Atmospheric Environment (v.41, #33)

A study of the variation of urban mixed layer heights by Matthew Simpson; Sethu Raman; Julie K. Lundquist; Martin Leach (6923-6930).
The AERMET model is used to estimate hourly mixing heights during the Joint URBAN (2003) experiment in Oklahoma City, Oklahoma. AERMET is a simple 2-D model that requires only routine meteorological observations and an early morning atmospheric sounding to estimate convective boundary layer (CBL) growth. Estimated mixing heights are compared with observed mixing heights measured during Joint URBAN 2003. Observed CBL heights are derived from profiler data using a peak signal-to-noise ratio method. The method of deriving mixing heights from profiler data is validated using daily atmospheric sounding data. Estimated mixing heights using AERMET show good agreement with observations on days of varying temperature and cloud cover. AERMET was able to estimate the rapid boundary layer growth observed in the late morning and early afternoon hours during highly convective conditions. CBL heights of over 3000 m are observed in sounding data during the late afternoon. Estimated CBL heights of over 3000 m during the late afternoon agreed well with observations from the sounding and profiler data.
Keywords: AERMET; Convective boundary layer; Profiler; Joint URBAN (2003); Signal-to-noise ratio;

Using three sets of satellite data for burned areas together with the tree cover imagery and a biogeochemical component of the Integrated Science Assessment Model (ISAM) the global emissions of CO and associated uncertainties are estimated for the year 2000. The available fuel load (AFL) is calculated using the ISAM biogeochemical model, which accounts for the aboveground and surface fuel removed by land clearing for croplands and pasturelands, as well as the influence on fuel load of various ecosystem processes (such as stomatal conductance, evapotranspiration, plant photosynthesis and respiration, litter production, and soil organic carbon decomposition) and important feedback mechanisms (such as climate and fertilization feedback mechanism). The ISAM estimated global total AFL in the year 2000 was about 687 Pg AFL. All forest ecosystems account for about 90% of the global total AFL. The estimated global CO emissions based on three global burned area satellite data sets (GLOBSCAR, GBA, and Global Fire Emissions Database version 2 (GFEDv2)) for the year 2000 ranges between 320 and 390 Tg CO. Emissions from open fires are highest in tropical Africa, primarily due to forest cutting and burning. The estimated overall uncertainty in global CO emission is about ±65%, with the highest uncertainty occurring in North Africa and Middle East region (±99%). The results of this study suggest that the uncertainties in the calculated emissions stem primarily from the area burned data.
Keywords: Open fire; CO; Inventory; ISAM;

The aim of this paper is to present preliminary results of aerosol modeling over Greater Paris. We briefly describe the parameterizations and the numerical methods related to the general dynamic equation for aerosols: a size resolved aerosol model (SIREAM) has been hosted by the chemistry transport model Polair3D inside the Polyphemus system. Then the results of a simulation over Greater Paris are presented with preliminary comparisons to measured data for aerosols ( PM 10 and PM 2.5 ) and gases (ozone, NO 2 ) for six months of 2001 (from April to the end of September). A sensitivity analysis is also performed in order to assess the robustness of the model configuration.
Keywords: Air quality; Aerosol; Regional scale; Sensitivity analysis;

A variable K-model has been proposed for the dispersion in low winds in the surface-based inversion by expressing the eddy diffusivities as a linear function of downwind distance from the source. The resulting partial differential equation with variable coefficients along with the physically relevant boundary conditions is solved analytically. For the accuracy of the so-obtained solution, an analysis of the convergence and error estimation has been carried out. It is shown that the series converges absolutely. An upper bound for the error based on the partial sum of the series is estimated and it is described that the error tends to zero as the number of terms in the expansion are sufficiently large.The solution has been used to simulate the field tracer data sets collected from Hanford and IIT diffusion experiments in stable and unstable conditions, respectively. It predicts 41% cases in stable and 35% cases in unstable conditions within a factor of two to observations.
Keywords: Mathematical model; Variable eddy diffusivities; Eigen-function expansion; Low wind dispersion; Inversion/mixing layer;

