Fast airborne aerosol size and chemistry measurements with the high resolution aerosol mass spectrometer during the MILAGRO Campaign

P. F. DeCarlo, E. J. Dunlea, J. R. Kimmel, A. C. Aiken, D. Sueper, J. Crounse, P. O. Wennberg, L. Emmons, Y. Shinozuka, A. Clarke, J. Zhou, J. Tomlinson, D. R. Collins, D. Knapp, A. J. Weinheimer, D. D. Montzka, T. Campos, and J. L. Jimenez Cooperative Institute for Research in Environmental Science (CIRES) University of Colorado, Boulder, CO, USA Dept. of Atmospheric and Oceanic Science, Univ. of Colorado at Boulder, Boulder, CO, USA Dept. of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder, CO, USA California Institute of Technology, Pasadena, CA, USA National Center for Atmospheric Research, Boulder, CO, USA Department of Oceanography, University of Hawaii, USA Department of Meteorology, Texas A&M University, College Station, TX, USA

some very large plumes that were not associated with organics. The inorganic acids were neutralized by ammonium most of the time, although some periods with ammonium deficit were also observed. Volkamer et al. (2006) show that secondary organic aerosol (SOA) from urban sources is produced rapidly and about 8 times more efficiently in the city than an SOA model predicted. Although only one case study is 10 presented, the paper states that similar conclusions were obtained for several other days simulated. This study extended previous results about SOA underprediction over regional scales (de Gouw et al., 2005;Johnson et al., 2006a;Heald et al., 2005) to shorter time scales and the urban environment. Kleinman et al. (2007b) analyzed the aerosol evolution with photochemical age during MILAGRO over the urban area and 15 near outflow and confirm the results of Volkamer et al. (2006).
Wildfires and biomass burning (BB) have been also identified as important sources of particulate matter in Mexico City. Bravo et al. (2002) found a correlation between estimated emissions from wildfires and both Total Suspended Particles (TSP) and PM 10 for the years 1992-1999. During the Mexico City Metropolitan Area campaign in 2003 20 (MCMA-2003) biomass burning was identified as an important source of fine particles, and especially for organic species (Johnson et al., 2006b;Molina et al., 2007;. Recent results from the MILAGRO campaign also suggest that biomass burning contributed significantly to the gas and particle pollution in the city basin and outflow, although the magnitude of the impacts of fires is the subject of debate (Moffet Introduction EGU source (e.g., WSOC, water-soluble K, levoglucosan) were not included in this study, and thus biomass burning emissions were probably mixed with other emission sources in the source apportionment analysis (M. Viana, personal communication, 2007). A different source apportionment study of fine organic aerosol reports that BB made a highly variable contribution in time accounting on average for 16% and 32% of the am-5 bient organic carbon (OC) at the urban (T0) and rural (T1) ground supersites respectively (Stone et al., 2007). 14 C measurements indicate a significant fraction of modern carbon in the organic aerosol measured at the T0 site, part of which likely originates from urban sources such as trash burning and cooking (Aiken et al., 2007a). An overview of the large-scale meterology during the MILAGRO campaign is pre- 10 sented in Fast et al. (2007) and identifies 3 general regimes. 1-14 March is the first regime and is characterized by sunny and dry conditions. The second regime from the 14-23 March saw an increase in humidity and the development of afternoon convection, which slowly diminished as the atmosphere became drier. The third regime (24-31 March) began with a cold surge, saw increased precipitation, and a decrease 15 in the frequency and intensity of fires. The C-130 performed 12 research flights (RFs) during the MILAGRO campaign. For this paper the focus will be on RFs 2, 3, and 12 (the 8, 10, and 29 March 2006 respectively). Data will also be presented from RFs 1, 9, 10, and 11 from the 4, 23, 26, and 28 March 2006. RFs 2, 3, and 12 all had city and regional components in their flight 20 patterns. RF 1 had a city and regional component, but not all instruments were working on the C-130. RF 9 was a flight to the Yucatan Pennisula, and did not contain a city component. RF 10 was cut short due to mechanical problems with the plane, and the majority of the flight was spent at high altitude. RF 11 was a flight designed to measure the morning transition of photochemistry and did not sample in the city basin. During 25 RFs 4-8 a leak in the shared inlet system did not allow quantitative measurements of aerosol chemistry; when possible mass fractions of the measured AMS chemical species were reported to the database, however they have not been included in this paper. Introduction

EGU
In this paper, we present, the results of the measurements of non-refractory submicron aerosol chemistry from the HR-ToF-AMS in the C-130 which indicate that organic aerosols (OA) are the largest NR chemical component, with nitrate, sulfate, ammonium, and chloride also making significant contributions. Different aerosol species have different spatial distributions and tracer correlations, indicating influences from dif-5 ferent sources and processes. A large fraction of the organic aerosol is oxidized, with an oxygen-to-carbon atomic ratio (O/C) that increases with distance from the city approaching a ratio of 0.9 away from the basin. Organic O/C can be used as a qualitative "photochemical clock." Finally a detailed case study of RF 2 is presented to assemble all of these results into a coherent and detailed picture of submicron aerosol over and 10 around Mexico City. Quantitative apportionment of OA sources will be addressed in a subsequent publication.

