Particulate polycyclic aromatic hydrocarbon spatial variability and aging in Mexico City

Particulate polycyclic aromatic hydrocarbon spatial variability and aging in Mexico City D. A. Thornhill, S. C. Herndon, T. B. Onasch, E. C. Wood, M. Zavala, L. T. Molina, J. S. Gaffney, N. A. Marley, and L. C. Marr Department of Civil and Environmental Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia, USA Aerodyne Research Inc., Billerica, Massachusetts, USA Molina Center for Energy and the Environment, La Jolla, California and Massachusetts Institute of Technology, Cambridge, Massachusetts, USA Department of Chemistry, University of Arkansas, Little Rock, Arkansas, USA Received: 17 October 2007 – Accepted: 24 October 2007 – Published: 9 November 2007 Correspondence to: L. C. Marr (lmarr@vt.edu)


Introduction
The Mexico City Metropolitan Area (MCMA) is home to some of the highest measured concentrations of particulate polycyclic aromatic hydrocarbons (PAHs) in the world Oberdorster, 2000;Stoeger et al., 2006;Tran et al., 2005). Especially for low-solubility particles, surface area may be a more appropriate measure of exposure (Maynard, 2003). Tandem measurements of both PAHs and surface area in laboratory and field experiments have been shown to discriminate between different types of combustion sources and to indicate the degree of particle aging (Bukowiecki et al., 2002;Burtscher 15 et al., 1993;Marr et al., 2004;Ott and Siegmann, 2006;Siegmann et al., 1999).
In April 2003, a multi-national team of scientists conducted an intensive five-week field campaign in the Mexico City Metropolitan Area (MCMA-2003) to contribute to the understanding of air quality problems in megacities . Measurements of PAHs by three different techniques suggested that aerosol surface PAH con- 20 centrations diminish rapidly during the mid-morning hours due to coating by secondary aerosol in the highly photochemically active environment of Mexico City (Dzepina et al., 2007;Marr et al., 2006). However, detailed PAH measurements in 2003 were limited to a single site, so the spatial and temporal variations in their concentrations, which are important from a standpoint of exposure and control, are not known. 25 In March 2006, an even larger field campaign in Mexico City took place to study air pollution in megacities not only at the local scale, but also at the regional and global scales. The Megacity Initiative: Local and Global Research Observations (MILAGRO) campaign consisted of four components whose goals ranged from providing the sci-Introduction EGU entific basis for policies that would reduce pollutant levels in Mexico City itself to de-scribing the long-range transport of pollution emitted by a megacity. As part of the MCMA-2006 ground-based component focusing on local impacts, we measured particulate PAHs, aerosol active surface area (AS), and other gaseous, particulate, and meteorological parameters at six locations throughout Mexico City. Measurements 5 were situated at the Instituto Mexicano del Petróleo supersite near the city center and on board the Aerodyne Mobile Laboratory (AML). In addition to visiting the supersite, the AML also traveled to five other suburban, exurban, and rural sites ( Fig. 1) that encompassed residential, industrial, commercial, undeveloped, and mixed settings. The objective of this study is to describe the temporal and spatial variations in PAH 10 and AS concentrations in Mexico City. Furthermore, we investigate the relationships of ambient PAH and AS concentrations with other pollutants to gain new knowledge about their sources and evolution as they are transported throughout the megacity atmosphere. Transformations are important because they could affect the particles' toxicity and long-range transport impacts. We compare and contrast concentrations in 15 fresh, mixed, and aged emissions by considering a busy downtown location, suburban areas, the city outskirts, and a mountaintop location at the edge of the city. The knowledge gained from the study will provide the scientific basis for the development of risk assessments for exposure to these pollutants in Mexico City and the crafting of control strategies to reduce their emissions and health impacts. Introduction EGU estimate of total PAHs adsorbed on particles' surfaces at 10-s resolution with a detection limit of 1 ng m −3 . Although the technique does not provide speciation information, its strengths are its sensitivity and high time resolution, both of which are limitations of traditional filter-based methods.

Conclusions References
Aerosol active surface area, or Fuchs surface, is defined as that which is accessible 5 to a molecule that might diffuse to a particle's surface. It was measured by diffusion charging (EcoChem DC 2000 CE). The DC analyzer generates a corona discharge which produces a cascade of electrons and ions that can attach to particles. As with the photoemission aerosol sensor for PAHs, a sensitive electrometer is then used to measure the current generated by the flow of charged particles. The analyzer reports 10 active surface area of particles smaller than ∼100 nm at 10-s resolution with a detection limit of 1 mm 2 m −3 . The simultaneous measurement of particle surface properties with the PAS and DC sensors has been described as a technique for fingerprinting different types of combustion particles (Bukowiecki et al., 2002).
