Urban Aerosols and Their Impacts - American Chemical Society

Chapter 16

Estimates of the Vertical Transport of Urban Aerosol Particles

Downloaded by PURDUE UNIV on May 26, 2018 | https://pubs.acs.org Publication Date: November 15, 2005 | doi: 10.1021/bk-2006-0919.ch016

Edward E. Hindman Earth and Atmospheric Sciences Department and NOAA-Cooperative Remote Sensing Science and Technology Center, The City College of New York, 138 Street and Convent Avenue, New York, NY 10031 th

Diurnal measurements obtained in urban Kathmandu, Nepal and rural Steamboat Springs, Colorado are presented to illustrate the role of convection in the vertical transport of aerosol particles in the atmosphere. At both locations, C N concentrations were a maximum during the morning and evening rush periods and a minimum during early afternoon. The vertical transport of aerosols was estimated for Steamboat Springs based on elementary meteorological principles and a method is presented for determining the verticle transport that makes use of archived meteorologocal data and analysis tools available from the National Oceanic and Atmospheric Administration, Air Resources Laboratory. The boundary layer depth was seen to peak each day at about 21 UTC. The vertical extent of the plume from the collapse of the World Trade Center on September 11, 2001 was estimated using this method and results indicated that the plume could have risen to between 1.7 and 2 km above sea level, consistent with the observed depth. The method presented here can also be used to forecast the depth and estimate vertical mixing in the atmospheric boundary layer.

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© 2006 American Chemical Society

Gaffney and Marley; Urban Aerosols and Their Impacts ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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Introduction The diurnal-cycle of solar heating and nighttime cooling is confined to a shallow layer in contact with the ground called the atmospheric boundary layer (ABL) (7). As a result of the heating and cooling, the ABL experiences variations in temperature, humidity, wind speed, air pollutants and depth, hi the absence of major storm systems, the ABL is shallow, stable and most polluted in the early morning and is deep, unstable and least polluted in the afternoon. The increasing depth of the ABL results in vertical transport and, hence, dilution of the pollutants. Diurnal pollution and meteorological measurements are presented here to illustrated this behavior of the ABL In the absence of major storm systems, vertical transport is accomplished largely by convection. Air in contact with the ground heats more rapidly than the air above and it becomes positively buoyant. As this convective "bubble" rises, it expands and cools. When the bubble" cools to the temperature of the surrounding environment, the upward motion ceases and the "bubble" mixes with the surrounding air. Thus, heat, moisture, aerosol particles and gases from the surface are transported aloft by convection. Convection typically reaches a maximum in the early afternoon when the maximum amout of heat has been transferred from the surface to the air in contact with the surface (7). This fact is illustrated by the diurnal variation of pollutants mesured by Hindman and Upadhyay in urban Kathmandu, Nepal (2) and by Hindman in rural Steamboat Springs, Colorado (3). In this chapter, diurnal measurements obtained in urban Kathmandu, Nepal and rural Steamboat Springs, Colorado are presented to illustrate the role of convection in the vertical transport of aerosol particles in the atmosphere. The vertical transport of aerosols is estimated based on elementary meteorological principles and a method is presented for determining the verticle transport that makes use of archived meteorologocal data and analysis tools available from the National Oceanic and Atmospheric Administration (NOAA) , Air Resources Laboratory (ARL). The depth of the ABL on September 11, 2001 over Manhattan, New York is estimated by using this method and this estimated depth of is consistent with the observed depth. Thus, the method appears useful for estimating the vertical transport of air pollutants fl

Experimental Methods Diurnal measurements obtained in urban Kathmandu, Nepal and rural Steamboat Springs, Colorado are presented here to illustrate the role of


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convection in the vertical transport of aerosol particles in the atmosphere. The Steamboat Springs site was located near the floor of the Yampa River Valley (40.5N, 106.72W) near a major roadway. At both the Kathmandu and Steampoat Springs sites, the concentrations of aerosol particles with diameters around 10 nm, called condensation nuclei (CN), were continuously measured during the diurnal-cycle using standard instrumentation (TSI Incorporated). Motorized vehicles were a significant source of CN in both of these mountain valley locations. Consequently, the number of moving vehicles were counted as a function of time-of-day at both sites. Finally, the surface meteorological conditions were continuously measured.

