Photochemical Pollution Episodes Research Articles

Origins of formaldehyde in a mountainous background atmosphere of southern China

Formaldehyde (HCHO) is one of the key indicators of severe photochemical pollution and strong atmospheric oxidation capacity in southern China. However, current information on the origins of regional HCHO and the impacts of polluted air masses remains scarce and unclear. In this study, an intensive observation of HCHO was conducted at a mountainous background site in southern China during typical photochemical pollution episodes. The concentrations of HCHO reached up to 6.14 ppbv and averaged at 2.68 ± 1.11 ppbv. Source appointment using a photochemical age-based parameterization method revealed significant contributions of secondary formation (50 %) and biomass burning (42 %). Meanwhile, under the influence of the East Asian Winter Monsoon, polluted air masses from central and western China can significantly increase the regional HCHO levels. The simulation results adopting the Rapid Adaptive Optimization Model for Atmospheric Chemistry model further demonstrated that the intrusion of active anthropogenic pollutants (e.g., small-molecule alkenes) can accelerate the net production rate of HCHO, particularly through BVOC-oxidation pathways. This study suggests a potential enhanced mechanism of HCHO production resulting from anthropogenic-biogenic interactions. It highlights that polluted air masses carrying abundant HCHO from upwind areas may facilitate severe photochemical pollution in the Greater Bay Area.

Observation and modeling of organic nitrates on a suburban site in southwest China

Here we report the measurements of two types of organic nitrates (ONs), peroxy nitrates (PNs) and alkyl nitrates (ANs), in Chengdu, China, during summer 2019. The average concentrations of PNs and ANs were 1.3 ± 1.1 ppbv and 0.5 ± 0.3 ppbv during the day, with peaks of 7.7 ppbv and 1.9 ppbv, respectively, which were in the middle and upper end of the reported levels in China. Much higher PNs and ANs concentrations were found during the photochemical pollution period than during the clean period. Box model simulation was capable of reproducing PNs during photochemical pollution episodes but showed overestimation in other periods, which was likely caused by the simplification of PNs sinks. The OH oxidation of aldehydes and ketones was the most important source of the PNs precursors, PAs (peroxyacyl radicals), except for the thermal decomposition of PNs, which was further confirmed by the relative incremental reactivity (RIR) analysis. The model basically reproduced the observed ANs by the refinement of related mechanisms, with isoprene contributing to its formation by 29.2 %. The observed PNs and total oxidants (Ox = NO2 + O3) showed a good positive correlation, with a ratio of PNs to Ox of 0.079, indicating a strong suppression of PNs chemistry to ozone formation. The model quantified the suppression of PNs chemistry on the peak ozone production rate by 21.3 % on average and inhibited ozone formation up to 20 ppbv in total. The RIR analysis suggests that the production of both O3 and ANs was in the VOC-limited regime and highlights the importance of VOC control (especially aromatics) to mitigate photochemical pollution in Chengdu. The study deepens the understanding of photochemical pollution in urban areas of western China and further emphasizes the impacts of ONs chemistry on ozone pollution.

Open biomass burning emissions and their contribution to ambient formaldehyde in Guangdong province, China.

Formaldehyde (HCHO) plays a vital role in atmospheric chemistry and O3 formation. Open biomass burning (OBB) is considered to be an important source of HCHO; however, its quantitative contribution to ambient HCHO remains poorly understood due to the lack of reliable high-resolution emission inventories. In this study, a satellite-based method coupled with local emission factors was developed to estimate the hourly primary emissions of HCHO and volatile organic compound (VOC) precursors from OBB in Guangdong (GD) Province of southern China. Furthermore, the contribution of OBB to ambient HCHO was quantified using the Community Multi-scale Air Quality model. The results suggested that in average OBB emissions contributed 5293 tons of primary HCHO per year, accounting for ~14% of the total anthropogenic HCHO emissions in GD. The ambient HCHO concentration ranged from 0.3 ppbv to 8.7 ppbv during normal days, and from 8 ppbv to 45 ppbv in downwind area during OBB impacted days. The monthly contribution of OBB to local HCHO levels reached up to 50% at locations with frequent fires and over 70% during a forest fire event. Ambient HCHO was heavily affected by primary OBB emissions near the source region and by the oxidation of OBB-emitted VOCs in the downwind area. Secondary HCHO formation from OBB emissions was enhanced during photochemical pollution episodes, especially under conditions of high O3 and low NOx. OBB-emitted ethene was identified as the most important VOC precursor of HCHO and contributed to the formation of ~50% of the secondary HCHO. The HCHO formation potential of cropland fires was 26% higher than that of forest fires. Our results suggest that OBB can elevate ambient HCHO levels significantly. Thus, strict control policies on OBB should be implemented, especially for open burning agricultural residues in upwind areas on serious photochemical pollution days.