15
The HR-ToF-AMS has been described in detail previously (DeCarlo et al., 2006). It improves upon previous versions of the AMS (Drewnick et al., 2005;Canagaratna et al., 2007;Jayne et al., 2000) by the use of a high-resolution mass spectrometer that allows the determination of the elemental composition of most ions, while previous AMS versions can only determine the total signal at each integer mass. In the rest of 20 the paper we will refer to the HR-ToF-AMS as "AMS" for brevity. The AMS sampled nonrefractory submicron aerosol (NR-PM 1 ) during the research flights. Data were acquired in two acquisition modes : Particle Time-of-Flight mode (PToF) which allows for particle sizing, and Mass Spec (MS) mode, which produces species concentrations and a complete mass spectrum of the non-refractory submicron mass Introduction EGU with no size information, but with higher sensitivity than the PToF mode. For most of the campaign data was averaged and saved every 12 seconds. Data was also saved at longer time intervals (∼30-45 s) early in the campaign, and sometimes a "plume-mode" was utilized which consisted of 3 second save intervals of MS mode only (no particle sizing). The HR-ToF-AMS was run exclusively in the V-mode of ion flight for the mass 5 spectrometer, which is about a factor of 10 more sensitive than the alternative W-mode. V-mode operation has lower resolution than W-mode, but still maintains high enough resolution to separate most ions of the same nominal mass, and especially reduced vs. oxidized organic fragments (DeCarlo et al., 2006 EGU AMS data. The residence time between the inlet tip and the AMS was ∼4 s (of which 3.2 s was in the PCI), and the flow warmed up to cabin temperature due to ram heating in the inlet and heat transfer (∆T ∼2-35 • C for plane altitudes below 4 km). This may have led to some evaporation of aerosol components (Murphy, 2006), which would be more important for ammonium nitrate and chloride, less important for organics, and 5 negligible for ammonium sulfate (Huffman et al., 2007 3 ). Further information on AMS data processing is given in Sect. 2.2.

Nephelometer
Total and submicrometer aerosol scattering coefficients were measured at 450, 550 and 700 nm using two TSI model 3563 3-wavelength integrating nephelometers (Anderson et al., 1996;Heintzenberg and Charlson, 1996;Anderson et al., 2003). The submicrometer TSI nephelometer employed a 1-µm aerodynamic impactor maintained at 30 lpm by an Alicat Scientific volumetric flow controller. While the measurements were made every second, sample air residence time inside the nephelometers was about 10 s. The noise over 10 s at 550 nm is estimated to be 0.3 Mm −1 (Anderson et 15 al., 1996). The instrument relative humidity was usually lower than 30%. The scattering coefficients were detected over 7-170 • , and corrected for 0-180 • using the measured wavelength dependence as a surrogate for the particle size after Anderson and Ogren (1998). For the submicron (PM 1 ) scattering, this angular truncation correction is typically less than 10%, and contributes negligible (<3%) uncertainty. Gas calibration 20 results in a smaller (+/−1%) systematic error (Anderson et al., 1996). The aerosol inlet for the nephelometer was a shrouded solid diffuser maintained at isokinetic flow ±5% and aspirated at about 100 lpm. The aerodynamic 50% size cut was 5 µm (Mc-Naughton et al., 2007). Introduction The Texas A&M SMPS (Wang and Flagan, 1990) measured the size distribution of particles between 0.012 and 0.67 µm mobility diameter once every 1.5 min. A high flow differential mobility analyzer (Stolzenburg et al., 1998) is used in the SMPS with sample and sheath flow rates of 1.5 and 15 liters per minute, respectively. The sampled 5 aerosol was dried to below 10% RH prior to classification using a Nafion tube bundle. The SMPS was located adjacent to, and shared an inlet with, the nephelometer and SP2.

HCN
The Caltech CIMS measured selected product ions on the C-130 via reaction of the 10 reagent ion CF 3 O − with analytes directly in air. HCN is measured by monitoring the product ion at m/z 112, which is the cluster of CF 3 O − with HCN. The sensitivity to HCN is dependent on the water vapor mixing ratio. Sensitivity changes due to water vapor changes are corrected for using the dewpoint hygrometer water measurement from the C-130 aircraft, and a water calibration curve that has been generated though 15 laboratory measurements. Non-water sensitivity changes are corrected for using inflight standard addition calibrations of H 2 O 2 and HNO 3 (other species measured by the CIMS) and proxied to laboratory calibrations of HCN. The detection limit (S/N=1) for HCN for a 0.5 s integration period is better than 15 pptv for moderate to low water vapor levels (H 2 O mixing ratio ≤0.004) (Crounse et al., 2006).

