Source Of OH Radicals Research Articles

Acidic northern soils as sources of atmospheric nitrous acid (HONO)

The photolysis of nitrous acid (HONO) is an important source of OH radical, the key oxidizing agent in the atmosphere. Recently it has been reported that nitrite (NO2−) in soil can also be a source of atmospheric HONO, in addition to the other pathways, e.g. the hydrolysis of nitrogen dioxide (NO2) on wet surfaces. However, the production rates of HONO in various soils are unknown. We selected a range of dominant northern acidic soils and showed in microcosm experiments that soils which have the highest nitrous oxide (N2O) and nitric oxide (NO) emissions (drained peatlands) also have the highest HONO production rates. It is known that in natural peatlands with high water table and in boreal coniferous forest soils, low nitrification activity (microbial production of nitrite and nitrate) limits their N2O production. Low availability of nitrite in these soils is the likely reason also for their low HONO production rates. Lowering of water table of peatlands as a result of drainage or climate change enhances their nitrification activity and N2O/NO production and as shown here also their HONO production. Also nitrogen deposition, or nitrogen fertilization and other land-use practices increasing availability of mineral nitrogen can be expected to enhance HONO emissions from soils.

Unexpectedly high indoor hydroxyl radical concentrations associated with nitrous acid

The hydroxyl (OH) radical is the most important oxidant in the atmosphere since it controls its self-oxidizing capacity. The main sources of OH radicals are the photolysis of ozone and the photolysis of nitrous acid (HONO). Due to the attenuation of solar radiation in the indoor environment, the possibility of OH formation through photolytic pathways indoors has been ignored up to now. In the indoor air, the ozonolysis of alkenes has been suggested as an alternative route of OH formation. Models and indirect measurements performed up to now according to this hypothesis suggest concentrations of OH radicals on the order of 10(4)-10(5) molecules per cubic centimeter. Here, we present direct measurements of significant amounts of OH radicals of up to 1.8⋅10(6) molecules per cubic centimeter during an experimental campaign carried out in a school classroom in Marseille. This concentration is on the same order of magnitude of outdoor OH levels in the urban scenario. We also show that photolysis of HONO is an important source of OH radicals indoors under certain conditions (i.e., direct solar irradiation inside the room). Additionally, the OH concentrations were found to follow a linear dependence with the product J(HONO)⋅[HONO]. This was also supported by using a simple quasiphotostationary state model on the OH radical budget. These findings force a change in our understanding of indoor air quality because the reactivity linked to OH would involve formation of secondary species through chemical reactions that are potentially more hazardous than the primary pollutants in the indoor air.

Bionic radical generation and antioxidant capacity sensing with photocatalytic graphene oxide–titanium dioxide composites under visible light

OH radicals as reactive oxygen species in an organism were applied to assay antioxidant capacity since the obtained results present high biological relevance. As a proper photocatalyst, titanium dioxide was employed to generate OH radicals under ultraviolet light irradiation. However, ultraviolet light can damage molecular probe (DNA or protein) during the detection of antioxidant capacity, which interferes with the results. In this article, a novel composite graphene oxide-titanium dioxide (GO-TiO2) nanostructure was synthesized, which can generate numerous OH radicals just under visible light irradiation. In addition, a novel electrochemical antioxidant capacity sensor was designed with GO-TiO2 composites as source of OH radicals and DNA as a molecular probe. Antioxidants were measured by using the suppression of the decline of reduction current of methylene blue used as an intercalating agent for DNA after irradiation and ˙OH-mediated DNA damage. Using gallic acid (GA) as a mode antioxidant species, the detection of GA at levels as low as 0.85 mg L(-1) was possible. The antioxidant capacity of other antioxidants was also assayed. Finally, the novel sensor was applied to the determination of antioxidant capacity in tea.

