Statewide Air Pollution Research Center Research Articles

A review of gas-phase chemical mechanisms commonly used in atmospheric chemistry modelling

The atmospheric chemical mechanism is an essential component of airshed models used for investigating the chemical behaviors and impacts of species. Since the first tropospheric chemical mechanism was proposed in the 1960s, various mechanisms including Master Chemical Mechanism (MCM), Carbon Bond Mechanism (CBM), Statewide Air Pollution Research Center (SAPRC) and Regional Atmospheric Chemistry Mechanism (RACM) have been developed for different research purposes. This work summarizes the development and applications of these mechanisms, introduces their compositions and lumping methods, and compares the ways the mechanisms treat radicals with box model simulations. CBM can reproduce urban pollution events with relatively low cost compared to SAPRC and RACM, whereas the chemical behaviors of radicals and the photochemical production of ozone are described in detail in RACM. The photolysis rates of some oxygenated compounds are low in SAPRC07, which may result in underestimation of radical levels. As an explicit chemical mechanism, MCM describes the chemical processes of primary pollutants and their oxidation products in detail. MCM can be used to investigate certain chemical processes; however, due to its large size, it is rarely used in regional model simulations. A box model case study showed that the chemical behavior of OH and HO2 radicals and the production of ozone were well described by all mechanisms. CBM and SAPRC underestimated the radical levels for different chemical treatments, leading to low ozone production values in both cases. MCM and RACM are widely used in box model studies, while CBM and SAPRC are often selected in regional simulations.

Secondary organic aerosol formation from camphene oxidation: measurements and modeling

Abstract. While camphene is one of the dominant monoterpenes measured in biogenic and pyrogenic emission samples, oxidation of camphene has not been well-studied in environmental chambers and very little is known about its potential to form secondary organic aerosol (SOA). The lack of chamber-derived SOA data for camphene may lead to significant uncertainties in predictions of SOA from oxidation of monoterpenes using existing parameterizations when camphene is a significant contributor to total monoterpenes. Therefore, to advance the understanding of camphene oxidation and SOA formation and to improve representation of camphene in air quality models, a series of experiments was performed in the University of California Riverside environmental chamber to explore camphene SOA mass yields and properties across a range of chemical conditions at atmospherically relevant OH concentrations. The experimental results were compared with modeling simulations obtained using two chemically detailed box models: Statewide Air Pollution Research Center (SAPRC) and Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A). SOA parameterizations were derived from the chamber data using both the two-product and volatility basis set (VBS) approaches. Experiments performed with added nitrogen oxides (NOx) resulted in higher SOA mass yields (up to 64 %) than experiments performed without added NOx (up to 28 %). In addition, camphene SOA mass yields increased with SOA mass (Mo) at lower mass loadings, but a threshold was reached at higher mass loadings in which the SOA mass yields no longer increased with Mo. SAPRC modeling of the chamber studies suggested that the higher SOA mass yields at higher initial NOx levels were primarily due to higher production of peroxy radicals (RO2) and the generation of highly oxygenated organic molecules (HOMs) formed through unimolecular RO2 reactions. SAPRC predicted that in the presence of NOx, camphene RO2 reacts with NO and the resultant RO2 undergoes hydrogen (H)-shift isomerization reactions; as has been documented previously, such reactions rapidly add oxygen and lead to products with very low volatility (i.e., HOMs). The end products formed in the presence of NOx have significantly lower volatilities, and higher O : C ratios, than those formed by initial camphene RO2 reacting with hydroperoxyl radicals (HO2) or other RO2. Further analysis reveals the existence of an extreme NOx regime, wherein the SOA mass yield can be suppressed again due to high NO / HO2 ratios. Moreover, particle densities were found to decrease from 1.47 to 1.30 g cm−3 as [HC]0 / [NOx]0 increased and O : C decreased. The observed differences in SOA mass yields were largely explained by the gas-phase RO2 chemistry and the competition between RO2+ HO2, RO2+ NO, RO2+ RO2, and RO2 autoxidation reactions.

