DOE Atmospheric Science Program

**Project Overview: The Cumulus Humilis Aerosol Processing Study
(CHAPS)**

**Proposed Summer 2007 ASP Field Campaign**

Date of this Version: May 18, 2006

Carl Berkowitz, Larry Berg

( <carl.berkowitz@pnl.gov> ; <larry.berg@pnl.gov> )

Atmospheric Chemistry and Meteorology Group

**Pacific Northwest National Laboratory**

John Ogren

([[john.a.ogren@noaa.gov]{.underline}](mailto:john.a.ogren@noaa.gov) )

Earth System Research Laboratory

**National Oceanic and Atmospheric Administration**

Chris A. Hostetler and Rich Ferrare

(
[[chris.a.hostetler@nasa.gov]{.underline}](mailto:chris.a.hostetler@nasa.gov)
,
[[richard.a.ferrare@nasa.gov]{.underline}](mailto:r.ferrare@larc.nasa.gov)
)

Science Directorate

**NASA Langley Research Center**

[1. Abstract: 2](#abstract)

[1. Rationale of the Campaign: 2](#rationale-of-the-campaign)

[2. Overview of Campaign: 4](#overview-of-campaign)

[3. The Importance of Aerosol Transformation and Transport by Clouds
5](#the-importance-of-aerosol-transformation-and-transport-by-clouds)

[4. Scientific Questions: 6](#scientific-questions)

[5. Approach: Basic Flight Plans and Sampling Strategy
7](#approach-basic-flight-plans-and-sampling-strategy)

[6. Why Oklahoma City? Science, logistics and collaboration
7](#why-oklahoma-city-science-logistics-and-collaboration)

[7. Measurements 7](#measurements)

[7.1. Proposed Measurements from the G-1 aircraft:
7](#proposed-measurements-from-the-g-1-aircraft)

[7.2. Proposed measurements from the NASA King Air;
7](#proposed-measurements-from-the-nasa-king-air)

[7.3. Proposed measurements from the surface sites:
7](#proposed-measurements-from-the-surface-sites)

[8 Data policy 7](#data-policy)

[9. References: 7](#references)

# Abstract:

The primary goal of this campaign is to characterize and contrast
freshly emitted aerosols above, within and below [fields]{.underline} of
cumulus humilis (or fair-weather cumulus, FWC) and to use these
observations to address how below-cloud and above-cloud aerosol optical
and cloud nucleating properties differ downwind of a mid-size city
relative to similar aerosols in air less affected by emissions. The
observations from this campaign will also be used to aid in the
development and evaluation of parameterizations of the transformation
and transport of these aerosols by FWC for use in regional-scale and
Global Climate Models. This final product has the potential to reduce
the uncertainties associated with the treatment of aerosols by these
models. Supporting the *in situ* observations to be made by the DOE
Gulfstream-1 (G‑1) aircraft will be profiles of aerosol backscatter and
extinction in the vicinity of these clouds to be made with a High
Spectral Resolution Lidar (HSRL) onboard the NASA Langley Research
Center's King Air Be-200. Measurements will also be made of the
composition of aerosols inside and outside of the urban plume in order
to characterize both activated and interstitial aerosols. Due to the
transient nature and the small spatial scale of the individual FWC, this
study will look at the aerosol properties in aggregate, rather then data
from individual transects through a cloud. Surface measurements will be
used to continuously characterize aerosol and chemical features within
the boundary layer. Preliminary discussions with the Cloud and Land
Surface Interaction Campaign (CLASIC) science team of the DOE ARM
Program have identified overlap between the science questions posed for
the CLASIC Intensive Operation Period (IOP) and the proposed ASP
campaign suggesting collaboration would benefit both teams.

# Rationale of the Campaign:

As a result of their relatively short lifetime, anthropogenic aerosols
have a high degree of variability over both space and time (see Kaufman
et al., 2002; IPCC,2001; Andreae et al., 1986 and references therein).
Whereas greenhouse gases such as CO~2~ can have a lifetime characterized
by years, the residence time for a typical aerosol is characterized by
days to weeks. Kleinman et al. (2006) have found that aerosols undergo
rapid changes in size, chemical properties and related hygroscopic and
optical properties following their release to the atmosphere. For these
reasons, a contrast in aerosol properties would be expected to be found
between the upwind and downwind regions of an isolated source.
Measurements made downwind of an isolated source of aerosols offer an
opportunity to study the transformations of aerosols as they first
encounter cloud fields as part of their evolution into 'aged' aerosols.

Metropolitan areas with populations of order one-million people are
known to be associated with sub-micron hygroscopic particles produced by
automotive exhaust, cooking and industry (Husar et al., 1997). Field
observations of the composition and the associated hygroscopic and
optical properties of aerosols produced from a 'typical' North American
city are therefore directly relevant to the scientific goals of ASP
since it is thought that aerosols may offset \~ 50% of the warming
attributed to greenhouse gases (see Kiehl and Briegleb, 1993, IPCC, 2001
and references therein). The relative contribution of aerosol
cooling/warming relative to the warming associated with greenhouse gases
is a central focus of the ASP program.

Recognition of the complexity of the composition of urban aerosols is
now making its way into the modeling community, including an increased
appreciation of the significance of carbonaceous material and black
carbon (Novakov et al., 1997; Allen et al., 2003a, 2003b). Adding to the
growing complexity now known to be associated with these aerosols is
recognition that black carbon inside a particle can have a large effect
on the absorptive properties of these particles producing a much more
strongly absorbing particle than if such material is absent (Ackerman et
al., 1981, Martins et al. 1998, Haywood et al. 2000 and Jacobson 2001).
Laboratory studies by Chylek et al. (1992) show that a soot kernel
within water drops of size 0.1 to 1 mm results in enhanced absorption
compared to the situation in which the soot is uniformly distributed
throughout the drop volume. Identifying the extent to which black carbon
is found in aerosols associated with an urban plume, relative to the
extent to which black carbon is found in aerosols associated with
'clean', air will help assess the climatological significance of this
enhanced absorption.

