Session Overview
POSTER 7: UCP - Air quality in urban boundary layer
Tuesday, 21/Jul/2015:
3:00pm - 4:00pm


Air quality in São Paulo – Brazil: temporal evolution and spatial distribution of carbon monoxide, coarse particulate matter and ozone

Flávia Noronha Dutra Ribeiro, Luana Antunes Tolentino Souza, Delhi Teresa Paiva Salinas, Jacyra Soares, Amauri Pereira de Oliveira, Regina Maura de Miranda

University of São Paulo, Brazil

Air pollutants over the Metropolitan Region of São Paulo (MRSP) frequently exceed the standards set by the Environmental National Council, increasing the mortality and morbidity of its population (21 millions of inhabitants). The pollutants concentration are influenced by emission and meteorological factors, however, public policies implemented to decrease pollutants concentration focus only in emissions. A better understanding of the temporal evolution and spatial distribution of the pollutants concentration, as well as its relation with emission and meteorological processes, is key to assess the efficiency of these policies and to improve the knowledge of pollutants transport and dispersion. This worked aimed to study the temporal evolution, spatial distribution and correlation to meteorological variables, as wind speed, temperature and relative humidity, of carbon monoxide (CO), coarse particulate matter (PM10, aerodynamic diameter under 10 μm), and ozone (O3). The investigation was performed using the data sets recorded by 33 monitoring stations of the environmental agency of São Paulo (CETESB) from 1996 to 2013. CO in the MRSP is 97 % emitted by vehicles and its concentration is strongly related to emission patterns and proximity to heavy traffic. Since 1996, CO concentration has drastically decreased, in response to a federal resolution, that forced the development and utilization of new technologies of fuels, engines and car parts. This pollutant has not exceed the air quality standard since 2008, however, the tendency recently indicates stabilization of its concentration, probably caused by the increase of the vehicular fleet. CO also presented weak correlation to meteorological variables and strong influence of the planetary boundary layer development, showing mainly semidiurnal, diurnal and seasonal patterns, and may be used as a reference pollutant to assess the impact of public policies that aim to reduce traffic emissions. In the MRSP, PM10 is only 40 % emitted by vehicles and presented a subtler decreasing tendency than CO over the years. Its correlation to the meteorological variables is higher, particularly to relative humidity, probably related to precipitation. Particulate matter also presented diurnal and seasonal patterns, as well as a weekly cycle. Ozone is not emitted and is formed in the atmosphere in the presence of volatile organic compounds (VOC), nitric oxides (NOx) and solar radiation, however its concentration depends also on the emission of its precursors (VOCs and NOx). Therefore O3 concentration is higher during spring and summer and in the afternoon, contrary to CO and PM10. In the MRSP, it is the pollutant that, recently, exceeds more frequently the air quality standards, and presents the highest concentrations in the Ibirapuera station, that is located inside the biggest park of São Paulo city. The year 2014 presented higher concentrations of ozone during austral summer (Dec 2013, Jan and Feb 2014) and spring (Sep, Oct and Nov 2014) than previous years. The causes probably are: a drier and sunnier summer and more subsidence during spring. Overall, the results indicate that other forms of public policies are needed to decrease the pollutants concentration, for instance, motivating the usage of public transportation instead of private vehicles.

POSTER 7-1-1671081_a.pdf
POSTER 7-1-1671081_b.pdf

Particle Matters (PM) air pollution in the metropolitan area of Haifa, Israel - The role of synoptic conditions and wind regime

Hadas Saaroni1, Eldad Levi1, Reuven Givati1, Baruch Ziv2

1Tel Aviv University, Israel; 2The Open University of Israel

Relatively high Particle Matter (PM) concentrations are detected over the Middle East and Israel, attributed to both natural dust outbreaks as well as to local and remote anthropogenic sources. Haifa metropolitan area has local anthropogenic pollution sources such as the major industrial plants including the national petroleum refineries, petrochemical and agrochemical industries. Yuval and Broday (2006) showed that while dust outbreak events are excluded, heavy traffic load is a main source for PM10 in this area. The spatio-temporal distribution of the pollution is highly dependent on the geographical characteristics of the region, such as the complex terrain and the Haifa bay structure, and the atmospheric conditions determined by the implied meso- and local-scale circulations and the synoptic systems.

