# SENSITIVITY OF PARTICULATE MATTER NITRATE 

# FORMATION TO PRECURSOR EMISSIONS IN THE

# CALIFORNIA SAN JOAQUIN VALLEY

Betty K. Pun\* and Christian Seigneur

Atmospheric and Environmental Research, Inc.

2682 Bishop Drive, Suite 120

San Ramon, CA 94583

2 April 2001

**ABSTRACT**

The formation of secondary ammonium nitrate during the 1995 Integrated
Monitoring Study (IMS95) in San Joaquin Valley, CA was investigated
using a box model that simulates the atmospheric chemistry and
gas/particle partition of inorganic compounds. The concentration of
particulate matter (PM) nitrate was found to be sensitive to reductions
in VOC emissions. Nitric acid, rather than ammonia, was the limiting
reagent in the formation of PM nitrate. The formation of nitric acid was
more sensitive to the availability of oxidants than that of NO~x~.
Oxidant chemistry in wintertime conditions in the San Joaquin Valley was
shown to be VOC-sensitive. In fact, a decrease in NO~x~ emissions may
have the counter-intuitive effect of increasing PM nitrate.

**INTRODUCTION**

The 1995 Integrated Monitoring Study (IMS95) was a planning study for
the California Regional PM~10~/PM~2.5~ Air Quality Study (CRPAQS).
Details of the IMS95, including maps of the study domain, can be found
in a special issue of Atmospheric Environment (Volume 33, Issue 29,
1999). Ambient data from the IMS95 show that areas in the California San
Joaquin Valley (SJV) exceed the short-term PM~2.5~ National Ambient Air
Quality Standard (24-hour average concentration of 65 μg/m^3^)^1^. Much
of the PM~2.5~ observed during winter is secondary in origin. Of the key
components of PM~2.5~, ammonium nitrate (NH~4~NO~3~) typically accounts
for close to 20 μg/m^3^ of PM material, corresponding to 30% of urban
PM~2.5~ and 60% of PM~2.5~ in rural areas^2^. In contrast, ammonium
sulfate, the key component in PM~2.5~ observed in may parts of the
eastern United States, only accounted for less than 5% of the PM~2.5~
mass during IMS95^2^. Therefore, it is important to investigate the
PM-precursor relationships of NH~4~NO~3~ for the formulation of
effective PM~2.5~ control strategies, especially in rural areas.

In their conceptual model of PM formation, Pun and Seigneur^1^
postulated that the formation of NH~4~NO~3~ is limited by the
availability of nitric acid (HNO~3~), because ammonia (NH~3~) emissions
seem abundant in the SJV. This conclusion is supported by other recent
analyses^3^. HNO~3~ is itself a secondary component, formed in the
atmosphere as a product of photochemical reactions involving nitrogen
dioxide (NO~2~), hydroxyl radicals (OH), and ozone (O~3~) (the O~3~
reaction involves intermediate species nitrate radicals, NO~3~, and
dinitrogen pentoxide, N~2~O~5~). While nitrogen oxides (NO~x~) are
directly emitted, the radical species and O~3~ are produced from
precursors NO~x~ and volatile organic compounds (VOC). Therefore,
oxidant formation may be sensitive to NO~x~ or to VOC. Clearly, the
chemistry regime has important implications towards the choice of
effective emission controls.

This modeling study was performed to investigate the sensitivity of PM
nitrate formation under conditions prevalent in the SJV during the
winter season. This work was designed to complement field measurements
that may help unravel the details of nitrate chemistry in the ambient
environment. Our objectives were to: (1) study the sensitivities of
oxidants and PM to precursors, and (2) corroborate the modeling results
with indicator species approaches^4,5,6^ for predicting the sensitivity
of wintertime PM formation.

# SIMULATION METHODS

##### Box Model. A box model was selected to study the sensitivity of PM nitrate to NO~x~ and oxidants. Although a three-dimensional (3-D) model should ultimately be used for this investigation, existing databases were insufficient for the reliable application of a 3-D model (e.g., aloft concentrations needed to define boundary and initial conditions were not available). A box model, with carefully chosen initial conditions and emissions, can provide valuable information on the major processes that govern the dynamics of nitrate formation during the winter PM episodes.

Winter PM accumulation is primarily associated with stagnant conditions
with low wind speeds (less that 2 m s^-1^). Therefore, advection did not
need to be treated. The box model treats the following processes using
an operator splitting approach: (1) emissions of precursor gases and PM;
(2) gas-phase chemistry using the Carbon Bond Mechanism IV^7^ (CMB-IV),
augmented with isoprene chemistry and heterogeneous nitrate
chemistry^8,9^; (3) dilution by and entrainment of aloft air as the
mixing height rises; (4) dry deposition of gases and PM and wet
deposition of PM associated fog; (5) gas/particle partitioning using
SCAPE2^10^, a thermodynamic equilibrium aerosol module.