Urban aerosols are known to affect clouds. Wintertime stratiform clouds
associated with the urban plumes of Denver (CO) and Kansas City (MO)
were found to have a larger number of droplets and smaller median volume
diameter than clouds that had not been affected by these urban plumes
(Alkezweeny et al., 1993). Such observations have important implications
on climate and climate change. A reduction in droplet size has also been
postulated to result in brighter clouds with an associated cooling
effect to the atmosphere (Nakajima, et al. 2001). As a result of
observations such as these, understanding mesoscale downwind effects is
requisite to understanding their regional and global effects (Twomey et
al., 1984; Charlson et al., 1992). Another link between urban aerosols
and cloud has been suggested by Rosenfeld (1999) who noted that smaller
cloud droplets have smaller coalescence efficiencies, resulting in
reduced precipitation and longer cloud lifetimes, providing yet another
link to urban aerosols and climate.

Previous studies have noted that aerosol optical properties are not
homogenous in the presence of clouds. Satellite observations have shown
increases in aerosol optical depth (AOD) near clouds (Wetzel and Stowe,
1999; Nakajima et al., 2001) although these results may simply be an
effect of cloud contamination in seemingly cloud-free pixels. This
uncertainty in the behavior of aerosol extinction near clouds and the
uncertainty in cloud cover fraction leads to increased uncertainties in
determining direct radiative forcing (Coakley et al., 2005). In
developing an estimate of the direct effects of anthropogenic aerosols
on solar radiation, Chung et al. (2005), through the use of satellite
observations, ground-based observations and models of aerosol chemistry,
transport and radiative transfer, noted that the major contributor to
the uncertainty in evaluating atmospheric absorption comes from the
uncertainty in the vertical distribution of aerosols and the single
scattering albedo of aerosols.

Airborne measurements have also observed small scale variability in AOD.
During the Chesapeake Lighthouse and Aircraft Measurements for
Satellites (CLAMS) experiment, airborne sun photometer measurements
indicated that AOD typically varied by 25-30% over distances of 50 km
(Redemann et al., 2005). The lack of spectral variability of AOD
indicated little variability in the aerosol size distribution, so it was
concluded that over the east coast of the U.S. during the summer, AOD
variability is caused primarily by the transport and diffusion of
similar aerosol types rather than the mixing of aerosol types of
different size and composition. It is not clear whether these same
results apply to the central U.S. or whether they are unique to the east
coast.

# Overview of Campaign:

The primary goal of this field campaign is to characterize freshly
emitted aerosols above, within and below [fields]{.underline} of cumulus
humilis (fair-weather cumulus, FWC) in the vicinity of a mid-size,
mid-latitude city. Observations will be made through an measurement
program with *in situ* instrumentation onboard the DOE Gulfstream‑1
(G‑1) and profiles of aerosol backscatter and extinction measured using
the NASA High Spectral Resolution Lidar (HSRL) flown on the NASA Langley
King Air Be-200. The King Air will fly above the cloud fields sampled by
the G‑1. Measurements will be made inside and outside of the urban
plume, with the in-cloud observations using a counterflow virtual
impactor system to allow separate sampling of a selected set of
parameters for activated and interstitial aerosols. Surface measurements
will be used to continuously characterize aerosol and chemical features
of the boundary layer.

Aircraft sampling will be conducted within fields of FWC in the vicinity
of Oklahoma City. Area emissions of organic aerosols around Oklahoma
City are on the order of 2 tons day^-1^ km^-2^, and area emissions of
SO~2~ are on the order of 8 X 10^‑4^ tons day^-1^ km^-2^ (based on the
NEI-99 version 3 emissions inventory for emissions between 8 am and 9 am
CST). By sampling close to the source of this isolated urban plume, we
will observe aerosols that will have had few interactions with clouds
since their release or formation. Oklahoma City was selected as the site
for this campaign for several reasons, foremost of which is its
relatively isolated nature, which will make its plume more
distinguishable from regional-scale features. Also, FWC are common in
the Southern Great Plains (occurring on 40% of the days; Warren et al.
1986), and once the clouds form they tend to persist throughout the day
in a fairly steady fashion (Berg et al. 2006). Finally, collaboration
with the ARM CLASIC campaign, also set in Oklahoma, is anticipated to
yield additional benefits to both ASP and ARM investigators (see Section
7, 'Why Oklahoma City? Logistics, science and collaboration.')

While *fields* of FWC are quite extensive and can have a long lifetime,
*individual* FWC are transient, with a lifetime on the order of only
15-30 minutes, and relatively small in size, generally less then 1 km in
diameter (e.g. Berg et al. 2006). These two factors, in conjunction with
the minimum sampling speed of the G-1, will of necessity result in a
statistical characterization of the in-cloud and out-of-cloud properties
within the cloud layer. Assuming that the airspeed of the G-1 is
approximately 100 ms^-1^, the aircraft will spend on the order of 10 s
or less within each FWC. Thus, it is expected that only a subset of the
instrumentation to be deployed will be able to differentiate between
clear and cloudy air. Two sets of fast response instruments (described
in Section 7) will be carried on the G-1 to separately sample both
interstitial aerosol and aerosols within cloud droplets (cloud droplet
nuclei, CDN) as the interstitial aerosols and droplets are separated by
a CVI inlet.

# The Importance of Aerosol Transformation and Transport by Clouds

The transformation and transport of aerosols from the boundary layer to
the free atmosphere by fields of FWC is expected to affect the radiation
budget, and hence be of interest to ASP, for a number of reasons.

