References

Measurements of HNO3 and N2O5 using Ion drift – Chemical Ionization Mass Spectrometry during the MCMA – 2006 Campaign J. Zheng, R. Zhang, E. C. Fortner, L. Molina, A. C. Aiken, J. L. Jimenez, K. Gäggeler, J. Dommen, S. Dusanter, P. S .Stevens, and X. Tie Dept. Atmospheric Sciences, Texas A & M University, College Station, Texas 77843, TX, USA Molina Center for Energy and the Environment, La Jolla, CA, USA Dept. Chemistry and Biochemistry, University of Colorado, Boulder, CO, USA Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, 5232 Villigen, Switzerland Dept. Chemistry, Indiana University, Bloomington, IN, USA Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO, USA now at: Dept. Chemistry and Biochemistry, Montana State University, Bozeman, MT, USA Received: 18 January 2008 – Accepted: 22 January 2008 – Published: 6 March 2008 Correspondence to: R. Zhang (zhang@ariel.met.tamu.edu) Published by Copernicus Publications on behalf of the European Geosciences Union.


Introduction
For decades, the Mexico City Metropolitan Area (MCMA), the biggest megacity in the world, has suffered from severe air pollution, especially in ozone (O 3 ) and particu-Figures

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Printer-friendly Version Interactive Discussion late matter (Molina and Molina, 2002;Molina et al., 2007).As the home of 20 million residents, 3.5 million vehicles, and 35 000 industries, the MCMA emits annually near 206 kilotons of nitrogen oxides (NO x =NO+NO 2 ), 22.5 kilotons of sulfur dioxide (SO 2 ), and 1769 kilotons of volatile organic compounds (VOCs) (Molina and Molina, 2002) into the atmosphere, due to both domestic and industrial fossil fuel consumptions.Furthermore, the unique combination of geographical conditions, i.e. the basin topography, tropical solar radiation (19 • 25 N and 99 • 10 W), and high elevation (2240 m a.m.s.l.), makes air pollutants easier to accumulate in the MCMA and is favorable for photochemical production of O 3 and aerosols (Molina and Molina, 2002;Zhang et al., 2004a).Since 1988, this region annually experiences an hourly average O 3 concentration higher than 110 ppb for over 80% days and a peak O 3 concentration of frequently more than 300 ppb (Molina and Molina, 2002).Both VOCs and NO x play critical roles during O 3 formation in the troposphere (Finlayson-Pitts and Pitts, 1999) and the variations in their concentrations and the ensuing effects on O 3 production rate can be characterized as either NO x -sensitive or VOC-sensitive (Sillman, 1999;Lei et al., 2004;Zhang et al., 2004b).Recently, based on a model study, Tie et al. (2007) suggested that the O 3 formation in the MCMA was VOC-limited.The lifecycle of NO x and its budget in the MCMA is one of the critical pre-required information to develop effective O 3 control strategies (Lei et al., 2007).The dominant daytime sink of NO x is through the oxidation of NO 2 by the hydroxyl radical (OH) to form nitric acid (HNO 3 ), which is removed from the atmosphere by dry and wet depositions (Finlayson-Pitts and Pitts, 1999).Generally, HNO 3 deposition is considered as an irreversible sink for NO x , but recent studies have suggested that HNO 3 deposited on the surface can be recycled back into the atmosphere as NO x by heterogeneous reactions with NO (Saliba et al., 2001) and photolysis in the presence of water (Ramazan et al., 2006).Under certain meteorological conditions, HNO 3 can be transported over 1000 km downwind from the plume origin, and a HNO 3 enriched plume can potentially contribute to O 3 production far away from its origin.During the nighttime, NO x will titrate O 3 to form the nitrate radical (NO 3 ), which can further react reversibly with NO 2 to form dinitrogen pentoxide (N 2 O 5 ) (for Reaction 4, K eq =4.5×10 −14 exp[−1260/T ] cm 3 molecule −1 s −1 , JPL, 2006).At night, NO 3 can exist in significant concentrations and undergo H-atom abstraction or addition reactions with hydrocarbons in a similar way as OH radicals during daytime (Suh et al., 2001).Comparing with NO 3 , N 2 O 5 is relatively unreactive, but it can act as a reservoir of NO 3 and undergo heterogeneous hydrolysis to form HNO 3 (Zhang et al., 1995;Brown et al., 2006).Therefore, in-situ observations of HNO 3 and N 2 O 5 are indispensable to fully characterize the NO x chemistry and budget in the troposphere and to develop effective control strategies.Several measurement techniques have been developed to measure gaseous HNO 3 in the troposphere, including a nylon filter (Anlauf et al., 1988), a mist chamber (Talbot et al., 1990), a denuder technique (Perrino et al., 1990;Simon et al., 1995), a luminol method (Hering et al., 1988), and chemical ionization mass spectrometry (CIMS) (Huey et al., 1998).Among the different approaches, the CIMS technique has the advantages of high sensitivity and fast time-response.Ambient HNO 3 mixing ratio varies considerably temporally and spatially, with the reported values ranging from a few tens of ppt in clean remote environment to tens of ppb in aged urban plumes (Furutani and Akimoto, 2002;Huey et al., 2004).Despite the importance of N 2 O 5 in the nocturnal NO x chemistry, only recently in situ N 2 O 5 measurements have become possible.In situ measurements of N 2 O 5 have been performed by a cavity ring-down spectroscopy technique (CRDS) (Brown et al., 2002) and a CIMS technique (Huey et al., 1995).observed N 2 O 5 concentration ranges from a few ppt to hundreds of ppt (Brown et al., 2001;2006;Slusher et al., 2004).A series of field campaigns have been conducted MCMA such as the 2002, 2003 and 2006 field campaigns.The objectives of these campaigns are to fully characterize the up-to-date air quality, to investigate the underline chemical processes that are responsible for secondary air pollutants formations, and to develop effective control strategies.In this paper, we present measurements of HNO 3 and N 2 O 5 during the MCMA 2006 campaign from a recently developed ion drift-chemical ionization mass spectrometry (ID-CIMS) technique and its first field deployment.The results provide insights into the production and gas/particle partitioning of HNO 3 in MCMA.

