High concentrations of N 2 O 5 and NO 3 observed in daytime with a TD-CIMS : chemical interference or a real atmospheric phenomenon ?

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Introduction
The nitrate radical (NO 3 ) and dinitrogen pentoxide (N 2 O 5 ) play important roles in the nocturnal tropospheric chemistry.NO 3 is among the most important oxidants in the Figures

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Full atmosphere, particularly for biogenic hydrocarbons and sulfur-containing compounds (Atkinson, 1991).N 2 O 5 has long been recognized as a key intermediate in the transformation of nitrogen oxides (NO x = NO + NO 2 ) to aerosol nitrates (Riemer et al., 2003;Aldener et al., 2006;Chang et al., 2011;Brown and Stutz, 2012).Recent studies have also demonstrated an important role of N 2 O 5 hydrolysis at night in chlorine activation and the subsequent effect on the next-day's ozone formation (Osthoff et al., 2008;Simon et al., 2009;Thornton et al., 2010).Due to the low ambient abundances and high reactivity, accurate measurements of atmospheric N 2 O 5 and NO 3 have been challenging.Based on the strong absorption of NO 3 in the visible spectrum at 662 nm, several optical techniques have been developed to measure the ambient NO 3 , including long-path differential optical absorption spectroscopy (DOAS) (Platt et al., 1980;Atkinson et al., 1986), cavity ring-down spectroscopy (CRDS) (Brown et al., 2001(Brown et al., , 2002)), laser-induced fluorescence (LIF) (Wood et al., 2003(Wood et al., , 2005;;Matsumoto et al., 2005), and cavity enhanced absorption spectroscopy (CEAS) (Venables et al., 2006;Langridge et al., 2008).N 2 O 5 is determined by using a heated channel to decompose it into NO 3 or from the calculation according to the fast equilibrium between N 2 O 5 with NO 3 and NO 2 .
Another emerged technique for detecting ambient N 2 O 5 and NO 3 is the chemical ionization mass spectrometry (CIMS) which combines the ion-molecule chemistry with mass spectrometry detection.This technique was originally used in the laboratory to study the heterogeneous uptake kinetics of N 2 O 5 and ClNO 2 (e.g., Hu and Abbatt, 1997;Thornton et al., 2003;Thornton and Abbatt, 2005), and later on was applied in field measurements (Slusher et al., 2004;Zheng et al., 2008).The fundamental of this method is the reaction of I − (the reagent ion) with N 2 O 5 (and/or NO 3 ) forming the NO − 3 ion that can be detected at 62 amu.Previous laboratory studies suggested that the N 2 O 5 /NO 3 measurement at 62 amu may be subject to interferences from other Ncontaining trace gases such as HNO 3 , HO 2 NO 2 , ClONO 2 and others (see references listed in Table 1).On the other hand, field inter-comparison of a TD-CIMS and a cavity ring-down system showed a high degree of correlation between the two methods Introduction

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Full  (Slusher et al., 2004;Huey et al., 2007) indicating the capability of this method in measuring N 2 O 5 in the atmosphere.Although non-negligible and varying background signals at 62 amu have been observed during field studies (Chang et al., 2011), there have been no reports of detailed assessment of these potential interferences under different atmospheric conditions and for different configurations of CIMS.Kercher et al. (2009) developed a method to detect N 2 O 5 via the I(N 2 O 5 ) − cluster ion at 235 amu with an unheated inlet to address the interference at 62 amu in their CIMS, however this method has its own limitation including a lower sensitivity and larger impact of water vapor than that at 62 amu.
In autumn 2010, a TD-CIMS (thermal dissociation-CIMS), which is the same type used by Slusher et al. (2004) and Huey et al. (2007), was deployed to an urban site in Hong Kong which is characterized by large quantities of NO x , ozone and particulate matters.Unexpectedly, concentration peaks of N 2 O 5 + NO 3 were frequently observed in daytime in our study.To investigate this unusual observation, we have conducted a series of laboratory and field tests, including testing interferences individually and in combination from PAN, NO 2 , O 3 , HNO 3 , detection of N 2 O 5 by using a cold inlet via the detection of I(N 2 O 5 ) − cluster ion at 235 amu, and examination of daytime ClNO 2 which is a product of N 2 O 5 .In this paper, we will first present the ambient observations, and then the detailed test results on interferences and other evidence for daytime N 2 O 5 .
A surprising result from the tests is that PAN + NO 2 can have large interference to the TD-CIMS at 62 amu, which has not been reported in previous studies.Despite large chemical interference, we show that the observed daytime N 2 O 5 signal may be in part due to real contribution from NO 3 and N 2 O 5 .Introduction