1)  The transport by clouds of 0.1 to 1 μm aerosols from within the
    boundary layer to the free atmosphere results in moving optically
    important particles from a region of relatively low wind speeds and
    high net deposition rates to a region of higher wind speeds and
    negligible deposition, greatly enhancing the particle lifetime and
    hence aerosol mass loading in areas downwind of the source region.
    The importance of just the transformation aspect of this process was
    shown by Lu et al. (2003) who reported that particles above the tops
    of marine FWC had a unimodal size distribution (with most particles
    smaller than 0.1 micron) in contrast to particles below these cloud
    fields that displayed a bimodal distribution (with peaks at \~45nm
    and 200nm). They also found that the total dry aerosol number was
    greater above cloud top than within the (marine) boundary layer. The
    importance of just the transport aspect was demonstrated by Cotton
    et al. (1995) who used Large Eddy Simulations (LES) to show that 30%
    of the boundary-layer air passed through a region of FWC in a
    one-hour period.

> In addition, lifting strongly absorbing aerosols from below clouds
> into and above the cloud layer will alter the energy budget by
> effectively reducing the insolation reaching lower levels of the
> atmosphere (Harshvardhan, 1993; Ghan and Penner, 1992). The presence
> of moderately absorbing aerosols above a bright cloud surface rather
> than a darker land surface can also change the top of atmosphere (TOA)
> forcing from cooling to warming.

1)  Aerosol particles activated as cloud condensation nuclei are subject
    to aqueous phase chemistry in cloud drops which can lead to changes
    in the optical properties of the aerosols. An aqueous phase reaction
    might add sulfate to the cloud drop with the result that once the
    cloud drop has evaporated, the scattering efficiency, E~scat~, of
    the aerosol will have increased and the mass absorption efficiency,
    E~abs~, will have decreased because of the additional sulfate. The
    potential importance of these changes stems from the sensitivity of
    radiative forcing to these quantities. Depending on the surface
    reflectance and the backscattering fraction, very small changes in
    the single scattering albedo, ω~0~ = E~scat~/\[ E~scat~ + E~abs~\],
    are believed to be associated with a change in sign (±) of the
    direct forcing (Haywood and Boucher, 2000; Hansen et al., 1997).

2)  Scavenging and activation is a function of aerosol composition with
    the result that some types of aerosols that are more readily
    scavenged (e.g. sulfate) are also more likely to scatter radiation
    while others that more resistant to scavenging (e.g. soot) are more
    likely to absorb radiation. Reductions in aerosol scattering
    efficiency, increases in aerosol absorption and increases in aerosol
    number density above clouds relative to those below clouds will have
    a substantial effect on the back scatter of solar radiation to
    space. It has also been shown that FWCs play an important role in
    determining the chemical composition of air above cloud top (Taylor
    et al., 1997; Edy et al. 1996; Thompson et al., 1994; Vukovich and
    Ching, 1990; Ching and Alkezweeny, 1986; Greenhut, 1986; Ching et
    al., 1984) suggesting that FWC will also play an important role in
    determining the characteristics of particles above cloud top.

3)  The relative humidity is much lower in the free troposphere then in
    the boundary layer so reductions would be expected in the size
    distribution of aerosols associated with hygroscopic swelling and
    possible changes in the composition as they move from below cloud
    base to above cloud top. While this is an apparently obvious fact,
    it is not obvious that models can adequately simulate such changes.

4)  Finally, the mass concentration of aerosol particulate matter is the
    single most important factor in determining the amount of
    particulate scattering and absorption under clear sky conditions.
    Cloud processes will affect both aerosol mass of particles processed
    by the clouds and the total mass concentrations.

# Scientific Questions:

The CHAPS campaign will address the following scientific questions:

[Question 1:]{.underline} How do below-cloud and above-cloud aerosol
optical and cloud nucleating properties differ downwind of a typical
North American city from those properties in air unperturbed by the
urban emissions? What are the differences in the:

> a\. radiative properties (e.g., single scattering albedo, mass
> scattering efficiencies),
>
> b\. the chemical composition,
>
> c\. hygroscopic properties (including capability to serve as CCN) and
>
> d\. size distribution?

One process that might be important is the dissolution of SO~2~ into
aqueous solution. This process is a strong function of the concentration
of H~2~O~2~ and the pH, necessitating the measurement of both SO~2~ and
H~2~O~2~. The addition of SO~2~ to the drops is likely to increase the
aerosol mass when the droplets evaporate upon exiting the cloud (through
the oxidation and the production SO~4~^2-^). The additional aerosol mass
will likely change the optical properties of the aerosols, as well as
change their hygroscopic properties.

Addressing these questions requires identifying air containing aerosols
that have passed through FWC. Aerosols found above cloud top need not
have passed through clouds immediately below; they may have arrived at
cloud top through advection or subsidence from higher in the atmosphere.
These air parcels will be identified through the use of conserved
thermodynamic tracers that do not change their value during adiabatic
lifting and that mix in a nearly linear manner. These tracers have been
employed by many investigators to identify air parcels that have passed
through clouds (e.g., Paluch, 1979; Hanson 1984) and it is proposed that
such tracers be used to identify air that has been lifted through the
FWC. Because the focus of this campaign is on warm non-precipitating
clouds, a number of variables can be used that are conserved for both
dry and moist adiabatic processes, including total water mixing ratio
(defined as the sum of the water vapor, liquid water, and ice in a
parcel; (*q~t~*), equivalent potential temperature and liquid-water
potential temperature (*θ~l~*)[^1].

