Designing monitoring networks to represent outdoor human exposure

Measurements of outdoor human exposure to suspended particulate matter (PM) are always constrained by available resources. An effective network design requires tradeoffs between variables measured, the number of sampling locations, sample duration, and sampling frequency. Sampling sites are needed to represent neighborhood and urban spatial scales with minimal influences from nearby sources. Although most PM measurements for determining compliance with standards are taken over 24-h periods every third to sixth day, outdoor human exposure assessment requires measurements taken continuously throughout the day, preferably over durations of 1 h or less. More detailed particle size and chemistry data are also desirable, as smaller size fractions and specific chemicals may be better indicators of adverse health effects than total mass samples.     

Continuous and filter-based measurements of PM2.5 nitrate and sulfate at the Fresno Supersite

PM(2.5) nitrate [Formula: see text] and sulfate ([Formula: see text]) were measured continuously with R&P8400N and R&P8400S instruments, respectively, and compared with filter-based measurements at the Fresno Supersite from October, 2000 through December, 2005. [Formula: see text] concentrations were higher in winter than summer with a long-term decreasing trend. Correlations between 24-h average continuous and filter-based [Formula: see text] were greater than 0.96 in 4 out of 5 years. Continuous [Formula: see text] was generally lower than filter-based [Formula: see text] although the difference decreased over time, from -52% in 2001 to +13% in 2005. These differences were similar in winter (-23%) and summer (-19%) while the corresponding differences between ambient and instrument temperature were -12 and 0.7 degrees C, respectively. Neither seasonal nor long-term trends in [Formula: see text] can be explained by variations in ambient temperature, the difference between ambient and instrument temperature, or changes in aerosol chemical composition. There were no seasonal or long-term trends in [Formula: see text] concentrations, partially due to low concentrations observed in Fresno. Long-term variability in the performance of R&P8400 [Formula: see text] and [Formula: see text] instruments suggest that collocation with filter measurements is needed for long-term measurements.     

Evaluation of OC/EC speciation by thermal manganese dioxide oxidation and the IMPROVE method

Ambient particulate samples are routinely analyzed for organic and elemental carbon (OC/EC) using either thermal manganese dioxide oxidation (TMO) or thermal volatilization-pyrolysis correction methods, such as the Interagency Monitoring of PROtected Visual Environments (IMPROVE) method with correction by reflectance, or a variation of the National Institute of Occupational Safety and Health (NIOSH) Method 5040 using thermal optical transmittance (TOT). With TMO, EC is modeled after the oxidation properties of submicron graphite and needle coke by MnO2, and is the fraction of total carbon (TC) that is not oxidized at >525 degrees C. In thermal volatilization methods, EC is the fraction of TC that accounts for the light extinction properties of the sample at the start of analysis. Chow et al. (2001) compared IMPROVE and NIOSH methods implemented on the same instrument using 60 samples of various types and found that NIOSH EC was lower than IMPROVE. This study compares total, organic, and elemental carbon measurements from the TMO and IMPROVE thermal optical reflectance (TOR) methods using a sample set consisting of 60 IMPROVE nonurban, 16 Korean urban, 10 Hong Kong urban, and 14 synthetic carbon black samples.     

A process-based model designed for filling of large data gaps in tower-based measurements of net ecosystem productivity

In this paper we present a simple hybrid gap-filling model (GFM) designed with a minimum number of parameters necessary to capture the ecological processes important for filling medium-to-large gaps in Flux data. As the model is process-based, the model has potential to be used in filling large gaps exhibiting a broad range of micro-meteorological and site conditions. The GFM performance was evaluated using “Punch hole” and extrapolation experiments based on data collected in west-central New Brunswick. These experiments indicated that the GFM is able to provide acceptable results (r² > 0.80) when >500 data points are used in model parameterization. The GFM was shown to address daytime evolution of NEP reasonably well for a wide range of weather and site conditions. An analysis of residuals indicated that for the most part no obvious trends were evident; although a slight bias was detected in NEP with soil temperature. To explore the portability of the GFM across ecosystem types, a transcontinental validation was conducted using NEP and ancillary data from seven ecosystems along a north-south transect (i.e., temperature–moisture gradient) from northern Europe (Finland) to the Middle East (Israel). The GFM was shown to explain over 75% of the variability in NEP measured at most ecosystems, which strongly suggests that the GFM maybe successfully applied to forest ecosystems outside Canada.