The research analyzes the role of synoptic conditions and wind regime in the temporal and spatial distribution of PM10 and PM2.5. The ‘environment to circulation’ approach is adopted (following Yarnal 1993, Yarnal et al. 2001 and Dayan and Tubi 2012), defining the 'pollution potential' regarded as percentage of exceeding days for the pertinent regional synoptic types. This is based on the classification of Alpert et al. (2004). In order to get insight into the relevant mechanisms, the diurnal evolution of the wind field for each synoptic type is derived and analyzed.

The database includes PM measurements of 9 monitoring stations and wind measurements of 5 stations, for 10 years. Analysis of the days in which the concentrations exceeded the Israeli standard (daily average of 130 microgram/m3 for PM10 and 37.5 microgram/m3 for PM2.5) indicates their dominance in the spring and winter seasons. The North African Low has the highest pollution potential (22% and 33% for PM2.5 and PM10, respectively) and the second is the Cyprus Low (12% for both). However, the contribution of the Cyprus Low to exceeding occurrences is 7 times higher due to its higher frequency. Moreover, the highest concentrations observed belong also to Cyprus Lows. The remaining synoptic systems, typically associated with static stability has lower pollution potential of <5% for PM10 and 5-10% for PM2.5. It should be noted that in spite of the persistent stability and absence of rain along the entire summer season, there were no exceeding days for PM10 and only 4% for PM2.5.

Exceeding days of PM are dominated by natural dust outbreaks even in this highly industrial area. However, its spatial distribution within the study area may point at the contribution of local sources.

POSTER 7-2-3491372_a.pdf

Evaluation of the mesoscale effect of photocatalytic pavements and vegetation on air quality

Alberto Martilli, Magdalena Palacios, Jose Luis Santiago


In the last decade, new technologies have been developed to reduce air pollution levels without changing emissions. One of them is to put photocatalytic materials (p.m.) in the streets and building facades of cities. Such materials, in presence of solar radiation, chemically react to destroy NOx. So far, focus has been put on the analysis of the depolluting effect of these materials at microscale, e. g. in one street or square. The first scientific question of this contribution is: what could be the effect of p.m. applied in the streets of a whole city (or in a fraction of them) on pollution levels in the city itself, and in the downwind plume? To answer, it is essential to estimate the deposition velocity from the capacity to destroy NOx of the p.m.. In this study the velocities estimated in the framework of the LIFE+ project MINOX-street, from a series of laboratory and real scale experiments, are implemented in the mesoscale atmospheric model WRF, with a detailed urban paramterization, to represent the depolluting effect of the p.m. Another strategy to decrease pollutant levels is the urban vegetation, through the deposition on leaves. So, the second set of scientific questions is: to what extent deposition on urban vegetation affects pollutant levels in the city and in the downwind plume? Can it be compared to the p.m.? To answer it is necessary to account for the effect of urban vegetation on mean and turbulent transport. Starting from a series of microscale simulations, a simple parameterization to represent this effect is derived and implemented in WRF, together with one for the deposition on leaves. Given that the answers are a function of the urban morphology, a series of idealized simulation for a city with different building densities and heights and with different configurations of urban vegetation (type, Leaf Area Density, and position relatively to the building) is used for the study. Finally the findings deduced from the idealized simulations are tested over a real city (Madrid metropolitan area).

POSTER 7-3-5021464_a.pdf

The ClearfLo project – The influence of turbulent mixing properties of London’s urban boundary layer on surface and elevated O3 and NOX pollutant concentrations

SI Bohnenstengel1,2, SE Belcher2,3, JF Barlow2, O Coceal2,4, CH Halios2, C Helfter5, J Lee6,7

1MetOffice@Reading, Reading, UK; 2Department of Meteorology, University of Reading, Reading, UK; 3Met Office Hadley Centre, Exeter, UK; 4National Centre for Atmospheric Science, Department of Meteorology, University of Reading, UK; 5Centre for Ecology & Hydrology (CEH), Edinburgh Research Station, Scotland, UK; 6Department of Chemistry, University of york, York, UK; 7National Centre for Atmospheric Science, Department of Chemistry, University of York, UK

The ClearfLo project aimed to understand the processes generating pollutants like O3, NOx and particulate matter and their interaction with the urban atmospheric boundary layer. ClearfLo (, a large multi-institution NERC-funded project, established integrated measurements of the meteorology, composition and particulate loading of London’s urban atmosphere accompanied by modeling of urban meteorology and air pollution.