Plots of two conserved variables are known as a conserved variable
diagram and can be used to identify air that has passed through clouds.
An example of such a diagram, using *θ~l~* and *q~t~* is given in
Figure 1. These distributions were computed from a LES of the BOMEX
field campaign; Figure 1A shows the frequency distribution function
(FDF) of all points 1 km above the surface, while 1B shows the *θ~l~*
and *q~t~* of the cloudy updrafts. Points of the FDF that lie between
the updraft points in 1B and the layer mean indicate mixtures that have
some air that has passed through clouds. Although total water mixing
ratio can be difficult to measure within clouds, accurate measurements
are needed only below and outside the clouds to determine if air has
been processed by FWC.

In addition to thermodynamic variables, measurements of the mixing
ratios of insoluble trace gases having only a surface source can be used
to indicate air parcels that have recently passed through a cloud field.
A number of investigators (Lu et al., 2003; Perry and Hobbs, 1996; Radke
and Hobbs, 1991) have also used regions of high relative humidity around
clouds to identify parcels that have been processed by clouds

![](media/image1.png){width="6.770833333333333in"
height="3.3854166666666665in"}

**Figure 1**. Frequency distribution function of *θ~l~* and *q~t~* with
the range of cloudy updrafts highlighted with the blue oval and likely
mixtures of cloudy and clear air marked with the red oval (A), and
scatter plots of *θ~l~* and *q~t~* for convective updrafts for a LES of
BOMEX (B) (after Wang and Stevens 2000).

[Question 2:]{.underline} How does the distribution of aerosol
extinction vary in relation to proximity to individual clouds and fields
of clouds and why?

Aerosol extinction and optical depth are highly variable in the vicinity
of clouds at least in part because of variable RH and also because of
intrinsic changes in aerosol properties due to interactions with clouds.
This variability can be important within the context of evaluating the
radiative forcing associated with these aerosols and in evaluating the
aerosol properties via remote sensing (Coakley et al., 2005). CHAPS will
use a High Spectral Resolution Lidar (HSRL) carried by the NASA Langley
Research Center's King Air Be-200 to measure profiles of aerosol optical
thickness and aerosol intensive parameters (backscatter color ratio,
extinction/backscatter ratio, and depolarization). These measurements
can be used to investigate the spatial variability of aerosol optical
properties over the southern Great Plains within the geographical domain
to be sampled via *in situ* measurements from the G-1. Furthermore, the
aerosol extensive (extinction, backscatter) and intensive parameters
derived from the HSRL measurements can be used in conjunction with
profiles of RH from radiosondes or the G-1 to infer whether AOD
variability is due to changes in aerosol type (e.g. composition, size),
aerosol amount or levels of atmospheric moisture.

[Question 3:]{.underline} What are the differences between activated
aerosols within the urban plume, and those outside the urban plume? What
are the differences between aerosols that have [not]{.underline} been
activated within and outside the urban plume?

Observations to address this question will follow from the basic flight
plan illustrated in Figure 2. Flight plans for the Be-200 and G-1 will
consist of transects at right angles to the dominant winds that define
the direction of the urban plume. The lengths of these cross-wind
transects will be several times the length scale of Oklahoma City thus
assuring that the transect ends are outside the plume. Instruments
carried by the G-1 will be used to determine which part of the transect
is within and outside the plume, e.g., carbon monoxide, thus allowing
the associated measurements to be partitioned according to whether they
were made within or outside the plume.

[Question 4]{.underline}. To what extent can large-scale models with
state of the art cloud parameterizations capture the statistical
features of the below-above cloud aerosol fields?

One of the ASP program's deliverables is the addition of parameterized
aerosol physics and chemistry to an existing cumulus parameterization
suitable for inclusion in regional-scale models or GCMs. Because current
global and regional-scale models poorly simulate the effects of FWC on
aerosols PNNL is developing a module to better treat this aspect of the
transformation and transport of aerosols. This code builds on the
Cumulus Potential Scheme of Berg and Stull (2004; 2005) consisting of
one module representing boundary-layer physics and another module
representing the macrophysical properties of the clouds (such as mixing
between the cloud and environment, total cloud water, cloud-base height
and cloud-top). New approaches in climate modeling like the Multi-Scale
Modeling Framework, in which two-dimensional cloud-resolving models are
each run inside GCM grid box (Randall et al. 2003) still do not
explicitly resolve FWC. In most other applications the vertical
transport associated with the clouds is represented using
parameterizations that were designed for deep convection. Even models
which do include a parameterization for FWC, like the NCAR Community
Atmosphere Model (CAM3), ignore the sub-grid variability of temperature
and moisture in the model grid cell with the result that little can be
said about the evolution of aerosol properties within these simulated
domains (Hack 1994). The observations of this campaign are tailored to
evaluating this modeling product

The vertical distribution of aerosols remains one of the largest source
of disagreement among models, as shown in several global model
intercomparison activities including the Comparison of Large Scale
Sulphate Aerosol Models (COSAM; Barrie et al., 2001; Lohmann et al.,
2001), Intergovernmental Panel on Climate Change (IPCC, 2001) and the
Global Aerosol Model Intercomparison (AEROCOM) in 2004 (Textor et al.,
2005; Kinne et al., 2005. Some of the large variations between the
models noted by Textor included differences in aerosol composition,
aerosol lifetime and water content due to hygroscopic growth in ambient
air. Although models have shown reasonably good agreement among
themselves and with measurements in terms of aerosol optical thickness
(AOT), the AEROCOM model intercomparisons have shown that there are
large differences in how the various models partition aerosol mass and
optical depth among these various components (Kinne et al., 2005). As a
result it was found that the models differed in their description of
aerosol vertical distribution and optical depth Observations made at low
altitudes by the G-1 in conjunction with the observations from the HSRL
Be-200 aircraft will provide a rich data set for future modeling studies
of the vertical distribution of aerosols.