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Full 2 Experiment and methodology

Measurement site
The field study was conducted in the urban center of Hong Kong ( 22• 18 N, 114 • 11 E, ∼ 15 m a.s.l.).The sampling site was located on the rooftop of a seven-story teaching building (about 20 m above the ground) on the campus of the Hong Kong Polytechnic University (HKPU).To the southeast of the campus, there is a cross-harbor tunnel in the north-south direction with a large flow of vehicles, especially during rush hours (see Fig. 1).Victoria Harbor is located about 1 km south of the measurement site.Thus emissions from vehicles and marine vessels are the most important local anthropogenic sources.The sampling site is surrounded by business districts, tourism and residential areas, with no large industrial sources nearby.The field measurements were carried out from 15 October to 4 December 2010, which is the season with the most severe photochemical pollution in Hong Kong (Wang et al., 2009).

The CIMS apparatus
A TD-CIMS was deployed to measure the sum of N 2 O 5 and NO 3 in this study.The system was developed at the Georgia Institute of Technology and is based on a soft and selective ionization process resulting from the reaction between a reagent ion and the target compounds, with the generated ions detected by a mass spectrometer.In the present study, the measurement method and operating parameters of TD-CIMS were the same as those described by Slusher et al. (2004)  was directed to the exhaust.Immediately before the flow tube, the last 14.8 cm of inlet was heated to 180 • C. The temperature of the air on the axis of the heated inlet was estimated at ∼ 117 • C (Slusher et al., 2004), under which condition more than 99 % of the N 2 O 5 would decompose into NO 3 with an ambient NO 2 level of 40 ppbv.The produced and the original NO 3 then reacted with I − to produce NO − 3 ions in the flow tube, which were subsequently detected and quantified by a quadruple mass spectrometer at 62 amu with a time resolution of 6 s.The NO − 3 signal is proportional to the concentration of total NO 3 , the I − signal, the effective reaction rate constant between them, and the effective reaction time (Huey, 2007).Figure 3  In the present study, the TD-CIMS instrument was calibrated once a week using the on-line N 2 O 5 synthesis method (Bertram et al., 2009).The calibration source was generated from the reactions of NO 2 with O 3 and subsequently NO 3 with NO 2 .The concentrations of the prepared N 2 O 5 were determined via the change in NO 2 concentrations after adding ozone, and conversely verified by the change in ozone after adding NO 2 .Zero air that was free from moisture and aerosols served as the diluent so as to prevent the hydrolysis of N 2 O 5 during the calibrations (see Fig. 2).NO 2 was monitored with a chemiluminescence analyzer equipped with a photolytic converter, which ensured measurements of the true NO 2 (Xu et al., 2012).Based on the relative standard deviation of the sample signal, the precision of our TD-CIMS was 3 % for 1000 pptv N 2 O 5 .The sensitivity of N 2 O 5 during the campaign was 2.8 ± 0.2 (mean ± SD) Hz pptv −1 .The instrument background was automatically measured for 2 min once an hour by adding a small flow (5 mL min exhibited relatively low background compared to ambient N 2 O 5 + NO 3 during both day and night.According to three times the standard deviation of the background signal, the typical detection limit of N 2 O 5 for 6 s average time was estimated to be 13 pptv for our TD-CIMS.