CO
The NCAR/NSF C-130 CO vacuum UV resonance fluorescence instrument is similar to that of Gerbig et al. (1999). The MILAGRO data have a 3 ppbv precision, 1-s resolution, and a typical accuracy of ±10% for a 100 ppbv ambient mixing ratio.  (NO and NO 2 ) and NO y (total reactive nitrogen) were measured (along with O 3 ) using the NCAR 4-channel chemiluminescence instrument, previously flown on the NASA WB-57F (Ridley et al., 2004). NO y was measured via Au-catalyzed conversion of reactive nitrogen species to NO, with a time response of about 1 s. NO 2 was mea-5 sured as NO following photolytic conversion of NO 2 , with a time response of about 3 s due to the residence time in the photolysis cell. NO was measured with an identical time response due to use of a cell with an identical residence time. NO and NO 2 are reported at 1-s. For NO, NO 2 , and NO y , the precision of a 1-s value is near 15 pptv. The overall estimated uncertainties of 1-sec values are ±(15 + 7% of the mixing ratio) 10 pptv for NO, ±(15+10% of the mixing ratio) pptv for NO 2 , ±(15+15% of mixing ratio) pptv for NO y .  (Kimmel et al., 2006;Jimenez et al., 2003). The amplification factor of the microchannel plate detector (MCP) was measured every day the instrument was in use ("single ion calibration").
Due to the stability and reproducibility of the single ion throughout the campaign a single average value was used for the entire campaign. The lack of MCP degradation is likely due to the very limited instrument use, ∼120 h total, equivalent to only 5 days of operation in a ground-based field campaign. Ionization efficiency (IE) calibrations were performed 6 times during the campaign. Due to customs issues the calibration equipment was not delivered until 10 days into the campaign, thus the IE for the first 3 flights was assumed to be the same, and was determined by a calibration made on 12 March 2006. Over the course of the campaign the IE calibration values varied by 20%, but varied by smaller amounts from flight to flight. 20% is therefore an upper limit to the uncertainty introduced by not having calibration equipment for the first three flights.

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IE values for the rest of the flights were determined from calibrations bracketing the flights. All AMS data was converted to mass loading at STP (273 K and 1013.25 hPa). Measured AMS mass was converted to volume using the assumption that species volume is additive (Eq. 4 of DeCarlo et al., 2004). For this conversion the densities of the species were assumed to be 1.78, 1.72, and 1.52 g cm −3 for ammonium sulfate, am-Introduction EGU particle collection efficiency (CE) for the AMS was assumed to be 0.5 unless there was evidence of acidic aerosol. In cases of acidic aerosol, the CE was increased proportial to the mass fraction of sulfuric acid (CE=1) to ammonium bisulfate (CE=0.5) (Canagaratna et al., 2007;Takegawa et al., 2005;Quinn et al., 2006). The integration of total signals for individual ions ("high-resolution sticks," e.g. C 2 H 3 O + and C 3 H + 7 at m/z 43) from raw mass spectral data was carried out using the "Pika" module of Squirrel developed by our group (http://cires.colorado.edu/jimenez-group/ ToFAMSResources/ToFSoftware/PikaInfo/), which follows the procedures described in DeCarlo et al. (2006). Sticks for spectra acquired in open (particles + air + mass spectrometer background) and closed (only background) modes were calculated. Particle 10 + air signal for each ion was determined by the difference of the open signal and the closed signal. Air contributions to individual ions (e.g. CO + 2 at m/z 44) were subtracted from the total signal at that ion. A correction at m/z 29 was also necessary, as the 2 most intense peaks CHO + and 15 NN + (29.00274 and 29.00318 respectively), cannot be separated at the resolution of the instrument. N 15 N + was calculated as a constant 15 fraction of the N + 2 signal at m/z 28 from calibrations with a particle filter, with the remaining signal assigned to the CHO + ion. Two other peaks, corresponding to C 2 H + 5 and CH 3 N + at m/z 29 were also resolved from the above two ions. Figure 1 shows ion signals at m/z 29 and 43 during research flight 3. Traces of the ion signals for this flight are shown and insets show the mass spectra for these m/z for one 12 s data point. The 20 inset for m/z 43 shows 2 different mass spectra corresponding to 2 different portions of the flight one city and one in the outflow. From this it is clear that the C 2 H 3 O + fragment is proportionately larger than the C 3 H + 7 fragment, indicative of the larger contribution of oxygenated species (likely SOA) in the outflow and of more reduced species (such as traffic exhaust and less aged SOA) over the city.

