3.1. Altitude Profiles
 On several flights, the aircraft performed spiral ascents and descents (altitude profiles) over California and the Pacific Ocean to determine boundary layer heights and examine vertical gradients in tropospheric composition. Climb rates and descent rates were typically 5–10 m/s, so 1-s measurements give a vertical resolution of ≤10 m. During altitude profiles, the vertical resolution of the PILS measurements was limited by the several-minute sampling period to be approximately 1 km, which was not sufficient to fully capture the observed vertical gradients. Hence, when the aircraft encountered rapidly changing ambient conditions during altitude profiles, the bulk aerosol ionic composition measurements are used to indicate qualitatively regions where particulate NO3− and NH4+ were enhanced. Frequently the boundary layer was well mixed, as indicated by CO and O3 mixing ratios that were independent of altitude within the boundary layer. In contrast, HNO3 and fine-particle mixing ratios were often strongly dependent on altitude, where HNO3 was depleted and fine particles were enhanced at higher altitudes within the boundary layer.
 Gas-to-particle conversion occurred in the upper portion of the boundary layer over the San Joaquin Valley. Meteorological parameters, trace gas mixing ratios, and particle mixing ratios are shown in Figure 2 when the aircraft descended from the free troposphere into the boundary layer near Visalia California on 17 May 2002 (Figure 1). The boundary layer height was indicated by the weak (<1°C) inversion in the temperature profile between 1.9 and 2.3 km (Figure 2a). Below the temperature inversion, the temperature lapse rate was nearly adiabatic at −8.9°C/km. The relative humidity (RH) increased with altitude within the boundary layer, but remained below the deliquescence RH [Stelson and Seinfeld, 1982; Mozurkewich, 1993], which varied from 62% to 68%. When the aircraft descended from the free troposphere into the planetary boundary layer, there was a sharp change in trace gas mixing ratios. Above the temperature inversion in the free troposphere, measurements of CO and O3 between 2.3 and 4 km altitude averaged 107 ppbv and 43 ppbv respectively. Below the temperature inversion, the mixing ratios of CO and O3 were nearly constant and were approximately twice as high as the values in the free troposphere, as shown in Figure 2b. O3 measurements (not shown) discussed here were obtained during the ascent from the San Joaquin Valley, since the descent data were compromised by an instrument malfunction.
Figure 2. The 1 s measurements recorded during the descent of the aircraft over the San Joaquin Valley from 4 to 0.6 km altitude on 17 May (Table 1, row 7). (a)Ambient temperature (black) and RH (green) are shown versus aircraft altitude. (b) CO (black) versus aircraft altitude and (c) HNO3 (dark blue), NO3− (light blue), and fine-particle mixing ratios (red). Fine-particle mixing ratios are calculated from fine-particle volume, assuming the particle composition to be ammonium nitrate.
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 HNO3 and fine-particle mixing ratios, in contrast to CO and O3, exhibited large changes not only between the free troposphere and boundary layer but also within and below the temperature inversion (Figure 2c). HNO3 was depleted and fine particles were enhanced at higher altitudes within the boundary layer. During this time, the fine particles were dominated by the accumulation mode with diameters between 0.2 and 0.6 μm, and fine-particle volume was approximately half the coarse-particle volume. Coarse-particle composition was not measured here. Bulk particle composition measurements were not obtained in the upper part of the boundary layer during this altitude profile (Figure 2c), so that NO3− measurements were not coincident with the observed HNO3 depletion and fine-particle enhancement. However, bulk particle composition measurements during other portions of the altitude profile showed that the inorganic fine-particle ionic composition was primarily NH4+, NO3−, and sulfate.