Other instruments
In addition to the TD-CIMS, a large number of other instruments were deployed concurrently.Here, we briefly describe those that were used to aid the presentation of the N 2 O 5 and NO 3 data.O 3 was measured by a commercial UV photometric analyzer (Model 49i, Thermo Environmental Instruments (TEI), USA).NO and NO 2 were analyzed with a chemiluminescence instrument (Model 42i, TEI) equipped with a photolytic NO 2 -converter (Air Quality Design, USA) (Xu et al., 2012).Solar radiation was measured using a LI-200 Pyranometer Sensor (LI-COR, USA).The ambient RH and temperature were monitored with a RH/temperature probe (Model 41382VC/VF, M.R. YOUNG, USA).During the field measurements, the minute-average data of trace gases and meteorological parameters were collected in real time by a data logger (Model 8816, Environmental Systems Corporation, USA).

Observation results
The time series of hourly mixing ratios of N 2 O 5 + NO 3 measured in urban Hong Kong from 15 October to 4 December 2010 is shown in Fig. 3. Similar to the measurement results obtained in other locations, several night-time concentration peaks of N 2 O 5 +NO 3 were noticed in our study (e.g., 31 October, 1 and 10 November).However, very high mixing ratios of N 2 O 5 + NO 3 were frequently observed during the daytime (e.g., 23 and 24 October, 19, 22 and 30 November, and 2 December).During the 50 day measurement period (excluding 11 November due to a lack of data), the average hourly concentration of N 2 O 5 + NO 3 was 86.9 (± 85.6) pptv with the maximum value of 1033 pptv Introduction

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To learn more about the atmospheric conditions associated with the elevated daytime N 2 O 5 and NO 3 signals at 62 amu, we present six cases during which the hourly N 2 O 5 +NO 3 values exceeded 400 pptv.Figure 4 presents the 5 min data of N 2 O 5 +NO 3 , O 3 , O x (O 3 + NO 2 ), NO, NO 2 , RH, and solar radiation for these episodes.The ratio of NO 3 /N 2 O 5 , calculated based on the temperature dependent equilibrium among N 2 O 5 , NO 3 and NO 2 , is also given.Inspection of the figure reveals that the daytime N 2 O 5 + NO 3 peaks appeared when both ozone and NO 2 were in high levels together with low levels of NO.However, daytime N 2 O 5 + NO 3 concentrations calculated using both steady state (Osthoff et al., 2006) and non-steady state approaches (McLaren et al., 2010) were much lower (by a factor of 1-100) than observations.The daytime concentrations in our study are also much higher than those of previous studies which reported daytime N 2 O 5 and NO 3 concentrations of only a few to over ten pptv (Geyer et al., 2003;Brown et al., 2005;Osthoff et al., 2006).

Chemical interferences
A major drawback of the TD-CIMS technique is that there could be potential interferences to the selected NO − 3 ion that can give rise to significant background noise at 62 amu (Chang et al., 2011).Previous laboratory studies have proposed many possible ion-molecule reactions yielding the NO − 3 ion, as summarized in Table 1.As stated earlier, the detailed results of these interferences in the real atmospheres have not been reported to date.To examine the influences of these potential interferences to our N 2 O 5 + NO 3 measurements, a series of tests were conducted in various atmospheric conditions and in laboratory during and after the measurement campaign.compounds that are unreactive to NO, including most potential ones other than PAN and HO 2 NO 2 .As stated above, the background was periodically measured once an hour throughout the campaign.Considerable amount of background signals at 62 amu in our CIMS was indeed seen during the present study with a mean counts (±SD) of 71.7 (±36.0)Hz, and they also exhibited a diurnal pattern with higher values in the late afternoon (see Fig. 5).This indicated that the N 2 O 5 + NO 3 measurements via the 62 amu channel in our TD-CIMS were subject to some interference during the present study.However, the background signals were much lower than the ambient signals.
For the six cases with daytime N 2 O 5 + NO 3 concentrations exceeding 400 pptv, the instrument background only accounted for on average 10 % of the ambient values.This background signal due to most of the gases shown in Table 1 and has been accounted for (i.e., subtracted from the total signals at 62 amu) in our final data.The interference from PAN and HO 2 NO 2 could not be determined because they also reacted with NO during zeroing.Their interferences are addressed below.