The key feature of CBM-IV is the lumping of organic compounds based on
their molecular structures (model species represent paraffin carbons,
olefin bonds, etc). Inorganic reactions represented in CBM-IV are
similar to those used in the other gas-phase mechanisms, such as SAPRC
and RADM. Due to the abundance of biogenic emissions in the SJV, the
most recent treatment for isoprene chemistry was implemented to ensure
the proper representation of gas-phase chemistry. Isoprene reacts with
oxygen atoms (O), OH, O~3~, NO~3~, and NO~2~. A surrogate isoprene
reaction product, ISPD, may undergo photolysis or react with OH, O~3~,
and NO~3~. Therefore, the version of CBM-IV used in this study simulates
the chemistry of 34 species (25 molecular species and 9 radicals) with
88 reactions. Photolysis rates were calculated based on cloud-free
conditions, although fog sometimes persisted after sunrise.
Heterogeneous chemistry of N~2~O~5~, NO~3~, and HO~2~ was treated using
the reaction probability approach recommended by Jacob^8^. These
reactions were simulated when fog was present using an average droplet
diameter of 20 μm^11^. Aqueous-phase sulfate chemistry was not included.
Sulfate is not a key component of PM~2.5~ in SJV and is not the focus of
this study. The omission of aqueous-phase sulfur chemistry is not
expected to have significant impacts on the simulation, since
SO~2~/sulfate chemistry has little effect on nitrate formation in an
ammonia-rich and sulfate-poor environment.

The gas-phase chemical kinetic equations are solved using the Young and
Boris^12^ ordinary differential equation solver. Pseudo-steady-state
assumptions are made for all radical species (with the exception of
NO~3~, whose reaction time scale dictates whether or not steady state is
assumed at any time). This approach provides a good balance between
numerical robustness and computational efficiency.

SCAPE2 simulates the composition of atmospheric particles at equilibrium
given the total (i.e., gas and particulate) amounts of sulfate, nitrate,
ammonium, sodium, and chloride. At each time step, the concentrations of
NH~3~, sodium, chloride, and sulfate change as a result of direct
emissions. In addition, sulfuric acid and nitric acid are formed from
chemical reactions in the gas phase. SCAPE2 calculates the thermodynamic
equilibrium of the gas/particulate system based on time-varying inputs
of temperature and relative humidity (RH). At each time step, SCAPE2
outputs the gaseous concentrations of NH~3~, HNO~3~, and HCl, and
particulate concentrations of sodium, sulfate, ammonium, nitrate, and
chloride.

Typical dry deposition velocities were derived for SO~2~, NO~2~, O~3~,
HNO~3~, H~2~O~2~, formaldehyde, higher aldehydes, and sulfate from the
SARMAP air quality model (SAQM) and from Models-3 for NH~3~. The dry
deposition velocity of sulfate was used for all particulate species in
the simulation. Wet deposition was modeled when fog was present using an
average particle deposition rate of about 3% per hour^13^.

**Base case simulation inputs.** Conditions during the 4-6 January 1996
episode were generally cool, calm, and stagnant^14^. Surface
temperatures fluctuated between 7 and 16 °C. Surface wind velocities
were below 0.5 m s^-1^ 40% to 50% of the time, and day time maximum
mixing heights ranged from 450 m to 1250 m at several stations. Fog was
present for an average 12 hours per day during the episode^14^. PM~2.5~
concentrations rose from 35 μg m^-3^ to 80 μg m^-3^ in Fresno during the
three-day episode.

The box model requires emissions of ammonia, nitrogen oxides (NO~x~),
and VOC, which are the precursors of PM nitrate and oxidants. The
California Air Resources Board (CARB) prepared gridded emission inputs
from the IMS95 inventories, which were evaluated by Magliano et al.^2^
The emission files obtained from CARB for a typical weekday in the IMS95
domain contained gaseous species NO~x~, VOC (speciated), NH~3~, SO~2~,
and several particulate species including Na^+^, Cl^-^, SO~4~^=^,
organic carbon (OC), elemental carbon (EC), and a category representing
all other particulate compounds. The diurnal emission profiles of NO~x~
and VOC (point plus area sources) show strong diurnal variations, but
that of NH~3~ is fairly constant throughout the day. Eight classes of
VOC (2 alkanes, 2 aromatics, 4 olefins) were converted to the lumped
CBM-IV structure groups (PAR, TOL, XYL, OLE) and isoprene for use in the
box model. For the box model simulations, the emissions in the modeling
domain were extracted from these files.

