Large particulate nitrate formation from N 2 O 5 uptake in a 1 chemically reactive layer aloft during wintertime in Beijing

12 Particulate nitrate (pNO3 ) is a dominant component of secondary aerosols in urban 13 areas. Therefore, it is critical to explore its formation mechanism to assist with the 14 planning of haze abatement strategies. Simultaneous ground-based and tower-based 15 measurements were conducted during a winter heavy haze episode in urban Beijing, 16 China. We found pNO3 formation via N2O5 heterogeneous uptake was negligible at 17 ground level, due to the presence of high NO concentrations limiting the production 18 of N2O5. In contrast, the contribution from N2O5 uptake was larger at higher altitudes 19 (e.g., > 150 m), which was supported by the observed large total oxidant (NO2 + O3) 20 missing aloft compared with ground level. The nighttime integrated production 21 potential of pNO3 for the higher altitude air mass overhead urban Beijing was 22 estimated to be 50 μg m, and enhanced the surface pNO3 significantly with 28 μg 23 m after nocturnal boundary layer broken in the next morning. In this case, the 24 oxidation of NOX to nitrate was maximized once N2O5 uptake coefficient over 0.0017, 25 since N2O5 uptake dominated the fate of NO3 and N2O5 with the presence of large 26 aerosol surface concentrations. These results highlight that pNO3 formation via N2O5 27 Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-1217 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 11 January 2018 c © Author(s) 2018. CC BY 4.0 License.

heterogeneous hydrolysis at higher altitude air masses aloft could be an important source for haze formation in the urban airshed during wintertime.Accurately describing the formation and development of reactive air masses aloft is a critical task for improving current chemical transport models.

Introduction
Winter particulate matters (PM) pollution events occur frequently in China, and have drawn widespread and sustained attention in recent years (Guo et al., 2014;Zhang et al., 2015;Huang et al., 2014).PM pollution reduced visibility (Lei and Wuebbles, 2013) and had harmful effects on public health (Cao et al., 2012).Particulate nitrate (pNO3 -) is an important component of secondary inorganic aerosols, and contributed to 15-40% of the PM2.5 mass concentration in China (Sun et al., 2013(Sun et al., , 2015a(Sun et al., , 2015b;;Chen et al., 2015;Zheng et al., 2015;Wen et al., 2015).The main atmospheric pathways of nitrate formation are (1) the reaction of OH with NO2 and (2) N2O5 heterogeneous hydrolysis (Seinfeld and Pandis, 2006).The first reaction (OH + NO2) was a daytime pathway since OH is severely limited at night, and N2O5 uptake was refer to a nighttime pathway as NO3 and N2O5 is easily photo-labile.
Nitrate formation via N2O5 heterogeneous hydrolysis was proved efficient by ground based observation in summer in North China (H.C.Wang et al., 2017b; Z. Wang et al., 2017), which is comparable with or even high than the daytime formation.
Several model studies showed N2O5 hydrolysis is responsible for nocturnal pNO3 - enhancement in Beijing (Pathak et al., 2009(Pathak et al., , 2011;;H.C. Wang et al., 2017a).Although the pNO3 -formation via N2O5 uptake is significant in summertime, the importance of this pathway in wintertime was not well characterized.As there are many differences of N2O5 chemistry between winter and summer.First, as the key precursor of NO3 and N2O5, O3 level are much lower in winter than in summer due to short daytime length and weak solar radiation.Second, colder temperatures and high NO2 levels favor partitioning towards N2O5.Third, nighttime lasts much longer in winter, making N2O5 heterogeneous hydrolysis potentially more important in pNO3 -formation.Finally, Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2017-1217Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 11 January 2018 c Author(s) 2018.CC BY 4.0 License.N2O5 uptake coefficient, the most important parameter in N2O5 heterogeneous hydrolysis, is likely very different from that in summer.Since the particle characteristics and meteorological conditions (e.g.organic compounds, particle nitrate, liquid water contents, solubility, viscosity and relative humidity) are different in summer and winter (Chen et al., 2015;Zhang et al., 2007).These differences would result in the N2O5 uptake coefficient has large variation (Wahner et al., 1998;Mentel et al., 1999;Kane et al., 2001;Hallquist et al., 2003;Thornton et al., 2003;Bertram and Thornton, 2009;Grzinic et al., 2015;Wagner et al., 2013).
In addition to seasonal differences, previous studies have also shown a potential altitude dependence in pNO3 -production.In the evening, vertical mixing is strong suppressed in nocturnal boundary layer (NBL) due to the reduction of sunlight diminishes the heating of the earth's surface, leading to vertical layer occur with NO3 and N2O5 gradient (Brown et al., 2007), as well as the particle compounds percentage and size distribution (Ferrero et al., 2010;2012).On nights when NO3 radical production in the surface layer is negligible due to high NO emissions, N2O5 uptake may still be active aloft without NO titration (Pusede et al., 2016;Baasandorj et al., 2017).The N2O5 uptake aloft leads to elevated pNO3 -formed in the upper layer with effective NOX removal (Watson et al .,2002;S.G. Brown et al., 2006;Lurmann et al., 2006), which was reported with field observations at a high altitude site in Kleiner Feldberg, Germany (Crowley et al., 2010a), London British Telecommunications tower, UK (Benton et al., 2010), Boulder Atmospheric Observatory (BAO) tower in Colorado, USA (Wagner et al., 2013) and so on.Model studies also proposed nitrate formation varied in different height (Kim et al., 2014;Ying, 2011;Su et al., 2017).
The mass fraction and concentration of pNO3 -in Beijing was reported higher aloft (260 m) than at the ground in Beijing (Chan et al., 2005;Sun et al., 2015b), and they explained the favorable gas−particle partitioning aloft under lower temperature conditions.
To explore the possible sources of pNO3 -and the dependence of altitude in wintertime, we conducted vertical profile measurements of NO, NO2, and O3 with a tower platform in combination with simultaneous ground measurements of these parameters in urban Beijing.A box model was used to investigate the reaction rate of N2O5 heterogeneous hydrolysis and impact on pNO3 -formation at different altitudes during a heavy haze episode over urban Beijing.Additionally, the dependence of pNO3 -formation on the N2O5 uptake coefficient was probed.