Interference of PAN
PAN is the most possible compound to interfere the TD-CIMS measurements in this study, not only due to its relatively high ambient abundances but also because it can escape from background determination by adding NO.In the previous studies, higher N 2 O 5 signals than the steady-state predictions were usually observed with elevated mixing ratios of PAN (Brown et al., 2005;Osthoff et al., 2006).The interference from PAN to the NO − 3 signals in our TD-CIMS was evaluated post the field campaign by adding synthetic PAN to zero air and in the ambient air samples.The PAN was generated from a PAN calibrator (Meteorologie Consult GmbH), which is based on the reaction sequence of NO and acetone in ultrapure air with a Penray lamp.The concentrations of PAN that were added to the CIMS were quantified simultaneously by a NO y analyzer (TEI 42CY).We also generated PAN using a conventional wet chemistry method by reacting peracetic acid with HNO 3 (Gaffney et al., 1984).The test Introduction

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Full results from the two PAN sources are consistent, thus this study only shows the result which used the PAN generated from the photolytic source.
For tests of PAN in zero air and in relatively clean ambient air at a coastal site (Hok Tsui), measurable interference from PAN was observed at 62 amu.The result in zero air indicates 17 ∼ 25 (22.8 ± 4.1) pptv from NO 3 + N 2 O 5 for every ppbv of PAN (see intercepts in Fig. 7).Adding PAN to ambient air at the coastal site yielded similar results.Table 2 shows the results from four tests conducted with varying ambient pollution levels and meteorological conditions.Introducing 5.6-6.3ppb of PAN to the ambient air resulted in an increase in the NO  (Veres et al., 2008;Roberts et al., 2010).This appeared not to be the case in our CIMS as additions of both PAN and HNO 3 didn't lead to any increase at 62 amu compared to the addition of PAN alone (see Fig. 6).
A surprising finding was much large interferences at 62 amu when the same spike tests were conducted at the PolyU site, which has very high NO x concentrations, and the interference appeared to increase with ambient NO 2 suggesting that reaction between PAN and NO 2 leads to significant interference at 62 amu.To confirm this, a series of tests were conducted.Figure 7 shows the signal at 62 amu as a function of PAN and NO 2 concentrations in zero, which clearly shows that the interference increases with both PAN and NO 2 .For example, at 5 ppbv of PAN, adding 60 ppbv of NO 2 produces 400 ppt equivalent NO 3 signal, compared to 150 pptv without NO 2 , indicating an amplifying effect of NO 2 on the previously reported PAN interference.The exact chemical reaction that leads to the interference is not clear.We believe that interference of PAN + NO 2 is related to the thermal dissociation of PAN followed by radical reactions with NO 2 in the heated inlet.The radical reactions might serve as a source of NO 3 and N 2 O 5 , or the products could react with I − to produce NO − 3 ions which is detected at 7482 Introduction

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Full 62 amu.We will evaluate the possible contribution of PAN + NO 2 to the observed high daytime N 2 O 5 + NO 3 for the 2010 autumn study later.

