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Evaluation of the SAPRC-07 mechanism against CSIRO smog chamber data
Atmospheric Environment 44 (2010) 1707e1713
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Evaluation of the SAPRC-07 mechanism against CSIRO smog chamber data Merched Azzi a, *, Stephen J. White a, b, Dennys E. Angove a, Ian M. Jamie b, Ajith Kaduewela c a
CSIRO Energy Technology, Locked Bag 2007, Kirrawee, NSW 2232, Australia Department of Chemistry and Biomolecular Sciences, Macquarie University, NSW 2109, Australia c Air Resources Board, California Environmental Protection Agency, 1001 I Street, Sacramento, CA 95812, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 July 2009 Received in revised form 26 November 2009 Accepted 9 February 2010
The updated SAPRC-07 mechanism was evaluated against data from experiments performed in the CSIRO smog chamber. The mechanism predictions have been compared to experimental results as well as predictions by SAPRC-99. Experiments were performed using either toluene or m-xylene in the presence of NOx at sub-0.1 ppmv concentrations. For the majority of m-xylene experiments, the modelled D(O3eNO) concentration was within 20% of observed values for both SAPRC mechanisms. However during the oxidation of toluene the production of radicals was poorly predicted, with final D(O3eNO) concentration under-predicted by up to 60%. The predictions of major oxidants from isoprene oxidation were in good agreement with observed values. For the NOx-limited conditions however, the ozone concentration predicted by both mechanisms were under-predicted by approximately 20% in the five experiments tested. The performance of the SAPRC-07 mechanism was also evaluated against twelve evaporated fuel experiments. Two types of evaporative mode experiments were performed: headspace evaporated fuel and wholly evaporated fuel. The major difference was a significantly higher concentration of aromatic hydrocarbons and larger alkane products in wholly evaporated fuels. For headspace evaporated fuel experiments both SAPRC mechanisms were in good agreement with experimental results. For wholly evaporated experiments the average D(O3eNO) model error was 25% with SAPRC-07 compared to less than 5% for SAPRC-99. Updates to the photolysis data for dicarbonyls, the light source used and the experimental conditions under which these experiments were performed are possible causes for the discrepancy between SAPRC-99 and -07 predictions for wholly evaporated experiments. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: SAPRC-07 Smog chamber Ozone Evaporated fuel Chemical mechanism
1. Introduction Air quality model (AQM) simulations have been used extensively to predict and study the formation of photochemical smog (Hess et al., 2004). Within AQMs, the chemical mechanism component simulates the reaction, photolysis and deposition of volatile organic compounds (VOCs) (Dodge, 2000). The choice of chemical mechanisms used in AQMs requires that the limitations and applicability of the selected mechanism are known, as the choice of chemical mechanism (Faraji et al., 2008; Luecken et al., 2008) and the manner in which the mechanism is used (Mallet and Sportisse, 2006) can lead to large discrepancies in model predictions. Due to their importance in air quality model predictions, chemical mechanisms need to be evaluated against experimental data to ensure that they can describe the formation of ozone and other species of interest accurately.
* Corresponding author. Tel.: þ61 2 9710 6870; fax: þ61 2 9710 6800. E-mail address: [email protected] (M. Azzi). 1352-2310/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2010.02.013
Smog chambers have been used extensively to investigate and simulate VOC oxidation, as they provide a controlled environment where VOC concentrations can be controlled and monitored precisely (Doyle, 1970; Jeffries et al., 1975; Leone et al., 1985). Comparison can be made between observed concentrations and mechanism predictions for important species to confirm the mechanism is able to replicate the most important photochemical reactions accurately. Where mechanism results fail to match observation, insight may be provided into where possible changes to the mechanism need to be made to provide accurate predictions, or to determine which future experiments should be performed to gather more experimental data. Chemical mechanisms vary from those comprising very few reactions, such as the generic reaction set (GRS) mechanism as used in The Air Pollution Model (TAMP) (Azzi et al., 1992; Hurley, 2008), to the Master Chemical Mechanism which comprises over 13,500 reactions (Bloss et al., 2005). The Statewide Air Pollution Research Centre mechanism (SAPRC-99) developed by Carter (2000), is a widely used chemical mechanism which has recently been updated to SAPRC-07 (Carter, 2009). SAPRC-07 has been evaluated
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against approximately 2500 chamber experiments performed in 11 chambers using different irradiation sources, including sunlamps, arc lamps, blacklight lamps and sunlight. This mechanism has therefore been evaluated against a much larger experimental database than any other existing chemical mechanism. The primary objective of this work was to collate experimental data from the CSIRO smog chamber and to determine the ability of SAPRC-07 to predict oxidant formation compared to SAPRC-99. Experiments were performed using toluene, m-xylene and isoprene, all in the presence of NOx. For the aromatic hydrocarbon experiments both VOC and NOx initial concentrations were less than 0.1 ppmv for all experiments, concentrations which are closer to typical ambient concentrations. Evaporated petroleum mixtures experiments were also modelled to evaluate the performance of both versions of the SAPRC mechanism in predicting oxidant formation from hydrocarbon mixtures. 2. Experimental 2.1. CSIRO chamber A series of chamber experiments designed to study the photooxidation of selected hydrocarbons were performed in the CSIRO smog chamber facility. The design and operation of this facility has been previously described in detail by Angove et al. (2000), Hynes et al. (2005) and White et al. (2010). Light was provided by mercury blacklight blue lamps, which emit radiation between 350 and 390 nm. Some advantages of using these lamps include lower ozone photolysis and a higher NO2 photolysis rate than can be provided by blacklights while still maintaining temperature below 40 C. They also provide a more consistent emission spectrum than sunlight or regular blacklight lamps. Disadvantages compared to chambers using arc lamps or other blacklights is that light emissions below 350 nm are limited, which reduces the photolysis of ozone to O(1D), an important source of the OH radical. The limited emission spectrum may also effect the relative photolysis rates of other species such as NO3 and aldehydes (Carter et al., 1995). These lamps therefore provide a unique light source compared to chambers evaluated in the SAPRC experiment database. 2.2. Chamber experiments Experiments were performed using either a single evaporated hydrocarbon or an evaporated fuel mixture. Both sets of experiments were performed in the presence of NOx. Single hydrocarbon experiments were performed using toluene, m-xylene or isoprene, with each hydrocarbon introduced into the chamber by injecting a known liquid volume of the hydrocarbon into a glass bulb. The bulb was gently heated with hot water and the vapour was streamed into the chamber using nitrogen. These experiments were performed for between 5 and 14 h at a relative humidity of between 2% and 5% when measured at 21 2 C. The temperature rose from 21 2 C at the beginning of each experiment to 33 4 C after 1 h, after which time it was constant until the end of the experiment. Evaporated fuel was injected into the chamber either as fully evaporated fuel (using the method above) or as headspace evaporated fuel, where the headspace vapour from fuel equilibrated at 38 C was injected. For the evaporated fuel mixtures experiments NOx was provided by both initial nitric oxide injection, and a HONO generator run continuously throughout the experiment at 1.5 ppbv h1. These experiments were performed for 6 h at a relative humidity of 50% when measured at 21 2 C. A stable temperature of 30 2 C was reached after 1 h and maintained
until the end of the experiment. A full description of the fuel experiments, as well as the ability of the SAPRC-99, MCMv3.1 and CB05 chemical mechanisms to replicate the results of these experiments, is reported elsewhere (White et al., 2010).
2.3. Chemical mechanisms The SAPRC-99 (S99) and SAPRC-07 (S07) mechanisms were used to simulate chamber experiments and the results obtained were compared to experimental data. In some cases, comparisons were extended to include the simulated results obtained by the Master Chemical Mechanism (MCM). The auxiliary chamber mechanism, which describes the formation and loss of important species on the chamber walls, is shown in Table 1 (Azzi et al., 2008). For experiments containing a single major hydrocarbon, the base S99 or S07 mechanism was used, with the reaction of the hydrocarbon of interest included in each mechanism. For experiments containing mixtures, each hydrocarbon was specified in the model input files by their detailed model species name (DMS) (Carter, 2000). The species were lumped by the SAPRC software by the adjustable parameter mechanism (Carter, 2009). A greater number of species had explicit DMS names in S07 compared to S99. The input file for each mechanism included as much detail as was possible, such that the input files for each mechanism varied slightly. For example, m-cymene and m-diethyl benzene were designated by the detailed model species M-CYMENE and M-DE-BEN respectively in S07. In S99 only M-DE-BEN was included as a DMS, so the m-cymene concentration was included in the C10-BEN2 detailed model species along with other C10 di-substituted benzenes that did not have an explicit DMS name. A full list of speciation is included in supplementary materials. Although slightly different inputs were used, using the S99 hydrocarbon input in S07 made less than 2% difference to the final ozone predictions.