 The strong variations apparent in HNO3 and particle volume were a consequence of gas-to-particle conversion. At the higher altitudes and lower temperatures, the ammonium nitrate equilibrium was driven from the gas phase to the particulate phase, such that gas-phase HNO3 was removed from the air mass and particle mass was enhanced. The connection between HNO3 depletion and particle mass growth is indicated by the strong anticorrelation between HNO3 and fine particles shown in Figure 3. The linear least squares fit to the fine particle versus HNO3 mixing ratio had a correlation coefficient of −0.92 and a slope of −1.12 (Table 1, row 7). If the fine-particle enhancement were a consequence of ammonium nitrate formation from the conversion of gas phase HNO3 and NH3, then each mole of HNO3 removed would result in 1 mole of ammonium nitrate formed, and the fine particle:HNO3 slope would be −1. Within the combined measurement uncertainty, the measured slope is consistent with the assumption that the fine-particle enhancements were caused by ammonium nitrate formation. Mixing processes can also affect the measured slope. Mixing often produced a positive correlation between HNO3 and fine particles (as seen at higher and lower altitudes in Figure 3), since both are secondary pollutants with common sources from polluted urban areas and low values in remote areas and the free troposphere. The preservation of the strong anticorrelation between fine particles and HNO3 and the correlation slope of approximately −1 indicates that the gas-to-particle conversion of NH3 and HNO3 to ammonium nitrate occurred rapidly compared to the mixing of different air masses within the boundary layer.
Figure 3. The 1 Hz measurements of fine-particle versus HNO3 mixing ratios during the descent of the aircraft from 4 to 0.6 km over the San Joaquin Valley on 17 May. The open squares are measurements below 1 km altitude, the open circles are measurements above 2 km, and the solid circles are measurements between 1–2 km. The line is the linear least squares fit to the data between 1–2 km (Table 1, row 7).
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Table 1. Location, Time, and Altitude During Periods When HNO3 and Fine-Particle Mixing Ratios Were Anticorrelateda
|Location (Latitude, Longitude)||Date, Time||Altitude, km||RH, %||Temperature, °C||HNO3, ppbv||Fine Particle, ppbv||CO, ppbv||Fine Particle:HNO3 Correlation|
|Mojave Desert (34.5, 117.0)||25 April, 10:50–11:10 AM||1.7–2.2||60–80||6–10||1.5–3.0||2.6–5.1||176 ± 8||−1.1 ± 0.2||−0.80|
|San Joaquin Valley (37.1, 120.4)||8 May, 3:35–3:40 PM||0.55||22–27||19–20||0.7–2.2||1.2–3.2||161 ± 7||−0.9 ± 0.1||−0.68|
|Rubidoux (34.0, 117.3)||13 May, 12:15–12:23 PM||0.7||10–19||26–29||0.9–7.0||1.5–13||305 ± 63||−1.58 ± 0.06||−0.92|
|Redlands (34.1, 117.2)||13 May, 12:34–12:45 PM||0.8–1.5||11–14||22–29||1.3–5.3||1.9–12||292 ± 59||−2.5 ± 0.2||−0.88|
|San Bernardino Mountains (34.1, 116.9)||13 May, 2:49–2:54 PM||3.7||30–43||3–5||0–1.4||1.2–35||215 ± 51||−10 ± 1||−0.72|
|Mojave Desert (33.9, 116.5)||13 May, 3:02–3:14 PM||1.9–3.5||11–25||5–20||0.6–6||1.0–3.9||175 ± 13||−0.37 ± 0.03||−0.78|
|San Joaquin Valley (36.4, 119.1)||17 May, 4:20–4:40 PM||1–2||35–68||14–20||0.2–5.0||3–9||210 ± 14||−1.12 ± 0.09||−0.92|
 The conversion of gas phase HNO3 to fine-particle mass was observed on several other altitude profiles in other locations, when ambient conditions differed considerably from the San Joaquin Valley (Table 1). Similar vertical gradients in HNO3 mixing ratios and particle concentrations were observed in altitude profiles over the Mojave Desert east of Los Angeles. On 13 May, a spiral profile was performed east of Palm Springs. From 0.2 km to the top of the boundary layer at 3.6 km (Figure 4), HNO3 decreased dramatically and fine particles increased at higher altitudes. Ammonium nitrate formation was indicated by enhancements of fine-particulate NO3− and NH4+ of ∼0.8 ppbv that was coincident with the HNO3 depletion and increase in fine particles. During this time, the sulfate concentration was unchanged at approximately 0.2 ppbv. The fine-particle volume was approximately 15% greater than the coarse-particle volume, and fine particles with diameters between 0.2–0.6 μm dominated the fine-particle mass.