Formation of carbonyls and hydroperoxyenals (HPALDs) from the OH radical reaction of isoprene for low-NOx conditions: influence of temperature and water vapour content

The formation of gas-phase products from the reaction of OH radicals with isoprene for low-NOx conditions ([NOx] ≤ 1010 molecule cm−3) has been studied in an atmospheric pressure flow tube (Institute for Tropospheric Research-Laminar Flow Tube, IfT-LFT) operating in the temperature range of 293–343 K and a relative humidity of < 0.5 % up to 50 %. The photolysis of H2O2 or ozone photolysis in the presence of water vapour served as the NOx-free OH radical sources. For dry conditions at 293 K, the measured yields of methyl vinyl ketone (MVK), 0.07 ± 0.02, and methacrolein (MACR), 0.12 ± 0.04, were in reasonable agreement with literature data. Beside the C4-carbonyls, further product signals have been attributed tentatively to glycolaldehyde, methylglyoxal, hydroxyacetone, 3-methylfuran, C5-hydroperoxyenals (HPALDs) and C5-hydroxy-hydroperoxides. A simplified, “classical” reaction mechanism without efficient HPALD production describes well the observed yield for MVK and MACR. Unexpected high MVK and MACR yields of up to 0.65 in total were measured under conditions of a relative humidity of 50 % using both OH radical sources and two different measurement techniques for organics (proton transfer reaction mass spectrometry and gas chromatography with flame ionization detector). The reaction mechanism applied is not able to describe the strong increase of MVK and MACR yields with increasing water vapour content. The signal attributed to the HPALDs showed a distinct rise of about one order of magnitude increasing the temperature from 293 K to 343 K. A rough estimate leads to a HPALD yield of 0.32 at 343 K with an uncertainty of a factor of two. The results of this study do not support a predominant formation of HPALDs under atmospheric conditions in low-NOx areas. The surprisingly high MVK and MACR yields measured for a relative humidity of 50 % and the formation of glycolaldehyde, methylglyoxal and hydroxyacetone necessitate further research.

Mechanism and kinetic study on the OH-initiated degradation of 2,3,7,8-tetrachlorinated dibenzofuran in atmosphere

High-accuracy molecular orbital calculation has been performed to investigate the atmospheric oxidation reaction of 2,3,7,8-tetrachlorodibenzofuran (2,3,7,8-TeCDF) initiated by the OH radical in the presence of O2 and NO/H2O. The possible channels involved in the reaction are discussed, and the favorable reaction pathways are revealed. The degradation occurs easily once the OH radical initiates the reaction. Two aspects need to be mentioned: one is that H2O in atmosphere is a source of OH radical which will initiate a new round of degradation and improve the degradation efficiency; the other is that the furan ring of 2,3,7,8-TeCDF can be turned into dioxin ring, which may explain the experimental hypothesis that polychlorodibenzofurans can be transformed to polychlorodibenzo-p-dioxins. Rate constants of the elementary reactions are calculated over a temperature range of 250–400K. Arrhenius formulas are fitted and the atmospheric lifetimes of reaction species in the troposphere are discussed for the first time, which can be applied to the study on the model simulation and the management of the hazardous materials.

Photochemical Aging of α-Pinene Secondary Organic Aerosol: Effects of OH Radical Sources and Photolysis

This study addresses photochemical aging of secondary organic aerosol (SOA) produced from α-pinene ozonolysis. The SOA is aged via hydroxyl radical (OH) reactions with first-generation vapors and UV photolysis. OH radicals are created through tetramethylethylene ozonolysis, HOOH photolysis, or HONO photolysis, sources that vary in OH concentration and the presence or absence of UV illumination. Aging strongly influences observed SOA mass concentrations, but the behavior is complex. In the dark or with high concentrations of OH, vapors are functionalized, lowering their volatility, resulting in an increase in OA by a factor of 2-3. However, with lower concentrations of OH under UV illumination SOA mass concentrations decrease over time. We attribute this decrease to evaporation driven by photolysis of the highly functionalized second-generation products. The photolysis rates are rapid, a few percent of the NO(2) photolysis frequency, and can thus be highly competitive with other aging mechanisms in the atmosphere.

Rate Constant Measurements for the Overall Reaction of OH + 1-Butanol → Products from 900 to 1200 K

