Perry J. Samson

Impact of temperature on oxidant photochemistry in urban, polluted rural and remote environments

The impact of temperature on formation of O3 and odd nitrogen photochemistry is investigated using urban-, regional- and global-scale simulations. Urban and polluted rural environments are explored with a regional simulation derived from a specific episode in the midwestern United States. The simulations predict that O3 increases with temperature in both urban and polluted rural environments. The O3-temperature relation is driven largely by chemistry of peroxyacetylnitrate (PAN) which represents an increased sink for both NOx and odd hydrogen at low temperatures. Isoprene emissions, H2O, and solar radiation also contribute to the O3-temperature relation. Possible correlations between temperature and anthropogenic emissions or stagnant meteorology were not included. Observations at urban and rural sites in the United States suggests that O3 increases with temperature at a faster rate than the models predict. Calculations with a one-dimensional global model suggest that increased temperature in the polluted boundary layer does not lead to increased O3 in the free troposphere, because increased export of O3 is balanced by decreased export of odd nitrogen species.

Uncertainty in Trajectory Calculations Due to Low Resolution Meteorological Data

Abstract Meteorological observations conducted during the Cross Appalachian Tracer Experiment (CAPTEX) were utilized to quantify the uncertainty in boundary layer trajectory calculations due to low-resolution meteorological data [the current National Weather Service (NWS) rawinsonde network). Evaluation of several spatial and temporal interpolation techniques against high-resolution measurements revealed mean absolute errors of 2–4 m s−1 in estimation of horizontal wind components. A trajectory of errors procedure is introduced that allows the quantification of probable errors in transport calculation due to imprecise interpolation. Our results, based on the observed distributions of spatial and temporal interpolation errors during CAPTEX, indicate dust boundary layer trajectories calculated using the current NWS network with 12 h resolution contain a 50% chance of exceeding horizontal displacement errors of 350 km after 72 h travel time. An increase in spatial resolution is shown to improve the accuracy ...

PHOTOCHEMISTRY OF OZONE FORMATION IN ATLANTA, GA-MODELS AND MEASUREMENTS*

Chemical measurements made during an air pollution event in Atlanta, GA have been compared with results from several photochemical simulations. Measurements included Os, primary reactive organic gases (ROG), aldehydes, PAN, total reactive nitrogen (NO,,) and HzOz, with vertical profiles for primary ROG. Photochemical models using two different chemical representations and a range of assumptions about winds, vertical mixing and emissions were used to simulate the event. Results show that assumptions about vertical mixing can cause a variation in simulated surface concentrations of primary hydrocarbons of a factor of two or more. A tendency to underestimate isoprene was found in comparison with measured vertical profiles. The models tend to overestimate concentrations of HCHO, H,02 and PAN in comparison with measurements. Peak 0s and concurrent NO, from helicopter measurements was used as a basis for evaluating individual model scenarios. Scenarios were developed with different Op-NO,-ROG sensitivity, but only the NO,-sensitive scenarios are consistent with measured 09, NO, and isoprene. Key word index: Ozone, nitrogen oxides, hydrocarbons, photochemical smog, hydrogen, peroxide.

Trajectory Analysis of Summertime Sulfate Concentrations in the Northeastern United States

Abstract This paper presents a technique for quantifying the relationships between observed concentrations of atmospheric sulfate aerosol and their corresponding upstream history of sulfur dioxide emissions, wind speed and mixing height. Using reported sulfate concentrations from several sampling sites in the northeastern United States, 72 h upstream trajectories have been computed for winds in the mixed layer of the atmosphere over the duration of their respective sampling periods. Trajectories from one site were computed for four sublayers, each 400 m thick, extending from the surface to 1600 m. The deviations in along-trajectory and cross-trajectory directions of each of the sublayers from the position of the whole mixed layer were computed. From this the functions σy(t) and σx(t) for travel times of 6 to 72 h were derived for each layer individually and collectively for the whole layer. The values of σy(t) and σx(t) for the whole mixed-layer were found to be roughly equivalent over this time period an...

THE INFLUENCE OF ATMOSPHERIC TRANSPORT ON PRECIPITATION CHEMISTRY AT TWO SITES IN THE MIDWESTERN UNITED STATES

Cluster analyses of two-dimensional mixed layer back trajectory data were used to determine what fraction of chemical variability in precipitation composition could be related to differences in atmospheric transport. Trajectories arriving at two different sites, Rockport, Indiana, and Gaylord, Michigan, were clustered to identify events occurring with similar transport patterns. It was found that certain transport situations resulted in significantly higher concentrations and depositions of the major ions, H+, SO;, NO; and NHf. At Rockport, the greatest fraction of acid deposition was associated with low wind speeds. At Gaylord, transport direction played a greater role than transport speed in influencing precipitation composition. Results presented here suggest that lo%+% of the variability in ion concentrations may be related to differences in atmospheric transport. The residual variation in concentra- tions was correlated with differences in the occurrence of upwind precipitation, precipitation type., and variation in precipitation amount.

