ARM TR-014

> Aerosol Observing System (AOS) Handbook
>
> November 2005
>
> A. Jefferson
>
> Work supported by the U.S. Department of Energy,
>
> Office of Science, Office of Biological and Environmental Research

# Contents {#contents .Heading-Front}

[1. General Overview 1](#general-overview)

[2. Contacts 1](#contacts)

[3. Deployment Locations and History
2](#deployment-locations-and-history)

[4. Near-Real-Time Data Plots 2](#near-real-time-data-plots)

[5. Data Description and Examples 2](#data-description-and-examples)

[6. Data Quality 9](#data-quality)

[7. Instrument Details 9](#instrument-details)

# Figures {#figures .Heading-Front}

[1. Aerosol Absorption and Scattering Coefficients at Three Wavelengths.
7](#__RefHeading___Toc94672499)

[2. Single-Scattering Albedo Before and After Rain Episode.
8](#__RefHeading___Toc94672500)

[3. Seasonal and Annual Aerosol Submicron Scattering and Absorption
Coefficient Averages. 8](#__RefHeading___Toc94672501)

# Tables {#tables .Heading-Front}

[1. Primary Variables: Quantities Found 2](#__RefHeading___Toc119297923)

[2. Instrument Noise, Drift, and Uncertainty Factors.
4](#__RefHeading___Toc119297924)

[3. Diagnostic Variables in the AOS Data Stream.
5](#__RefHeading___Toc119297925)

# General Overview

The aerosol observing system (AOS) is the primary Atmospheric Radiation
Measurement (ARM) platform for in situ aerosol measurements at the
surface. The principal measurements are those of the aerosol absorption
and scattering coefficients as a function of the particle size and
radiation wavelength. Additional measurements include those of the
particle number concentration, size distribution, hygroscopic growth,
and inorganic chemical composition. The AOS measures aerosol optical
properties to understand how particles interact with solar radiation and
influence the earth\'s radiation balance. The measurements are useful
for calculating parameters used in radiative forcing calculations such
as the aerosol single-scattering albedo, asymmetry parameter, mass
scattering efficiency, and hygroscopic growth. The system is located at
the Southern Great Plains (SGP) site in Oklahoma and has been
operational since the beginning of April 1996. Since 1997, the [Aerosol
Group](http://www.cmdl.noaa.gov/aero/index.html) at the National Oceanic
and Atmospheric Administration's (NOAA)/Climate Monitoring and
Diagnbostics Laboratory (CMDL), now the Global Monitoring Division
(GMD), has had mentorship of the AOS.

In March 2000, a long-term [vertical profiling
campaign](http://www.cmdl.noaa.gov/aero/net/iap/index.html) began over
the SGP site using a light aircraft. The measurements from the vertical
profiles complement the surface measurements because the instrumentation
on the airplane is similar to that at the SGP surface site. The aircraft
measurements provide information on the aerosol vertical mixing. The AOS
system will undergo some major upgrades in 2006 with the addition of two
Radiance nephelometers. The sampling inlet was changed in 2005 to allow
for sampling of large particles, \< 5 *u*m.

# Contacts

## Mentor

John Ogren

NOAA/GMD

R/GMD -1

325 Broadway

Boulder, CO 80305

Phone: 303-497-6210

<john.a.ogren@noaa.gov>

Anne Jefferson

NOAA/GMD

Phone: 303-497-6493

<Anne.jefferson@noaa.gov>

Richard Eagan (data acquisition)

Argonne National Laboratory

Phone: 630-252-3435

<dick.eagan@anl.gov>

## Instrument Developer

NOAA/GMD Aerosol Group

325 Broadway

Boulder, CO 80305

<http://www.cmdl.noaa.gov/aero/>

# Deployment Locations and History 

See inst_log_gaw.xls.

# Near-Real-Time Data Plots

Daily plots of the unedited raw data can be found at
<http://www.cmdl.noaa.gov/aero/net/sgp/qcplots.html>.

# Data Description and Examples

## Data File Contents

### Primary Variables and Expected Uncertainty

Table 1 describes the primary variables of the AOS.

