Optimization of Polyhydroxyalkanoates Production from Thermus thermophilus HB8 Using Response Surface Methodology

The thermophilic bacterium Thermus thermophilus HB8 was used for the overproduction of polyhydroxyalkanoates (PHAs) using a mathematical approach for the first time for optimization of process variables. In addition, the combined effect of nitrogen and phosphate concentrations on PHAs production was also investigated. A five-level-three-factor central composite rotary design was employed in combination with response surface methodology (RSM) to optimize the process variables for the production of PHAs in Thermus thrermophilus HB8. The three independent variables studied in the work were cultivation time, C/N ratio and phosphate concentration. Two second-order polynomial equations were obtained for biomass and PHA production by multiple regression analysis using RSM. The statistical analyses of the results showed that all the three variables had significant impact both on the cell growth and polymer accumulation. The model predicted a maximum PHA production of 0.47?g/L which represents the 42?% of dry cell weight (DCW) after 55?h of cultivation and with on setting the C/N ratio at 9:1?g/g and phosphate concentration at 20?mM. Verification of the predicted value resulted into a PHA production of 0.44?g/L (40.36?% of DCW).     

Municipal Wastes Treatment and Production of Polyhydroxyalkanoate by Modified Two-Stage Batch Reactor

The municipal wastes were utilized as substrate for polyhydroxyalkanoate (PHA) using two strains of Bacillus licheniformis (PHAs-007, wild type and M2-12, mutant). Municipal wastes were subjected to separate wastewater and biosolid. Municipal biosolid was digested by anaerobic bacteria thereafter only the supernatant with soluble organic compounds was subjected into the PHA-producing reactor containing municipal wastewater. The mutant strain M2-12 gave the highest value of biomass (42.0 ± 2.0 g/L) and PHA concentration (37.4 ± 1.0 g/L with 88.9 % of dry cell weight, DCW) and reduced 76.5 % of soluble chemical oxygen demand after 60 h of cultivation. The value of pH, biochemical oxygen demand and total solid of the reclaimed wastewater after PHA recovery was 7.1, 20 and 97 mg/L, respectively. Moreover, the polymers produced by both strains of B. licheniformis were characterized. The resultant polymer from B. licheniformis PHAs-007 and M2-12 cultivated in the PHA-producing reactor was identified as poly-3-hydroxybutyrate-co-3-hydroxyvalerate [P(3HB-co-3HV)] and poly-3-hydroxybutyrate-co-4-hydroxybutyrate [P(3HB-co-4HB)], respectively. The results suggesting that the production of PHA by municipal wastes is feasible thus the PHA production stage can be integrated in waste treatment to produce PHA and treated municipal wastes at the same time.     

Cost Effective Production of Poly-β-Hydroxybutyrate by Bacillus subtilis NG05 Using Sugar Industry Waste Water

The present study describes the treatment of sugar industry waste water and its use as a potential low cost substrate for production of bioplastic by Bacillus subtilis NG05. The B. subtilis NG05 grow at the rate of 0.14 g h^?1 L^?1 of production media used and accumulate the 50.1 % of poly β-hydroxybutyrate (PHB). The phase contrast microscopy revealed the presence of PHB granules in B. subtilis NG05 which was further confirmed by Fourier transform infrared spectroscopy and ¹H-nuclear magnetic resonance. The polymer was further analysed by differential scanning calorimetry. PHB production yield was achieved up to 4.991 g L^?1 with Sugar industry waste water as sole nutrient source. Thus the process provided dual benefits of conversion of a waste material into value added product, PHB and waste management.     

Evaluation of buckwheat and barley tea wastes as ethanol fermentation substrates

Buckwheat tea waste (BWTW) and barley tea waste (BTW), by-products of the beverage industry, are alternative carbohydrate sources for ethanol production. In this study, optimal enzyme loading for enzymatic saccharification of BWTW and BTW was determined, and simultaneous saccharification and fermentation (SSF) was performed by Saccharomyces cerevisiae and Mucor indicus to produce ethanol. Optimal enzyme loading for enzymatic saccharification of 2?% w/v BWTW and BTW was 0.5?% (weight of enzyme/weight of tea wastes) for BWTW and 0.1?% for BTW. Ethanol production from BWTW by S. cerevisiae and M. indicus after 48?h of SSF was 49.9/100?g of BWTW and 47.9/100?g of BWTW, respectively, with 0.5?% enzyme loading. Ethanol production from BTW by S. cerevisiae and M. indicus after 48?h of SSF was 20.5/100?g of BTW and 21.6/100?g of BTW, corresponding to 62 and 66?% of the theoretical yield based on starch content, respectively, with 0.1?% of enzyme loading. Furthermore, S. cerevisiae produced 76?% of the theoretical yield based on the total glucose from starch in BWTW and BTW when a mixture of BWTW and BTW was used as a substrate, with 0.2?% enzyme loading and no additional nitrogen or mineral sources.     

