Error Analysis and Uncertainty in the Determination of Aerosol Optical Properties Using Cavity Ring-Down Spectroscopy, Integrating Nephelometry, and the Extinction-Minus-Scattering Method

<p>Despite the substantial improvements in the measurements of aerosol physical and chemical properties and in the direct and indirect radiative effects of aerosols, there is still a need for studying the properties of aerosols under controlled laboratory conditions to develop a mechanistic and quantitative understanding of aerosol formation, chemistry, and dynamics. In this work, we present the factors that affect measurement accuracy and the resulting uncertainties of the extinction-minus-scattering method using a combination of cavity ring-down spectroscopy (CRDS) and integrating nephelometry at a wider range of optical wavelengths than previously attempted. Purely scattering polystyrene latex (PSL) spheres with diameters from 107–303 nm and absorbing polystyrene spheres (APSL) with 390 nm diameter were used to determine the consistency and agreement, within experimental uncertainties, of CRDS and nephelometer values with theoretical calculations derived from Mie theory for non-absorbing spheres. Overall uncertainties for extinction cross-section were largely 10%–11% and dominated by condensation particle counter (CPC) measurement error. Two methods for determining <i>σ</i>_ext error are described, and they were found to produce equivalent results. Systematic uncertainties due to particle losses, RD cell geometry (<i>R_L</i>), CPC counting efficiency, ring-down regression fitting, blank drift, optical tweezing, and recapturing of forward scattered light are also investigated. The random error observed in this work for absorbing spheres is comparable to previous reported measurements. For both absorbing and non-absorbing spheres, a statistical framework is developed for including the contributions to random error due to CPC measurement uncertainty, <i>R_L</i>, statistical fluctuations in particle counts, fluctuations in the blank, and mass flow controller flow error.</p> <p>Copyright 2014 American Association for Aerosol Research</p

<i>Sclerotinia homoeocarpa</i> Overwinters in Turfgrass and Is Present in Commercial Seed

<div><p>Dollar spot is the most economically important disease of amenity turfgrasses in the United States, yet little is known about the source of primary inoculum for this disease. With the exception of a few isolates from the United Kingdom, <i>Sclerotinia homoeocarpa</i>, the causal agent of dollar spot, does not produce spores. Consequently, it was assumed that overwintering of this organism in soil, thatch, and plant debris provides primary inoculum for dollar spot epidemics. Overwintering of <i>S. homoeocarpa</i> in roots and shoots of symptomatic and asymptomatic creeping bentgrass turfgrass was quantified over the course of a three-year field experiment. Roots did not consistently harbor <i>S. homoeocarpa</i>, whereas <i>S. homoeocarpa</i> was isolated from 30% of symptomatic shoots and 10% of asymptomatic shoots in the spring of two out of three years. The presence of stroma-like pathogen material on leaf blades was associated with an increase in <i>S. homoeocarpa</i> isolation and colony diameter at 48 hpi. Commercial seed has also been hypothesized to be a potential source of initial inoculum for <i>S. homoeocarpa</i>. Two or more commercial seed lots of six creeping bentgrass cultivars were tested for contamination with <i>S. homoeocarpa</i> using culture-based and molecular detection methods. A viable, pathogenic isolate of <i>S. homoeocarpa</i> was isolated from one commercial seed lot and contamination of this lot was confirmed with nested PCR using <i>S. homoeocarpa</i> specific primers. A sensitive nested PCR assay detected <i>S. homoeocarpa</i> contamination in eight of twelve (75%) commercial seed lots. Seed source, but not cultivar or resistance to dollar spot, influenced contamination by <i>S. homoeocarpa</i>. Overall, this research suggests that seeds are a potential source of initial inoculum for dollar spot epidemics and presents the need for further research in this area.</p></div

Sampling dates for <i>S. homoeocarpa</i> overwintering sample collections.

<p>Sampling dates for <i>S. homoeocarpa</i> overwintering sample collections.</p

Results for <i>S. homoeocarpa</i> contamination of creeping bentgrass commercial seed lots by culture-based and molecular detection methods.