3. Results 3.1. Method of comparison Model results were evaluated by two methods in this work: by comparison between S99 and S07 predictions; and by comparison between mechanism predictions and experimental results. The oxidative capacity of a chamber experiment was determined using D(O3eNO) (1) (Johnson, 1983; Carter and Atkinson, 1987). Table 1 Auxiliary mechanism for CSIRO smog chamber. SAPRC rate
Reaction
Rate
P(1) ea RN-I
JNO2 ðmin1 Þ Generator ¼ HONO hv þ wall ¼ HONO
Measured by actinometry 1.5 103 ppmv h1 No generator: 4.0 106 ppmv HONO generator: 4:0 106 ppmv þ 2:5 105 =JNO2 ppmv
K(NO2W)
NO2 ¼ (yhono)$HONO þ (1 yhono).wall NOx e N2O5 ¼ wall NOx N2O5 þ H2O ¼ wall NOx O3 ¼ wall OH ¼ HO2 HNO3 / wHNO3
7.2 105 min1
yhono K(N25I) K(N25S) K(O3W) K(XSHC) K(NAW)
0.42 6.0 104 min1 1.48 105 ppmv1 min1 1.2 105 min1 250 min1 1.98 104 min1
a Continuous HONO generator used in evaporated fuel experiments. Alterations were made to the RN-I for each JNO2 experimental value used.
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This describes the ability of the entire system to form ozone in the presence of NOx, by including the reaction of NO to NO2.
DðO3 eNOÞ [ ð½O3 t L ½O3 0 Þ L ð½NOt L ½NO0 Þ:
(1)
The model error (2) describes the relative difference between modelled and experimental values. fDðO3 eNOÞmod L DðO3 eNOÞexp g=DðO3 eNOÞexp 3 100%:
(2)
3.2. m-Xylene Thirteen m-xylene smog chamber experiments were modelled using S99 and S07. These experiments were carried out for ROC/ NOx varying between 1.8 and 21.0, measured in ppmC/ppmN. Twelve of these experiments form a 4 by 3 matrix, with four initial hydrocarbon measurements of approximately 90, 64, 37 and 13 ppbv, and initial NOx measurements of approximately 35, 48 and 69 ppbv. The light intensity used was equal to a photolysis rate constant for NO2 ðJNO2 Þ of 0.41 0.02 min1, with measurements performed using actinometry experiments (Hynes et al., 2005). Comparison of the calculated model errors is shown in Fig. 1. For both mechanisms D(O3eNO) was predicted to within 15% at 6 h for experiments with ROC/NOx above 7, with the exception of S99 predictions for higher ROC/NOx experiments which were slightly over-predicted. For experiments with ROC/NOx below 7, the mechanism predictions for D(O3eNO) were between 5 and 45%. Minimal change was observed in D(O3eNO) model error for high ROC/NOx experiments between 3 and 6 h. This was due to minimal ozone formation after 3 h in these experiments. For low ROC/NOx experiments where NO oxidation was still occurring, some improvement was observed between 3 and 6 h for both mechanisms. For the majority of experiments the D(O3eNO) predictions made by S99 were higher than S07, however the differences between the two mechanisms was no more than 15% for any experiment after 6 h. The results for D(O3eNO) from m-xylene oxidation in this chamber compare favourably with those performed at UCR chambers (Fig. 2). For this comparison only experiments with initial NOx and m-xylene below 300 ppbv each, and initial ROC/NOx below 23 were used for comparison. The prediction of D(O3eNO) for all but 4 of the 28 experiments with ROC/NOx above 7 are accurate to within 10% regardless of the light source used. The predictions for experiments with low initial ROC/NOx were less accurate, with all but one of the 15 experiments under-predicting D(O3eNO) model error greater than 10%. So for m-xylene experiments with low initial
Fig. 2. D(O3eNO) model error for m-xylene experiments at 6 h for experiments performed in the CSIRO chamber; and in the UCR-EPA, CTC and DTC chambers using arc and blacklight lamps.
concentrations, the predictions of D(O3eNO) were similar for experiments performed using arc lamps, blacklight lamps and blacklight blue lamps. 3.3. Toluene Fifteen toluene smog chamber experiments were performed at two different light intensities, with nine experiments using a JNO2 of 0.41 0.02 min1 and six experiments using a JNO2 of 0.80 0.02 min1. Initial toluene concentrations varied from 35 to 90 ppbv and initial NOx between 33 and 73 ppbv, producing ROC/ NOx varying between 3.5 and 18.9. The D(O3eNO) model error for S07 and S99 is shown for both high and low JNO2 (Fig. 3). Unlike m-xylene, toluene photooxidation was not well predicted for this chamber by either mechanism. After 3 h the low ROC/NOx experiments were performing more satisfactorily, but that was due to low ozone formation in these experiments (less than 20 ppbv). For these experiments, the formation of ozone from auxiliary processes such as wall effects was proportionally larger than the amount of ozone formed from the reaction of toluene. After 6 h a positive model error to ROC/NOx dependence was present for all but the low ROC/NOx experiments, consistent with other chamber modelling of aromatic photooxidation (Metzger et al., 2008). The results from these experiments
Fig. 1. D(O3eNO) model error for m-xylene experiments at (a) 3 h and (b) 6 h.
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Fig. 3. D(O3eNO) model error for toluene experiments at (a) 3 h and (b) 6 h; for two different light intensity subsets (JNO2 ¼ 0:41 or 0:80 min1 ). The experiments performed in the UCR-EPA chamber were performed using arc lamps (ROC/NOx ¼ 19.8; JNO2 ¼ 0:26 min1 ) or blacklight lamps (ROC/NOx ¼ 12.7; JNO2 ¼ 0:172 min1 ). Note that lines are visual aids only.
contrast with those observed by Carter (2009), where the average D(O3eNO) model error for S07 was within 20% of experimental values for toluene experiments performed using arc lights and blacklights. This discrepancy in model error when compared to m-xylene photooxidation may be explained by the parameterisation process used by the SAPRC mechanism. Unsaturated dicarbonyls were represented in the SAPRC-07 mechanism by three species, AFG1 and AFG2 for photoreactive mono-unsaturated dicarbonyls, and AFG3 for less photoreactive unsaturated dicarbonyls (Carter, 2009). The yield of AFG3, and the combined yield of AFG1 and AFG2, were both estimated given experimental yield data. The AFG1 and AFG2 products share exact mechanisms and absorption cross-sections, but differ in that the former photolyses to form radical species whereas the latter does not. The relative yields of AFG1 and AFG2 were adjusted to predict ozone formation accurately for experiments in the SAPRC experiment database. As such the characteristics of the experiments within the database used for optimisation will impact on the yields determined for each product, and by extension the performance of the mechanism. For m-xylene, there were 30 experiments for which the initial VOC and NOx concentrations were each below 300 ppbv, and where the initial ROC/NOx was less than 23. For toluene there were only 3 experiments used that fit the same criteria, for an equivalent ROC/NOx less than 20. This criterion was chosen as many higher concentration experiments reach the NOx-limited stage very early in the experiment. For these the onset and rate of ozone formation is more influenced by the effect of high NOx concentrations or high RO2 to NO concentrations during oxidation. Due to the uncertainty of the photolysis data for aromatic dicarbonyls (Calvert et al., 2002), the results from any chamber using blacklights cannot be used alone to assess aromatic photooxidation. However in collaboration with chamber results using other light sources, they can provide a valuable tool in testing details of models (Carter et al., 1995). In the SAPRC-07 mechanism, the lumped aromatic products arising from toluene and m-xylene are represented by the same product (AFG), although differ in their respective yields. As no differences were observed between light sources for m-xylene including the CSIRO chamber, it is possible that the poor performance for toluene experiments in this chamber is related to the lack of low concentration experiments used in toluene mechanism optimisation. Further investigation is required to determine the exact cause of this discrepancy.