Kinetic Analysis of the Temperature Effect on Polyhydroxyalkanoate Production by <Emphasis Type="Italic">Haloferax mediterranei</Emphasis> in Synthetic Molasses Wastewater

Haloferax mediterranei is an extremely halophilic archaeon that is able to synthesize polyhydroxyalkanoate (PHA) in high salt environment with low sterility demand. In this study, a mathematical model was validated and calibrated for describing the kinetic behavior of H. mediterranei at 15, 20, 25, and 35 °C in synthetic molasses wastewater. Results showed that the production of PHA by H. mediterranei, ranging from 390 to 620 mg h^?1 L^?1, was strongly dependent on the temperature. The specific growth rate (µ _max), specific substrate utilization rate (q _max), and specific decay rate (k _d) of H. mediterranei increased with temperature following Arrhenius equation prediction. The estimated activation energy was 58.31, 25.59, and 22.38 kJ mol^?1 for the process of cell growth, substrate utilization, and cell decay of H. mediterranei, respectively. The high temperature triggered the increased PHA storage even without nitrogen limitation. Thus, working at high temperatures seems a good strategy for improving the PHA productivity of H. mediterranei.     

Anaerobic co-digestion of municipal solid organic waste with melon residues to enhance biodegradability and biogas production

Management of solid organic waste has become a major challenge in developing countries. Raw solid organic waste can be converted into biogas through anaerobic digestion; however, the efficiency of the process is influenced by various factors including the composition of the substrate. The present study was designed with the objective of enhancing the biodegradability of the organic fraction of municipal solid waste (OFMSW) and biogas production through co-digestion of the substrate with melon residues. The study was conducted in batch mode in four phases. The results revealed that an addition of melon waste at the rate of 300?g?kg^?1 OFMSW substantially increased the biodegradation rate and biogas production compared to OFMSW alone. The removal of up to 57.2?% volatile solids and a carbon to nitrogen (C/N) ratio of 15.9 was achieved at a 60?% water level when the digestion mixture was treated with inocula collected from partially-degraded food waste. The findings of this study reveal that melon residues could be used as a potential co-substrate to enhance the biodegradability of OFMSW and biogas production.     

Production and degradation of polyhydroxyalkanoates in waste environment

Polyhydroxyalkanoates (PHAs) are energy/carbon storage materials accumulated under unfavorable growth condition in the presence of excess carbon source. PHAs are attracting much attention as substitute for non-degradable petrochemically derived plastics because of their similar material properties to conventional plastics and complete biodegradability under natural environment upon disposal. In this paper, PHA production and degradation in waste environment as well as its role in biological phosphorus removal are reviewed. In biological phosphorus removal process, bacteria accumulating polyphosphate (poly P) uptake carbon substrates and accumulate these as PHA by utilizing energy from breaking down poly P under anaerobic condition. In the following aerobic condition, accumulated PHA is utilized for energy generation and for the regeneration of poly P. PHA production from waste has been investigated in order to utilize abundant organic compounds in waste water. Since PHA content and PHA productivity that can be obtained are rather low, PHA production from waste product should be considered as a coupled process for reducing the amount of organic waste. PHAs can be rapidly degraded to completion in municipal anaerobic sludge by various microorganisms. ©     

Fate of mutagenic chemicals in soil amended with petroleum and wood preserving sludges