All the PAH and AS analyzers were factory calibrated three months prior to the field 15 campaign. At the beginning of the field campaign, we co-located and cross-calibrated the instruments against each other while measuring ambient air in Mexico City and then applied the resulting correction factors to all data. To facilitate analysis using diagnostic ratios and multivariate statistics, we averaged all data over a common 2-or 10-min interval. Black carbon (BC), operationally defined as the light-absorbing component 20 of particles, was measured at 2-min intervals using an aethalometer (Magee Scientific AE-3) at a wavelength of 880 nm.

Measurement sites
During the month-long MCMA field campaign in March 2006, we conducted measurements at the Instituto Mexicano del Petróleo (T0 supersite) and on board the Aerodyne 25 Mobile Laboratory (AML), which visited six sites including the T0 supersite (Fig. 1) EGU light-duty vehicles and modern heavy-duty diesel buses. The PAH and AS analyzers were situated on a building rooftop, approximately 15 m above ground level. The nearest major roads were 40 m away. To fulfill the objective of observing aged plumes, we selected the location and timing of the AML visits (Table 1) on the basis of meteorological analyses that identified sites that were generally downwind of the urban plume on 5 certain days (de Foy et al., 2007). The AML was designed and built by Aerodyne Research Inc. (Kolb et al., 2004). It was equipped with a comprehensive suite of gas and particle analyzers that measure carbon monoxide (CO), carbon dioxide (CO 2 ), nitric oxide (NO), nitrogen dioxide (NO 2 ), total nitrogen oxides (NO y ), speciated volatile organic compounds (VOCs), PAHs, and AS, among others. The AML's PAH analyzer was identical to that used at T0. During the field campaign, the AML drove to six sites and remained parked for 2-12 days at each location (Table 1). These sites encompassed varying environments, including residential, commercial, industrial, undeveloped, and mixed land use areas.
The AML visited the T0 and T1 supersites, Pedregal, Pico Tres Padres, Santa Ana, 15 and PEMEX (Fig. 1). Intended to represent the partially aged Mexico City plume as it drifts downwind under certain meteorological conditions, the T1 supersite is located at the Universidad Technológica de Tecámac ∼30 km northeast of T0. Tecámac is a suburb in the State of Mexico and has a mixture of commercial and residential areas. The supersite is within 2 km of the town center, and the nearest road is several hun-20 dred meters away. Pedregal is located ∼25 km southwest of downtown Mexico City in a suburban residential area whose roads are lightly traveled. The sampling site was the JFK Elementary School, which is also one of the routine air quality monitoring sites for the environmental agency of the Mexico City Federal District Government. Pico Tres Padres, the site of the main television transmitters for the MCMA, is an isolated moun- 25 tain ∼15 km north-northeast of T0 and 3000 m above sea-level, or ∼800 m above the valley floor. A single, mostly unused road runs up the mountain, and the surrounding area is not well traveled. Santa Ana is located in a residential area at the southern tip of the city, ∼40 km southeast of T0. Its roads are lightly traveled. Under certain meteo-Introduction EGU rological conditions, Santa Ana represents an outflow receptor for air pollution coming from the city center. Located in the extreme north of the city, the PEMEX site is in a highly industrialized area closely situated to a major oil refinery (5 km away), cement plants, chemical factories, agricultural activities, and a power plant. During the MCMA-2006 field campaign, air flow trajectories within the Mexico City 5 basin and the fate of the urban plume were simulated, and five types of wind circulation patterns were identified (de Foy et al., 2005;de Foy et al., 2007) were O 3 -North (9)(10)(11)(18)(19)(20)22 March), and eight were Convection (24-31 March). The prevailing meteorological conditions can strongly influence ambient pollutant concentrations for given emission levels and also determine the regional impacts of the urban plume (de Foy et al., 2006).

