United States National Exposure

Environmental Protection Research Laboratory

Agency Research Triangle Park, NC 27711

EPA Project Summary

Research and Development EPA/600/SR-95/117 August 1995

Air bor ne measur ements near

Champaign, IL and Milwaukee, WI

were made during the summers of

1990 and 1992 to study pollutant transport and transformation by clouds.

Measurements of the aerosol size distributions, wind, turbulence, cloud microphysics parameters and trace gases

were made from 31 research flights.

During the 1990 study SF6 was used as

a tracer to determine cloud transport

and entrainment.

In large clouds air from below the

cloud bases was transported without

dilution through the mid-levels of the

clouds. On the other hand, in smaller

clouds a more uniform dilution was

observed as a result of outside air entrainment. The dilutions in the lower

levels of the small clouds could be

explained by a simple buoyancy sorting model.

An increase in the relative sizes of

aerosol in the accumulation mode was

observed in an area that was likely affected by the venting of cumuli in the

area. Similar increases in size were not

observed in evaporating regions of

stratiform clouds. A hypothesis is proposed to explain the measurements.

The cumuli activate much smaller aerosol which, after aqueous phase reactions and evaporation, have a much

greater relative increase in size than

the larger aerosol activated by stratus

clouds.

The results from the entrainment experiments suggested that, during the

early stages of entrainment, air from

above the rising cloud is carried alongside the upper cloud region by the circulation present there. Later, the air is

mixed into the main portion of the cloud

and rapidly diluted with the cloud interior. The observations are consistent

with the hypothesis that entrainment

occurs through a vortex-like circulation that brings air from above the rising cloud top into the central region of

the cloud.

The eddy correlation method was

used to determine the transfer velocities of gases and aerosols over Lake

Michigan downwind of Chicago. The

results show downward transfer velocities (deposition) of 0.15 and 0.86 cm s-1

for 03 and aerosols in the size range of

0.1 to 3.0 μm in diameter and upward

transfer velocities of 0.04 and 0.54 cm

s

-1 for CO2

 and water vapor about 7.5

km from the shoreline. At mid-lake

much lower transfer velocities were

measured. The turbulence intensity, in

the subrange, was found to decrease

as the air traveled over the cooler water.

This Project Summary was developed

by the National Exposure Research

Laboratory’s Atmospheric Modeling Division, Research Triangle park, NC, to

announce key findings of the research

project that is fully documented in a

separate report of the same title (see

Project Report ordering information at

back).

Introduction

Clouds cover about half of the earth’s

surface and occupy much of the troposphere. During their formation and dissipation, they interact in several ways with

atmospheric pollution. Cumuliform type

Tracer Studies of Transport and

Transformation in Cumuli

J. L. Stith, A. J. Alkezweeny, and D. A. Burrows
2

clouds (Cumuli) are especially effective at

vertically redistributing pollutants. The largest variety, cumulonimbus, transport large

amounts of polluted lower level air into

the upper troposphere and bring cleaner

mid- and upper-level air downward.

Smaller cumuli play a similar role, except

that they influence the vertical distribution

of pollutants in the lower atmosphere. Although the effects from these smaller

clouds are not as dramatic as those from

the larger storms, their much greater number makes them very important in atmospheric chemistry. Both types of clouds

transport pollutants away from the earth’s

surface, a major sink for most pollutants.

Dry deposition is reduced, leading to long

range transport of pollutants thus impacting further away receptors. On the other

hand, precipitating clouds remove aerosols and trace gases from the troposphere.

Clouds also change the chemical and

physical states of pollutants. For instance,

the formation of acidic species by oxidation of SO2

 in clouds is much faster than

in clear air. Clouds modify the size spectra of the atmospheric aerosol, changing

its physical properties (e.g. light scattering, residence time, etc.). Pollutants that

are lifted to higher elevations will experience higher light intensity which may alter

their photochemistry.

Modeling the fate of pollutants on regional and larger scales is a major area of

research for the US EPA. These models

must properly account for the effects of

clouds. For example, RADM, the Regional

Acid Deposition Model, must account for

cloud processes as well as gas-phase

chemistry and deposition.

This report summarized the work that

was done under a cooperative agreement

between the US EPA and the University

of North Dakota (UND). The objectives of

the program were to better understand

cloud processes (entrainment, cloud -

aerosol interactions, etc.), and to collect

measurements in support of the EPA regional modeling, especially the cloud module for the RADM and related models.

During the period of the program the

RADM and related code was extended by

the EPA to address other concerns besides acid deposition (e.g. the fate of atmospheric aerosols). Consequently, the

data from this program also support this

broader effort.

Cloud and Aerosol Interactions

Measurements of in-cloud scavenging

were made using the University of North

Dakota Cessna Citation research aircraft

on June 12, 1992, approximately 60 nautical miles southwest of Green Bay, WI.

The aircraft was instrumented to measure

several cloud physics and standard meteorological parameters. The cloud droplet size distributions were measured using

the Particle Measuring System (PMS) forward-scattering spectrometer probe model

100 (FSSP). The FSSP is capable of measuring droplets in the diameter range of

about 3-50 μm. Another PMSs probe, the

passive cavity aerosol spectrometer probe

model PCASP-100X, was used to measure the aerosol size distributions in the

range from 0.1 to 3.0 μm. The probes

complement each other and cover the

range from 0.1 to 50 μm in a total of 30

channels. A 2DC PMS probe was used to

determine ice crystal concentrations in the

cloud.