Understanding the formation of high-ozone episodes at Raoyang, a rural site in the north China plain

Abstract An observational field campaign was carried out from 20 June to July 31, 2016 at Raoyang, a rural site in the North China Plain (NCP). In-situ measurements of O3 and related gases were obtained and analyzed in this work. Heavy photochemical pollution episodes lasting for a few days were observed in the campaign. The episodes could be divided into two categories, with one being influenced by plume from northeast (NE) via Beijing-Tianjin cities and the other from southeast (SE) via Shandong province. The SE episodes were more frequent with higher O3 concentrations and lagged daily peaks of O3 despite of lower concentrations of precursors. Such episodes could last for several days or even a week. In contrast, the NE episode was associated with relatively lower O3 levels and earlier daily O3 peaks though the levels of precursors were higher. The WRF-Chem model was used to study the differences between typical SE and NE episodes in regional distributions of O3 concentrations, chemical regimes of O3 formation, ozone production efficiency (OPE), photochemical age of plume, and evolution of O3 concentration within the boundary layer. Our results suggest that the O3 production regimes were different for the Raoyang site, with the NE episode being VOCs-sensitive and the SE episode in the VOC-sensitive to NOx-sensitive transition regime. In both episodes, a major part of the NCP was under the VOCs-sensitive regime. Plumes in the SE episodes were more aged than those in the NE episode. The OPE values were higher in more aged SE plumes, resulting in higher O3 concentrations despite of lower precursor concentrations. Our results also indicate that downward mixing of O3 in the nocturnal residual layer played an important role in rapid buildup of surface O3 in the next morning in both NE and SE episodes. The formation of photochemical smog seems to be particularly effective in the SE episode since O3 is rapidly produced during daytime, stored aloft at night, and mixed downward to the ground-level, and the process can circulate for several consecutive days. Our study highlights the complicated role of chemical reaction, transport and boundary layer processes in the formation of severe photochemical smog in the region.

Characteristics and Reactivity of VOCs in Hangzhou During a Typical Photochemical Pollution Episode

An intensive observation of ambient volatile organic compounds (VOCs) was carried out in Hangzhou, a key city in the Yangtze River Delta, during a typical photochemical pollution episode from September 14-23, 2018. The analysis results of 80 effective samples showed that the average concentration of 122 compounds of VOCs was (59.5±19.8)×10-9 during the observation period, and oxygenated VOCs (OVOCs) were the most abundant component. The assessment results of atmospheric reaction activity with ozone formation potential (OFP) showed that the average value of OFP was 145×10-9 during the observation period, of which alkenes and carbonyl compounds were the most abundant components. The chemical reactivity of VOCs in Hangzhou was equivalent to acrylonitrile. Based on the positive matrix factorization (PMF) model, five major sources of VOCs in Hangzhou were identified, including secondary formation (25.2%), combustion and industrial processing (27.2%), solvent use (17.3%), biogenic sources (9.2%), and vehicular exhaust (21.2%). The results can provide guidance for further understanding of VOC characteristics and the basis for scientific prevention and control measures in Hangzhou.

Impact of VOCs emission from iron and steel industry on regional O3 and PM2.5 pollutions.