  --------------------------------- ---------------------------------------- ----------
  []{#__RefHeading___Toc119297923                                            
  .anchor}**Table 1.** Primary                                               
  Variables: Quantities Found.                                               

  **Variable Name**                 **Quantity Measured**                    **Unit**

  Ozone                             Ozone concentration                      ppm

  CPCPartConc                       CPC Particle concentration               1/cm\^3

  BapBlu_I_1μm                      1 μm absorption coefficient at 400 nm    1/m

  BapBlu_I_10μm                     10 μm absorption coefficient at 400 nm   1/m

  BapGrn_I_1μm                      1 μm absorption coefficient at 500 nm    1/m

  BapGrn_I_10μm                     10 μm absorption coefficient at 500 nm   1/m

  BapRed_I_1μm                      1 μm absorption coefficient at 700 nm    1/m

  BapRed_I_10μm                     10 μm absorption coefficient at 700 nm   1/m

  BluTScatCoef_1μm \_LRH            Low RH Neph. 450 nm total scat. coef. at 1/m
                                    1 μm                                     

  BluTScatCoef_10μm \_LRH           Low RH Neph. 450 nm total scat coef. at  1/m
                                    10 μm                                    

  GrnTScatCoef_1μm \_LRH            Low RH Neph. 550 nm total scat. coef. at 1/m
                                    1 μm                                     

  GrnTScatCoef_10μm \_LRH           Low RH Neph. 550 nm total scat. coef. at 1/m
                                    10 μm                                    

  RedTScatCoef_1μm \_LRH            Low RH Neph. 700 nm total scat. coef. at 1/m
                                    1 μm                                     

  RedTScatCoef_10μm \_LRH           Low RH Neph. 700 nm total scat. coef. at 1/m
                                    10 μm                                    

  BluBScatCoef_1μm \_LRH            Low RH Neph. 450 nm total backscat.      1/m
                                    coef. at 1 μm                            

  BluBScatCoef_10μm \_LRH           Low RH Neph. 450 nm total backscat.      1/m
                                    coef. at 10μm                            

  GrnBScatCoef_1μm \_LRH            Low RH Neph. 550 nm total backscat.      1/m
                                    coef. at 1 μm                            

  GrnBScatCoef_10μm \_LRH           Low RH Neph. 550 nm total backscat.      1/m
                                    coef. at 10 μm                           
  --------------------------------- ---------------------------------------- ----------

  ----------------------- ---------------------------------------- ----------
  **Table 1**. (contd)                                             

  **Variable Name**       **Quantity Measured**                    **Unit**

  RedBScatCoef_1μm \_LRH  Low RH Neph. 700 nm total backscat.      1/m
                          coef. at 1 μm                            

  RedBScatCoef_10μm \_LRH Low RH Neph. 700 nm total backscat.      1/m
                          coef. at 10 μm                           

  BluTScatCoef_1μm \_HRH  High RH Neph. 450 nm total scat. coef.   1/m
                          at 1 μm                                  

  BluTScatCoef_10μm \_HRH High RH Neph.450 nm total scat coef. at  1/m
                          10 μm                                    

  GrnTScatCoef_1μm \_HRH  High RH Neph. 550 nm total scat. coef.   1/m
                          at 1 μm                                  

  GrnTScatCoef_10μm \_HRH High RH Neph. 550 nm total scat. coef.   1/m
                          at 10 μm                                 

  RedTScatCoef_1μm \_HRH  High RH Neph. 700 nm total scat. coef.   1/m
                          at 1 μm                                  

  RedTScatCoef_10μm \_HH  High RH Neph. 700 nm total scat. coef.   1/m
                          at 10 μm                                 

  BluBScatCoef_1μm \_HRH  High RH Neph. 450 nm total backscat.     1/m
                          coef. at 1 μm                            

  BluBScatCoef_10μm \_HRH High RH Neph. 450 nm total backscat.     1/m
                          coef. at 10 μm                           

  GrnBScatCoef_1μm \_HRH  High RH Neph. 550 nm total backscat.     1/m
                          coef. at 1 μm                            

  GrnBScatCoef_10μm \_HRH High RH Neph. 550 nm total backscat.     1/m
                          coef. at 10 μm                           

  RedBScatCoef_1μm \_HRH  High RH Neph. 700 nm total backscat.     1/m
                          coef. at 1 μm                            

  RedBScatCoef_10μm \_HRH High RH Neph. 700 nm total backscat.     1/m
                          coef. at 10 μm                           