Figure 4. The same as Figure 2, but for the spiral ascent from 0.2 to 4.1 km altitude over the Mojave Desert on 13 May (Table 1, row 6). Additionally, O3 is shown in pink on Figure 4b. Gaps in the HNO3 data occur during instrument background measurements, which were performed approximately once every 20 min, and calibrations, which were performed once every 45 min.
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 The boundary layer height was indicated by the weak (<1°C) inversion in the temperature profile that began at approximately 3.6 km (Figure 4a). The RH increased with altitude within the boundary layer, but remained well below the deliquescence RH, which increased from ∼58% at 0.2 km (34°C ambient temperature) to 75% at 4 km (1°C ambient temperature). Below the temperature inversion, the temperature lapse rate was approximately adiabatic at −9.7°C/km. When the aircraft entered the free troposphere, there was a sharp change in trace gas mixing ratios. In the free troposphere above 3.9 km altitude, O3 mixing ratios were 42 ± 2 ppbv, and CO mixing ratios were 89 ± 4 ppbv. Between 0.2–3.6 km, O3 and CO were nearly constant and were approximately twice as high as the values in the free troposphere, as shown in Figure 4b.
 From 0.2–3.6 km above the Mojave Desert, large vertical gradients in fine-particle mass and HNO3 were caused by gas-to-particle conversion. Neither O3 nor CO mixing ratios (Figure 4b) exhibited the large vertical variations that were apparent in the HNO3 and fine-particle mixing ratios (Figure 4c). HNO3 and fine particles were anticorrelated between 1.9 and 3.5 km, whereas they were positively correlated at lower altitudes in the boundary layer and at higher altitudes in the free troposphere. In this example, HNO3 and fine particles were less strongly anticorrelated (r = −0.78), and the linear least squares fit to the fine particle to HNO3 relationship had a slope of −0.37. In this case, mixing of differing air masses may have reduced both the anticorrelation and the slope of the fine particle to HNO3 relationship. NO3− was enhanced when HNO3 was depleted, though the 0.8 ppbv NO3− enhancement was much smaller than the ∼2 ppbv depletion of HNO3. Since coarse-particle composition was not measured, it is not possible to exclude the possibility that some nitrate growth occurred in the coarse mode. The gas-to-particle conversion over the desert occurred at considerably lower RH, higher altitude, and lower ambient temperature (Table 1, row 6) compared to the San Joaquin Valley (Table 1, row 7). Similarly, gas-to-particle conversion during the altitude profile over the Mojave Desert on April 25 (Table 1, row 1) occurred at low temperatures, although the RH was much greater than the example described above. The conversion of gas phase HNO3 to ammonium nitrate was observed over a range of locations, ambient temperatures, and relative humidities.
3.2. Los Angeles Basin
 Gas-to-particle conversion was also observed at lower altitudes in the boundary layer, especially in air masses located downwind from large NH3 emissions. The flight in the Los Angeles Basin on 13 May 2002 examined both urban and agricultural source regions of pollutants and locations at the periphery of the basin where pollutants were likely to be transported [McElroy and Smith, 1986]. In the Los Angeles Basin, a sea breeze and coastal temperature inversion often determine the distribution of pollutants. A strong temperature inversion (∼7°C) [Schultz and Warner, 1982] caused by the cooling of air over the Pacific Ocean often exists near the coast, and an afternoon sea breeze regularly carries pollutants emitted in the urban area eastward. As air is advected inland, it is warmed such that the temperature inversion weakens and the boundary layer grows. The transport of pollutants from the Los Angeles Basin is often inhibited by the temperature inversion and the surrounding San Gabriel and San Bernardino Mountains. When the mixed layer stabilizes in the late afternoon, layers of pollutants can remain aloft above the boundary layer and within the temperature inversion. During sea breeze conditions, ventilation occurs when an air mass is lifted over the mountains at the eastern (downwind) end of the basin [Lu and Turco, 1995].