Iron and steel industry emission is an important industrial source of air pollution. However, little is known about the relationship between volatile organic compounds (VOCs) emitted and regional air pollution. In this study, VOCs emissions from a typical iron and steel plant in Yangtze River Delta (YRD, China) were monitored from April 2018 to March 2019. The ozone formation potential (OFP) and secondary organic aerosol (SOA) formation of VOCs were calculated to reveal the influence of VOCs emissions on regional ozone and particulate pollution, and the sensitivity analysis approach was performed to explore the qualitative and quantitative relationships between VOCs and O3, as well as VOCs and PM2.5. The VOCs concentration was 93.76 ± 266.97ppbv during the study. The OFP was 760.08 ± 2391.90μgm-3, and aromatics were the predominant precursors, contributing 54.05% of the total OFP. Furthermore, the SOA estimated by fractional aerosol coefficient (FAC) and time-resolved (TR) methods were 6.032 ± 13.347μgm-3 and 0.971 ± 4.650μgm-3, accounting for 8.65-26.39% (13.78 ± 7.46%) and 1.55-4.20% (2.22 ± 1.23%) of the PM2.5 concentrations, respectively. The results indicated that VOCs were more sensitive to O3 pollution in high pollution domains, whereas VOCs were more sensitive to PM2.5 pollution in low pollution domains. We concluded that reducing VOCs emissions might be effective in alleviating photochemical pollution episodes in areas around iron and steel industry, and the haze pollution occurred in these regions may be caused by the primary emission of PM, and the contribution of SOA was relatively small.

The effect of ozone fumigation on the biogenic volatile organic compounds (BVOCs) emitted from Brassica napus above- and below-ground.

The emissions of BVOCs from oilseed rape (Brassica napus), both when the plant is exposed to clean air and when it is fumigated with ozone at environmentally-relevant mixing ratios (ca. 135 ppbv), were measured under controlled laboratory conditions. Emissions of BVOCs were recorded from combined leaf and root chambers using a recently developed Selective Reagent Ionisation—Time of Flight—Mass Spectrometer (SRI-ToF-MS) enabling BVOC detection with high time and mass resolution, together with the ability to identify certain molecular functionality. Emissions of BVOCs from below-ground were found to be dominated by sulfur compounds including methanethiol, dimethyl disulfide and dimethyl sulfide, and these emissions did not change following fumigation of the plant with ozone. Emissions from above-ground plant organs exposed to clean air were dominated by methanol, monoterpenes, 4-oxopentanal and methanethiol. Ozone fumigation of the plants caused a rapid decrease in monoterpene and sesquiterpene concentrations in the leaf chamber and increased concentrations of ca. 20 oxygenated species, almost doubling the total carbon lost by the plant leaves as volatiles. The drop in sesquiterpenes concentrations was attributed to ozonolysis occurring to a major extent on the leaf surface. The drop in monoterpene concentrations was attributed to gas phase reactions with OH radicals deriving from ozonolysis reactions. As plant-emitted terpenoids have been shown to play a role in plant-plant and plant-insect signalling, the rapid loss of these species in the air surrounding the plants during photochemical pollution episodes may have a significant impact on plant-plant and plant-insect communications.

Characterization of atmospheric trace gases and particulate matter in Hangzhou, China

Abstract. The Yangtze River Delta (YRD) is one of the most densely populated regions in China with severe air quality issues that have not been fully understood. Thus, in this study, based on 1-year (2013) continuous measurement at a National Reference Climatological Station (NRCS, 30.22° N, 120.17° E; 41.7 m a.s.l.) in the center of Hangzhou in the YRD, we investigated the seasonal characteristics, interspecies relationships, and the local emissions and the regional potential source contributions of trace gases (including O3, NOx, NOy, SO2, and CO) and particulate matter (PM2.5 and PM10). Results revealed that severe two-tier air pollution (photochemical and haze pollution) occurred in this region, with frequent exceedances in O3 (38 days) and PM2.5 (62 days). O3 and PM2.5 both exhibited distinct seasonal variations with reversed patterns: O3 reaching a maximum in warm seasons (May and July) but PM2.5 reaching a maximum in cold seasons (November to January). The overall results from interspecies correlation indicated a strong local photochemistry favoring the O3 production under a volatile organic compound (VOC)-limited regime, whereas it moved towards an optimum O3 production zone during warm seasons, accompanied by the formation of secondary fine particulates under high O3. The emission maps of PM2.5, CO, NOx, and SO2 demonstrated that local emissions were significant for these species on a seasonal scale. The contributions from the regional transport among inland cities (Zhejiang, Jiangsu, Anhui, and Jiangxi Province) on a seasonal scale were further confirmed to be crucial to air pollution at the NRCS site by using backward trajectory simulations. Air masses transported from the offshore areas of the Yellow Sea, East Sea, and South Sea were also found to be highly relevant to the elevated O3 at the NRCS site through the analysis of potential source contribution function (PSCF). Case studies of photochemical pollution (O3) and haze (PM2.5) episodes both suggested the combined importance of local atmospheric photochemistry and synoptic conditions during the accumulation (related with anticyclones) and dilution process (related with cyclones). Apart from supplementing a general picture of the air pollution state in the city of Hangzhou in the YRD region, this study specifically elucidates the role of local emission and regional transport, and it interprets the physical and photochemical processes during haze and photochemical pollution episodes. Moreover, this work suggests that cross-regional control measures are crucial to improve air quality in the YRD region, and it further emphasizes the importance of local thermally induced circulation for air quality.