Evaluation of a highly condensed SAPRC chemical mechanism and two emission inventories for ozone source apportionment and emission control strategy assessments in China

The response of summertime O3 to changes in the nitrogen oxides (NOx) and volatile organic compounds (VOC) emissions, and contributions of different NOx and VOC sources to O3 in China are studied using a highly condensed photochemical mechanism in the Statewide Air Pollution Research Center (SAPRC) family (CS07A) and two popular anthropogenic emission inventories, the Multi-resolution Emission Inventory for China (MEIC) and Regional Emission inventory in ASia (REAS). Although CS07A predicts slightly lower O3 concentrations than the standard fix-parameter version of the SAPRC-11 mechanism, the two mechanisms predict almost identical relative responses to daily maximum 8-hour O3 (O3-8h) due to NOx and VOC emission reductions. A source-oriented version of the CS07A is applied to determine source contributions of NOx and VOCs to O3 using MEIC and REAS. The two inventories lead to similar model performance of O3, with MEIC predicting higher O3 in Beijing and Shanghai, especially on high O3 days. Source apportionment results show that industry and transportation are the top two contributors to non-background O3 for both inventories, followed by power and biogenic emissions. In general, the two inventories lead to similar source contribution estimations to O3 attributable to NOx. However, their estimations of relative contributions to VOC-related O3 differ for the industrial and transportation sectors. Differences in the source apportionment results are more significant in some urban areas, although both emissions capture the spatial variations in the source contributions. Our results suggest that future emission control policies should be assessed using multiple emission inventories, and the condensed CS07A is suitable for policy applications when a large number of simulations are needed.

Development of ozone reactivity scales for volatile organic compounds in a Chinese megacity

Abstract. We developed incremental reactivity (IR) scales for 116 volatile organic compounds (VOCs) in a Chinese megacity (Guangzhou) and elucidated their application in calculating the ozone (O3) formation potential (OFP) in China. Two sets of model inputs (emission-based and observation-based) were designed to localize the IR scales in Guangzhou using the Master Chemical Mechanism (MCM) box model and were also compared with those of the US. The two inputs differed in how primary pollutant inputs in the model were derived, with one based on emission data and the other based on observed pollutant levels, but the maximum incremental reactivity (MIR) scales derived from them were fairly similar. The IR scales showed a strong dependence on the chemical mechanism (MCM vs. Statewide Air Pollution Research Center), and a higher consistency was found in IR scales between China and the US using a similar chemical mechanism. With a given chemical mechanism, the MIR scale for most VOCs showed a relatively small dependence on environmental conditions. However, when the NOx availability decreased, the IR scales became more sensitive to environmental conditions and the discrepancy between the IR scales obtained from emission-based and observation-based inputs increased, thereby implying the necessity to localize IR scales over mixed-limited or NOx-limited areas. This study provides recommendations for the application of IR scales, which has great significance for VOC control in China and other countries suffering from serious O3 air pollution.

Global tropospheric effects of aromatic chemistry with the SAPRC-11 mechanism implemented in GEOS-Chem version 9-02.

The Goddard Earth Observing System with chemistry (GEOS-Chem) model has been updated with the Statewide Air Pollution Research Center version 11 (SAPRC-11) aromatics chemical mechanism, with the purpose of evaluating global and regional effects of the most abundant aromatics (benzene, toluene, xylenes) on the chemical species important for tropospheric oxidation capacity. The model evaluation based on surface and aircraft observations indicates good agreement for aromatics and ozone. A comparison between scenarios in GEOS-Chem with simplified aromatic chemistry (as in the standard setup, with no ozone formation from related peroxy radicals or recycling of NOx) and with the SAPRC-11 scheme reveals relatively slight changes in ozone, the hydroxyl radical, and nitrogen oxides on a global mean basis (1 %–4 %), although remarkable regional differences (5 %–20 %) exist near the source regions. NOx decreases over the source regions and increases in the remote troposphere, due mainly to more efficient transport of peroxyacetyl nitrate (PAN), which is increased with the SAPRC aromatic chemistry. Model ozone mixing ratios with the updated aromatic chemistry increase by up to 5 ppb (more than 10 %), especially in industrially polluted regions. The ozone change is partly due to the direct influence of aromatic oxidation products on ozone production rates, and in part to the altered spatial distribution of NOx that enhances the tropospheric ozone production efficiency. Improved representation of aromatics is important to simulate the tropospheric oxidation.