A field study using monoliths (lysimeters) of a sandy clay loam soil was conducted to assess the fate of mutagenic chemicals after refinery and wood preserving bottom sediment sludges were land treated. The Ames Salmonella/microsome assay¹ was used to determine the direct (without metabolic activation, −S9) and indirect (with metabolic activation, + S9) mutagenicity of the wastes, unamended soil, waste amended soils, and leachate. Extracts having a mutagenic ratio (MR) (MR= No. colonies from sample extract/No. colonies from DMSO solvent control) of ⩾ 2 were considered positively mutagenic. Extracts of the wood preserving waste sludge without activation were non-mutagenic (MR < 2) but extracts with activation ( + S9) produced very strong indirect mutagenicity (MR = 7.9). After soil incorporation, the waste amended soil produced very strong direct (MR = 8.9) and indirect (MR = 11.9) mutagenicity by day 180 and remained mutagenic (MR = 5.7, −S9; MR = 3.95, + S9) through day 350. The amount of residue in leachate from the wood preserving waste amended lysimeters was significantly greater (P <0.05) than the unamended soil during the first 90 days after waste application, but was not different after 90 days. The leachate residue from wood preserving waste amended lysimeters in the 90–180-day period produced mutagenic responses both with (MR = 2.24 and 2.51) and without (MR = 2.29) activation. Polynuclear aromatic hydrocarbons were the main constituents identified in the leachate residues that produced a mutagenic response. Soil treatment of the refinery sludge reduced its weak indirect mutagenicity before soil incorporation (MR = 2) to non-mutagenic (MR = 1.4) immediately following soil treatment. The MR of the waste amended soil increased to 1.7 by day 180 but by day 350 decreased to a level equal to that observed at day 0 (MR = 1.4). Leachate from the refinery amended lysimeters had significantly greater (P < 0.05) amounts of organic residue than unamended lysimeters 180 to 350 days after waste application. The leachate from one refinery waste amended lysimeter (90–180 days after waste application) produced a mutagenic response (MR = 3.16). The refinery sludge was detoxified shortly after soil treatment, but the wood preserving sludge required > 350 days to detoxify in the soil environment. The possibility exists that mobile mutagenic chemicals may leach into underlying groundwater from the treatment zone of soils amended with refinery and wood preserving sludges.     

Optimization of Eisenia fetida stocking density for the bioconversion of rock phosphate enriched cow dung–waste paper mixtures

Vermitechnology is gaining recognition as an environmental friendly waste management strategy. Its successful implementation requires that the key operational parameters like earthworm stocking density be established for each target waste/waste mixture. One target waste mixture in South Africa is waste paper mixed with cow dung and rock phosphate (RP) for P enrichment. This study sought to establish optimal Eisenia fetida stocking density for maximum P release and rapid bioconversion of RP enriched cow dung–paper waste mixtures. E. fetida stocking densities of 0, 7.5, 12.5, 17.5 and 22.5 g-worms kg^?1 dry weight of cow dung–waste paper mixtures were evaluated. The stocking density of 12.5 g-worms kg^?1 resulted in the highest earthworm growth rate and humification of the RP enriched waste mixture as reflected by a C:N ratio of <12 and a humic acid/fulvic acid ratio of >1.9 in final vermicomposts. A germination test revealed that the resultant vermicompost had no inhibitory effect on the germination of tomato, carrot, and radish. Extractable P increased with stocking density up to 22.5 g-worm kg^?1 feedstock suggesting that for maximum P release from RP enriched wastes a high stocking density should be considered.     

Production of Curdlan Grown on Cassava Starch Waste Hydrolysates

Cassava starch waste hydrolysates (CSWHs) with different degrees of polymerisation, i.e., CSWHs-1, CSWHs-2 and CSWHs-3, were prepared through the hydrolysis of cassava starch waste with thermostable a-amylase from Thermococcus sp. HJ21. The prepared CSWHs were then used as a carbon source for curdlan production with Alcaligenes faecalis ATCC 31749. The amount of curdlan produced and the glucosyltransferase activity during curdlan synthesis increased more obviously when CSWHs-2 was used as the carbon source than when glucose was used. Using both carbon sources, the maximum curdlan production was observed at day 5, and the maximum glucosyltransferase activity was observed at day 4. Glucosyltransferase activity decreased thereafter, and biomass continued to increase until the end of the experiment (day 6). Results indicated that the enhanced curdlan production with CSWHs as the carbon source was highly correlated with glucosyltransferase activity.     