Diurnal profiles of temperature, RH, and mixing layer height are needed
to define the meteorological conditions used in the box model
simulation. Temperature and RH are used in both the gas-phase chemistry
and aerosol thermodynamics calculations. The mixing layer height is used
to define the dilution and entrainment characteristics of the modeling
domain. Meteorological data were downloaded from the CARB-maintained
IMS95 data base (http://www.arb.ca.gov/themis). Very humid conditions
were observed during IMS95; the average relative humidity was above 90%
from 7 p.m. to 9 a.m. and minimum relative humidity during the day was
about 65%. Mixing layer heights were determined from the vertical
temperature profiles at four stations (Corcoran, El Nido, Bakersfield,
and Fresno) within the SJV for the January 1996 episode (Ajith Kaduwela,
CARB, personal communication, 1999). Spatially-averaged mixing height
profiles were used in this study to represent typical episode
conditions. Due to limited data, an averaged profile was used for all
days. The mixing layer height ranged from less than 100 m during predawn
hours to about 750 m in the late afternoon.

Observed concentrations were obtained from the IMS95 data base and were
used to drive the box model as initial conditions (Table 1). Note that
the model repartitioned gas-phase and particle-phase species that were
not in equilibrium in the first time step.

Based on Magliano et al.^2^, the ambient NH~3~/NO~x~ ratio compared well
with the emissions inventory values over a 25 km radius of the
monitoring station (i.e., an area of about 2000 km^2^). Therefore, a 44
km x 44 km area around Fresno was chosen for the box model simulations.
Sensitivity simulations with domains of 4 km x 4 km (urban scale) and
216 km x 288 km (entire IMS95 domain) were also performed. These
simulations showed chemical dynamics that were not characteristic of the
ambient conditions in the SJV. Because of high emissions concentrated
over a small area without advection flow, the urban scale simulation
resulted in significant build-up of pollutants, such as NO~x~, VOC, and
PM, and a depletion of NH~3~ within a couple of simulated days, which
was not observed during IMS95. The chemical dynamics of the regional
scale simulation indicated that the oxidant chemistry was too slow
(because of the dilution of emissions over a large area) in rural areas
to account for the observed PM nitrate and oxidant concentrations.

A 3-day simulation was performed for the Fresno domain, based loosely on
the conditions found during the 4-6 January 1996 episode. We assumed
that pollutants are trapped and preserved aloft when the nocturnal
inversion isolates the surface from the aloft layer. Therefore, the
modeled aloft concentrations on each day are equal to the concentrations
of pollutants in the previous afternoon at the time of maximum mixing
height. Aloft concentrations are also required for the first of the
modeled days. Since aloft concentrations were not measured during IMS95,
characteristic aged emissions (the concentrations on the third day of a
simulation without initial conditions) were assigned as the initial set
of aloft concentrations.

Magliano et al.^2^ found significant uncertainties in the emissions
inventory (e.g., underestimation by a factor of 4 for NMOC/NO~x~) based
on comparisons of inventory ratios of VOC/NO~x~, NH~3~/NO~x~, and
PM/NO~x~ to ambient values. Therefore, the emissions of organics were
adjusted to obtain a base case that best matches the ambient
concentrations. In the base case simulation, the organic emissions were
doubled from 2.3 x 10^6^ mole C/day to 4.7 x 10^6^ mole C/day within the
modeling domain in order to produce O~3~ concentrations similar to those
observed in the during IMS95. Such an adjustment is commonplace in air
quality studies^15,16,17^ Total emissions used in the simulations are
listed in Table 1.

###### Sensitivity Simulations. Simulations were conducted to test the changes in PM~2.5~ nitrate concentrations resulting from changes in the emissions of NO~x~ and VOC within the modeling domain. The results are summarized in an isopleth plot (Figure 1). Since the responses of 24-hour average PM nitrate concentrations to reductions in VOC and NO~x~ emissions are very consistent over a range of reduction levels, we only discuss in detail a sensitivity simulation with a 50% reduction in VOC emissions and another one with a 50% NO~x~ reduction.