  Pa1000Conc              PCASP Chan. 0 (Dp \> 10 micrometers)     1/cm\^3

  pa10_p12conc            PCASP Chan. 1 (0.10 micrometers \< Dp \< 1/cm\^3
                          0.12)                                    

  pa12_p14conc            PCASP Chan. 2 (0.12 micrometers \< Dp \< 1/cm\^3
                          0.14)                                    

  pa14_p16conc            PCASP Chan. 3 (0.14 micrometers \< Dp \< 1/cm\^3
                          0.16)                                    

  pa16_p18conc            PCASP Chan. 4 (0.16 micrometers \< Dp \< 1/cm\^3
                          0.18)                                    

  pa18_p20conc            PCASP Chan. 5 (0.18 micrometers \< Dp \< 1/cm\^3
                          0.20)                                    

  pa20_p23conc            PCASP Chan. 6 (0.20 micrometers \< Dp \< 1/cm\^3
                          0.23)                                    

  pa23_p26conc            PCASP Chan. 7 (0.23 micrometers \< Dp \< 1/cm\^3
                          0.26)                                    

  pa26_p30conc            PCASP Chan. 8 (0.26 micrometers \< Dp \< 1/cm\^3
                          0.30)                                    

  pa30_p35conc            PCASP Chan. 9 (0.30 micrometers \< Dp \< 1/cm\^3
                          0.35)                                    

  pa35_p40conc            PCASP Chan. 10 (0.35 micrometers \< Dp   1/cm\^3
                          \< 0.40)                                 

  pa40_p45conc            PCASP Chan. 11 (0.40 micrometers \< Dp   1/cm\^3
                          \< 0.45)                                 

  pa45_p50conc            PCASP Chan. 12 (0.45 micrometers \< Dp   1/cm\^3
                          \< 0.50)                                 

  pa50_p60conc            PCASP Chan. 13 (0.50 micrometers \< Dp   1/cm\^3
                          \< 0.60)                                 

  pa60_p70conc            PCASP Chan. 14 (0.60 micrometers \< Dp   1/cm\^3
                          \< 0.70)                                 

  pa70_p80conc            PCASP Chan. 15 (0.70 micrometers \< Dp   1/cm\^3
                          \< 0.80)                                 

  pa80_p90conc            PCASP Chan. 16 (0.80 micrometers \< Dp   1/cm\^3
                          \< 0.90)                                 

  pa90_p100conc           PCASP Chan. 17 (0.90 micrometers \< Dp   1/cm\^3
                          \< 1.00)                                 

  pa100_p130conc          PCASP Chan. 18 (1.00 micrometers \< Dp   1/cm\^3
                          \< 1.30)                                 

  pa130_p140conc          PCASP Chan. 19 (1.30 micrometers \< Dp   1/cm\^3
                          \< 1.40)                                 

  pa140_p160conc          PCASP Chan. 20 (1.40 micrometers \< Dp   1/cm\^3
                          \< 1.60)                                 

  pa160_p180conc          PCASP Chan. 21 (1.60 micrometers \< Dp   1/cm\^3
                          \< 1.80)                                 

  pa180_p200conc          PCASP Chan. 22 (1.80 micrometers \< Dp   1/cm\^3
                          \< 2.00)                                 

  pa200_p230conc          PCASP Chan. 23 (2.00 micrometers \< Dp   1/cm\^3
                          \< 2.30)                                 
  ----------------------- ---------------------------------------- ----------

  ---------------------- ---------------------------------------- ----------
  **Table 1**. (contd)                                            

  **Variable Name**      **Quantity Measured**                    **Unit**

  pa230_p260conc         PCASP Chan. 24 (2.30 micrometers \< Dp   1/cm\^3
                         \< 2.60)                                 

  pa260_p300conc         PCASP Chan. 25 (2.60 micrometers \< Dp   1/cm\^3
                         \< 3.00)                                 

  pa300_p350conc         PCASP Chan. 26 (2.60 micrometers \< Dp   1/cm\^3
                         \< 3.00)                                 

  pa350_p400conc         PCASP Chan. 27 (3.50 micrometers \< Dp   1/cm\^3
                         \< 4.00)                                 

  pa400_p500conc         PCASP Chan. 28 (4.00 micrometers \< Dp   1/cm\^3
                         \< 5.00)                                 

  pa500_p650conc         PCASP Chan. 29 (5.00 micrometers \< Dp   1/cm\^3
                         \< 6.50)                                 

  pa650_p800conc         PCASP Chan. 30 (6.50 micrometers \< Dp   1/cm\^3
                         \< 8.00)                                 

  pa800_p1000conc        PCASP Chan. 31 (8.00 micrometers \< Dp   1/cm\^3
                         \< 10.0)                                 
  ---------------------- ---------------------------------------- ----------