Updating the SAPRC Maximum Incremental Reactivity (MIR) scale for the United States from 1988 to 2010

ABSTRACTOzone reactivity scales play an important role in selecting which chemical compounds are used in products ranging from gasoline to pesticides to hairspray in California, across the United States and around the world. The California Statewide Air Pollution Research Center (SAPRC) box model that calculates ozone reactivity uses a representative urban atmosphere to predict how much additional ozone forms for each kilogram of compound emission. This representative urban atmosphere has remained constant since 1988, even though more than 25 years of emissions controls have greatly reduced ambient ozone concentrations across the United States during this time period. Here we explore the effects of updating the representative urban atmosphere used for ozone reactivity calculations from 1988 to 2010 conditions by updating the meteorology, emission rates, concentration of initial conditions, concentration of background species, and composition of volatile organic compound (VOC) profiles. Box model scenarios are explored for 39 cities across the United States to calculate the Maximum Incremental Reactivity (MIR) scale for 1,233 individual compounds and compound-mixtures. Median MIR values across the cities decreased by approximately 20.3% when model conditions were updated. The decrease is primarily due to changes in atmospheric composition ultimately attributable to emissions control programs between 1998 and 2010. Further effects were caused by changes in meteorological variables stemming from shifting seasons for peak ozone events (summer versus early fall). Lumped model species with the highest MIR values in 1988 experienced the greatest decrease in MIR values when conditions were updated to 2010. Despite the reduction in the absolute reactivity in the updated 2010 atmosphere, the relative ranking of the VOCs according to their reactivity did not change strongly compared to the original 1988 atmosphere. These findings indicate that past decisions about ozone control programs remain valid today, and the ozone reactivity scale continues to provide relevant guidance for future policy decisions even as new products are developed.Implications: Updating the representative urban atmosphere used for the Maximum Incremental Reactivity (MIR) scale from 1988 to 2010 conditions caused the reactivity of 1223 individual compounds and combined mixtures to decrease by an average of 20.3% but the relative ranking of the VOCs was not strongly affected. This means that previous guidance about preferred chemical formulations to reduce ozone formation in cities across the United States remain valid today, and the MIR scale continues to provide relevant guidance for future policy decisions even as new products are developed.

Ozone sensitivity to isoprene chemistry and emissions and anthropogenic emissions in central California

Ozone formation in California in year 2017 was simulated using three chemical mechanisms: the Statewide Air Pollution Research Center 2007 mechanism with updated toxics and isoprene chemistry (S07TIC); the Carbon Bond 6, revision 2 (CB6r2) mechanism; a modified CB6r2 mechanism with increased hydroxyl radical (OH) production from isoprene oxidation at low nitrogen oxide (NOx) concentrations (CB6r2OH). The simulations with a photochemical grid model (PGM) showed little difference in ozone between CB6r2 and CB6r2OH. Further analysis with a box model found little sensitivity of ozone to OH production in the isoprene chemistry of all three mechanisms under conditions representative of California. The S07TIC generally predicts greater ozone than the CB6r2, and the difference in the predicted concentrations exceeds the uncertainty estimated from uncertainties in the emissions inventory. The sensitivity of ozone to anthropogenic volatile organic compound (VOC), NOx and carbon monoxide (CO) emissions and to isoprene emissions was also calculated with the PGM. Accounting for uncertainties in the emissions inventory, the sensitivity to NOx emissions is significantly different between S07TIC and CB6r2 in central and northern California; the sensitivity to isoprene emissions is significantly different in southern California. All mechanisms give a negative sensitivity of ozone to isoprene emissions in areas of northern California where NOx emissions are small. The CB6r2 and S07TIC differ in the sensitivity of ozone to organic nitrate (ON) formation from isoprene oxidation, likely because all ONs recycle to NOx in the S07TIC mechanism but only first-generation ONs do so in the CB6r2. For current California conditions, uncertainties in OH production from isoprene have little impact on ozone, but the extent to which isoprene ONs recycle NOx does impact ozone concentration and ozone sensitivity to NOx emissions.

Main ozone-forming VOCs in the city of Sao Paulo: observations, modelling and impacts

High-ozone concentrations currently represent the main air pollution problem in the city of Sao Paulo, Brazil. To elucidate the main volatile organic compounds (VOCs), which act as ozone precursors, samples from air quality monitoring stations were evaluated. Thirty-five samples were collected in August–September of 2006 and 43 in July–August of 2008, when the consumption of ethanol was about 50 % of the total fuel used in the Sao Paulo Metropolitan Area. Samples were collected using electropolished stainless canisters. Chemical analyses were performed on pre-concentrated samples followed by gas chromatograph with flame ionization and mass spectrometry detection. The incremental reactivity scale was used to rank the ozone precursors using the Ozone Isopleth Package for Research (OZIPR) trajectory model coupled with chemical mechanism Statewide Air Pollution Research Center (SAPRC). Sixty-nine species of VOCs were quantified, and the ten main ozone precursors identified in 2008 were as follows: formaldehyde (42.8 %), acetaldehyde (13.9 %), ethene (12.2 %), propene (5.1 %), 1-methylcyclopentene (3.0 %), p-xylene (2.4 %), 1-butene (2.1 %), trans-2-pentene (1.9 %), 2-methyl 2-butene (1.7 %) and trans-2-butene (1.6 %). Volatile organic compound mass distribution showed that in 2008 alkanes represented 46 % of the total VOCs, alkenes 27 %, aromatics 14 %, alkadienes 1 % and aldehydes 12 %.