Anaerobic co-digestion of food waste and landfill leachate in single-phase batch reactors

In order to investigate the effect of raw leachate on anaerobic digestion of food waste, co-digestions of food waste with raw leachate were carried out. A series of single-phase batch mesophilic (35 ± 1 °C) anaerobic digestions were performed at a food waste concentration of 41.8 g VS/L. The results showed that inhibition of biogas production by volatile fatty acids (VFA) occurred without raw leachate addition. A certain amount of raw leachate in the reactors effectively relieved acidic inhibition caused by VFA accumulation, and the system maintained stable with methane yield of 369–466 mL/g VS. Total ammonia nitrogen introduced into the digestion systems with initial 2000–3000 mgNH₄–N/L not only replenished nitrogen for bacterial growth, but also formed a buffer system with VFA to maintain a delicate biochemical balance between the acidogenic and methanogenic microorganisms. UV spectroscopy and fluorescence excitation–emission matrix spectroscopy data showed that food waste was completely degraded.We concluded that using raw leachate for supplement water addition and pH modifier on anaerobic digestion of food waste was effective. An appropriate fraction of leachate could stimulate methanogenic activity and enhance biogas production.     

Utilization of molasses spentwash for production of bioplastics by waste activated sludge

Present study describes the treatment of molasses spentwash and its use as a potential low cost substrate for production of biopolymer polyhydroxybutyrate (PHB) by waste activated sludge. Fluorescence microscopy revealed the presence of PHB granules in sludge biomass which was further confirmed by fourier transform-infra-red spectroscopy (FT-IR) and ¹³C nuclear magnetic resonance (NMR). The processing of molasses spentwash was carried out for attaining different ratios of carbon and nitrogen (C:N). Highest chemical oxygen demand (COD) removal and PHB accumulation of 60% and 31% respectively was achieved with raw molasses spentwash containing inorganic nitrogen (C:N ratio = 28) followed by COD removal of 52% and PHB accumulation of 28% for filtered molasses containing inorganic nitrogen (C:N ratio = 29). PHB production yield (Y_p/s) was highest (0.184 g g^?1 COD consumed) for deproteinized spentwash supplemented with nitrogen. In contrast, the substrate consumption and product formation were higher in case of raw spentwash. Though COD removal was lowest from deproteinized spentwash, evaluation of kinetic parameters suggested higher rates of conversion of available carbon to biomass and PHB. Thus the process provided dual benefit of conversion of two wastes viz. waste activated sludge and molasses spentwash into value-added product-PHB.     

Comparison of different liquid anaerobic digestion effluents as inocula and nitrogen sources for solid-state batch anaerobic digestion of corn stover

Effluents from three liquid anaerobic digesters, fed with municipal sewage sludge, food waste, or dairy waste, were evaluated as inocula and nitrogen sources for solid-state batch anaerobic digestion of corn stover in mesophilic reactors. Three feedstock-to-effluent (F/E) ratios (i.e., 2, 4, and 6) were tested for each effluent. At an F/E ratio of 2, the reactor inoculated by dairy waste effluent achieved the highest methane yield of 238.5 L/kgVS_feed, while at an F/E ratio of 4, the reactor inoculated by food waste effluent achieved the highest methane yield of 199.6 L/kgVS_feed. The microbial population and chemical composition of the three effluents were substantially different. Food waste effluent had the largest population of acetoclastic methanogens, while dairy waste effluent had the largest populations of cellulolytic and xylanolytic bacteria. Dairy waste also had the highest C/N ratio of 8.5 and the highest alkalinity of 19.3 g CaCO₃/kg. The performance of solid-state batch anaerobic digestion reactors was closely related to the microbial status in the liquid anaerobic digestion effluents.     

Ethanol production from potato peel waste (PPW)

Considerable concern is caused by the problem of potato peel waste (PPW) to potato industries in Europe. An integrated, environmentally-friendly solution is yet to be found and is currently undergoing investigation. Potato peel is a zero value waste produced by potato processing plants. However, bio-ethanol produced from potato wastes has a large potential market. If Federal Government regulations are adopted in light of the Kyoto agreement, the mandatory blending of bio-ethanol with traditional gasoline in amounts up to 10% will result in a demand for large quantities of bio-ethanol. PPW contain sufficient quantities of starch, cellulose, hemicellulose and fermentable sugars to warrant use as an ethanol feedstock. In the present study, a number of batches of PPW were hydrolyzed with various enzymes and/or acid, and fermented by Saccharomyces cerevisae var. bayanus to determine fermentability and ethanol production. Enzymatic hydrolysis with a combination of three enzymes, released 18.5 g L^?1 reducing sugar and produced 7.6 g L^?1 of ethanol after fermentation. The results demonstrate that PPW, a by-product of the potato industry features a high potential for ethanol production.     