Calculation of light scattering from AMS data
AMS total size distributions from PToF mode were averaged to a 5 min timebase for RFs 1, 2, 3, 9, 10 11, and 12. The total NR size distribution was converted to a number 18280 Introduction EGU distribution vs. volume-equivalent diameter (dN/dlogd ve ), assuming spherical particles and using the bulk density of the aerosol calculated from the chemical composition for each 5 min average, according to the conversions detailed in DeCarlo et al. (2004). Scattering was calculated for this distribution using the routine of Bohren and Huffman (1983) translated into Igor (C. Brock, NOAA, personal communication, 2006). We used 5 a refractive index of 1.54, which is the average of the refractive indices for Ammonium Sulfate (1.55), and organics (1.53) (Hand et al., 2002;Kleinman et al., 2007a) and calculated scattering at 550 nm to compare to the Nephelometer, which measured submicron scattering at that wavelength.

10
Elemental analysis was performed on the high-resolution data following the method described in Aiken et al. (2007b). Inorganic and air ions were removed so only organic ions were included in the calculation of elemental ratios. Oxygen-to-carbon atomic ratios of the organic aerosol (O/C) and hydrogen-to-carbon (H/C) were determined, as well as the organic mass-to-organic carbon ratio (OM/OC). The nitrogen-to-carbon ra-15 tio was calculated, however the V-mode does not have enough resolution to reliably quantify some N-containing ions, so the absolute N/C ratio is not reported, although it is used in the OM/OC calculation. W-mode data from the T0 supersite in Mexico City show that organic N/C is typically more than an order of magnitude smaller than the O/C, comparable to the N/C ratio found on the C-130. Thus errors in the reported 20 OM/OC due to imprecision in the N/C ratios derived here should be small. Organonitrates and organosulfates can produce nominally "inorganic" ions in the AMS (e.g. NO + , Ignoring these ions will result in a negative bias on the O/C and OM/OC. However the analysis of the stoichiometric neutralization of ammonium vs. nitrate, sulfate, and chloride (discussed below, see also Zhang et al., 2007b) suggests 25 that the contribution of these types of species (and thus the associated errors in O/C and OM/OC) is small. To avoid the effect of noise at low OA concentrations, ratios are only reported when the OA mass was larger than 2 µg m −3 . tions. Note that data from takeoff and landing were excluded from these comparisons since the inlet for the AMS was in a different location on the plane than the Nephelometer and SMPS, and particles generated by the landing gear and exhaust are sampled differently for these portions of flights. to lack of complete overlap in the measured size ranges, effects on sizing of particle non-sphericity, and since SMPS is measuring a different particle size at each point in space (time), while the AMS does an interleaved average as discussed above. Based on preliminary data, black carbon makes up 1-3% of the submicron mass during MI-15 RAGE (R. Subramanian, DMT, personal communication), and would make up slightly less of the submicron volume due to the higher density values for black carbon (Park et al., 2004) in relation to the dominant organic constituents, consequently the AMS calculated volume is not expected to be significantly impacted by the exclusion of black carbon. At high mass loadings the AMS is above the fit line, which could be due to 20 ammonium nitrate evaporation in the SMPS (Gysel et al., 2006), which was maintained at 40 • C to improve RH control in the tandem differential differential mobility analyzer (TDMA) that was located in the same enclosure as the SMPS.

Calculated scattering vs measured scattering
Scattering calculated from AMS size-resolved composition was compared to the aver-25 aged nephelometer measurements. Figure 2d shows that agreement is good for most  Figure 3a-c show this information. Figure 3d shows the average aerosol composition as measured at the ground during the entire 10 campaign at the T0 supersite (19.48973 N, −99.1501 W) at the typical times of day (noon-6 PM local time) when the C-130 flew over the city. The average for the whole campaign was used for T0 as there were only a few direct flyovers and both instruments were not always operating during those times, and also because the bulk aerosol composition was not highly variable at T0. The relative concentrations of organic and total 15 inorganic species are very similar. The even larger fraction of organics reported during the MCMA-2003 study  are due to the average reported for that campaign to be from a full day. This is because there are higher primary organics and lower ammonium nitrate concentrations during evenings, nights, and early mornings. The spatial and vertical distribution of the species showed significant differences.