ID-CIMS
The ID-CIMS method has been previously described (Fortner et al., 2004), and only details pertinent to this work are provided.Figure 1 shows a schematic diagram of the ID-CIMS that consists of an ion-drift tube, an ion source, and a quadrupole mass spectrometer.Also depicted in Fig. 1 is the HNO 3 calibration device, which consists of a U-shape 1-inch outer diameter (OD) and 7-inch long glass tube wrapped with a temperature-regulated heating jacket (the dashed line).The U-tube has two symmetrical compartments divided by a glass grid in the middle, with one side housing a 5.0-cm long Teflon permeation tube (VICI Metronics Inc.) and the other filled with 0.5-cm diameter glass beads.A corona discharge is used to produce the ion source; it consists of a stainless steel needle typically biased by about −1200 volts (V 1 ) and a grounded 1/4" OD stainless steel tube.A dry scroll pump (Varian) with a 500 liter per minute pumping speed draws the ambient air into the ID-CIMS system.A small portion of the air is introduced into the drift region through an orifice of about 0.5 mm.Within the drift tube, the reagent ion is produced and the ion-molecule reaction occurs to ionize where A corresponds to the neutral species to be analyzed and quantified, R represents the reagent ion, k is the reaction rate constant, and P denotes the product ion.The concentration of the neutral species after the reaction time ∆t is expressed, where [P (+or−) ] and [R (+or−) ] correspond to the intensities of product and reagent ions measured by the mass spectrometer, respectively.k can be obtained by laboratory measurements or theoretical calculations (Zhao et al., 2004).∆t is determined by the length of the drift tube and the velocity of the reagent ions, U.While moving along the drift region with the carrier gas at a flow velocity of (U f ), ions are also driven by a controllable electronic field to achieve a drift velocity (U d ), which is determined by where µ is the ionic mobility and E is the electronic field intensity.µ can be calculated from the reduced ionic mobility, µ 0 , where p and T are the pressure and temperature inside the drift tube, respectively.