Both forward and backward transport modeling methods are being developed for characterization of sources in atmospheric releases of toxic agents. Forward modeling methods, which describe the atmospheric transport from sources to receptors, use forward-running transport and dispersion models or computational fluid dynamics models which are run many times, and the resulting dispersion field is compared to observations from multiple sensors. Forward modeling methods include Bayesian updating and inference schemes using stochastic Monte Carlo or Markov Chain Monte Carlo sampling techniques. Backward or inverse modeling methods use only one model run in the reverse direction from the receptors to estimate the upwind sources. Inverse modeling methods include adjoint and tangent linear models, Kalman filters, and variational data assimilation, among others.This survey paper discusses these source estimation methods and lists the key references. The need for assessing uncertainties in the characterization of sources using atmospheric transport and dispersion models is emphasized.
Keywords: Atmospheric transport and dispersion models; Bayesian updating and inference methods; Inverse modeling; Adjoint and tangent linear models; Kalman filtering; Variational data assimilation;

Carbonaceous aerosol particles were observed in a residential area with wood combustion during wintertime in Northern Sweden. Filter samples were analyzed for elemental carbon (EC) and organic carbon (OC) content by using a thermo-optical transmittance method. The light-absorbing carbon (LAC) content was determined by employing a commercial Aethalometer and a custom-built particle soot absorption photometer. Filter samples were used to convert the optical signals to LAC mass concentrations. Additional total PM10 mass concentrations and meteorological parameters were measured. The mean and standard deviation mass concentrations were 4.4±3.6 μg m−3 for OC, and 1.4±1.2 μg m−3 for EC. On average, EC accounted for 10.7% of the total PM10 and the contribution of OC to the total PM10 was 35.4%. Aethalometer and custom-built PSAP measurements were highly correlated (R 2=0.92). The hourly mean value of LAC mass concentration was 1.76 μg m−3 (median 0.88 μg m−3) for the winter 2005–2006. This study shows that the custom-built PSAP is a reliable alternative for the commercial Aethalometer with the advantage of being a low-cost instrument.
Keywords: Aethalometer; Particle soot absorption photometer; Elemental carbon; Organic carbon; PM10;

Data from two of the United States Environmental Protection Agency's Speciation Trends Network fine particulate matter sites within Chicago, Illinois were used to examine the influence that the results and profiles of the Chemical Mass Balance (CMB) receptor model have on the source contributions and profiles of the Positive Matrix Factorization (PMF) model. This was accomplished using the target shape technique, which utilizes a priori information from the CMB source profiles inputted into the PMF model. The target shape methodology involves inputting specific information for the source profiles into the PMF model as it is resolving source profile and contribution matrices. The target shape results demonstrated it is possible to determine in both the CMB and PMF source profiles those species, which do not influence the solutions of either model.A second method utilizing information from the CMB results was used to impose a condition where the Motor Vehicles source never had a zero contribution as was applied to the CMB model. This involved utilizing an edge rotation to rotate the PMF results to yield a different solution without worsening the fit of the original results. The purpose of this work is to achieve a rotation, which produced a PMF solution where all of the Motor Vehicles contributions were greater than zero. Comparing the rotated Motor Vehicles and Sulfates source contributions in PMF to those obtained from CMB showed a better correlation between the PMF Motor Vehicles contributions to the original CMB results than those prior to rotation.
Keywords: Source apportionment; Positive Matrix Factorization; Chemical Mass Balance; PM2.5; Speciation Trends Network (STN); Rotations; Influential species; Receptor modeling;