Assessing photochemical ozone formation in the Pearl River Delta with a photochemical trajectory model

A photochemical trajectory model (PTM), coupled with the Master Chemical Mechanism (MCM) describing the degradation of 139 volatile organic compounds (VOCs) in the troposphere, was developed and used for the first time to simulate the formation of photochemical pollutants at Wangqingsha (WQS), Guangzhou during photochemical pollution episodes between 12 and 17 November, 2007. The simulated diurnal variations and mixing ratios of ozone were in good agreement with observed data ( R 2 = 0.80, P < 0.05), indicating that the photochemical trajectory model – an integration of boundary layer trajectories, precursor emissions and chemical processing – provides a reasonable description of ozone formation in the Pearl River Delta (PRD) region. Calculated photochemical ozone creation potential (POCP) indices for the region indicated that alkanes and oxygenated organic compounds had relatively low reactivity, while alkenes and aromatics presented high reactivity, as seen in other airsheds in Europe. Analysis of the emission inventory found that the sum of 60 of the 139 VOC species accounted for 92% of the total POCP-weighted emission. The 60 VOC species include C 2–C 6 alkenes, C 6–C 8 aromatics, biogenic VOCs, and so on. The results indicated that regional scale ozone formation in the PRD region can be mainly attributed to a relatively small number of VOC species, namely isoprene, ethene, m-xylene, and toluene, etc. A further investigation of the relative contribution of the main emission source categories to ozone formation suggested that mobile sources were the largest contributor to regional O 3 formation (40%), followed by biogenic sources (29%), VOC product-related sources (23%), industry (6%), biomass burning (1%), and power plants (1%). The findings obtained in this study would advance our knowledge of air quality in the PRD region, and provide useful information to local government on effective control of photochemical smog in the region.

Contribution of atmospheric processes affecting the dynamics of air pollution in South-Western Europe during a typical summertime photochemical episode

Abstract. The southern Mediterranean region frequently experiences critical levels of photochemical pollutants during summertime. In order to account for the contribution of different atmospheric processes during this type of episodes, the WRF-ARW/HERMES/CMAQ modelling system was applied with high resolution (1 km2, 33 sigma vertical layers, 1 h) to assess the different dynamics in a coastal environment and an inland-continental zone: the North-Eastern and Central Iberian Peninsula (NEIP and CIP, respectively). The former is characterized by a very complex terrain, while the latter behaves as a flat area, which clearly affects the pattern of local flows. A representative type of photochemical pollution episode (occurring over 78% of summer days) which occurred during 17–18 June, 2004 is selected as the study period. The CMAQ Integrated Process Rate provides the hourly contributions of atmospheric processes to net O3, NOx and NMVOCs concentrations. The O3 photochemical formation occurs mainly in downwind areas from the main NOx emission sources during midday. At surface level it accounts for 50 to 75 μg m−3 h−1. The urban areas and main roads, as main sources of NOx emissions, act as O3 sinks, quenching up to −200 μg m−3 per hour during the traffic circulation peaks. The O3 concentration gradient generated, larger during daytime, increases the contribution of diffusion processes to ground-level O3 (up to 200 μg m−3 h−1 fluxes, mainly from upper vertical layers). The maximum positive contributions of gas-phase chemistry to O3 occur in the coastal domain at high levels (around 500 to 1500 m a.g.l.), while in the continental domain they take place in the whole atmospheric column under the PBL. The transport of ozone precursors by advective flows determines the location of the maximum O3 surface concentrations. The O3 chemical formation involves the oxidation of less NMVOCs in the NEIP than in the CIP domains, due to differences in chemical sensitivity between these areas. The dry deposition is an important sink in the lowest layer of the model, together with vertical diffusion flows. Finally, the contributions from cloud processes, wet deposition and heterogeneous chemistry are negligible during the whole episode, characterized by a high solar radiation and neither precipitation nor cloudiness. This process analysis provides new quantitative information about the origin of the peaks of O3 and its precursors, aiding the design of abatement strategies in South-Western Europe.

