4.1. Spatial WSOC Distribution
 Figure 2 shows the cumulative spatial distribution of WSOC concentration measured on the Twin Otter during May 2010. Average levels in specific portions of the region are shown in Figure 3a. WSOC was fairly well-correlated with particle number concentration (r2 = 0.49; slope = 0.85 ng C m−3/cm−3; n = 34655) and volume concentration (r2 = 0.53; slope = 0.12 μg C m−3/(μm3 cm−3); n = 33975), as measured by the PCASP. The highest WSOC concentrations were consistently observed in the Los Angeles Basin, as compared to desert outflow regions and the San Joaquin Valley, reaching values as high as 5.3 μg C m−3. High concentrations were often observed along the north-south corridor extending between Pasadena and the downtown Los Angeles area, coincident with the highest levels of BC, CO, and m/z 57 (>0.3 μg m−3, >0.35 ppmv, >80 ng m−3, respectively). Comparable or higher concentrations were also observed during some flights on the eastern side of the Basin near San Bernardino and Redlands. The lowest concentrations of WSOC in the Los Angeles Basin were usually observed near Torrance when the dominant wind direction was westerly from the ocean (i.e., upwind of major emissions sources in Long Beach), and to the southeast/east of the Puente and Chino Hills, which serve as a barrier to the transport of pollution from the western side of the Basin. Outflow regions were characterized by lower WSOC levels than those observed in the Basin mainly owing to dilution during urban plume transport and a lack of significant WSOC sources in the desert. The general west-to-east transect of highest WSOC levels coincided with the average westerly/southwesterly wind patterns during the period of flights, with increases from the western side of the Basin near downtown Los Angeles toward maximum levels near Whittier, then decreasing to the northeast near San Bernardino. Smoke plumes were occasionally intercepted in the Los Angeles Basin, usually near the eastern side in the vicinity of Riverside, owing to small-scale fires. These plumes resulted in the highest WSOC concentrations during the respective flights, with one being the highest of the campaign (maximum levels observed in separate fire plumes: 1.7 μg C m−3, Flight 6; 5.3 μg C m−3, Flight 8; 1.8 μg C m−3, Flight 12; 1.6 μg C m−3, Flight 18). Chemical ratios that can be used to trace secondary production (e.g., WSOC:CO; WSOC:BC) become perturbed in these cases owing to primary production (i.e., direct emission) of WSOC in these fresh plumes.
Figure 2. (left) Spatial distribution of WSOC concentrations during the CalNex campaign in the San Joaquin Valley, the Los Angeles Basin, and outflow regions extending north through the Cajon Pass and east through the Banning Pass toward Indio and the Salton Sea. (right) Close-up of WSOC in the Los Angeles Basin.
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Figure 3. Cumulative spatial averages of (a) WSOC, (b) surface O3 concentration, (c) WSOC:BC, (d) WSOC:PCASP volume concentration, (e) WSOC:nitrate, (f) WSOC:sulfate, (g) WSOC:organic, and (h) ambient temperature and RH. Numbers in boxes correspond to true values that exceed the range shown in color bars. Sizes of the boxes correspond to the geographic area within which the data were averaged. City labels are included in Figure 3a.
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 A number of flights focused on Banning Pass, owing to the role of this area as an exit for the Basin pollution. Table 2 shows a summary of all the WSOC measurements in this area with relevant meteorological information. Out of eleven flight legs through this outflow-pass, nine were characterized by westerly winds and higher WSOC levels than periods with easterly winds. For example, during Flight 4 the first transect was characterized by easterly winds and lower concentrations of WSOC and particle concentration. Approximately two hours later, winds shifted to be westerly resulting in enhancements in WSOC (0.37 to 0.50 μg C m−3) and fine particle number concentration (2480 to 7720 cm−3). Of the other parameters shown in Table 2, WSOC was best correlated with RH and PCASP particle volume concentration (r2 = 0.79 for both).
4.2. Vertical WSOC Distribution
 The airborne measurements were usually conducted below an altitude of 1 km in the Los Angeles Basin and San Joaquin Valley and at higher altitudes through the outflow-passes (Figure 4a). On 11 of the 16 flights, WSOC exhibited its highest concentration above 500 m, usually near the eastern end of the Basin by Riverside, San Bernardino, and Banning Pass. Two representative flights demonstrating this behavior are shown in Figures 4b and 4c, where WSOC peaks between 600 and 1100 m in altitude near Banning Pass and Riverside. The maximum concentration during Flight 8 (5.3 μg C m−3, 14 May 2010) corresponded to a Riverside fire plume and was the highest Twin Otter WSOC measurement of the entire CalNex field study. A cluster of points in Figure 4a (color-coded as blue and yellow-orange) are relatively lower in WSOC concentration than the rest below 1 km as they are from the San Joaquin Valley and outflow areas.