Ion chemistry
The ion chemistry used to detect HNO 3 is similarly to that described by Huey and Lovejoy (1996).The reagent ion, SiF − 5 , is produced in two steps.A flow of about 300 standard cubic centimeters per minute (sccm) N 2 doped with <0.1% SF 6 flows through the ion source region, where SF 6 is attached by one electron, A trace amount of SiF 4 is introduced downstream of the ion source and reacts with SF N 2 O 5 subsequently reacts with I − to produce NO − 3 , with a rate constant of Huey et al., 1995).
The reduced ionic mobility µ 0 of SiF − 5 and I − are determined according to Mason and McDaniel (1988), where T eff is the effective temperature of the carrier gas (in K), m and M are the masses (in atomic mass units) of the ion (SiF − 5 or I − ) and the carrier gas (N 2 ), respectively, and Ω is the momentum-transfer collision integral (in cm 2 ).T eff is given by where k b is Boltzmann's constant and T is the temperature of the carrier gas.Ω is obtained from the tabulated values (Viehland et al., 1975) based on ion-neutral interaction potentials.Because no experimental data are available to characterize the ion-neutral interactions in the SiF − 5 /N 2 or I − /N 2 system, we perform ab initio calculations to obtain the ion-neutral interaction potentials using the Gaussian 03 software package on an SGI Origin 3800 supercomputer (Lei et al., 2000;Lei and Zhang, 2001).The geometry optimization and energy calculations at a series of center-to-center distances for SiF 31G(d,p) and B3LYP/LANL2DZ, respectively.The results are then fitted with a potential model where the B term represents the short-range repulsion energy, C 6 denotes the chargeinduced quadrupole attraction plus the London dispersion attraction, and C 4 represents the attraction between the ion and the dipole induced in N 2 .Based on the calculated potential well depth, ε, and the minimum position, r m , the corresponding value of Ω is obtained.

Instrument calibrations
The ID-CIMS, in principle, can quantitatively determine the concentration of neutral species using Eq. ( 7).The accuracy of the calculation is affected by uncertainties associated with several parameters, including the reduced ionic mobility, the ion-molecule reaction rate constants, the transmission efficiencies of the quadrupole mass filter, fragmentation in the ion-molecule reaction, and etc.These parameters are invariant under given experimental conditions and their uncertainties are minimized by calibration with known concentration gas standards.
The HNO 3 calibration employs a permeation device, as shown in Fig. 1.The permeation device (Fig. 1a) is heated and maintained at 40.0 • C.About 400 to 500 sccm N 2 carrier gas is fed into the side filled with glass beads and warmed to the same temperature as the entire device before entering the permeation tube side.After the permeation device, the concentrated HNO 3 /N 2 stream is injected into a 1-inch OD Teflon tube in which they are mixed with a 20 to 150 slpm dilution flow.The concentrations of the exiting HNO 3 flow ranges between 0.25 and 2 ppb.Prior to each calibration, the permeation device is maintained at an operational condition for more than 6 h to achieve equilibrium.Due to the "sticky" nature of HNO 3 , it is necessary to verify the effective permeation rate under the normal operation condition to account for any possible wall loss during the preparation of the calibration standards.The procedure to verify the permeation rate is similar to the calibration process, but, instead of introducing the HNO 3 standards into the ID-CIMS for calibration, the HNO 3 standard is introduced to a glass bubbler containing a specific amount of pure water (R>17 MΩ) for a specific time.All tubing is passivated by the HNO 3 standard for a few hours before the final solution is collected.The HNO 3 solution is analyzed by ion chromatography (DIONEX), which is calibrated by ultra pure sodium nitrate (Sigma-Aldrich) solutions.The measured permeation rate is 109±1.2ng/min, within 10% of the manufacture certified value (116 ng/min).
Figure 2 shows a correlation between the volumetrically determined HNO 3 standard concentration and the calculated concentration using the procedure described in Sect.2.1.The error bars represent the systematic variation in measurements.The value of the slope of Fig. 2 corresponds to the calibration factor used to compensate the difference between the true values and the adapted values of the calculation parameters, i.e., the ion-molecule reaction rate constant, reduced ionic mobility, quadrupole transmission efficiency, and etc.For five independent calibrations at 25 • C, a calibration factor of 2.2±0.2 is obtained.N 2 O 5 calibration is conducted with laboratory synthesized samples (Huey et al., 1995).N 2 O 5 is formed through two sequential reactions (Reactions 3 and 4 detected, we estimate its potential interference and find that only less than 6% of initial N 2 O 5 decomposes into NO 3 at room temperature.Figure 3 shows the plots of the N 2 O 5 concentrations measured by ID-CIMS against the concentrations determined from UV absorption.The slope in Fig. 3 represents the calibration factor.
During the field measurements, HNO 3 background checks are performed once a few hours by directing the ambient air flow through a two-inch OD nylon filter.N 2 O 5 background signal is checked by passing the ambient air mixed with several hundreds of ppb NO through a 12-in-long heated metal tubing.Detection limits of HNO 3 and N 2 O 5 for 10 s integration time are estimated about 100 ppt and 20 ppt, respectively.

