Number range CHAPTER**FACT SHEET FOR NO~y~ MONITORING**

**Introduction**

Reactive nitrogen compounds (NO~y~) have been identified as precursors
for both ozone and fine particulate matter (PM~2.5~). The National
Ambient Air Monitoring Strategy (NAAMS) calls for deployment of NO~y~
monitors at approximately 75 locations. OAQPS has begun an effort to
gain knowledge and experience with NO~y~ monitoring in order to resolve
a number of technical issues that have been identified with NO~y~
monitoring, as well as to be prepared to provide support to State and
Local monitoring agencies as they begin to purchase, install, and
operate NO~y~ monitors.

**Definition of NO~y~**

NO~y~ consists of all oxides of nitrogen in which the oxidation state of
the N atom is +2 or greater, ie, the sum of all reactive nitrogen oxides
including NO~x~ (NO + NO~2~) and other nitrogen oxides referred to as
NO~z~. The major components of NO~z~ include nitrous acids \[nitric acid
(HNO~3~), and nitrous acid (HONO)\], organic nitrates \[peroxyl acetyl
nitrate (PAN), methyl peroxyl acetyl nitrate (MPAN), and peroxyl
propionyl nitrate, (PPN)\], and particulate nitrates.

NO + NO~2~ + NO~z~ = NO~y~

**Principle of Operation**

The current recommendation for the measurement of NO~y~ is the use of a
thermal converter to convert reactive nitrogen species to NO followed by
detection of NO by its chemiluminescence reaction with an excess of
O~3~. NO is measured by bypassing the converter. The combination of
NO~2~ and NO~z~ can be then determined by difference. This procedure is
similar to the current methodology used to measure NO~x~, however, the
converter temperature is higher in order to more completely convert
NO~z~ species, and the converter has been moved to very near the sample
inlet to avoid line losses of "sticky" NO~y~ species such as HNO~3~.

**Literature Search**

Based on the results of a literature search, three manufacturers of
trace level NO~y~ monitors have been identified. The specifications for
the three analyzers are provided in Table 1. The price for these
analyzers are in the range of \$15,000 and \$20,000. See the internet
websites below for additional information on these analyzers, and vendor
contacts in your area:

[[http://www.teledyne-api.com/products/index.asp]{.underline}](http://www.teledyne-api.com/products/index.asp)

[[http://www.thermo.com/]{.underline}](http://www.thermo.com/)

[[http://www.ecotech.com.au/ml9800.htm]{.underline}](http://www.ecotech.com.au/ml9800.htm)

**Trace Level Modifications**

A number of modifications are typically made to the standard NOx/NOy
monitor to improve the detection limit for trace gas monitoring.
Depending on the particular make, modifications may include the
following:

1.  Pre-reactor to eliminate non-NOy interferent,

2.  Increase sample flow rate, and

3.  Improved temperature controls in photomultiplier tube.

**Calibration and Zero**

Calibration and zero tests should be performed through the remote
converter. The Thermo NO~y~ analyzer has a separate connection for the
calibration gas at the inlet of the converter. An adequate supply of
calibration gasses must be supplied to ensure that excess calibration
gas passes out the inlet. Currently, OAQPS recommends that a zero and
span check be performed daily, and multi-point calibrations be performed
on a monthly basis. OAQPS is currently conducting a study to determine
if more frequent checks are needed, or if less frequent checks are
acceptable.

**Issues with the Method**

*1) Reaction of NO with ambient ozone.*

It is well known that ambient ozone will react with NO in the sample
line, resulting in a reduction in the NO reading. For this reason, it is
recommended that the residence time in the sample line be as short as
possible, and no more than 2 seconds. For a 1/4 inch OD, 1/8 inch ID
tubing and a total sample rate of 1.5 liters per minute, the maximum
sample line length is approximately 20 feet. Thick wall tubing and
increased bypass flow can be used if longer sampling lines are needed.

*2) Conversion efficiency of converter.*

The molybdenum converter is consumed, and has an average life of
approximately 1 year. The efficiency of the converter will reduce over
time, and as such must be checked periodically. At this time,
differences of opinion exist between researchers as to the appropriate
NO~y~ species to use to check converter efficiency. Potential species
include NO~2~, HNO~3~, isopropyl nitrate, and n-propyl nitrate. OAQPS is
performing a study to determine which species should be used in the
NCore program to test converter efficiency.

*3) Ammonia interference*.

Depending on the converter temperature, the converter may convert a
small amount of NH~3~ to NO, resulting in increased NO readings.
However, under normal circumstances NH~3~ concentrations are low
compared to NO and this positive interference is negligible.
Nonetheless, care should be taken when siting the monitor to be sure
that it is not located near significant NH~3~ sources which could cause
elevated NH~3~ concentrations (e.g., concentrated animal feeding
operations).

**Table 1. Summary of Commercially Available NO~y~ Monitors**

+----------+------+----+------+-------+-------+------+-------+---------+
| **Inst   | **Lo | *  | **Re | *     | *     | **   | **P   | *       |
| rument** | west | *L | spon | *Zero | *Span | Auto | recis | *Approx |
|          | Ran  | DL | se** | Dr    | Dr    | R    | ion** | imate** |
| **Make   | ge** | ** |      | ift** | ift** | angi |       |         |
| /Model** |      |    | **Ti |       |       | ng** |       | **      |
|          |      |    | me** |       |       |      |       | Price** |
+----------+------+----+------+-------+-------+------+-------+---------+
| API      | 0-   | 50 | 120  | \<0.1 | \     | Yes  | 0.5%  | \$15    |
| 501NOY   | 5ppb | p  |      | pp    | <0.5% |      |       | -19,000 |
|          |      | pt |      | b/day | FS    |      |       |         |
+----------+------+----+------+-------+-------+------+-------+---------+
| Thermo   | 0-   | 50 | 120  | Neg.  | +/-   | Yes  | NA    | \       |
| 42C-YNOy | 5ppb | p  |      |       | 1% FS |      |       | $19,490 |
|          |      | pt |      |       |       |      |       |         |
+----------+------+----+------+-------+-------+------+-------+---------+
| EcoTech  | 0-2  | 50 | Vari | \<0.1 | +/-   | Yes  | 5     | NA      |
| EC9      | 0ppb | p  | able |       | 1% FS |      | 0ppt, |         |
| 841A-NOy |      | pt |      | pp    |       |      | or    |         |
|          |      |    |      | b/day |       |      | 0.5%  |         |
|          |      |    |      |       |       |      | of    |         |
|          |      |    |      |       |       |      | re    |         |
|          |      |    |      |       |       |      | ading |         |
|          |      |    |      |       |       |      | above |         |
|          |      |    |      |       |       |      | 50    |         |
|          |      |    |      |       |       |      | ppb   |         |
+----------+------+----+------+-------+-------+------+-------+---------+