Field measurement
Ground measurement (15 m above the ground) was carried out in the campus of Peking University (PKU, 39°59'21"N, 116°18'25"E) in Beijing, China.The location of the PKU site was shown in Fig. 1, as well as the vertical measurement site (IAP, introduced below).At PKU site, dry-state mass concentration of PM2.5 was measured by a TEOM 1400A analyzer.NOx was measured by chemiluminescence analyzer (Thermo Scientific, TE-42i-TR) and O3 was measured by a UV photometric O3 analyzer (Thermo Scientific, TE-49i).Dry state particle number and size distribution (PNSD) was measured from 0.01 to 0.6 µm with a Scanning Mobility Particle Sizer (SMPS, TSI Inc. 3010).These parameters were listed in Table S1.The data were collected from December 16 to 22, 2016.Additionally, relative humidity (RH), temperature (T), wind direction and speed were available during the measurement period.
Vertical profile measurements were conducted from December 18 to 20, 2016 at the tower-based platform (maximum height: 325 m) on the campus of the Institute of Atmospheric Physics, Chinese Academy of Sciences (IAP, 39°58'28"N, 116°22'16"E), during a heavy PM pollution episode.The IAP site is just with 4 km distance from the PKU site.The measurement instruments were installed on board a movable cabin on the tower.The ambient NOx and O3 concentrations were measured with two low-power, light-weight instruments (Model 405 nm and Model 106-L; 2B Technologies, USA).The Model 405 nm instrument measures NO2 directly based on the absorbance at 405 nm, and NO is measured by adding excess O3 (conversion efficiency ~100%).The limit of detection of both NO and NO2 is 1 part per billion volume (ppbv), with an accuracy of 2 ppbv or 2% of the reading, and the time resolution is 10 s.The Model 106-L instrument measures O3 based on the absorbance at 254 nm with a precision of 1 ppbv or 2% of the reading and a limit of detection of 3 ppbv.Height information was retrieved from the observed atmospheric pressure measured by Model 405 nm instrument.The cabin ascended and descended at a rate of 10 m min -1 , with a height limit of 260 m at daytime and 240 m at night.The cabin stopped after reaching the peak and parameters were measured continually for the last 10 min of each cycle.One vertical cycle lasted for approximately 1 h.We measured two cycles per day, one in the morning and the other in the evening.Homogeneous hydrolysis of N2O5 and NO3 heterogeneous are neglected in this analysis because there is little absolute humidity and extremely low NO3 concentration during wintertime (Brown and Stutz, 2012).The corresponding rate constants of R1-R3 are those reported by Sander et al., (2011).