3.4. Isoprene Five isoprene experiments were performed in the CSIRO chamber at a light intensity equivalent to JNO2 ¼ 0:40 0:03 min1 . Initial isoprene varied between 88 and 360 ppbv, with initial NOx measured between 38 and 114 ppbv. The only major update to the organic isoprene mechanism in S07 was the increase in the photolysis rate of methacrolein, which had little impact due to its relatively slow rate compared to its reaction with OH. Very little difference was observed between S99 and S07 predictions, with all major product concentrations almost identical. The oxidation of isoprene and nitric oxide, as well as the formation of formaldehyde, PAN and formic acid, were predicted to within 20% of experimental values at any given time by both S99 and S07. Little change was observed in predictions of methacrolein and methyl vinyl ketone. The final ozone predictions made by all mechanisms were approximately 20% lower than observed values for all experiments (Fig. 4). The deviation between modelled and experimental ozone occurred as the nitric oxide concentration became negligible. This was likely related therefore to the uptake of nitrogen into organic
Fig. 4. Ozone predictions for isoprene oxidation using SAPRC-07, SAPRC-99 and the MCM.
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compounds, such as organic nitrates (RONO2) and peroxy acyl nitrates (PANs). Given the importance of isoprene in global VOC budgets (Guenther et al., 2006) the oxidation of isoprene must be well represented in chemical mechanisms. These experiments use higher concentrations of isoprene and NOx than would be encountered in the environment. Despite some discrepancy in final ozone concentration predictions, the oxidation of isoprene is equally well simulated by S99 and S07 for these experiments. 3.5. Petrol experiments The photooxidation of evaporated fuel experiments was simulated using S99 and S07. Two types of evaporated fuel experiments were performed using three varieties of fuel. The two evaporation modes used were headspace vapour, and wholly evaporated fuel. The three fuel varieties were unleaded petrol (ULP) and two ethanol blends (E5 and E10). In summary of results previously reported for these experiments for SAPRC-99, MCM and CB05 (White et al., 2010), S99 predicted ozone formation to within 15% for both injection modes and all fuel varieties. The variability of the three mechanisms to the HONO generation rate was addressed, and the variability in S07 was found to be similar to that of S99 (refer to Supplementary materials). Six headspace experiments were performed, and the vapour mixtures used were characterised by high concentrations of alkanes and low concentrations of aromatics. For S99 the D(O3eNO) model error was found to be correlated to the initial carbon monoxide concentration, which varied between 120 and 770 ppbv. This initial amount of CO was due to CO not being removed in the zero air system, but the concentrations used are within the range expected in the ambient atmosphere. The same correlation to initial CO was found for S07, with model error for each experiment approximately 5e10% lower than the equivalent S99 prediction (Fig. 5). For headspace experiments the final D(O3eNO) predictions for both SAPRC mechanisms were within 15% of the experimental values for almost all experiments. Six wholly evaporated fuel experiments were performed, and the vapour mixtures used in these experiments contained significantly higher concentrations of aromatics and larger alkanes when compared to the equivalent headspace fuel experiments. The average D(O3eNO) model error for S07 was 25%, which contrasts with less than 5% for S99 and 19% for the MCM (Fig. 6). The differences
Fig. 5. D(O3eNO) model error for headspace experiments at 6 h compared to initial carbon monoxide concentration.
Fig. 6. D(O3eNO) model error for wholly experiments at 6 h compared to initial ethanol concentration. The error bars represent the difference between model errors for replicate experiments.
between the mechanisms were reflected in the OH and HO2 predictions, which were significantly lower in S07 compared to S99. 3.6. Sensitivity to changes in updated mechanism In the SAPRC-07 mechanism, a positive increase in overall biases for experiments using synthetic hydrocarbon mixtures was observed with S07 compared to S99 (Carter, 2009). This is in contrast to these results which show a negative trend for experimental bias. In the S07 mechanism, key reaction rates and photolysis data were updated given advances in experimental data. These changes will have been accounted for in the optimisation of dicarbonyl yields for aromatics, however they may have varying impact on chamber experiments performed under different conditions. The sensitivity of over 20 changes to key reactions was tested by altering a single reaction rate in the S07 mechanism to the rate that was used for that reaction in the S99 mechanism. For photolysis rates this involved using the absorption spectrum and quantum yields that were used in S99 for that species. The rate changes which produced the largest change in final D(O3eNO) predictions are shown in Fig. 7. The changes shown are the absolute changes in
Fig. 7. Sensitivity of final D(O3eNO) model error to rate changes in SAPRC-07. Reaction rates or absorption spectra were changed to their equivalent values in SAPRC-99.
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model error between the regular S07 mechanism and the ratealtered S07 mechanism. This method provides a basis for determining which changes to the base mechanism may have resulted in a greater change for wholly evaporated experiments than for headspace fuel experiments for S07. The reaction rate which effected the largest change in this subset of experiments was the reaction of NO2 and OH to form nitric acid. Changes to this reaction rate resulted in similar changes to the D(O3eNO) model error in both headspace and wholly evaporated experiments. The photolysis of NO2 and the reaction of O3 and NO also resulted in similar changes to model error for headspace and wholly evaporated experiments. The other reaction rates alterations shown in Fig. 7, the photolysis of methylglyoxal and glyoxal (both channels), resulted in much larger changes to model error in wholly evaporated experiments than in headspace experiments. This is related to the concentrations of glyoxal and methylglyoxal, which are a direct product of aromatics, present in higher concentrations in wholly evaporated experiments. For headspace experiments the modelled concentrations of both glyoxal and methylglyoxal were less than 0.4 ppbv, compared with between 2 and 5 ppbv for wholly evaporated experiments. Toluene experiments were much more sensitive to these reaction rate changes than petrol experiments. This was typical of the poor performance of S07 in predicting toluene photooxidation in this chamber. The average D(O3eNO) error for toluene experiments was 34% compared to an average of 13% for non-toluene experiments. As most toluene experiments had not reached the NOx-limited regime after 6 h, any small increase in reactivity had a proportionally larger effect on model predictions for toluene than for m-xylene or petrol, where the regime had been reached or was closer to being reached in a greater number of experiments. Despite the high sensitivity of toluene experiments to these reaction rate changes, the final D(O3eNO) predictions in the CSIRO chamber were similar for both S99 and S07. This is because the optimisation of dicarbonyl product yields for each aromatic hydrocarbon ensured that D(O3eNO) was predicted similarly by both SAPRC mechanisms. This was true for m-xylene and toluene experiments in the CSIRO smog chamber, where D(O3eNO) was predicted similarly by both mechanisms. However for