Key features of new particle formation events at background sites in China and their influence on cloud condensation nuclei

New particle formation (NPF) event at multi rural sites in China Identifying the characteristics of NPF event Comparing NPF event between clean and polluted conditions Quantifying contribution to the cloud condensation nuclei Implication of climate and air quality Long-term continuous measurements of particle number size distributions with mobility diameter sizes ranging from 3 to 800 nm were performed to study new particle formation (NPF) events at Shangdianzi (SDZ), Mt. Tai (TS), and Lin’an (LAN) stations representing the background atmospheric conditions in the North China Plain (NCP), Central East China (CEC), and Yangtze River Delta (YRD) regions, respectively. The mean formation rate of 3-nm particles was 6.3, 3.7, and 5.8 cm⁻³·s⁻¹, and the mean particle growth rate was 3.6, 6.0, and 6.2 nm·h⁻¹ at SDZ, TS, and LAN, respectively. The NPF event characteristics at the three sites indicate that there may be a stronger source of low volatile vapors and higher condensational sink of pre-existing particles in the YRD region. The formation rate of NPF events at these sites, as well as the condensation sink, is approximately 10 times higher than some results reported at rural/urban sites in western countries. However, the growth rates appear to be 1–2 times higher. Approximately 12%–17% of all NPF events with nucleated particles grow to a climate-relevant size (>50 nm). These kinds of NPF events were normally observed with higher growth rate than the other NPF cases. Generally, the cloud condensation nuclei (CCN) number concentration can be enhanced by approximately a factor of 2–6 on these event days. The mean value of the enhancement factor is lowest at LAN (2–3) and highest at SDZ (~4). NPF events have also been found to have greater impact on CCN production in China at the regional scale than in the other background sites worldwide.     

A comparative study of particle size distribution from two oxygenated fuels and diesel fuel

Oxygenated fuels are known to reduce particulate matter (PM) emissions from diesel engines. In this study, 100% soy methyl ester (SME) biodiesel fuel (B100) and a blend of 10% acetal denoted by A-diesel with diesel fuel were tested as oxygenated fuels. Particle size and number distributions from a diesel engine fueled with oxygenated fuels and base diesel fuel were measured using an Electrical Low Pressure Impactor (ELPI). Measurements were made at ten steady-state operational modes of various loads at two engine speeds. It was found that the geometric mean diameters of particles from SME and Adiesel were lower than that from base diesel fuel. Compared to diesel fuel, SME emitted more ultra-fine particles at rated speed while emitting less ultra-fine particles at maximum speed. Ultra-fine particle number concentrations of A-diesel were much higher than those of base diesel fuel at most test modes.     

Dry deposition and particle size distributions of nitrate and sulfate in ambient air

This study reports the dry deposition pollutants of anion species (NO₃ ^‐ and SO₄ ^‐2) in the Ping Tung City of Southern Taiwan. Several deposition properties are discussed in this paper. It included dry deposition flux, anion species size distribution and deposition velocities. Noll Rotary Impactor (NRI) and Microorifice Uniform Deposit Impactor (MOUDI) were used to collect ambient air coarse and fine particulate. Dry deposition plate was applied to collect particle deposition flux. Dionex 2000i/SP Ion Chromatography equipped with 4 mm AG4A‐SC and AS4A‐SC column was employed to analyze the anion species. The eluent solution is 1.8 mM sodium carbonate/1.7 mM sodium bicarbonate. The measured dry deposition flux of nitrate ranged from 0.63 to 3.96 mg/m²‐day and averaged 2.12 mg/m²‐day, while the measured dry deposition of sulfate ranged from 1.17 to 9.53 mg/m²‐day and averaged 3.92 mg/m²‐day. The particle size distribution of nitrate has bimodal particle size distributions. However, the sulfate displayed uniform particle size distribution for all four sampling sites. Mean cumulative fraction (F%) of nitrate in the particle size range below 1, 2.5, 10 and 25 μm, in sequence, were 34.6%, 60.9%, 91.0% and 97.6%, respectively. However, the mean F% of sulfate in the particle size range below 1, 2.5, 10 and 25 um, in sequence, were 57.3%, 82.6%, 90.3% and 98.0%, respectively. The sulfate has more F% in the submicron particles. The mean MMD _o of nitrate and sulfate are 2.35 and 0.87 μm, respectively. The mean dry deposition velocities of nitrate and sulfate are 0.45 and 0.38 cm/sec, respectively.     