Ozone production in urban plumes transported over water: Photochemical model and case studies in the northeastern and midwestern United States

Abnormally high concentrations of O3 have been observed in rural locations on the shore of Lake Michigan and on the Atlantic coast in Maine, at a distance of 300 km or more from major anthropogenic sources. We hypothesize that this O3 is associated with transport from major urban centers and with the suppression of vertical mixing as urban plumes are transported over water. A dynamical/photochemical model is developed that represents formation of O3 in shoreline environments and is used to simulate case studies for Lake Michigan and the northeastern United States. Results suggest that a broad region with elevated O3, NOx and volatile organic carbon (VOC) forms as the Chicago plume travels over Lake Michigan, a pattern consistent with observed O3 at surface monitoring sites. Near-total suppression of dry deposition of O3 and NOx over the lake is needed to produce high O3. Results for the east coast suggest that the observed peak O3 can only be reproduced by a model that includes suppressed vertical mixing and deposition over water, 2-day transport of a plume from New York, and superposition of the New York and Boston plumes. An investigation of the sensitivity of O3 to emissions of NOx and VOC suggests that results vary greatly between cities, even when the composition of urban emissions is similar. An index for VOC versus NOx sensitivity is shown to correlate with total reactive nitrogen (NOy) at the time of peak O3.

Aggregation of Selected Three-Day Periods to Estimate Annual and Seasonal Wet Deposition Totals for Sulfate, Nitrate, and Acidity. Part I: A Synoptic and Chemical Climatology for Eastern North America

Abstract Running 3-day periods from 1979 to 1985 were categorised into one of 20 meteorological categories. These categories were developed through the cluster analysis of 3-day progressions of 85-kPa wind flow over eastern North America. The purpose for developing the categories was to identify recurring atmospheric transport patterns that were associated with differing amounts of wet sulfate (SO2−4) and nitrate (NO−3) deposition at a variety of locations in eastern North America. Identification of these patterns was necessary to facilitate the selection of time periods for simulation by the Regional Acid Deposition Model and in the development of a method for estimating long-term acidic deposition over eastern North America from a limited number of model runs. The effectiveness of this method (referred to as the aggregation method) was expected to be dependent on the ability of the categories to separate structure in wet deposition patterns. This paper describes the determination of the 20 meteorologica...

Uncertainty in Estimating Boundary-Layer Transport during Highly Convective Conditions

Abstract Routine and supplemental rawinsonde observations collected during the Preliminary Regional Experiment for Storm-Central (PRE-STORM) were analyzed to assess the uncertainty in boundary-layer trajectory calculations due to imprecise interpolation of the horizontal wind field. This study was designed to complement our earlier analysis of rawinsonde data collected during the Cross Appalachian Tracer Experiment (CAPTEX; Kahl and Samson 1986). The present study is representative of widespread convective conditions, while our previous study was representative of fairly persistent, undisturbed flow. Spatial autocorrelation analysis revealed significant wind field variability on scales less than 100 km. Evaluation of several spatial and temporal interpolation techniques yielded mean absolute errors in estimation of u and v wind components ranging from 3.3–6.5 m −1. Spatial interpolation accuracy improved only slightly when supplemental measurements were included in the interpolation procedure. Estimates o...

Ozone and Visibility Reduction in the Midwest: Evidence for Large-Scale Transport

Abstract For June and July 1975, ozone concentrations throughout the Midwest showed a consistent dependence onwind direction with the highest concentrations associated with winds from the southeast. This systematicpattern suggests that large-scale transport rather than local sources control the general trends of the ozonelevel in the study area.Ozone and meteorological data covering an episode period 29 July-2 August have been analyzed usingweather maps, visibility reports, upper air soundings, trajectories and aircraft measurements to ascertainthe scale of the ozone problem. Results indicate that the highest ozone concentrations occurred within regions of reported haze, and that these regions had definite bounds which could be followed over the country.Moreover, distinct areas of reported obscured sky formed in the vicinity of St. Louis and Pittsburgh whichcould be followed over thousands of kilometers reducing visibilities substantially even in upper Ontario.Trajectories indicate that adverse conditions...

Trajectory sensitivity to rawinsonde data resolution

Abstract Meteorological measurements collected during the Cross Appalachian Tracer Experiment (CAPTEX) were used to evaluate the sensitivity of trajectory calculations to variations in the spatial and temporal density of rawinsonde data. Data resolution was systematically adjusted in order to separately evaluate the effects of enhanced spatial and temporal resolution. Variable mixed layer, constant layer, and constant level trajectories were more sensitive to increases in spatial resolution than to increases in temporal resolution. For mixed layer, 0–400-m layer and 400-m level trajectories, sensitivity to combined increases in both spatial and temporal rawinsonde data resolution was dominated by the effects of spatial resolution alone. This dominance was not observed for trajectories calculated at higher levels and layers. Trajectory case studies corresponding to two CAPTEX tracer releases were presented. These examples illustrate the potential effects of a single additional measurement at an existing National Weather Service rawinsonde station, and the importance of specifying the initial transport wind accurately.