1<p>Two representative lots of each cultivar were selected by for detection of <i>S. homoeocarpa</i> contamination by each method; additional lots of ‘Shark’ and ‘Penncross’ seed were tested by the nested PCR detection method only.</p>2<p>Three 1,000 seed replicates for all lots were performed by plating 500 seeds from each lot onto two different media semi-selective for <i>S. homoecarpa</i> (1,000 seeds total) and repeating three times (3,000 seeds/lot total).</p>3<p>Nested PCR was repeated twice for each seed lot; numbers shown in parentheses are the number of subsamples testing positive for <i>S. homoeocarpa</i> contamination in each run.</p>4<p>Q-PCR detection was ceased after it was determined that PCR inhibitors in seed DNA samples rendered the assay ineffective.</p><p>Results for <i>S. homoeocarpa</i> contamination of creeping bentgrass commercial seed lots by culture-based and molecular detection methods.</p

<i>Sclerotinia homoeocarpa</i> isolation and weather data for pathogen overwintering studies.

<p>Roots and shoots were collected from symptomatic and asymptomatic turf in the fall of each year to determine starting <i>S. homoeocarpa</i> populations. The same areas were resampled in the spring to assess pathogen survival. ANOVA values for season, treatment, and season by treatment effects are reported for each year. Weather data includes daily air temperature (dashed line) and precipitation (solid line) averages. The average temperature in 2011–2012 was c. 4°C higher than in the other two years and a major snow events occurred in early November 2011–2012 and early February of 2012–2013 (arrows).</p

Results of nested PCR detection of <i>Sclerotinia homoeocarpa</i> DNA in creeping bentgrass commercial seed lots.

<p>Columns reflect the average proportion of samples positive for <i>S. homoeocarpa</i> contamination across the two lots of each cultivar. Shading indicates cultivar source: ‘96-2’, ‘Focus,’ and ‘SR1150’ were from a single source while ‘Shark,’ ‘Penncross,’ and ‘Declaration’ were from different seed distributors. Error bars represent ± one standard error of the mean. Source and cv(source) P-values are from ANOVA of the data with the normal distribution and α = 0.05. The P-value for R versus S cultivars was obtained from a pre-planned orthogonal contrast.</p

Seed sources, cultivars, and National Turf Evaluation Program performance results for creeping bentgrass commercial seed lots used in <i>S. homoeocarpa</i> culture-based and molecular seed detection studies.

1<p>The experimental name for cv. ‘Focus’ in NTEP trials was ‘GMC comp’.</p>2<p>NTEP ratings provided are the average from dollar spot trials in 2009, 2010, and 2010 that included all six cultivars.</p>3<p>Ratings are based on turfgrass quality and are given on a 0–9 scale; 9 = highest turf quality/resistance to dollar spot and 0 = lowest turf quality/resistance to dollar spot.</p><p>Seed sources, cultivars, and National Turf Evaluation Program performance results for creeping bentgrass commercial seed lots used in <i>S. homoeocarpa</i> culture-based and molecular seed detection studies.</p

Primers used for molecular detection of <i>S. homoeocarpa</i> in creeping bentgrass commercial seed lots.

1<p>Primers developed in the present research were designed using the NCBI Primer-BLAST tool with the DNA sequence for the <i>S. homoeocarpa</i> EF1α input as the sole target sequence. Primer specificity was checked by sequencing of primary and nested PCR products and qPCR melt curve analysis.</p><p>Primers used for molecular detection of <i>S. homoeocarpa</i> in creeping bentgrass commercial seed lots.</p

Stroma on creeping bentgrass leaf blades and its effect on isolation on <i>S. homoeocarpa</i>.

<p><b>A</b>, typical stroma-like material observed on a turfgrass leaf blade was brown in color and appeared as flaky, plate-like material. <b>B</b>, The percentage of CRB leaf blades with and without visible stroma from which <i>S. homoeocarpa</i> was isolated. P-value from ANOVA using the binary distribution for presence/absence of <i>S. homoeocarpa</i> and α = 0.05. <b>C</b>, Average diameter of <i>S. homoeocarpa</i> colonies 48 h after plating leaf blades with or without visible stroma. P-value is from ANOVA with the normal distribution and α = 0.05. Error bars represent ± one standard error of the mean (n = 5).</p

Final Model Build Brief Description

A brief text summary of the model build approac