Relation of particle-size spectrum and food abundance to particle selectivity in the mud snail Hydrobia totteni (Prosobranchia: Hydrobiidae)

We studied the relationship between particle-size selectivity and variable particle-size distribution in the American mud snail Hydrobia totteni Morrison (Prosobranchia: Hydrobiidae) collected in the summer of 1979 and 1982, from saltmarsh mudflats at Flax Pond, Old Field, New York, USA. Using individual size fractions of native sediment, snails fed fastest on intermediate-sized particles (41 to 63 m); this pattern was related to diatom abundance, which was similarly greatest on the intermediate particle-size classes. These results conform to another study, which found a quite different relationship between feeding rate and particle size, but a similarly strong correlation between particle size and diatom abundance. Snails were fed a range of particle-size distributions of glass beads. As median particle size increased, preference shifted towards smaller particles. This shift cannot be explained by feeding rates determined from individual particle-size classes. The most reasonable alternative model is a shift towards feeding on fine particles that occur among the coarser particles that are not ingested. Our data suggest that diatom growth on certain particle sizes is as important to particle selectivity as the particle sizes themselves. Particle size preference cannot be extrapolated from studies of feeding rates on individual size classes since selectivity is qualitatively different in mixtures of various size classes.     

Effects of molecular weight and concentration of carboxymethyl cellulose on morphology of hydroxyapatite nanoparticles as prepared with one-step wet chemical method

Nano-sized apatite particles (nAP) synthesized with carboxymethyl cellulose (CMC) have shown great application potentials in in situ heavy metal remediation. However, differences in CMC’s properties effects on the size of nAP produced are not well understood. In this paper, two types of CMC, with respective molecular weights (MW) of ～120000 and ～240000 Dalton or respective polymerization degrees of 500 (CMC-500) and 1050 (CMC-1050), were studied in a concentration range of 0.05%–0.5% (w/w) for nAP synthesis. Morphology of the particles was characterized with transmission electron microscopy (TEM). Results showed that 0.05% CMC-500 solution gave an average particle size of 148.7±134.9 nm, 0.25% CMC-500 solution produced particles of 21.8±20.4 nm, and, 0.5% CMC-500 solution contained particles of 15.8±7.7 nm. In comparison, 0.05% CMC-1050 solution produced nanoparticles of 6.8±3.2 nm, 0.25% CMC-1050 produced smaller nAP of 4.3±3.2 nm, and 0.5% CMC-1050 synthesized the smallest nanoparticles in this study, with an average diameter of 3.0±2.1 nm. Chemical composition of the products was identified with X-ray diffraction (XRD) as pure hydroxyapatite. Interactions between nAP and CMC were discussed with help of attenuated total reflection Fourier transform infrared (ATRFTIR) spectroscopic data. This study showed that CMC at higher concentration as well as higher MW facilitated to produce finer nanoparticles, showing that nAP size could be manipulated by selecting appropriate CMC MW and/or applying appropriate CMC concentration.     

Adsorption and partition of benzo(a)pyrene on sediments with different particle sizes from the Yellow River

Experiments have been carried out to study the sorption of Benzo(a)pyrene(Bap) on sediment particles from the Yellow River using a batch equilibration technique. Effects of particle size on the adsorption and partition of Bap were investigated with the particle content of 3 g/L. Several significant results were obtained from the study. (1) Isotherms of Bap could be fitted with the dual adsorption-partition model under different particle sizes, and the measured value of the adsorption and partition was in agreement with the theoretical value of the dual adsorption-partition model. (2) When the particle diameter was d ? 0.025 mm, the adsorption was predominant in the sorption of Bap, which accounted for 68.7%–82.4% of the sorption. For the particles with the size of 0.007 mm?d<0.025 mm, the adsorption was predominant when the equilibrium concentration of Bap was 0–8.87 μg/L in the water phase; and the partition was predominant when the equilibrium concentration of Bap was higher than 8.87 μg/L in the water phase. When the particle diameter was d<0.007 mm, the partition was predominant. (3) On the point of particle size, the contribution of adsorption to sorption followed the order: “d?0.025 mm”>“0.007 mm ?d<0.025 mm” >“d<0.007 mm”. (4) The partition coefficients of Bap in solids with different particle sizes were linearly correlated with the organic content, and the K _oc of Bap was about 1.26 × 10⁵ (L/kg).     