#### Definition of Uncertainty

The calculation of the measurement uncertainty of each nephelometer
follows the protocol of Anderson et al. (1999). The measurement
uncertainty associated with the TSI 3563 nephelometer was calculated
from five known sources and is expressed as a linear combination of the
following terms: du~total~^2^ = du~noise~^2^ + du~drift~^2^ +
du~cal~^2^ + du~trunc~^2^ + du~stp~^2^. Here, du~p~ designates the
uncertainty in u associated with the parameter, p. These arise from the
following:

-   instrument noise in the filtered air scattering coefficient

-   instrument drift in the calibration

-   uncertainty in the instrument calibration to Rayleigh scattering of
    dry air and CO~2~

-   instrument truncation of near forward scattered light

-   uncertainty in the instrument pressure and temperature in conversion
    of the data to STP. The associated uncertainties for each parameter
    for one-minute averages are listed in Table 2 as a function of the
    scattering coefficient magnitude.

  --------------------------------- --------- -------- ------------- ------------- --------- -----------
  []{#__RefHeading___Toc119297924                                                            
  .anchor}**Table 2**. Instrument                                                            
  Noise, Drift, and Uncertainty                                                              
  Factors.                                                                                   

  bsp                               noise     drift    calibration   truncation    Stp       **total**

  1                                 1.25      0.44     0.08          0.02          0.003     **1.33**

  10                                1.56      0.80     0.75          0.22          0.03      **1.92**

  20                                1.84      1.20     1.50          0.44          0.07      **2.70**

  50                                2.50      2.40     3.75          1.10          0.17      **5.23**

  100                               3.32      4.40     7.51          2.20          0.34      **9.58**
  --------------------------------- --------- -------- ------------- ------------- --------- -----------

Uncertainty associated with differences in the aerosol inlets and tubing
is expected to be insignificant for submicron aerosol. Losses within the
nephelometer itself were found to be negligible for submicron particles
and are 5%-10% for super micrometer particles (Anderson and Ogren 1998).
The variation in particle size with relative humidity (RH) and hence,
the particle transmission through a submicron impactor, operating
upstream of the nephelometer, will vary with the particle type. For an
RH below 50%, we estimate this uncertainty to be less than 5% based on
Berner impactor efficiency curves and estimates of the scattering size
distribution. In addition to RH, the flow rate affects the 50%
aerodynamic cut off diameter of the impactor. Running the Berner-type
impactors at a flow 10% lower than 30 lpm yields a 5% change in cut
size. Typically, flows are within 1%-2% of the expected flow rate.

For low-scattering values, instrument noise is the prevalent source of
uncertainty, while for higher scattering coefficients both noise and
instrument truncation uncertainties dominate. Uncertainty for low signal
values can be greatly reduced by increasing the signal averaging time.
For a 10-min averaging time, the uncertainty associated with noise for a
bsp of 1 Mm^-1^ is 0.40 Mm^-1^.

Uncertainties in the particle soot absorption photometer (PSAP), have
been described by Bond et al. (2001) and the appropriate corrections
have been applied to PSAP measurements made at the SGP site. Corrections
have been made for spot size, flow rate, interpretation of scattering as
absorption and instrument response to absorption. Uncertainty in the
measurements also stems from the variability of each PSAP unit and from
instrument noise. Bond et al. found instrumental variability to be 6% of
the measured absorption. Instrument noise (i.e., detection limit),
determined by measuring particle-free air, is 0.1 Mm^-1^ for hourly
averaged data and 0.9 Mm^-1^ for minute-averaged data. The total
uncertainty in aerosol absorption coefficient from the PSAP usually
varies from 1 to 4 Mm^-1^ for one minute average data and depends on the
magnitudes of the absorption and scattering coefficients.