The rate constant for the overall reaction OH + 1-butanol → products was determined in the temperature range 900 to 1200 K from measurements of OH concentration time histories in reflected shock wave experiments of tert-butyl hydroperoxide (TBHP) as a fast source of OH radicals with 1-butanol in excess. Narrow-linewidth laser absorption was employed for the quantitative OH concentration measurement. A detailed kinetic mechanism was constructed that includes updated rate constants for 1-butanol and TBHP kinetics that influence the near-first-order OH concentration decay under the present experimental conditions, and this mechanism was used to facilitate the rate constant determination. The current work improves upon previous experimental studies of the title rate constant by utilizing a rigorously generated kinetic model to describe secondary reactions. Additionally, the current work extends the temperature range of experimental data in the literature for the title reaction under combustion-relevant conditions, presenting the first measurements from 900 to 1000 K. Over the entire temperature range studied, the overall rate constant can be expressed in Arrhenius form as 3.24 × 10(-10) exp(-2505/T [K]) cm(3) molecule(-1) s(-1). The influence of secondary reactions on the overall OH decay rate is discussed, and a detailed uncertainty analysis is performed yielding an overall uncertainty in the measured rate constant of ±20% at 1197 K and ±23% at 925 K. The results are compared with previous experimental and theoretical studies on the rate constant for the title reaction and reasonable agreement is found when the earlier experimental data were reinterpreted.

Nitrous acid (HONO) during polar spring in Barrow, Alaska: A net source of OH radicals?

[1] HONO was measured by a LOPAP instrument (LOng Path Absorption Photometer) for one month during the OASIS spring 2009 campaign in Barrow, Alaska. HONO concentrations between ≤ 0.4 pptv (DL) and ∼500 pptv were measured. The very high concentrations observed on several days were caused by local direct emissions and were highly correlated with the NOx and CO data. When only “clean days” were considered, average HONO concentrations varied between ≤ 0.4 - 10 pptv. Average HONO/NOx and HONO/NOy ratios of ∼6% and ∼1% were observed, respectively, in good agreement with other remote LOPAP measurement data, but lower than measured in most other polar regions by other methods. The strong correlation between sharp peaks of OH and HONO during daytime, which was not observed for any other measured radical precursor, suggested that HONO photolysis was a major source of OH radicals in Barrow. This was supported by calculated net OH radical production by HONO and O3 photolysis for which the contribution of O3 (2%) could be neglected compared to that of HONO (98%). A net extra HONO/OH source necessary to explain elevated HONO levels during daytime of up to 90 pptv/h was determined, which was highly correlated with the actinic flux. Accordingly, a photochemical HONO source is proposed here, in good agreement with recent studies. From the higher correlation of the net HONO source with and [NO2] compared to and [NO3−], photosensitized conversion of NO2 on humic acid containing snow surfaces may be a more likely source of HONO in the polar atmosphere of Barrow than nitrate photolysis.

Heterogeneous reactions of HONO formation from NO2 and HNO3: a review

The photolysis of nitrous acid (HONO) is an important reaction of atmospheric chemistry due to the fact that it can be the source of OH radical in the troposphere. Despite its role as a radical precursor, the chemical mechanisms leading to HONO formation are not well understood. It is commonly assumed that HONO formation is due to both homogeneous and heterogeneous processes involving NOx (mixture of NO and NO2) in which the kinetic and mechanistic details are still under investigation. In this discussion, we would like to highlight the formation of HONO from NO2 and nitric acid (HNO3) in the presence of organic particulate. We understood that in the real case, many parameters can influence the reaction mechanism; however, this is just an effort to have a better understanding of the study of HONO formation in the atmospheric process.

Volatility of secondary organic aerosol during OH radical induced ageing

Abstract. The aim of this study was to investigate oxidation of SOA formed from ozonolysis of α-pinene and limonene by hydroxyl radicals. This paper focuses on changes of particle volatility, using a Volatility Tandem DMA (VTDMA) set-up, in order to explain and elucidate the mechanism behind atmospheric ageing of the organic aerosol. The experiments were conducted at the AIDA chamber facility of Karlsruhe Institute of Technology (KIT) in Karlsruhe and at the SAPHIR chamber of Forchungzentrum Jülich (FZJ) in Jülich. A fresh SOA was produced from ozonolysis of α-pinene or limonene and then aged by enhanced OH exposure. As an OH radical source in the AIDA-chamber the ozonolysis of tetramethylethylene (TME) was used while in the SAPHIR-chamber the OH was produced by natural light photochemistry. A general feature is that SOA produced from ozonolysis of α-pinene and limonene initially was rather volatile and becomes less volatile with time in the ozonolysis part of the experiment. Inducing OH chemistry or adding a new portion of precursors made the SOA more volatile due to addition of new semi-volatile material to the aged aerosol. The effect of OH chemistry was less pronounced in high concentration and low temperature experiments when lower relative amounts of semi-volatile material were available in the gas phase. Conclusions drawn from the changes in volatility were confirmed by comparison with the measured and modelled chemical composition of the aerosol phase. Three quantified products from the α-pinene oxidation; pinonic acid, pinic acid and methylbutanetricarboxylic acid (MBTCA) were used to probe the processes influencing aerosol volatility. A major conclusion from the work is that the OH induced ageing can be attributed to gas phase oxidation of products produced in the primary SOA formation process and that there was no indication on significant bulk or surface reactions. The presented results, thus, strongly emphasise the importance of gas phase oxidation of semi- or intermediate-volatile organic compounds (SVOC and IVOC) for atmospheric aerosol ageing.