Experimental and numerical investigation of density current over macro-roughness

The dynamics of density current over a bottom covered by macro-roughness elements were investigated by laboratory experiments and a computational model using large eddy simulations. The macro-roughness considered had significant size in comparison with the scale of density current. Five different roughness conditions were considered, namely flat bottom (for reference), half spheres, fine gravels, medium gravels, and large gravels. These bottom conditions had variations in roughness element size, shape, angularity, and spatial configuration. The density current was a lock-exchange type with a density difference of 1% between the two fluids initially separated by a gate in the middle. In the computational model, the roughness was captured using two different methods depending on the size of the roughness elements. For the large roughness elements, i.e., the half spheres and the medium and large gravels, an immersed boundary method was used to resolve the surface of each gravel, which was obtained through 3D laser scanning. The realistic and physically correct placement of these scanned objects in the simulation domain was achieved using a computer tool which can detect the collision of rigid bodies and simulate their dynamics. For the fine gravels, a rough wall function was used. The computational model was validated with the data measured in the experiments, including front position and velocity, and point velocity measurement within the current. The results show that density currents over macro-roughness have distinct behavior from those over a smooth boundary. The characteristics (size, angularity, and pavement pattern) of the macro-roughness play a key role in the current development. Macro-roughness significantly retards the front propagation and enhances entrainment.     

北京市地面街尘与径流中重金属的污染特征（Characteristics of Heavy Metals in Street Dusts and Urban Runoff in Beijing）

街尘作为城市各种污染物的载体和地表径流污染物的主要来源对水环境的影响日益受到关注.论文分析了北京市城区街尘与地表径流中重金属浓度和颗粒的粒径分布.结果表明:不同城市土地利用类型对街尘和径流中的重金属含量、颗粒粒径分布具有重要影响.在商业区、主要道路、住宅区、城中村4种土地利用类型中,街尘重金属浓度和地面单位面积重金属质量均以主要道路最高;径流重金属浓度由高到低顺序依次为:主要道路>城中村>居民区;主要道路和商业区街尘颗粒中细粒径占的比例较高,在全部土地利用类型的径流水样中颗粒物粒径分布差别不大;随着街尘中颗粒物粒径的减小,重金属浓度增加;街尘中小于149μm的颗粒物质量百分比和重金属浓度均较高,且在径流中这个粒径段的颗粒物含量也高,体积比达80%以上.建议在今后的城市街尘面源污染控制中应特别关注土地利用类型和街尘粒径的影响.     

The influence of physicochemical parameters on bioaccessibility-adjusted hazard quotients for copper,lead and zinc in different grain size fractions of urban street dusts and soils

When the hazard quotient for ingestion (HQI) of a trace element in soil and dust particles is adjusted for the element’s bioaccessibility, the HQI is typically reduced as compared to its calculation using pseudo-total element concentration. However, those studies have mostly used bulk particles (<2 mm or <250 µm), and the reduction in HQI when expressed as bioaccessible metal may not be similar among particle size fractions, the possibility probed by the present study of street dusts and soils collected in Tehran. The highest Cu, Pb and Zn near-total concentrations occurred in the finest particles of dusts and soils. Bioaccessible concentrations of Cu, Pb and Zn in the particles (mg kg^?1) were obtained using simple bioaccessibility extraction test (SBET). The bioaccessibility (%) did not vary much among near-total concentrations. In the bulk (<250 µm) sample, the bioaccessible concentration of Cu and Pb increased as the pH of sample increased, while Zn bioaccessibility (%) in the bulk particles was influenced by organic matter and cation exchange capacity. X-ray diffraction identified sulfide and sulfate minerals in all of the size-fractionated particles, which are insoluble to slightly soluble in acidic conditions and included most of the Cu and Pb in the samples. The only Zn-bearing mineral identified was hemimorphite, which would be highly soluble in the SBET conditions. The calculated HQI suggested potential non-carcinogenic health risk to children and adults from ingestions of soils and dusts regardless of particle size consideration, in the order of Zn > Pb ≥ Cu. The HQI calculated from near-total metal was not much different for particle size classes relative to bulk particles; however, the bioaccessibility percent-adjusted HQI for Pb was higher for the smaller particles than the bulk. This work is novel in its approach to compare HQI for a bulk sample of particles with its composite particle size fractions.     