Box model simulation
Following the work of Wagner et al., (2013), the box model can be solved by four equations (Eq.1-4).In the framework, O3 only losses via the reaction of NO2+O3 and the change of the O3 concentration can express as Eq. 1.Since the ratio of N2O5 to NO3 was calculated to be larger than 150:1 in a typical urban region in wintertime (NO2 = 15 ppbv, nighttime temperature = 0°C), N2O5 is proposed to be dominate the NO3 loss, than means two molecules of NO2 lost (convert to nitrate or ClNO2) for one molecule NO3 formed (Eq.2).In the model we need to know the mixing ratio of NO2 and O3 at time zero (here set to sunset).According to Eq. 1 and Eq. 2, the NO2 (t=0) and O3 (t=0) concentration can derived from the duration time and the vertical measured NO2 and O3 at each height.Assuming the equilibrium between NO3 and N2O5 is maintained after a time period, the sum concentration of NO3 and N2O5 can be described by Eq. 3. Using the temperature dependent equilibrium rate constant (keq) and the modeled NO2 at a certain time, Eq. 4 can be used to determine the ratio of N2O5 and NO3.Combined, Eq. 1-4 allow for the calculation of N2O5 concentrations, given a constant of NO3 and N2O5 rate constant (kNO3 and kN2O5).Modeled N2O5 concentrations and given kN2O5 are then used to estimate pNO3 -formation, here the HNO3 produced in R4 is not accounted, as a large part of the products are organic nitrates (Brown and Stutz, 2012).Here kNO3 and kN2O5 denotes the pseudo first order reaction rate constant of the total NO3 reactivity caused by ambient volatile organic compounds (VOCs) and N2O5 heterogeneous uptake, respectively.kN2O5 is given in Eq. 5. Sa is aerosol surface area, C is the mean molecular speed of N2O5, and γN2O5 is N2O5 uptake coefficient.The model is run from sunset to sunrise, where the length of night was about 14.5 h.
(Eq. 1) Dry-state Sa at PKU site was calculated based on the PNSD measurement, which was corrected to ambient (wet) Sa for particle hygroscopicity by a growth factor (Liu et al., 2013).The uncertainty of the wet Sa was estimated to be ~30%, which was associated from the error from dry PNSD measurement (~20%) and the growth factor PM measurements by National Monitoring Sites proved this heavy haze pollution episode was a typical regional event (Fig. S1).Furthermore, synchronous study on the night of December 19, 2016 shown small variation of vertical particle number concentration, with the boundary layer height below 340 m (Zhong et al., 2017).
Therefore, the Sa at PKU site is representative to the urban Beijing conditions and applied in the model is reasonable.
N2O5 uptake coefficient was regard as the main uncertainty of the N2O5 loss, along with the ClNO2 yield lead to the uncertainties in the estimation of particulate nitrate formation (Thornton et al., 2010;Riedel et al., 2013;Wagner et al., 2013;Phillips et al., 2016).Wagner et al., (2013) shows the significant particular nitrate suppression aloft in the wintertime in Denver, USA, with the uptake coefficient of about 0.005 when particulate nitrate fraction in the PM2.5 mass concentration about 40%.
Considered the high particle nitrate content in North China in wintertime is similar to that in Denver (Sun et al., 2013(Sun et al., , 2015a;;Chen et al., 2015;Zheng et al., 2015;Wen et al., 2015), here we used a constant uptake coefficient of 0.005 as the model initial input in the base case, and the uncertainty of N2O5 uptake coefficients will be discussed later.Since the model input of ClNO2 yield only affect the value of produced particulate nitrate concentration, and would not change the modeled N2O5 concentration, here we set the initial fClNO2 to zero.The impact of ClNO2 yield will be further discussed later.
Respect to kNO3, the average value in summertime was estimated to be 0.024 s -1 in 2006 (H.C.Wang et al., 2017a).While in wintertime, the kNO3 should be smaller as the intensity of plant emissions reduced in the lower temperature and weak solar radiation.The model input k(NO3) was set to an relative moderate value of 0.02 s -1 (equivalent to 0.2 ppbv isoprene + 40 pptv monoterpene + 1.0 ppbv cis-2-butene).A series of sensitivity tests were conducted to study the uncertainties to the model simulation, and the detailed test set were listed in Table 1, included the test of N2O5 uptake coefficient and kNO3.The γN2O5 sensitivity tests were set to a lower limit of 0.001 to a upper limit of 0.05, as well as the kNO3 set to 0.001, 0.01 and 0.1 s -1 .