ASP scientists at NASA have begun investigating the extent to which the
intensive aerosol parameters (backscatter and extinction color ratios,
depolarization) derived from lidar measurements can be used to infer the
vertical distribution of these features and thereby help evaluate models
(Ferrare et al., 2006). As part of the 2007 ASP campaign, it is proposed
to extend these investigations by using the vertical profiles of aerosol
extinction and aerosol intensive parameters from the HSRL to help
evaluate the ability of models to reproduce aerosol extinction and
optical thickness profiles as well as to help determine how well models
can represent horizontal and vertical variations in aerosol types. Lidar
measurements made using the HRSL, accompanied with the *in situ*
measurements of aerosol size distribution and relative humidity would be
of interest to the ASP work being led by Drs. Steven Ghan and Jerome
Fast. These modeling studies include, among other objectives, assessing
how well global- and regional-scale models simulate a variety of
cloud/aerosol interactions, and therefore depend on observations of the
type proposed for CHAPS.

# Approach: Basic Flight Plans and Sampling Strategy

The proposed campaign will consist of an integrated sampling program
involving both the G-1 and Be-200 aircraft. The G-1 will make *in situ*
measurements of aerosol concentrations, composition, size distributions,
optical properties, cloud nucleating properties below, within and above
clouds downwind of Oklahoma City while the Be-200 will use the HSRL to
provide remotely observed profiles of aerosol extinction and
backscatter. In addition, there will be a surface station to provide
observations of boundary layer aerosols and meteorological conditions.

The *in situ* aircraft sampling will consist of cross-wind legs made
below, within and above fields of FWC with the spatial scale of the
sampling intended to provide statistics against which to test the
fidelity of parameterizations used in large-scale models describing
aerosol transport over typical GCM grid cells (Δx \~ 100km). By
centering the campaign in the near-field downwind region of Oklahoma
City the G-1 can be used to make cross-wind transects that intersect the
urban plume. Such a strategy, used by ASP investigators on a number of
past campaigns (e.g., Nashville and Houston) will enable *in situ*
sampling of air that is influenced by both local emissions and air that
is relatively clean. A schematic of the general sampling strategy is
shown in the figure below.

**Figure 2:** Schematic showing generic flight plan for sampling fields
of cumulus within the plume emanating from Oklahoma City. Arrows
identify cross-wind transects to be made below, within and above the
cloud fields. See text (below).

The wind direction is shown in this figure by the large arrow on the
right with fields of FWC extending over the region. Solid vertical lines
represent cross-wind transects to be made below cloud base,
dashed-dotted lines represent transects made within the clouds and
dashed lines represent transects flown above cloud top (these three sets
of lines have been displaced slightly for purposes of clarity).
Characterization of background fields will be made via short cross-wind
transects upwind of Oklahoma City and by measurements made on the tails
of the downwind transects. The majority of flight time would be spent
making a series of cross-wind transects below- , within and above-cloud
directly downwind of Oklahoma City but still relatively close (\<50 km)
to its edge. Identification of air that had passed over the city would
be made via onboard measurements of compounds associated with urban
emissions, e.g., CO. The number of transects will be determined by the
availability of G-1 flight hours (both total and per mission), with a
balance to be struck between the number of in-cloud transects and the
total number of sets of downwind transects.

While the G-1 is making its surveys upwind and downwind of Oklahoma
City, the NASA Be-200 will fly directly over the G-1 at an altitude of
25-28 kft, acquiring lidar profile data to provide vertical context on
aerosol and cloud structure above and below G-1. The G‑1, in turn will
provide *in situ* measurements of moisture and aerosol microphysics to
support the interpretation of the airborne HSRL. The NASA HSRL will also
provide guidance for the day to day variations in the G-1 flight plan,
directing the G-1 to different altitudes based on real-time lidar
profiles of aerosol and cloud vertical distribution. Spirals by the G-1
will be added over ground-sites to be deployed in conjunction with the
CLASIC IOP (see Section 5). These spirals will provide profiles of the
aerosols and aerosol optical properties, as well as profile of the
thermodynamic variables.

It is expected that the study area will, on occasion, have low stratus
or stratocumulus, and that sampling within these cloud fields would
serve the interests of the ASP program. Aircraft sampling will be made
under these conditions as allowed by Federal Aviation Regulations, and
agency policies governing the deployment of the DOE and NASA aircraft.

# Why Oklahoma City? Science, logistics and collaboration 

*Science:*

Oklahoma City is relatively isolated from other urban areas which will
simplify distinguishing between 'new' aerosols within the Oklahoma City
plume and 'old' aerosols outside the plume, where the plume boundaries
will be defined from ancillary observations of carbon monoxide. It is
anticipated that close to Oklahoma City new aerosols will dominate over
'old' aerosols and that the measurements will show a contrast between
the new and old, the latter dominating outside the urban plume. In
addition, the terrain in this region is relatively uniform which
minimizes the likelihood that local dynamic forcing by terrain will
cause a systematic bias in the formation of clouds.

*Logistics*

The aircraft will be within heavily controlled radar air space which is
an added safety feature for any airborne campaign.

*Collaboration: CLASIC*

Another advantage of a summer 2007 campaign in Oklahoma is the potential
for collaboration with the ARM Cloud and Land Surface Interaction
Campaign (CLASIC) Intensive Operation Period (IOP). The focus of CLASIC
is to advance the understanding of cumulus convection and its controls,
particularly those associated with land surface processes. The CLASIC
IOP will cover a period of 1-3 months and will straddle the winter wheat
harvest when large changes in the land surface lead to large changes in
the surface albedo, latent heat flux, and sensible heat flux. A central
part of this campaign will be the Center for Inter-Disciplinary Remotely
Piloted Aircraft Studies (CIRPAS) Twin Otter and the NASA ER2. The Twin
Otter will make *in situ* observations of cloud and aerosol
microphysics, while the ER2 will make high-altitude over flights with a
suite of remote sensing instruments. CLASIC may bring two mobile SMART
C-Band radars which will be deployed at various locations within the ARM
SGP site. Discussions between ASP and CLASIC scientists have identified
a number of areas of mutual interest, including:

-   [Cloud microphysical measurements.]{.underline} The ASP team's
    interest in microphysics stems from wanting to better understand
    in-cloud transformations of aerosols. The CLASIC team's interest in
    microphysics stems from wanting to better understand changes to the
    cloud properties associated with varying land use and boundary layer
    properties.