Estimates of non-traditional secondary organic aerosols from aircraft SVOC and IVOC emissions using CMAQ

Abstract. Utilizing an aircraft-specific parameterization based on smog chamber data in the Community Multiscale Air Quality (CMAQ) model with the volatility basis set (VBS), we estimated contributions of non-traditional secondary organic aerosols (NTSOA) for aircraft emissions during landing and takeoff (LTO) activities at the Hartsfield–Jackson Atlanta International Airport. NTSOA, formed from the oxidation of semi-volatile and intermediate volatility organic compounds (S/IVOCs), is a heretofore unaccounted component of fine particulate matter (PM2.5) in most air quality models. We expanded a prerelease version of CMAQ with VBS implemented for the Carbon Bond 2005 (CB05) chemical mechanism to use the Statewide Air Pollution Research Center 2007 (SAPRC-07) chemical mechanism and added species representing aircraft S/IVOCs and corresponding NTSOA oxidation products. Results indicated that the maximum monthly average NTSOA contributions occurred at the airport and ranged from 2.4 ng m−3 (34 % from idle and 66 % from non-idle aircraft activities) in January to 9.1 ng m−3 (33 and 67 %) in July. This represents 1.7 % (of 140 ng m−3) in January and 7.4 % in July (of 122 ng m−3) of aircraft-attributable PM2.5 compared to 41.0–42.0 % from elemental carbon and 42.8–58.0 % from inorganic aerosols. As a percentage of PM2.5, impacts were higher downwind of the airport, where NTSOA averaged 4.6–17.9 % of aircraft-attributable PM2.5 and, considering alternative aging schemes, was as high as 24.0 % – thus indicating the increased contribution of aircraft-attributable SOA as a component of PM2.5. However, NTSOA contributions were generally low compared to smog chamber results, particularly at idle, due to the considerably lower ambient organic aerosol concentrations in CMAQ compared to those in the smog chamber experiments.

수도권 지역에서 기상-대기질 모델링을 위한 VOC와 PM2.5의 화학종 분류 및 시간분배계수 산정

The purpose of this study is to assign emission source profiles of volatile organic compounds (VOCs) and particulate matters (PMs) for chemical speciation, and to correct the temporal allocation factor and the chemical speciation of source profiles according to the source classification code within the sparse matrix operator kernel emission system (SMOKE) in the Seoul metropolitan area. The chemical speciation from the source profiles of VOCs such as gasoline, diesel vapor, coating, dry cleaning and LPG include 12 and 34 species for the carbon bond IV (CBIV) chemical mechanism and the statewide air pollution research center 99 (SAPRC99) chemical mechanism, respectively. Also, the chemical speciation of PM2.5 such as soil, road dust, gasoline and diesel vehicles, industrial source, municipal incinerator, coal fired, power plant, biomass burning and marine was allocated to 5 species of fine PM, organic carbon, elementary carbon, , and . In addition, temporal profiles for point and line sources were obtained by using the stack telemetry system (TMS) and hourly traffic flows in the Seoul metropolitan area for 2007. In particular, the temporal allocation factor for the ozone modeling at point sources was estimated based on emission inventories of the stack TMS data.

Assessment of SAPRC07 with updated isoprene chemistry against outdoor chamber experiments

Isoprene, the most emitted non-methane hydrocarbon, is known to influence ozone (O3) formation in urban areas rich with biogenic emissions. To keep up with the recent advance on isoprene oxidation chemistry including the identification of isoprene epoxydiols (IEPOX) as a precursor to secondary organic aerosol (SOA), Xie et al. (2013) updated the SAPRC (Statewide Air Pollution Research Center)-07 chemical mechanism. It is currently unknown how the Xie modification of SAPRC07 impacts the ability of the model to predict O3. In this study we will evaluate the Xie mechanism with simulations of 24 isoprene experiments from the UNC Dual Gas-phase Chamber. Our results suggest that the new mechanism increases NOx (nitrogen oxides) inter-conversion and produces more O3 than SAPRC07 for all experiments. In lower-NOx experiments, the new mechanism worsens O3 performance in the wrong direction, increasing bias from 4.92% to 9.44%. We found increased NOx recycling from PANs accounts for that. This could be explained by more PANs made due to increased first generation volatile organic compound (VOC) products and hydroxyl radical (OH) production.