Field setup and characterization of the inlet
The ID-CIMS instrument was deployed at the T0 supersite, located at the Instituto Mexicano del Petr óleo (IMP) near the center of the Mexico City Basin ( 19• 29.400 N,

99
• 08.911 W).The ID-CIMS was housed inside an air-conditioned hut on the roof of Bldg.32, which was about 30 m above the ground and among a cluster of buildings with the similar height.A 2.20 cm ID PFA tubing was used as the inlet, which had been proved to be the best inlet material (Neuman et al., 1999).In order to minimize the surface effect, ambient air was sampled from two feet above the hut ceiling, and the inlet length was about 12 feet to bring the air sample into the ID-CIMS, located a few inches away from the sampling window.During most times of the field campaign, the weather was dry and relatively cold.Because HNO 3 gas-aerosol partitioning was sensitive to temperature variations, no heating was applied to the HNO 3 inlet.Instead, the sampling flow rate was kept at 200 slpm to minimize the sampling residence time (0.42 s) and the entire inlet was kept under the ambient conditions.In order to characterize the inlet performance, several tests were conducted by exposing the inlet to HNO 3 (performed at 760 torr, 25 • C, RH=50%).The front of the inlet was exposed to a solution of 68 wt% HNO 3 for less than one second and the ID-CIMS was set to collect data at 0.4 Hz.As shown in Fig. 4 by about 80% after 9 s.The decay in the HNO 3 signal was well-fixed by an exponential decay function ([HNO 3 ]=2.4+97.5 exp(−0.237t),showing that the inlet had little memory effect.The influence between adjacent data points was less than 20% relative to each other, if data were collected within every 9 s.The inlet for N 2 O 5 measurements was also made from 2.2 cm ID Teflon tubing except that its length was only 2 ft to minimize surface loss.An Aerodyne aerosol mass spectrometer (AMS) was used to measure the chemical composition of non-refractory aerosols from 35 nm to 1.5 µm in size.This version of AMS was equipped with a high-resolution time-of-flight mass analyzer and was capable of analyzing organic species and most nitrate and sulfate compounds of different elemental compositions at the same nominal m/z.More details about the AMS were described by DeCarlo et al. (2006).
An ICMS was set up at T0 to measure both gas and aerosol phase acids.Air was aspired through a wet effluent diffusion denuder (WEDD) at 2 L/min to sample the gas phase acids.Water was continuously pumped through the denuder at a flow rate of 2 mL/min at counter flow to the air.The air to the aerosol collector (AC) first passed at 4 L/min through an activated charcoal denuder to remove the gas phase.The air stream was mixed with heated water vapor ( 100• C and a flow rate of 0.6 ml/min), which condensed on the aerosol particles.These droplets impacted on a cooled maze (Fisseha et al., 2006).The gas phase as well as the particle phase extracts were collected on a concentrator column (TAC-LP1, Dionex) and analyzed alternately using ion chromatography (conductivity detector) with a mass spectrometer in a quasi-continuous mode.The mass spectrometer (MSQ from Dionex) used electro-spray ionization and had a single quadrupole mass detector.NOx and O 3 data used in this work were measured by commercial instruments (Thermo Scientific).Both instruments were regularly calibrated during the campaign.The detection limits were 0.2 ppb for each species and the data were collected at 1-min interval.during the daytime and after sunset it was switched to the N 2 O 5 measurement.All data was collected at 0.1 Hz and presented in CST.
Figure 5 shows a diurnal profile of HNO 3 observed on 22 March, a sunny and hazy day.Due to its photochemical production nature, HNO 3 started to slowly accumulate right after sunrise.From 08:30 a.m. to 10:00 a.m., about 200 ppt HNO 3 was observed and no significant increase occurred before 11:00 a.m.After a steadily increase, HNO 3 reached near 3 ppb at 02:00 p.m., when the photochemical activity reached the maximum.From 02:00 p.m. to 04:00 p.m., HNO 3 reached a peak, and started to decrease sharply after 04:00 p.m. Within one and half hours, only about 400 ppt was observed.HNO 3 did not totally disappear even after it was completely dark, indicating that there were likely other HNO 3 sources sustaining its present.However, when the ID-CIMS was switched into the N 2 O 5 mode, no detectable N 2 O 5 was observed.
Figure 6 depicts another HNO 3 diurnal profile observed on 23 March, which was relatively cleaner and windier than the previous day.Before 09:00 a.m., HNO 3 was close the instrument detection limit.Shortly after, HNO 3 increased steadily to about 1 ppb at 11:30 a.m.and, then, started to decrease to 600 ppt at 01:00 p.m., when HNO 3 production was supposedly to be maximized.Around 02:45 p.m., HNO 3 started to increase again and reached a daily maximum 1.5 ppb.HNO 3 decreased steadily to 400 ppt at 04:00 p.m. Similar to the previous day observation, near 300 ppt HNO 3 was still observable at about 06:30 p.m. Also, when the ID-CIMS was switched into the N 2 O 5 mode, no N 2 O 5 was observable.Note that 22 March and 23 represented the typical polluted and clean conditions encountered during the field campaign, respectively.
Figure 7 shows the HNO 3 diurnal profile averaged over the entire campaign.Typ-4889 Introduction