Nitrogen removal from landfill leachate via ex situ nitrification and sequential in situ denitrification

Ex situ nitrification and sequential in situ denitrification represents a novel approach to nitrogen management at landfills. Simultaneous ammonia and organics removal was achieved in a continuous stirred tank reactor (CSTR). The results showed that the maximum nitrogen loading rate (NLR) and the maximum organic loading rate (OLR) was 0.65 g N l^?1 d^?1 and 3.84 g COD l^?1 d^?1, respectively. The ammonia and chemical oxygen demand (COD) removal was over 99% and 57%, respectively. In the run of the CSTR, free ammonia (FA) inhibition and low dissolved oxygen (DO) were found to be key factors affecting nitrite accumulation. In situ denitrification was studied in a municipal solid waste (MSW) column by recalculating nitrified leachate from CSTR. The decomposition of MSW was accelerated by the recirculation of nitrified leachate. Complete reduction of total oxidized nitrogen (TON) was obtained with maximum TON loading of 28.6 g N t^?1 TS d^?1 and denitrification was the main reaction responsible. Additionally, methanogenesis inhibition was observed while TON loading was over 11.4 g N t^?1 TS d^?1 and the inhibition was enhanced with the increase of TON loading.     

Microbial Degradation of UV-Pretreated Low-Density Polyethylene Films by Novel Polyethylene-Degrading Bacteria Isolated from Plastic-Dump Soil

Eleven effective low-density polyethylene (LDPE)-degrading bacterial strains were isolated and identified from landfill soil containing large amounts of plastic materials. The isolates belonged to 8 genera, and included Pseudomonas (areroginosa and putida), Sphingobacterium (moltivorum), Delftia (tsuruhatansis), Stentrophomonas (humi and maltophilia), Ochrobacterum (oryzeae and humi), Micrococcus (luteus), Acinetobacter (pitti), and Citrobacter (amalonaticus). Abiotic degradation of LDPE films by artificial and natural ultraviolet (UV)-exposure was analyzed by FT-IR spectroscopy. LDPE films treated with UV-radiation were also inoculated with the isolates and biofilm production and LDPE degradation were measured. Surface changes to the LDPE induced by bacterial biofilm formation were visualized by Scanning Electron Microscopy. The most active bacterial isolate, IRN19, was able to degrade polyethylene film by 26.8?±?3.04% gravimetric weight over 4 weeks. Analysis of 16S rRNA sequence of this isolate revealed 96.97% similarity in sequence to Acinetobacter pitti, which has not previously been identified as a polyethylene-degrading bacterium. Also, most the effective biofilm forming isolate, IRN11, displayed the highest cell mass production (6.29?±?0.06 log cfu/cm²) after growth on LDPE films, showed 98.74% similarity to Sphingobacterium moltivourum.     

A novel method for the preparation of silver/chitosan-O-methoxy polyethylene glycol core shell nanoparticles

Uniformly sized silver/chitosan-O-methoxy polyethylene glycol (chitosan-O-MPEG) core shell nanoparticles with different degree of substitution were synthesized. Thus, N-phthaloyl chitosan is reacted with polyethylene glycol monomethyl ether iodide in the presence of silver oxide by the following steps. At first, amino groups of chitosan are protected by fourfold excess of phthalic anhydride. Then N-phthaloyl chitosan is reacted with an appropriate amount of monomethyl ether iodide in the presence of silver oxide and lastly N-phthaloyl groups are removed to yield silver/chitosan-O-MPEG core shell nanoparticles. Structure of prepared silver/chitosan-O-MPEG core shell nanoparticles have been characterized by UV/Vis spectroscopy, transmission electron microscopy (TEM), Fourier transform infrared (FT-IR), X-ray diffraction, and scan electron microscopy (SEM-EDX). Experimental results revealed that the prepared silver core particles had the size 18?±?2?nm. Core shell structure with chitosan-O-MPEG-coating had the size 40?±?2?nm.     