Application of Weather Research and Forecasting Model with Chemistry (WRF/Chem) over northern China: Sensitivity study, comparative evaluation, and policy implications

Abstract An extremely severe and persistent haze event occurred over the middle and eastern China in January 2013, with the record-breaking high concentrations of fine particulate matter (PM 2.5 ). In this study, an online-coupled meteorology-air quality model, the Weather Research and Forecasting Model with Chemistry (WRF/Chem), is applied to simulate this pollution episode over East Asia and northern China at 36- and 12-km grid resolutions. A number of simulations are conducted to examine the sensitivities of the model predictions to various physical schemes. The results show that all simulations give similar predictions for temperature, wind speed, wind direction, and humidity, but large variations exist in the prediction for precipitation. The concentrations of PM 2.5 , particulate matter with aerodynamic diameter of 10 μm or less (PM 10 ), sulfur dioxide (SO 2 ), and nitrogen dioxide (NO 2 ) are overpredicted partially due to the lack of wet scavenging by the chemistry-aerosol option with the 1999 version of the Statewide Air Pollution Research Center (SAPRC-99) mechanism with the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) and the Volatility Basis Set (VBS) for secondary organic aerosol formation. The optimal set of configurations with the best performance is the simulation with the Gorddard shortwave and RRTM longwave radiation schemes, the Purdue Lin microphysics scheme, the Kain-Fritsch cumulus scheme, and a nudging coefficient of 1 × 10 −5 for water vapor mixing ratio. The emission sensitivity simulations show that the PM 2.5 concentrations are most sensitive to nitrogen oxide (NO x ) and SO 2 emissions in northern China, but to NO x and ammonia (NH 3 ) emissions in southern China. 30% NO x emission reductions may result in an increase in PM 2.5 concentrations in northern China because of the NH 3 -rich and volatile organic compound (VOC) limited conditions over this area. VOC emission reductions will lead to a decrease in PM 2.5 concentrations in eastern China. However, 30% reductions in the emissions of SO 2 , NO x , NH 3 , and VOC, individually or collectively, are insufficient to effectively mitigate the severe pollution over northern China. More aggressive emission controls, which needs to be identified in further studies, are needed in this area to reach the objective of 25% PM 2.5 concentration reduction in 2017 proposed in the Action Plan for Air Pollution Prevention and Control by the State Council in 2013.

2007년 6월 수도권 오존모사 I - 광화학측정자료를 이용한 SAPRC99 화학종별 휘발성유기물질 배출량 입력자료 평가

Volatile organic compound (VOC) emissions in the 2007 CAPSS (Clean Air Protection Supporting System) emissions inventory are chemically speciated for the SAPRC99 (Statewide Air Pollution Research Center 99) mechanism, following the Source Classification Code (SCC) matching method to borrow the U.S.EPA's chemical speciation profiles. CMAQ simulations with High-order Direct Decoupled Method (HDDM) are in turn applied to evaluate uncertainty in the method by comparing the simulated model VOC species to the observations in the Seoul Metropolitan Area (SMA) for a 2007 June episode. Simulations under-predicted ALK1 to ALK4 in SAPRC99 by a factor of 2 to 5 and over-predicted ALK5 by a factor of 7.5 while ARO1, ARO2, OLE1, and ethylene (ETH) are comparable to the observations, showing relative difference by 10 to 30%. OLE2 emissions are roughly 4 times overestimated. Emission rates for individual VOC model species are revised referring to the ratio of simulated to observed concentrations. Impact of the VOC emission changes on the overall ozone prediction was insignificant for the days of which 1-hr maximum ozone are lower than 100 ppb. However, simulations showed ozone difference by 5 to 10 ppb when high ozone above 120 ppb was observed in the vicinity of Seoul. This result suggests that evaluations on individual model VOC emissions be necessary to lead ozone control plans to the right direction. Moreover, the simulated ratios of ARO1 and ARO2 to <TEX>$NO_x$</TEX> are roughly 50% lower than the observed ones, which imply that adjustment in <TEX>$NO_x$</TEX> and VOC emission rates may be required to mimic the real VOC/<TEX>$NO_x$</TEX> condition over the area.