The project established a new long-term measurement infrastructure in London encompassing measurement capabilities at street level, in the urban background, at elevated levels and in the rural surrounding to determine the urban increment in meteorology and air pollution. Two intensive observation periods (IOPs) in January/February 2012 and during the Olympics in summer 2012 measured London’s atmosphere with higher level of detail.

Results from the campaign showed that pollutant concentrations over heterogeneous urban areas such as London vary in space and with time. This is partly caused by local emissions, traffic patterns, chemical pathways but is also governed by local and meso-scale meteorology. The general processes leading to NOx and O3 concentrations in the urban environment are well understood. However, the role of the turbulent mixing properties of the urban boundary layer and their influence on concentrations in the urban background and at elevated levels are less well studied. In this talk we combine the long-term air pollution measurements of O3 and NOx taken at an urban background site in North Kensington (NK) and at an elevated height at BT Tower with turbulence measurements and analyse the role the meteorology plays for air pollution concentrations. Results show that on days characterized by a well-mixed urban boundary layer O3 concentrations at NK and BT Tower follow the rural background concentrations; under less well-mixed conditions vertical differences in measured O3 concentrations are partly caused by suppressed turbulent mixing.

Reduction of pollutant concentrations within the urban canopy and indoor environment

Misaki Tanimoto1, Ryozo Ooka2, Hideki Kikumoto2

1Graduate School of Engineering, The University of Tokyo; 2Institute of Industrial Science, The University of Tokyo

Oxidants including ozone (O3) are a major component of photochemical smog. High concentrations of O3 can irritate mucous membranes and negatively affect respiratory organs. Because people tend to stay indoors for large amounts of time, indoor O3 exposures can be greater than outdoor exposures for many people. Therefore, indoor concentrations deserve considerable research attention. Outdoor O3 concentrations can be reduced during transport processes within the urban canopy and within the indoor environment by surface removal and reactions between O3 and other chemicals in the air. When we investigate air pollution and damage to human health, it is useful to evaluate the relationship between outdoor and indoor pollutant concentrations and the degree to which pollutant concentrations are reduced during transport.

In this study, we carried out measurements of O3 and nitrogen oxide (NOx) concentrations in outdoor and indoor environments at our university campus in Tokyo for 5 days starting on July 7, 2014. For the outdoor measurements, we setup measurement points outside and inside the urban canopy. We specifically measured pollutant concentrations at three points including on a building rooftop, outside of the window at the fourth floor lecture room, and at the center of the lecture room; we assumed that these three points individually represent values outside of the urban canopy, inside of the urban canopy, and in the indoor environment, respectively. Based on results from the measurements, we calculated the ratio of concentrations outside and inside the urban canopy, as well as an I/O (indoor/outdoor) ratio. In addition, on the last day of July 11, we opened and closed the window in the lecture room in order to analyze the reductive processes for O3 and NOx concentrations in the indoor environment. We also calculated the ventilation rate, which is considered to significantly affect the indoor environment, from the concentration variations at the time the window was opened and closed.

The results showed that O3 concentrations outside of the urban canopy were higher than the concentrations at the other two measurement points; additionally, the concentrations inside of the urban canopy were higher than those indoors. In regards to the I/O ratio, O3 and NOx indoor concentrations and their outdoor concentrations were strongly correlated at all measurement points. The ratio of O3 concentrations inside and outside of the urban canopy was approximately 0.85, and the value for NOx was 0.79. Furthermore, the ratios of O3 and NOx concentrations inside of the urban canopy and in the indoor environment were approximately 0.95 and 0.91, respectively.

POSTER 7-5-5671384_b.pdf

Aerosol Pollution over the largest cities of Russia

Elizaveta P. Malinina1, Natalia Ye. Chubarova1, Mikhail A. Sviridenkov2

1Lomonosov Moscow State University, Russian Federation; 2A. M. Obukhov Institute of Atmospheric Physics RAS, Russian Federation