Numerical modeling of a continuous photochemical pollution episode in Hong Kong using WRF–chem

A continuous and heavy photochemical pollution episode when Typhoon Nari occurred during Sep. 14–19, 2001 in Hong Kong was studied by use of the new generation of regional air quality model WRF–chem V2.1, in which the meteorological model (WRF) and chemical model (chem) are fully coupled on line. In this episode, the regional air quality monitoring stations in Hong Kong recorded high concentrations of O 3, CO and NO x for six days, the maximum concentration of O 3 reached 191 ppb, and the atmospheric visibility dropped to 1.8 km. Investigations showed that WRF–chem can basically simulate this continuous photochemical pollution episode, with the best correlation coefficient of the simulated and the observed O 3 concentrations being 0.84. The production and loss of O 3 were significant in this episode, and the largest rates were 22.7 and −17.6 ppb h −1, respectively. The simulated meteorological conditions showed high temperature, lower relative humidity, strong solar radiation, northerly airstream, and stable boundary layer structure in Hong Kong during the episode, these weather factors were conducive to formation and maintenance of photochemical pollution. It was also clear that the vertical transport played an important role in this episode. As controlled by downdraft at the outside of Typhoon system with more stable atmosphere, air pollutants were limited and accumulated in the low level of boundary layer, leading to high O 3 concentration from photochemical formation.

Air quality models sensitivity to on-road traffic speed representation: Effects on air quality of 80 km h −1 speed limit in the Barcelona Metropolitan area

Atmospheric modeling permits to quantitatively assess the effects of emissions abatement strategies in urban areas, which are mainly oriented to reduce on-road traffic sector contributions. Nowadays the emissions inventories are one of the causes of uncertainties in air quality modeling. This work explores the improvements in urban air quality assessment by using the WRF-ARW/HERMES/CMAQ modeling system when switching from constant on-road traffic speed to hourly variable speeds taken from measurement campaigns in the emissions model. Furthermore, the effects of limiting the speed circulation in the Barcelona Metropolitan area to 80 km h −1 are assessed. A photochemical pollution episode representative of summertime conditions (corresponding to the year 2004) in the northeastern Iberian Peninsula and fitting in a usual traffic circulation pattern (working day) is selected to carry out the study. The introduction of variable speed in the HERMES emissions model affect the model evaluation in the urban area, improving specifically the O 3 precursors estimates, and consequently O 3 predictions in the area (reductions in mean normalized gross error (MNGE) around 1%). The speed limitation effects assessed with the modified modeling system represent improvements in air quality levels; specifically it reduces local NO 2, SO 2 and PM10 levels, up to 5.7%, 5.3% and 3.0% over the area affected by the speed limit, respectively. The O 3 concentrations slightly increase in the urban area (up to 2.4%), due to the chemical regime (NO x emissions reduction in a VOCs limited area), but the effects in the urban plume are negligible. The need for air quality models as management tools makes essential to improve the emissions models accuracy, by introducing changes to better represent real conditions.

Predictability of European air quality: Assessment of 3 years of operational forecasts and analyses by the PREV'AIR system

For the first time, the long‐term evaluation of an operational real‐time air quality forecasting and analysis system is presented, using error statistics over 3 consecutive years. This system, called PREV'AIR, is the French air quality forecasting and monitoring system. It became operational in 2003 as a result of a cooperation between several public organizations. The system forecasts and analyzes air quality throughout Europe, with a zoom over France, for regulatory pollutants: ozone (O3), particulate matter with diameter smaller than 10 μm (PM10), and nitrogen dioxide (NO2). The ability of PREV'AIR to forecast, up to 3 days ahead, photochemical and particle pollution over the domains considered is demonstrated: daily ozone maxima forecasts correlate with observations with 0.75–0.85 mean coefficients; U.S. Environmental Protection Agency acceptance criteria relative to the forecast accuracy for high concentrations and daily maxima are met for more than 90% of the measurement sites. For NO2 and PM10, the performance corresponds to the state of the art. The contribution of weather forecast errors to air quality predictability is addressed: ozone daily maxima forecast errors are not dominated by meteorological forecast errors; for rural stations, only 6% (15% and 25%, respectively) of the error variance is due to meteorological forecast errors on the first 24 (48 and 72, respectively) hours. The Model Output Statistics procedure, implemented in PREV'AIR, is proved to improve ozone forecasts, especially when photochemical pollution episodes occur. The PREV'AIR real‐time analysis procedure, based on a kriging method, provides an accurate and comprehensive description of surface ozone fields over France.