20 Figure 3c shows that away from the city the total average concentration is ∼1/4 of that over the city, confirming the importance of the MCMA as a regional source, and with an increased fraction of sulfate at the expense of nitrate. In general, sulfate was more of a regional component to the aerosol, while nitrate was localized to the city and in the near-outflow. This is consistent with the conclusions of , 25 who based them on the rapid variability and strong diurnal cycles of nitrate in the city, versus the much more constant and slowly varying levels of sulfate, as well as the fact that OH + NO 2 could explain the nitrate increases observed in the mornings, while 18284 Introduction EGU sulfate formation from OH + SO 2 was small compared to the concentrations observed. Spatial maps of NR-PM 1 sulfate, nitrate, and chloride are shown in Fig. 4a-c. Both industrial sources and volcanic sources have been identified as potentially contributing to the aerosol sulfate in the basin Johnson et al., 2006b;Raga et al., 1999;. The maps show the more regional distribution 5 of sulfate with significant structure indicating the influence of the large SO 2 sources from industrial complexes (e.g. Tula and refineries near Veracruz) and active volcanoes (e.g. Popocateptl). Submicron aerosol nitrate appears to be in the form of ammonium nitrate, based on the ammonium balance and observed fragmentation pattern (see below), while the fractional contribution of organic nitrates to total nitrate appears to be 10 small. Nitrate shows a dominant source in the city basin, and correlates strongly with CO in the city basin (R 2 =0.79) but not with HCN (R 2 =0.10) indicating the dominance of the urban non-fire source for this species. Mexico City is characterized by high concentrations of gas-phase NH 3 that favor its co-condensation with nitric acid to form semivolatile ammonium nitrate . In contrast, fires plumes 15 around Mexico City have a large excess of NO x vs. NH 3 (Yokelson et al., 2007), which may explain the their limited ammonium nitrate formation. Nitrate shows a very large fractional reduction in the outflow, most likely due to evaporation upon dilution with regional air with low HNO 3 and NH 3 . HNO 3 also reacts with dust in the Mexico City area forming mineral nitrates in the supermicron mode (Fountoukis et al., 2007;Moffet 20 et al., 2007;Querol et al., 2007) that would not be detected by the AMS. However the dust spatial and temporal distribution is highly variable and is unlikely to be the only cause of the pronounced decrease in nitrate away from the city observed in this study. Figures 5a-c show the ratio of organic and nitrate aerosol to excess CO to remove the effect of dilution in the cleaner regional air. Clearly nitrate shows a large 25 reduction with distance from the Mexico City urban area, while OA does not. Although aerosol nitrate does not completely disappear, its ratio to CO decreases quickly and has dropped by nearly a factor of 4 by the time the aircraft is 200 km from the city basin. The OA/CO ratio in the outflow near the city is about 80 µg m −3 STP ppm −1 , which is The fact that similar asymptotic values are observed despite lower biogenic emissions during MILAGRO versus the summer in New England could be due to the higher BB emissions in Mexico during this study (Yokelson et al., 2007). NR-Chloride shows sources both in and outside of the Mexico City basin. Often the NR-Chloride is associated with HCN indicating a fire source, however there are 10 instances of chloride plumes without an associated increase in HCN, indicating other sources as well, consistent with previous results  and studies at other locations (Tanaka et al., 2003). The nitrate fraction showed an enhancement in the plane data at altitude as compared with the ground data consistent with previous studies (Morino et al., 2006;Neuman et al., 2003) likely due to additional ammonium 15 nitrate condensation with lower temperature and higher relative humidity aloft, whereas the average mass fraction of aerosol sulfate at the T0 ground site was higher than it was aloft over the city basin measured by the C-130 during the MIRAGE campaign. Figure 6 shows two vertical profiles performed outside of the city basin to the east of the Popocatepetl volcano during RF 12 (29 March 2006). During the first vertical 20 profile (17:30-18:00 UTC) 2 acidic sulfate layers are seen at high altitude, which have increased in vertical separation later in the flight when a second vertical profile (2100-2120 UTC) was made in the same area. Size distributions for the parts of the vertical profile marked I, II, and III are shown to the right of the vertical profile. The first vertical profile shows that the highest sulfate layer (I) has small aerosol size modes of highly 25 acidic sulfate most likely from recent nucleation and growth in the volcanic plume, similar to the nucleation and growth study of Zhang et al. (2004). The second, sulfate layer (II), lower in altitude, consists of larger particles, indicating a more aged layer but still very acidic, and the lowest layer (III) extending to the bottom of the profile contains EGU a mix of organics, ammonium sulfate and ammonium nitrate is likely pollution transported from the Puebla basin based on back trajectories. The second vertical profile performed 3.5 h later indicates that the most of the particles in the highest layer (I) have grown to or coagulated with accumulation mode sizes and increased the sulfate mass loading. However there is still a significant tail of mass at particle sizes less than 5 100 nm. The other 2 size distributions related to layers II and III have not changed significantly during that time. No enhancement of organics is found in the acidic sulfate layers, consistent with other recent field studies that find no evidence for significant acid-catalyzed SOA formation in the atmosphere (Peltier et al., 2007b;Zhang et al., 2007b). This case study demonstrates the vertical heterogeneity of the aerosol com-10 position, as well as the high signal-to-noise of the HR-ToF-AMS in the V MS mode, and the ability of the instrument to rapidly determine meaningful chemically-resolved size distributions.