Interference of HNO 3
Another possible source of interference to the NO − 3 ion is the HNO 3 since it has a NO 3 group.Several studies have proposed the production of NO − 3 from ion-molecule reactions involving HNO 3 , with somewhat inconsistent results.Huey and co-workers showed that the reaction of I − with HNO 3 (producing NO − 3 ) was rather inefficient.Thornton and coworkers on the other hand found a significant background signal (10-50 Hz) at the NO − 3 mass in a cold iodide CIMS under conditions of long ion-molecule reaction time, and attributed this to the reaction of HNO 3 with I − (Thornton et al., 2003;Thornton and Abbatt, 2005).Roberts and coworkers reported that the HNO 3 was sensitive to the acetate ions and react to produce NO − 3 ions at the 62 amu (Veres et al., 2008;Roberts et al., 2010).
The relatively low background signals by adding NO in our study suggest insignificant interference from HNO 3 to our CIMS, which has been corrected during data reduction.This was further confirmed by the addition of HNO 3 .The test was carried out three times for varying ambient conditions to check the repeatability of the results, which are listed in Table 2 with an example being shown in Fig. 6.It can be seen that after introducing a trace amount of HNO 3 to the inlet tube of our TD-CIMS, there was no significant increase in the NO − 3 signal compared to the ambient air.Additionally, adding HNO 3 to a trace level of PAN which resulted in an acetate ion signal of ∼ 0.9 × 10 4 Hz, the NO − 3 signal showed no apparent change compared to that for only adding PAN.These results suggest that the HNO 3 itself and its mixture with acetate ions have no significant interference to the detection of N 2 O 5 +NO 3 via the NO (negative-ion proton-transfer CIMS) (Veres et al., 2008;Roberts et al., 2010), possibly due to different configurations and operation conditions.

Interference of other possible compounds
ClONO 2 , BrONO 2 and HO 2 NO 2 also react efficiently with I − to produce NO − 3 (Huey et al., 1995;Hanson et al., 1996;Zhang et al., 1997;Amelynck et al., 2001).Again, the relatively low instrument background indicated small interference from ClONO 2 and BrONO 2 and any interference from them would have been corrected in our measurements because they cannot be removed by adding NO.The zeroing would not work for HO 2 NO 2 .However, it is impossible for HO 2 NO 2 to pass through the heated inlet tube in our TD-CIMS considering its thermally unstable nature.
In addition, the NO − 3 ion may also come from the ion-molecule reactions involving Cl − (35 amu), C 2 H 2 N − (40 amu), NO − 2 (46 amu), O − 3 (48 amu), CO − 3 (60 amu), and CO − 4 (76 amu) (listed in Table 1).These reagent ions were observed in quite low levels (i.e., < 20 Hz for 40 and 48 amu, and 10-500 Hz for 35, 46, 60 and 76 amu) in our TD-CIMS during the field measurements (see Fig. 3).Therefore, the ion-molecule reactions in- In summary, according to the above tests and discussions, reactions between PAN and NO 2 in the heated inlet are found to have significant interference to signal at 62 amu in our TD-CIMS, while HNO 3 and other compounds are not believed to have contributed to signal at 62 amu, although our tests are not exhaustive for including all other chemicals.Introduction