Major to ultra trace elements in rainfall collected in suburban Tokyo by Tadashi Shimamura; Masato Iwashita; Satoe Iijima; Megumi Shintani; Yuichi Takaku (6999-7010).
Major to ultra trace elements such as rare earth elements (REEs), platinum group elements (PGEs) in 20 rainfall events from suburban Tokyo were determined by inductively coupled plasma mass spectrometry (ICP-MS). Anion species were also determined by an ion chromatography (IC). The concentrations of PGEs were so low that only Pt was detected in some rainfall events. Enrichment factors (EFs), refer to soil and sea salt components, were calculated for the measured elements (with Al and Na as references). Be, (Na), Mg, (Al), Si, Cl, K, Fe, Rb, Sr, REEs (except La, Gd), Ta, and U were mostly originated from natural materials (soil and sea salt). For Li, B, Ca, Mn, Sr, Ba, and Cs, the contribution of natural materials was significant. EFs for Cu, Zn, As, Se, Sb, Cd, Pb, Bi, Ag, Te, Au, Pt, SO4-S and NO3-N exceeded 100 indicating non-crustal, non-sea salt origin, presumably anthropogenic; however, contribution of volcanic gases could not be excluded for As, Se, Te and Bi. Pt seemed to be uniformly distributed worldwide and a catalyst for automobile emission control may be the main source. Au also showed uniform distribution. On the other hand, EFs for Zr, Nb, Hf and Th were less than unity. Probably these elements resided in acid resistant refractory fine minerals that did not decompose with acid treatment, and did not evaporate and ionize in the ICP. An alternative explanation is that the concentration of these elements was lower in the soil of the sampling area than the average crust. In the crust normalized REE pattern plot, La, Eu and Gd showed clear positive anomalies. La and Gd could have anthropogenic components. A possible source of La and Gd is cracking catalyst for petrol refining, but this source does not fully explain the anomaly. The source of Gd may also be Gd-DTPA (Gadolinium (III) diethyltriaminepentaacetic acid) used for Magnetic Resonance Imaging (MRI) contrast agents. The Eu origin may be soil with higher concentration than the crust average.
Keywords: Rainwater; Trace element; Rare earth element; Platinum group element; Enrichment factor;

Long-term trends in atmospheric pollen levels in the city of Thessaloniki, Greece by Athanasios Damialis; John M. Halley; Dimitrios Gioulekas; Despina Vokou (7011-7021).
We examined the long-term trends in pollen atmospheric levels in Thessaloniki, the second largest city of Greece. On the basis of data collected during the period 1987–2005, we estimated trends in the atmospheric pollen levels for the 16 different taxa, each of whose contribution to the total atmospheric pollen concentration was at least 0.5%. We also tested for trends towards earlier, longer or more highly peaked pollen seasons. The salient feature of these data is that the levels of pollen have been increasing; this is true for the majority of the individual taxa examined (12 out of 16) and for their aggregate. On average, the atmospheric pollen concentration is doubling every decade, but for some species the rate is much higher, with doubling times less than 5 years. Among the taxa with the highest rate of long-term trend in atmospheric pollen concentration, four belong to the group of woody plants (Cupressaceae, Quercus, Platanus, Pinaceae) and only one to that of herbs (Urticaceae). For the pollen-season-related attributes (onset, peak, end and duration), there was no systematic tendency and the changes were more nuanced. The observed increase in pollen abundance coincides with a rise in air temperature, which is the only meteorological factor to have experienced a sustained and significant change over the same period in Thessaloniki. Our results suggest that changes in pollen distributions are dominated by increases in pollen production rather than changes in flowering phenology and that several species showing strong trends might serve as bio-indicators of expected climate change. Given that the pollen-producing reservoir around the city has not increased, these results provide further evidence of the impact of climate change on plant biota.
Keywords: Climate change; Flowering phenology; Global warming; Pollen production; Respiratory allergy;

Trajectory source apportionment (TSA) methods have been used in many research projects to attempt to identify the sources of pollution. Hybrid Single Particle Lagrangian Integrated Trajectories (HYSPLIT) is a popular model for use in various TSA methods. One of the options in this model is to choose a starting height. Very little research is available to assist a user in making this choice. This paper evaluates starting heights of 10, 50, 100, 250, and 500 m on the accuracy of the Multi-Receptor (MURA) method using artificial sources for three different simulations. It was found that using ensembles of trajectories in the MURA method appear to average out most of the biases found from different trajectory starting heights up to the 500 m tested.
Keywords: Trajectory analysis; HYSPLIT; Source apportionment; MURA; Trajectory starting height;

Aerosol indirect effect over Indo-Gangetic plain by S.N. Tripathi; A. Pattnaik; Sagnik Dey (7037-7047).
Moderate resolution imaging spectroradiometer (MODIS) data are analyzed over the Indo-Gangetic plain (IGP) to study the effect of aerosol optical depth (AOD) on the water (R eff , w) and ice (R eff , i) cloud effective radius for the period 2001–2005. The temporal variation of R eff , w and R eff , i shows reverse trend as that of AOD for most of the time. The intensity of positive indirect effect (i.e. increase of R eff , w/i with decrease of AOD and vice versa) is the highest in winter (ΔR eff , w/ΔAOD∼−9.67 μm and ΔR eff , i/ΔAOD∼−12.15 μm), when the role of meteorology is the least. The positive indirect effect is significant in 43%, 37%, 68% and 54% of area for water clouds in winter, pre-monsoon, monsoon and post-monsoon seasons, respectively, whereas the corresponding values for ice clouds are 42%, 35%, 53% and 53% for the four seasons, respectively. On the contrast, R eff , i in some locations shows increment with the increase in AOD (negative indirect effect). The negative indirect effect is significant at 95% confidence level in 7%, 18%, 9% and 6% grids for winter, pre-monsoon, monsoon and post-monsoon seasons, respectively. The restricted spatial distribution of negative indirect effect in IGP shows that the cloud microphysical processes are very complex. Our analyses clearly identify the contrasting indirect effect, which requires further in situ investigations for better understanding of the aerosol–cloud interaction in the region.
Keywords: Indo-Gangetic plain; Aerosol indirect effect; Clouds;

