Regional Atmospheric Chemistry Mechanism Research Articles

Inter-comparison of the Regional Atmospheric Chemistry Mechanism (RACM2) and Master Chemical Mechanism (MCM) on the simulation of acetaldehyde

Acetaldehyde (CH3CHO) is a key player of atmospheric chemistry, an important air pollutant, and hence a major target of air quality modeling and management. The Regional Atmospheric Chemistry Mechanism (RACM) is a highly lumped gas-phase chemical mechanism that has been widely applied in atmospheric chemistry modeling studies. A significant update of the latest version of RACM (RACM2) is the addition of CH3CHO as an explicit aldehyde species, facilitating the direct simulation of CH3CHO. In this study, we compared the performances of RACM2 and Master Chemical Mechanism (MCM; v3.3.1) on the simulation of CH3CHO. Zero-dimensional chemical box models based on these two independent mechanisms were prescribed to a polluted scenario to simulate the evolution of ozone (O3), hydroxyl radical (OH), C2H5O2 (ETHP) and CH3CHO, as well as their detailed chemical budgets. Overall, both mechanisms agreed with the simulation of O3 and OH, but the RACM2 model simulated significantly higher levels of ETHP and CH3CHO than the MCM model. The difference in the chemical kinetic data in both mechanisms is not the reason for this discrepancy. The oversimplification of the lumped peroxy acyl radicals (RCO3) and ≥C3 aldehydes chemistry of RACM2 should be responsible for its higher simulated ETHP and CH3CHO. We caution the use of RACM2 or any other highly aggregated chemical mechanism for the simulation of C2H5O2 and CH3CHO. Better methods are needed to represent the chemistry of peroxy acyl radicals and ≥C3 aldehydes for aggregated chemical mechanisms. More experiments are required to directly validate and further improve the current chemistry mechanisms.

Simulation of organic nitrates in Pearl River Delta in 2006 and the chemical impact on ozone production

Organic nitrates are secondary atmospheric pollutants that play a key role in ozone and aerosol production. This study focused on the simulation of organic nitrates through a box model coupled with RACM2 (Regional Atmospheric Chemistry Mechanism, version 2), based on data from the PRIDE-PRD2006 campaign in Backgarden, China. Our study found that an overestimation of organic nitrate production rate was generated by the model. Furthermore, the effective production ratio ( α eff) of organic nitrates was around 0.033 after optimizing its chemical production module. The chemical impacts of organic nitrates on ozone production were related to VOC-OH reactivity and α eff. We found that VOC-OH reactivity was positively related to α eff, resulting in the suppression of ozone production caused by organic nitrates, which showed that P (O x =O3+NO2) increased initially and subsequently decreased with VOC-OH reactivity. These results highlight the importance of organic nitrate’s impact on ozone production in strategies to control ozone pollution, specifically regarding the reduction of low-molecular-weight VOCs in the Pearl River Delta.

How the OH reactivity affects the ozone production efficiency: case studies in Beijing and Heshan, China

Abstract. Total OH reactivity measurements were conducted on the Peking University campus (Beijing) in August 2013 and in Heshan (Guangdong province) from October to November 2014. The daily median OH reactivity was 20 ± 11 s−1 in Beijing and 31 ± 20 s−1 in Heshan, respectively. The data in Beijing showed a distinct diurnal pattern with the maxima over 27 s−1 in the early morning and minima below 16 s−1 in the afternoon. The diurnal pattern in Heshan was not as evident as in Beijing. Missing reactivity, defined as the difference between measured and calculated OH reactivity, was observed at both sites, with 21 % missing reactivity in Beijing and 32 % missing reactivity in Heshan. Unmeasured primary species, such as branched alkenes, could contribute to missing reactivity in Beijing, especially during morning rush hours. An observation-based model with the RACM2 (Regional Atmospheric Chemical Mechanism version 2) was used to understand the daytime missing reactivity in Beijing by adding unmeasured oxygenated volatile organic compounds and simulated intermediates of the degradation from primary volatile organic compounds (VOCs). However, the model could not find a convincing explanation for the missing reactivity in Heshan, where the ambient air was found to be more aged, and the missing reactivity was presumably attributed to oxidized species, such as unmeasured aldehydes, acids and dicarbonyls. The ozone production efficiency was 21 % higher in Beijing and 30 % higher in Heshan when the model was constrained by the measured reactivity, compared to the calculations with measured and modeled species included, indicating the importance of quantifying the OH reactivity for better understanding ozone chemistry.