15
Figure 2 displays time series of AS and total particulate PAH concentrations at the T0 supersite. The raw 1-min measurements are shown by colored lines, whose color indicates the wind transport episode defined for each day. The black lines represent 1-h averages and are intended to highlight diurnal patterns in the measurements. The highest AS concentrations occurred on O 3 -North and Convection days, while the high-20 est average PAH concentrations occurred on Convection days, which are defined by weak winds aloft (de Foy et al., 2007). Strong diurnal patterns are evident in both AS and PAHs. AS concentrations at T0 averaged 80 mm 2 m −3 during the campaign, with a maximum 10-s value of 760 mm 2 m −3 on 30 March at 09:58. Typically, concentrations rose above 100 mm 2 m −3 between 06:30-08:30 and then decreased throughout the remainder of the morning and afternoon to ∼50 mm 2 m −3 . PAH concentrations averaged 50 ng m 3 throughout the cam- EGU paign with a maximum value of 3660 ng m 3 on 30 March at 10:02, within minutes of the maximum AS observation. During the morning rush hour, PAH concentrations generally rose to a maximum of ∼250 ng m −3 between 06:30-08:30 and then decreased throughout the remainder of the morning and afternoon to ∼20 ng m −3 . The daily minima in AS were more variable than in PAHs. Overnight concentrations rose as high as 5 75 ng m 3 . The daily maxima were nearly twice as high as observed at a site 17 km to the southeast in 2003  and 1.5 times as high as observed at a site 13 km to the southwest in 2003 and 2005 (Baumgardner et al., 2007). In all three studies, the PAH concentration measured by aerosol photoionization falls off more rapidly between 08:00-10:00 than do concentrations of other primary pollutants. Table 1 summarizes PAH concentrations at each site visited by the AML in chronological order. The highest average PAH concentration was observed at T0 and the lowest at Pico Tres Padres and Santa Ana, the mountaintop and southern outflow sites, respectively. The maximum PAH concentration occurred at T0. Figure 3 shows PAH time series at each site. At Pedregal, PAH concentrations were highest between 06:00-15 08:00 on Monday 6 March. They were slightly elevated in the hours before midnight on the evening of Saturday 4 March. These periods probably correspond to the times of heaviest traffic and lowest boundary layer height. At Pico Tres Padres, PAH concentrations remained below 10 ng m −3 , even though fine particulate mass concentrations (PM 2.5 ), not shown, routinely rose at approximately 10:00 each day, as the boundary 20 layer lifted up past the site. At T1, PAH concentrations increased to 100-200 ng m −3 in the morning hours, well before 06:00.
Santa Ana lies at the southern edge of the basin, and here PAH concentrations were always less than 30 ng m −3 . Cold Surge conditions, in which the wind flushes from the center of the basin toward the south and past the site late into the evening (de Foy Table 2 displays the relationships between PAHs and carbon monoxide (CO), carbon dioxide (CO 2 ), and total nitrogen oxides (NO y ) measured by the AML. The slope and standard error of the least-squares linear regression and correlation coefficient (R 2 ) are shown in the table. For CO, the correlations are weak everywhere except Pedregal and T1. PAHs are strongly correlated with NO y at T1, PEMEX, and T0, and weakly 5 correlated with NO y at the other sites. The regressions between PAHs and true NO x are not significantly different from those with NO y , so henceforth, we will refer to the relationship as with NO x . This notation will facilitate comparison with other studies, the majority of which use chemiluminescence and report results as NO x . The slopes are highest and similar at T1 and T0, which are also the most urbanized sites. For CO 2 , 10 the correlations are strongest at T0 and weak elsewhere. All correlation factors are relatively weak at Pico Tres Padres (R 2 ≤0.26) and Santa Ana (R 2 ≤0.44).
The simultaneous measurement of PAHs at T0 and other sites allows examination of their spatial variability in the MCMA. Figure 1 presents the Pearson correlation coefficients of 10-min PAH concentrations at various sites visited by the AML against those 15 measured continuously at the T0 supersite during periods of simultaneous measurements. A correlation factor could not be calculated for Pedregal because monitoring at T0 had not yet begun. Of course, the correlation was strongest when the AML was parked at T0. It was moderate at T1 and poor at all other sites.