Formation of excited uranyl in oxidation of tetravalent uranium with oxygen in HClO4 aqueous solutions: IV. Enhancement of the chemiluminescent reaction in the presence of Fe2+

Experimental data that support the hypothesis on the determining role of •OH radicals in the emergence of luminescence during the oxidation of U(IV) with atmospheric oxygen in aqueous HClO4 solutions have been obtained using the H2O2-FeSO4 system as a source of OH radicals. It has been found that brighter chemiluminescence (CL) is observed in the presence of 10−5 mol/l Fe2+ in a 5 × 10−4 mol/l U(IV) solution in 0.1 mol/l HClO4 compared with the FeSO4-free solution. The CL yield in the presence of Fe2+ (ηCL = 3.9 × 10−8) is 2.8 times that in the solution without iron (ηCL = 1.4 × 10−8). These results can be regarded as a further piece of evidence for the idea that the elementary event of the formation of a CL emitter—electronically excited uranyl ion *(UO22+)—in radical chain U(IV) oxidation reactions is electron transfer from the uranoyl ion (UO2+) to the oxidant, the •OH radical. Thus, one of the main prerequisites for light emission during U(IV) oxidation reactions is a high generation efficiency of •OH radicals and their easy access to the uranoyl UO2+ ion.

Effect of Nitrite and Nitrate as the Source of OH Radical in the O3/UV Process with or without Benzene

This study suggests the prediction model for the concentration variation of and along with the rate constants of all reactions during ozonation under UV radiation (/UV process). While was completely converted into during the -only process, the production of radical or was expected in the /UV process. In addition, the quenching of OH radicals, by radical in the /UV process, resulted in regeneration of . However, the regeneration of was not observed in the /UV process in the presence of where the concentrations of and were significantly reduced compared to in the process without . The pseudo-first order rate constants of all species were calculated with and without the presence of to predict the variation of concentrations of all species during the /UV process. It was suggested that and in the /UV process can be more effectively removed from an aqueous system with an OH radical scavenger such as .

Soil Nitrite as a Source of Atmospheric HONO and OH Radicals

Hydroxyl radicals (OH) are a key species in atmospheric photochemistry. In the lower atmosphere, up to ~30% of the primary OH radical production is attributed to the photolysis of nitrous acid (HONO), and field observations suggest a large missing source of HONO. We show that soil nitrite can release HONO and explain the reported strength and diurnal variation of the missing source. Fertilized soils with low pH appear to be particularly strong sources of HONO and OH. Thus, agricultural activities and land-use changes may strongly influence the oxidizing capacity of the atmosphere. Because of the widespread occurrence of nitrite-producing microbes, the release of HONO from soil may also be important in natural environments, including forests and boreal regions.

Direct observation of OH radicals after 565 nm multi-photon excitation of NO 2 in the presence of H 2O

The study of a controversial reaction, NO 2 ∗ + H 2 O as a potential new source of OH radicals in the atmosphere, has been performed using the coupling of a FAGE instrument to a laser photolysis cell. No OH radicals were observed using an unfocused excitation laser beam at 565 nm, but OH formation was observed by focusing the beam even though this reduces the instrument sensitivity. The dependence of the OH-fluorescence signal with the laser energy shows that the OH radicals originate from a complex mechanism including multiphoton absorption of NO 2.