### Secondary/Underlying Variables

The secondary variables are value-added products (VAPs) calculated from
the primary variables. These aerosol-intensive parameters depend on the
intrinsic aerosol properties and not on the aerosol quantity. Further
information on the AOS VAPs is located at
<http://science.arm.gov/vaps/aip1ogren.stm>. The AOS VAP is as follows:

> f(RH) or aerosol hygroscopic growth factor is the ratio of the
> scattering coefficient at 85% RH to that at 40% RH. f(RH) provides an
> indication of the amount of water absorbed by the aerosol, which
> depends on the aerosol size and chemical composition. This factor is
> necessary to calculate the scattering coefficient at ambient RH. f(RH)
> data are available in the AOS ARM archive data stream under the
> filename: sgpcmdlaosfitrhC1.c1.

### Diagnostic Variables

The diagnostic variables in the AOS data stream include those of the
instrument temperature, pressure, and RH. A description of each
parameter is given in Table 3.

  --------------------------------- ---------------------------------------- ----------
  []{#__RefHeading___Toc119297925                                            
  .anchor}**Table 3**. Diagnostic                                            
  Variables in the AOS Data Stream.                                          

  **Variable Name**                 **Quantity Measured**                    **Unit**

  SampTemp                          Sample temperature at base of inlet      C
                                    heater                                   

  SampRH                            Sample RH at base of inlet heater        \%

  AmbTemp                           Outdoor air temperature                  C

  TSINephTin_HRH                    High RH Neph. inlet temperature          C

  TSINephPres_HRH                   High RH Neph. Pressure                   hPa

  TSINephTSamp_HRH                  High RH Neph. Sample Temperature         C

  TSINephRHSamp_HRH                 High RH Neph. Relative humidity          \%

  HGdownstream_RH                   RH downstream of humidograph             \%

  HGupstream_RH                     RH upstream of humidograph               \%
  --------------------------------- ---------------------------------------- ----------

  ---------------------- ---------------------------------------- ----------
  **Table 3**. (contd)                                            

  **Variable Name**      **Quantity Measured**                    **Unit**

  HGupstream_T           Temp. upstream of humidograph            C

  HGdownstream_T         Temp. downstream of humidograph          C

  TSINephTin_LRH         Low RH Neph. inlet temperature           C

  TSINephPres_LRH        Low RH Neph. Pressure                    hPa

  TSINephTSamp_LRH       Low RH Neph. Sample Temperature          C

  TSINephRHSamp_LRH      Low RH Neph. Relative humidity           \%
  ---------------------- ---------------------------------------- ----------

### Data Quality Flags

The data quality flags are set below. Of particular note are flags 4 and
12. Every hour the system is in a 'zero' mode for 6 min. During this
time, filtered air passes through the instruments. The aerosol size
alternates between submicron and sub 10-micron size cuts every 6 min.

0 =\> \'Local contamination (Not set by ingest)

1 =\> \'Wind speed \< min (Not set by ingest)

2 =\> \'Wind direction out of sector (Not set by ingest)

3 =\> \'CN spike (Not set by ingest)

4 =\> \'System Zero Mode

5 =\> \'Stack offline (tilted)

6 =\> \'Zero/Stack flow \> max

7 =\> \'Zero/Stack flow \< min

8 =\> \'Value \> max

9 =\> \'Value \< min

10 =\> \'Std. Dev. \> max (Not set by ingest)

11 =\> \'Std. Dev. \< min (Not set by ingest)

12 =\> \'1-um impactor

13 =\> \'Instrument Zero mode

14 =\> \'Calibration mode

15 =\> \'Offline/inop/no data

16 =\> \'Power Off, Flow Out of Range

17 =\> \'Dependant Min/Max Field Out of Range (not set by ingest)

18 =\> \'Incorrect Valve Position

19 =\> \'Instrument Bypass

20 =\> \'TSI lamp power out/CPC Low Vacuum

21 =\> \'TSI valve fault/CPC Low Liquid

22 =\> \'TSI chopper fault

23 =\> \'TSI shutter fault

24 =\> \'TSI cal air

25 =\> \'TSI span gas (CO~2~)

26 =\> \'Not Used

27 =\> \'Not Used

28 =\> \'Data not flagged, but invalid

29 =\> \'Data not flagged, judged suspicious

30 =\> \'Data flagged, judged valid

### Dimension Variables

This section is not applicable to this instrument.