 Typical sea breeze conditions existed during the study period. The aircraft entered the Los Angeles basin at approximately 12:00 PM local standard time (LST) and departed at approximately 4:00 PM LST. The prevailing winds were from the west at 5 ± 2 m/s. The aircraft flew at an altitude of approximately 700 m over the valley regions and up to 4 km during spiral altitude profiles and over the San Gabriel and San Bernardino Mountains. The flight track and locations discussed in the text are shown in Figure 5. During the flight, HNO3, NOx, NOy, O3, and CO mixing ratios were elevated downwind of the Los Angeles urban area (the loop in the flight track at 34.0 N, −118.0 W) where mixing ratios reached 15, 47, 67, 93, and 700 ppbv, respectively. On this clear late spring afternoon with steady winds from the west, a small fraction of the emitted NOx was oxidized, and most of the NOx oxidation produced HNO3. The highest O3 (113 ppbv) and HNO3 (17 ppbv) levels were observed downwind from the Los Angeles urban area near Ontario. While slightly more photochemical processing of emissions had occurred at this inland location compared to locations closer to the urban center, the ratios of CO/NOy and NOx/NOy did not vary dramatically across the basin. Throughout the basin, the NOx:NOy correlation slope was approximately 0.7, the HNO3:NOy correlation slope was approximately 0.2, and O3 mixing ratios were generally 80 ± 10 ppbv. The ratios of these trace gases were relatively constant across the basin because they were all strongly influenced by the same tailpipe area source.
Figure 5. The flight track of the NOAA P-3 aircraft, color-coded according to aircraft altitude, over the Los Angeles Basin on 13 May 2002. The dark lines are the California coast, and the gray lines are roads. The circles, which indicate the center of the 2 min 40 s period when bulk fine-particle composition measurements were obtained, are sized according to the fraction of nitrate in the particle phase. The winds were from the west at 5 ± 2 m/s. The dotted line separates the basin into western and eastern parts. The numbers in boxes are the times (local standard time) that the aircraft was at the indicated location along the flight track.
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 In contrast to the trace gases, particle concentrations exhibited substantial variability within the Los Angeles Basin. Figure 6 shows fine-particle mixing ratio versus CO for the flight track shown in Figure 5. In the western part of the Los Angeles Basin, fine particles and CO were positively correlated, whereas fine particles and CO were often uncorrelated in the eastern part of the Los Angeles Basin. This indicates that many fine particles in the eastern Los Angeles Basin had a source different from the source of CO. Additionally, NO3− was especially enhanced in the eastern part of the basin, and the highest NO3− was 5.0 ppbv (12.7 μg m−3) inland near Rubidoux. Nitrate was the most abundant of the measured water-soluble inorganic fine-aerosol species, and enhancements in NH4+ were coincident with the NO3− enhancements. Fine-sulfate aerosol was always less than 0.2 ppbv (1 μg m−3) over the Los Angeles Basin and much less than NO3− or NH4+ in the polluted areas.
Figure 6. Fine-particle mixing ratio versus CO, measured in the eastern Los Angeles Basin (red circles) and in the western Los Angeles Basin (open squares).
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 The gas-to-particle conversion processes that caused the elevated particle layers observed during the altitude profiles also contributed to the particle mass enhancements in the eastern Los Angeles Basin. The circles along the flight track shown in Figure 5 indicate NO3− measurements and are sized according to the fraction of nitrate in the particle phase relative to the total nitrate [NO3−/(NO3− + HNO3)]. In the western part of the Los Angeles Basin, most of the total nitrate was in the gas phase as HNO3, whereas most of the total nitrate was in the particle phase as NO3− in many areas in the eastern part of the Basin. Downwind of the regions of agricultural NH3 emissions near Rubidoux in the eastern part of the Los Angeles Basin [Russell and Cass, 1986], ammonium nitrate was formed from the reaction of gas phase HNO3 with NH3.