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Full To check the measurement results by at 62 amu, we attempted to measure ambient N 2 O 5 with a cold version of CIMS immediately after the present campaign at the same site.The CIMS was configured similar to that described by Kercher et al. (2009).When an unheated inlet tube is used, the reactions of N 2 O 5 with I − produce both the NO  (Kercher et al., 2009).In our study, the sensitivity of N 2 O 5 from the I(N 2 O 5 ) − ion was determined at 0.55 ± 0.003 Hz pptv −1 , which is smaller than the value of 0.93 Hz pptv −1 obtained by Kercher et al. (2009)  not re-scaled according to the I(H 2 O) − /I − ratio as described by Kercher et al. (2009), because the I − ion at 127 amu was not detected in real-time in our study.The rescaled N 2 O 5 concentrations during the daytime would be even higher, because the daytime I(H 2 O) − signals were generally lower than (by a factor of 0.4-1.0)those during the calibrations.Despite the above uncertainty in determining the absolute value, the general variation pattern of N 2 O 5 should be trustworthy.Consistent with the setup with the heated inlet and the detection at 62 amu, signal at 235 amu with the cold CIMS also showed a daytime peak.Figure 9 gives an example of the ambient results taken on 20 December 2010.
Another independent piece of evidence for the daytime N 2 O 5 + NO 3 in Hong Kong is concurrent increase in the mixing ratios of ClNO 2 (a product of N 2 O 5 hydrolysis) observed in a follow-up study in western Hong Kong (Tung Chung; see SI for the experiment information).At this site, elevated N 2 O 5 + NO 3 concentrations were also found at daytime during photochemical episodes, with the ClNO 2 signals (208 amu) showing concurrent increases.Figure 10 shows an example for 28 August 2011.On that day, N 2 O 5 + NO 3 from 62 amu exhibited an afternoon peak of 670 pptv (5 min average, at 14:00 LT), and ClNO 2 had a concurrent enhancement to 120 pptv.To estimate the levels of N 2 O 5 that would be needed to sustain such amount of ClNO 2 , we assumed a photostationary steady state for ClNO 2 in the afternoon with an uptake coefficient of 0.03 for N 2 O 5 hydrolysis on aerosol surfaces and a ClNO 2 yield of 10 %.
The photolysis rate of ClNO 2 was estimated as 7.8 × 10 −4 s −1 using the method by Simon et al. (2009), and the aerosol surface area was 979 mm 2 m −3 from concurrent measurements of aerosol size distribution.The calculation showed that to produce the observed 120 pptv of ClNO 2 , at least 518 pptv of N 2 O 5 would be required.This result provides additional corroboration of the possible elevated daytime concentrations of N 2 O 5 +NO 3 in Hong Kong.

AMTD Introduction
Full  Full  Full 1. Line 2 on Page 3, add a blank between "N 2 O 5 " and "has".
2. Line 4 on Page 8, delete the word of "for" immediately after "with".
Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | which was configured to simultaneously measure ambient N 2 O 5 and PANs.The schematic diagram of our TD-CIMS is shown in Fig. 2. I − , which was produced from passing a flow of 2 sccm of 0.3 % CH 3 I/N 2 through an alpha ion source (Po-210), served as the reagent ion.Ambient air samples were drawn through a PFA-Teflon tube (I.D., 9.5 mm; O.D., 12.7 mm; length, Discussion Paper | Discussion Paper | Discussion Paper | depicts the mass spectrums of ambient air in urban Hong Kong obtained both at day and at night, which clearly shows the I − signal at 127 amu, NO − 3 (N 2 O 5 + NO 3 ) signal at 62 amu, and CH 3 C(O)O − (PAN) signal at 59 amu, etc.
−1 ) of NO (1000 ppm) to the sample flow (diluted to 9.2 SLPM), titrating NO 3 and thus N 2 O 5 .The background signal of the NO Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | The background determination by adding excess NO provided a first examination of the interferences.Such determined background reflects the interferences from Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

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3 signal of 42-75 pptv of NO 3 +N 2 O 5 in our TD-CIMS, again indicating interference from PAN to the field NO 3 /N 2 O 5 measurements via the NO − 3 by TD-CIMS.The mechanism how PAN interferes the NO − 3 detection is unclear.Some researchers proposed that the CH 3 C(O)O − ion (produced from the reaction of I − with CH 3 C(O)O 2 -the product of PAN thermo-dissociation) likely reacts with HNO 3 to produce NO − 3 Discussion Paper | Discussion Paper | Discussion Paper |