-   [Aerosol radiative properties.]{.underline} The Twin Otter and G-1
    will be equipped with identical instruments for measuring aerosol
    radiative properties, specifically, aerosol light scattering,
    hemispheric backscattering, and absorption at three visible
    wavelengths. Combining the data sets from the two aircraft, along
    with identical measurements from the ARM Cessna 206 over the SGP
    Central Facility, will provide a unique evaluation of the spatial
    variability of aerosol radiative properties.

-   [Location]{.underline}. The ASP team is proposing a campaign in the
    area in and around Oklahoma City because this area provides a
    location to sample within (and outside) of an urban plume coming
    from a relatively isolated medium size city in the presence of FWC.
    The CLASIC team has proposed a campaign in Oklahoma as a result of
    their interest in abrupt change in surface characteristics (e.g.,
    change in land use associated with the harvest of winter wheat) on
    fluxes and cloud microphysics.

-   [Season]{.underline}: The ASP team is proposing a campaign during
    the period of time most closely associated with FWC over central
    North America. This coincides with the time selected by the CLASIC
    team based on considerations of winter wheat harvest and associated
    changes in land surface processes.

There appear to be at least three collaborative efforts that would
benefit both the ASP and CLASIC campaigns. First, both programs will
benefit by deploying aircraft on the same days. Having both the G-1 and
the CIRPAS Twin Otter make early morning observations in the same area
to the north of Oklahoma City, followed by having the G-1 continue to
make observations in this area throughout the late morning/early
afternoon as the Twin Otter extends its observations to the north and
south, will provide information on local change to cloud microphysical
and aerosol properties (from the combined early morning observations
from both aircraft) and regional-scale changes (as the Twin Otter
samples to the north and south). There have also been discussions about
having the G-1 and Twin Otter fly in a stacked pattern, providing
simultaneous observations aerosol properties below and above a field of
clouds. This would directly benefit ASP's interest to compare and
contrast aerosols in these two domains.

ASP and ARM would like to deploy ground based aerosol instrumentation at
the ARM CLASIC super site within the Little Washita Watershed
[and]{.underline} a ground site with both aerosol and meteorological
measurements closer to Oklahoma City. If this can be done then a natural
addition to the 'basic aircraft strategy' described earlier would be to
add profiles over the surface sites or to have concurrent flyovers by
the aircraft above the ground sites. Such coordinated efforts would
serve the scientific interests of both programs as it would provide
additional information on the vertical structure of aerosol
concentration, size spectra, optical properties and composition within
columns of air extending from the surface to the highest altitude
sampled by an aircraft; such observations would serve a number of
scientific questions.

![](media/image2.png){width="5.990972222222222in"
height="4.4534722222222225in"}

**Figure 3:** Map illustrating the geographic relation of key areas for
the CHAPS campaign and the ARM CLASIC IOP. The CLASIC IOP plan calls for
aircraft transects between the Little Washita Watershed (northeast of
Lawton, OK) and the SGP/Central Facility, both indicated by the large
circles. The ASP plan calls for repeated flights in the vicinity of
Oklahoma City.

A third area of common interest is the planetary boundary layer (PBL)
depth. The PBL contains the roots of the FWC of interest to the ASP
investigators and is the area of active mixing of interest to the CLASIC
scientists. ASP scientists from NASA/Langley will provide measurements
of PBL height using the aerosol backscatter and extinction profiles
measured by the HSRL.

There are differences in the tentative logistical plans for both
campaigns (Figure 3). The G‑1 flights will be centered near Oklahoma
City, with most of the flights being made less than 50km downwind of the
city (where 'downwind' would be determined by that days' synoptic
pattern). In contrast, the Twin Otter deployments for the CLASIC IOP
will focus on a north-south transect between the SGP Central Facility
and the Little Washita Watershed, which is located about 100 km south of
Oklahoma City. There is not perfect overlap in the sampling domains of
the two campaigns, but the overlap is significant enough that
observations from one aircraft should be able complement those from the
other.

# Measurements

## Proposed Measurements from the G-1 aircraft: 

The priority for instrument deployment, given limitations on space and
power on the G-1, is to focus on the optical and chemical properties of
aerosols. The core package consists of two identical sets of instruments
to measure aerosol scattering and absorption with one set of
measurements to be made from a counter-flow virtual impactor (CVI) inlet
and another set of measurements to be made from an inlet that allows
passage of interstitial aerosols. The CVI system takes advantage of the
higher inertia of cloud drops to draw them through a slight counter-flow
into the measurement system while smaller particles are unable to
overcome the counter-flow. Downstream of the CVI the cloud drops will be
evaporated and the resulting cloud droplet nuclei (CDN) fed to aerosol
instrumentation to characterize their optical, chemical and physical
(size, number and shape) and cloud activation properties.

The interstitial inlet will also feed an airstream to an Aerodyne
Aerosol Mass Spectrometer which will let us characterize the composition
of these aerosols. The airstream from the interstitial inlet will also
be fed into both a fast tandem DMA system and a PILS system. The size
distribution of cloud drops will come from a DMT CAPS probe (cloud
droplet size between .3 and 50 microns, precipitation droplet sizing
between 25 -- 1550 microns). The G-1 PCASP system will also be employed
to measure particles between 0.1 and 3 microns at ambient RH. State
parameters (temperature, pressure, moisture) are part of the standard
suite of G‑1 measurements. A 5-port gust probe built into the G-1 will
provide vertical wind speeds and turbulence measurements. Gas phase
measurements will consist of CO (Vacuum UV), O~3~ (the NOAA B2B system),
SO~2~ and H~2~O~2~. Onboard radiometric measurements will include UV
radiation (up- and downwelling), short-wave irradiance (up and down),
long-wave irradiance (up and down) and short-wave spectral irradiance
(up and down).