The relationship between PAHs and AS has been shown to be related to the source 20 type and aging of the particles (Bukowiecki et al., 2002;Marr et al., 2004;Siegmann et al., 1999). Figure 4 illustrates the relationship between PAH and AS (10- EGU line is 39 times higher in the morning compared to the afternoon. There is no clear relationship between the ratio and wind transport episode. Figure 4 also shows PAHs versus BC, classified the time of day. PAHs and BC are expected to be correlated since both originate from combustion sources. For all times, the correlation between PAHs and BC is stronger (R 2 =0.77) than between PAHs and 5 AS, and the slope of the line, 11.7±0.1 ng µg −1 , indicates that PAHs are 1% of BC by mass. As with PAH/AS, higher PAH/BC ratios tend to occur during the morning rush hour period. As the day progresses, PAH/BC ratios tend to decrease and are lowest between 12:00-18:00. The equation of the regression line between 05:00-07:00 is PAH=14.4×BC-13.8 with R 2 =0.82. Later in the afternoon, between 12:00-14:00, the relationship is PAH=0.76×BC+16.2 with R 2 =0.02. The slope is 19 times higher in the morning. Table 3 contrasts PAH/BC ratios in Mexico City with those measured in three other cities, where the same aerosol photoionization method was used to measure PAHs. The mass ratio of PAHs to BC in Mexico City is similar to that measured along a freeway loop in the Los Angeles area and approximately 8-30 times higher than 15 in diluted vehicle exhaust in Ogden, Utah and ambient air in Fresno, California. The correlation factors between PAHs and BC are similar in all cities. Figure 5 shows average weekday diurnal patterns of the ratios PAH/AS and BC/AS. The latter is an indicator of the fraction of particles that are of combustion origin at any time (Burtscher et al., 1993), as BC is expected to be minimally reactive. The BC/AS 20 ratio peaks around the morning rush hour and then falls off steadily throughout the late morning and early afternoon. During this period, growth of secondary aerosol in Mexico City is considerable Salcedo et al., 2006;Volkamer et al., 2006) and contributes to AS but not BC. Even though both PAHs and BC are of combustion origin, their ratios to AS diverge between 07:00-12:00, with PAH/AS falling off 25 more rapidly than BC/AS. Whether this observation is due to chemical transformations, physical coating of the particles, or other mechanisms is discussed below.  (Zheng and Fang, 2000). These values compare more closely with Mexico City; 10 however, PAH concentrations in Mexico City are still higher. PAHs have also been measured in different environments of Greece (Mantis et al., 2005), where total particulate concentrations ranged from 2 ng m −3 to 52 ng m −3 in rural to urban areas. Again, these values compare more closely with the results seen in Mexico City, but the values for Mexico City are higher yet. 15 The database of measurements of ambient AS in other cities is considerably smaller. Mexico City's mean AS concentration of 80 mm 2 m −3 at T0 is comparable to that found in Los Angeles, where mean concentrations of 69 and 53 mm 2 m −3 were recorded at two ambient locations (Ntziachristos et al., 2007). AS in a residential area of Redwood City ranged between 40-300 mm 2 m −3 , with the higher concentrations attributed to 20 wood burning and fireplaces in the neighborhood (Ott and Siegmann, 2006). It appears that while particulate PAH loading in Mexico City is higher than in many other cities, its aerosol surface area loading is not comparatively extraordinary. Vehicular traffic has been recognized as the major contributor to PAH emissions in urban areas (Kittleson et al., 2004;Lee et al., 1995), and therefore it is not surprising 25 that the highest average PAH and AS concentrations are found at T0, where traffic is heaviest. The timing of PAH and AS peaks at the more urban locations corresponds to periods of rush hour traffic. Burtscher et al. (1993)  EGU values in Zurich to occur during rush hour and ascribed them to motor vehicles. At some locations, including Pedregal, T1, and PEMEX, increases in PAH concentrations occur at nighttime between 23:00 and 04:00 (Fig. 3). The increase in PAHs may be due to transport of particles emitted earlier in the evening during times of high traffic density, or it may indicate the presence of other nighttime sources. Speciation measurements in 2003 suggest that wood and trash burning contribute to PAHs observed at night . The impact of emissions at nighttime can be magnified because of stable atmospheric conditions. Some industries are thought to switch to using dirtier fuels and processes at night, when enforcement of regulations is less likely. Furthermore, at T1 during the first two weeks of the field campaign, Doran et al. (2007) 10 observed increased organic and elemental carbon during nighttime hours with peak values attained in the morning hours near sunrise. A similar pattern occurred at T1 on 21 March. The temporal variations imply that at night a buildup of pollution from nearby urban sources is occurring, followed by a subsequent dilution during the next morning as the boundary layer expands. 15 Spatial and temporal patterns in concentrations indicate not only potential sources of PAHs but also the degree of atmospheric processing the particles undergo. As emissions are transported, there are subject to dilution and other transformations. This behavior is supported by Fig. 3, which shows that in general, higher PAH concentrations occur at T0 and PEMEX, which are dominated by fresh emissions. Lower con-20 centrations occur at Pico Tres Padres and Santa Ana, which are receptor sites where emissions have undergone dilution and aging by the time they arrive. Intermediate concentrations occur at T1 and Pedregal, which lie between the two extremes.