**RESULTS AND DISCUSSION**

# Base Case Simulation. The first day is treated as a "spin-up" period, to minimize the effects of initial conditions on the results of the simulation. Therefore, only the results of the second and third days are compared against 24-hour average ambient concentrations in Table 2 to ensure that the box model captures the general dynamics of the formation of secondary pollutants. Figure 2 shows the average diurnal profiles of key secondary species, including O~3~, NO~2~, PM nitrate and ammonium. Available observations are also displayed to ensure that the box model provides proper representations of the physical and chemical processes in SJV. The O~3~ concentration peaks at 2 p.m. on both the second and third days, with values of 23 and 27 ppb, respectively (observed peaks of 20 to 35 ppb occurred between 2 and 3 p.m.). The simulated concentrations of O~3~ are within the range of values observed during the January 1996 episode of IMS95.

The NO~x~ concentration profiles do not match the observed
concentrations as well. The ambient data for NO typically display a peak
value (60 -- 100 ppb) between 8 and 9 a.m. and high concentrations (30
-- 50 ppb) throughout the night. A morning maximum concentration of
about 20 ppb is predicted at about 10 a.m., and NO concentrations are
typically low during the night. The diurnal range of observed NO~2~
concentrations is smaller than that of NO concentrations. NO~2~
concentrations fluctuate between 10 and 30 ppb, with a midday minimum
slightly before the time of maximum O~3~. The simulated NO~2~
concentrations reproduced this profile well, although they are typically
10 ppb higher than the observed values. Model predictions of VOC
compared reasonably well with the afternoon VOC samples. However, the
box model was not able to predict the peak morning concentrations. The
simulated concentrations of the primary precursors (NO~x~ and VOC) were
lower than the observations. Since all emissions are well mixed in a box
model, the size of the modeling domain is probably too large to
represent the emission-driven variability in NO~x~ and VOC observations
at the Fresno core site.

The 24-hour average concentrations of PM~2.5~ and major PM~2.5~
components are summarized in Table 2^18^. The underprediction of PM~2.5~
was due to primary emissions. Schauer and Cass^19^ analyzed the source
contributions of PM~2.5~ in Fresno and found that 43% of the observed
PM~2.5~ was primary in origin. In the box model, the contribution of
primary compounds is smaller. Secondary compounds are more regionally
distributed. Therefore, the model provides better estimates of secondary
inorganic components and the base case concentrations of sulfate,
nitrate, and ammonium are quite similar to those measured in Fresno^18^.

The diurnal profiles of predicted particulate ammonium and nitrate, as
well as their precursors, NH~3~ and HNO~3~, are shown in Figure 3. The
concentration of HNO~3~ predicted by the model is much lower than the
IMS95 observations (Table 2). This result is consistent with the
modeling results of Kumar et al.^18^, who alluded to measurement
difficulties for HNO~3~. Some PM nitrate seems to be formed during the
day; however, the accumulation of PM nitrate and ammonium also takes
place in the evening, probably as a result of favorable partitioning of
inorganic nitrate toward the particulate phase (due to colder
temperatures and higher RH), as well as chemical production. The
relatively high concentrations of NH~3~ and the build-up of NH~3~ during
the night (especially early morning) indicate that the formation of
particulate nitrate is limited by the availability of HNO~3~, with a
possible exception at the end of the simulation when NH~3~ is close to
depletion. Since the increase in PM nitrate in the evening exceeds the
available HNO~3~ in the gas phase for partitioning (the day time peak of
HNO~3~ is about 0.3 ppb), we conclude that the chemical production of
nitric acid and PM nitrate is significant in the evening. Two chemical
pathways exist for the production of HNO~3~. The OH pathway takes place
primarily during the day, when OH is more abundant.

OH + NO~2~  HNO~3~ (1)

The NO~3~ and N~2~O~5~ pathways consist of Reactions 2 to 5. Since NO~3~
photolyzes rapidly during the day, these pathways take place primarily
at night.

NO~2~ + O~3~  NO~3~ (2)

NO~3~   HNO~3~ (3)

NO~2~ + NO~3~  N~2~O~5~ (4)

N~2~O~5~ + H~2~O  2 HNO~3~ (5)

Reaction 3 is a heterogeneous reaction that takes place on fog droplets.
Reaction 5 is favored when the RH is high and when lower temperatures
increase the stability of the combination product N~2~O~5~. When fog is
present, a heterogeneous mechanism of Reaction 5 is also viable. The
conclusion that the NO~3~ and N~2~O~5~ pathways play a significant role
in PM production is inferred from the predicted concentrations of the
intermediates N~2~O~5~ and NO~3~ during the evening. The production of
PM nitrate via the N~2~O~5~ pathway ceased later at night when N~2~O~5~
and NO~3~ are depleted, because O~3~, a key ingredient of NO~3~
(Reaction 2) is depleted. Figure 4a shows the relative contributions of
the two chemical pathways and initial and boundary conditions to the
observed PM nitrate. As seen in Figure 4a, excluding initial conditions
and boundary conditions, 80% of the daytime concentration of PM nitrate
is attributed to Reaction 1. While the PM nitrate formed from the
OH+NO~2~ reaction persists into the night, about 50% of the nighttime
nitrate produced in situ is attributed to NO~3~ reactions (Reaction 2 to
5).