 The effects of NH3 emission upon the conversion of gas phase HNO3 to particulate ammonium nitrate in the eastern Los Angeles Basin is evident in the segment of flight shown in Figure 7. Substantial enhancements in fine particles, NH4+, and NO3−, and the depletion of HNO3, occurred after the aircraft flew downwind of the livestock operations located in the Rubidoux area. O3 was nearly constant at 80 ± 2 ppbv, while CO varied considerably between 150–450 ppbv. Neither CO nor O3 were correlated with the enhancements in particles or the depletion of HNO3. Fine-particulate magnesium, chloride, nitrite, potassium, and sodium were <0.05 μg m−3, and calcium was measured to be 0.15 μg m−3, indicating a negligible contribution of mineral and sea-salt particles to the fine-particle mass. Most of the fine-particle mass was in the accumulation mode with diameters between 0.2 and 0.6 μm. Coarse-particle volume was approximately 20% larger than the fine-particle volume.
Figure 7. The 1 s measurements of HNO3 (blue) and fine-particle mixing ratio (red) obtained near Rubidoux in the eastern Los Angeles Basin on 13 May 2002. Fine-particulate NO3− (light blue), NH4+ (pink), and sulfate (yellow) concentrations are shown as bars, where the length indicates the 2 min 40 s averaging time for each sample. CO mixing ratios (right axis) are shown in gray. The aircraft was flying at 0.7 km altitude in a NE direction (from approximately 33.0 N, −118.0 W to 34 N, −117 W) traveling from Fullerton to Redlands (Table 1, row 3).
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 Ammonium nitrate formation is demonstrated by comparing the magnitude of the HNO3 depletion to the NO3− and NH4+ enhancement. If ammonium nitrate were formed from HNO3 depletion, the molar enhancements of NO3− and NH4+ would be equivalent to the reduction of gas phase HNO3. When HNO3 values decreased from 6 ppbv to 1.5 ppbv near Rubidoux, the NO3− enhancement was 4.8 ppbv. Within the combined uncertainties of the measurements, the HNO3 depletion was equivalent to the enhancement in fine-particulate NO3−. The NH4+ molar enhancements were 17 % larger than the NO3− enhancements (Figure 7). Within the 20% inaccuracy of the bulk particle composition measurements, these results are consistent with fine-particulate ammonium nitrate formation. Although coarse-particle composition was not measured here, the bulk fine-particle composition measurements demonstrated that the conversion of HNO3 to ammonium nitrate occurred mostly on fine particles here.
 Particle growth is studied further by examining the relationship between fine particles and HNO3 (Figure 8). When NO3− was enhanced, the linear least squares fit to the fine particle versus HNO3 data had a correlation coefficient of −0.92 and a slope of −1.58. The measured enhancement in fine particles was greater than can be accounted for solely from ammonium nitrate formation. Similarly, in the eastern Los Angeles Basin near Redlands (Table 1, row 4), the correlation slope between fine particles and HNO3 was significantly steeper than −1. Secondary organic aerosol formation can contribute to the fine-particle mixing ratio to produce this difference. Organic material has been shown to account for a substantial fraction of fine-particle mass in this area [e.g., Chow et al., 1994].
Figure 8. The 1 s measurements of fine-particle versus HNO3 mixing ratios for the section of flight shown in Figure 7. The open circles are measurements between 12:10–12:15 PM LST and 12:22–12:30 PM LST, and the solid circles are measurements between 12:15–12:22 PM LST. The solid line is a linear least squares fit to the solid circles (Table 1, row 3).
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 Although gas phase NH3 was not measured during this study, NH3 concentration can be calculated when ammonium nitrate particles were detected. During the measurements near Rubidoux (Table 1, row 3), the RH was considerably lower than the 60% deliquescence relative humidity. Using the measured gas phase HNO3 mixing ratio (1 ppbv) that existed with the ammonium nitrate aerosol and the dissociation constant of 60 ppbv2 at 28°C, gas phase NH3 is calculated to be approximately 60 ppbv here, consistent with previous reports of large NH3 emissions and concentrations in this region of the Los Angeles Basin [Russell and Cass, 1986].