ACPD
PAHs as detected by the surface-specific photoemission method may diminish due to coating by secondary aerosol, and Figs. 4 and 5 support this hypothesis. PAH loss 25 by photodegradation, heterogeneous reactions, or volatilization is less likely because measurements in 2003 showed that even when surface-bound PAHs diminish, PAHs are still detected by aerosol mass spectrometry, a method that is able to detect them anywhere in the particles, not just on the surface  EGU in secondary aerosol in the mid-morning hours contributes to aerosol surface area and is expected to cause a reduction in both PAH/AS and BC/AS ratios, but it does not explain the decrease in PAH/BC ratio (Fig. 4) or the divergence between the two (Fig. 5). Studies using a variety of techniques have shown that primary combustion particles are rapidly coated by secondary aerosol within a few hours in Mexico City 5 (Baumgardner et al., 2007;Dzepina et al., 2007;Johnson et al., 2005;Marr et al., 2006;Salcedo et al., 2006). This finding could explain the temporal patterns observed in PAH/AS and PAH/BC ratios because secondary aerosol formation would not increase the total mass of PAHs but could contribute to it being coated. After condensation of secondary aerosol on primary combustion particles, the PAHs on the surfaces of 10 primary particles would no longer be detectable by the photoionization method, but BC would remain detectable by the light absorption method. In contrast, the ratios of PAH/AS and PAH/BC were found to be much more constant throughout the day at a port south of Los Angeles . The difference may be due to the upwind location of this site, which experiences relatively clean inflow from the Pacific 15 Ocean and thus less secondary aerosol formation. Pico Tres Padres is of special interest because it sits 800 m above the valley floor, and during the morning hours, it is above the mixing (boundary) layer that contains freshly emitted pollutants. Its diurnal patterns of particulate mass loading differ from those at sites on the valley floor. PM 2.5 concentrations increase around 10:00 each 20 day, coinciding with the rising of the boundary layer, verified visually, up to the AML's location on the mountain. Particle surface PAH concentrations, however, do not rise concomitantly, further supporting the proposition that initially fresh combustion emissions from the valley below have undergone transformations that inhibit the detection of surface-bound PAHs. 25 The rapid coating of primary combustion particles in the megacity environment could have important implications for PAH longevity in the atmosphere. Experiments have shown that particulate PAHs can decay in the presence of sunlight (Kamens et al., 1988) and can undergo heterogeneous reactions with the hydroxyl radical, ozone, and  (Esteve et al., 2006;Kwamena et al., 2007;Molina et al., 2004). However, if the PAHs are coated by secondary aerosol, they may be less susceptible to degradation and may persist long enough to be transported to remote areas. Figure 1 shows that intersite correlation coefficients calculated for other sites versus 5 T0 are quite weak, except for T1. While strong intersite correlations would indicate spatially uniform emission patterns and sources and regional-scale mixing of pollutants, the results for Mexico City suggest that PAHs vary considerably in space. Concentrations at individual sites are largely independent of one another and are instead dominated by local sources, and/or fresh combustion particles have been sufficiently transformed that surface-bound PAHs are no longer present by the time the particles reach other sites. This conclusion is further supported by the lack of a consistent relationship between PAH and AS concentrations and wind transport episodes (Fig. 2); regional-scale meteorological patterns do not have a strong effect on concentrations. Mantis et al. (2005) and Siegmann et al. (1999) report generally higher correla-15 tion coefficients for their intersite comparisons during studies of PAHs in Greece and Switzerland, respectively. The study in the Greater Athens area of Greece found an intersite correlation of r=0.61 between two urban locations, r=0.76 between an urban location and a background location, and r=0.57 between an urban location and a mixed-urban industrial location. An important implication for risk assessment studies 20 is that a single monitoring site in Mexico City will not adequately represent the population's exposure.

