Appendix I 

Recommended Conversion Factors for Secondary Formation of 

PM- Nitrate from NOx Emissions 
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Recommended Conversion Factors for Secondary Formation of 

PM- Nitrate from NOx Emissions 

Air Resources Board Research Division 

September 2005 

Chemical Formation of Ammonium Nitrate from NOX Emissions 

In urban areas of California, nitrate represents a larger fraction of particulate matter 

(PM) mass compared to the rest of the nation due to wide use of low-sulfur fuels for 

mobile and stationary sources. For example, electricity generation in California 

primarily depends on clean-burning natural gas rather than coal or oil, which typically 

has higher sulfur content. Based on ambient measurements of PM10 (particles with an 

aerodynamic diameter of 10 microns or less), it can be shown that the distribution of 

PM10-nitrate is different in summer and winter (see Figures 1a and 1b) and can vary 

significantly between summer and winter at the same site. As is shown in the figures, 

nitrate predominates in urban areas, particularly during winter, when cool, humid 

conditions facilitate the formation of ammonium nitrate from ammonia and nitric acid. In 

the South Coast Air Basin (SoCAB - North Long Beach, Los Angeles, Anaheim, Azusa, 

Riverside, and Fontana), although nitrate production is greatest during the winter 

months, the nitrate fraction remains significant during the summer due to extremely high 

ammonia concentrations from dairy feedlots. 

The formation of secondary ammonium nitrate (NH4NO3) begins with the oxidation of 

oxides of nitrogen (NOx) into nitric acid (HNO3). The nitric acid then reacts with 

gaseous ammonia to form NH4NO3 . NOx can be oxidized into nitric acid through both 

daytime and nighttime reactions involving the hydroxyl radical and ozone respectively. 

Although volatile organic compounds (VOCs) can play a role in the oxidation of NOx to 

HNO3, studies in the San Joaquin Valley have shown that NH4NO3 is most responsive 

to reductions in NOx emissions, with minimal response to changes in VOC emissions. 

The sea influences the chemical composition of aerosols in the coastal zone. Sodium 

chloride (NaCl) is always present in aerosols in the form of large particles originating 

from seawater. Several studies have indicated the importance of HNO3 reaction on the 

sea salt particles, leading to thermally stable sodium nitrate (NaNO3) production in the 

particle phase accompanied by liberation of gaseous hydrochloric acid (HCl) from the 

particles. This reaction may be the principal source of coarse (2.5 to 10 μm) nitrate, and 

plays an important role in atmospheric chemistry because it is a permanent sink for gasphase nitrogen oxide species. 

In a study, a numerical model simulation of marine boundary layer (MBL) was 

performed to investigate chemical reactions in the gas-phase and aerosol particles in 

the MBL. Results indicate that under horizontally homogenous conditions with nearneutral stability, transfer of HNO3 to particle phase to form NaNO3 may decrease the 

deposition velocity of nitrogen by over 50%, leading to greater horizontal transport prior 

to deposition to the sea surface. Conversely, for wind speed above 10 m/s, transfer of 
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nitrogen to the particle phase would increase the deposition rate and hence decrease 

transport prior to sea surface removal. However, several of theoretical studies have 

stated the need for experimental data to better elucidate the dynamical and chemical 

process in the marine boundary layer, in particular emission from oceangoing ships in 

terms of their impact on the chemistry of the marine atmosphere. 

0

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10

12

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Figure 1a. PM10-Nitrate 

Winter, Nov-Feb 1998-2000

 
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0

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4

6

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10

12

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Figure 1b. PM10-Nitrate

Summer, Jun-Sep 1998-2000

 

To estimate how much of the total PM10 nitrate can be attributed to NOX emissions, two 

parameters need to be considered: (1) the fractional conversion of the total emitted NOx 

to HNO3 and (2) the fractional conversion of the HNO3 to NH4NO3 aerosol. It is difficult 

to determine an accurate estimate of these parameters without performing a 

comprehensive air quality modeling simulation. Reactions of the precursors with other 

chemical entities, often formed from photolysis, begin immediately upon the introduction 

of emissions into ambient air, and depending on emission rate, weather, and air 

concentration of all reactants, the conversion process may proceed at different rates. 

Some reactions will take place in minutes, while others in days. In the meantime, the 

pollutants and their products are being transported, diluted, deposited, and augmented 

by new emissions. Below ARB staff describes three methodologies that can be used to 

estimate the conversion rate: the use of photochemical modeling, the analysis of 

ambient monitoring data, and a simplified box model analysis. 

Photochemical Modeling: Dr. Kleeman of the University of California Davis campus, 

has recently conducted an air quality modeling study in the San Joaquin Valley (SJV). 