Dust events are common air pollution events in parts of the world with arid, semi-arid, or desert areas. There is little research on the association between respiratory and cardiovascular health and dust events in places which are close to the deserts. The aim of this study is to evaluate the health effects of dust events in a location where traffic and industry are underdeveloped and dust events are most frequent in China. The setting allows the opportunity to reduce confounding by anthropogenically derived particulate matter and to confirm the health effects of dust events. The present study was done using daily counts of hospitalizations in Minqin (1994–2003) for respiratory and cardiovascular diseases (International Classification of Diseases, Tenth revision) for males and females. Using a semi-parametric generalized additive model and controlling for long-term temporal trends, day of the week, meteorological factors, and seasonal influence, counts of hospitalization were analyzed for dust events in a Poisson regression.Relative risks (RRs) were used to estimate the risk of dust events for respiratory and cardiovascular hospitalizations. In the year-round model, dust events with a lag of 3 days were significantly associated with total respiratory hospitalization for males and females, with RRs of 1.14 (95% confidence interval (CI) 1.01–1.29) and 1.18 (95% CI 1.00–1.41); dust events with a lag of 4 days were significantly associated with upper respiratory tract infection (URTI) in males (RR 1.28, 95% CI 1.04–1.59), and dust events with a lag of 6 days were significantly associated with pneumonia in males, with an RR of 1.17 (95% CI 1.00–1.38). A significant association between dust events with a lag of 3 days and hypertension in males was also found (RR 1.30, 95% CI 1.03,1.64). In the seasonal analysis model, the associations between the dust events and respiratory and cardiovascular hospitalizations were stronger in spring and in winter, respectively.The results show the health effects of dust events on respiratory and cardiovascular hospitalizations, and the characteristic seasonal distribution of the health effects. In addition, the health effects of dust events are consistent with recent animal and human data showing the respiratory and cardiovascular effects of particulate matter.
Keywords: China; Dust events; Desert dust; Dust storm; Respiratory diseases; Upper respiratory tract infection; Pneumonia; Cardiovascular disease; Ischemic heart disease; Hypertension; Daily hospitalization;

Employing Heisenberg's turbulent spectral transfer theory to parameterize sub-filter scales in LES models by Gervásio A. Degrazia; André B. Nunes; Prakki Satyamurty; Otávio C. Acevedo; Haroldo F. de Campos Velho; Umberto Rizza; Jonas C. Carvalho (7059-7068).
A turbulent subfilter viscosity for large eddy simulation (LES) models is proposed, based on Heisenberg's mechanism of energy transfer. Such viscosity is described in terms of a cutoff wave number, leading to relationships for the grid mesh spacing, for a convective boundary layer (CBL). The limiting wave number represents a sharp filter separating large and small scales of a turbulent flow and, henceforth, Heisenberg's model agrees with the physical foundation of LES models. The comparison between Heisenberg's turbulent viscosity and the classical ones, based on Smagorinsky's parameterization, shows that both procedures lead to similar subgrid exchange coefficients. With this result, the turbulence resolution length scale and the vertical mesh spacing are expressed only in terms of the longitudinal mesh spacing. Through the employment of spectral observational data in the CBL, the mesh spacings, the filter width and the subfilter eddy viscosity are described in terms of the CBL height. The present development shows that Heisenberg's theory naturally establishes a physical criterium that connects the subgrid terms to the large-scale dimensions of the system. The proposed constrain is tested employing a LES code and the results show that it leads to a good representation of the boundary layer variables, without an excessive refinement of the grid mesh.
Keywords: LES subfilter; Heisenberg model; Convective boundary layer;