## Annotated Examples

The AOS data contain both the aerosol-scattering and absorption
coefficients measured at the SGP surface site as well as vertical
aircraft profiles of these values over the site. The data below show
typical values of the aerosol-scattering and absorption coefficients for
the two aerosol size cuts as well as the calculated intensive parameters
of the single-scattering albedo at 550 nm and the Angstrom exponent,
calculated for the 550 and 700 nm wavelength pair.

Figure 1 shows the aerosol absorption coefficient at 550 nm (top frame)
and the aerosol-scattering coefficient at three wavelengths, 450, 550,
and 700 nm for six days in September 2004. Both size cuts, submicron
(dark colors) and sub 10 micron (light colors) are plotted, but because
most of the aerosol resides in the submicron size mode there is little
difference in the scattering and absorption coefficients of these size
modes. On September 23 (day 267), a sharp decline occurred in the
aerosol scattering that accompanied rain. The single-scattering albedo
at 550 nm (Figure 2) was relatively high during the rain and
substantially declined after the rain episode. The aerosol Ǻngstrom
exponent (550/700 nm pair) initially declined at the onset of rain,
indicating the presence of large particles, and subsequently increased
during the rain to high values. On days 269 and 270, a low abundance of
relatively large and dark aerosols was present as indicated by the low
single-scattering albedo and low values of the Ǻngstrom exponent.

![](media/image1.png)

[]{#__RefHeading___Toc94672499 .anchor}Figure 1. Aerosol Absorption and
Scattering Coefficients at Three Wavelengths.

![](media/image2.png)

![](media/image3.png)

[]{#__RefHeading___Toc94672500 .anchor}Figure 2. Single-Scattering
Albedo Before and After Rain Episode.

On average, biweekly in situ aerosol profiles are measured over the SGP
site with a Cessna 172 aircraft. Seasonal and annual averages of the
aerosol submicron scattering and absorption coefficients for these
flights are shown in Figure 3.

![](media/image4.wmf){width="3.0771653543307087in"
height="2.9681102362204723in"}![](media/image5.wmf){width="3.0826771653543306in"
height="2.9744094488188977in"}

[]{#__RefHeading___Toc94672501 .anchor}Figure 3. Seasonal and Annual
Aerosol Submicron Scattering and Absorption Coefficient Averages.

## User Notes and Known Problems

This section is not applicable to this instrument.

## Frequently Asked Questions

This section is not applicable to this instrument.

# Data Quality

## Data Quality Health and Status

The following links go to current data quality health and status
results.

[DQ HandS](http://dq.arm.gov/) (Data Quality Health and Status)

[NCVweb](http://dq.arm.gov/ncvweb/ncvweb.cgi) for interactive data
plotting using

The tables and graphs shown contain the techniques used by ARM's data
quality analysts, instrument mentors, and site scientists to monitor and
diagnose data quality.

## Data Reviews by Instrument Mentor

Monthly reports on the data and instrument performance are provided by
the mentor and can be found at
[www.db.arm.gov/IMMS/](http://www.db.arm.gov/IMMS/)

## Data Assessments by Site Scientist/Data Quality Office

This section is not applicable to this instrument.

## Value-Added Procedures and Quality Measurement Experiments

See Section 5.1.2 on Secondary/Underlying Variables.

# Instrument Details

## Detailed Description

### List of Components

Aerosol instrumentation inside the trailer consists of the following:

1.  Two nephelometers. These are 3-wavelength (450, 550, and 700 nm) TSI
    (Model 3563) nephelometers that measure total angular scattering and
    hemispheric backscattering coefficients from 90° to 170°. The second
    TSI nephelometer is connected to a humidity scanning system to
    provide measurements of the scattering coefficients as a function of
    RH. Calibration with CO~2~ gas

> for the nephelometers is done weekly. Until summer 2001, there also
> was a 1-wavelength (at 545 nm) Radiance Research (Model M-903)
> nephelometer that measured the total light-scattering coefficient.

2.  Light absorption photometer. The Radiance Research (Model PSAP)
    Particle Soot Absorption Photometer measures the particle absorption
    coefficient at the wavelength of 550 nm. In April 2005, this
    instrument was upgraded to a 3-wavelength instrument.