Regional calculations for the SJV based on the IMS95 episode of January 4-6, 1996 

show that approximately 13-18% of the reactive nitrogen emissions in the SJV are 

converted to nitrate at the ground level on a mole basis. Accounting for the difference 
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in molecular weight between NOx and NH4NO3, each gram of emitted NOx produces 

approximately 0.35 - 0.48 grams of NH4NO3. 

1

 

Analysis of Ambient Data: In addition, a simple analysis of ambient concentration data 

can be performed. Assuming that the sum of the ambient NOx and PM-nitrate 

concentrations measured at the same place and time represents the emissions that led 

to the formation of the particles, monthly average conversion factor at several sites in 

the South Coast Air Basin were calculated. The results show the month-to-month 

variations in the ambient concentrations and the conversion factors and it generally 

depicts the increase of the conversion factors in the South Coast Air Basin from the 

source sites (e.g., Long Beach) to the downwind sites (e.g., Glendora, Upland) to 

Riverside. This supports the use of a range for the conversion of NOX to PM nitrate 

Box Model: A third estimate of NOX to nitrate conversion can also be obtained in an air 

basin considering the air basin as one well-mixed box in which NOX is being oxidized to 

nitrate by hydroxyl radical and ozone. The ratio of residence times of the ambient air in 

an air basin to ambient chemical lifetime of NOX to nitrate yields conversion factor for 

secondary formation of PM nitrate from NOX emissions of about 0.5. 

Recommendation

Emissions of NOX, and ammonia (NH3) interact in the atmosphere in presence of other 

ambient air pollutants to form NH4NO3 particles. Because this “secondary” particulate 

matter can constitute a sizable fraction of the total ambient particulate matter measured 

at air quality monitors, and because emissions of the precursors can vary substantially 

from source to source, it is important to have at least a simple methodology of the 

formation of secondary particulate matter from emissions of NOx and NH3. 

Unfortunately, but not surprisingly, it is not easy to model secondary aerosol chemistry. 

Reactions of the precursors with other chemical entities, often formed from photolysis, 

begin immediately upon the introduction of emissions into ambient air, and depending 

on emission rate, weather, and air concentration of all reactants, the conversion process 

may proceed at different rates. Some reactions will take place in minutes, others in 

days. In the meantime, the pollutants and their products are being transported, diluted, 

deposited, and augmented by new emissions along their path. 

In spite of the complexity of the issue and recognizing the importance of taking 

secondary PM formation into consideration, we have attempted to construct a 

quantitative relationship between precursor emissions and secondary ammonium nitrate 

as described briefly in this document. To estimate the conversion of NOx to PM nitrate, 

three independent methodologies suggest that the fraction of NOx emissions converted 

to nitrate ranged from 30 to 50% over entire San Joaquin Valley Air Basin or South 

Coast Air Basin. Thus, as an approximation, each gram of emitted NOx produces 

 

1

 Kleeman found that the conversion ratio was highly variable depending on time and 

location. The 0.35 to 0.48 g of NH4NO3 value reported in this study is the average over 

the entire SJV. 
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approximately 0.30 - 0.50 grams of secondary PM (i.e., PM-Nitrate). It is important to 

note that the conversion ratio can be highly variable depending on time and location and 

that a conversation rate range should be applied. 

A strong argument against the application of a single conversion rate is an assumption 

of linearity as it suggests that decreases and increases in NOX emissions will lead to 

proportional reductions and gains in PM-nitrate mass. However, the rate of NOX

oxidation and the branching ratio between inorganic and organic nitrates are known to 

depend strongly on environmental conditions and reactant concentrations. The 

partitioning of inorganic nitrate between gaseous nitric acid, ammonium nitrate, and 

non-volatile nitrate is known to depend in a non-linear manner on a number of factors, 

such as relative humidity, temperature, and ammonia. Also, the application of a single 

conversion factor implies that the conversion of NOX emissions to PM-nitrate is 

essentially complete. However, this may not be true as some portion of the measured 

NOX concentrations are due to “fresh” emissions, which have not had sufficient time to 

react. This effect can be seen in the SoCAB where detailed air quality models show a 

change in percent conversion of NOX to PM-nitrate as one moves from source sites 

(e.g., Hawthorne, Long Beach, Los Angeles, Burbank), to downwind sites (e.g., Azusa, 

Upland, Fontana), to Rubidoux, which sits downwind of large ammonia sources. 

In addition, it is not recommended to use these conversion factors for emissions that 

would occur over water such as emissions from oceangoing ships at sea. ARB staff is 

unable at this time to adequately evaluate the potential for the formation of secondary 

PM-nitrate at sea due to a lack experimental data under marine conditions, in particular 

ammonia concentrations and chemical processes. In addition, NOX loss processes in 

the marine environment need further investigation as do various aspects of ship plume 

chemistry.