 Gas-to-particle conversion was observed over a wide range of ambient conditions at several locations in the Los Angeles Basin. For example, at higher altitudes and lower temperatures, nearly all HNO3 was removed from an air mass when the aircraft flew over the San Bernardino Mountains from Cajon Pass to Palm Springs (Table 1, row 5). HNO3 decreased from 2 ppbv to below 20 pptv. HNO3 levels were below instrument detection limits, and over 99% of the HNO3 was removed from this air mass. High NO3− levels (>2 ppbv) coincident with low HNO3 levels (<20 pptv) demonstrated that the HNO3 instrument discriminated against aerosol NO3− by >99%. The variability in trace gas and particle mixing ratios in this region make a quantitative analysis of the observed gas-to-particle conversion less precise. The HNO3 depletion varied from 2–5 ppbv, and the NO3− enhancement varied from 1.7–3.4 ppbv, in qualitative agreement with the measured HNO3 depletion. Although the RH was high compared to the low altitude measurements near Rubidoux, it was still well below the 74% deliquescence relative humidity. The dissociation constant at these lower temperatures was 0.1 ppbv2, and nearly all NO3− was in the particle phase. The fine-particle volume was approximately twice as large as the coarse-particle volume, so that fine particles accounted for most of the particle mass. The anticorrelation between fine particles and HNO3 had a slope of −10. This suggests that the fine particles measured here include substantial contributions from other species, and are less likely to be pure ammonium nitrate particles compared to the particles sampled near Rubidoux.
3.3. San Joaquin Valley
 Spatial variability in gas-to-particle conversion was observed on 8 May 2002 during level flight at 0.55 km in the boundary layer over the Pacific Ocean, San Francisco Bay area, and northern San Joaquin Valley (Figure 1). In the San Joaquin Valley, winds were from 20° west of north at 6 ± 1 m s−1. The measurements over the San Joaquin Valley were enhanced in aerosol NO3− and NH4+ and fine particles compared to the measurements over the ocean and San Francisco. Over the ocean NO3− was <0.07 ppbv (0.2 μg m−3), over San Francisco NO3− was <0.12 ppbv (0.3 μg m−3), while NO3− levels exceeded 3.3 ppbv (1.3 μg m−3) over the San Joaquin Valley from Stockton to Merced. Sulfate levels over the San Joaquin Valley and San Francisco urban area were ∼0.1 ppbv (0.4 μg m−3) and were comparable to the levels measured over the Los Angeles Basin. HNO3 levels were higher over San Francisco (1.5–4 ppbv) than over the San Joaquin Valley (1–2 ppbv). Over San Francisco, the fine- and coarse-particle volumes were approximately equivalent, whereas the coarse-particle volume was approximately four times the fine-particle volume over the San Joaquin Valley. These results are consistent with previous measurements in the summertime in rural areas in the San Joaquin Valley, where coarse particles containing crustal materials have been shown to sometimes account for the majority of the particle mass [Chow et al., 1996].
 Although the HNO3, NO3−, and fine-particle mixing ratios were approximately an order of magnitude lower over the San Joaquin Valley than measured over the Los Angeles Basin, and fine particles did not account for the majority of the particle mass, the same gas-to-particle conversion process was observed. When fine-particulate NO3− was substantially enhanced, HNO3 was anticorrelated with fine particles with a slope of −0.9 (Table 1, row 2), and NH4+ was also enhanced. This result supports the assumption that the enhancement in fine particles over the San Joaquin Valley was from ammonium nitrate. Furthermore, gas-to-particle conversion was not observed over the Pacific Ocean or San Francisco, but only over the San Joaquin Valley, which is influenced by agricultural NH3 emissions.