Model simulation of NO3, N2O5 and ClNO2 at a rural site in Beijing during CAREBeijing-2006

A chemical box model was used to study nitrate radical (NO3), dinitrogen pentoxide (N2O5) and nitryl chloride (ClNO2) in a rural site during the Campaign of Air Quality Research in Beijing 2006 (CAREBeijing-2006). The model was based on regional atmospheric chemistry mechanism version 2 (RACM2) with the heterogeneous uptake of N2O5 and the simplified chloride radical (Cl) chemistry mechanism. A high production rate of NO3 with a mean value of 0.8ppbv/h and low mixing ratios of NO3 and N2O5 (peak values of 17 pptv and 480 pptv, respectively) existed in this site. Budget analysis showed that NO emission suppressed the NO3 chemistry at the surface layer, the reaction of NO3 with VOCs made a similar contribution to NO3 loss as N2O5 heterogeneous uptake. The NO3 chemistry was predominantly controlled by isoprene, and NO3 oxidation produced organic nitrate with a mean value of 0.06ppbv/h during nighttime. The organic nitrate production initiated by NO3 was equal to that initiated by OH, implying the importance of nighttime chemistry for secondary organic aerosol (SOA) formation. We confirmed that the N2O5 heterogeneous reaction accounted for nighttime particle NO3− enhancement, with a large day to day variability, and made less of a contribution to NOx loss compared to that of OH reacting with NO2. Additionally, abundant ClNO2, up to 5.0 ppbv, was formed by N2O5 heterogeneous uptake. ClNO2 was sustained at a high level until noon in spite of the gradually increasing photolysis of ClNO2 after sunrise. Chlorine activation caused by N2O5 heterogeneous uptake increased primary ROx formation by 5% and accounted for 8% of the net ozone production enhancement in the morning.

Strong deviations from the NO-NO2-O3 photostationary state in the Pearl River Delta: Indications of active peroxy radical and chlorine radical chemistry

Simultaneous measurements of meteorological data, trace gases, and volatile organic compounds were made in two regional sites, viz. Backgarden and Kaiping, in the Pearl River Delta (PRD) during summer and autumn, respectively. The strong deviations from the NO-NO2-O3 Photostationary State, quantified by the leighton ratios, are carefully deduced through a comprehensive data set consist of the high-quality measurements of NO, NO2, O3 and JNO2 as well as the peroxy radical measurements. This is the first report of the Leighton ratio in China, with relatively high recorded values of 2.3 ± 0.4 (Backgarden) and 3.1 ± 1.4 (Kaiping), suggesting a strongly oxidising atmosphere in the PRD, typical of the ozone pollution season. A sensitivity analysis using a zero-dimensional chemical box model based on the regional atmospheric chemistry mechanism, version 2 (RACM2) constrained by the experimental measurements, indicated that peroxy radicals account for 70 (Backgarden) and 66% (Kaiping) of the observed positive deviations from the NOx photostationary state (characterized by a Leighton ratio of 1) on average. We consider that the remaining deviations result from neglecting the effects of chlorine chemistry, so we introduced a Cl chemistry module into RACM2, and the modelled results for Cl were as follows: 4.7 × 10−4 pptv in Backgarden and 1.3 × 10−3 pptv in Kaiping; these results are lower than the Cl concentration derived from the NOx photostationary state. More work is required to confirm the role of additional peroxy radical sources at both high and low NOx regimes, as well as that of the halogen radicals, in perturbing the NO-NOx-O3 cycle, which would significantly enhance trace gas removal and photochemical ozone production.

