Simple photochemical modelling of NOX pollution in a street canyon

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Simple photochemical modelling of NOXpollution in a

street canyon

L. Soulhaca, S. Fellinia, C.V. Nguyenb, P. Salizzonib

aUniv. Lyon, INSA Lyon, CNRS, Ecole Centrale de Lyon, Univ. Claude Bernard Lyon 1,

LMFA, UMR5509, 69621, Villeurbanne France

bUniv. Lyon, Ecole Centrale de Lyon, CNRS, Univ. Claude Bernard Lyon 1, INSA Lyon,

LMFA, UMR5509, 69130, Ecully, France

Abstract

To predict pollutant concentration in urban areas, it is crucial to take into

account the chemical transformations of reactive pollutants in operational dis-

persion models. In this work, we derive and discuss diﬀerent NO– NO2– O3

chemical street canyon models with increasing complexity and we analytically

evaluate their applicability in diﬀerent urban contexts. We then evaluate the

performance of the models in predicting NO2concentration at diﬀerent locations

within an urban district by comparing their predictions with measurements ac-

quired in a ﬁeld campaign. The results are in line with analytical speculations

and give indications as to which model to use according to the conditions of

the urban street canyon. In courtyards with limited ventilation and without

direct emissions, the performance of the photostationary model is satisfactory.

On the other hand, the application of a non-photostationary model signiﬁcantly

improves the predictions in urban canyons with direct vehicular emissions. The

applicability of the proposed models in operational tools at the city scale is

ﬁnally discussed.

Keywords

Photochemical smog, Urban air quality, Non-photostationary chemical model,

Air pollution measurement campaign

Preprint submitted to Elsevier October 24, 2022

arXiv:2210.11859v1 [physics.ao-ph] 21 Oct 2022

Introduction

The time scales related to pollutant transfer over large urban agglomerations

range from a few minutes to several hours. During this period, a large num-

ber of physico-chemical processes take place and determine the concentration of

pollutants in the urban atmosphere (Sillman, 1999). When the focus is on dis-

persion at the local district scale, the rate of turbulent transport is considerably

high compared to the rate of chemical transformation and most of the atmo-

spheric compounds can be treated as inert tracers. There are however chemical

reactions which are suﬃciently fast to signiﬁcantly aﬀect the concentration of

pollutants during their residence time in the streets. This is notably the case

for NOX, i.e. the nitrogen oxides that are most relevant for air pollution.

The emissions of NOXresult from combustion processes, especially from

motor vehicle engines or from power stations and industries. They are there-

fore a tracer of anthropogenic activity in urban areas and their trends are used

to assess the eﬀectiveness of regulations on air pollution, or to evaluate the

eﬀects of sudden changes in emissions, such as during COVID-19 restrictions

(e.g., Toscano and Murena, 2020; Lovarelli et al., 2020; Misra et al., 2021). It is

generally assumed that the partition of NOXat the point of emission is approx-

imately between 10% to 15% for NO2(nitrogen dioxide) and 85% to 90% for

NO (nitrogen monoxide) (Ntziachristos et al., 2000). Acute exposure to NOX

causes respiratory disease and compromises lung functioning when inhaled at

high concentrations. Children are the most vulnerable, with a demonstrated

increased incidence of childhood asthma due to NO2emissions from vehicular

traﬃc (Khreis et al., 2017; Anenberg et al., 2022). Despite being the major

contributor to NOX, NO is less toxic than NO2. However, as most radicals, it is

extremely unstable and forms NO2through photochemical oxidation. Nitrogen

dioxide is then converted back to NO as a result of photolysis which also leads

to the regeneration of ozone (O3). When the photostationary state is reached,

these reactions result in a cycle with zero net chemistry and the chemical com-

pounds reach the equilibrium composition, which can be easily derived in terms

of kinetic reaction parameters by the Leighton relation (Leighton, 1961). De-

viations from this state occur when (i) the residence time of polluants in the

reference volume (i.e. the street) is shorter than the time needed for reach-

ing the photostationary equilibrium, (ii) turbulent motions mix the reactants so

slowly that they remain segregated rather than reacting (Li et al., 2021), (iii) the

transformation of nitrogen monoxide into NO2is altered by the role of complex

reactions with radicals resulting from the oxidation of Volatile Organic Com-

pounds (VOCs) and CO (Jenkin and Clemitshaw, 2000). The concentrations of

NO and NO2are also aﬀected by reactions involving the hydroxyl radical and

leading to the production of nitric acid.

The coupling of turbulent and chemical dynamics to assess photochemi-

cal pollution in urban areas has been explored extensively in the past two

decades by means of Computational Fluid Dynamics (CFD) simulations. Baker

et al. (2004) extended a Large Eddy Simulation (LES) for turbulent ﬂow in

a street canyon with a simple NOX-O3chemical model. The same reaction

scheme was adopted by Baik et al. (2007), who instead used Reynolds-averaged

Navier–Stokes (RANS) simulations. By introducing a photostationary state de-

fect index, both studies highlighted the regions of a street canyon most prone

to chemical instability. The chemistry of VOC has been included in RANS sim-

ulations by Kwak and Baik (2012) and Kim et al. (2012), while Bright et al.