We have estimated the mixing height (MH) and investigated the relationship between vertical mixing and ground-level ozone concentrations in Seoul, Korea, by using three ground-based active remote sensing instruments operating side by side: micro-pulse lidar (MPL), differential absorption lidar (DIAL), and differential optical absorption spectroscopy (DOAS). The MH is estimated from MPL measurements of aerosol extinction profiles by the gradient method under convective conditions. Comparisons of the MHs estimated from MPL and radiosonde measurements show a good agreement (r 2=0.99). Continuous MPL measurements with high temporal and vertical resolution reveal the diurnal variations of the MH under convective conditions and the presence of a residual layer during the nighttime. Comprehensive measurements of ozone and aerosol by MPL, DIAL and DOAS during an high ozone episode (24–26 May 2000) in Seoul, Korea, reveal that (1) photochemical ozone production and advection from upwind regions (the western part of Seoul) contribute two peaks of ozone concentrations at the ground around 14:00 and 18:00 local time on 25 May 2000, respectively, and (2) the entrainment and the fumigation processes of ozone aloft in the nighttime residual layer into the ground is a major contributor of high concentrations of ground-level ozone observed on the following day (26 May 2000).
Keywords: Mixing height (MH); Ground-level ozone; Micro-pulse lidar (MPL); Differential absorption lidar (DIAL); Differential optical absorption spectroscopy (DOAS);

Effectiveness of urban shelter-in-place—II: Residential districts by Wanyu R. Chan; William W Nazaroff; Phillip N. Price; Ashok J. Gadgil (7082-7095).
In the event of a short-term, large-scale toxic chemical release to the atmosphere, shelter-in-place (SIP) may be used as an emergency response to protect public health. We modeled hypothetical releases using realistic, empirical parameters to explore how key factors influence SIP effectiveness for single-family dwellings in a residential district. Four classes of factors were evaluated in this case study: (a) time scales associated with release duration, SIP implementation delay, and SIP termination; (b) building air-exchange rates, including air infiltration and ventilation; (c) the degree of sorption of toxic chemicals to indoor surfaces; and (d) the shape of the dose–response relationship for acute adverse health effects. Houses with lower air leakage are more effective shelters, and thus variability in the air leakage of dwellings is associated with varying degrees of SIP protection in a community. Sorption on indoor surfaces improves SIP effectiveness by lowering the peak indoor concentrations and reducing the amount of contamination in the indoor air. Nonlinear dose–response relationships imply substantial reduction in adverse health effects from lowering the peak exposure concentration. However, if the scenario is unfavorable for indefinite sheltering (e.g. sheltering in leaky houses for protection against a nonsorbing chemical with a linear dose–response), the community must implement SIP without delay and exit from shelter when it first becomes safe to do so. Otherwise, the community can be subjected to even greater risk than if they did not take shelter indoors.
Keywords: Sorption; Infiltration; Air-exchange rate; Toxic chemical; Emergency response;

The effect of diet manipulation on nitrous oxide and methane emissions from manure application to incubated grassland soils by L.M. Cardenas; D. Chadwick; D. Scholefield; R. Fychan; C.L. Marley; R. Jones; R. Bol; R. Well; A. Vallejo (7096-7107).
Changes to agricultural management, particularly of the nitrogen (N) input to farms, have great potential for mitigating emissions of N containing gases, especially the greenhouse gas nitrous oxide (N2O). Manipulating diets fed to livestock is a potential method for controlling N excretion and emissions of greenhouse gases (GHG's) to the atmosphere. We selected three slurries derived from sheep that had been fed, either ensiled ryegrass (Lolium hybridicum), lucerne (Medicago sativa) or kale (Brassica oleracea) and applied them to a grassland soil from the UK in a laboratory experiment using a special He/O2 atmosphere incubation facility. The resulting fluxes of N2O, CH4 and N2 were measured, with the largest total N fluxes generated by the ryegrass slurry treatment (14.23 ryegrass, 10.84 lucerne, 13.88 kale and 4.40 kg N ha−1 from the control). Methane was emitted only from the ryegrass slurry treatment. The isotopomer signatures for N2O in the control and lucerne slurry treatments indicated that denitrification was the main process responsible for N2O emissions.
Keywords: Nitrous oxide; Denitrification; Diet; Sheep slurry; Methane; Nitrogen;