Correlation of PAHs with AS, NO x , CO 2 , CO, and BC
The ratio of PAH to AS concentrations has been described as a fingerprint for different types of combustion particles (Bukowiecki et al., 2002;Matter et al., 1999;Siegmann et 25 al., 1999). The relationship between these parameters provides a qualitative means of EGU identifying different sources and describing the physical and chemical properties of particles. In contrast to previous studies which have shown tighter relationships between PAHs and AS for specific sources such as diesel exhaust, roadway vehicle emissions, candles, fires, and cigarettes (Bukowiecki et al., 2002;Marr et al., 2004;Siegmann et al., 1999), the relationship shown in Fig. 4 contains significantly more scatter. For a 5 single source, the relationship between PAHs and AS is expected to be linear with a characteristic slope. The spread of the data indicates that the aerosol represents a mixture of different sources and particles of different ages. Ambient measurements in complex environments are expected to produce such results. The stronger correlation of PAHs with NO x , rather than with CO and CO 2 (Table 2), 10 likely reflects the importance of diesel engines as sources of both PAH and NO x emissions (Harley et al., 2005;Marr et al., 1999). CO is emitted mainly by gasoline-powered vehicles, which emit far lower particulate PAHs than do diesel engines (Marr et al., 2002). Weak positive correlations between PAHs and NO x have also been reported in Brisbane, Australia (Muller et al., 1998). 15 Pollutant ratios can be useful for estimating emissions and for describing the evolution of source strengths over decadal time scales (Marr et al., 2002). The mass ratio of particulate PAH/NO x measured along roads during the MCMA-2003 field campaign was 4.7±5.9×10 −4 (Jiang et al., 2005). During the MCMA-2006 field campaign, this ratio was 1.09±0.05 ng m −3 ppb −1 , or 7.7±0.4×10 −4 in mass terms, at T0 (and simi-20 lar at the other urbanized sites T1 and PEMEX). The ratio in 2006 has not changed significantly from that measured in 2003, within the precision of the methods used. While the mass ratio of PAH/BC at T0 is similar to that observed along a freeway loop in the Los Angeles area (Westerdahl et al., 2005), lower ratios were found in ambient air in Fresno, California and diluted vehicle exhaust in Ogden, Utah (Arnott et al., 2005). 25 The similarity in PAH/BC between T0 and the Los Angeles freeway may indicate that vehicular sources are similar in the two locations and dominate measurements at T0 or alternatively, if particles have aged slightly by the time they reach the elevated T0 measurement site, may indicate that the PAH/BC ratio in fresh emissions in Mexico City EGU is actually higher than in Los Angeles. The ambient ratio in Mexico City is nearly 10 times higher than in Fresno. Direct measurements of exhaust are needed to determine whether particulate emissions from Mexico City's vehicles contain higher amounts of PAHs than in the U.S. If so, PAH emissions could be minimized by reducing the PAH content of fuels (Marr et al., 1999).

Conclusions
It is apparent that PAH pollution is a major problem in the more heavily trafficked areas of the MCMA. PAH concentrations near downtown exhibit a consistent diurnal pattern and routinely exceed 200 ng m −3 during the morning rush hour. Weak correlations between PAH and AS are indicative of the wide variety of sources and ages of particles present in Mexico City. PAH concentrations are poorly correlated in space, and therefore PAHs should not be treated as a regional-scale pollutant. An important implication of this result is that for risk assessment studies, a single monitoring site will not adequately represent an individual's exposure. The stronger correlation of PAHs with NO x , rather than with CO and CO 2 , probably reflects the importance of diesel engines as 15 sources of both PAH and NO x emissions. Mexico City's PAH/BC ratio is similar to that found along freeways in Los Angeles and 8-30 times higher than that found in two other cities. Aging of primary combustion particles by coating with secondary aerosol appears to result in a decrease in surface PAH/AS and PAH/BC ratios over the course of the day and may prolong the lifetime of PAHs in the atmosphere.  Environ. Sci. Technol., 32, 450-455, 1998. Molina, L. T., Kolb, C. E., de Foy, B., Lamb, B. K., Brune, W. H., Jimenez, J. L., Ramos-Villegas, R., Sarmiento, J., Paramo-Figueroa, V. H., Cardenas, B., Gutierrez-Avedoy, V., and Introduction  (Theochem), 458, 191-201, 1999. Stoeger, T., Reinhard, C., Takenaka, S., Schroeppel, A., Karg, E., Ritter, B., Heyder, J., and Schulz, H Inhalation Toxicol., 12, 1113-1126 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29