Conclusions References
Tables Figures

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Full ically, no significant HNO 3 was observed before 08:00 a.m., and HNO 3 started to accumulate after 09:00 a.m. and reached daily high, 1±0.65 ppb, between 02:00 to 03:00 p.m.After 04:00 p.m., HNO 3 decreased rapidly to less than 0.4 ppb at 06:00 p.m. and gradually approached 200 ppt, before the ID-CIMS was switched to N 2 O 5 measurements.The occurrence of the HNO 3 daily peak was consistent with its photochemical production mechanism, however the magnitude of the HNO 3 peak was lower than that expected given the overwhelming NO x emission inventory in the MCMA and the strong tropical solar radiation.Another interesting observation was that several hundred ppt HNO 3 was still detected several hours after sunset.Since no significant N 2 O 5 was present, the nighttime production of HNO 3 from the hydrolysis of N 2 O 5 (through Reactions 2 to 5) could not proceed.
It is evident that the gas-phase chemistry alone could not explain the observed slow rise in the HNO 3 diurnal profiles after sunrise and the residual HNO 3 amount after sunset during MCMA 2006.Heterogeneous processing of HNO 3 in the particle phase needs to be accounted for to explain the HNO 3 measurements (Zhang et al., 1993).
It has been suggested that the heterogeneous reaction between HNO 3 and NH 3 in the particle-phase represents an important process to modulate the gaseous HNO 3 concentration (Seinfeld and Pandis, 1998), campaign (Moya et al., 2004).Although no direct measurements of NH 3 was available at T0 during MCMA 2006, its concentration is expected to be high, given that both population and traffic activity inside the MCMA maintained an increasing trend since 2003.
Figure 8 shows a comparison between gaseous HNO 3 and aerosol nitrate (NO − 3 ) mass concentration measured by an Aerodyne aerosol mass spectrometer (AMS) on 22 and 23 March.The calculated equilibrium constant (K p ) of Reaction 18 is also plotted in Fig. 8, which is a function of ambient temperature.There are several prominent features in Fig. 8.The aerosol nitrate NO − 3 started to increase right after sunrise and reached a maximum in the early morning.The aerosol nitrate peak occurred several hours earlier than that of the gaseous HNO 3 concentration.The aerosol nitrate dropped sharply after the morning peak and remained low throughout the afternoon and night hours.During each day, the measured HNO 3 concentration and the calculated K p were anti-correlated with NO − 3 .On the other hand, there appeared to have a good correlation between the measured HNO 3 concentration and calculated K p : both slowly increased at the morning hours and their peaks coincided.Hence, the slow rise in the measured HNO 3 concentration is likely explained by gas/aerosol partitioning.HNO 3 photochemically produced from Reaction 1 was scavenged from the gas phase by the particle-phase reaction with NH 3 to form aerosol nitrate, when ambient temperature was low and aerosol nitrate was favorable (K p <6).The gas/aerosol partitioning hence accounts for the delayed rise and daily maximum of measured HNO 3 compared to those of measured aerosol nitrate.As the temperature rose during the course of the day, the equilibrium favored gaseous HNO 3 .HNO 3 was released back to gas-phase from the evaporation of aerosol nitrate, even when its photochemical production decreased.After sunset, the HNO 3 photochemical production ceased, as reflected in the sharp decrease in measured HNO 3 , while a few hundreds of ppt HNO 3 were still measured.Since detectable NO aerosol nitrate corresponded to a higher HNO 3 peak, when the difference in K p was insignificant.This explains the difference in the measurements of NO − 3 and HNO 3 between 22 and 23 March 2006.Therefore, we concluded that during MCMA 2006 at T0 gas/particle partitioning played a key role to account for the measurements in gaseous HNO 3 and aerosol nitrate.
The magnitude and diurnal cycle of aerosol nitrate during the MCMA 2003 was reported by Salcedo et al. (2006).In the morning, particulate nitrate increased and was consistent with the production of nitric acid.However, after 11:00 a.m.aerosol nitrate decreased while HNO3 production continued.This observation was explained by the increase in the PBL height and temperature and the decrease in the RH as the day progressed.Also, it has been suggested that gas-particle partitioning plays a dominant role in the fate of aerosol nitrate over larger spatial scales (DeCarlo et al., 2007).