petrol mixtures a significant difference was observed in D(O3eNO) predictions between the two mechanisms depending on the composition of the mixture. The rate changes which produced the largest difference between model predictions for headspace and wholly evaporated experiments were the light dependent dicarbonyl photolysis rates. The light source used in these experiments may have had some impact on the difference in predictions between the two mechanisms. For S07 the quantum yields for all three photolysis pathways had been updated from their values in S99 (Chen et al., 2000; Volkamer et al., 2005), although the absorption spectra used in both mechanisms were similar. The change in each photolysis rate is shown for three light sources in Table 2. Further information on the calculation of photolysis rates is available in the supplementary materials. The change in the photolysis rates of dicarbonyls between S99 and S07 was substantially greater for the CSIRO light source than either the blacklight or arc light source. Likewise the change in the photolysis rates between the SAPRC mechanisms when using blacklight lamps was significantly greater than using arc lamps. However the absolute rates for these reactions in S07, for the typical JNO2 values used in the UCR-EPA chamber experiments, are quite similar for each light source. The discrepancy between SAPRC mechanism predictions for wholly evaporated experiments therefore may be due to the light
Table 2 Photolysis rates in S07 for glyoxal and methylglyoxal. Rates have been shown for CSIRO lamps (blacklight blue), blacklight (BL) lamps and arc lamps. The relative change from the equivalent rate in S99 is also shown. Rates are calculated at the typical JNO2 values used for these experiments. Rate
CSIRO S99a
CSIRO S07a (change)
BL S07b (change)
Arc S07c (change)
GLY / 2 HCO (% change from S99) GLY / HCHO þ H2 (% change from S99) MGLY / products (% change from S99) AFG / products (% change from S99)d
1.0 104
0.39 104 (61%) 8.8 106 (1100%) 0.58 104 (48%) 2.1 103 e
0.25 104 (8.6%) 11.5 106 (1800%) 0.27 104 (34%) 1.8 103 e
0.46 104 (10%) 9.9 106 (58%) 0.85 104 (32%) 1.7 103 e
0.73 106 1.1 104 e
CSIRO photolysis rates were calculated for JNO2 ¼ 0:42 min1 and measured in s1. Blacklight photolysis rates were calculated for JNO2 ¼ 0:18 min1 and measured in s1. c Arc photolysis rates were calculated for JNO2 ¼ 0:26 min1 and measured in s1. d Product representation not comparable between SAPRC-99 and SAPRC-07. a
b
source used being different from those used in the SAPRC experiment database. However the predictions of D(O3eNO) from m-xylene photooxidation in the smog chamber at CSIRO were similar to predictions made using chambers with different light sources. Likewise the prediction of D(O3eNO) from toluene photooxidation, although severely under-predicted, was similar for both SAPRC mechanisms. It is not clear therefore that the light source is responsible for these differences in predictions, given its similar predictions for the two most common aromatic hydrocarbons. However it is possible that a combination of experimental conditions, including light source, higher OH concentration (due to HONO injection), higher water content and the photooxidation mechanisms of other aromatic hydrocarbons could be responsible for this variation in mechanism predictions. 4. Conclusions An evaluation of the SAPRC-07 chemical mechanism was carried out using data collected from experiments performed in the CSIRO smog chamber. For the majority of experiments SAPRC-07 performed similarly to SAPRC-99 in the CSIRO smog chamber, with the exception of aromatic-containing mixtures. The prediction of D(O3eNO) from m-xylene photooxidation was within 10% with higher ROC/NOx ratios, and between 5 and 45% for lower ROC/NOx ratios. These predictions matched those made in the various UCR chambers performed using different light sources. Conversely, the photooxidation of toluene was poorly represented by both mechanisms with ozone formation under-predicted by all experiments. There was a large difference between the number of low concentration toluene and m-xylene experiments in the SAPRC database and this may explain some of the discrepancy between the model results for these species. Experiments performed using petrol headspace vapour were predicted accurately by both SAPRC mechanisms. However large differences were observed between mechanism predictions for experiments performed using wholly evaporated petrol vapour. The changes made to photolysis data for dicarbonyls in the SAPRC07 mechanism resulted in larger changes in photolysis rates for blacklight-irradiated chambers compared to arc lamp irradiated chambers. It is possible that the light source chosen may be responsible for the difference in predictions for SAPRC-07 compared to SAPRC-99 in the CSIRO chamber. However the similar performance in mechanism predictions for single aromatic experiments, which would also be effected by changes in the photolysis data for these species, indicate that other unidentified problems may be responsible for the change in mechanisms predictions.
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The SAPRC-07 mechanism represents a substantial update to the SAPRC-99 mechanism. The performance of the SAPRC-07 in predicting oxidant formation for single hydrocarbon smog chamber experiments was equivalent to SAPRC-99 for the majority of experiments evaluated. Differences in the predictions from the wholly evaporated experiments however show the importance of light irradiation and photolysis in mechanism predictions for aromatics, as well as the importance of evaluating the performance of aromatic chemical mechanisms in predicting the oxidation of synthetic mixtures. Acknowledgements The authors would like to acknowledge the California Air Resources Board for organising and funding the SAPRC peer review. We would also like to thank William P.L. Carter for assistance with the SAPRC mechanism and for providing experimental data, and John Carras for helpful discussions and funding support. Appendix. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.atmosenv.2010.02.013. References Angove, D.E., Halliburton, B.W., Nelson, P.F., 2000. Development of a new indoor environmental chamber at MRL, North Ryde. In: Proceedings of the 15th International Clean Air & Environment Conference. Clean Air Society of Australia & New Zealand, pp. 270e274. Azzi, M., Johnson, G.M., Cope, M., 1992. An introduction to the generic reaction set photochemical smog mechanism. In: Proceedings of the 11th International Clean Air and Environment Conference, Brisbane, Australia. Clean Air Society of Australia and New Zealand, pp. 451e462. Azzi, M., White, S.J., Angove, D.E., November 2008. Review of the SAPRC-07 Chemical Mechanism Prepared for Dr Ajith Kaduwela, California Air Resources Board (CARB). Available at: http://www.arb.ca.gov/research/reactivity/reactivity.htm. Bloss, C., Wagner, V., Jenkin, M.E., Volkamer, R., Bloss, W.J., Lee, J.D., Heard, D.E., Wirtz, K., Martin-Reviejo, M., Rea, G., Wenger, J.C., Pilling, M.J., 2005. Development of a detailed chemical mechanism (MCMv3.1) or the atmospheric oxidation of aromatic hydrocarbons. Atmospheric Chemistry and Physics 5, 641e664. Calvert, J.G., Atkinson, R., Becker, K.H., Kamens, R.M., Seinfeld, J.H., Wallington, T.J., Yarwood, G., 2002. The Mechanisms of Atmospheric Oxidation of Aromatic Hydrocarbons. Oxford University Press, Oxford. Carter, W.P.L., May 8, 2000. Documentation of the SAPRC-99 Chemical Mechanism for VOC Reactivity Assessment. Report to the California Air Resources Board, Contracts 92-239 and 95-308. Carter, W.P.L., June 22, 2009. Development of the SAPRC-07 Chemical Mechanism and Updated Ozone Reactivity Scales. Final Report to the California Air Resources Board, Contract No. 03-318.