The presence of fog at night enhances the production of HNO~3~ (via the
heterogeneous reactions of NO~3~ and N~2~O~5~). However, it also
increases the removal rate of PM due to wet deposition. Compared to a
sensitivity case where fog was not simulated, it was found that the net
effect of fog was the removal of about 10% nitrate over a 24-hour
period. PM nitrate removal as the net effect of fog is consistent with
the fog modeling results of Lillis et al.^14^ Because fog removes HO~2~
radicals via heterogeneous reaction, daytime O~3~ was also reduced when
fog is present because of the reduced production of OH from HO~2~ and
NO~x~ the next morning.

**VOC Emission Reduction.** Figures 5a and 5b show the O~3~-NO~x~
dynamics and the PM and precursor time series, respectively, for the
sensitivity case with a 50% reduction in VOC. The key result is that the
24-hour average PM nitrate concentration is reduced from 16.5 and 22.5
μg/m^3^ on days 2 and 3 in the base case (Table 2) to 13.5 and 14.5
μg/m^3^, respectively. Given that the initial condition is 11.7 μg/m^3^,
the production of secondary PM is greatly reduced when the VOC emissions
are halved. Particulate ammonium, which is associated with particulate
nitrate, is also reduced (24-hour average concentrations are 5.0 and 5.5
μg/m^3^ on days 2 and 3, respectively; down from the base case values of
5.9 and 7.8 μg/m^3^, respectively).

Figure 5b shows that, as in the base case, NH~3~ is abundant in the
system relative to HNO~3~. In fact, the gaseous concentrations of HNO~3~
are lower in this sensitivity simulation than in the base case. The
general features of Figure 5a are quite similar to those of the base
case (Figure 3a). O~3~ concentrations are lower (maximum O~3~
concentrations reduced from 23 and 27 ppb on days 2 and 3 in the base
case to less than 20 ppb in this sensitivity simulation), and NO~x~
concentrations are generally higher, consistent with slower chemical
removal of NO~x~ by oxidation. The concentrations of NO~2~, the
nitrogen-containing reagent in Reactions 1 and 2, are very similar in
the 50% VOC simulation and in the base case. The reduction in PM is
therefore caused by the limited availability of oxidants, OH and O~3~.
Figure 6 compares the concentrations of OH, O~3~, and N~2~O~5~ between
the base case and the 50% VOC reduction case. A 50% reduction of VOC
emissions reduces peak OH and O~3~ concentrations by as much as 20%. The
resulting N~2~O~5~ concentrations are more than proportionately reduced,
and consequently, the rate of HNO~3~ production by this pathway is
considerably reduced. Since the N~2~O~5~ route contributes significantly
to the production of HNO~3~ in the base case, PM nitrate is similarly
reduced. The change in the relative importance of Reaction 1 vs.
Reactions 2 to 5 is reflected in the difference in the PM nitrate
build-up pattern between the base case and the 50% VOC case. The base
case shows a relatively small increase in the nitrate concentrations
following the day-time minimum that resulted from the entrainment of
cleaner air, followed by a substantial increase in PM nitrate after
sunset. In the reduced VOC case, PM nitrate increased gradually from mid
morning to the mid afternoon due to the NO~2~ + OH reaction (see Figure
4b). On the other hand, little nitrate formation takes place at night
because, by sunset, O~3~ has been nearly depleted; therefore, the
N~2~O~5~ pathway for nitrate formation (which depends on O~3~ to form
NO~3~) is negligible in this case. The smaller contribution of the NO~3~
and N~2~O~5~ reactions to the evening concentrations of PM nitrate can
also be seen in Figure 4b. This is a major difference from the base
case.

**NO~x~ Emission Reduction.** The results of the 50% NO~x~ emission
reduction case are presented in Figure 7. Figure 7a shows the dynamics
of O~3~ and NO~x~. As is quite frequently the case with VOC-sensitive
regimes, reducing NO~x~ actually increases the formation of O~3~ because
less NO is available to titrate O~3~. The maximum O~3~ concentrations
are 28 and 38 ppb on days 2 and 3, higher than those observed in the SJV
in the wintertime. The night-time NO~2~ concentrations decreased from 35
to 37 ppb in the base case to 27 and 22 ppb on the first two nights. NO
concentrations are also low, even during the morning rush hour.