We evaluated the urban aerosol pollution over the largest Russian cities, which are located in various climatic zones. To detect the urban aerosol pollution we used the aerosol optical thickness (AOT) data, collected from MODIS at 550 nm during the warm period of 2000-2014. To validate MODIS data over the territory of Russian Federation we compared its data with the data of measurements from the AERONET network. This comparison showed a good quality of MODIS AOT retrievals and proved that urban aerosol pollution could be detected by this radiometer. To measure the urban aerosol pollution we used the value of ΔAOT, which we defined as the difference between AOT in the city and the value of 5%-percentile AOT over the surrounding region within 7°×7°. According to our estimates the average ΔAOT values vary from 0.01 to 0.08 for the different Russian cities. Our research showed, that urban aerosol pollution depends mainly on nitrogen dioxide emissions and, thus, on the population of the city. Besides, we received the dependence of the AOT value on the latitude of the city. To assess the dominating aerosol components and the sources of aerosol pollution in the Russian cities we used the data on emissions and concentrations of main aerosol pollutants from 1988 till 2011, collected by the Russian air pollution monitoring network. We evaluated the ratio of sulfur dioxide to nitrogen dioxide emissions and its trends to assess the dominating aerosol properties and temporal variability and their possible role on aerosol properties.

In addition, we have made more detailed research of aerosol pollution for Moscow megalopolis with more than 12 million population. We used the data of the two AERONET sites: one is located at the Lomonosov Moscow State University Meteorological Observatory (Mosocw MSU MO site) and the other one is located at the Zvenigorod Scientific Station (ZSS) located 55 km to the west from Moscow (Zvenigorod site) at the same latitude 55.7N. We used the simultaneous AOT measurements from September 2006 till July 2013 within 5 minutes. On the average, the difference in aerosol optical thickness between these two sites is ΔAOT =0.02 at 500 nm with some seasonal variations and maximum in winter. We also analyzed the difference in volume particle distribution, single scattering albedo and other aerosol characteristics simultaneously measured at the sites. Our results mainly confirm the previous assessments which were fulfilled earlier for the shorter period. In addition, we analyzed the interannual variability of Moscow aerosol pollution ΔAOT at 500 nm according to these data since 2006 and compared the obtained results with the MODIS AOT retrievals.


cornelius oyetunji akanni

olabisi onabanjo university, Nigeria

Ambient air quality resulting from urbanization, Land-use changes and weather variability is one of the micro-climatic problems faced in many major urban centers. The ambient air where majority of residents in the formal and informal sectors subsist daily could hardly be regarded as fresh; there is paucity of information on urbanization, land use changes, weather variability, environmental and health implications of the declining air quality in Lagos metropolis.

Hence, this paper examines an aspect of urban micro-climate by considering the influence of urbanization and weather variability on ambient air quality of Lagos Metropolis, Nigeria.

The micro-climate of Lagos Metropolis is characterized by relatively high air temperature, low wind speed and air pollution that varied according to traffic density. Average air temperature values during the wet and dry seasons were 26.63 0.13 oC and 28.18 0.14 oC, average wind-speed during the wet and dry seasons were 4.8 0.31 m/min and 4.12 0.24 m/min respectively. These values were significantly different (P<0.05). Step-wise regression analyses further showed that air temperature and land-use contributed significantly to air quality degradation (P<0.05). Concentrations of pollutants were higher than the WHO Air Quality standards and were also higher along traffic corridors than further away.

Key words: Urban-Micro-climate, Weather variability, Traffic Corridor, Ambient Air pollution, Land-use.

Aircraft measurements and WRF-FARM modeling of ozone, particles and pollutants in the city of Naples-Caserta

Bhishma Tyagi1, Daniele Gasbarra1, Giuseppe Agrillo1, Vincenzo Magliulo1, Beniamino Gioli2, Angelo Riccio3, Pantaleone Carlucci1, Luca Shindler1, Piero Toscano2, Alessandro Zaldei2, Andrea Esposito1

1National Research Council, Institute for Agricultural and Forestry Systems in the Mediterranean (CNR-Isafom), Napoli, Italy; 2National Research Council, Institute of Biometeorology (CNR-Ibimet), Firenze, Italy; 3Dept. of Applied Sciences, University of Napoli “Parthenope”, Napoli, Italy

Tropospheric ozone in urban areas is the result of complex photochemical interactions between radicals, NOx (anthropogenic emissions), VOCs (anthropogenic + biogenic emission), UV-irradiance. Moreover, actual levels of ozone, particles and other pollutants are mediated by atmospheric transport and re-circulation, leading to concentration spatial patterns that may be de-coupled from anthropogenic emission areas, leading to high concentrations in peri-urban or sea areas that may be further advected into cities. This poses a challenge of elucidating these mechanisms and assessing to which extent emission reduction policies can aim at reaching pollution reduction targets. In this study we deploy a modelling framework (WRF + FARM) and validate it on specific intensive aircraft measurements of wind and turbulence patterns (speed, direction, turbulence intensity), O3 and particles in the city area of Naples-Caserta (4,434,136 on area of 2,300 km²), the largest metropolitan area in the Mediterranean where the highest population density of Europe is reached. We will show criticalities for air quality (FARM) and atmospheric transport (WRF) modelling in light of spatialized measurements, to assess which conditions are better represented, how PBL height and scalar temporal and spatial concentration patterns are reproduced, and define an operational framework to elucidate the complex interactions between air quality and atmospheric transport in complex urban Mediterranean areas.