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Carter, W.P.L., Atkinson, R., 1987. An experimental study of incremental hydrocarbon reactivity. Environmental Science and Technology 21, 670e679. Carter, W.P.L., Luo, D., Malkina, I.L., Pierce, J.A., March 26, 1995. Environmental Chamber Studies of Atmospheric Reactivities of Volatile Organic Compounds. Effects of Varying Chamber and Light Source. Final Report to the National Renewable Energy Laboratory, Coordinating Research Council, Inc., California Air Resources Board and South Coast Air Quality Management District. Chen, Y., Wang, W., Zhu, L., 2000. Wavelength-dependent photolysis of methylglyoxal in the 290e440 nm region. Journal of Physical Chemistry A 104, 11126e11131. Dodge, M.C., 2000. Chemical oxidant mechanisms for air quality modelling: critical review. Atmospheric Environment 34, 2103e2130. Doyle, G.J., 1970. Design of a facility (smog chamber) for studying photochemical reactions under simulated tropospheric conditions. Environmental Science and Technology 4, 907e916. Faraji, M., Kimura, Y., McDonald-Buller, E., Allen, D., 2008. Comparison of the carbon bond and SAPRC photochemical mechanisms under conditions relevant to southeast Texas. Atmospheric Environment 42, 5821e5836. Guenther, A., Karl, T., Harley, P., Wiedinmyer, C., Palmer, P.I., Geron, C., 2006. Estimates of global terrestrial isoprene emissions using MEGAN (Model of Emissions of Gases and Aerosols from Nature). Atmospheric Chemistry and Physics 6, 3181e3210. Hess, G.D., Tory, K.J., Cope, M.E., Lee, S., Puri, K., Manins, P.C., Young, M., 2004. The Australian Air Quality Forecasting System. Part ii: case study of a Sydney 7-day photochemical smog event. Journal of Applied Meteorology 43, 663e679. Hurley, P., October 2008. TAPM V4. Part 1: Technical Description CSIRO Marine and Atmospheric Research Paper No. 25. Available at: http://www.cmar.csiro.au/ research/tapm/. Hynes, R.G., Angove, D.E., Saunders, S.M., Haverd, V., Azzi, M., 2005. Evaluation of two MCM v3.1 alkene mechanisms using indoor environmental chamber data. Atmospheric Environment 39, 7251e7262. Jeffries, H., Fox, D., Kamens, R., 1975. Outdoor Smog Chamber Studies: Effect of Hydrocarbon Reduction on Nitrogen Dioxide. Prepared for U.S. Environmental Protection Agency, EPA-650/3-75-011. Johnson, G.M., 1983. Factors affecting oxidant formation in Sydney air. In: Carras, J. N., Johnson, G.M. (Eds.), The Urban Atmosphere e Sydney, a Case Study. CSIRO, Melbourne, pp. 393e408. Leone, J.A., Flagan, R.C., Grosjean, D., Seinfeld, J.H., 1985. An Outdoor smog chamber and modeling study of tolueneeNOx photooxidation. International Journal of Chemical Kinetics 17, 177e216. Luecken, D.J., Phillips, S., Sarwar, G., Jang, C., 2008. Effects of using the CB05 vs. SAPRC99 vs. CB4 chemical mechanism on model predictions: ozone and gasphase photochemical precursor concentrations. Atmospheric Environment 42, 5805e5820. Mallet, V., Sportisse, B., 2006. Uncertainty in a chemistry-transport model due to physical parameterizations and numerical approximations: an ensemble approach to applied ozone modelling. Journal of Geophysical Research 111 (D01302). doi:10.1029/2005JD006149. Metzger, A., Dommen, J., Gaeggeler, K., Duplissy, J., Prevot, A.S.H., Kleffman, J., Elshorbany, Y., Wisthaler, A., Baltensperger, U., 2008. Evaluation of 1,3,5-trimethylbenzene degradation in the detailed tropospheric chemistry mechanism, MCMv3.1, using environmental chamber data. Atmospheric Chemistry and Physics 8, 6453e6468. Volkamer, R., Spietz, P., Burrow, J., Platt, U., 2005. High-resolution absorption crosssection of glyoxal in the UVevis and IR spectral ranges. Journal of Photochemistry and Photobiology A: Chemistry 172, 35e46. White, S.J., Azzi, M., Angove, D.E., Jamie, I.M., 2010. Modelling the photooxidation of ULP, E5 and E10 in the CSIRO smog chamber. Atmospheric Environment. doi:10. 1016/j.atmosenv.2009.11.050
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Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv
Evaluation of the SAPRC-07 mechanism against CSIRO smog chamber data Merched Azzi a, *, Stephen J. White a, b, Dennys E. Angove a, Ian M. Jamie b, Ajith Kaduewela c a
CSIRO Energy Technology, Locked Bag 2007, Kirrawee, NSW 2232, Australia Department of Chemistry and Biomolecular Sciences, Macquarie University, NSW 2109, Australia c Air Resources Board, California Environmental Protection Agency, 1001 I Street, Sacramento, CA 95812, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 July 2009 Received in revised form 26 November 2009 Accepted 9 February 2010
The updated SAPRC-07 mechanism was evaluated against data from experiments performed in the CSIRO smog chamber. The mechanism predictions have been compared to experimental results as well as predictions by SAPRC-99. Experiments were performed using either toluene or m-xylene in the presence of NOx at sub-0.1 ppmv concentrations. For the majority of m-xylene experiments, the modelled D(O3eNO) concentration was within 20% of observed values for both SAPRC mechanisms. However during the oxidation of toluene the production of radicals was poorly predicted, with final D(O3eNO) concentration under-predicted by up to 60%. The predictions of major oxidants from isoprene oxidation were in good agreement with observed values. For the NOx-limited conditions however, the ozone concentration predicted by both mechanisms were under-predicted by approximately 20% in the five experiments tested. The performance of the SAPRC-07 mechanism was also evaluated against twelve evaporated fuel experiments. Two types of evaporative mode experiments were performed: headspace evaporated fuel and wholly evaporated fuel. The major difference was a significantly higher concentration of aromatic hydrocarbons and larger alkane products in wholly evaporated fuels. For headspace evaporated fuel experiments both SAPRC mechanisms were in good agreement with experimental results. For wholly evaporated experiments the average D(O3eNO) model error was 25% with SAPRC-07 compared to less than 5% for SAPRC-99. Updates to the photolysis data for dicarbonyls, the light source used and the experimental conditions under which these experiments were performed are possible causes for the discrepancy between SAPRC-99 and -07 predictions for wholly evaporated experiments. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: SAPRC-07 Smog chamber Ozone Evaporated fuel Chemical mechanism
1. Introduction Air quality model (AQM) simulations have been used extensively to predict and study the formation of photochemical smog (Hess et al., 2004). Within AQMs, the chemical mechanism component simulates the reaction, photolysis and deposition of volatile organic compounds (VOCs) (Dodge, 2000). The choice of chemical mechanisms used in AQMs requires that the limitations and applicability of the selected mechanism are known, as the choice of chemical mechanism (Faraji et al., 2008; Luecken et al., 2008) and the manner in which the mechanism is used (Mallet and Sportisse, 2006) can lead to large discrepancies in model predictions. Due to their importance in air quality model predictions, chemical mechanisms need to be evaluated against experimental data to ensure that they can describe the formation of ozone and other species of interest accurately.
* Corresponding author. Tel.: þ61 2 9710 6870; fax: þ61 2 9710 6800. E-mail address: [email protected] (M. Azzi). 1352-2310/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2010.02.013
Smog chambers have been used extensively to investigate and simulate VOC oxidation, as they provide a controlled environment where VOC concentrations can be controlled and monitored precisely (Doyle, 1970; Jeffries et al., 1975; Leone et al., 1985). Comparison can be made between observed concentrations and mechanism predictions for important species to confirm the mechanism is able to replicate the most important photochemical reactions accurately. Where mechanism results fail to match observation, insight may be provided into where possible changes to the mechanism need to be made to provide accurate predictions, or to determine which future experiments should be performed to gather more experimental data. Chemical mechanisms vary from those comprising very few reactions, such as the generic reaction set (GRS) mechanism as used in The Air Pollution Model (TAMP) (Azzi et al., 1992; Hurley, 2008), to the Master Chemical Mechanism which comprises over 13,500 reactions (Bloss et al., 2005). The Statewide Air Pollution Research Centre mechanism (SAPRC-99) developed by Carter (2000), is a widely used chemical mechanism which has recently been updated to SAPRC-07 (Carter, 2009). SAPRC-07 has been evaluated
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against approximately 2500 chamber experiments performed in 11 chambers using different irradiation sources, including sunlamps, arc lamps, blacklight lamps and sunlight. This mechanism has therefore been evaluated against a much larger experimental database than any other existing chemical mechanism. The primary objective of this work was to collate experimental data from the CSIRO smog chamber and to determine the ability of SAPRC-07 to predict oxidant formation compared to SAPRC-99. Experiments were performed using toluene, m-xylene and isoprene, all in the presence of NOx. For the aromatic hydrocarbon experiments both VOC and NOx initial concentrations were less than 0.1 ppmv for all experiments, concentrations which are closer to typical ambient concentrations. Evaporated petroleum mixtures experiments were also modelled to evaluate the performance of both versions of the SAPRC mechanism in predicting oxidant formation from hydrocarbon mixtures. 2. Experimental 2.1. CSIRO chamber A series of chamber experiments designed to study the photooxidation of selected hydrocarbons were performed in the CSIRO smog chamber facility. The design and operation of this facility has been previously described in detail by Angove et al. (2000), Hynes et al. (2005) and White et al. (2010). Light was provided by mercury blacklight blue lamps, which emit radiation between 350 and 390 nm. Some advantages of using these lamps include lower ozone photolysis and a higher NO2 photolysis rate than can be provided by blacklights while still maintaining temperature below 40 C. They also provide a more consistent emission spectrum than sunlight or regular blacklight lamps. Disadvantages compared to chambers using arc lamps or other blacklights is that light emissions below 350 nm are limited, which reduces the photolysis of ozone to O(1D), an important source of the OH radical. The limited emission spectrum may also effect the relative photolysis rates of other species such as NO3 and aldehydes (Carter et al., 1995). These lamps therefore provide a unique light source compared to chambers evaluated in the SAPRC experiment database. 2.2. Chamber experiments Experiments were performed using either a single evaporated hydrocarbon or an evaporated fuel mixture. Both sets of experiments were performed in the presence of NOx. Single hydrocarbon experiments were performed using toluene, m-xylene or isoprene, with each hydrocarbon introduced into the chamber by injecting a known liquid volume of the hydrocarbon into a glass bulb. The bulb was gently heated with hot water and the vapour was streamed into the chamber using nitrogen. These experiments were performed for between 5 and 14 h at a relative humidity of between 2% and 5% when measured at 21 2 C. The temperature rose from 21 2 C at the beginning of each experiment to 33 4 C after 1 h, after which time it was constant until the end of the experiment. Evaporated fuel was injected into the chamber either as fully evaporated fuel (using the method above) or as headspace evaporated fuel, where the headspace vapour from fuel equilibrated at 38 C was injected. For the evaporated fuel mixtures experiments NOx was provided by both initial nitric oxide injection, and a HONO generator run continuously throughout the experiment at 1.5 ppbv h1. These experiments were performed for 6 h at a relative humidity of 50% when measured at 21 2 C. A stable temperature of 30 2 C was reached after 1 h and maintained
until the end of the experiment. A full description of the fuel experiments, as well as the ability of the SAPRC-99, MCMv3.1 and CB05 chemical mechanisms to replicate the results of these experiments, is reported elsewhere (White et al., 2010).
