James L. McElroy

Lidar Descriptions of Mixing-Layer Thickness Characteristics in a Complex Terrain/Coastal Environment

Abstract Airborne lidar and supplementary measurements made during a major study of air chemistry in southern California (SCCCAMP 1985) provided a rare opportunity to examine atmospheric boundary-layer structure in a coastal area with complex terrain. This structure results from a combination of daytime heating or convection in the boundary layer (CBL), the intrusion of a marine layer into the inland areas, the thermal internal boundary layer (TIBL) formed within the marine onshore flow, inland growth of the TIBL, interactions of the CBL and the TIBL, and airflow interactions with terrain features. Measurements showed offshore mixing-layer thicknesses during SCCCAMP to be quite uniform spatially and day to day at 100–200 m. Movement of this layer onshore occurred readily with terrain that sloped gradually upward (e.g., to 300 m MSL at 50 km inland), but was effectively blocked by a 400–500 m high coastal ridge. In the higher terrain beyond the coastal ridge, aerosol layers aloft were often created as a re...

Methodology for designing air quality monitoring networks: I. Theoretical aspects.

An objective methodology is presented for determining the number and disposition of ambient air quality stations in a monitoring network for the primary purpose of compliance with air quality standards. The methodolgy utilizes a data base with real or simulated data from an air quality dispersion model for application with a two-step process for ascertaining the optimal monitoring network. In the first step, the air quality patterns in the data base are collapsed into a single composite pattern through a figure-of-merit (FOM) concept. The most desirable locations are ranked and identified using the resultant FOM fields. In the second step the network configuration is determined on the basis of the concept of spheres of influence (SOI) developed from cutoff values of spatial correlation coefficients between potential monitoring sites and adjacent locations. The minimum number of required stations is then determined by deletion of lower-ranked stations whose SOIs overlap. The criteria can be set to provide coverage of less than some fixed, user-provided percentage of the coverage of tha SOIs of the higher ranked stations and for some desired level of minimum detection capability of concentration fluctuations.The methodology is applied in a companion paper (McElroy et al., 1986) to the Las Vegas, Nevada, metropolitan area for the pollutant carbon monoxide.

Vertical pollutant distributions and boundary layer structure observed by airborne lidar near the complex southern California coastline

Abstract Airborne lidar flights to determine air pollutant transport in the complex coastal environment of S California reveal details of vertical pollutant distributions, atmospheric boundary layer structure, and pollutant transport and diffusion unique to such an area. The lidar data show two (or more) layers of aerosols offshore each separated by relatively clear areas. Typically the top of the lower layer corresponds to the base of the temperature inversion over the water, and the upper layer lies within the temperature inversion. Little intermixing of the layers occurs unless onshore air flow carries both layers over the coastline. A short distance inland, afternoon heating and mechanical turbulence eliminate the layered structure. The upper layers which may be formed by sea-breeze undercutting, by convergence in complex terrain or at the land/sea interface, or by interaction of heated upslope flow with terrain features can flow offshore with existing winds or at night with the land-breeze. The origin of the layers aloft is an essential factor in determining the pollutant impact. Thus, transport and layer structure phenomena observed by lidar within the complex S California coastline environment may have an important bearing on any proposed land-use changes. Finally, the results of the study suggest the need for diffusion climatologies that take into account the frequency and duration of the wind regimes aloft in addition to the low-level thermal structure.

Creation and fate of ozone layers aloft in Southern California

Abstract Data obtained during the 1987 Southern California Air Quality Study (SCAQS) were used to examine the creation and fate of ozone layers aloft. The ultimate objective was to estimate the potential impact of the layers on surface ozone concentrations. The smog episode of 13–15 July 1987 formed the basis for a case study. Several ozone layers aloft were created over the northern part of the Los Angeles Basin (e.g. over El Monte) during the day on 13 July. These layers were attributed to the action of slope flows (including convergence), convective elements transported into the inversion layer and undercutting by the seabreeze flow. A peak value of 34 pphm was observed aloft in one of the layers. The principal layers aloft were still present intact over the northern part of the Basin on the morning of 14 July. Their movement eastward (from areas in the vicinity of El Monte and Upland) was estimated by trajectory and isentropic analysis. The results indicated that a likely impact area of the ozone layer aloft near Upland during the day of the 14th was near San Bernardino. The ozone layer above El Monte on the morning of 14 July appears to have been incorporated into the mixed layer near or to the southeast of Riverside. A maximum ozone impact of 3–4 pphm was estimated although a portion of this impact was probably due to an overnight reservoir of surface-based precursors of ozone.

A numerical study of the nocturnal heat island over a medium-sized mid-latitude city (Columbus, Ohio)

A numerical investigation is conducted of the nocturnal heat island over Columbus, Ohio, a relatively flat mid-latitude city. Specifically, a cross-sectional steady-state numerical model to simulate the (thermal) structure of the nocturnal urban boundary layer is developed from a one-dimensional, time-dependent model due to Estoque. The model is applied to Columbus for special periods in September 1968 and March 1969 during which comparable experimental data were collected. The numerical simulations agree well with the observed data with respect to the detailed spatial form of the surface-based thermal boundary layer across the city. The use of the model for other metropolitan areas, for ascertaining the relative effect of city size and building geometry on the development of the thermal boundary layer, and for determining the effect of alternate land-use strategies on the thermal stratification are also discussed.

