Warren H. White

Anatomy of a Los Angeles smog episode: Pollutant transport in the daytime sea breeze regime

Abstract The three-dimensional distribution of aerosols and trace gases in the Los Angeles air basin was mapped out during a smoggy day by instrumented aircraft. Strong vertical and horizontal gradients were observed in the concentrations of both primary and secondary pollutants. The day began with much of the basin occupied by polluted air carried over from the day before. New emissions accumulated in the stagnant air until late morning, when a well-organized sea breeze developed at the surface. Onshore flow during the afternoon carried heavily polluted air into inland receptor areas, ahead of a shallow, well-defined, advancing layer of cleaner marine air. The highest ozone concentrations of the day were observed just above the marine layer in stagnant air decoupled from the surface and just ahead of the marine front in photochemically aged air transported into low emission density areas. The data show that air pollution in the basin is a regional problem and that emissions in the western portion of the basin can result in high concentrations of secondary pollutants (e.g. > 0.25 ppm ozone) in areas over 50 km downwind. Layers of well aged pollutants are also shown to occur aloft. These layers can remain overnight and be re-entrained the next day by a deepening mixing layer, contributing to surface concentrations.

Spatial resolution of a transport inversion technique

A spatial calibration for conditional frequency analysis, the inversion technique applied to routine back trajectories to study the association between geographical regions and conditions at a receptor, is discussed for trajectories arriving at Hopi Point, Arizona. Several 0.5° × 0.5° cells were selected at three different distances from Hopi Point. These cells were used to mimic sources, with air residing over them considered contaminated by imaginary emissions. The distribution of back trajectory segment endpoints over a latitude-longitude grid was then examined as a function of the trajectories sojourn over each imaginary source cell on their way to the receptor. The areal extent of significant associations, evaluated for different seasons and the various ideal source positions, suggests that this inversion technique can resolve the direction of potential sources but has limited resolution for their radial distance from the receptor.

A tracer calibration of back trajectory analysis at the Grand Canyon

The potential of routine back trajectory analyses to locate sources of contaminants in air at Grand Canyon is investigated with data on methylchloroform (CH 3 CCl 3 ) concentrations collected during the Subregional Cooperative Electric Utility, Department of Defense, National Park Service, and Environmental Protection Agency Study(SCENES). Following a now-standard approach, the distribution of back trajectory segment endpoints over a latitude-longitude grid is examined as a function of measured concentrations at the fixed monitoring site. Grid cells in which segment endpoints are preferentially associated with high concentrations are then identified as candidate emissions sources. The method correctly identifies southern California as a source of CH 3 CCl 3 . An objective rationale is developed for screening out chance associations, attributable to statistical fluctuations, and the results are evaluated for real and hypothetical tracers with known, simple distributions.

Estimating Precision Using Duplicate Measurements

Abstract Precision is a concept for which there is no universally accepted metric. Reports of precision vary depending on the formula and inclusion criteria used to calculate them. To properly interpret and utilize reported precisions, the user must understand exactly what the precision represents. This paper uses duplicate Interagency Monitoring of Protected Visual Environments (IMPROVE) measurements to illustrate distinctions among different approaches to reporting precision. Three different metrics are used to estimate the precision from the relative differences between the duplicate measurements: the root mean square (RMS), the mean absolute value, and a percentile spread. Precisions calculated using the RMS relative difference yield wide distributions that tend to overestimate most of the observed differences. Precisions calculated using percentiles of the relative differences yield narrower distributions that tend to fit the bulk of the observed differences very well. Precisions calculated using the mean absolute relative difference lie between the other two precision estimates. All three approaches underestimate the observed differences for a small percentage of outliers.

Visibility: an evolving issue

The task of protecting visual air quality requires an operational understanding of what haze is, how it develops, and what the responsible pollutants are. In the past decade these issues have been addressed through intensive modeling, field experimentation, laboratory simulation, and trend analysis. The fundamental physical and chemical characteristics of haze have been identified, with the result that many of the questions that spurred visibility research have been transformed into a second generation of inquiry. Their capacity to design effective control strategies, to mount effective monitoring efforts, and to predict with accuracy the visual effect of changing emissions patterns hinges on the resolution of these new questions. This article identifies some of these critical issues in visual air quality.