2.3. Chemical mechanisms The SAPRC-99 (S99) and SAPRC-07 (S07) mechanisms were used to simulate chamber experiments and the results obtained were compared to experimental data. In some cases, comparisons were extended to include the simulated results obtained by the Master Chemical Mechanism (MCM). The auxiliary chamber mechanism, which describes the formation and loss of important species on the chamber walls, is shown in Table 1 (Azzi et al., 2008). For experiments containing a single major hydrocarbon, the base S99 or S07 mechanism was used, with the reaction of the hydrocarbon of interest included in each mechanism. For experiments containing mixtures, each hydrocarbon was specified in the model input files by their detailed model species name (DMS) (Carter, 2000). The species were lumped by the SAPRC software by the adjustable parameter mechanism (Carter, 2009). A greater number of species had explicit DMS names in S07 compared to S99. The input file for each mechanism included as much detail as was possible, such that the input files for each mechanism varied slightly. For example, m-cymene and m-diethyl benzene were designated by the detailed model species M-CYMENE and M-DE-BEN respectively in S07. In S99 only M-DE-BEN was included as a DMS, so the m-cymene concentration was included in the C10-BEN2 detailed model species along with other C10 di-substituted benzenes that did not have an explicit DMS name. A full list of speciation is included in supplementary materials. Although slightly different inputs were used, using the S99 hydrocarbon input in S07 made less than 2% difference to the final ozone predictions.
3. Results 3.1. Method of comparison Model results were evaluated by two methods in this work: by comparison between S99 and S07 predictions; and by comparison between mechanism predictions and experimental results. The oxidative capacity of a chamber experiment was determined using D(O3eNO) (1) (Johnson, 1983; Carter and Atkinson, 1987). Table 1 Auxiliary mechanism for CSIRO smog chamber. SAPRC rate
Reaction
Rate
P(1) ea RN-I
JNO2 ðmin1 Þ Generator ¼ HONO hv þ wall ¼ HONO
Measured by actinometry 1.5 103 ppmv h1 No generator: 4.0 106 ppmv HONO generator: 4:0 106 ppmv þ 2:5 105 =JNO2 ppmv
K(NO2W)
NO2 ¼ (yhono)$HONO þ (1 yhono).wall NOx e N2O5 ¼ wall NOx N2O5 þ H2O ¼ wall NOx O3 ¼ wall OH ¼ HO2 HNO3 / wHNO3
7.2 105 min1
yhono K(N25I) K(N25S) K(O3W) K(XSHC) K(NAW)
0.42 6.0 104 min1 1.48 105 ppmv1 min1 1.2 105 min1 250 min1 1.98 104 min1
a Continuous HONO generator used in evaporated fuel experiments. Alterations were made to the RN-I for each JNO2 experimental value used.
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This describes the ability of the entire system to form ozone in the presence of NOx, by including the reaction of NO to NO2.
DðO3 eNOÞ [ ð½O3 t L ½O3 0 Þ L ð½NOt L ½NO0 Þ:
(1)
The model error (2) describes the relative difference between modelled and experimental values. fDðO3 eNOÞmod L DðO3 eNOÞexp g=DðO3 eNOÞexp 3 100%:
(2)
3.2. m-Xylene Thirteen m-xylene smog chamber experiments were modelled using S99 and S07. These experiments were carried out for ROC/ NOx varying between 1.8 and 21.0, measured in ppmC/ppmN. Twelve of these experiments form a 4 by 3 matrix, with four initial hydrocarbon measurements of approximately 90, 64, 37 and 13 ppbv, and initial NOx measurements of approximately 35, 48 and 69 ppbv. The light intensity used was equal to a photolysis rate constant for NO2 ðJNO2 Þ of 0.41 0.02 min1, with measurements performed using actinometry experiments (Hynes et al., 2005). Comparison of the calculated model errors is shown in Fig. 1. For both mechanisms D(O3eNO) was predicted to within 15% at 6 h for experiments with ROC/NOx above 7, with the exception of S99 predictions for higher ROC/NOx experiments which were slightly over-predicted. For experiments with ROC/NOx below 7, the mechanism predictions for D(O3eNO) were between 5 and 45%. Minimal change was observed in D(O3eNO) model error for high ROC/NOx experiments between 3 and 6 h. This was due to minimal ozone formation after 3 h in these experiments. For low ROC/NOx experiments where NO oxidation was still occurring, some improvement was observed between 3 and 6 h for both mechanisms. For the majority of experiments the D(O3eNO) predictions made by S99 were higher than S07, however the differences between the two mechanisms was no more than 15% for any experiment after 6 h. The results for D(O3eNO) from m-xylene oxidation in this chamber compare favourably with those performed at UCR chambers (Fig. 2). For this comparison only experiments with initial NOx and m-xylene below 300 ppbv each, and initial ROC/NOx below 23 were used for comparison. The prediction of D(O3eNO) for all but 4 of the 28 experiments with ROC/NOx above 7 are accurate to within 10% regardless of the light source used. The predictions for experiments with low initial ROC/NOx were less accurate, with all but one of the 15 experiments under-predicting D(O3eNO) model error greater than 10%. So for m-xylene experiments with low initial
Fig. 2. D(O3eNO) model error for m-xylene experiments at 6 h for experiments performed in the CSIRO chamber; and in the UCR-EPA, CTC and DTC chambers using arc and blacklight lamps.
concentrations, the predictions of D(O3eNO) were similar for experiments performed using arc lamps, blacklight lamps and blacklight blue lamps. 3.3. Toluene Fifteen toluene smog chamber experiments were performed at two different light intensities, with nine experiments using a JNO2 of 0.41 0.02 min1 and six experiments using a JNO2 of 0.80 0.02 min1. Initial toluene concentrations varied from 35 to 90 ppbv and initial NOx between 33 and 73 ppbv, producing ROC/ NOx varying between 3.5 and 18.9. The D(O3eNO) model error for S07 and S99 is shown for both high and low JNO2 (Fig. 3). Unlike m-xylene, toluene photooxidation was not well predicted for this chamber by either mechanism. After 3 h the low ROC/NOx experiments were performing more satisfactorily, but that was due to low ozone formation in these experiments (less than 20 ppbv). For these experiments, the formation of ozone from auxiliary processes such as wall effects was proportionally larger than the amount of ozone formed from the reaction of toluene. After 6 h a positive model error to ROC/NOx dependence was present for all but the low ROC/NOx experiments, consistent with other chamber modelling of aromatic photooxidation (Metzger et al., 2008). The results from these experiments
Fig. 1. D(O3eNO) model error for m-xylene experiments at (a) 3 h and (b) 6 h.