排水管道内不同粒径沉积颗粒物冲刷率的分析与计算

为探究和量化在水流冲刷下,排水管道中不同粒径颗粒物冲刷沉积的过程,本文模拟排水管道内沉积颗粒的冲刷过程.冲刷过程中,粒径较小的悬移质颗粒(小于0.1 mm),在管道沿线取样测得水流中悬浮固体质量浓度(SS);粒径较大的推移质颗粒(0.1—2 mm),测得管道沿线不同位置沉积的颗粒质量.并建立两个数学模型分别计算排水管道内两类颗粒物的冲刷率.研究发现,悬移质颗粒,以水流中SS为依据,可计算出输送通量和冲刷率;推移质颗粒,以管道不同位置沉积下来的颗粒质量为依据,拟合出了管道中的沉积分布函数,是以e为底数的指数函数,可计算出冲刷量和冲刷率.模型计算出的冲刷率呈现出规律性:悬移质颗粒被冲刷成悬浮状态,随水流迁移过程中部分会再次沉积,使得悬移质颗粒的冲刷率从管道前段至中后段逐步降低,如初始沉积质量为100 g的0.045 mm悬移质颗粒,在0.30 m·s^-1的冲刷流速下,计算出管道前段冲刷率为78.94%,最终在管道后段降至13.89%;对于两类颗粒而言,颗粒物粒径越小,冲刷流速越大,初始沉积质量越小,冲刷率越高.     

Particle size distribution and pollutants in road-deposited sediments in different areas of Zhenjiang,China

An understanding of road-deposited sediment (RDS) characteristics on an impervious surface is essential to estimate pollutant washoff characteristics and to minimise the impacts of pollutants on the water environment. A total of 62 RDS samples were collected from four different land-use types (commercial, residential, intense traffic and riverside park) in Zhenjiang City, China. The samples were fractionated into seven grain-size classes and analysed for particle size distribution and concentrations of pollutants. The samples are found to consist predominantly of fine particles (60–80%, <250 μm). The maximum mean concentrations of zinc, lead and copper were 686.93, 589.19 and 158.16 mg/kg, respectively, with the highest metal concentrations found in samples from the intense traffic area. The maximum mean contents of organic matter (12.55%), nitrogen (6.31 mg/g) and phosphorus (5.15 mg/g) were found in samples from the commercial area. The concentrations of heavy metals were highest in the smallest particle size fraction analysed (63 μm). The organic matter and nitrogen content generally increased with decreasing particle sizes in the <500-μm particle size range. The results also revealed that most of the total nitrogen (TN) is attached to the finer sediments and that to effectively reduce TN loads in particulates, treatment facilities must be able to remove the finer particles (down to 125 μm for TN).     

Grazing by adult estuarine calanoid copepods of the Chesapeake Bay

Grazing by adult female Eurytemora affinis, Acartia tonsa and A. clausi on natural distributions of particles from the Chesapeake Bay has been investigated. During the course of a year's sampling, a wide variety of particle size-biomass distributions were observed as seasonal shifts in detritus, and over 150 algal species occurred. These distributions were grouped into 5 basic types in the analyses of feeding. All three species demonstrated similar capabilities for feeding over a broad range of particle size with selection (higher filtering rates) on larger particles and on biomass peaks. Feeding on multiple-peak distributions resulted in strong selection or tracking of each biomass peak with reduced filtering rates between peaks. Evidence is presented which suggests that the copepods first feed on large particles and then successively switch to biomass peaks of the smaller size categories. Comparisons of the feeding behavior of Eurytemora affinis and the Acartia species showing that the Acartia species have greater capabilities for taking large particles may be associated with modifications of their mouth parts for raptorial feeding. The results suggest considerable flexibility in copepod feeding behavior which cannot be explained solely by the mechanism of a fixed sieve.     