The charge balance of ammonium to inorganic anions (sulfate, nitrate, chloride) can be used to evaluate the acidity or neutralization of the aerosol as long as the contri-15 bution of mineral cations (Ca + , K + etc.) is small (Zhang et al., 2007b). This balance is shown in Fig. 7. Large relative concentrations of supermicron volume to submicron volume can influence submicron ion balance due to tailing of the supermicron size distribution (and associate mineral cations) into the submicron. A comparison of the supermicron volume (likely dominated by mineral dust) and submicron volume 20 from the Optical Particle Sizer, operated by the University of Hawaii, indicate that with the exception of a few short plumes submicron volume was typically a factor of 2 or more larger than supermicron volume. This suggests that the contribution of mineral cations to the charge balance due to the submicron tail of the dust mode was small. Charge balance is observed within the scatter for most datapoints, which indicates that 25 ammonium nitrate and sulfate are the predominant forms of nitrate and sulfate in the submicron aerosol. There are, however, instances of highly acidic sulfate plumes originating in some instances from the Popocatepetl volcano (as shown in Fig. 5) and in other instances from petrochemical or power plant complexes such as Tula. O/C atomic ratios for the total organic aerosol were calculated from the HR spectra with the procedure described by Aiken et al. (2007b, c 5 ). Figure 8 shows flight tracks for RFs 1, 2, 3, 11, and 12 colored by the organic O/C ratio. This figure clearly shows an increase in the O/C ratio away from the city, with maximum values around ∼0.9. The 5 range of values observed for OM/OC is 1.5-2.3, with the lower values corresponding to air over the city and the larger ones to aged regional air. It is also important to notice that even above Mexico City the O/C ratio is already ∼0.4 and above, which already represents a highly oxygenated aerosol. This is likely due to flights through the city occurring in the early to late afternoon when photochemical SOA formation has already 10 been active for hours (Kleinman et al., 2007b;Volkamer et al., 2006). Primary urban emissions from vehicles have much lower O/C ratios around 0.06-0.1 (Aiken et al., 2007c 5 ). This indicates that a significant amount of secondary formation has already occurred by the time the C-130 reaches the city. The influence of BB on the organic O/C ratio is investigated in Fig. 9  EGU were sampled, consequently the range of values may be broader than what is given here, but this is useful as a indicator of approximate O/C ratios for biomass burning. From Fig. 8 it is clear that organic O/C ratios increase significantly away from the city, with distance being a proxy for the age of the aerosol; i.e. aerosol outside the city can have a city origin and have been aged during transport, or a regional non-city origin.
Regional air typically shows aged characteristics due to the longer average residence time of the OA sampled in the regional air (Zhang et al., 2005b;Zhang et al., 2005a). Since thermodynamics drives atmospheric organics towards increased oxidation, the increase in the organic O/C ratio is indicative of aging of the aerosol, and in the absence of calibration or kinetic modeling, it is a qualitative indicator of photochemical age. 10 Recent results by Huffman et al. (2007) 3 indicate that SOA is less volatile than urban POA and most P-BBOA, so some of the increase in the O/C ratio in the regional scale may be due to evaporation losses of more reduced species. This is likely not the dominant effect since primary O/C ratios are low for urban combustion emissions, and around 0.4 for biomass burning plumes, and to reach O/C ratios of 0.6-0.8 addition 15 of SOA of higher O/C ratios, or heterogeneous oxidation of the aerosol need to occur. However heterogeneous oxidation proceeds with longer timescales of the order of 1 week or longer (Molina et al., 2004;Murphy et al., 2007;Schauer et al., 1996;Zhang et al., 2005b). Heterogeneous reactions are too slow by a factor of ∼100 to explain the rate of O increase observed here, assuming that 2 O atoms are taken up by the OA 20 after reaction with OH with an uptake coefficient of 0.3 (assuming a particle diameter of 300 nm and an OH concentration of 2×10 6 molec cm −3 ). In contrast formation has a timescale around 1 day (de Gouw et al., 2005;Peltier et al., 2007a), so additional SOA formation (from urban and BB precursors) or additional gas-phase oxidation of semivolatile SOA species are the most likely reasons for the observed increase in O/C. 25 An increase in the O/C ratio while OA/CO stays approximately constant implies that there is a loss of carbon from the OA.
Another qualitative measure of photochemical age is the -log 10 (NO x /NO y ) ratio (Kleinman et al., 2007a). This and other such ratios work best if there is an isolated ACPD 7, 2007 Aerosol size and chemistry measurements during MILAGRO P. F. DeCarlo et al. EGU source or source region, which does not mix with other sources from emission to the time of sampling. This can be a problem when using this ratio when urban pollution mixes with fire plumes, as fire plumes from the pine forests near Mexico City had high NO x emissions (Yokelson et al., 2007). When air from different sources mixes this photochemical clock is "reset" losing information on the age of pollution from both sources.