Keywords: Aircraft measurements, Ozone, Vertical profile, Weather and air quality

POSTER 7-9-7561610_a.pdf


Daniela Famulari1, Ida Russo1, Giuseppina Tirimberio1, Raffaella Vuolo1, Paul Di Tommasi1, Andrea Esposito1, Beniamino Gioli2, Vincenzo Magliulo1, Adriano Mazzarella3, Angelo Riccio4, Alesandro Zaldei2, Piero Toscano2

1National Research Council, Institute for Agricultural and Forestry Systems in the Mediterranean (CNR-Isafom),Via Patacca 85, 80040 Ercolano (Na), Italy; 2National Research Council, Institute of Biometeorology (CNR-Ibimet), Via Caproni 8, 50145 Firenze, Italy; 3Meteorological Observatory, Department of Science of the Earth, Environment and Resources, University of Naples “Federico II", Naples, Italy; 4Department of Applied Science, University of Naples “Parthenope”, Via De Gasperi 5, 80133, Naples, Italy

A super-site for the measurement of atmospheric pollutants from urban sources has been established in Naples where the complex layout of the coasts and surrounding mountains favours the development of combined sea breeze upslope winds and the evolution of return flows with several layers of pollutants and subsidence.

The metropolitan area of Naples has one of the highest population densities in Europe with an important impact in terms of emissions associated with diesel/gasoline exhaust, industrial emissions, agricultural burning and waste disposal.

In the super-site, located at the meteorological observatory of San Marcellino, an eddy covariance tower has been installed recently on the rooftop of the building: a fast response ultrasonic anemometer (Gill WindMaster) has been mounted on a 10-m mast, alongside three insulated inlet lines through which the air is sampled for reactive, non-reactive gases and particulate. The height of the terrace is on average 35 m above the irregular street level, resulting in an overall measuring height of 45 m. Mixing ratios of CO2, CH4 and H2O are measured by an infrared spectrometer (10 Hz, Los Gatos Research); O3 mixing ratios are measured by a fast analyser (10Hz, FOS Sextant) for the calculation of fluxes, and referred to concentrations measured by a slower analyser (2B-Technologies, 205). NO and NOx are continuously quantified (1 Hz) using a NOx analyser Eco Physics model (CLD 88p) associated with a photolytic converter (PLC 860). Size segregated aerosol are measured by gravimetric method at an hourly/daily frequency through a SWAM 5A Dual Channel (PM10 and PM2.5, FAI Instruments). A faster optical particle counter (4 Hz, FAI Instruments) allows the estimate of fluxes - as well as concentrations – of 22 classes of particles diameter. All analysers outputs are synchronised with the sonic anemometer through a common acquisition at 10 Hz using a CR3000 datalogger (Campbell Scientific). A full weather station provides ancillary measurements at the site including two webcams to record exceptional events to aid interpretation of the results.

The fluxes are representative of varying footprint source areas, covering the historical centre of Naples, the harbour, and some main traffic arteries of the city.

Preliminary results show that, during a stable event in the month of November, the mean urban levels of CO2 are between 420-520 ppm; the mean levels of CH4 span between 1.85-2.48 ppm, and the O3 levels are extremely variable, between 2 and 62 ppb. These follow NOx abundance, with values ranging between 0.5 and 84 ppb for NO, and 5 and 29 ppb for NO2, with an average NO2/NO ratio around 30%. Daily PM10 levels vary between 18 and 55 micro-g/m3, and PM2.5 between 12 and 47 micro-g/m3. The largest concentrations were measured from air coming from the harbour, coinciding with the presence of cruise ships.

The year-long planned measurements will allow to establish relationships between the fluxes of greenhouse gases and the other pollutant species measured, to investigate the controls of the emission and provide relative emission factors for the urban sources.