3.  One condensation nuclei counter. The TSI (Model 3010) measures the
    total number concentration of condensation particles of diameter in
    the size range of 10 nm to 3 μm.

4.  Optical particle counter (OPC). The PMS (Model PCASP-X) OPC measures
    the particle number concentration in 31 size channels from 0.1 to 10
    μm. This instrument was inoperable and removed in 2004 because of
    poor instrument reliability.

5.  Ozone monitor. The Dasibi (Model 1008) ozone monitor measures ozone
    mixing ratios between 1 and 1000 ppbv using monochromatic
    ultraviolet (UV) absorption spectrophotometry.

### System Configuration and Measurement Methods

Figure 4 shows a flow schematic for the AOS. Atmospheric aerosol is
sampled at a volumetric flow rate of 1,000 m^3^ min^-1^. The main flow
into the sampling stack is split into five lines:

-   Three spare lines

-   Condensation nuclei counter (CNC)

-   TSI nephelometers and PSAP

-   Chemical sampling.

![](media/image6.png){width="3.2625in" height="3.4479166666666665in"}

Figure 4. Flow Schematic for AOS.

The flow rate through each of these five instrument lines is 30 l
min^-1^. Ozone is sampled from a separate line coated with Teflon and
the ozone sample line is mounted on the main aerosol sampling stack. The
Dasibi ozone monitor is preceded by a particle filter because particles
interfere with the ozone measurement. The aerosol inlet stack can be
tilted down for regular maintenance and periodic filtered air checks.
Note that the RR Neph and OPC are no longer in the sample line.

The aerosol sample stream is heated to an RH of 40% or less, depending
on ambient conditions, before it enters the sample lines. The sample air
is dried to measure the aerosol optical properties with minimal
influence from water vapor. Care is taken not to over heat the aerosol
and evaporate semi-volatile compounds such as nitric acid or organics.
An RH control unit conditions the air stream before it enters the second
3-wavelength nephelometer to provide a measurement of scattering
coefficient as a function of RH. The ambient scattering coefficients can
be derived from the measured dry scattering coefficients and the
hygroscopic growth factor, fRH.

### Specifications

The following describes the observational specifications of the AOS.

-   **AOS Size Cuts**

> The TSI nephelometers and Radiance Research absorption photometer are
> preceded by two switched impactors; a 10-um impactor removes particles
> with aerodynamic diameter \> 10 μm and a 1-μm impactor removes super
> micron size particles. The size cut switches every 6 min.
>
> The chemical sampling instrumentation has a 10-um size cut before the
> weekly sample and a 1‑μm size cut before the daily filter samples.

-   **TSI (Model 3563) nephelometer**

> wavelengths: 450 nm, 550 nm, 700 nm
>
> sensitivity (TSI specs): 1.0 x 10-7 m-1 for 450 & 550 nm; 3.0 x 10-7
> m-1 for 700 nm

-   **Radiance Research (Model PSAP) absorption photometer**

> wavelength: 550 nm

-   **TSI (Model 3010) condensation particle counter**

> wavelength: 780 nm
>
> concentration range: 0.0001 to 10000 particles cm-3 particle size
> range: 0.01 μm to 3 μm particles

-   **PMS (Model PCASP-X) optical particle counter**

> wavelength: 633 nm
>
> particle size range: 31 size channels from 0.1 to 10 μm and 1 channel
> for diameter \>10 μm
>
> maximum counting rate: 10,000 particles sec-1

-   **Dasibi (Model 1008) ozone monitor**

> mixing ratio range: 1 to 1000 ppbv
>
> UV light source wavelength: 253.7 nm

## Theory of Operation

The individual instruments are described on their own web pages at
[Nephelometers](http://www.cmdl.noaa.gov/aero/net/instrumentation/neph_desc.html)
and\
[Absorption
Photometer](http://www.cmdl.noaa.gov/aero/net/instrumentation/psap_desc.html).

## Calibration

### Theory and Procedures

**Nephelometers**: The instrument calibration is checked weekly using
low-span and high-span gases (filtered air and CO~2~, respectively).
Rayleigh scattering from these gases is a known quantity, and the
performance characteristics of each nephelometer are derived from these
calibrations. The instrument calibration values are readjusted annually
or more frequently, if needed. A weekly span check is performed where
the instruments are filled first with filtered air and then CO~2~, and
the agreement between measured and calculated CO~2~ Rayleigh scatter is
determined. As long as the span checks show good agreement, the
calibration is believed to be good.