5
For this reason we compared the organic O/C atomic ratio to the -log 10 (NO x /NO y ) from RF 12 (29-March-2006), when biomass burning was low, and gas phase data of HCN and CO did not indicate recent biomass burning influence. Figure 10 shows a comparison between the -log 10 (NO x /NO y ) ratio and the O/C ratio. There is a clear trend which shows the increase in the -log 10 (NO x /NO y ) ratio with increasing O/C ratio, indicating that this is a good qualitative clock for photochemical age. The x-intercept of the regression occurs at approximately an O/C ratio of 0.55. The reason for this high value is potentially due to the C-130 arrival into the city in the mid afternoon, by which time a significant fraction of the total SOA formation has already occurred. The -log 10 (NO x /NO y ) clock can also be "reset" by mixing fresh high NO x plumes with older 15 pollution. Since secondary OA mass exceeds primary OA the mixing of a fresh plume with low concentrations of OA (and low O/C) would only minimally affect the O/C ratio which would be dominated by the higher O/C ratio of the secondary OA. The group of points above the value of 1.5 for -log 10 (NO x /NO y ) come from a leg of the flight at 18.67 N from 95.88 to 95.16 W longitude, just inland from the Gulf of Mexico and in 20 the vicinity of petrochemical refineries and processing facilities. It is likely that the relationship of -log 10 (NO x /NO y ) to O/C atomic ratio for this section of the flight is more indicative of an aged industrial plume than an urban plume.

Case study: research flight 2, 8 March 2006
RF 2 provides the opportunity to combine the analysis techniques discussed above 25 into a detailed picture of the aerosol for this flight. AMS sampling for this flight began 1 h into the flight due to pump down requirements after lack of ground power. The flight consisted of 3 general sampling regimes. The first regime sampled aged outflow from 18290 Introduction EGU the previous day southwest of the city basin where the forecast models predicted it to be located. The second portion of the flight involved a standard city flight pattern with the C-130 entering the city basin from the northwest and flying south along the western edge of the basin. The plane then turned east and flew along the southern edge of the basin, turning around in the southeast corner of the basin and flying northwest across 5 the city. A pass over the T0 supersite and stacked legs over the T1 supersite were then performed. The final portion of the flight consisted of stacked legs to the east of the city basin sampling same day pollution which had been advected to that vicinity based on the meteorological predictions. Figure 11 shows the time traces of many pollutants and properties of the pollutants, 10 and is discussed in detail in the following text. Figure 11e shows the day tagged CO emissions from the MOZART model, and qualitatively indicates the age of the sampled air mass. Early in the flight the air mass is clearly more aged than the latter portion of the flight, and is generally characterized by lower aerosol loadings dominated by organics with sulfate as the dominant inorganic component ( Fig. 11c and d), with the 15 highest aerosol O/C atomic ratios (up to 0.7) and OM/OC ratios (around 2.0) of any portion of the flight (Fig. 11b). The average size distribution for this portion of the flight shows the mass diameter peaking around 300 nm d va for organics and slightly larger at 400 nm d va for ammonium sulfate, which is the dominant inorganic component. The ammonium size distributions shown in Fig. 11f-i, are noisy due to the water back-20 ground in the mass spectrometer resulting from limited pumping time, but are shown for completeness. The OA/CO ratio for the first portion of the flight (Fig. 11a) averages 70 µg m −3 STP ppmv −1 with a standard deviation of +/−11 µg m −3 STP ppmv −1 . As the C-130 enters the city basin around 21:00 UTC (03:00 p.m. local time), MOZART CO age indicates that ∼80% of the CO is from the same day emissions 25 (Fig. 11e). Aerosol nitrate becomes the dominant inorganic aerosol component, with organic aerosol still dominating the submicron mass ( Fig. 11c and d) EGU at 350 nm d va . Sulfate is a minor component of the aerosol, but shows a slightly higher mass diameter around 450 nm d va indicative of its more aged and regional character (Fig. 11g). The OA/CO ratios (Fig. 11a) are highly variable indicating the influence of fire emissions on the aerosol. Lower values of approximately 40 µg m −3 STP ppmv −1 CO are reached during the periods marked I and III in Fig. 11d. These periods are 5 associated with high CO, high aerosol nitrate, and are indicative of "fresh" urban SOA. The period marked II in Fig. 11d has a higher OA/CO ratio averaging 100 µg m −3 STP ppmv −1 CO, and is associated with high levels of HCN, and NR-aerosol chloride. AMS m/z 60 is used as a marker for biomass burning (Alfarra et al., 2007;Schneider et al., 2006), however, it is not completely unique to fire emissions, and during non-fire peri-10 ods m/z 60 makes up approximately 0.3% of the organic mass. By subtracting 0.3% of the organic mass from m/z 60 we have a good indication of the fire contribution to signal at this m/z. This scaled m/z 60 also increased during period II, adding yet another indication of the influence of a fire source. This marker appears to be a good tracer for fresh fire plumes during this study. Although it is possible that this marker is