Despite the lower concentrations of NO~x~, more PM nitrate is formed, as
shown in Figure 7b. Twenty-four hour average PM nitrate concentrations
rose from 21.3 to 28.6 from day 2 to day 3, a 30% increase over the base
case values. Although this result seems counter-intuitive, it is easily
explained if one considers the dynamics of the VOC-sensitive chemistry.

The NO~2~ concentrations are always higher in the base case than in the
sensitivity case. As shown in Figure 6b, the concentration of OH
radicals during the day is about 23% to 28% higher in the 50% NO~x~
reduction case than in the base case. The increase in the radical
concentration occurs due to increased production from the photolysis of
O~3~. As shown in Figure 7b, daytime formation of HNO~3~ increased
slightly with respect to the base case because the decrease in NO~2~
concentrations (Figure 7a) is compensated by the increase in OH
concentrations (Figure 6b). The concentration of N~2~O~5~ is about 50%
higher in the evening compared to the base case. In the previous
section, we have shown that a decrease in O~3~ results in a more than
proportional reduction in N~2~O~5~. The converse is also true; the
increase in O~3~ in the reduced NO~x~ simulation relative to the base
case triggers a more than proportional increase in N~2~O~5~ in the
evening (Figure 6c). Indeed, significant PM nitrate formation is
observed at night in Figure 4c, indicating the importance of the NO~3~
and N~2~O~5~ pathways in this system. The midnight increase of gaseous
HNO~3~ on the last day follows a depletion of NH~3~, which is converted
to particulate ammonium to neutralize the particulate nitrate. Once
NH~3~ is depleted, the pH of the aqueous particles quickly drops (to 1.2
at the conclusion of the simulation), preventing further partitioning of
HNO~3~ from the gas phase into the particles.

**Photochemical Indicators.** Several photochemical indicators have been
proposed to determine the sensitivity of O~3~ to VOC vs. NO~x~. These
indicators include H~2~O~2~/HNO~3~, NO~y~, HCHO/NO~y~, O~3~/(NO~y~ --
NO~x~) and (NO~y~ -- NO~x~)/NO~y~^4,5^. They represent dominant products
under VOC- or NO~x~-sensitive regimes or ratios of these products, or
chain length in the radical reactions that produce O~3~. For example,
the ratio H~2~O~2~/HNO~3~ represents the competition of the HO~2~
radical termination product (H~2~O~2~ dominant in NO~x~-sensitive
regime) and the OH + NO~2~ termination product (HNO~3~ dominant in
VOC-sensitive regime). Since HNO~3~ partitions between the gas and
particle phases, PM nitrate and HNO~3~ are considered together in the
denominator of the ratio. NO~y~ is the total oxidized nitrogen. (NO~y~
-- NO~x~) is a measure of the amount of NO~x~ oxidation products
(HNO~3~, HNO~2~, PAN, etc.). The ratio of O~3~ and (NO~y~ -- NO~x~) is
an indication of the chain length of the radical reaction, i.e., the
propagation reactions that produce O~3~ vs. the termination reactions
that remove radicals and NO~x~ from the system.

Lu and Chang^5^ proposed numerical thresholds for the above
photochemical indicators to characterize between VOC-sensitive and
NO~x~-sensitive regimes (see Table 3) based on a summertime modeling
study using the three-dimensional model SAQM. There may be slight
variations in the thresholds used to define VOC vs. NO~x~ sensitivity
for a winter vs. a summer simulation, but the underlying chemical trends
should be the same. Our base case simulation shows that the wintertime
O~3~ concentrations should be sensitive to VOC. The simulated indicator
values (Table 3) are different enough from the thresholds that this
conclusion is insensitive to seasonal variations in the thresholds. In
the sensitivity runs, O~3~ also decreases with decreasing VOC (and
increases with decreasing NO~x~). Since both O~3~ and HNO~3~ are formed
from reactions involving radicals (HO~2~ and OH) and NO~x~, this result
was further extended in our simulations to the fact that inorganic
nitrate production (i.e., HNO~3~) is also VOC-sensitive.