Modelling the impact of elevated primary NO 2 and HONO emissions on regional scale oxidant formation in the UK

Recent increases in the fractional contribution of NO 2 to NO x emissions from the road transport sector in Europe are well documented in the literature. A photochemical trajectory model has been used to simulate the impact of increasing the NO 2 fraction on the chemical evolution of air masses arriving at the TORCH field campaign site in the southern UK during late July and August 2003, a period which included a widespread photochemical pollution episode associated with a heat-wave. The impact of partial emissions of NO x in the form of nitrous acid (HONO) has also been considered. The model incorporates emissions of NO x , CO, SO 2, methane and a detailed speciation of non-methane volatile organic compounds (VOC), coupled with a comprehensive description of the chemistry of secondary pollutant formation. An increase in the fractional contribution of NO 2 to NO x emissions from 0% to 30% (v/v) results in a 2.49 ppb increase in the simulated campaign mean mixing ratio of oxidant (defined as the sum of O 3 and NO 2). This is almost exclusively in the form of O 3, and represents a ca. 7% increase in the simulated campaign mean O 3 mixing ratio. Consideration of 156 events, at 6-hourly resolution throughout the campaign period, indicates that oxidant increments during the heat-wave period are generally simulated to be greater than those for the remainder of the campaign, with a maximum increment of ca. 12 ppb. The increases in oxidant mixing ratios are shown to derive from both the direct effect of increased NO 2 input, and indirectly from the enhanced regional-scale chemical processing that this promotes. An illustrative increase in the fractional contribution of HONO to NO x emissions from 0% to 5% (v/v) results in increases in the simulated campaign mean mixing ratios of O 3 and oxidant of 1.51 and 1.15 ppb, respectively, with the smaller increment for oxidant reflecting a decrease in the NO 2 mixing ratio resulting from a notably enhanced NO x oxidation rate. The oxidant increments during the heat-wave period are once again simulated to be greater than those for the remainder of the campaign, with a maximum increment of ca. 11 ppb, which results exclusively from the enhanced regional-scale chemical processing that the HONO emissions promote. The impact of increased fractional NO 2 and HONO emissions on the rate of oxidation of NO x to nitrate is also illustrated and discussed.

Photochemical pollution under sea breeze conditions, during summer, at the Portuguese West Coast

Photochemical pollution was studied at three sampling places located on the Portuguese West Coast along the coast-to-continent breeze direction, during the summer period. A first basic campaign (June–July 2001) covered mainly the monitoring of trace gases, volatile organic compounds (VOCs) and the vertical profiles of O 3. During a second auxiliary campaign (July 2002), the vertical profiles of pollutants (O 3 and VOCs) at the west coast point were further assessed. The important role of marine breezes on the formation and transport of photochemical pollutants has been confirmed. Vertical profiles of O 3 revealed a depletion of ozone at altitudes lower than 100–300 m during the night and the early morning and a good vertical mixture during the day in the sea breeze boundary layer. The analysis of vertical profiles of VOCs revealed that lower concentrations tended to exist at the top of the sea breeze layer. However some compounds, such as n-hexane, showed a higher concentration at higher levels. The concentration of O 3 in the diurnal boundary layer, resulting from photochemical reactions of both the anthropogenic and biogenic precursors, gradually increased with the sea breeze inland, ranging from around 30 ppb at midday on the coast, to 70 ppb in the inland rural zone. However, in all sampling places, the photochemical pollution episodes at 80–100 ppb level took place under particular conditions of high photochemical activity and see breeze circulation. As revealed from analyses of propylene equivalent concentrations, the photochemical formation of O 3 in the inland rural zone is strongly affected by biogenic VOCs (isoprene and monoterpenes), whereas a major contribution to the photochemical pollution with ozone in the coastal zone issued from anthropogenic VOCs (alkenes and aromatics).