Figure 4. (a) Vertical profile of WSOC mass concentrations during 16 flights over the spatial domain shown in Figure 1. The majority of the data collected below 1 km are from the Los Angeles Basin, with the minority of the yellow-red and blue markers corresponding to San Joaquin Valley and outflow areas to the east of the Basin, respectively. The majority of the markers near 1 km correspond to transits through outflow-passes (i.e., Banning Pass and Cajon Pass), while higher altitudes correspond to transits north of the Los Angeles Basin. (b and c) Examples of vertical profiles during two flights in the Los Angeles Basin showing that the highest WSOC levels were observed above the first few hundred meters. The maximum WSOC level during Flight 8 corresponded to a local fire plume, while the peak WSOC level in Flight 13 was observed over the eastern edge of the Basin near Banning Pass.
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 Multiple pollution layers have been observed aloft in this region in past airborne measurements [Blumenthal et al., 1978; McElroy and Smith, 1986; Collins et al., 2000]. These layers arise by horizontal and vertical displacement of the morning inversion layer and orographic uplift [Lu and Turco, 1995]. It is possible that such layers, which can undergo continued chemical processing while separated from the mixing layer, contribute to surface concentrations through turbulent mixing as the boundary layer deepens [Husar et al., 1977; Blumenthal et al., 1978]. Vertical profiles of WSOC were obtained during two flights up to an altitude of approximately 3.2 km (Figure 5). The ascents and descents occurred at the northern edge of the Basin. These profiles offer a direct comparison of WSOC separated by 2–4 h of aging time. The ascents out of Los Angeles at the beginning of the flights (Figures 5a and 5c) were characterized by a decrease in WSOC with altitude until a point where a series of vertical layers became evident with enhanced WSOC levels and PCASP particle number concentrations. The descents into the Basin (Figures 5b and 5d) more than two hours later exhibited systematically larger WSOC and PCASP number concentrations in the bottom 1.5 km above the surface with similar layers of WSOC enrichment at higher altitudes. This is likely due to continuous emissions during the daytime and higher photochemical activity occurring to generate more WSOC.
Figure 5. Vertical profiles of WSOC during two flights: (a and b) Flight 10 on 18 May 2010 and (c and d) Flight 12 on 20 May 2010. Colored markers correspond to WSOC and dashed black lines represent ambient temperature. Local time = UTC – seven hours.
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 To identify the level of vertical WSOC variation in the mixing layer, simultaneous ground and airborne measurements in the same vertical column are compared at Pasadena (Figure 6a) and Riverside (Figure 6b). A total of 13 and 12 simultaneous measurements were conducted in Pasadena and Riverside, respectively, between 11:00–15:30 (LT). The airborne measurements during the overpasses were conducted within the mixing layer at altitudes between 500–800 m. At both sites, WSOC levels were generally higher aloft, and the difference is more evident in Pasadena. The greatest enhancements aloft relative to ground measurements at both sites were coincident with the highest ratios of RH aloft relative to at the surface, which is most evident at Riverside. The average RHs during the overpasses at Pasadena and Riverside were 67 ± 10% (max = 78%) and 52 ± 9% (max = 65%), respectively. This may be indicative of production of WSOC by processes that are more efficient as a function of increasing aerosol-phase water. High RHs (>70%) have been shown to coincide with enhanced particulate WSOC and organic acid concentrations owing to more effective partitioning of these species to the aerosol phase and multiphase chemistry [e.g., Hennigan et al., 2008b, 2009; Ervens and Volkamer, 2010, Sorooshian et al., 2010]. In addition, direct photolytic processing has also been suggested to be an important mechanism to generate SOA in particles residing in humid air [Bateman et al., 2011].
Figure 6. Comparison of simultaneous ground and airborne WSOC measurements in the same vertical column at (a) Pasadena and (b) Riverside.
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 Vertical profiles of ratios of WSOC to PCASP particle volume concentration and AMS organic mass are shown in Figure 7. The range of the average WSOC:particle volume concentration ratio (Figure 7a) was between 0.11 and 0.19 between the surface and 3.2 km, while the WSOC:organic ratio (Figure 7b) ranged widely between 0.26 and 0.63. Both ratios exhibit variable behavior as a function of altitude, with an average reduction from the surface to ∼800 m, and with maximum values occurring at altitudes exceeding 1500 m. The large variability in these ratios is largely a result of day-to-day variability; however, the absolute range of the ratios is indicative of WSOC being a significant component of the aerosol from the surface up to 3.2 km.