Blanchard et al.^3^ determined that there was no ammonia limitation in
the SJV during IMS95. We explore whether our results for the sensitivity
of PM nitrate formation from its precursors, HNO~3~ and NH~3~, are
consistent with the generic analysis conducted by Ansari and Pandis^6^.
Ansari and Pandis defined five variables that govern the inorganic PM
formation system, as shown in Table 4. Using these variables, the
wintertime condition in SJV is characterized by low temperature and high
relative humidity with sufficient free ammonia relative to total
nitrate. According to Ansari and Pandis^6^, wintertime PM concentrations
in the SJV should be very sensitive to a change in HNO~3~ concentrations
but should not be sensitive to NH~3~ concentrations. This result is
consistent with Blanchard et al.^3^ and with our simulation results.

**Implications.** Our box model simulations point to the fact that PM
formation in the SJV during winter is HNO~3~-sensitive, that HNO~3~
formation is oxidant-sensitive, and that oxidant formation is sensitive
to reductions in VOC emissions. In fact, a decrease in NO~x~ emissions
leads to an increase in PM due primarily to an increase in O~3~
concentrations. The conclusion that PM formation is HNO~3~-sensitive is
also obtained if one uses the generic analysis of Ansari and Pandis^6^.
The indicator species of Lu and Chang^5^ also indicate that oxidant
formation is VOC-sensitive.

It should be noted that the box model represents some domain-averaged
chemistry but cannot characterize the locally specific chemical regimes.
Other assumptions include stagnant conditions and aloft carry-over of
gaseous and PM pollutants. Further work should extend this box model
analysis to a three-dimensional modeling study so that transport
processes can be simulated and the spatial variability of the response
of PM to precursors can be addressed. However, an extensive reliable
database is needed for the application of a 3-D model. The forthcoming
California Regional PM Air Quality Study (CRPAQS) database may provide
such an opportunity.

**ACKNOWLEGMENTS**

This work was supported by Pacific Gas & Electric Company
(PG&E)/California Energy Commission (CEC), under PG&E Contract No.
4600008160. The authors thank Dr. Steve Ziman, Chevron Research and
Technology, for suggesting the research project.

**LITERATURE CITED**

1\. Pun, B.K., Seigneur, C., *Atmos. Environ*. **1999**, *33*,
4865-4875.

2\. Magliano, K.L., Hughes, V.M., Chinkin, L.R., Coe, D.L., Haste, T.L.,
Kumar, N., Lurmann, F.W., *Atmos. Environ.,* **1999**, *31*, 4757-4773.

3\. Blanchard, C.L., Roth, P.M. , Tanenbaum, S.J., Ziman, S.D.,
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Table 1. Base case emissions and initial conditions for box model
simulations.

  ----------------------------------- -----------------------------------
  **Chemical Species**                **Emissions**

  NO~x~                               9.3 x 10^5^ mol/day

  NH~3~                               4.3 x 10^5^ mol/day

  SO~2~                               5.0 x 10^5^ mol/day

  VOC                                 2 x 2.3 x 10^6^ molC/day

  Isoprene                            2.2 10^4^ mol/day

  **Chemical Species**                **Initial Concentration**

  NO                                  36 ppb

  NO~2~                               22 ppb

  O~3~                                8 ppb

  NH~3~                               4 ppb

  SO~2~                               1.6 ppb

  CO                                  1.9 ppm

  HNO~3~                              1.2 ppb

  VOC                                 218 ppbC

  PM chloride                         0.32 ppb (0.49 μg/m^3^)

  PM sulfate                          0.54 ppb (2.2 μg/m^3^)

  PM ammonium                         5.5 ppb (4.3 μg/m^3^)

  PM nitrate                          4.4 ppb (11.7 μg/m^3^)
  ----------------------------------- -----------------------------------

Table 2. Daily average measured concentrations in the Fresno Area
January 4-6, 1996 (Source: Kumar et al.^18^) and results of base case
simulation.

  ----------------------- ----------------------- ------------------ -----------
  **Chemical species**    **Average               **Predicted        
                          concentration**         concentrations**   

                                                  **Day 2**          **Day 3**

  O~3~                    7.5 ppb                 7.9                10.3

  NO                      56.4 ppb                7.6                6.5

  NO~2~                   27.0 ppb                30.4               31.3

  HNO~3~                  1.9 ppb                 0.07               0.09

  NH~3~                   6.6 ppb                 8.0                6.0

  VOC ^(1)^               0.46 ppmC               0.29               0.36

  PM nitrate              19.5 μg/m^3^            16.5               22.5

  PM ammonium             6.3 μg/m^3^             5.9                7.8

  PM~2.5~                 55 μg/m^3^              30                 40
  ----------------------- ----------------------- ------------------ -----------

\(1\) Average of morning (6-9 a.m.) and afternoon (3-6 p.m.) samples.