Results Figures 1 and 2, illustrate, respectively, the diurnal pattern of C N concentrations in Kathmandu between October 18-23, 1995 and in Steamboat Springs between January 11-25,2001. At both locations, the CN concentrations were a maximum during the morning and evening rush-periods when the maximum vehicle movements occurred and a minimum during early afternoon. The C N concentrations were larger in Steamboat Springs due to the shallow wintertime ABL. The corresponding Kathmandu and Steamboat Springs temperature and moisture measurements (dew-point and relative humidity) are illustrated, respectively, in Figures 3 and 4. The diurnal variation in temperature and moisture is clearly illustrated at both locations. The ABL was moist at both locations. Kathmandu was moist due to the recent ending of the summer monsoon and Steamboat Springs was moist due to snow cover. The diurnal variations of wind speed and direction at Kathmandu and Steamboat Springs are illustrated in Figures 5 and 6. The air flow was controlled by the topography at both locations. At Kathmandu, in the early morning, a lowspeed flow from widely varying directions was caused by "drainage" from the mountains surrounding the valley while in the afternoon, high-speed flow from 270 degrees was a component of the up-slope flow along the Himalayas to the north and east as recently modeled by Regmi, et al. (4). In contrast, the aroundthe-clock, low-speed flow from approximately 90 degrees in Steamboat Springs was "drainage" from the snow-covered Park Range to the east. There were, however, higher speed winds around noontime with a corresponding shift in the wind direction to 180 degrees indicating some weak up-slope flow along the Range. The diurnal CN and temperature patterns measured in the Kathmandu and Steamboat Springs urban areas, while much smaller than the New York City (NYC) area, are expected to resemble the NYC area in fair weather conditions.


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Figure 1. Condensation nucleus (CN) concentrations in Kathmandu, Nepal between October 18-23, 1995.

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Figure 2. Condensation nucleus (CN) concentrations in Steamboat Springs, Colordo between January 11-25, 2001.



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Figure 4. Temperature (black) and relative humidity (gray) values in Steamboat Springs, Colordo between January11-25, 2001.



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Figure 5. Wind speed (top) and direction (bottom) in Kathmandu, Nepal between October 18-23, 1995.


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315 The mountain-vallley flows in Kathmandu and in Steamboat Springs are replaced by the land-sea breezeflowsin NYC (/). The depth of the ABL over Steamboat Springs for the period January 11 -25, 2001 was estimated using archived meteorological data available from the NOAA ARL Real-time Environmental Applications and Display System (READY) web site (5). The latitude and longitude for the site was entered and the section on "Stability Time-series, EDAS 80km" was interrogated using first the "edas.subgrid.janO 1.001" data (EDAS = Eta Data Assimilation System). The month (01), day (11) and hour (15) and plot duration of 72 h was entered. The resulting plot for January 11, 12, and 13 is shown in Figure 7. The boundary layer depth is seen to peak each day at about 21 UTC (15 MDT) near the time of maximum convection. The remaining days of the period were analyzed using the "edas.subgrid.janOl .002" data. The average maximum depth of the ABL for the entire period was 558 m, double the 250 m minimum value. This result indicates a weak convection occurred over the snow covered Yampa River Valley during the period and is consistant with the modest mid-day reduction in CN concentrations shown in Figure 2. The reductions were due to the weak convection becasue the maximum vehicle movements occurred about noontime.

Figure 7. The Atmospheric Boundary Layer Depth and Stability Parameters (D = neutral, E = slightly stable, F = moderately stable, G = extremely stable)for Steamboat Springs, C O , January 11-13,2001.