In order to quantify the contribution of solvent use and road traffic to the total non-methane volatile organic compound (NMVOC) emissions in Germany, the composition of air in the city of Wuppertal was investigated during three campaigns at different locations. The measurements covered NMVOCs in the range of C3–C10 hydrocarbons and C1–C6 oxygenated compounds. An assessment of the contribution from different emission sources to the observed NMVOC concentrations was attempted with the chemical mass balance (CMB) modelling technique. Emission profiles for traffic were obtained from measurements performed in a traffic tunnel, at a downtown street intersection and during drives through the city and on motorways. Solvent emission profiles were investigated in the vicinity of different factories and workshops using solvents in Wuppertal. Apportionment analyses were performed for several receptor points located down-wind from the city centre, in residential, dense traffic and industrial areas.The results of the present work show that traffic emission rather than solvent use determines the ambient NMVOC composition. The maximum contribution of solvent use to the NMVOC emission estimated on the basis of experimentally obtained results amounts to about 23% in the whole area of Wuppertal. It can be concluded that the contribution of solvent use to the NMVOC concentrations also in other German cities falls in the range of few to about 20%, assuming that Wuppertal can be considered as a typical German urban area with certain proportions of domestic, traffic and various industrial activities. These results are in strong disagreement with the German Emission Inventory, which states, that in the reference year 2003 about 51% of the total NMVOC emissions originate from solvent use and only 14% from traffic.
Keywords: Volatile organic compounds; Source profiles; Chemical mass balance model; Source contribution; Traffic emission; Solvent use emission;

The effects of meteorology on ozone in urban areas and their use in assessing ozone trends by Louise Camalier; William Cox; Pat Dolwick (7127-7137).
The United States Environmental Protection Agency issues periodic reports that describe air quality trends in the US. For some pollutants, such as ozone, both observed and meteorologically adjusted trends are displayed. This paper describes an improved statistical methodology for meteorologically adjusting ozone trends as well as characterizes the relationships between individual meteorological parameters and ozone. A generalized linear model that accommodates the nonlinear effects of the meteorological variables was fit to data collected for 39 major eastern US urban areas. Overall, the model performs very well, yielding R 2 statistics as high as 0.80. The analysis confirms that ozone is generally increasing with increasing temperature and decreasing with increasing relative humidity. Examination of the spatial gradients of these responses show that the effect of temperature on ozone is most pronounced in the north while the opposite is true of relative humidity. By including HYSPLIT-derived transport wind direction and distance in the model, it is shown that the largest incremental impact of wind direction on ozone occurs along the periphery of the study domain, which encompasses major NO x emission sources.
Keywords: Ozone trends; Generalized linear model; Meteorological adjustment; HYSPLIT; Spatial patterns;

Evaluation of the MODIS aerosol optical depth retrieval over different ecosystems in China during EAST-AIRE by Lili Wang; Jinyuan Xin; Yuesi Wang; Zhanqing Li; Guangren Liu; Jing Li (7138-7149).
The accuracy of the Moderate Resolution Imaging Spectroradiometer's (MODIS) aerosol products is still uncertain in China, due to a lack of validation by long-term and large-scale ground-based observations. In this paper, the MODIS aerosol optical depth (AOD) product is evaluated using Chinese Sun Hazemeter Network (CSHNET) data as ground truths over different ecological regions in China during the East Asian Study of Tropospheric Aerosols—an International Regional Experiment (EAST-AIRE). The evaluation results show very large differences in the MODIS AOD retrieval between different ecosystems and geographic locations. The most agreement between the MODIS data and that of the CSHNET was in farmland sites in central-southern China, where high correlation (R>0.82) and large percentages (R2>72%) within the expected error lines issued by NASA were found. In temperate forest, coastal regions, and northeast and central farmlands, there appeared moderate agreement, with R∼0.64–0.80 and 45–73% of retrieval data falling within the expected errors. The poorest agreement existed in northern arid and semiarid regions, in remote northeast farmlands, in the Tibetan and Loess Plateau, and in southern forests, with 13–54% of retrieval data falling within the expected errors. In addition, the MODIS AOD retrievals were significantly overestimated in the northern arid and semiarid regions and underestimated in remote northeast farmlands and southern forests.
Keywords: MODIS; CSHNET; Aerosol optical depth (AOD); Ecosystem; China;