Examining the major contributors of ozone pollution in a rural area of the Yangtze River Delta region during harvest season

Abstract. Open biomass burning (OBB) emits significant amounts of non-methane hydrocarbons (NMHCs), and the mixing of OBB with urban plumes could exacerbate regional ozone (O3) pollution. In the present study, an observational field campaign was performed in a rural area at the northern edge of the Yangtze River Delta region (YRDR) from 15 May to 24 June 2010, during intensive open burning of wheat residues. The net photochemical production rate of oxidant (Ox = O3 + NO2) at the site was evaluated based on a box model (Regional Atmospheric Chemical Mechanism, Version 2) constrained by real-time ambient measurements (e.g., O3, volatile organic compounds (VOCs), NOx (NO2 + NO), J values). Our results showed that both in situ photochemistry and direct transport from urban areas in the YRDR were responsible for the high Ox concentration at the site. During an OBB-impact case, net photochemical production of Ox in the daytime was pronounced, with a 6 h averaged Ox production rate of 13 ± 4 ppbv h−1 (maximum value of 21 ppbv h−1 at 12:00 CST). Photochemical Oxproduction changed from VOC-limited in the morning to NOx-limited in the afternoon due to the rapid photochemical consumption of NOx during the day. A combined analysis with positive matrix factorization demonstrated that O3 pollution in the rural area of the YRDR was largely affected by urban emission, and OBB-related emissions also contributed to in situ photochemical production, particularly in the afternoon. Our study suggested that a joint effort in reducing both NMHCs (e.g., aromatics) and NOx emissions in the urban area, as well as local OBB activities, may be effective in eliminating high-O3 pollution risk in the rural areas of the YRDR.

Twelve-month, 12 km resolution North American WRF-Chem v3.4 air quality simulation: performance evaluation

Abstract. We present results from and evaluate the performance of a 12-month, 12 km horizontal resolution year 2005 air pollution simulation for the contiguous United States using the WRF-Chem (Weather Research and Forecasting with Chemistry) meteorology and chemical transport model (CTM). We employ the 2005 US National Emissions Inventory, the Regional Atmospheric Chemistry Mechanism (RACM), and the Modal Aerosol Dynamics Model for Europe (MADE) with a volatility basis set (VBS) secondary aerosol module. Overall, model performance is comparable to contemporary modeling efforts used for regulatory and health-effects analysis, with an annual average daytime ozone (O3) mean fractional bias (MFB) of 12% and an annual average fine particulate matter (PM2.5) MFB of −1%. WRF-Chem, as configured here, tends to overpredict total PM2.5 at some high concentration locations and generally overpredicts average 24 h O3 concentrations. Performance is better at predicting daytime-average and daily peak O3 concentrations, which are more relevant for regulatory and health effects analyses relative to annual average values. Predictive performance for PM2.5 subspecies is mixed: the model overpredicts particulate sulfate (MFB = 36%), underpredicts particulate nitrate (MFB = −110%) and organic carbon (MFB = −29%), and relatively accurately predicts particulate ammonium (MFB = 3%) and elemental carbon (MFB = 3%), so that the accuracy in total PM2.5 predictions is to some extent a function of offsetting over- and underpredictions of PM2.5 subspecies. Model predictive performance for PM2.5 and its subspecies is in general worse in winter and in the western US than in other seasons and regions, suggesting spatial and temporal opportunities for future WRF-Chem model development and evaluation.

A comparison of atmospheric composition using the Carbon Bond and Regional Atmospheric Chemistry Mechanisms

Abstract. We incorporate the recently developed Regional Atmospheric Chemistry Mechanism (version 2, RACM2) into the Community Multiscale Air Quality modeling system for comparison with the existing 2005 Carbon Bond mechanism with updated toluene chemistry (CB05TU). Compared to CB05TU, RACM2 enhances the domain-wide monthly mean hydroxyl radical concentrations by 46% and nitric acid by 26%. However, it reduces hydrogen peroxide by 2%, peroxyacetic acid by 94%, methyl hydrogen peroxide by 19%, peroxyacetyl nitrate by 40%, and organic nitrate by 41%. RACM2 enhances ozone compared to CB05TU at all ambient levels. Although it exhibited greater overestimates at lower observed concentrations, it displayed an improved performance at higher observed concentrations. The RACM2 ozone predictions are also supported by increased ozone production efficiency that agrees better with observations. Compared to CB05TU, RACM2 enhances the domain-wide monthly mean sulfate by 10%, nitrate by 6%, ammonium by 10%, anthropogenic secondary organic aerosols by 42%, biogenic secondary organic aerosols by 5%, and in-cloud secondary organic aerosols by 7%. Increased inorganic and organic aerosols with RACM2 agree better with observed data. Any air pollution control strategies developed using the two mechanisms do not differ appreciably.