Earlier onset of the spring fine dust season in the southwestern United States

Particulate matter (PM)2.5 dust concentrations (mineral particles with aerodynamic diameters less than 2.5u2009µm) typically peak in spring and early summer at rural and remote sites across the southwestern United States. Trend analyses indicate that springtime regional mean PM2.5 dust concentrations have increased from 1995 to 2014, especially in March (5.4%u2009yr−1, pu2009<u20090.01). This increase reflects an earlier onset of the spring dust season across the Southwest by 1 to 2u2009weeks over the 20u2009year time period. March dust concentrations were strongly correlated with the Pacific Decadal Oscillation index (ru2009=u2009−0.65, pu2009<u20090.01), which was mostly in its negative phase from 2007 to 2014, during which the region was drier, windier, and less vegetated. The positive spring trend and its association with large-scale climate variability have several important implications for visibility, particulate matter, health effects, and the hydrologic cycle in the region.

Three-Dimensional Distribution of Air Pollutants in the Los Angeles Basin

Abstract Data from a three-dimensional pollutant mapping program, conducted in the Los Angeles basin, wereanalyzed to obtain grand average vertical profiles sampled on 24 summer days in 1973. Morning andafternoon profiles at four locations show an erosion of the nighttime radiation inversion, increased temperatures, more intense mixing in the inland areas, and a semi-permanent subsidence inversion at higher levels.High values of primary pollutant parameters (NO, and condensation nuclei) are seen in the western part ofthe basin at Hawthorne. Secondary pollutant parameters (Os and light scattering coefficient) were dominating at the inland receptor site, Riverside. Ozone concentrations in the morning were consistently higheraloft. The deficit near the surface is attributed to ozone scavenging by primary emissions.

Assessing the Suitability of Historical PM2.5 Element Measurements for Trend Analysis

The IMPROVE (Interagency Monitoring of Protected Visual Environments) network has characterized fine particulate matter composition at locations throughout the United States since 1988. A main objective of the network is to evaluate long-term trends in aerosol concentrations. Measurements inevitably advance over time, but changes in measurement technique have the potential to confound the interpretation of long-term trends. Problems of interpretation typically arise from changing biases, and changes in bias can be difficult to identify without comparison data that are consistent throughout the measurement series, which rarely exist. We created a consistent measurement series for exactly this purpose by reanalyzing the 15-year archives (1995-2009) of aerosol samples from three sites - Great Smoky Mountains National Park, Mount Rainier National Park, and Point Reyes National Seashore-as single batches using consistent analytical methods. In most cases, trend estimates based on the original and reanalysis measurements are statistically different for elements that were not measured above the detection limit consistently over the years (e.g., Na, Cl, Si, Ti, V, Mn). The original trends are more reliable for elements consistently measured above the detection limit. All but one of the 23 site-element series with detection rates >80% had statistically indistinguishable original and reanalysis trends (overlapping 95% confidence intervals).

Considerations in the use of ozone and PM2.5 data for exposure assessment

The US national ambient-air monitoring network, created to verify compliance with health-based standards, now doubles as an important source of exposure data for the epidemiological analyses on which these standards increasingly rest, particularly in the case of ozone and PM2.5. This paper was written for a workshop called to facilitate and inform the use of routine ozone and PM2.5 data by the Environmental Public Health Tracking Network. It examines the fit between priorities that shape regulatory monitoring and modeling and the data needs of public health tracking.

Reanalysis of archived IMPROVE PM2.5 samples previously analyzed over a 15-year period.

The IMPROVE (Interagency Monitoring of Protected Visual Environments) network has collected airborne particulate matter (PM) samples at locations throughout the United States since 1988. These samples have been analyzed for elemental content using analytical methods that evolved over the years. Changes in analytical methods sometimes introduced shifts in reported concentrations that are evident in the historical record. We sought to illuminate the effects of methodological changes by reanalyzing archived samples with current methods. To test the feasibility of this approach, the 15-year archive of PM samples from Great Smoky Mountains National Park was selected for reanalysis as a single analytical batch using a common protocol and calibration. Comparisons of the reanalyses and original analyses indicate that concentrations of all but one measured element, Br, remained stable on the filters over years of storage. The agreement between the two analyses varied with element and original measurement method. For elements measured well above their contemporary detection limits - S, K, Ca, Fe, and Zn - the reanalysis established that method changes had limited impacts on reported concentrations, generally <10%. For elements originally measured near their detection limits, reanalysis confirmed the presence of discontinuities in the data record, many of which were previously recognized and documented as method-related.