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Fig. 3. D(O3eNO) model error for toluene experiments at (a) 3 h and (b) 6 h; for two different light intensity subsets (JNO2 ¼ 0:41 or 0:80 min1 ). The experiments performed in the UCR-EPA chamber were performed using arc lamps (ROC/NOx ¼ 19.8; JNO2 ¼ 0:26 min1 ) or blacklight lamps (ROC/NOx ¼ 12.7; JNO2 ¼ 0:172 min1 ). Note that lines are visual aids only.
contrast with those observed by Carter (2009), where the average D(O3eNO) model error for S07 was within 20% of experimental values for toluene experiments performed using arc lights and blacklights. This discrepancy in model error when compared to m-xylene photooxidation may be explained by the parameterisation process used by the SAPRC mechanism. Unsaturated dicarbonyls were represented in the SAPRC-07 mechanism by three species, AFG1 and AFG2 for photoreactive mono-unsaturated dicarbonyls, and AFG3 for less photoreactive unsaturated dicarbonyls (Carter, 2009). The yield of AFG3, and the combined yield of AFG1 and AFG2, were both estimated given experimental yield data. The AFG1 and AFG2 products share exact mechanisms and absorption cross-sections, but differ in that the former photolyses to form radical species whereas the latter does not. The relative yields of AFG1 and AFG2 were adjusted to predict ozone formation accurately for experiments in the SAPRC experiment database. As such the characteristics of the experiments within the database used for optimisation will impact on the yields determined for each product, and by extension the performance of the mechanism. For m-xylene, there were 30 experiments for which the initial VOC and NOx concentrations were each below 300 ppbv, and where the initial ROC/NOx was less than 23. For toluene there were only 3 experiments used that fit the same criteria, for an equivalent ROC/NOx less than 20. This criterion was chosen as many higher concentration experiments reach the NOx-limited stage very early in the experiment. For these the onset and rate of ozone formation is more influenced by the effect of high NOx concentrations or high RO2 to NO concentrations during oxidation. Due to the uncertainty of the photolysis data for aromatic dicarbonyls (Calvert et al., 2002), the results from any chamber using blacklights cannot be used alone to assess aromatic photooxidation. However in collaboration with chamber results using other light sources, they can provide a valuable tool in testing details of models (Carter et al., 1995). In the SAPRC-07 mechanism, the lumped aromatic products arising from toluene and m-xylene are represented by the same product (AFG), although differ in their respective yields. As no differences were observed between light sources for m-xylene including the CSIRO chamber, it is possible that the poor performance for toluene experiments in this chamber is related to the lack of low concentration experiments used in toluene mechanism optimisation. Further investigation is required to determine the exact cause of this discrepancy.
3.4. Isoprene Five isoprene experiments were performed in the CSIRO chamber at a light intensity equivalent to JNO2 ¼ 0:40 0:03 min1 . Initial isoprene varied between 88 and 360 ppbv, with initial NOx measured between 38 and 114 ppbv. The only major update to the organic isoprene mechanism in S07 was the increase in the photolysis rate of methacrolein, which had little impact due to its relatively slow rate compared to its reaction with OH. Very little difference was observed between S99 and S07 predictions, with all major product concentrations almost identical. The oxidation of isoprene and nitric oxide, as well as the formation of formaldehyde, PAN and formic acid, were predicted to within 20% of experimental values at any given time by both S99 and S07. Little change was observed in predictions of methacrolein and methyl vinyl ketone. The final ozone predictions made by all mechanisms were approximately 20% lower than observed values for all experiments (Fig. 4). The deviation between modelled and experimental ozone occurred as the nitric oxide concentration became negligible. This was likely related therefore to the uptake of nitrogen into organic
Fig. 4. Ozone predictions for isoprene oxidation using SAPRC-07, SAPRC-99 and the MCM.
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compounds, such as organic nitrates (RONO2) and peroxy acyl nitrates (PANs). Given the importance of isoprene in global VOC budgets (Guenther et al., 2006) the oxidation of isoprene must be well represented in chemical mechanisms. These experiments use higher concentrations of isoprene and NOx than would be encountered in the environment. Despite some discrepancy in final ozone concentration predictions, the oxidation of isoprene is equally well simulated by S99 and S07 for these experiments. 3.5. Petrol experiments The photooxidation of evaporated fuel experiments was simulated using S99 and S07. Two types of evaporated fuel experiments were performed using three varieties of fuel. The two evaporation modes used were headspace vapour, and wholly evaporated fuel. The three fuel varieties were unleaded petrol (ULP) and two ethanol blends (E5 and E10). In summary of results previously reported for these experiments for SAPRC-99, MCM and CB05 (White et al., 2010), S99 predicted ozone formation to within 15% for both injection modes and all fuel varieties. The variability of the three mechanisms to the HONO generation rate was addressed, and the variability in S07 was found to be similar to that of S99 (refer to Supplementary materials). Six headspace experiments were performed, and the vapour mixtures used were characterised by high concentrations of alkanes and low concentrations of aromatics. For S99 the D(O3eNO) model error was found to be correlated to the initial carbon monoxide concentration, which varied between 120 and 770 ppbv. This initial amount of CO was due to CO not being removed in the zero air system, but the concentrations used are within the range expected in the ambient atmosphere. The same correlation to initial CO was found for S07, with model error for each experiment approximately 5e10% lower than the equivalent S99 prediction (Fig. 5). For headspace experiments the final D(O3eNO) predictions for both SAPRC mechanisms were within 15% of the experimental values for almost all experiments. Six wholly evaporated fuel experiments were performed, and the vapour mixtures used in these experiments contained significantly higher concentrations of aromatics and larger alkanes when compared to the equivalent headspace fuel experiments. The average D(O3eNO) model error for S07 was 25%, which contrasts with less than 5% for S99 and 19% for the MCM (Fig. 6). The differences
Fig. 5. D(O3eNO) model error for headspace experiments at 6 h compared to initial carbon monoxide concentration.
Fig. 6. D(O3eNO) model error for wholly experiments at 6 h compared to initial ethanol concentration. The error bars represent the difference between model errors for replicate experiments.
between the mechanisms were reflected in the OH and HO2 predictions, which were significantly lower in S07 compared to S99. 3.6. Sensitivity to changes in updated mechanism In the SAPRC-07 mechanism, a positive increase in overall biases for experiments using synthetic hydrocarbon mixtures was observed with S07 compared to S99 (Carter, 2009). This is in contrast to these results which show a negative trend for experimental bias. In the S07 mechanism, key reaction rates and photolysis data were updated given advances in experimental data. These changes will have been accounted for in the optimisation of dicarbonyl yields for aromatics, however they may have varying impact on chamber experiments performed under different conditions. The sensitivity of over 20 changes to key reactions was tested by altering a single reaction rate in the S07 mechanism to the rate that was used for that reaction in the S99 mechanism. For photolysis rates this involved using the absorption spectrum and quantum yields that were used in S99 for that species. The rate changes which produced the largest change in final D(O3eNO) predictions are shown in Fig. 7. The changes shown are the absolute changes in
Fig. 7. Sensitivity of final D(O3eNO) model error to rate changes in SAPRC-07. Reaction rates or absorption spectra were changed to their equivalent values in SAPRC-99.