OH and HO<sub>2</sub> radical chemistry during PROPHET 2008 and CABINEX 2009 – Part 1: Measurements and model comparison

Abstract. Hydroxyl (OH) and hydroperoxyl (HO2) radicals are key species driving the oxidation of volatile organic compounds that can lead to the production of ozone and secondary organic aerosols. Previous measurements of these radicals in forest environments with high isoprene, low NOx conditions have shown serious discrepancies with modeled concentrations, bringing into question the current understanding of isoprene oxidation chemistry in these environments. During the summers of 2008 and 2009, OH and peroxy radical concentrations were measured using a laser-induced fluorescence instrument as part of the PROPHET (Program for Research on Oxidants: PHotochemistry, Emissions, and Transport) and CABINEX (Community Atmosphere-Biosphere INteractions EXperiment) campaigns at a forested site in northern Michigan. Supporting measurements of photolysis rates, volatile organic compounds, NOx (NO + NO2 and other inorganic species were used to constrain a zero-dimensional box model based on the Regional Atmospheric Chemistry Mechanism, modified to include the Mainz Isoprene Mechanism (RACM-MIM). The CABINEX model OH predictions were in good agreement with the measured OH concentrations, with an observed-to-modeled ratio near one (0.70 ± 0.31) for isoprene mixing ratios between 1–2 ppb on average. The measured peroxy radical concentrations, reflecting the sum of HO2 and isoprene-based peroxy radicals, were generally lower than predicted by the box model in both years.

The regional atmospheric chemistry mechanism, version 2

The Regional Atmospheric Chemistry Mechanism (RACM) is a gas-phase chemical mechanism that is widely used for the modeling of regional atmospheric chemistry. Much new data has been published since the original RACM was completed (Stockwell et al., 1997). The RACM mechanism was updated to create the Regional Atmospheric Chemistry Mechanism, version 2 (RACM2). Like the RACM1 mechanism, the RACM2 mechanism is designed to simulate remote to polluted conditions from the Earth's surface through the upper troposphere. The RACM2 mechanism includes updated reaction schemes, rate constants and product yields. It has been tested against environmental chamber data and compared with previous RACM scenario simulations. The aromatic chemistry was expanded to include a greater number of species with highly revised reaction schemes. The reaction mechanism for isoprene was expanded to include a more explicit treatment of methyl vinyl ketone. Alcohols were speciated to more accurately reflect peroxy–peroxy reactions in the remote atmosphere. Acetone was speciated due to its importance in the upper troposphere.

Global Sensitivity Analysis of the Regional Atmospheric Chemical Mechanism: An Application of Random Sampling-High Dimensional Model Representation to Urban Oxidation Chemistry

Chemical mechanisms play a crucial part for the air quality modeling and pollution control decision-making. Parameters in a chemical mechanism have uncertainties, leading to the uncertainties of model predictions. A recently developed global sensitivity analysis (SA) method based on Random Sampling-High Dimensional Model Representation (RS-HDMR) was applied to the Regional Atmospheric Chemical Mechanism (RACM) within a zero-dimensional photochemical model to highlight the main uncertainty sources of atmospheric hydroxyl (OH) and hydroperoxyl (HO(2)) radicals. This global SA approach can be applied as a routine in zero-dimensional photochemical modeling to comprehensively assess model uncertainty and sensitivity under different conditions. It also highlights the parameters to which the model is most sensitive during periods when the model/measurement OH and HO(2) discrepancies are greatest. Uncertainties in 584 model parameters were assigned for measured constituents used to constrain the model, for photolysis and kinetic rate coefficients, and for product yields of the reactions. With simulations performed for the hourly field data of two typical days, modeled and measured OH and HO(2) generally agree better for polluted conditions than for cleaner conditions, except during morning rush hour. Sensitivity analysis shows that the modeled OH and HO(2) depend most critically on the reactions of xylenes and isoprene with OH, NO(2) with OH, NO with HO(2), and internal alkenes with O(3) and suggests that model/measurement discrepancies in OH and HO(2) would benefit from a closer examination of these reactions.