Photochemical Modeling in California with Two Chemical Mechanisms: Model Intercomparison and Response to Emission Reductions

ABSTRACT An updated version of the Statewide Air Pollution Research Center (SAPRC) chemical mechanism (SAPRC07C) was implemented into the Community Multiscale Air Quality (CMAQ) version 4.6. CMAQ simulations using SAPRC07C and the previously released version, SAPRC99, were performed and compared for an episode during July– August, 2000. Ozone (O3) predictions of the SAPRC07C simulation are generally lower than those of the SAPRC99 simulation in the key areas of central and southern California, especially in areas where modeled concentrations are greater than the federal 8-hr O3 standard of 75 parts per billion (ppb) and/or when the volatile organic compound (VOC)/nitrogen oxides (NOx) ratio is less than 13. The relative changes of ozone production efficiency (OPE) against the VOC/NOx ratio at 46 sites indicate that the OPE is reduced in SAPRC07C compared with SAPRC99 at most sites by as much as approximately 22%. The SAPRC99 and SAPRC07C mechanisms respond similarly to 20% reductions in anthropogenic VOC emissions. The response of the mechanisms to 20% NOx emissions reductions can be grouped into three cases. In case 1, in which both mechanisms show a decrease in daily maximum 8-hr O3 concentration with decreasing NOx emissions, the O3 decrease in SAPRC07C is smaller. In case 2, in which both mechanisms show an increase in O3 with decreasing NOx emissions, the O3 increase is larger in SAPRC07C. In case 3, SAPRC07C simulates an increase in O3 in response to reduced NOx emissions whereas SAPRC99 simulates a decrease in O3 for the same region. As a result, the areas where NOx controls would be disbeneficial are spatially expanded in SAPRC07C. Although the results presented here are valuable for understanding differences in predictions and model response for SAPRC99 and SAPRC07C, the study did not evaluate the impact of mechanism differences in the context of the U.S. Environmental Protection Agency's guidance for using numerical models in demonstrating air quality attainment. Therefore, additional study is required to evaluate the full regulatory implications of upgrading air quality models to SAPRC07. IMPLICATIONS CMAQ simulations of gas-phase pollutants over California were conducted using the newly released SAPRC07C gas-phase chemical mechanism and the previous released version, SAPRC99. SAPRC07C predicted slightly lower concentrations of O3 and important radical species than did SAPRC99 in key polluted regions of California. In certain regions, SAPRC07C simulates an increase in O3 in response to 20% reductions in NOx emissions, whereas SAPRC99 simulates a decrease in O3. A consequence of this difference is that areas where NOx controls would be disbeneficial are spatially expanded for SAPRC07C compared with SAPRC99.

Evaluation of alkene degradation in the detailed tropospheric chemistry mechanism, MCM v3, using environmental chamber data

The representation of alkene degradation in version 3 of the Master Chemical Mechanism (MCM v3) has been evaluated, using environmental chamber data on the photo-oxidation of ethene, propene, 1-butene and 1-hexene in the presence of NOx, from up to five chambers at the Statewide Air Pollution Research Center (SAPRC) at the University of California. As part of this evaluation, it was necessary to include a representation of the reactions of the alkenes with O(3P), which are significant under chamber conditions but generally insignificant under atmospheric conditions. The simulations for the ethene and propene systems, in particular, were found to be sensitive to the branching ratios assigned to molecular and free radical forming pathways of the O(3P) reactions, with the extent of radical formation required for proper fitting of the model to the chamber data being substantially lower than the reported consensus. With this constraint, the MCM v3 mechanisms for ethene and propene generally performed well. The sensitivity of the simulations to the parameters applied to a series of other radical sources and sink reactions (radical formation from the alkene ozonolysis reactions and product carbonyl photolysis; radical removal from the reaction of OH with NO2 and β-hydroxynitrate formation) were also considered, and the implications of these results are discussed. Evaluation of the MCM v3 1-butene and 1-hexene degradation mechanisms, using a more limited dataset from only one chamber, was found to be inconclusive. The results of sensitivity studies demonstrate that it is impossible to reconcile the simulated and observed formation of ozone in these systems for ranges of parameter values which can currently be justified on the basis of the literature. As a result of this work, gaps and uncertainties in the kinetic, mechanistic and chamber database are identified and discussed, in relation to both tropospheric chemistry and chemistry important under chamber conditions which may compromise the evaluation procedure, and recommendations are made for future experimental studies. Throughout the study, the performance of the MCM v3 chemistry was also simultaneously compared with that of the corresponding chemistry in the SAPRC-99 mechanism, which was developed and optimized in conjunction with the chamber datasets.