**Light Absorption Photometer**: Calibration of this instrument
absorption measurement can be done using an extinction cell and a
nephelometer to provide an independent measure of suspended state
aerosol absorption. This calibration and the resulting instrument
corrections are found in Bond et al. 2001. CMDL calibrates the
volumetric flow rate through the instrument with a gilibrator or Bios
flowmeter, and does not attempt to calibrate the absorption measurement.

**Optical Particle Counter**: The optical particle counter is calibrated
using polystyrene latex spheres of known size and index of refraction.
The calibration is performed once a year or as needed.

**Ozone Monitor**: The ozone monitor is periodically shipped back to our
laboratory and calibrated with a National Institute of Standards and
Technology (NIST)-certified standard ozone calibration source.

### History

Calibration data are available from NOAA/CMDL upon request.

## Operation and Maintenance

### User Manual

This section is not applicable to this instrument.

### Routine and Corrective Maintenance Documentation

**Daily Maintenance**

System - Check system variables to see within range.

Humidity System - Check water level and add water if necessary.

PSAP - Change aerosol sample filter.

**Weekly Maintenance**

PSAP - Change reference filter.

Nephelometer - CO~2~ calibration span check.

Impactors - Change impactor films.

Leak test - Test for leaks in stack/tubing/fittings.

Chemical Filters -- change filters in aerosol chemical filter sampler.

**Annual Maintenance**

Instrument maintenance and calibration is done annually; further
maintenance is done on an as-needed basis. The annual maintenance
includes:

-   Pumpbox - Replace carbon vanes and diaphragms in various pumps;
    check and calibrate pitot tube, replace or clean the pump exhaust
    filter.

-   Stack - Leak test with HEPA filter; clean flow splitter; check T/RH
    probes; inspect tubing.

-   PSAP - Check tubing and connectors; check internal filter; calibrate
    flow.

-   CNC - Replace drier tube; fill with butanol; flush aerosol line,
    compare counts to another TSI 3010 CNC.

-   Impactors - Clean impactor surfaces; regrease with high vacuum
    grease; replace impactor films.

-   Nephelometer - Perform CO­~2~ calibration; do zero air test over
    night, replace filters.

-   Pressure Sensors - Check pressures and calibrate.

-   Mass Flow Meters - Change upstream filters; calibrate with flow
    calibrator.

-   Miscellaneous - Check wind vane direction; clean solenoid valves;
    update station inventory log.

### Software Documentation

ARM netCDF file header descriptions may be found at
[AOS](http://science.arm.gov/tool/dod/showdod.php?Inst=aos) Data Object
Design Changes.

### Additional Documentation

This section is not applicable to this instrument.

## Glossary

See the ARM Glossary at <http://www.arm.gov/about/glossary.stm>.

## Acronyms

AOS aerosol observing system

ARM Atmospheric Radiation Measurement (Program)

CMDL Climate Monitoring and Diagnostics Laboratory

CNC condensation nuclei counter

CPC condensation particle counter

IAP in situ aerosol profiling

Lpm liters per minute

MFC mass flow controller

NOAA National Oceanic and Atmospheric Administration

OPC optical particle counter

Ppbv parts per billion by volume

PSAP particle soot absorption photometer

PSL polystyrene latex spheres

RH relative humidity

SGP Southern Great Plains

VAP value-added product

## Citable References

Anderson, TL, and JA Ogren. 1998. "Determining aerosol radiative
properties using the TSI 3563 integrating nephelometer." *Aerosol
Science and Technology* 29, 57-69.

Anderson, TL, DS Covert, JD Wheeler, JM Harris, KD Perry, BE Trost, DJ
Jaffe, and JA Ogren. 1999. "Aerosol backscatter fraction and
single-scattering albedo: Measured values and uncertainties at a coastal
station in the Pacific Northwest." *Journal of Geophysical Research*
104, 26793-26807.

Bond, TC, TL Anderson, and D Campbell. 2001. "Calibration and
intercomparison of filter-based measurements of visible light absorption
by aerosols." *Aeroso Science and Technology* 30, 582-600.