Figure 7. Vertical distribution of (a) the ratio of WSOC to PCASP particle volume concentration (data from all flights) and (b) the ratio of WSOC to total AMS non-refractory organic aerosol mass (Flight 11, 13, 16–18). Numbers beside each point in each panel correspond to the sample size.
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4.3. WSOC Ratios to Other Aerosol Mass and Volume Measurements
 To gain more insight into the relative importance of WSOC to the total aerosol budget and the sources and sinks of WSOC, the ratios of WSOC to other aerosol parameters are examined in Figures 3 and 8. Despite the existence of vertical gradients in aerosol composition and mass, it is useful to constrain airborne-measured WSOC with simultaneous ground-based PM2.5 measurements (PM2.5 was not quantified on the Twin Otter). For the limited sites at which PM2.5 measurements were available within the Basin, WSOC contributed typically between 6 and 11% to PM2.5 mass. Note that converting WSOC to an organic mass equivalent concentration requires a conversion factor, which was previously assumed to be ∼1.8 for the region [Docherty et al., 2008]. The ratio of WSOC to PM2.5 was highest in the western portion of the Los Angeles Basin near Long Beach, downtown Los Angeles, and Glendora (0.10–0.11) and was lowest near Banning Pass (∼0.06) owing partly to the major enhancement in ammonium nitrate levels near the eastern side of the Basin. The closest measure of how WSOC contributes to total PM2.5 using aircraft measurements is the comparison to PCASP volume concentration, where the PCASP measures particles in a similar size range as the PILS-TOC (<2.5 μm). The WSOC:PCASP volume ratio (Figure 3d) was typically between ∼0.1–0.15 μg C m−3/(μm3 cm−3) in the Los Angeles Basin, while higher average values were observed in the outflow regions. Ratios reached an average of 0.30 ± 0.14 near the Salton Sea to the east of the Basin and 0.21 ± 0.12 near Barstow to the northeast of the Basin. As a basis for comparison, the ratio of WSOC to fine particle volume concentration was 0.12 and 0.10–0.22 μg C m−3/(μm3 cm−3) in non-biomass and biomass burning plumes, respectively, in the northeastern United States [Sullivan et al., 2006; Peltier et al., 2007b]. An increase in this ratio can be due to a variety of reasons including losses in other aerosol components (e.g., nitrate volatilization), increased production of WSOC during transport, local sources such as fires, or entrained air masses from aloft that are enriched with WSOC relative to other aerosol components.
Figure 8. Ratio of m/z 44:WSOC as a function of m/z 44:43. Markers are color-coded with longitude where red corresponds to western side of the Los Angeles Basin and purple corresponds to outflow desert regions to the east of the Basin.
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 Examining the variation of relative concentrations of WSOC and other particle and gas-phase constituents along the sea breeze trajectory from west to east provides insight into the relative importance of volatilization and secondary formation of WSOC (Figure 3). The highest O3 levels were observed farther inland and were highest in the outflows to the east of the Basin (Figure 3b). This is indicative of increased photochemical processing during sea breeze transport of air masses from the coast to the desert. This would likewise favor secondary production of WSOC. Black carbon is a primary species and thus an increase in the WSOC:BC ratio is a tracer for secondary production. This ratio was highest near the southwestern portion of the study region, presumably owing to the low levels of BC in marine air and upwind of major BC sources in the Basin (Figure 3c). This ratio did not show any clear trend as a function of downwind distance, likely a result of the abundance of BC and organic aerosol sources in the Basin, including the fires observed on the eastern side of the Basin. Unlike the majority of the Basin, there are fewer BC sources along trajectories to the east of the outflow channels; WSOC:BC initially decreases and then increases owing possibly to a combination of WSOC volatilization initially and secondary production afterwards.