Composition and production rate of medical waste from a small producer in Greece

The objective of this work was to determine the composition and production rate of medical waste from the health care facility of social insurance institute, a small waste producer in Xanthi, Greece. Specifically, produced medical waste from the clinical pathology (medical microbiology) laboratory, the X-ray laboratory and the surgery and injection therapy departments of the health facility was monitored for six working weeks. A total of 240 kg medical solid waste was manually separated and weighed and 330 L of liquid medical waste was measured and classified. The hazardous waste fraction (%w/w) of the medical solid waste was 91.6% for the clinical pathology laboratory, 12.9% for the X-ray laboratory, 24.2% for the surgery departments and 17.6% for the injection therapy department. The infectious waste fraction (%w/w) of the hazardous medical solid waste was 75.6% for the clinical pathology laboratory, 0% for the X-ray laboratory, 100% for the surgery departments and 75.6% for the injection therapy department. The total hazardous medical solid waste production rate was 64 ± 15 g/patient/d for the clinical pathology laboratory, 7.2 ± 1.6 g/patient/d for the X-ray laboratory, 8.3 ± 5.1 g/patient/d for the surgery departments and 24 ± 9 g/patient/d for the injection therapy department. Liquid waste was produced by the clinical pathology laboratory (infectious-and-toxic) and the X-ray laboratory (toxic). The production rate for the clinical pathology laboratory was 0.03 ± 0.003 L/patient/d and for the X-ray laboratory was 0.06 ± 0.006 L/patient/d. Due to the small amount produced, it was suggested that the most suitable management scheme would be to transport the hazardous medical waste, after source-separation, to the Prefectural Hospital of Xanthi to be treated with the hospital waste. Assuming this data is representative of other small medical facilities, medical waste production can be estimated for such facilities distributed around Greece.     

Production of Poly(3-hydroxyalkanoate)s by Pseudomonas putida Cultivated in a Glycerol/Nonanoic Acid-Containing Medium

The biosynthesis of poly(3-hydroxyalkanoate) (PHA) by Pseudomonas putida (JCM6160) cultivated in a medium containing glycerol, nonanoic acid, or a glycerol/nonanoic acid mixture as the sole carbon sources was investigated. The PHA content was ~20 % when glycerol was the carbon source. This relatively low content can be attributed to the glycerol end-cap effect and the absence of enzymes that can directly synthesize PHA from acetyl CoA, which is the major metabolite of glycerol. Fatty acids, containing even numbered carbons, are synthesized from acetyl CoA, and they can be used as substrates for PHA synthesis. However, this process also results in decreasing PHA content as fatty acids are siphoned off into other pathways. However, addition of 5 mM nonanoic acid into a 20 mM glycerol-containing medium dramatically increased the PHA content in P. putida, which was 1.3 times larger than the sum of the values found when glycerol and nonanoic acid were each used as the sole carbon source. The PHA, synthesized in the glycerol/nonanoic acid medium, contains 3-hydroxy alkanoate units that have 5, 6, 7, 8, 9, or 10 carbons. The units that contain the even numbered carbons are derived from fatty acids that were produced from glycerol; whereas, the PHA units with the odd numbered carbons are derived from nonanoic acid. Pentanoate units were also found in the polyester derived from glycerol and nonanoic acid, and must have been synthesized indirectly via β-oxidation of nonanoic acid with the assistance of glycerol because pentanoate units were not found in PHA when P. putida was cultivated in the presence of only nonanoic acid.     

Atmospheric Levels of Nitrogen Oxides at a Greek Oil Refinery Compared with the Urban Measurements in Athens

Oil refining is among the industrial activities that emit considerable amounts of air pollutants into the atmosphere. Nitrogen oxides are important air pollutants that are emitted by oil refineries as products of combustion processes. The ambient air concentrations of nitrogen oxide (NO) and nitrogen dioxide (NO₂) were monitored continuously at a site close to an oil refinery, near the city of Corinth in Greece, during autumn 1997 together with the main meteorological parameters. The contribution of the oil refinery to the measured atmospheric levels of nitrogen oxides was estimated. The ambient air concentration of nitrogen oxides in the area surrounding the oil refinery were generally lower than the ambient air concentrations in the urban area of Athens in Greece, and the NO₂ levels were always below the existing air quality standards. The influence of the refinery emitted NO_x in the photochemical production of ozone seems to be more important in terms of human and vegetation exposure given the high ozone backgrounds measured in the area.