The horizontal cross sections of individual FWC are relatively small, on
the order of 1 km. Given the G-1 travels at approximately 100 ms^-1^,
this gives a residence time of 10 s or less within the clouds.
Measurements of the aerosols optical properties both inside and outside
of the clouds will be possible because of the fast response times of
these instruments. Other measurements, such as those of the AMS and PILS
will not be able to distinguish between the clear and cloudy air. But
the majority of each flight leg will be in clear air, so that it should
be possible to measure the chemical properties of the aerosols in the
cloud layer.

**Table 1**: Duplicate instruments sampling from both the CVI and
non-CVI inlet.

  ----------------- ------------ ------------------------- ------------------
  **Parameter**     **Response   **Proposed Method**       **Detection Limit
                    Time (s)**                             or Measurement
                                                           Range**

  Light scattering  1            3-wavelength              0.1 Mm^-1^
  coefficient                    nephelometers             

  Aerosol           4            3-wavelength PSAP         0.2 Mm^-1^
  absorption                                               

  Cloud             1            Droplet Measurement       Supersaturation
  condensation                   Tecnology (DMT) Cloud     0.07-3%
  nuclei at fixed                Condensation Nuclei       
  supersaturation                Counter (CCN)             
  (2 per inlet, 4                                          
  total)                                                   

  Aerosol size      1            Fast Integrated Mobility  15-1000 nm
  distribution                   Spectrometer              

  Aerosol           30           Time-Resolved Aerosol     0.2 - 7μm
  composition                    Collector (TRAC)          

  Water Vapor       1            TDL H20                   \~0-10 g/kg

  Particle Number   1            TSI Condensation Particle \>3 nm
                                 Counter (CNC) x 2         
  ----------------- ------------ ------------------------- ------------------

**Table 2**: Unique instruments sampling from the non-CVI inlet.

  ---------------- ------------ ------------------------- ------------------
  **Parameter**    **Response   **Proposed                **Detection Limit
                   Time (s)**   Method/contributor**      or Measurement
                                                          Range**

  Hygroscopicity   120          Hygroscopicity analyzer   
                                (H-FIMS)                  

  Aerosol          30           Time-Resolved Aerosol     0.2 - 7μm
  composition                   Collector (TRAC)          

  Submicron        30           SMPS                      0.1-3 3μm; 20 --
  aerosol size                                            10^7^
                                                          particles/cm^3^

  Size-resolved EC 1            Droplet Measurement       \~5000
  mass                          Technology Single         particles/s.
                                Particle Soot Photometer  
                                (SP2)/                    

  Submicron bulk   30           Aerosol mass spectrometry variable
  OC, SO~4~,                                              
  NO~3~, Cl, NH~4~                                        
  g m(^-3^)                                               

  Particle ionic   180          Particle-in-liquid system 0.1 μg/m^3^
  composition                                             

  Carbon Monoxide  1            VacuumUV                  5ppbv
  (CO)                                                    

  Sulfur Dioxide   2            UV Pulsed Fluorescence    200-300pptv
  (SO~2~)                                                 

  Ozone (O~3~)     10           B2B Ozone Analyzer        0-1000 ppb

  Hydrogen         60           3-Channel Peroxide System 
  Peroxide                                                
  (H~2~O~2~)                                              
  ---------------- ------------ ------------------------- ------------------

**Table 3:** Aerosol and cloud droplet sizing instruments mounted on the
noise of the G-1

  ---------------- ------------ ------------------------- ------------------
  **Parameter**    **Response   **Proposed Method**       **Detection Limit
                   Time (s)**                             or Measurement
                                                          Range**

  Aerosol size     1            PCASP                     0.17 - 3μm
  distribution                                            

  Aerosol and                   DMT Cloud aerosol and     0.5-50 μm &
  Droplet Size                  precipitation             25-1550 μm
  Distribution                  spectrometer              
  ---------------- ------------ ------------------------- ------------------