Ground-based observations.
A severe winter PM pollution event was captured from the ground observations from December 16 to 22, 2016 in Beijing.Figure 2a shown the mass concentration of PM2.5 began to increase on December 16 and reached the maximum value of 480 μg m -3 December on 20.A fast PM growth event happened on the night of December 19-20 (colored red in Fig. 2a), the PM2.5 mass concentration increased continuously throughout the night, with an overall increment of 100 μg m -3 .During the episode, the meteorological condition is featured with high RH (50% ± 16%) and low temperature (2 ± 3 o C).The slow surface wind (< 3 m s -1 ) indicated the atmosphere was static stabilized (Fig. 2c, 2d).The daytime O3 concentration was low due to high NO emission and weak solar radiation.After sunset, O3 was rapidly titrated to zero by the elevated NO.The presence of high NO concentrations would have strongly suppressed the concentration of NO3, and further suppressed N2O5 near the ground.
Figure 2b depicted large amounts of NO and NO2 were observed throughout the whole PM pollution episode, suggesting that pNO3 -production via N2O5 uptake was not important near the ground during the winter haze episode.

Tower observations.
Six vertical measurements of Ox (< 50 m) was consist with that measured at ground level and shown in Fig. S2, confirmed the two sites are comparable at ground level at last.On the night of December 20 (Fig. 3a), the NO2 and NO from 0-240 m were abundant and conservative around 21:00, with the concentration of 80 ppbv and 100 ppbv, respectively.The O3 concentrations keep zero during the nighttime (Fig. 3b).
The vertical profile on December 20 suggests that at least below 240 m, the N2O5 chemistry is also not important like ground level as mentioned above.The case on the night of December 18 is similar to the night of December 20.
The vertical measurement on December 19 did not like those happened on December 18 and 20. Figure 4a shows the vertical profiles around 21:00 on December 19 that NO was abundant from the ground to 100 m, then gradually decreased to zero from 100 m to 150 m, and stay zero above 150 m.The observed NO2 concentration was 85 ± 2 ppbv below 100 m, which gradually decreased from 100 m to 150 m, and was 50 ± 2 ppbv from 150 m to 240 m.The observed O3 concentrations were below the instrument limit of detection below 150 m (Fig. 4b).Above 150 m, the O3 concentration was 20 ± 2 ppbv, corresponding to the greatly diminished NO concentration.With respect to the total oxidants (Ox = O3 + NO2), the mixing ratio of Ox was 85 ± 2 ppbv at lower altitudes, while the Ox concentration at higher altitudes was 15 ppbv lower than that at lower altitude (Fig. 4b).The OX missing at the higher altitude air mass indicated an additionally nocturnal removal of Ox aloft.
Figure 5 depicted the vertical measurement at 09:30 on the morning of December 20 has similar features with those observed at 21:00 on December 19, and the NBL still not be broken.The OX missing aloft in the morning increased to 25 ppbv at 240-260 m, demonstrated that an additional 25 ppbv of Ox was removed or converted to other compounds at higher altitudes than surface layer on the night of December 19-20.Figure S3 shows the vertical profiles of NO, NO2, O3 and Ox at ~12:00 on December 18, when the solar radiation is strong enough to drive the trace gas mixing well in vertical direction.NOx and O3 were observed well mixed indeed, with small variation from ground level to 260 m.