N 2 O 5 measurements
The ID-CIMS instrument was typically switched to conduct N 2 O 5 measurements after sunset during the 2006 MCMA campaign.During most times of the campaign, nighttime N 2 O 5 was below the detection limit of the instrument.Because NO concentration frequently exceeded 100 ppb after 07:00 p.m. and O 3 was quickly depleted by the freshly emitted NO, Reactions 3 and 4 were inhibited at the surface level.Therefore, NO 3 and N 2 O 5 did not play a major role during the nighttime chemistry on the surface.It was still likely that some HNO 3 might be formed above the planetary boundary layer (PBL) where the N 2 O 5 chemistry was still occurring at night and mixed downward (Stutz et al., 2004).However, as an exception, two N 2 O 5 peaks were observed in the late afternoon and early evening on March 26 (Fig. 9).There was a scattered shower started in the early afternoon and no HNO 3 was observed thereafter.At 04:00 p.m., the rain stopped and the sky remained cloudy, and O 3 and NO 2 were about 60 ppb and 20 ppb, respectively.Meanwhile, the NO concentration was less than 2 ppb.The only 3 ppb.Background checks were performed at the beginning and the end of the measurement.Two N 2 O 5 peaks near 40 ppt were observed around 05:00 p.m. and 08:00 p.m., when both NO 2 and O 3 were still substantial, but no fresh NO emission was present.From 05:45 p.m. to 07:20 p.m., the T0 site was hit by another intermittent shower, and no N 2 O 5 was observed and the NO x and O 3 instruments were off-line for calibrations.After 07:30 p.m., O 3 was anti-correlated with NO 2 due to the NO titration.About 08:50 p.m., NO started to increase significantly and nearly all O 3 was depleted, as N 2 O 5 disappeared.Although the ID-CIMS could also response to NO 3 radical as NO − 3 at the same m/e=62 peak, NO 3 radical was estimated to be less than 4% of the N 2 O 5 signal assuming that Reaction 4 was under thermodynamic equilibrium and using the observed NO 2 and O 3 concentrations.