Evaluation of MM5-EMICAT2000-CMAQ performance and sensitivity in complex terrain: High-resolution application to the northeastern Iberian Peninsula

The complex configuration of the northeastern Iberian Peninsula (NEIP) provokes a complex behavior of photochemical pollutants, which demands a high spatial resolution when applying an air quality model. CMAQ has been used for air quality assessment in the NEIP coupled with the MM5 meteorological model and EMICAT2000 emission model, and has been extensively evaluated against available ambient data during a typical summertime photochemical pollution episode. Simulations with different resolutions were evaluated to select the needed grid resolution. Meteorological inputs are sensitive to the degree of topographical smoothing. Fine-resolution simulations present the best scores during the development of the sea breeze. The performance of statistical parameters for ground-level O 3 greatly improves when decreasing the horizontal and vertical grid spacing. Statistical parameters indicate that decreasing the horizontal grid spacing to 2 km greatly improves the critical success index, the false alarm ratio and the probability of detection. Furthermore, sensitivity studies provide the opportunity to check whether O 3 values react consistently to similar changes in emissions. The model sensitivity was evaluated by performing simulations to represent O 3 formation with baseline emission rates for VOCs and NO x , and reducing anthropogenic VOC and NO x emissions by 35%. Evaluation of ground-level O 3 shows a good agreement when the model predicts dominant VOC-sensitive chemistry. Statistical parameters of O 3 evaluation worsen when reducing VOCs emissions and improve in the—35% NO x case, indicating that the O 3-production chemistry may not be sufficiently reactive.

Simulating regional scale secondary organic aerosol formation during the TORCH 2003 campaign in the southern UK

Abstract. A photochemical trajectory model has been used to simulate the chemical evolution of air masses arriving at the TORCH field campaign site in the southern UK during late July and August 2003, a period which included a widespread and prolonged photochemical pollution episode. The model incorporates speciated emissions of 124 non-methane anthropogenic VOC and three representative biogenic VOC, coupled with a comprehensive description of the chemistry of their degradation. A representation of the gas/aerosol absorptive partitioning of ca. 2000 oxygenated organic species generated in the Master Chemical Mechanism (MCM v3.1) has been implemented, allowing simulation of the contribution to organic aerosol (OA) made by semi- and non-volatile products of VOC oxidation; emissions of primary organic aerosol (POA) and elemental carbon (EC) are also represented. Simulations of total OA mass concentrations in nine case study events (optimised by comparison with observed hourly-mean mass loadings derived from aerosol mass spectrometry measurements) imply that the OA can be ascribed to three general sources: (i) POA emissions; (ii) a "ubiquitous" background concentration of 0.7 µg m-3; and (iii) gas-to-aerosol transfer of lower volatility products of VOC oxidation generated by the regional scale processing of emitted VOC, but with all partitioning coefficients increased by a species-independent factor of 500. The requirement to scale the partitioning coefficients, and the implied background concentration, are both indicative of the occurrence of chemical processes within the aerosol which allow the oxidised organic species to react by association and/or accretion reactions which generate even lower volatility products, leading to a persistent, non-volatile secondary organic aerosol (SOA). The contribution of secondary organic material to the simulated OA results in significant elevations in the simulated ratio of organic carbon (OC) to EC, compared with the ratio of 1.1 assigned to the emitted components. For the selected case study events, [OC]/[EC] is calculated to lie in the range 2.7-9.8, values which are comparable with the high end of the range reported in the literature.

Integrated studies of a photochemical smog episode in Hong Kong and regional transport in the Pearl River Delta of China

Air pollution in Hong Kong has close relations to synoptic systems, especially continental high pressure in cold season and tropical storms in the warm season. Here, a case study was performed of the characteristics of a photochemical pollution episode during March 27‐31, 2000, in which the maximum concentrations of ozone (O3), nitrogen oxides (NOx), carbon monoxide (CO), respirable suspended particulates (RSP) and sulphur dioxide (SO2) reached 265 μgm −3 , 2166 μgm −3 (roadside), 5405 μgm −3 , 349.3 μgm −3 and 221 μgm −3 , respectively. The O3/CO and O3/NOx ratios were found to be 0.1 and 6.29, respectively, at rural site without significant correlation. At an urban site, moderate negative correlation was observed for O3, CO, NOx, where the ratios of O3/CO and O3/NOx were relatively lower (0.03 and 0.21, respectively). The episode lasted 2 d in the Hong Kong territory and the Pearl River Delta (PRD) region. Synchronous meteorological and air monitoring data were adopted for analysis with focus on concentration variability, weather pattern, mechanisms of local photochemical production and regional transport. Our results indicate that clear skies with high temperature and low humidity caused by the strong high-pressure system over South China as well as local emissions were attributable to the high O3 level observed. In addition, backward trajectories and the O3/NOx, O3/CO ratios provided evidence that regional transport did play an important role in the formation of the episode. The MM5/STEM-2K1 and TAPM model systems were used to simulate the regional and urban O3 pattern in PRD and Hong Kong. The modeling surface concentrations of O3 were compared with the measurement from Hong Kong air quality monitoring network, which showed that the ground O3 level was underestimated 10‐20% due to the uncertainty in emission data, simple chemical mechanism as well as MM5 meteorological prediction. Sensitivity study revealed the strong cross-border transport over PRD region, which accounted for 50‐90% of the observed O3 level at nighttime and 40‐70% at daytime in Hong Kong territory depending on site locations.