Particulate nitrate formation aloft.
N2O5 uptake is one of the two most important pathways of the ambient NOX losses, and is consequently an important pathway of pNO3 -formation (Wagner et al., 2013;Stutz et al., 2010;Tsai et al., 2014).At higher altitudes (e.g.> 150 m), NO3 and N2O5 chemistry can be initiated in the co-presence of high NO2 and significant O3 levels.
Therefore, N2O5 uptake could represent a plausible explanation for the observed OX missing in the higher-altitude air masses on the night of December 19.To explore this phenomenon, a time-step box model was used to simulate the NO3 and N2O5 In the base case, the modeled N2O5 concentration is zero below 150 m, as the high level NO consumed NO3 formation fast.While the modeled N2O5 concentrations at 21:00 were in the range of 400-600 parts per trillion volume (pptv) above 150 m (Fig. 5a).pNO3 -accumulation via N2O5 heterogeneous hydrolysis from sunset to the measurement time was significant, yielding a maximum of 24 μg m -3 within 4.5 hours after sunset (Fig. 5(b)).
The box model enabled the analysis of the integrated pNO3 -and ClNO2 via N2O5 uptake over the whole night.As shown in Fig. 5c, the modeled integrated pNO3 -went up to as high as 50 μg m -3 .The integrated pNO3 -at sunrise was equal to the loss of 27 ppbv OX, shows a good agreement with the observed OX missing aloft at the morning hours.During the nighttime, the formed pNO3 -aloft via N2O5 uptake would lead to the particle nitrate concentration much higher than that in the surface layer, which has been reported many field observations (Watson et al .,2002;S.G. Brown et al., 2006;Lurmann et al., 2006;Ferrero et al., 2012;Sun et al., 2015b).In addition, during the morning time when NBL was broken, the elevated pNO3 -aloft will vertically mixed and enhanced the PM concentration at surface layer, this phenomenon also been observed in previous studies (Watson et al .,2002;S.G. Brown et al., 2006;Lurmann et al., 2006).In this case, the planetary boundary layer (PBL) height during the daytime is about 340 m on December 20 (Zhong et al., 2017).Assuming that the height of NBL and planetary boundary layer (PBL) are the same, and the air mass was well mixed in the following morning in the PBL.The nighttime N2O5 uptake aloft would be enhanced the ground pNO3 -mass concentration significantly with 28 μg m -3 in the morning, which is in good agreement with the observed PM peak in the morning on December 20, with the PM enhancement of ~60 μg m -3 .The result demonstrated that the nocturnal N2O5 uptake aloft and downward transportation are really importance in understanding the PM growth process.

