly different diurnal and seasonal emission patterns. Seasonal adjustment factors (in addition to the light and temperature factors) should be incorporated into future empirical models of isoprene emissions. Comparison of measured isoprene emissions with estimates of anthropogenic volatile organic compound emissions suggests that isoprene is more important for ozone production in much of Massachusetts on hot summer days when the highest ozone events occur.

Understanding the nature of methane emission from rice ecosystems as basis of mitigation strategies

Methane is considered an important greenhouse gas and rice fields are one of the major atmospheric methane sources. This paper develops sampling strategies and formulates mitigation options based on diel (day and night) and seasonal patterns of methane emissions. The design of sampling strategies and identification of abatement strategies were based on data obtained using automatic closed-chamber systems. Diel patterns of methane emissions from irrigated rice fields displayed similar patterns from planting to flowering. Fluxes at 0600, 1200, and 1800 h were important components of the total diel flux. Methane fluxes sampled during these times are sufficient to capture most of the diel variation observed throughout the growing season. Seasonal patterns of methane emissions indicated the intensity of flux measurement that should be done within the growing season. The characterization of seasonal emission patterns according to ecologies, fertilizer amendments, and water management provides a focus for mitigation strategies. Mitigation strategies, based on seasonal patterns, may be grouped into two approaches: preventive measures and reducing measures. Preventive measures may take into consideration, prior to planting rice, the ecology and fertilizer management that will give fewer methane emissions. Reducing measures become necessary when the option to prevent high emissions is not possible under a given condition. Other factors may be considered as preventive or reducing measures to mitigate methane emission. These factors, however, should be evaluated as to how they influence the diel and seasonal patterns of methane emission. Two things can be derived from the evaluation: (i) an improvement of the emission factor for better extrapolation; and (ii) a better understanding of how and when mitigation strategies can best be applied.