Photochemical Ozone Creation Potentials Values Research Articles

Photochemical ozone creation potentials for volatile organic compounds: Rationalization and estimation

The Photochemical Ozone Creation Potential (POCP) scale quantifies the relative abilities of volatile organic compounds (VOCs) to produce ground level ozone. POCP values are usually calculated using atmospheric boundary layer models containing detailed representations of atmospheric VOC degradation chemistry. The sensitivity of POCP values to variation of a number of kinetic and mechanistic parameters has been investigated here. It is shown that POCP values for VOCs can be rationalized in terms of their molecular structure and OH reactivity. As a result, a simple method has been developed and optimized that allows POCP values for north-west European and USA urban reference conditions to be estimated for alkanes, alkenes, aromatic hydrocarbons, and several oxygenated VOC classes without the requirement to construct a detailed chemical mechanism or run an atmospheric model. The procedure for determining the estimated POCP value (POCPE) is described, and the results are presented and discussed.

Environmental risks of new-generation fluorocarbons in replacement of potent greenhouse gases

This article aimed to address two categories of environment friendly fluorocarbons (i.e., hydrofluoroethers and hydrofluoro-olefins), which are used as replacements for potent greenhouse gases like hydrochlorofluorocarbons (HCFC) and hydrofluorocarbons (HFC). In the work, the author will introduce the new-generation fluorocarbons on the basis of their excellent data on the environmental properties (e.g., atmospheric lifetime and global warming potential) and safety (e.g., flammability and exposure limit/level). Further, the values of photochemical ozone creation potential (POCP) for these fluorocarbons and some HFC compounds are estimated using the Wallington method and the rate constant data for the reaction of OH radical, showing that their POCP values are largely lower than those of common hydrocarbons. Based on the molecular structure and possible fate mechanism, these fluorocarbons and their decomposition products (especially in trifluoroacetic acid and fluoride) may pose adversely environmental risks, including flammability and fluoride hazards. Currently, hydrofluoroethers (HFE) such as HFE-7000 (C3H7OCH3), and hydrofluoro-olefins (HFO), including HFO-1234yf (CH2 = CFCF3) and HFO-1234ze (CHF = CHCF3), are considered as the probable candidates to replace HCFC and HFC.

Protocol for the development of the Master Chemical Mechanism, MCM v3 (Part B): tropospheric degradation of aromatic volatile organic compounds

Abstract. Kinetic and mechanistic data relevant to the tropospheric degradation of aromatic volatile organic compounds (VOC) have been used to define a mechanism development protocol, which has been used to construct degradation schemes for 18 aromatic VOC as part of version 3 of the Master Chemical Mechanism (MCM v3). This is complementary to the treatment of 107 non-aromatic VOC, presented in a companion paper. The protocol is divided into a series of subsections describing initiation reactions, the degradation chemistry to first generation products via a number of competitive routes, and the further degradation of first and subsequent generation products. Emphasis is placed on describing where the treatment differs from that applied to the non-aromatic VOC. The protocol is based on work available in the open literature up to the beginning of 2001, and some other studies known by the authors which were under review at the time. Photochemical Ozone Creation Potentials (POCP) have been calculated for the 18 aromatic VOC in MCM v3 for idealised conditions appropriate to north-west Europe, using a photochemical trajectory model. The POCP values provide a measure of the relative ozone forming abilities of the VOC. These show distinct differences from POCP values calculated previously for the aromatics, using earlier versions of the MCM, and reasons for these differences are discussed.

Protocol for the development of the Master Chemical Mechanism, MCM v3 (Part A): tropospheric degradation of non-aromatic volatile organic compounds