Inter-comparison with ICMS
An ICMS instrument was set up side-by-side with the ID-CIMS during the MCMA 2006 field campaign.The ICMS utilized a denuder to selectively collect gas phase HNO 3 into aqueous solutions, which were then analyzed by an electrospray mass spectrometer.The ICMS generated one data point about every two hours, thus its time series could not match with the temporal resolution of about 10 s from the ID-CIMS measurements.We compared the results between the two instruments for periods when both instruments were collecting data and the ID-CIMS data were averaged to represent the data during a similar period.The results were plotted in Fig. 10.A good correlation in performance was found (R 2 =0.75) between the two techniques.We also performed a t-test for these two set of data and a P value of 0.68 further proved that statistically these two techniques reported the same results.The ID-CIMS data set was about 17% lower than the ICMS data set, and the intersection was well below the detection limit of either instrument.

Conclusions
We deployed an ID-CIMS during the MCMA 2006 campaign to measure HNO 3 and N 2 O 5 at the T0 site.The objective of this work is to characterize the NO x chemistry and its budget in the MCMA.Diurnally, HNO 3 was less than 200 parts per trillion (ppt) during the night and in the early morning, increased steadily from around 09:00 a.m.CST, reached a peak value of 0.5 to 3 parts per billion (ppb) in the early afternoon, and declined sharply to less than half of the peak value near 05:00 p.m. CST.
The HNO 3 mixing ratio was found to anti-correlate with aerosol nitrate, suggesting that the gaseous HNO 3 concentration was regulated by the gas-particle partitioning process.Inter-comparison between the ID-CIMS and the ICMS showed a good agreement   :36, 10:38, 12:30, 14:39, 16:35, 19:20, and 20:35 Huey and Lovejoy, 1996) to form SiF − 5 •HNO 3 adduct.During the field campaign, both the isotope peak of the reagent ion ( 30 SiF − 5 , m/e=125) and the product ion, SiF detected as the I − reagent ion, which is generated inside the ion source through electron attachment reaction, CF 3 I + e − → I − + CF 3 (13) ). O 3 and NO 2 are mixed in a sealed glass reactor and the produced N 2 O 5 is collected in a cryotrap (−78.5 • C) as white crystal.The first batch of N 2 O 5 is discarded to remove the water residue inside the cryotrap.During calibration, pure N 2 O 5 is kept in an octanol/dry ice bath (−57 • C).A small amount of dry N 2 flows through the container and carries N 2 O 5 vapor into a 10-cm long absorption cell inside a UV/VIS spectrometer (Perkin Elmer), where the absolute concentration of N 2 O 5 is measured by its absorption at 215 nm (σ=2.95×10−18 cm 2 molecule −1 , JPL 2006).The concentrated N 2 O 5 flow is diluted into a 140 slpm flow and analyzed by the ID-CIMS.Because N 2 O 5 is in thermodynamic equilibrium with NO 2 and NO 3 (Reaction 4) and NO 3 can also be , a spike of about 40 ppb HNO 3 was detected by the ID-CIMS within 3 s and the HNO 3 signal immediately decreased 4887 18) Depending on the ambient relative humidity (RH), ammonium nitrate formed in Reaction 18 can exist as a solid or an aqueous solution of NH + equilibrium constant, K p , is dependent on the temperature, lnK p = 84.6 − (24220/T ) − 6.1 × ln(T/298) (19) For the measurements relevant to MCMA 2006, the RH was fairly low (<41% averaged over the campaign period).The dry environment at the MCMA prevented NH 4 NO 3 from deliquescence and NH 4 NO 3 was expected in a solid state.High ammonia (NH 3 ) concentration (>35 ppb in the early morning) was reported during the MCMA 2003 ID-CIMS was switched to the N 2 O 5 mode at 04:20 p.m. and continued the measurement until 09:40 p.m., when NO and NO 2 were about 40 ppb and 60 ppb but O 3
Table 1 provides a summary of the calculations.The predicted values of µ 0 for SiF −5 and I− are 1.89 and 2.09 cm 2

Table 1 .
=0.75) in HNO 3 measurements.During most times of the MCMA 2006 campaign, N 2 O 5 was below the detection limit of the ID-CIMS due to high NO mixing ratio at the surface, with one exception of transient N 2 O 5 peaks of 40 ppt were encountered on 26 March under cloudy conditions.The results reveal that during the MCMA 2006 campaign HNO 3 was primarily produced through the photochemical process and HNO 3 diurnal profile was largely controlled by the gas/particle partitioning process.Introduction Ion-neutral potentials of SiF − 5 /N 2 and I − /N 2 systems based on ab initio calculations.