**Table 4**: Standard meteorological measurements.

+---------------+---------+------------------------+------------------+
| **Parameter** | **R     | **Proposed             | **Detection      |
|               | esponse | Method/contributor**   | Limit or         |
|               | Time    |                        | Measurement      |
|               | (s)**   |                        | Range**          |
+---------------+---------+------------------------+------------------+
| Turbulent     | 0.1     | Gust Probe             | \<0.5 m s^-1^    |
| Wind          |         |                        |                  |
| Components    |         |                        |                  |
|               |         |                        |                  |
| (u-,v-,w-)    |         |                        |                  |
+---------------+---------+------------------------+------------------+
| Water Vapor   | 0.1     | Lyman Alpha Absorption | ±0.1 g m^-3^     |
| (H~2~O)       |         |                        | (est.)           |
+---------------+---------+------------------------+------------------+
| Liquid Water  | 1       | Gerber PVM-100A Probe  | 0 to 10 g/m3, 2  |
| Concentration |         | (optical light         | to 70 microns    |
| (LWC)         |         | scattering)            |                  |
+---------------+---------+------------------------+------------------+
| Air           | 0.1     | Platinum Resistance    | ±0.5°C           |
| Temperature   |         |                        |                  |
+---------------+---------+------------------------+------------------+
| Dewpoint      | 1       | Chilled Mirror         | Dew: ± 0.2°C     |
+---------------+---------+------------------------+------------------+
| Altitude      | 1       | Barometric             | \<1 mb           |
+---------------+---------+------------------------+------------------+
| Position      | 1       | GPS                    | \<3m             |
+---------------+---------+------------------------+------------------+
| \*UV          | 1       | Eppley radiometer      | 295-385 nm       |
| radiation,    |         |                        |                  |
| up- and       |         |                        |                  |
| downwelling   |         |                        |                  |
+---------------+---------+------------------------+------------------+
| \*Short-wave  | 1       | Eppley PSP             | 285-2800 nm      |
| irradiance,   |         | Pyranometer/PNNL       |                  |
| up- and       |         |                        |                  |
| downwelling   |         |                        |                  |
+---------------+---------+------------------------+------------------+
| \*Long-wave   | 1       | Eppley PIR             | 4-50 nm          |
| irradiance,   |         | Pyrgeometer/PNNL       |                  |
| up- and       |         |                        |                  |
| downwelling   |         |                        |                  |
+---------------+---------+------------------------+------------------+
| \*Short-wave  | 1       | Yankee MFR             | 415, 500, 615,   |
| spectral      |         | radiometer/PNNL        | 665, 862 and 940 |
| irradiance,   |         |                        | nm; 10 nm FWHM   |
| up- and       |         |                        |                  |
| downwelling   |         |                        |                  |
+---------------+---------+------------------------+------------------+
| \*Surface/Sky | 1       | PRT 5 IR               | 9.5-11.5 micron, |
| IR            |         | radiometry/PNNL        | -20 to 50C       |
| temperature   |         |                        |                  |
+---------------+---------+------------------------+------------------+

\* These airborne radiation measurements are being considered in the
event that 3-dimensaional radiative transfer modeling studies will be
part of the analysis, although no such studies are planned at this time.

## Proposed measurements from the NASA King Air;

Measurements to be made from the Langley Research Center King Air Be‑200
are shown in Table 4, below:

**Table 5:** Measurements to be made from the NASA King Air Be-200.

  ---------------------------------- ------------------- -----------------
  Parameter                          Horizontal          Vertical
                                     Resolution (m)      Resolution (m)

  532 nm backscatter                 50                  30

  532 nm aerosol depolarization      50                  30

  532 nm extinction                  1500                300

  1064 nm backscatter                50                  30

  1064 nm aerosol depolarization     50                  30
  ---------------------------------- ------------------- -----------------

A very useful meteorological measurement to be made by the HSRL is
height of the planetary boundary layer The PBL contains the roots of the
FWC of interest to the ASP investigators and the mixing of interest to
the CLASIC scientists. The aerosol backscatter and extinction profiles
measured by the HSRL can be used to measure PBL height. Lidar systems
have been widely used to examine the structure and variability of the
PBL top and to derive the entrainment zone depth (e.g. Cohn and
Angevine, 2000; Brooks, 2003). The long duration (\~3.5 hour, 1200 km
flight leg) of the NASA Langley King Air is well suited for extensive
mapping of the PBL using the lidar technique. Also, since this aircraft
will be flying at high (\~8.5 km) altitude exclusively, with little or
no changes in altitude, the lidar measurements can provide long,
uninterrupted measurements of the PBL and entrainment zone, and so will
not be interrupted due to changes in altitude required for in situ
sampling of aerosols.

## Proposed measurements from the surface sites: 

The possibility of adding an aerosol measurement station to the CLASIC
science team super site in the Little Washita Watershed is under
consideration. In addition, the ASP/CHAPS team is looking into
establishing a second surface site in the vicinity of Oklahoma City. The
purpose of both sites would be to provide continuous aerosol and
meteorological observations within the boundary layer in which the roots
of the FWC are located.

At a minimum, instruments similar to those put in the field at surface
stations deployed during past campaigns (e.g., at Pt. Reyes during the
2005 MASE campaign) would be deployed. These have included aerosol
microphysics and optics (nephelometer, PSAP), aerosol size distribution
(PCASP), and particle sampling for off-line electron microscope analysis
(Table 5). Specific measurements to be made at the surface site(s) will
be determined by the available resources for the addition of these
surface sites.

**Table 6:** Measurements to be made at the surface site(s).

  ------------------- ------------ ---------------------- ---------------------
  **Parameter**       **Response   **Proposed             **Detection Limit or
                      Time (s)**   Method/contributor**   Measurement Range**

  Light scattering    1            3-wavelength           0.1 Mm^-1^
  coefficient                      nephelometers          

  Aerosol absorption  4            3-wavelength PSAP      0.2 Mm^-1^

  Particle number     1            TSI Model 3010         10 nm -- 3μm
  concentration                    Condensation Particle  particles; 0-50,000
                                   Counter                particles/cm^3^

  Aerosol size        1            PCASP                  (0.17 - 3μm)
  distribution                                            

  Aerosol composition 30           Time-Resolved Aerosol  0.2 - 7μm
                                   Collector (TRAC)       

  Submicron bulk OC,  30           PNNL: Aerosol mass     variable
  SO~4~, NO~3~, Cl,                spectrometry           
  NH~4~                                                   

  Profiles of                      Visialia Digi-Cora     
  temperature and                  radiosonde system      
  water vapor                                             

  Profiles of wind    1800         Visialia radar wind    
  speed and direction              profiler               
  ------------------- ------------ ---------------------- ---------------------

# 8 Data policy {#data-policy .unnumbered}

All participants in this campaign will be expected to follow the ASP
Data Policy as given in <http://www.asp.bnl.gov/datapolicy050505.html>
('ASP Data Policy, Draft 2, 2005-05-05). The goal of this policy, and an
important part of CHAPS, is to widen the audience of potential end-users
and to foster collaborations among campaign participants and with
outside users.

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[^1]: Defined at
    <http://amsglossary.allenpress.com/glossary/browse?s=l&p=26>