Abstract. Kinetic and mechanistic data relevant to the tropospheric degradation of volatile organic compounds (VOC), and the production of secondary pollutants, have previously been used to define a protocol which underpinned the construction of a near-explicit Master Chemical Mechanism. In this paper, an update to the previous protocol is presented, which has been used to define degradation schemes for 107 non-aromatic VOC as part of version 3 of the Master Chemical Mechanism (MCM v3). The treatment of 18 aromatic VOC is described in a companion paper. The protocol is divided into a series of subsections describing initiation reactions, the reactions of the radical intermediates and the further degradation of first and subsequent generation products. Emphasis is placed on updating the previous information, and outlining the methodology which is specifically applicable to VOC not considered previously (e.g. a- and b-pinene). The present protocol aims to take into consideration work available in the open literature up to the beginning of 2001, and some other studies known by the authors which were under review at the time. Application of MCM v3 in appropriate box models indicates that the representation of isoprene degradation provides a good description of the speciated distribution of oxygenated organic products observed in reported field studies where isoprene was the dominant emitted hydrocarbon, and that the a-pinene degradation chemistry provides a good description of the time dependence of key gas phase species in a-pinene/NOX photo-oxidation experiments carried out in the European Photoreactor (EUPHORE). Photochemical Ozone Creation Potentials (POCP) have been calculated for the 106 non-aromatic non-methane VOC in MCM v3 for idealised conditions appropriate to north-west Europe, using a photochemical trajectory model. The POCP values provide a measure of the relative ozone forming abilities of the VOC. Where applicable, the values are compared with those calculated with previous versions of the MCM.

MODELLING STUDY OF PHOTOCHEMICAL OZONE CREATION POTENTIAL OF NON-METHANE HYDROCARBON

Using an atmospheric photochemical model, the photochemical ozonecreation potential (POCP) values of non-methane hydrocarbon(NMHC) species are studied under different NMHC/NOx ratios. The behavior of POCP values of NMHC changing with time is also discussed under the condition of higher NMHC/NOx ratios, referring to the situation of China. The following results can beobtained. Alkenes, high-reactivity alkanes and aromatics, andformaldehyde are the primary contributors of photochemicalozone; The POCP values of NMHC species vary withNMHC/NOx ratios systematically. The POCP values of highreactivity species decrease with NMHC/NOx ratio increasing,but the POCP values of low reactivity species changelittle. The POCP value for a given NMHC species varieswith time of photochemical reactions. The POCP values oflow reactivity species increase with time, but the POCPvalues of high reactivity species change little. NOx, alkenes,aromatics, and aldehydes have higher reactivity and shorterlifetimes, therefore their transportation distances arelimited and they mainly produce local pollution. Alkanes havelower chemical reactivity and longer lifetimes so that theycan transport farther distance from their sources.

Characterization of the Reactivities of Volatile Organic Compounds Using a Master Chemical Mechanism

ABSTRACT A comprehensive description of the ozone-forming potentials of 101 organic compounds has been constructed under North American urban "averaged conditions" using a detailed master chemical mechanism and a simple air parcel trajectory model. This chemical mechanism describes the reactions of 3603 chemical species taking part in more than 10,500 chemical reactions. An index value has been calculated for each organic compound, which describes the increment in ozone concentrations found downwind of an urban area following the emission of a fixed increment in the mass emission of each organic compound. These indices, termed photochemical ozone creation potentials (POCPs), have been expressed on a scale relative to ethylene (ethene) = 100, and a reactivity scale has been generated for alkanes, alkenes, and oxygenated and halogenated organic compounds. A high degree of correlation (R2 = 0.9) was found between these POCP values and the most widely accepted urban reactivity scale. While the reactivities of most of the 86 organic compounds compared fell within a consistent range, significant discrepancies were found for only 5 compounds.

An Alternative Approach to Photochemical Ozone Creation Potentials Applied under European Conditions

ABSTRACT Photochemical ozone creation potential (POCP) values for 83 different volatile organic compounds (VOCs), including CO and CH4, were calculated under different environmental conditions representative for Europe. These calculations show that variations in POCP values are large between different types of chemical environments and that POCP values for VOCs should be presented as ranges instead of single values. POCP ranges are based on the extremes of the POCP values and are defined with the intention to include all POCP values an individual VOC will obtain in any European environment where O3 formation is of environmental concern. The POCP ranges indicate large differences in O3 production between individual VOCs, which justifies the use of this ranking scale instead of treating all VOCs as a homogeneous group of species in abatement strategies. Both the average O3 production over 96 hr and the maximum contribution to the O3 concentration were studied. The most efficient O3 producers were found to be iso-prene, 2-methyl-2-butene, and acrolein. As a group, the alkenes are the most potent O3 producers, followed by higher alkanes and then the aromatics. The calculated values show a good agreement with previously calculated POCP values under northern European conditions.