Automobile exhaust gas as a source of aqueous phase OH radical in the atmosphere and its effects on physiological status of pine trees

Free radical generation potential of automobile exhaust gas was examined by measuring hydroxyl (OH) radical photo-formation rates in exhaust gas-scrubbing water. Effects of automobile exhausts on physiological status of Japanese red pine trees ( Pinus densiflora Sieb. et Zucc.) were also investigated to elucidate the mechanism how the free radicals derived from exhaust gas damage higher plants. Gasoline and diesel exhaust gases were scrubbed into pure water. Potential photo-formation rates of OH radical in aqueous phase (normalized to sun light intensity of clear sky midday on May 1 at 34°N) for gasoline and diesel cars were ave. 51 and 107 μM h −1 m −3 of exhaust gas, respectively. Nitrite was a dominant source (ca. 70–90%) of photochemical formation of OH radical in both gasoline and diesel car exhausts. The scrubbed solution of diesel car exhaust gas was sprayed for six times per week to needles of pine tree seedlings in open top chambers. Control, exhaust + mannitol (added as OH radical scavenger), and nitrite + nitrate standard solution (equivalent levels existed in the exhaust gas) were also sprayed. Two months sprays indicated that the sprayed solutions of diesel exhaust and nitrite + nitrate caused a decrease of maximum photosynthetic rate and stomata conductance in pine needles while the control and exhaust + mannitol solution showed no effects on photosynthetic activities of pine needles. These results indicated that OH radicals generated mainly from photolysis of nitrite occurring in the scrubbing solution of exhaust gas are responsible for the decrease of photosynthetic activities of pine needles.

Hydrogen peroxide in natural cloud water: Sources and photoreactivity

The photochemical behaviour of H 2O 2 in cloud aqueous phase was investigated from a large amount of in situ cloud droplet sampling data. Then we performed laboratory irradiation experiments of real cloud samples, free from gaseous chemistry, in order to factorise between gas phase and liquid phase photochemical reactions. H 2O 2 was revealed to be sensitive to solar radiation, with higher concentrations during the day. H 2O 2 was dependent on the degree of air mass pollution with anthropogenic influences leading to lower values of aqueous H 2O 2. We observed higher differences between day and night of H 2O 2 concentrations for air masses influenced by anthropogenic activities (mainly Northern direction) compared to those form remote areas (oceanic). Contrary to this field diurnal trend, during laboratory irradiation experiments of the cloud aqueous phase, the concentration showed a regular linear decrease leading to the conclusion that photolysis is more important than photoproduction in cloud aqueous phase. So the diurnal cycle observed during field measurements is due to the high mass transfer from gas phase to liquid phase favoured by the high dispersion of liquid phase and the high solubility of hydrogen peroxide. As a consequence, H 2O 2 can act as a photochemical source of OH radicals in cloud aqueous phase.

Vertical gradients of HONO, NOx and O3 in Santiago de Chile

Gradients of HONO, NO x (NO + NO 2 ) and O 3 were measured during an early summer campaign on a high-rise building in downtown Santiago de Chile. Distinct gradients of decreasing concentrations with altitude were observed for HONO and NO x , while O 3 concentrations were found to increase with altitude. From the observed daytime maximum of the HONO/NO x ratio the existence of a strong daytime source of HONO is proposed, thus confirming recent results from another measurement site in downtown Santiago, where a high contribution of HONO to the OH radical initiation sources of >50% was observed. Since the HONO/NO x ratio and its daytime maximum were found to be independent of the altitude, it is concluded that HONO is an important OH radical source in Santiago not only close to the ground surface, but also at higher altitudes of the boundary layer. ► HONO/NO x daytime maximum indicates daytime source of HONO in Santiago de Chile. ► No HONO/NO x gradients observed during daytime in Santiago de Chile. ► HONO important OH source in the whole boundary layer. ► HONO/NO x and NO 2 /NO x ratios from direct emissions of 0.6% and 13% were determined in Santiago de Chile.

ChemInform Abstract: Gas-Phase Reactions of Benzene and Derivatives Triggered by Hydrazine/ Ozone; Hydroxylation vs. Degradation.

The reactions of benzene derivatives C6H5X (X = H, Cl, CH3 and CF3) with hydrazine and ozone in the dark (a source of OH radicals) have been studied in a flow reactor at ambient temperature. (Substituted) phenols were formed, but these were relatively unimportant compared to N-heterocyclic compounds (condensation products of cis-butenedial and analogues with hydrazine) and other (degradation) products. Benzene and (trifluoromethyl)benzene follow a similar path as toluene, also with oxidative ring opening as major reactions.