partially 15 lost due to volatilization (Huffman et al., 2007 3 ) and/or photochemical reactions with aging, recent field results indicate that its lifetime is of at least two days in the boundary layer (Lee et al., 2008). Finally during period II, the presence of ∼5 µg m −3 STP of ammonium nitrate indicates that there is still a large urban influence in this plume. Figure 12a shows the mass spectrum for the period I. This mass spectrum is very 20 similar to observed organic spectra from other urban locations (Alfarra et al., 2004;Zhang et al., 2005b). A comparison of the fractional contribution of each m/z to the total organic signal using the procedure of Alfarra et al. (2006), shows differences in the spectra from periods I and II and the similarity for periods I and III (Fig. 11b and c). When comparing the fire-dominated period (II) with the urban-dominated period I, only 25 a few peaks below m/z 120 have a higher percent contribution to the aerosol mass. The most pronounced ions that are larger in BB plumes are m/z 60 and 73, which are found in the mass spectra of levoglucosan (a component of biomass burning aerosol), and have been shown to be marker peaks for biomass burning in ambient air (Alfarra et al., Schneider et al., 2006). Although not shown the same pattern emerges when comparing period II with period III. Figure 12c demonstrates the similarity between periods I and III with very little change in the relative contributions for the mass spectra. Deviations from the 0% change in contribution are mostly due to noise at m/z with low S/N. Also clear from this plot is the increase in the contribution to organic mass of 5 high m/z (above m/z 120) for biomass burning. While the majority of the signal in all cases (I-III) is below m/z 100 (91.4%, 90.6%, 91.5%, for Cases I-III respectively) this indicates that biomass burning plumes contain either larger molecules, or molecules more resistant to fragmentation than those in urban POA and SOA. The final portion of the flight showed increases in the O/C atomic ratio and OM/OC 10 ratio compared to the city (Fig. 11b), but these variables did not increase to the levels seen in the aged outflow from the early part of the flight, consistent with the lower average age of the air predicted by MOZART. This part of the flight also included some advected fire plumes indicated by increases in the OA/CO ratio (up to 250 µg m −3 STP ppmv −1 ), HCN, NR-chloride, and m/z 60 ( Fig. 11b and c). Size distributions for the 15 period marked same day outflow (OF), and fire plume are shown in Fig. 11h and i. The same day outflow size distribution shows a clear drop in aerosol nitrate from the city plume, reduced importance of ultrafine particles below 100 nm, and a peak in the distribution at 300 nm d va . The fire plume distribution shows the clear dominance of organics in the fire plume, and the presence of nitrate in the size distribution suggests 20 that the fire plume is adding to the urban pollution, although a some of the nitrate may also originate from fire emissions. The mass size distribution for the fire plume peaks at a slightly smaller size of 250 nm d va .

Conclusions
A HR-ToF-AMS was flown for the first time during the MILAGRO campaign. The per- 25 formance of the instrument was very good, providing 12-s quantitative aerosol size and chemistry data. Comparisons between submicron volume from a SMPS, and light EGU scattering confirm the quantification of the AMS data. Organic species dominated the submicron aerosol both near the source and in the outflow and regional air. Inorganic components also made a large contribution, with different species showing different spatial characteristics. Aerosol sulfate had a more regional but still somewhat structured character with clear volcanic and petrochemical/power plant sources. Submicron 5 nitrate showed a strong city source and a high correlation with CO that links it to urban emissions. The higher source strength of NH 3 in the city versus the fires may explain the preferential formation of nitrate in city air, given the comparable NO x emissions of both sources. Ammonium was associated with nitrate and sulfate, indicating the dominant forms of these species were ammonium nitrate and ammonium sulfate respec-10 tively. NR-aerosol chloride was often associated with HCN indicating a fire source, although some chloride plumes did not show this correlation, indicating other sources of NR chloride. Organic aerosol had strong correlations with both CO and HCN indicating both urban and biomass burning sources. The organic O/C atomic ratio showed more reduced aerosol near the city/source regions and higher values farther from the 15 city. O/C was also shown to be a qualitative photochemical clock by comparison to -log 10 (NO x /NO y ). SOA formation from urban sources quickly overwhelms urban POA emissions as shown by the increase of the O/C ratio to values much larger than those of urban POA, and consistent with the results of Volkamer et al. (2006) , 1992, Environ. Pollut., 117, 243-253, 2002 Table 1. Results of the Organic Aerosol to CO regression analysis for RFs 1, 2, 3, 9, 10, 11, 12. RFs 1, 2, 3, 12 had a city and regional component; the rest of the flights are included for completeness, but do not represent the same range of conditions. CO background is given by the x-axis intercept for the low CO (<200 ppbv) conditions. Slopes are the OA/CO ratio for each flight. The R 2 value is given to indicate the quality of fit.