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model error between the regular S07 mechanism and the ratealtered S07 mechanism. This method provides a basis for determining which changes to the base mechanism may have resulted in a greater change for wholly evaporated experiments than for headspace fuel experiments for S07. The reaction rate which effected the largest change in this subset of experiments was the reaction of NO2 and OH to form nitric acid. Changes to this reaction rate resulted in similar changes to the D(O3eNO) model error in both headspace and wholly evaporated experiments. The photolysis of NO2 and the reaction of O3 and NO also resulted in similar changes to model error for headspace and wholly evaporated experiments. The other reaction rates alterations shown in Fig. 7, the photolysis of methylglyoxal and glyoxal (both channels), resulted in much larger changes to model error in wholly evaporated experiments than in headspace experiments. This is related to the concentrations of glyoxal and methylglyoxal, which are a direct product of aromatics, present in higher concentrations in wholly evaporated experiments. For headspace experiments the modelled concentrations of both glyoxal and methylglyoxal were less than 0.4 ppbv, compared with between 2 and 5 ppbv for wholly evaporated experiments. Toluene experiments were much more sensitive to these reaction rate changes than petrol experiments. This was typical of the poor performance of S07 in predicting toluene photooxidation in this chamber. The average D(O3eNO) error for toluene experiments was 34% compared to an average of 13% for non-toluene experiments. As most toluene experiments had not reached the NOx-limited regime after 6 h, any small increase in reactivity had a proportionally larger effect on model predictions for toluene than for m-xylene or petrol, where the regime had been reached or was closer to being reached in a greater number of experiments. Despite the high sensitivity of toluene experiments to these reaction rate changes, the final D(O3eNO) predictions in the CSIRO chamber were similar for both S99 and S07. This is because the optimisation of dicarbonyl product yields for each aromatic hydrocarbon ensured that D(O3eNO) was predicted similarly by both SAPRC mechanisms. This was true for m-xylene and toluene experiments in the CSIRO smog chamber, where D(O3eNO) was predicted similarly by both mechanisms. However for petrol mixtures a significant difference was observed in D(O3eNO) predictions between the two mechanisms depending on the composition of the mixture. The rate changes which produced the largest difference between model predictions for headspace and wholly evaporated experiments were the light dependent dicarbonyl photolysis rates. The light source used in these experiments may have had some impact on the difference in predictions between the two mechanisms. For S07 the quantum yields for all three photolysis pathways had been updated from their values in S99 (Chen et al., 2000; Volkamer et al., 2005), although the absorption spectra used in both mechanisms were similar. The change in each photolysis rate is shown for three light sources in Table 2. Further information on the calculation of photolysis rates is available in the supplementary materials. The change in the photolysis rates of dicarbonyls between S99 and S07 was substantially greater for the CSIRO light source than either the blacklight or arc light source. Likewise the change in the photolysis rates between the SAPRC mechanisms when using blacklight lamps was significantly greater than using arc lamps. However the absolute rates for these reactions in S07, for the typical JNO2 values used in the UCR-EPA chamber experiments, are quite similar for each light source. The discrepancy between SAPRC mechanism predictions for wholly evaporated experiments therefore may be due to the light
Table 2 Photolysis rates in S07 for glyoxal and methylglyoxal. Rates have been shown for CSIRO lamps (blacklight blue), blacklight (BL) lamps and arc lamps. The relative change from the equivalent rate in S99 is also shown. Rates are calculated at the typical JNO2 values used for these experiments. Rate
CSIRO S99a
CSIRO S07a (change)
BL S07b (change)
Arc S07c (change)
GLY / 2 HCO (% change from S99) GLY / HCHO þ H2 (% change from S99) MGLY / products (% change from S99) AFG / products (% change from S99)d
1.0 104
0.39 104 (61%) 8.8 106 (1100%) 0.58 104 (48%) 2.1 103 e
0.25 104 (8.6%) 11.5 106 (1800%) 0.27 104 (34%) 1.8 103 e
0.46 104 (10%) 9.9 106 (58%) 0.85 104 (32%) 1.7 103 e
0.73 106 1.1 104 e
CSIRO photolysis rates were calculated for JNO2 ¼ 0:42 min1 and measured in s1. Blacklight photolysis rates were calculated for JNO2 ¼ 0:18 min1 and measured in s1. c Arc photolysis rates were calculated for JNO2 ¼ 0:26 min1 and measured in s1. d Product representation not comparable between SAPRC-99 and SAPRC-07. a
b
source used being different from those used in the SAPRC experiment database. However the predictions of D(O3eNO) from m-xylene photooxidation in the smog chamber at CSIRO were similar to predictions made using chambers with different light sources. Likewise the prediction of D(O3eNO) from toluene photooxidation, although severely under-predicted, was similar for both SAPRC mechanisms. It is not clear therefore that the light source is responsible for these differences in predictions, given its similar predictions for the two most common aromatic hydrocarbons. However it is possible that a combination of experimental conditions, including light source, higher OH concentration (due to HONO injection), higher water content and the photooxidation mechanisms of other aromatic hydrocarbons could be responsible for this variation in mechanism predictions. 4. Conclusions An evaluation of the SAPRC-07 chemical mechanism was carried out using data collected from experiments performed in the CSIRO smog chamber. For the majority of experiments SAPRC-07 performed similarly to SAPRC-99 in the CSIRO smog chamber, with the exception of aromatic-containing mixtures. The prediction of D(O3eNO) from m-xylene photooxidation was within 10% with higher ROC/NOx ratios, and between 5 and 45% for lower ROC/NOx ratios. These predictions matched those made in the various UCR chambers performed using different light sources. Conversely, the photooxidation of toluene was poorly represented by both mechanisms with ozone formation under-predicted by all experiments. There was a large difference between the number of low concentration toluene and m-xylene experiments in the SAPRC database and this may explain some of the discrepancy between the model results for these species. Experiments performed using petrol headspace vapour were predicted accurately by both SAPRC mechanisms. However large differences were observed between mechanism predictions for experiments performed using wholly evaporated petrol vapour. The changes made to photolysis data for dicarbonyls in the SAPRC07 mechanism resulted in larger changes in photolysis rates for blacklight-irradiated chambers compared to arc lamp irradiated chambers. It is possible that the light source chosen may be responsible for the difference in predictions for SAPRC-07 compared to SAPRC-99 in the CSIRO chamber. However the similar performance in mechanism predictions for single aromatic experiments, which would also be effected by changes in the photolysis data for these species, indicate that other unidentified problems may be responsible for the change in mechanisms predictions.
M. Azzi et al. / Atmospheric Environment 44 (2010) 1707e1713
The SAPRC-07 mechanism represents a substantial update to the SAPRC-99 mechanism. The performance of the SAPRC-07 in predicting oxidant formation for single hydrocarbon smog chamber experiments was equivalent to SAPRC-99 for the majority of experiments evaluated. Differences in the predictions from the wholly evaporated experiments however show the importance of light irradiation and photolysis in mechanism predictions for aromatics, as well as the importance of evaluating the performance of aromatic chemical mechanisms in predicting the oxidation of synthetic mixtures. Acknowledgements The authors would like to acknowledge the California Air Resources Board for organising and funding the SAPRC peer review. We would also like to thank William P.L. Carter for assistance with the SAPRC mechanism and for providing experimental data, and John Carras for helpful discussions and funding support. Appendix. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.atmosenv.2010.02.013. References Angove, D.E., Halliburton, B.W., Nelson, P.F., 2000. Development of a new indoor environmental chamber at MRL, North Ryde. In: Proceedings of the 15th International Clean Air & Environment Conference. Clean Air Society of Australia & New Zealand, pp. 270e274. Azzi, M., Johnson, G.M., Cope, M., 1992. An introduction to the generic reaction set photochemical smog mechanism. In: Proceedings of the 11th International Clean Air and Environment Conference, Brisbane, Australia. Clean Air Society of Australia and New Zealand, pp. 451e462. Azzi, M., White, S.J., Angove, D.E., November 2008. Review of the SAPRC-07 Chemical Mechanism Prepared for Dr Ajith Kaduwela, California Air Resources Board (CARB). Available at: http://www.arb.ca.gov/research/reactivity/reactivity.htm. Bloss, C., Wagner, V., Jenkin, M.E., Volkamer, R., Bloss, W.J., Lee, J.D., Heard, D.E., Wirtz, K., Martin-Reviejo, M., Rea, G., Wenger, J.C., Pilling, M.J., 2005. Development of a detailed chemical mechanism (MCMv3.1) or the atmospheric oxidation of aromatic hydrocarbons. Atmospheric Chemistry and Physics 5, 641e664. Calvert, J.G., Atkinson, R., Becker, K.H., Kamens, R.M., Seinfeld, J.H., Wallington, T.J., Yarwood, G., 2002. The Mechanisms of Atmospheric Oxidation of Aromatic Hydrocarbons. Oxford University Press, Oxford. Carter, W.P.L., May 8, 2000. Documentation of the SAPRC-99 Chemical Mechanism for VOC Reactivity Assessment. Report to the California Air Resources Board, Contracts 92-239 and 95-308. Carter, W.P.L., June 22, 2009. Development of the SAPRC-07 Chemical Mechanism and Updated Ozone Reactivity Scales. Final Report to the California Air Resources Board, Contract No. 03-318.
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