Photochemical ozone creation potentials (POCP) and replacement of solvents in Europe

Photochemical ozone creation potentials (POCP) have been calculated under 12 different emission conditions and the replacement of current solvents by oxygenates have been investigated under two European emission conditions. As expected the higher the NO x /VOC ratio, the more pronounced the deviation among POCP values for individual VOC, and the more useful the method of ranking VOC according to their ability to contribute to ozone. The replacement of current solvents by oxygenates significantly reduces the ozone levels in polluted air, such as the London plume. The replacement of current solvents in European air, plumes 1 and 2 in Poppe et al. (1996) and Kuhn et al. (1998), has a limited effect on the ozone levels. The influence on other species such as PAN and aldehydes is, however, also, under these conditions, significant when replacing current solvents by oxygenates.

Photochemical ozone creation potentials for oxygenated volatile organic compounds: sensitivity to variations in kinetic and mechanistic parameters

The sensitivity of Photochemical Ozone Creation Potentials (POCP) to a series of systematic variations in the rates and products of reactions of radical intermediates and oxygenated products is investigated for the C 4 alcohols, 1-butanol ( n-butanol) and 2-methyl-1-propanol ( i-butanol), using the recently developed Master Chemical Mechanism (MCM) as the base case. The POCP values are determined from the calculated formation of ozone in the boundary layer over a period of approximately five days along an idealised straight line trajectory, using a photochemical trajectory model and methodology described in detail previously. The results allow the relative impacts on calculated ozone formation of various classes of chemical reaction within the degradation chemistry to be assessed. The calculated POCP is found to be very insensitive to many of the changes investigated. However, it is found to be sensitive to variations in the rate coefficient for the initiating reaction with OH ( k OH ), although the sensitivity decreases with increasing k OH . The POCP appears to vary approximately linearly with k OH at low values (i.e. k OH less than ca. 4×10 -13 cm 3 molecule -1 s -1), whereas at high reactivities (i.e. k OH greater than ca. 4×10 -11 cm 3 molecule -1 s -1), the calculated POCP value is comparatively insensitive to the precise value of k OH . The POCP is also very sensitive to mechanistic changes which influence the yields of unreactive oxygenated products (i.e. those with OH reactivities below ca. 10 -12 cm 3 molecule -1 s -1), for example acetone. The propensity of the organic compound to produce organic nitrates (which act as comparatively unreactive reservoirs for free radicals and NO x ) also appears to have a notable influence on the calculated POCP. Recently reported information relevant to the degradation of oxygenated VOCs is then used to update the chemical schemes for the 17 alcohols and glycols, 10 ethers and glycol ethers, and 8 esters included in the MCM, and new schemes are incorporated for dimethoxy methane (CH 3OCH 2OCH 3) and dimethyl carbonate (CH 3OC(O)OCH 3), which are proposed fuel additives. New or updated POCP values are calculated for all 37 oxygenated VOCs and, where applicable, these are compared with the previous POCP values and reported Maximum Incremental Reactivity (MIR) values.

Photochemical ozone creation potentials for a large number of reactive hydrocarbons under European conditions

A photochemical trajectory model is used to describe the ozone production from the oxidation of methane and 95 other hydrocarbons in the presence of sunlight and NO x in air parcels advected across north west Europe towards the British Isles. By adding a small additional mass emission of each hydrocarbon in turn, additional ozone production was stimulated. A photochemical ozone creation potential (POCP) index was generated from the model results showing the relative importance of each hydrocarbon in ozone formation, on a mass emitted basis. Aromatic and olefinic hydrocarbons showed the highest POCP values with halocarbons the lowest. Using the POCP index, motor vehicle exhaust is seen to exhibit the highest ozone-forming potential of all the hydrocarbon emission source categories evaluated. Toluene, n-butane, ethylene and the xylenes, alone, account for over one third of the ozone forming potential of European emissions. Certain hydrocarbons, including acetone and methyl acetate, show significantly lower POCPs and have considerable potential as candidates for substitution in industrial or chemical processes and as solvents.

Hydrocarbon reactivity and ozone formation in Europe

Several studies have shown that in qualitative terms VOCs can be ranked in terms of their importance to ozone formation in Europe, using a so-called photochemical ozone creation potential (POCP). However, all studies showed significant variability in the quantitative POCP values assigned to each VOC species. In this paper, we make use of a complex photochemical oxidant model to quantify how the reaction time-scales of emitted VOC and their reaction products vary with geographical location and time of year. The model is used to evaluate monthly POCP values for 9 hydrocarbon and oxygenated VOC compounds for a cross-section across Europe. Calculations are presented to illustrate the gradients in POCP values of one species,n-butane, across the whole of Europe for April and June 1989. We discuss some of the factors responsible for these variations.