An observational case study of ozone and precursors inflow to South East England during an anticyclone

Summertime photochemical air pollution episodes within the United Kingdom have been proposed via modelling studies to be strongly influenced by regional scale inflow of air from the continental European boundary layer. We present a vertically resolved case study using measurements made from the NERC/Met Office BAe 146 research aircraft on 18th August 2005 over the South East of England and the North Sea during a weak anticyclone centred over Northern Europe. The vertical distribution of ozone, CO, NO(x), peroxyacetyl nitrate (PAN) and a wide range of both nonmethane hydrocarbons (NMHC) and oxygenated volatile organic compounds (OVOCs) were determined between 500 ft (approximately 152 m) and 7000 ft (approximately 2134 m) over the East Anglia coastline and 50 km inland. In excess of 80 ppbV ozone was observed within inflowing boundary layer air over the North Sea coast in a broad N-S sloping feature around 60 km wide. The inflowing feature of European origin was also observed further inland within the boundary layer albeit with lower, more variable, ozone mixing ratios. The increased variability in ozone over land was a product of titration by fresh surface emissions of NO via rapid upward transport in thermals, a hypothesis supported by the observed vertical wind speed component. Fast boundary layer mixing over land was further illustrated by a uniform distribution in reactive alkenes. A comparison between aircraft and surface O(3) UK AUN (Automatic Urban Network) measurements showed good agreement with the inland site, Sibton, but marked differences with the coastal monitoring site at Weybourne, potentially due to gradients established by ocean deposition in stably stratified marine air.

Integrated studies of a photochemical smog episode in Hong Kong and regional transport in the Pearl River Delta of China

Air pollution in Hong Kong has close relations to synoptic systems, especially continental high pressure in cold season and tropical storms in the warm season. Here, a case study was performed of the characteristics of a photochemical pollution episode during March 27–31, 2000, in which the maximum concentrations of ozone (O3), nitrogen oxides (NOx), carbon monoxide (CO), respirable suspended particulates (RSP) and sulphur dioxide (SO2) reached 265 μgm-3, 2166 μgm-3 (roadside), 5405 μgm-3, 349.3 μgm-3 and 221 μgm-3, respectively. The O3/CO and O3/NOx ratios were found to be 0.1 and 6.29, respectively, at rural site without significant correlation. At an urban site, moderate negative correlation was observed for O3, CO, NOx, where the ratios of O3/CO and O3/NOx were relatively lower (0.03 and 0.21, respectively). The episode lasted 2 d in the Hong Kong territory and the Pearl River Delta (PRD) region. Synchronous meteorological and air monitoring data were adopted for analysis with focus on concentration variability, weather pattern, mechanisms of local photochemical production and regional transport. Our results indicate that clear skies with high temperature and low humidity caused by the strong high-pressure system over South China as well as local emissions were attributable to the high O3 level observed. In addition, backward trajectories and the O3/NOx, O3/CO ratios provided evidence that regional transport did play an important role in the formation of the episode. The MM5/STEM-2K1 and TAPM model systems were used to simulate the regional and urban O3 pattern in PRD and Hong Kong. The modeling surface concentrations of O3 were compared with the measurement from Hong Kong air quality monitoring network, which showed that the ground O3 level was underestimated 10–20% due to the uncertainty in emission data, simple chemical mechanism as well as MM5 meteorological prediction. Sensitivity study revealed the strong cross-border transport over PRD region, which accounted for 50–90% of the observed O3 level at nighttime and 40–70% at daytime in Hong Kong territory depending on site locations.