Sources and Cycling of Tropospheric Hydroxyl Radicals – An Overview

Abstract The hydroxyl radical is the principal oxidizing agent in the troposphere. It controls and determines the oxidizing power of the atmosphere and thus governs the atmospheric lifetime of many species, and their potential to contribute to climate change, air pollution and ozone formation. Owing to the development of new measurement techniques and the discovery of new reaction mechanisms, the understanding of the OH chemistry has witnessed significant advances during the last decades. This article presents a comprehensive review of the current understanding of OH radical chemistry with an outlook to future work in this active research area of atmospheric chemistry.Among the different OH radical initiation sources, e.g. O3 photolysis, HCHO photolysis, ozonolysis of alkenes and the photolysis of nitrous acid (HONO), the latter has been shown in recent field and model studies to make a major contribution to tropospheric OH levels. New photochemical sources of HONO have been discovered in the laboratory which can explain unexpectedly high concentrations of HONO observed in the atmosphere during daytime. A detailed analysis of HONO sources in the atmosphere is presented. OH radicals react with volatile organic compounds (VOCs) in a complex cycle regenerating OH radicals in the propagation cycle. Recent field and model studies show that the OH radical propagation cycle is balanced for high NOx conditions. Thus, radical losses by reactions with VOCs are balanced by OH formation through the reaction HO2+NO. However, studies at low NO and high VOC levels imply that unknown radical sources of OH exist in rural and forested areas. Possible sources of OH radicals under these conditions are discussed.

Oxidative capacity of the Mexico City atmosphere – Part 1: A radical source perspective

Abstract. A detailed analysis of OH, HO2 and RO2 radical sources is presented for the near field photochemical regime inside the Mexico City Metropolitan Area (MCMA). During spring of 2003 (MCMA-2003 field campaign) an extensive set of measurements was collected to quantify time-resolved ROx (sum of OH, HO2, RO2) radical production rates from day- and nighttime radical sources. The Master Chemical Mechanism (MCMv3.1) was constrained by measurements of (1) concentration time-profiles of photosensitive radical precursors, i.e., nitrous acid (HONO), formaldehyde (HCHO), ozone (O3), glyoxal (CHOCHO), and other oxygenated volatile organic compounds (OVOCs); (2) respective photolysis-frequencies (J-values); (3) concentration time-profiles of alkanes, alkenes, and aromatic VOCs (103 compound are treated) and oxidants, i.e., OH- and NO3 radicals, O3; and (4) NO, NO2, meteorological and other parameters. The ROx production rate was calculated directly from these observations; the MCM was used to estimate further ROx production from unconstrained sources, and express overall ROx production as OH-equivalents (i.e., taking into account the propagation efficiencies of RO2 and HO2 radicals into OH radicals). Daytime radical production is found to be about 10–25 times higher than at night; it does not track the abundance of sunlight. 12-h average daytime contributions of individual sources are: Oxygenated VOC other than HCHO about 33%; HCHO and O3 photolysis each about 20%; O3/alkene reactions and HONO photolysis each about 12%, other sources &lt;3%. Nitryl chloride photolysis could potentially contribute ~15% additional radicals, while NO2* + water makes – if any – a very small contribution (~2%). The peak radical production of ~7.5 107 molec cm−3 s−1 is found already at 10:00 a.m., i.e., more than 2.5 h before solar noon. O3/alkene reactions are indirectly responsible for ~33% of these radicals. Our measurements and analysis comprise a database that enables testing of the representation of radical sources and radical chain reactions in photochemical models. Since the photochemical processing of pollutants in the MCMA is radical limited, our analysis identifies the drivers for ozone and SOA formation. We conclude that reductions in VOC emissions provide an efficient opportunity to reduce peak concentrations of these secondary pollutants, because (1) about 70% of radical production is linked to VOC precursors; (2) lowering the VOC/NOx ratio has the further benefit of reducing the radical re-cycling efficiency from radical chain reactions (chemical amplification of radical sources); (3) a positive feedback is identified: lowering the rate of radical production from organic precursors also reduces that from inorganic precursors, like ozone, as pollution export from the MCMA caps the amount of ozone that accumulates at a lower rate inside the MCMA. Continued VOC reductions will in the future result in decreasing peak concentrations of ozone and SOA in the MCMA.