Table 3. Indicator thresholds and San Joaquin Valley simulation results.

  ----------------------- -------------------- ---------------------------
  **Indicator Species**   **Threshold^5^**     **SJV Winter Simulation**

  H~2~O~2~/(HNO~3~ + PM   0.9 ^(1)^            H~2~O~2~/(HNO~3~ + PM
  nitrate)                                     nitrate) always
                                               substantially less than 0.9

  NO~y~                   4.5 ppb ^(2)^        NO~y~ \> 40 ppb at all
                                               times

  HCHO/NO~y~              0.6 ^(1)^            HCHO/NO~y~ is less than
                                               0.1, because of the
                                               abundance of NO~y~

  O~3~/(NO~y~ -- NO~x~)   27.5 ^(1)^           O~3~/(NO~y~ -- NO~x~) \<
                                               3.5 because O~3~ is
                                               typically quite low

  (NO~y~ -- NO~x~)/NO~y~  0.55 ^(1)^           (NO~y~ - NO~x~)/NO~y~ ratio
                                               less than 0.3 throughout
                                               simulation
  ----------------------- -------------------- ---------------------------

\(1\) high values = NO~x~ sensitive; low values = VOC sensitive.

\(2\) high values = VOC sensitive; low values = NO~x~ sensitive.

Table 4. Range of PM sensitivity variables for IMS95 (base case
simulation results).

+-----------------------------------+-----------------------------------+
| #### Sensitivity variables        | **Range of values**               |
+-----------------------------------+-----------------------------------+
| Free ammonia (NH~3~^F^) ^(1)^     | 9 to 20 ppb                       |
+-----------------------------------+-----------------------------------+
| Total nitrate (HNO~3~^T^) ^(2)^   | 3 to 14 ppb                       |
+-----------------------------------+-----------------------------------+
| Gas ratio (GR) = NH~3~^F^/        | 1.1 to 4.5                        |
| HNO~3~^T^                         |                                   |
+-----------------------------------+-----------------------------------+
| Temperature                       | 279 to 289 K (low)                |
+-----------------------------------+-----------------------------------+
| Relative humidity (RH)            | 65 to 95% (high)                  |
+-----------------------------------+-----------------------------------+

\(1\) total ammonia -- 2 x sulfate

\(2\) sum of gas- and particulate-phase inorganic nitrate

####### FIGURE CAPTIONS

Figure 1. Isopleths of 24-hour average PM nitrate (ppb).

Figure 2. Predicted and observed average diurnal profiles of key
secondary species (a) NO~2~ and O~3~ (b) PM amonium and nitrate.

Figure 3. Base case simulation: (a) O~3~, NO, and NO~2~ concentrations,
(b) PM ammonium and nitrate, and gaseous NH~3~ and HNO~3~
concentrations.

Figure 4. Contribution of the OH + NO~2~ reaction, the NO~3~ and
N~2~O~5~ reactions, and initial and top boundary conditions to PM
nitrate for the (a) base case, (b) 50% VOC case, and (c) 50% NO~x~ case.

Figure 5. Sensitivity Simulation, 50% VOC Reduction: (a) O~3~, NO, and
NO~2~ concentrations, (b) PM ammonium and nitrate, gaseous NH~3~ and
HNO~3~ concentrations.

Figure 6. Comparison the base case simulation and the sensitivity cases:
(a) O~3~, (b) OH, and (c) N~2~O~5~.

Figure 7. Sensitivity simulation, 50% NO~x~ reduction: (a) O~3~, NO, and
NO~2~ concentrations, (b) PM ammonium and nitrate, and gaseous NH~3~ and
HNO~3~ concentrations.

Figure 1.

![](media/image1.wmf){width="5.281944444444444in"
height="3.9159722222222224in"}

Figure 2.

![](media/image2.wmf){width="6.1715277777777775in"
height="3.792361111111111in"}

![](media/image3.wmf){width="6.647916666666666in"
height="3.792361111111111in"}

Figure 3.

![](media/image4.wmf){width="5.678472222222222in"
height="6.430555555555555in"}

Figure 4.

![](media/image5.wmf){width="5.399305555555555in"
height="8.111805555555556in"}

Figure 5.

![](media/image6.wmf){width="5.678472222222222in"
height="6.430555555555555in"}

Figure 6.

![](media/image7.wmf){width="5.394444444444445in"
height="7.911111111111111in"}

Figure 7.

![](media/image8.wmf){width="5.678472222222222in"
height="6.430555555555555in"}