POSTER 7-11-8141578_b.pdf

Contributions of Biomass Burning and Traffic Emissions to Particulate Matter at two Urban Sites within the Ruhr Area, Germany

Miriam Kuepper1, Astrid John1, Thomas Kuhlbusch1, Stephan Leinert2, Ludger Breuer2, Dieter Gladtke2, Tanja Schuck2, Stephan Weber3

1Institute of Energy and Environmental Technology e.V. (IUTA), Duisburg, Germany; 2North Rhine-Westphalian State Agency for Nature, Environment and Consumer Protection, Essen, Germany; 3Technical University of Braunschweig, Institute for Climatology and Environmental Meteorology, Braunschweig, Germany

In industrialized countries an increase in relative contributions from biomass burning to the particulate matter (PM) burden, most notably caused by the introduction of low emission zones as well as by the prevalent use of stoves and fireplaces, has been observed recently. For this reason, measurements of black carbon (BC), elemental carbon (EC), organic carbon (OC) and levoglucosan in particulate matter over a one year period at two urban monitoring sites in North Rhine-Westphalia (Germany) - a traffic site (Duisburg) and an urban background site (Mülheim) - have been initiated by the North Rhine-Westphalian State Agency for Nature, Environment and Consumer Protection in order to quantify the contributions of biomass burning and traffic emissions to the carbonaceous share of PM (CM = carbonaceous matter). For discrimination of these sources an algorithm developed by Sandradewi et al. (2008) was applied on BC, EC and OC data. To evaluate this approach levoglucosan was used as a mono-tracer of biomass burning emissions.

Contributions of carbonaceous components attributed to biomass burning processes were almost identical at both sites with higher concentrations during winter (approx. 13 ± 6 % of PM10 compared to 6 ± 4 % of PM10 in summer), occasionally even exceeding those from traffic emissions. In contrast, CM from traffic did not show any significant seasonal variation and revealed higher contributions at the traffic site (15 ± 5 % of PM10, urban background: 9 ± 3 % of PM10). Simultaneous observation of contributions from biomass burning to CM determined via levoglucosan revealed a good agreement between both approaches with an R² of 0.82 for the traffic site and 0.74 for the urban background site within the winter period, whereas during summertime lower correlation coefficients were observed. These might be due to faster decomposition of levoglucosan and a differing mixture of utilized fuels, e.g. influence of charcoal barbecues.

POSTER 7-12-8691696_b.pdf

A numerical study on the effects of street aspect ratio on reactive pollutants dispersion

Soo-Jin Park1, Jae-Jin Kim1, Rokjin Park2, Hankyung Lee1

1Pukyong National University, Korea, Republic of (South Korea); 2School of Earth and Environmental Sciences, Seoul National University, Seoul, Korea, Republic of (South Korea)

In this study, the effects of street‒canyon aspect‒ratio on the dispersion of reactive pollutants were investigated using a coupled CFD‒chemistry model. Flow characteristics for different aspect ratios were analyzed first. For each aspect ratio, six emission scenarios with different VOC‒NOX ratios were considered. One vortex was generated when the street‒canyon aspect ratio defined as the ratio of building height to street width was less than 1.6 (shallow street canyon). When the street‒canyon aspect ratio was greater than 1.6 (deep street canyon), two vortices were formed in the street canyons. In contrast to the previous studies on two‒dimensional street canyon, vortex center is tilted toward the upwind building and reverse and downward flows are dominant in street canyons. Near the street bottom, there is big difference in flow pattern between in shallow street canyons and deep street canyons. Near the street bottom, reverse and downward flows are dominant in shallow street canyon and flow convergence exists near the center of the street canyon in deep street canyon, which induces big difference in dispersion pattern of NOX and O3 in the street canyon. NOX concentration is high near the street bottom and it decreases with height overall. Due to NO titration, O3 concentration is low at high NO region. At small VOC‒NOX ratio, NO concentration is high enough to destruct large amount of O3 by titration and, resultantly, O3 concentration in the street canyon is much lower than the background concentration. At high VOC‒NOX ratio, small amount of O3 is destructed by NO titration even in lower layer of the street canyon and, in the upper layer, O3 is formed through the photolysis of NO2 by the degradation reaction of VOCs. As the aspect ratio increases, NOX (O3) concentration averaged over the street canyons decreases (increases) in shallow street canyons, because outward flow becomes strong and NOX flux toward outsides of the street canyon increases, resulting in less NO titration in the street canyon. In deep street canyons, outward flow becomes weak and outward NOX flux decreases, resulting in the increase (decrease) of NOX (O3) concentration.