The droplet concentration just above the

cloud base is only about half of the aerosol concentration measured in the cloud

inflow. This indicates that only about half

of the aerosols act as cloud condensation

nuclei active at the supersaturation of the

cloud under study. To further investigate

this problem we have examined aerosol

size distributions from all sampled altitudes. The distribution in the cloud inflow

(where the relative humidity was 96%)

shows the normalized concentration decreasing with increasing aerosol diameter.

However, within the cloud the size distributions are relatively flat. When compared

to the inflow region, the in-cloud distributions show a marked increase in the concentrations of aerosols greater than about

0.4 mm. This means that a significant

portion of the aerosols did not grow larger

than 3.0 mm. The cloud droplet size distribution is slightly shifted toward larger droplets at the higher altitudes.

The results show that for cumulus clouds

aerosol sizes are shifted toward slightly

larger sizes after evaporation. On the other

hand, no significant change in the aerosol

sizes was observed after the aerosols were

processed by stratiform clouds. We believe that this result is due primarily to the

differences in cloud supersaturation in

stratiform cloud droplets compared to droplets in cumuli.

Transfer Velocities of Gases

and Aerosols Across the Lake

Michigan Surface

On June 18, 1992 a constant altitude

flight was made at 300 m above Lake

Michigan near the Chicago shoreline and

about 50 km downwind of it. The eddy

correlation method was used to calculate

the fluxes of CO2, O3, water vapor and

aerosols in the diameter range of 0.1 to

3.0 μm. The fluxes near the shoreline

were found to be significantly higher than

those in the middle of the lake. The turbulence intensities, as measured by e1/3, were

2.92±0.75 cm2/3s

-1 and 1.34±0.39 cm2/3s-1

near the shoreline and mid-lake respectively. Fluxes measured near the shoreline were likely to be representative of

those at the surface of the lake, because

of the strong turbulence during the measurement. However, this is not true for the

fluxes measured at mid-lake.

The fluxes near the shoreline for O3

and aerosols were directed toward the

surface and corresponding to transfer

(deposition) velocities of 0.15 cm s-1 and

0.86 cm s-1, respectively. For CO2, and

water vapor, the fluxes were directed upward and corresponding to transfer velocities of 0.04 cm s-1 and 0.54 cm s-1

,

respectively.

A west-to-east constant altitude flight

over the lake, starting from Chicago,

showed that turbulence, as measured by

e

1/3, decayed slowly along the flight track.

The O3 concentration steadily increased

from 39 ppb to about 52 ppb as the air

moved away from the shoreline.

Transport of Air by Convective

Clouds

Large volumes of air from the mixed

layer were lifted by a severe convective

storm with very little mixing with air from

the mid and lower levels of the free troposphere. Thus, these very strong storms

may be best modeled with little or no

entrainment in their lower levels. On a

much smaller scale, the small clouds that

were sampled contained a more uniform

distribution of mixtures of air from below

the cloud with air from near the level of

entrainment. The upper portions of these

clouds seem to be more dilute than their

lower part, but this result may also be a

function of the age of the cloud.

The techniques of buoyancy sorting are

used in recent convective parameterization techniques. This approach was tested

against observations and used to explain

the behavior of a cloud base region that

was tagged with a tracer. The results of

the buoyancy sorting model are encouraging in that they offer a simple explanation for the behavior of the tagged region

and they are able to explain the distribution of conserved parameters fairly well in

the lower levels of the clouds that were

studied, and less well in the upper regions. This may be improved when more

information on the dynamics of the clouds

are included. The results to-date suggest

that a comparison of our results with the

results from a more dynamic model—that

also includes the buoyancy sorting

method—should be conducted.

Entrainment

SF6 tracer was released during single

aircraft passes above the top of growing
3

turrets associated with three different cumuli to study the entrainment of air by the

cloud. The clouds ranged in size from a

vigorous convective turret associated with

a small thunderstorm, to a small, fairweather cumulus.

The results suggest that, during the early

stages of entrainment, the tracer remained

mostly out of the cloud and was carried

outward and down alongside the upper

cloud regions by the circulation present

there. In each experiment, concentrated

SF6 was first found on the edges of the

cloud turrets. Later, the tracer mixed into

the main portion of the turrets and rapidly

diluted. The observations are consistent

with the hypothesis that entrainment occurs through a vortex-like circulation that

brings air from just above the rising cloud

top into the central region of the cloud.

Analysis of buoyancy considerations in

cumuli suggest that much of the entrained

air should remain relatively close to, or

just slightly lower than, the altitude of entrainment. This helps explain the behavior

of the tracer in the above study; a portion

of the entrained tracer should be found

just below the altitude where it was released after being entrained into the cloud.

The tracer was indeed found in these locations (although it may have also been

located in other parts of the cloud that

were not sampled).
4

Jeffrey L. Stith, Abdul J. Alkezweeny, and D.A. Burrows are with the University of

North Dakota, Dept. of Atmospheric Sciences, Grand Forks, ND 58201-9007.

EPA Project Officers, Dr. Jason K.S. Ching and Dr. Jonathan E. Pleim are on

assignment to the National Exposure Research Laboratory (formerly Atmospheric Research and Exposure Assessment Laboratory, see below), from the

National Oceanic and Atmospheric Administration.

The complete report, entitled “Tracer Studies of Transport and Transformation in

Cumuli,” (Order No. PB95-255717; Cost: $19.50, subject to change) will be

available only from:

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

Telephone: 703-487-4650

The EPA Project Officer can be contacted at:

Atmospheric Modeling Division

National Exposure Research Laboratory

U.S. Environmental Protection Agency

Research Triangle Park, NC 27711

• •

United States

Environmental Protection Agency

Technology Transfer and Support Division (CERI)

Cincinnati, OH 45268

EPA/600/SR-95/117

Official Business

Penalty for Private Use $300

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