Development of a cloud condensation nuclei (CCN) counter using a laser and charge-coupled device (CCD) camera

A continuous flow streamwise thermal gradient cloud condensation nuclei (CCN) counter with an aerosol focusing and a laser-charge-coupled device (CCD) camera detector system was developed here. The counting performance of the laser-CCD camera detector system was evaluated by comparing its measured number concentrations with those measured with a condensation particle counter (CPC) using polystyrene latex (PSL) and NaCl particles of varying sizes. The CCD camera parameters (e.g. brightness, gain, gamma, and exposure time) were optimized to detect moving particles in the sensing volume and to provide the best image to count them. The CCN counter worked well in the particle number concentration range of 0.6–8000 #·cm^-3 and the minimum detectable size was found to be 0.5 μm. The supersaturation in the CCN counter with varying temperature difference was determined by using size-selected sodium chloride particles based on K?hler equation. The developed CCN counter was applied to investigate CCN activity of atmospheric ultrafine particles at 0.5% supersaturation. Data showed that CCN activity increased with increasing particle size and that the higher CCN activation for ultrafine particles occurred in the afternoon, suggesting the significant existence of hygroscopic or soluble species in photochemically-produced ultrafine particles.     

重庆市主城区春夏季不同粒径颗粒物污染特征分析

采用多通道采样器,采集重庆市主城区PM1.0、PM2.5和PM103种粒径的颗粒物样品,结果显示采样期间PM1.0、PM2.5和PM10日浓度均值分别为0.057 mg/m3、0.075 mg/m3和0.120 mg/m3,主城区颗粒物污染较为严重。对3种粒径颗粒物样品中的碳组分、水溶性组分以及无机污染元素组分进行分析...     

Occurrence and particle-size distributions of polycyclic aromatic hydrocarbons in the ambient air of coking plant

The purpose of this study was to characterize the occurrence and size distributions of ten species of polycyclic aromatic hydrocarbons (PAHs) in the ambient air of coking plants. Particulate-matter samples of four size fractions, including ≤2.1, 2.1–4.2, 4.2–10.2, and ≥10.2 μm, were collected using a Staplex234 cascade impactor during August 2009 at two coking plants in Shanxi, China. The PAHs were analyzed by a gas chromatograph equipped with a mass-selective detector. The concentrations of total particulate-matter PAHs were 1,412.7 and 2,241.1 ng/m³ for plants I and II, and the distributions showed a peak within the 0.1–2.1 μm size range for plant I and the 0.1–4.2 μm for plant II. The size distributions of individual PAHs (except fluoranthene) exhibited a considerable peak within the 0.1–2.1 μm size range in coking plant I, which can be explained by the gas–particle partition mechanism. The ambient air of the coking plant was heavily polluted by PAHs associated with fine particles (≤2.1 μm), and benzo[b]fluoranthene made the largest contribution to total PAHs. The exposure levels of coking-plant workers to PAHs associated with fine particles were higher than to PAHs associated with coarse particles. Benzo[b]fluoranthene, benzo[a]pyrene, and dibenzo[a,h]anthracene should be the primary pollutants monitored in the coking plant. This research constitutes a significant contribution to assessing the exposure risk of coking-plant workers and providing basic data for PAH standards for ambient air in coking plants.     

焦作市大气颗粒物中水溶性砷的分布特征

利用原子荧光法对焦作市春、夏、秋、冬四季不同粒径大气颗粒物中水溶性重金属砷的分布特征及时空变化进行分析研究。研究表明：焦作市水溶性砷主要富集在PM2.1中,且颗粒物粒径越小,其富集砷的能力就越强,各级颗粒物中4级、5级、6级（0.41～2.1μm）范围内砷的含量最高,其总含量约为5.71～19.96 ng/m3,三者在不同月份所占比例高达73%～86%;由于形成机制与影响条件不同,不同粒径大气颗粒物中砷表现出明显的季节变化特征,总体趋势为冬季〉秋季〉夏季〉春季,主要原因为冬、夏季燃煤量增加,春季大气污染源减少,地表植被覆盖密集,空气环境质量较好;燃煤是焦作大气中砷的主要来源之一,且其对大气细颗粒物（粒径≤2.1μm）中砷的贡献最明显。     

Particle size distributions, PM2.5 concentrations and water-soluble inorganic ions in different public indoor environments: a case study in Jinan, China