(2013) combined LES simulations with a detailed chemical reaction mechanism

(Reduced Chemical Scheme) comprising 51 chemical species and 136 reactions.

Similarly, Garmory et al. (2009) used the Stochastic Fields (FS) method to sim-

ulate turbulent reacting ﬂows with a chemistry model comprising 28 species.

These studies showed that the eﬀect of turbulent ﬂuctuations (i.e. segregation)

on the chemistry is signiﬁcant for species with the highest transformation rates.

They also showed that increasing chemical complexity (i.e. simulating VOC

chemistry) could contribute to additional but modest NO2and O3formation in

the canyon.

CFD simulations, coupled with detailed chemical models, provide an accu-

rate prediction but are computationally expensive and require a large amount of

detailed input data. To simulate air quality in large urban domains, consisting

of hundreds to thousands of streets, a more eﬃcient way is adopting simpliﬁed

modelling approaches (Vardoulakis et al., 2007). These are usually Gaussian-

Lagrangian models integrated with box models to simulate the concentration in

the street canyons. In these operational tools, photostationarity is a convenient

assumption as it allows the modelling of O3and NOXas inert tracers and to

subsequently apply photochemical equilibrium in the streets. This is the case of

the Canyon Plume Box Model (CPBM) (Yamartino and Wiegand, 1986), and

the street network model Sirane (Soulhac et al., 2017).

Another widespread approach is the adoption of empirical models to esti-

mate NO-NO2conversion (Ravina et al., 2022). These are based on a photosta-

tionary assumption but are optimised to ﬁt observed concentrations. Hirtl and

Baumann-Stanzer (2007) investigated the performances of the two empirical

conversion schemes after Romberg et al. (1996) and after Derwent and Middle-

ton (1996), when implemented in the Gaussian model Atmospheric Dispersion

Modelling System (ADMS) and in the LAgrangian Simulation of Aerosol Trans-

port (LASAT) model. These dispersion models turned out to be quite successful

in predicting average concentrations measured in street canyons.

A step forward in modeling the interaction between the time scales of chem-

ical reactions and those of transport is represented by the model ADMS-Urban

(McHugh et al., 1997; Carruthers et al., 2000). In ADMS-Urban, NOXchem-

istry can be modelled by the Generic Reaction Set (GRS) (Azzi et al., 1992;

Venkatram et al., 1994) photochemical scheme which includes seven chemical

reactions. The GRS chemical model is applied to the emitted pollutants after

transport and dispersion. The chemistry calculation for the receptor is split in

two steps: the ﬁrst considers the contribution from far sources (source-receptor

travel time greater than 150 s), while the second one includes the contribution

from the nearest sources (source-receptor travel time less than 150 s) (CERC,

2022). In this way, the model takes into account the travel time of the pollution

plume and it assumes a time -or distance- dependence on the generation of NO2.

Finally, the Operational Street Pollution Model (OSPM) (Palmgren et al.,

1996; Berkowicz et al., 1997) is a street canyon model which includes NO-NO2-

O3chemistry by means of a non-photostationary model that takes into account

the interaction between the chemical reaction rates and the residence time of

the pollutants in the street.

The overview above suggests that operational modeling of reactive pollu-

tant concentration at the urban scale requires an adequate description of (i) the

chemistry, (ii) the turbulent transport, (iii) the interaction between these two

processes, all while minimizing the computational cost and required input data

in order to be applied to hundreds to thousands of streets. To date, empirical

relationships and photostationary models are the most commonly used for op-

erational purposes while non-photostationary schemes are rarely implemented.

This is especially true for street network models, where, to our knowledge, the

non-photostationary scheme has not yet been implemented. Furthermore, the

existing literature lacks a coherent formulation of the diﬀerent photochemical

models, with a clear statement of the underlying assumptions and a concurrent

validation with real data. To ﬁll these gaps, in this work, we derive, com-

pare and validate three models for NOXphotochemical pollution that can be

eﬃciently implemented in street network models at the city scale. In the ana-

lytical derivation, we focus on the time scales of pollutant transformation and

transport in order to highlight the range of application of the diﬀerent models.

To verify the reliability of the diﬀerent schemes we compare the model outputs

to ﬁeld data. The main objective is to evaluate whether the application of a

non-photostationary model can bring substantial advantages in the prediction of

pollutant concentration in the streets, with respect to photostationary models.

The formulation of a photochemical model, adopting box-model approach,

is presented in 1. A general presentation of the measurement campaign is given

in Section 2. Results are discussed in Section 3, while the conclusions are drawn

in Section 4.

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摘要：

SimplephotochemicalmodellingofNOXpollutioninastreetcanyonL.Soulhaca,S.Fellinia,C.V.Nguyenb,P.SalizzonibaUniv.Lyon,INSALyon,CNRS,EcoleCentraledeLyon,Univ.ClaudeBernardLyon1,LMFA,UMR5509,69621,VilleurbanneFrancebUniv.Lyon,EcoleCentraledeLyon,CNRS,Univ.ClaudeBernardLyon1,INSALyon,LMFA,UMR5509,69130,Ecu...

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Simple photochemical modelling of NOX pollution in a street canyon

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