Comparisons of observed and modeled OH and HO<sub>2</sub> concentrations during the ambient measurement period of the HO<sub>x</sub>Comp field campaign

Abstract. A photochemical box model constrained by ancillary observations was used to simulate OH and HO2 concentrations for three days of ambient observations during the HOxComp field campaign held in Jülich, Germany in July 2005. Daytime OH levels observed by four instruments were fairly well reproduced to within 33% by a base model run (Regional Atmospheric Chemistry Mechanism with updated isoprene chemistry adapted from Master Chemical Mechanism ver. 3.1) with high R2 values (0.72–0.97) over a range of isoprene (0.3–2 ppb) and NO (0.1–10 ppb) mixing ratios. Daytime HO2(*) levels, reconstructed from the base model results taking into account the sensitivity toward speciated RO2 (organic peroxy) radicals, as recently reported from one of the participating instruments in the HO2 measurement mode, were 93% higher than the observations made by the single instrument. This also indicates an overprediction of the HO2 to OH recycling. Together with the good model-measurement agreement for OH, it implies a missing OH source in the model. Modeled OH and HO2(*) could only be matched to the observations by addition of a strong unknown loss process for HO2(*) that recycles OH at a high yield. Adding to the base model, instead, the recently proposed isomerization mechanism of isoprene peroxy radicals (Peeters and Müller, 2010) increased OH and HO2(*) by 28% and 13% on average. Although these were still only 4% higher than the OH observations made by one of the instruments, larger overestimations (42–70%) occurred with respect to the OH observations made by the other three instruments. The overestimation in OH could be diminished only when reactive alkanes (HC8) were solely introduced to the model to explain the missing fraction of observed OH reactivity. Moreover, the overprediction of HO2(*) became even larger than in the base case. These analyses imply that the rates of the isomerization are not readily supported by the ensemble of radical observations. One of the measurement days was characterized by low isoprene concentrations (∼0.5 ppb) and OH reactivity that was well explained by the observed species, especially before noon. For this selected period, as opposed to the general behavior, the model tended to underestimate HO2(*). We found that this tendency is associated with high NOx concentrations, suggesting that some HO2 production or regeneration processes under high NOx conditions were being overlooked; this might require revision of ozone production regimes.

Comparison of Different Gas-Phase Mechanisms and Aerosol Modules for Simulating Particulate Matter Formation

ABSTRACT The effects of two gas-phase chemical kinetic mechanisms, Regional Atmospheric Chemistry Mechanism version 2 (RACM2) and Carbon-Bond 05 (CB05), and two secondary organic aerosol (SOA) modules, the Secondary Organic Aerosoi Model (SORGAM) and AER/EPRI/Caltech model (AEC), on fine (aerodynamic diameter ≤2.5 μm) particulate matter (PM2.5) formation is studied. The major sources of uncertainty in the chemistry of SOA formation are investigated. The use of all major SOA precursors and the treatment of SOA oligomerization are found to be the most important factors for SOA formation, leading to 66% and 60% more SOA, respectively. The explicit representation of high-NOx and low-NOx gas-phase chemical regimes is also important with increases in SOA of 30–120% depending on the approach used to implement the distinct SOA yields within the gas-phase chemical kinetic mechanism; further work is needed to develop gas-phase mechanisms that are fully compatible with SOA formation algorithms. The treatment of isoprene SOA as hydrophobic or hydrophilic leads to a significant difference, with more SOA being formed in the latter case. The activity coefficients may also be a major source of uncertainty, as they may differ significantly between atmospheric particles, which contain a myriad of SOA, primary organic aerosol (POA), and inorganic aerosol species, and particles formed in a smog chamber from a single precursor under dry conditions. Significant interactions exist between the uncertainties of the gas-phase chemistry and those of the SOA module. IMPLICATIONS The current state of the science is more advanced for the gas-phase chemistry of ozone formation than for the chemistry and gas/particle partitioning of particulate matter (PM) formation. As a result, there are larger uncertainties associated with aerosol modules than with gas-phase chemical kinetic mechanisms. Nevertheless, the uncertainties associated with those modules are not additive in an air quality model and there are close interactions between the gas-phase chemical mechanism and the secondary aerosol formation. In particular, the effect of the NOx regime on SOA formation should be explicitly treated in air quality models.