In this study, we collected particles with aerodynamic diameter ?2.5 μm (PM_2.5) from three different public indoor places (a supermarket, a commercial office, and a university dining hall) in Jinan, a medium-sized city located in northern China. Water-soluble inorganic ions of PM_2.5 and particle size distributions were also measured. Both indoor and outdoor PM_2.5 levels (102.3–143.8 μg·m^?3 and 160.2–301.3 μg·m^?3, respectively) were substantially higher than the value recommended by the World Health Organization (25 μg·m^?3), and outdoor sources were found to be the major contributors to indoor pollutants. Diurnal particle number size distributions were different, while the maximum volume concentrations all appeared to be approximately 300 nm in the three indoor locations. Concentrations of indoor and outdoor PM_2.5 were shown to exhibit the same variation trends for the supermarket and dining hall. For the office, PM_2.5 concentrations during nighttime were observed to decrease sharply. Among others, SO ₄ ^2? , NH ₄ ⁺ and NO ₃ ^? were found to be the dominant water-soluble ions of both indoor and outdoor particles. Concentrations of NO ₃ ^? in the supermarket and office during the daytime were observed to decrease sharply, which might be attributed to the fact that the indoor temperature was much higher than the outdoor temperature. In addition, domestic activities such as cleaning, water usage, cooking, and smoking also played roles in degraded indoor air quality. However, the results obtained here might be negatively impacted by the small number of samples and short sampling durations.     

The adsorption of lead and copper from aqueous solution on modified peat-resin particles

Raw peat was modified with sulfuric acid, then mixed modified with resin to prepare the modified peat-resin particles. Using the batch experimental systems, the removal of heavy metals (copper and lead) on the modified peat-resin particles was investigated. The data of the adsorption isotherm could be fitted by the Langmuir equation well. The adsorption rate of heavy metals on modified peat-resin particles was very swift. The removal processes of heavy metals on modified peat-resin particles could be well described by pseudo-second order model. The adsorption rate of lead was affected by the initial heavy metal concentration, initial pH, particle size, agitation speed and particle mass. In the adsorption of heavy metals (lead and copper) on the modified peat-resin particles, ion exchange was the major reaction mechanism. Desorption data showed that the lead adsorbed by modified peat-resin particle could be desorbed by 0.5 N or 1.0 N HNO3. The desorption rate was swift. The experiments indicated that the modified peat-resin particles have great potential for the removal of heavy metals from wastewater.     

辽宁本溪大气颗粒物浓度特征

利用本溪大气成分监测站2008年3月至2009年2月大气颗粒物监测仪GRIMM180的连续监测资料,对该地区大气颗粒物的质量浓度变化、谱分布特征以及大气颗粒物与气象因素的关系进行分析研究.结果表明,本溪PM10和PM2.5平均质量浓度分别为0.086 mg.m-3和0.058 mg.m-3,其日平均质量浓度变化幅度较大;冬季和夏季质量浓度日变化均呈现明显的双峰双谷特征;数浓度谱分布较好地符合Junge分布;PM10日平均值超标率为8.7%,且大气颗粒物主要是以细粒子的形式存在;随风速的增大大气颗粒物质量浓度基本呈现逐渐减小的趋势,当风速〉0.6 m.s-1时,大气颗粒物质量浓度随风速增大下降明显,风速〉3.0 m.s-1时,下降的趋势减缓;降水过程对大气颗粒物有清除作用,其中对粗粒子的清除效果非常明显.     

Size fractionation of natural aquatic populations associated with autotrophic and heterotrophic carbon uptake

Natural phytoplankton samples from the Gulf of California were incubated with radioactive bicarbonate or glucose, with or without the addition of gentamycin, a broad spectrum antibiotic which inhibits bacterial activity but has little effect on many algae. The relative distribution of chlorophyll and radioactivity in particles retained by filters of 3 and 0.45 μ mean pore size was measured. From 13 to 83% of the ¹⁴C-bicarbonate assimilated by natural populations appeared to be associated with organisms having a smallest dimension less than 3 μ. These were probably algae, since the distribution of chlorophyll on the filters was similar to that of the radioactivity. Furthermore, gentamycin treatment did not change the distribution of radioactivity from ¹⁴C-bicarbonate. In these waters, considerable photosynthesis may be carried out by ultra-nannoplankton (3 to 1 μ). Glucose was assimilated mainly by bacteria, of which more than 90% passed through 3 μ filters. Gentamycin significantly reduced glucose uptake in most cases. Although this method cannot be adopted uncritically for the estimation of carbon flux from photoautotrophs to heterotrophs, differential filtration can be used to elucidate the size distribution of various metabolic groups in the plankton.