Testing CMAQ chemistry sensitivities in base case and emissions control runs at SEARCH and SOS99 surface sites in the southeastern US

CMAQ was run to simulate urban and regional tropospheric conditions in the southeastern US over 14 days in July 1999 at 32, 8 and 2 km grid spacings. Runs were made with either of two older mechanisms, Carbon Bond IV (CB4) and the Regional Acid Deposition Model, version 2 (RADM2), and with the more recent and complete California Statewide Air Pollution Research Center, version 1999 mechanism (SAPRC99) in a sensitivity matrix with a full emissions base case and separate 50% control scenarios for emissions of nitrogen oxides (NO X ) and volatile organic compounds (VOC). Results from the base case were compared to observations at the Southeastern Aerosol Research and Characterization Study (SEARCH) site at Jefferson Street in Atlanta, GA (JST) and the Southern Oxidant Study (SOS) Cornelia Fort Airpark (CFA) site downwind of Nashville, TN. In the base case, SAPRC99 predicted more ozone (O 3) than CB4 or RADM2 almost every hour and especially for afternoon maxima at both JST and CFA. Performance of the 8 km models at JST was better than that of the 32 km ones for all chemistries, reducing the 1 h peak bias by as much as 30 percentage points; at CFA only the RADM2 8 km model improved. The 2 km solutions did not show improved performance over the 8 km ones at either site, with normalized 1 h bias in the peak O 3 ranging from 21% at CFA to 43% at JST. In the emissions control cases, SAPRC99 was generally more responsive than CB4 and RADM2 to NO X and VOC controls, excepting hours at JST with predicted increased O 3 from NO X control. Differential sensitivity to chemical mechanism varied by more than ±10% for NO X control at JST and CFA, and in a similar range for VOC control at JST. VOC control at the more strongly NO X - limited urban CFA site produced a differential sensitivity response of <5%. However, even when differential sensitivities in control cases were small, neither their sign nor their magnitude could be reliably determined from model performance in the full emissions case, meaning that the degree of O 3 response to a change in chemical mechanism can differ substantially with the level of precursor emissions. Hence we conclude that properly understanding the effects of changes in a model's chemical mechanism always requires emissions control cases as part of model sensitivity analysis.

Predicting secondary organic aerosol formation rates in southeast Texas

Rates of secondary organic aerosol (SOA) formation, due to the reactions of aromatics and monoterpenes, were estimated for southeast Texas by incorporating a modified version of the Statewide Air Pollution Research Center's chemical mechanism (SAPRC99) into the Comprehensive Air Quality Model with extensions (CAMx version 3.10). The model included explicit representation of the reactions of five SOA precursors (α‐pinene, β‐pinene, sabinene, d‐limonene, and Δ3‐carene). Reactions of each SOA precursor with O3, OH radical, and NO3 radical were included. The model also included separate reactions for low‐ and high‐SOA‐yield aromatic groups with the OH radical. SOA yields in the mechanisms were estimated using compound‐specific yield information (ΔSOA/ΔHC) derived from smog chamber experiments conducted by J. R. Odum and colleagues and R. J. Griffin and colleagues. The form of the SOA yield model was based on the work of J. R. Odum and colleagues and is a function of existing organic aerosol concentrations. Existing organic aerosol concentrations were estimated on the basis of ambient measurements of total organic carbon in southeast Texas. The reactions of monoterpenes (predominantly α‐pinene and β‐pinene) with ozone led to the most regional SOA formation, followed by monoterpenes with the nitrate radical. Aromatic‐OH reactions led to less regional SOA formation compared to monoterpenes; however, this formation occurs close to the urban and industrial areas of Houston. In contrast, SOA formation due to the reactions of monoterpenes occurred in the forested areas north of the urban area. The results of this study are in qualitative agreement with estimates of SOA formation based on ambient data from the same time period.