Measurements of OH and HO<sub>2</sub> concentrations during the MCMA-2006 field campaign – Part 2: Model comparison and radical budget

Abstract. Measurements of hydroxyl (OH) and hydroperoxy (HO2) radicals were made during the Mexico City Metropolitan Area (MCMA) field campaign as part of the MILAGRO (Megacity Initiative: Local and Global Research Observations) project during March 2006. These measurements provide a unique opportunity to test current models of atmospheric ROx (OH + HO2 + RO2) photochemistry under polluted conditions. A zero-dimensional box model based on the Regional Atmospheric Chemical Mechanism (RACM) was constrained by 10-min averages of 24 J-values and the concentrations of 97 chemical species. Several issues related to the ROx chemistry under polluted conditions are highlighted in this study: (i) Measured concentrations of both OH and HO2 were underpredicted during morning hours on a median campaign basis, suggesting a significant source of radicals is missing from current atmospheric models under polluted conditions, consistent with previous urban field campaigns. (ii) The model-predicted HO2/OH ratios underestimate the measurements for NO mixing ratios higher than 5 ppb, also consistent with previous urban field campaigns. This suggests that under high NOx conditions, the HO2 to OH propagation rate may be overestimated by the model or a process converting OH into HO2 may be missing from the chemical mechanism. On a daily basis (08:40 a.m.–06:40 p.m.), an analysis of the radical budget indicates that HONO photolysis, HCHO photolysis, O3-alkene reactions and dicarbonyls photolysis are the main radical sources. O3 photolysis contributes to less than 6% of the total radical production.

The impacts of reactive terpene emissions from plants on air quality in Las Vegas, Nevada

A three-part study was conducted to quantify the impact of landscaped vegetation on air quality in a rapidly expanding urban area in the arid southeastern United States. The study combines in situ, plant-level measurements, a spatial emissions inventory, and a photochemical box model. Maximum plant-level basal emission rates were moderate: 18.1 μgC gdw −1 h −1 ( Washingtonia spp., palms) for isoprene and 9.56 μgC gdw −1 h −1 ( Fraxinus velutina, Arizona ash) for monoterpenes. Sesquiterpene emission rates were low for plant species selected in this study, with no measurement exceeding 0.1 μgC gdw −1 h −1. The high ambient temperatures combined with moderate plant-level emission factors resulted in landscape emission factors that were low (250–640 μgC m −2 h −1) compared to more mesic environments (e.g., the southeastern United States). The Regional Atmospheric Chemistry Mechanism (RACM) was modified to include a new reaction pathway for ocimene. Using measured concentrations of anthropogenic hydrocarbons and other reactive air pollutants (NO x , ozone), the box model employing the RACM mechanism revealed that these modest emissions could have a significant impact on air quality. For a suburban location that was downwind of the urban core (high NO x ; low anthropogenic hydrocarbons), biogenic terpenes increased time-dependent ozone production rates by a factor of 50. Our study demonstrates that low-biomass density landscapes emit sufficient biogenic terpenes to have a significant impact on regional air quality.