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3 ion in our TD-CIMS.It should be noted that the result of non-reactivity of HNO 3 to the acetate ion in our TD-CIMS is different from that obtained by Roberts and coworkers using a NI-PT-CIMS Discussion Paper | Discussion Paper | Discussion Paper | believed to have no significant influence to the NO − 3 detection based on the relationship between the product ion and reactants.
Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |5 Contribution of interference to daytime concentrationsAs indicated above, laboratory and field tests revealed significant interference from PAN and NO 2 to 62 amu in our TD-CIMS.The contribution from this interference to the observed daytime N 2 O 5 + NO 3 in this study was examined and corrected based the tests results in Fig.7.During the field campaign at PolyU, PAN was concurrently measured by the same TD-CIMS and a GC-ECD instrument (gas chromatography with an electron capture detector).For the six episodes shown in Fig.4, the interference would contribute to 41.3-67.0%, 29.0-39.8%, 32.2-73.2%,25.6-49.2%, 19.4-43.4%, 20.4- 77.8 %, respectively, to the daytime signals in the 6 episodes.(Two examples on 23 October and 30 November were shown in Fig.8.)The remaining daytime signal may be a real contribution from N 2 O 5 and NO 3 , but interferences from other untested chemicals cannot be ruled out.As interferences at the 62 ion channel are large and variable, it is difficult to determine the fraction of real signal from N 2 O 5 + NO 3 in our study at 62 amu.6Other evidence of daytime NO 3 + N 2 O 5 in Hong Kong − cluster ions.The I(N 2 O 5 ) − ion (at 235 amu) is thought to be free from the chemical interferences that can perturb the NO − 3 ion, and thus provides a better measure of N 2 O 5 possibly due to a smaller sample flow rate and/or stronger electric field in the collisional dissociation chamber in our CIMS.It's also noteworthy that the N 2 O 5 data from I(N 2 O 5 ) − in our study were Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Zheng, J., Zhang, R., Fortner, E. C., Volkamer, R. M., Molina, L., Aiken, A. C., Jimenez, J. L., Gaeggeler, K., Dommen, J., Dusanter, S., Stevens, P. S., and Tie, X.: Measurements of HNO 3 and N 2 O 5 using ion drift-chemical ionization mass spectrometry during the MILAGRO/MCMA-2006 campaign, Atmos.Chem.Phys., 8, 6823-6838, doi:10.5194/acp-8DiscussionPaper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

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Line 8 on Page 14, change "Figure 8" into "Figure 9". 5. Line 15 on Page 14, change "Figure 9" into "Figure 10".6. Page 23, replace the current Fig. 1 by a corrected version as follows.Several words that are attached on the figure have been modified.7. Page 29, replace the current Fig. 7 by a corrected version as follows.Some words have been added to indicate the PAN concentrations.

Supplementary material related to this article is available online at http://www. atmos-meas-tech-discuss.net/6/7473/2013/amtd-6-7473-2013-supplement.pdf. Introduction
CIMS technique, which has been previously applied to field measurements in the US, was deployed to measure ambient NO 3 and N 2 O 5 in urban Hong Kong in a photochemical season.Surprisingly, concentration peaks of NO 3 + N 2 O 5 were frequently observed in daytime at 62 amu channel in the TD-CIMS, which is in contrast to our current understanding of reactive nitrogen chemistry.Our subsequent laboratory and field tests provide new insights into chemical interferences in the TD-CIMS.In particular, we have discovered that reaction between NO 2 and PAN can amplify the interference from PAN at 62 amu.This interference could have contributed 30-50 % to the average daytime NO 3 and N 2 O 5 at our site.Despite the large interference, evidence exists to suggest that the elevated N 2 O 5 in daytime may be in part due to real contribution from NO 3 or N 2 O 5 .In viewing of the large and variable interferences at 62 amu and difficulty in correcting them, we conclude that it is not suitable to use the TD-CIMS to measure NO 3 or N 2 O 5 at 62 amu in a high NO x environment like the present study site.Adoption of either 235 amu with a cold inlet or an optics-based technique is recommended.We also suggest more studies to examine the abundance of daytime NO 3 and N 2 O 5 in similar urban areas with co-existence of high ozone and NO 2 .

Table 2 .
Summary of the interference tests for the TD-CIMS.