3.4. Single Particle Measurements
 Single particle measurements from a laser ionization mass spectrometer (PALMS) are used to examine the abundance of organic aerosol relative to ammonium nitrate during the measurements in the boundary layer over California. The ion signal at mass 30 corresponded to NO+ resulting from the laser ionization of particles containing NH4+ or NO3− [Murphy and Thomson, 1997]. Laser ionization of organic particulate matter caused a signal in positive mass spectra at mass 12 from ionized carbon atoms (C+). Relative contributions from organic matter and ammonium nitrate are assessed qualitatively by examining the fraction of the total ion current from C+ and NO+, respectively. During most of the flight over the Los Angeles Basin on 13 May the signal from C+ accounted for a larger fraction of the ion signal than the signal from NO+. Only downwind from regions of large NH3 emissions was the ammonium nitrate signal (NO+) dominant over the organic aerosol signal (C+). Additionally, in the eastern Los Angeles Basin where ammonium nitrate formation was large, the absolute C+ signal was enhanced relative to the western Los Angeles Basin, where most of the nitrate was in the gas phase. This enhancement of the absolute C+ signal downwind from NH3 emissions is consistent with the formation of organic aerosols from organic acids (formic and acetic acids, for example).
 Large variations in aerosol composition apparent in the single particle measurements coincided with the large variations in HNO3 and fine-particle mixing ratios. Figure 9 shows the average fraction of the total ion current from NO+ (ammonium nitrate) and C+ (organics) as a function of fine-particle mixing ratio for periods when gas-to-particle conversion was observed (Table 1). During these periods when HNO3 was depleted and NO3−, NH4+, and fine-particle mass were enhanced, the average fraction of the total ion current from NO+ was positively correlated with the fine-particle mixing ratio (Figure 9a). At the highest fine-particle values (and lowest HNO3), NO+ accounted for the majority (∼80%) of the ion current, consistent with particles that contained primarily ammonium nitrate. The relative contribution from organics to the ion signal (Figure 9b) decreased at high fine-particle mixing ratios, but it did not drop to zero. Organic material always contributed to the particle mass. At the lowest fine-particle mixing ratios, when HNO3 depletion was small, the signal from particulate organics dominated that from ammonium nitrate (Figure 9b). These results are consistent with the presence of organic aerosols that could cause fine particle:HNO3 slopes to be steeper than −1. However, the range of measured slopes (Table 1) is not explained by the differences in particle composition between locations measured by the PALMS instrument.
Figure 9. Average fraction of total ion current from (a) NO+ and (b) C+ from laser-ionized single particles, as a function of fine-particle mixing ratio. Each point is an average of 30 particles over a period of approximately 6 s. Table 1 gives the times and locations for these measurements near Rubidoux (red circles, Table 1, row 3), San Bernardino Mountains (black bowties, Table 1, row 5), Mojave Desert (green squares, Table 1, row 6), and San Joaquin Valley (blue triangles, Table 1, row 7).
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 These results agree qualitatively with previous studies that have shown substantial contributions from secondary organic aerosols in addition to ammonium nitrate to the fine-particle mass near Riverside California in the eastern part of the Los Angeles Basin [Chow et al., 1994; Kleeman et al., 1999; Liu et al., 2000]. Although the particle mass measured here in the spring is lower than was reported in summer, fall, and winter measurements cited above, the particle composition appears to be consistent.
 The particle composition over the San Joaquin Valley on 17 May differed from the particle composition observed over the Mojave Desert and Los Angeles Basin. Over the San Joaquin Valley, the fraction of the ion current from C+ was larger and that from NO+ was smaller, indicating that organic carbon accounted for a larger fraction of the particle mass. Differences in particle size distributions between the measurements in the San Joaquin Valley and other locations were also apparent. Coarse particles accounted for the largest fraction of the particle mass over the San Joaquin Valley (discussed above). These results agree with previous measurements that have shown that large contributions to particle mass come from both secondary organic aerosol and coarse particles containing crustal materials in the San Joaquin Valley [Chow et al., 1996].