Diurnal peroxy radical chemistry at a remote coastal site over the sea of Japan

The Rishiri Fall Experiment (RISFEX) was performed in September 2003 at a remote coastal site (45.07°N, 141.12°E, and 35 m asl) on Rishiri Island in the Sea of Japan. Peroxy radicals were measured from 15–21 September using a peroxy radical chemical amplifier (PERCA) instrument. Simultaneously measured were O3, NO, NO2, CO, nonmethane hydrocarbons (NMHCs), biogenic volatile organic compounds (BVOCs), carbonyl compounds, aromatics, volatile organoiodines, organobromines, black carbon, aerosols and photolysis frequencies of j(O1D) and j(NO2) as well as meteorological parameters. The data set covers the measurements under relative background conditions and polluted episodes over the period. The midday 30‐min averages of peroxy radicals ranged from 10 to 40 pptv with the highest observed on the event when the air masses were transported over the polluted continent in northern China before reaching the site. The significant radical signals were observed repeatedly in the early morning and late afternoon when j(O1D) was greatly reduced, resulting in the diurnal cycles of peroxy radicals much broader than those expected from the photolysis of O3 alone. A box model based on RACM (Regional Atmospheric Chemistry Mechanism) was used to calculate OH, HO2 and RO2 concentrations from measured stable species and parameters. The calculated peroxy radical concentrations agree well with measured ones during 0900–1530 Japan Standard Time (JST) (JST = UT + 9 hours) from 19 to 21 September. However, in the early morning and later afternoon when monoterpenes were the dominant VOC, the model overpredicts peroxy radicals by up to 85%. The including of uptake of radicals to aerosols into the model cannot account for the discrepancy. An underprediction by the model up to 45% was observed during noon hours on 18 September when isoprene concentration was high. It appears that unknown terpenes are present, which might produce extra radicals beyond the predicted by the model. The measured‐to‐modeled ratio of peroxy radical concentrations is near unity for [NO] < 120 pptv, but rises up to 15 for [NO] > 120 pptv, suggesting the presence of an unknown radical production process which is related with high NO concentration.

Reduction of the RACM scheme using Computational Singular Perturbation Analysis

The modeling of dispersion of reacting pollutants in the atmosphere requires proper description of both the chemistry involved and the transport of pollutants. We present here highly reduced chemical schemes for rural and urban dispersion problems based on the Computational Singular Perturbation (CSP) technique. CSP identifies the steady‐state species for removal, by resolving the “fast” and the “slow” chemistry components. Here, the CSP method is employed for the reduction of the Regional Atmospheric Chemistry Mechanism (RACM), which includes 77 species and 237 reactions. Three cases were tested: (1) a polluted scenario including ground emissions, (2) a relatively cleaner environment with 10% of the emission levels of case 1, and finally (3) rural conditions without any emissions, for which the effects of emissions on the reduction procedure were directly studied. It was found that reduced mechanisms with high degree of reduction, down to a 16 step mechanism, i.e., 56 species in steady state, produced excellent results for most species for all cases. These reduced mechanisms did not change much between urban or rural conditions and the inclusion of emissions did not affect significantly the selection of major species. This indicates the persistence of a well‐defined fast subset of the mechanism, suggesting that the produced reduced mechanisms are valid for a wide range of conditions in the atmosphere.

OH, HO2, and OH reactivity during the PMTACS–NY Whiteface Mountain 2002 campaign: Observations and model comparison

Hydroxyl (OH), hydroperoxy (HO2) radicals, collectively known as HOx, and OH reactivity, were measured during the PMTACS–NY (PM2.5 Technology Assessment and Characteristics Study‐New York) summer 2002 intensive at Whiteface Mountain, Wilmington, New York. The measurement results of OH and HO2 for 4 weeks are presented. Diurnal cycles show that the average noontime maximum mixing ratios were about 0.11 pptv (2.6 × 106 cm−3) for OH and 20 pptv for HO2. Measured HO2 to OH ratios were typically between 40 and 400, which are greater than those obtained in polluted and semipolluted rural environments. Low but significant mixing ratios of OH and HO2 persisted into early evening and were frequently observed during nighttime, consistent with previous studies in different environments. Steady state OH and HO2 were calculated with a zero‐dimensional chemical model using a complete Regional Atmospheric Chemical Mechanism (RACM) and a parameterized RACM which was constrained to the measured OH reactivity. Good agreement was obtained between the complete RACM and the parameterized RACM models. On average, the complete RACM model reproduced the observed OH with a median measured‐to‐modeled OH ratio of 0.82 and daytime HO2 with a median measured‐to‐modeled HO2 ratio of 1.21. The reasonably good agreement in this study is inconsistent with the significant underestimation of OH in the Program for Research on Oxidants: Photochemistry, Emissions, and Transport in 1998 (PROPHET98) study at a similar forested site. HOx budget analysis indicates that OH was primarily from the photolysis of HONO and O3 during the day and from O3 + alkenes reactions at night. The main HOx loss was the self reaction of HO2. The good agreement between the measured and calculated OH reactivity in this environment contrasts with findings in the PROPHET2000 study, in which significant OH reactivity was missing and the missing OH reactivity was temperature‐dependent.