Design of an optimum air monitoring network for exposure assessments

Abstract Nonlinear programming techniques are frequently used to design optimum monitoring networks. These mathematically rigorous techniques are difficult to implement or cumbersome when considering other design criteria. This paper presents a more pragmatic approach to the design of an optimal monitoring network to estimate human exposure to hazardous air pollutants. In this approach, an air quality simulation model is used to produce representative air quality patterns, which are then combined with population patterns to obtain typical exposure patterns. These combined patterns are used to determine ‘figures of merit’ for each potential monitoring site, which are used to identify and rank the most favorable sites. The spatial covariance structure of the air quality patterns is used to draw a ‘sphere of influence’ around each site to identify and eliminate redundant monitoring sites. This procedure determines the minimum number of sites required to achieve the desired spatial coverage. This methodology was used to design an optimal ambient air monitoring network for assessing population exposure to hazardous pollutants in the southeastern Ohio River valley.

Methodology for designing air quality monitoring networks: II. Application to Las Vegas, Nevada, for carbon monoxide.

An objective methodology presented in a companion paper (Liu et al., 1986) for determining the optimum number and disposition of ambient air quality stations in a monitoring network for carbon monoxide is applied to the Las Vegas, Nevada, area. The methodology utilizes an air quality simulation model to produce temporally-varying air quality patterns for each of a limited number of meteorological scenarios representative of the region of interest. These air quality patterns in turn serve as the data base in a two-step procedure for the identification and ranking of the most desirable monitoring locations (step 1) and the removal of redundancies in spatial coverage among the desired locations (step 2.)The performance of the air quality simulation model, a key element in the design methodology, was evaluated in the Las Vegas area in a special field measurement program. In the Las Vegas demonstration for carbon monoxide, 19 stations covering concentration maxima and 3 stations covering background concentrations in rural areas were identified and ranked. A 10-station network, for example, consisting of 7 stations for peak average concentrations and 3 stations for background concentrations, was selected for a desired minimum detection capability of 50% of concentration variations. Networks with fewer stations would be selected if smaller minimum detection capabilities of concentration variations are acceptable, and vice versa. Background stations could, of course, be deleted for networks with the sole purpose of discerning peak concentrations.

Differential absorption lidar measurement of vertical ozone profiles in the troposphere that contains aerosol layers with strong backscattering gradients: a simplified version

A technique for determining approximate ozone-concentration profiles from differential absorption lidar (DIAL) data obtained in the troposphere with large gradients of aerosol backscattering is presented. The atmospheric interferences are defined as errors of the off-on DIAL signal ratio; the interferences are separated and removed before the ratio is differentiated. To facilitate the separation of the regular (subjected to differentiation) component of the signal ratio from random noise, the ratio is transformed into an intermediate function, and the measurement error is minimized by fitting of an analytical function to the transformed function. Simple criteria are used to demarcate atmospheric layering, for which a strong aerosol-backscattering gradient can result in an unacceptably large error in the measured ozone concentration.

Airborne downlooking lidar measurements during state 78

Abstract EPAs airborne downlooking dye lidar was operated during the STATE field program in western Kentucky/Tennessee in the summer of 1978. In this paper, lidar estimates of height of the atmospheric mixing layer are shown and compared with in situ measurements. Lidar estimates of crosswind and vertical dimensions of TVA power plant plumes are compared with appropriate literature values and with in situ estimates from data compiled by other STATE investigators; wherever necessary, data are adjusted to ensure compatibility with respect to sampling time. In addition, lidar measurements of plume rise are compared with model calculations. The lidar estimates of mixing layer height were usually slightly higher than in situ counterparts, presumably because aerosols may rise and become trapped above the base of the elevated stable layer. Values of plume rise computed using the Briggs models were generally similar to those indicated from lidar measurements although considerable scattering of data existed; supplementary data indicated that the scatter could be reduced with the inclusion of the vertical shear of the horizontal wind in the models. When adjustment with respect to sampling time was accomplished, the lidar values of plume dimensions compared reasonably well with values obtained using data collected by in situ measurement platforms. The situations sampled by the lidar were almost exclusively at night or in the daytime when the plume was above the top of the mixing layer. For such situations, effects of initial mixing due to buoyancy and diffusion are apparent in the vertical plume spread. Effects of initial mixing due to buoyancy, turning of the horizontal wind with height, and diffusion are apparent in the crosswind plume spread; this was surmised through comparison of the lidar plume dimensions with the Pasquill-Gifford dispersion curves. The data, thus, provide additional evidence that information on the plume buoyancy, turning of the wind with height and the height, of the plume in relation to that of the mixing layer must be included in any new scheme or any adaptation of an existing scheme for the estimation of the spread for large, elevated buoyant plumes.

Estimation of Pollutant Transport and Concentration Distributions over Complex Terrain of Southern California Using Airborne Lidar

Airborne lidar data collected during the South Coast Air Basin/Southeast Desert Oxidant Transport Study in summer 1981 were used to provide estimates of concentration distributions of particles and oxidants, and of pollutant transport in mountain passes and over mountain slopes. The results support the idea that for certain situations, given a few representative in situ measurements for use in calibration, airborne lidar might be used to develop detailed concentration fields. When combined with appropriate measurements of wind speed profiles with height, useful estimates of pollutant transport flux might also be provided.