Sensitivity studies.
Previous studies have emphasized that the N2O5 uptake coefficient varies greatly (0.001-0.1) in different ambient conditions (Brown and Stutz, 2012;H.C. Wang et al., 2016), which is the main source of uncertainties in the model.Sensitivity tests illustrate that the modeled N2O5 concentration varied from 3 ppbv to 60 pptv when the N2O5 uptake coefficients were varied from 0.001 to 0.05 (Fig. 6a), the N2O5 concentration is very sensitivity to the loss from heterogeneous reaction.Compared with the base case, the accumulated pNO3 -is evidently lower at γ = 0.001 with the accumulated pNO3 -of 44 μg m -3 , the low N2O5 uptake coefficient condition is corresponding to several kinds of aerosols, such as secondary organic aerosol (Gross et al., 2009), humic acid (Badger et al., 2006)) and certain solid aerosols (Gross et al., 2008).When the N2O5 uptake coefficient enlarges from 0.005 to 0.05 (Fig. 6b, c), the integral pNO3 -almost not change with negligible increasing, indicating that the conversion capacity of N2O5 uptake to pNO3 -was almost maximized at certain ClNO2 yield, the convert capacity of NOx to nitrate was not limited by N2O5 heterogeneous reaction rate, but the formation of NO3 by the reaction of NO2 with O3.
For describing the nocturnal NOx convert capacity to particulate nitrate via N2O5 uptake coefficient, here we defined the particle nitrate convert efficiency (ε) as Eq. 6.
The ∆ represents the time duration from time zero at sunset till the ending time at sunrise.
∆ 0 In the case, Sa is set to 3000 μm 2 cm -3 , the ClNO2 yield is zero and kNO3 is 0.02 s -1 .
Figure 7 shows the dependence of the particle nitrate convert efficiency varied from 10 -5 to 0.1.When γN2O5 is lower than 0.0017, the particle nitrate formation enhanced rapidly with the increasing of N2O5 uptake coefficient, here we defined as the γN2O5 sensitive region when γN2O5 < 1.7×10 -3 .While γN2O5 ≥1.7×10 -3 is defined as γN2O5 insensitive region, since the convert efficient is over 90% and not sensitive to the variation of N2O5 uptake coefficient.According to Eq. 3 and Eq. 5, higher aerosol surface concentration, higher NOx, lower kNO3 and temperature would further enlarging the insensitivity region with lower γN2O5 value, and make the N2O5 uptake more easily located in the γN2O5 insensitive region.Here the critical value of the N2O5 uptake coefficient (1.7×10 -3 ) is relative low compared with that IUPAC (International Union of Pure and Applied Chemistry) recommended on the surface of mineral dust (0.013, 290-300K) (Crowley et al., 2010b) or determined in many field experiments (e.g.S.S. Brown et al., 2006;2009;Wagner et al., 2013;Morgan et al., 2015;Phillips et al., 2016;Z. Wang et al., 2017;Brown et al., 2016;H.C. Wang et al., 2017b;X.F. Wang et al., 2017), suggesting the particulate nitrate formation via N2O5 uptake was easily maximized in polluted episode, and further worsen the PM pollution.
In the base case, the modeled particulate nitrate formation via N2O5 uptake is an upper limit result, as the ClNO2 yield set to zero.Since large coral combustion emitted chloride into the atmosphere in the heating period in Beijing (Sun et al., 2013), associated with the emission by power plants in North China.The enhanced anthropogenic emission of chloride provides abundant chloride-containing aerosol to form ClNO2 via N2O5 uptake aloft, implying that significant ClNO2 formed in the upper layer of NBL (Tham et al., 2016;Z. Wang et al., 2017).Assuming the ClNO2 yield is the average value of 0.28 determined at high altitude in North China (Z.Wang et al., 2017), the produced pNO3 -throughout the whole night will decreased 7 μg m -3 .
The ClNO2 formation aloft throughout the night reach up to 2.5 ppbv, which is comparable with that observed in the field measurement in North China (Tham et al., 2016;Z. Wang et al., 2017;X.F. Wang et al., 2017).As the error of pNO3 -formation simulation was subject to the ClNO2 yield, higher yield would increase the model uncertainty directly, probing the ClNO2 yield are warranted in future studies.As for NO3 reactivity, Figure 7 shows sensitivity tests of the integral pNO3 -formation in the whole night at kNO3 = 0.001 s -1 , 0.01 s -1 , 0.02 s -1 , 0.05 s -1 .The integral pNO3 - formation was decreased when kNO3 vary from 0.001 s -1 to 0.1 s -1 , but the variation ratio to base case was within ±5%.The result shows the NO3-N2O5 loss via NO3 react with VOCs in polluted wintertime is not important, which may only lead to relative small uncertainties to the integral pNO3 -formation calculation.Nevertheless, if N2O5 uptake was extremely low (e.g.γN2O5 < 10 -4 ), the uncertainty cased by NO3 oxidation will be enlarged significantly.

Conclusion
During the wintertime, ambient O3 is often fully titrated to be zero at the ground of urban Beijing due to fast reaction with NO emissions.Consequently, the near surface air masses were chemically inert.Nevertheless, the chemical information of the air masses at higher altitudes was indicative of a reactive layer above urban Beijing, which potentially drives fast pNO3 -production via N2O5 uptake.In this study, we Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2017-1217Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 11 January 2018 c Author(s) 2018.CC BY 4.0 License.

A
box model was used to model the NO3 and N2O5 mixing ratios and the nitrate formation potential in vertical scale at IAP site.A simple chemical mechanism (see R1-R5) is used to model the nighttime NO3 and N2O5 chemistry in NO free air masses, and the physical mixing, dilution, deposition, or interruption during the transport of the air mass were not considered.Here f represents the ClNO2 yield from N2O5 uptake.

Figure 1 .
Figure 1.Location of the monitoring sites used in this study, including PKU (red 581

Figure 4 .
Figure 4. OX missing case presented by the vertical profiles of (a) NO and NO2, (b)602

Table 1 .
List of the parameter sets in base case and sensitivity tests.