Evaluation of isoprene degradation in the detailed tropospheric chemical mechanism, MCM v3, using environmental chamber data

Abstract The isoprene degradation mechanism included in version 3 of the Master Chemical Mechanism (MCM v3) has been evaluated and refined, using the Statewide Air Pollution Research Center (SAPRC) environmental chamber datasets on the photo-oxidation of isoprene and its degradation products, methacrolein (MACR) and methylvinyl ketone (MVK). Prior to this, the MCM v3 butane degradation chemistry was also evaluated using chamber data on the photo-oxidation of butane, and its degradation products, methylethyl ketone (MEK), acetaldehyde (CH3CHO) and formaldehyde (HCHO), in conjunction with an initial evaluation of the chamber-dependent auxiliary mechanisms for the series of relevant chambers. The MCM v3 mechanisms for both isoprene and butane generally performed well and were found to provide an acceptable reaction framework for describing the NOx-photo-oxidation experiments on the above systems, although a number of parameter modifications and refinements were identified which resulted in an improved performance. All these relate to the magnitude of sources of free radicals from organic chemical process, such as carbonyl photolysis rates and the yields of radicals from the reactions of O3 with unsaturated oxygenates, and specific recommendations are made for refinements. In addition to this, it was necessary to include a representation of the reactions of O(3P) with isoprene, MACR and MVK (which were not previously treated in MCM v3), and conclusions are drawn concerning the required extent of free radical formation from these reactions. Throughout the study, the performance of MCM v3 was also compared with that of the SAPRC-99 mechanism, which was developed and optimized in conjunction with the chamber datasets.

Nonlinearity in atmospheric response: A direct sensitivity analysis approach

The decoupled direct method (DDM) is used for efficient and accurate calculation of the higher‐order sensitivity coefficients in a regional photochemical air quality model with detailed chemical mechanism (Statewide Air Pollution Research Center (SAPRC‐99)). High‐order DDM (HDDM) is an extension to a previous implementation of DDM in three‐dimensional air quality models (DDM‐3D) that directly calculates the higher‐order derivatives (with respect to one parameter, as well as cross derivatives) with similar computational efficiency as the first‐order implementation and is also modified for better accuracy. (H)DDM results show very good agreement with brute force (finite difference) sensitivity coefficients for the first‐ and second‐order derivatives, but the agreement deteriorates for higher‐order coefficients. The nature of the truncation errors and other inaccuracies in the brute force approximations are explored. The difference between the first‐order brute force and DDM derivatives is dominated (and largely explained) by the truncation errors as calculated from HDDM results. Taylor expansion is used for parametric scaling of the response with the use of sensitivity coefficients. Use of higher‐order coefficients can significantly improve the accuracy of such projections. Finally, higher‐order sensitivity coefficients of ozone with respect to NOx and volatile organic compound emissions (including cross derivatives) are used to create time‐ and location‐dependent ozone isopleths.

A modeling study of the nighttime radical chemistry in the lower continental troposphere: 1. Development of a detailed chemical mechanism including nighttime chemistry

We describe a detailed chemical mechanism (140 species, 440 reactions) representing atmospheric chemistry of the lower continental troposphere. We focus on chemical reactions potentially important for nighttime chemistry. The mechanism is based on the methods and concepts proposed by Carter [1990] for the development of the Statewide Air Pollution Research Center (SAPRC) mechanism to reduce the number of species and reactions. Peroxy radical chemistry is represented with chemical operators and secondary VOCs are lumped into a limited number of surrogate species. To account for new experimental data provided since the first formulation of the SAPRC mechanism, the whole set of reactions has been updated. Representation of peroxy radical chemistry has been modified to account for variability of organic peroxy radical‐peroxy radical reaction rates. New chemical operators and new secondary species have been added to the mechanism to introduce reactions of alkenes with NO3. Reactions of alkenes with O3 have been largely revised to represent largest radical yields suggested by recent experimental studies. Chemistry of three biogenic compounds (isoprene, α‐pinene and β‐pinene) has been included. The mechanism is coupled to a two‐layer box model (including anthropogenic and biogenic emissions of VOCs and NOx and dry deposition) in order to represent different chemical environments (from urban to rural cases). Comparisons with the SAPRC‐99 mechanism show good agreements especially for high NOx regime. Results from the model are evaluated with typical observed concentrations in the different environmental cases with a special focus on the representation of radical species concentations. For both daytime and nighttime, the calculated OH and peroxy radical concentrations are within the range of available observed values although the model tends to overestimate HO2 concentrations in daytime. At night, OH concentrations are found to vary from 1 to 6×106 molecules cm−3 in the urban environment, 5×104 to 2×105 molecules cm−3 in polluted rural environments and 4×104 molecules cm−3 in remote environments.