### Appendix I

Recommended Conversion Factors for Secondary Format ion of

PM- Nitrate from NO

x 

### EmissionsI - 1 Recommended Conversion Factors for Secondary Format ion of

PM- Nitrate from NO
x Emissions

Air Resources Board Research Division

September 2005

Chemical Formation of Ammonium Nitrate from NO X 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, electr icity generation in California primarily depends on clean-burning natural gas rath er than coal or oil, which typically has higher sulfur content. Based on ambient measur ements of PM 10 (particles with an aerodynamic diameter of 10 microns or less), it can be shown that the distribution of

PM 10 -nitrate is different in summer and winter (see Fig ures 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 d uring winter, when cool, humid

conditions facilitate the formation of ammonium nit rate from ammonia and nitric acid. In the South Coast Air Basin (SoCAB - North Long Beach , Los Angeles, Anaheim, Azusa, Riverside, and Fontana), although nitrate productio n is greatest during the winter months, the nitrate fraction remains significant du ring the summer due to extremely high ammonia concentrations from dairy feedlots.

The formation of secondary ammonium nitrate (NH 4 NO 3 ) begins with the oxidation of oxides of nitrogen (NOx) into nitric acid (HNO 3 ). The nitric acid then reacts with gaseous ammonia to form NH 4 NO 3 . NOx can be oxidized into nitric acid through both daytime and nighttime reactions involving the hydro xyl radical and ozone respectively. Although volatile organic compounds (VOCs) can play a role in the oxidation of NOx to

HNO 3 , studies in the San Joaquin Valley have shown that NH 4 NO 3 is most responsive to reductions in NOx emissions, with minimal respon se to changes in VOC emissions.

The sea influences the chemical composition of aero sols in the coastal zone. Sodium chloride (NaCl) is always present in aerosols in th e form of large particles originating

from seawater. Several studies have indicated the i mportance of HNO 3 reaction on the sea salt particles, leading to thermally stable sod ium nitrate (NaNO 3 ) production in the particle phase accompanied by liberation of gaseous hydrochloric acid (HCl) from the particles. This reaction may be the principal sour ce of coarse (2.5 to 10 μm) nitrate, and plays an important role in atmospheric chemistry be cause 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 h omogenous conditions with nearneutral stability, transfer of HNO 3 to particle phase to form NaNO 3 may decrease the deposition velocity of nitrogen by over 50%, leadin g to greater horizontal transport prior to deposition to the sea surface. Conversely, for w ind speed above 10 m/s, transfer ofI - 2

nitrogen to the particle phase would increase the d eposition rate and hence decrease transport prior to sea surface removal. However, se veral of theoretical studies have stated the need for experimental data to better elu cidate 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 marin e atmosphere.

0 2 4 6 8 10 12 14 Figure 1a. PM10-Nitrate
Winter, Nov-Feb 1998-2000I - 3 0 2 4 6 8 10 12 14 Figure 1b. PM10-Nitrate

Summer, Jun-Sep 1998-2000

To estimate how much of the total PM 10 nitrate can be attributed to NO X emissions, two parameters need to be considered: (1) the fractiona l conversion of the total emitted NOx

to HNO 3 and (2) the fractional conversion of the HNO 3 to NH 4 NO 3 aerosol. It is difficult to determine an accurate estimate of these paramete rs without performing a comprehensive air quality modeling simulation. Rea ctions of the precursors with other chemical entities, often formed from photolysis, be gin immediately upon the introduction of emissions into ambient air, and depending on emi ssion rate, weather, and air concentration of all reactants, the conversion proc ess may proceed at different rates. Some reactions will take place in minutes, while ot hers 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 photochem ical modeling, the analysis of ambient monitoring data, and a simplified box model analysis.

Photochemical Modeling : Dr. Kleeman of the University of California Davi s campus, has recently conducted an air quality modeling stud y in the San Joaquin Valley (SJV). Regional calculations for the SJV based on the IMS9 5 episode of January 4-6, 1996 show that approximately 13-18% of the reactive nitr ogen emissions in the SJV are converted to nitrate at the ground level on a mole basis. Accounting for the differenceI - 4

in molecular weight between NOx and NH 4 NO 3 , each gram of emitted NOx produces approximately 0.35 - 0.48 grams of NH 4 NO 3

. 1

Analysis of Ambient Data : In addition, a simple analysis of ambient concen tration data can be performed. Assuming that the sum of the amb ient 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 resu lts show the month-to-month

variations in the ambient concentrations and the co nversion factors and it generally depicts the increase of the conversion factors in t he South Coast Air Basin from the source sites (e.g., Long Beach) to the downwind sit es (e.g., Glendora, Upland) to Riverside. This supports the use of a range for th e conversion of NO X to PM nitrate

Box Model

: A third estimate of NO X to nitrate conversion can also be obtained in an a ir basin considering the air basin as one well-mixed b ox in which NO X 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 NO X to nitrate yields conversion factor for secondary formation of PM nitrate from NO X emissions of about 0.5.

Recommendation

Emissions of NO X , and ammonia (NH 3 ) interact in the atmosphere in presence of other ambient air pollutants to form NH 4 NO 3 particles. Because this “secondary” particulate

matter can constitute a sizable fraction of the tot al ambient particulate matter measured at air quality monitors, and because emissions of t he precursors can vary substantially from source to source, it is important to have at l east a simple methodology of the formation of secondary particulate matter from emis sions of NOx and NH 3

. Unfortunately, but not surprisingly, it is not easy to model secondary aerosol chemistry. Reactions of the precursors with other chemical ent ities, often formed from photolysis, begin immediately upon the introduction of emission s 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 wil l take place in minutes, others in days. In the meantime, the pollutants and their pro ducts are being transported, diluted, deposited, and augmented by new emissions along the ir path.

In spite of the complexity of the issue and recogni zing the importance of taking secondary PM formation into consideration, we have attempted to construct a quantitative relationship between precursor emissio ns 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 fr action of NOx emissions converted to nitrate ranged from 30 to 50% over entire San Jo aquin Valley Air Basin or South Coast Air Basin. Thus, as an approximation, each g ram 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 NH 4 NO 3 value reported in this study is the average over

the entire SJV.I - 5

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 variab le depending on time and location and that a conversation rate range should be applied.

A strong argument against the application of a sing le conversion rate is an assumption of linearity as it suggests that decreases and incr eases in NO X emissions will lead to proportional reductions and gains in PM-nitrate mas s. However, the rate of NO X oxidation and the branching ratio between inorganic and organic nitrates are known to depend strongly on environmental conditions and rea ctant concentrations. The partitioning of inorganic nitrate between gaseous n itric acid, ammonium nitrate, and non-volatile nitrate is known to depend in a non-li near 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 NO X emissions to PM-nitrate is essentially complete. However, this may not be tru e as some portion of the measured

NO X 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 NO X 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 con version factors for emissions that

would occur over water such as emissions from ocean going ships at sea. ARB staff is unable at this time to adequately evaluate the pote ntial for the formation of secondary PM-nitrate at sea due to a lack experimental data u nder marine conditions, in particular ammonia concentrations and chemical processes. In a ddition, NO X loss processes in the marine environment need further investigation a s do various aspects of ship plume chemistry.