Behavior of OH and HO 2 in the winter atmosphere in New York City

Hydroxyl (OH) and hydroperxy (HO 2) radicals, collectively known as HO x , were measured during an intensive field study in January and February 2004 in New York City. Much less OH and HO 2 levels were observed than in the summer of 2001 at the same site. On average, the maximum daytime mixing ratios were 0.05 pptv (1.4×10 6 cm −3) for OH and 0.7 pptv for HO 2, which were about one fifth of the levels in the summer of 2001. A zero-dimensional chemical model, based on the regional atmospheric chemical mechanism (RACM) and constrained by the measured concentrations of O 3, NO, NO 2, CO, SO 2, speciated volatile organic compounds (VOCs) and meteorological parameters, was used to study the HO x chemistry in this environment. The model generally reproduced the daytime OH well, with a median measured-to-model ratio of 0.98. However, HO 2 was significantly under-predicted both at day and at night, with a median measured-to-model ratio of 6.0 during daytime. The discrepancy is pronounced when NO concentrations were high, a result that is consistent with some previous studies in urban environments. Photolysis of HONO was the dominant calculated HO x source during daytime; O 3 reactions with alkenes became the main calculated HO x source at night. The main calculated HO x sink was the OH reaction with NO 2. The discrepancy between measured and modeled HO 2 may be caused by significant HO x production that is missing in the model. An additional HO 2 production of up to 3×10 7 cm −3 s −1 (1.1 pptv s −1), which is three times the calculated HO x production, is needed. This HO 2 production can come either from unknown new HO x production or from unknown HO 2 recycling that does not go through OH.

Hydroxyl and Peroxy Radical Chemistry in a Rural Area of Central Pennsylvania: Observations and Model Comparisons

Atmospheric hydroxyl (OH), hydroperoxy (HO2), total peroxy (HO2 and organic peroxy radicals, RO2) mixing ratios and OH reactivity (first order OH loss rate) were measured at a rural site in central Pennsylvania during May and June 2002. OH and HO2 mixing ratios were measured with laser induced fluorescence (LIF); HO2 + RO2 mixing ratios were measured with chemical ionization mass spectrometry (CIMS). The daytime maximum mixing ratios were up to 0.6 parts per trillion by volume (pptv) for OH, 30 pptv for HO2, and 45 pptv for HO2 + RO2. A parameterized RACM (Regional Atmospheric Chemistry Mechanism) box model was used to predict steady state OH, HO2 and HO2 + RO2 concentrations by constraining the model to the measured OH reactivity and previously measured volatile organic compound (VOC) distributions. The averaged model calculations are generally in good agreement with the observations. For OH, the model matched the observations for day and night, with an average observed-to-modeled ratio of 0.80. In previous studies such as PROPHET98, nighttime NO was near 0 pptv and observed nighttime OH was significantly larger than modeled OH. In this study, nighttime observed and modeled OH agree to within measurement and model uncertainties because the main source of the nighttime OH was the reaction HO2 + NO → OH + NO2, with the NO being continually emitted from the surrounding fertilized corn field. The observed-to-modeled ratio for HO2 is 1.0 on average, although daytime HO2 is underpredicted by a factor of 1.2 and nighttime HO2 is over-predicted by a factor of ∼2. The average measured and modeled HO2 + RO2 agree well during daytime, but the modeled value is about twice the measured value during nighttime. While measured HO2 + RO2 values agree with modeled values for NO mixing ratios less than a few parts per billion by volume (ppbv), it increases substantially above the expected value for NO greater than a few ppbv. This observation of the higher-than-expected HO2 + RO2 with the CIMS technique confirms the observed increase of HO2 above expected values at higher NO mixing ratios in HO2 measurements with the LIF technique. The maximum instantaneous O3 production rate calculated from HO2 and RO2 reactions with NO was as high as 10–15 ppb h−1 at midday; the total daily O3 production varied from 13 to 113 ppbv d−1 and was 48 ppbv d−1 on average during this campaign.