 The contribution of WSOC to total non-refractory organic mass was highest near the southwestern portion of the Los Angeles Basin (up to ∼60% just north of Santa Ana) and lowest in the desert outflow regions (<40%) (Figure 3g). Along the sea breeze trajectory on the northern side of the Basin, the WSOC:Organic ratio tended to increase toward Banning Pass reaching an average value of 53 ± 34%. Recent studies have utilized m/z 44 and m/z 43 [e.g., Ng et al., 2010] to track the aging of organic aerosol in the atmosphere. The ratio of m/z 44:43 provides information about the chemical functionality of oxygenated organic aerosol (OOA), with higher values indicative of a higher state of oxidation. Wonaschütz et al.  examined the ratio of m/z 44:WSOC as a function of m/z 44:43 during the PACO campaign as a way of understanding how the composition of the WSOC fraction of the aerosol changed with photochemical age. Aircraft measurements in Figure 8 show that the ratio of m/z 44:WSOC increased as a function of m/z 44:43, with increases in both ratios as a function of longitude (higher values to the east). This is consistent with a shift from semi-volatile OOA components to more low-volatility OOA components as a function of photochemical aging. The immediate sharp increase in m/z 44:WSOC followed by a plateau is consistent with the findings of Ng et al.  and previous measurements during PACO [Wonaschütz et al., 2011]. It is noted that m/z 44 levels reached as high as ∼1.8 μg m−3, which is roughly similar to peak levels observed in Pittsburgh during September (up to ∼1.2 μg m−3 [Zhang et al., 2005]), Tokyo in August (up to ∼2.29 μg m−3 [Takegawa et al., 2007]), but less than that in Tokyo during the summer (up to ∼3.5 μg m−3) [Kondo et al., 2007].
 More insight into the secondary production of organics arises from the ratio of WSOC:SO42− (Figure 3f) as a function of distance eastward from the western end of the Basin where the highest SO42− levels were observed, owing to marine shipping sources and other major point sources near the coast. The WSOC:SO42− ratio ranged between 0.9 and 5.0 in the study region, which is greater than the cumulative average ratio (∼0.55) observed by Peltier et al. [2007b] in the northeastern United States. The peak WSOC:SO42− ratios between Riverside and Banning are coincident with an area with intense ammonium nitrate production owing largely to the influence of agricultural activity as a source of NH3, including animal husbandry operations. Ammonium nitrate production results in enhanced aerosol hygroscopicity and aerosol-phase water, which would promote more partitioning of WSOC to the aerosol phase and both multiphase chemistry and photolytic processing to produce WSOC [Hennigan et al., 2008a; Ervens and Volkamer, 2010; Sorooshian et al., 2010; Bateman et al., 2011]. In addition, WSOC may have been produced in the emissions from the agricultural activity. The WSOC:SO42− ratio decreased immediately downwind of Banning by an average factor of nearly 2.5 and this may be due to volatilization of WSOC (note that SO42− is not volatile), which is consistent with the reduction in the WSOC:BC ratio (by an average factor of ∼2) in the same area. Production of SO42− in this area can likely be ruled out owing to the low RHs, high temperatures, and lack of aqueous-phase chemistry to produce SO42− during afternoons (Table 2). The average RH during traverses through Banning Pass ranged between 11 and 55%.
 Reductions in aerosol-phase water, such as what happens when air is advected from the Basin to the desert outflows, promotes re-partitioning of WSOC to the gas phase [Hennigan et al., 2008b; 2009]. Lower amounts of the aerosol-phase water are associated with reduced RH and aerosol hygroscopicity. As shown in Figure 3h, the average temperature generally increases from west to east, and vice versa for average RH. The sub-saturated hygroscopicity of the regional aerosol decreased in the outflows, largely owing to ammonium nitrate volatilization. The average hygroscopic growth factor (Dp,wet/Dp,dry) at an RH of 92% for fine aerosol was 1.72 ± 0.25 in the Basin and 1.57 ± 0.19 in the desert outflows. To further examine the likelihood that volatilization is a sink for WSOC downwind of the Los Angeles Basin, the ratio of WSOC:NO3− is also examined (Figure 3e), as NO3- is vulnerable to evaporation. The range of this ratio was 0.2–2.3, within which are values observed in Mexico City (∼0.2–1.0) [Hennigan et al., 2008a] but much less than the cumulative average ratio (>30) observed in the northeastern United States [Peltier et al., 2007b]. AMS NO3− mass concentrations are typically highest at San Bernardino (6.08 ± 5.87 μg m−3 for the box labeled with this city in Figure 3) and they decrease significantly immediately downwind of the outflow-passes. As a result, the WSOC:NO3− ratio increased sharply in the outflows (greater than a factor of six), in contrast to the WSOC:SO42− ratio. This indicates that WSOC is less sensitive to volatilization as compared to ammonium nitrate. Such results are consistent with observations in Mexico City [Hennigan et al., 2008a].
 The general picture drawn in Figure 3 involves the following: (i) the western Basin is an important anthropogenic source for WSOC and precursors; and (ii) pollutants are transported to the eastern side of the Basin and through outflow-passes with the sea breeze, with aerosol aging processes during this time including dilution, secondary formation of WSOC, and volatilization of some fraction of WSOC. The increasing temperatures (decreasing RH) and reduction of aerosol hygroscopicity and aerosol-phase water in the outflows is likely responsible for the loss of some WSOC mass.