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Esitmates of vertical transport in New York City on 9/11/01 The depth of the ABL on September 11, 2001 over NYC was estimated from archived meteorological data availablefromthe NOAA-ARL-READY web site (4). First, the latitude and longitude of Central Park was entered (40.78N, 73.97W) and then the section "Meteogram, EDAS 80km" was interrogated. The data "edas.subgrid.sepOl.001" was selected and the month (09), day (11), hour (12) and plot duration of 12 h were specified. The meteorological parameters chosen were temperature, relative humidity, wind speed and direction, sealevel pressure and precipitation. The resulting temporal display of the measurements is shown in Figure 8. The period displayed is from 12 UTC (0800EDT) September 11, 2001 to 12 UTC the next day. The figure illustrates the fine weather; warm temperatures with low relative humidities, wind from the north with an early morning shower left over from the deluge the previous evening. Next, the section on "Soundings, EDAS 80km" was interrogated also using the latitude and longitude of Central Park to produce an atmospheric sounding every three hours between 12Z (0800EDT), September 11, 200land and 12Z the next day. Plate 1 shows the 21Z (17EDT) sounding that illustrates the maximum depth of the ABL. This depth was reached at the time of the maximum surface air temperature of 28C (from Figure 8). The dashed line in Plate 1 represents the parcel of surface air rising and cooling. The parcel rose until it cooled to the environmental temperature denoted by the solid red line. The dotted line is the corresponding cooling of the dew point temperture as the parcel rose. The two lines almost intersect indicating the parcel nearly became saturated. Indeed, on the afternoon of September 11, there were no cumulus clouds observed by the author. The parcel rose to nearly 800 mb or almost 2 km above the surface indicating that the ABL was nearly 2 km deep on the afternoon of September 11. Finally, the READY section on "Stability Time-series, EDAS 80km" was once again interrogated using the latitude and longitude of Central Park and using the period between 12Z (0800EDT), 11 September 2001 and and 12Z the next day. The atmospheric stability time-series and ABL depth that were produced are illustrated in Figure 9. It can be seen in Figure 9 that the earlymorning stable air (Pasquill stability class E) with its shallow A B L was quickly replaced by slightly unstable air (Pasquill stability class C) with a rapidly deepening ABL. After sunset about OOZ (20EDT), the A B L quickly became shallow as the air became stable. The ABL was 1700 m deep according to this analysis, which is consistant with the nearly 2 km deep ABL from the earlier analysis of atmospheric soundings.



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Figure 8. Meteorological conditions (totalprecipitation, wind direction, relative humidity, temperature and pressure) for Central Park, NYCon September 11-12, 2001.



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Plate 1. The environmental soundingfor Central Park, NYC; September 11; 21Z (17EDT) (See page 32 of color inserts.)


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Figure 9.. The Atmospheric Boundary Layer Depth and Stability Parameters (D = neutral, E = slightly stable, F = moderately stable, G = extremely stable) forCentral Park, NYCfor September 11-12, 2001. The depth of the A B L on the afternoon of September 11, 2001 was estimated from studying photographs of the smoke rising from the WTC site (see Chapter 11, Figures 2 and 4). The smoke rose to well above the original height of the Twin-Towers which were about 500 m tall. Some of the plume-rise was due to the intense heat of the fires. Further, the author observed the plume on the afternoon of September 12 as he left Manhatten when the George Washington Bridge was reopened to bicycle traffic. Again, the plume rose to well above the original height of the Towers. Although these estimates are qualitative, they indicate the plume propably rose to the top of the ABL which was estimated to be between 1.7 and 2 km above sea level. It is fortunate that the ABL was so deep on the 11 and 12 to effectively disperse the potentially toxic plume, perhaps, saving more lives. th

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320 The most detailed transport and dispersion estimates from the WTC tragedy may have been provided by Georgopopoulos (6). The extremely high time and space resolved Colorado State University, Regional Atmospheric Modeling System (RAMS) was used to reproduce the WTC plume in this work.


Forecasting vertical transport The method outlined here for estimating the depth of the ABL can also be used to forecast the depth of the ABL and estimate the vertical mixing of particulate matter and other pollutants in the atmosphere. The forecast data are available on the NOAA-ARL-READY web site under "Current meteorology" (5). The procedures used with the forecast data are identical to those used with the archived data.

Conclusions The method presented here to estimate the vertical extent of the plume from the collapse of the Workd Trade Center indicated that the plume could have risen to between 1.7 and 2 km above sea level. The nearly 2 km depth of the layer rapidly diluted the plume and perhaps prevented further loss of life. This method also can be used to forecast the depth of the atmospheric boundary layer.

References 1.

Stull, R. B. Meteorology for Scientists and Engineers(2nd Ed.) Brooks/Cole Publishing Co., St. Paul, MN, 2000, pp. 65-76. 2. Hindman, E. E. and Upadhyay, B. P. Atmos. Environ. 2002,36,727-739. 3. Hindman, E.E. unpublished results. 4. Regmi, R., Kitada, T., Kurata, G. J. Appl. Meteor. 2003,42,389-403. 5. "NOAA ARL Real-time Environmental Applications and Display sYstem Archived Meteorology" [http://www.arl.noaa.gov/ready/amet.htmI] National Oceanic and Atmospheric Administration, Air Resources Laboratory, 7/26/2004. 6. Georgopopoulos, P. G. Paper presented in ACS Symposium: Urban Aerosols and their Impact: Lessons Learned from The World Trade Center Tragedy, New York City, Sept. 10, 2003. ACS Division of Environmental Chemistry Preprints of Extended Abstracts, paper 135,2003,43 (2), 1363.


Urban Aerosols and Their Impacts - American Chemical Society

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