Deviations from the O3eNOeNO2 photo-stationary state in Delhi,India
Dilip M. Chate a, *, Sachin D. Ghude a, Gurfan Beig a, Anoop S. Mahajan a, Chinmay Jena a,Reka Srinivas a, Anita Dahiya b, Nandini Kumar b
a Indian Institute of Tropical Meteorology, Pune, Indiab TERI University, Delhi, India
h i g h l i g h t s
� This study focuses on the O3eNOeNO2-triad Photo Stationary State (PSS).� Site-specific deviations in the Leighton Ratio (F) for Delhi are investigated during a short period in 2012.� Large variations were observed in the NO (<1e295 ppb), NO2 (<2e47 ppb) and O3 (4e95 ppb) mixing ratios.� The peroxy radical (PO2) concentrations are calculated for O3eNOeNO2-triad data (F > 1).� Results indicate a large presence of PO2 in Delhi, as those levels reported elsewhere in the world.
a r t i c l e i n f o
Article history:Received 24 February 2014Received in revised form26 July 2014Accepted 31 July 2014Available online 1 August 2014
A network of air quality and weather monitoring stations was set-up across Delhi, India, under theSystem of Air quality Forecasting And Research (SAFAR) project. The objective of this network was toenable better understanding of air quality in terms of atmospheric chemistry, emissions and forecastingin Delhi, one of the largest metropolises in the world. In this study, we focus on the O3eNOeNO2-triadPhoto Stationary State (PSS), and investigate site-specific deviations in the Leighton Ratio (F) during ashort period in 2012 (1e31 December). Large variations were observed in the NO (<1 ppbv to a peak of295 ppbv), NO2 (<2 ppbve47 ppbv) and O3 (4 ppbve95 ppbv) mixing ratios, all of which showed strongdiurnal variation. The F values showed large deviations from unity over the measurement period, withmostly negative deviations (F < 1), showing that the air masses were dominated by local sources of NOx
and that the PSS was not achieved. Positive deviations (F > 1) were also observed occasionally, and thesedata were used to estimate the total peroxy radical (PO2) mixing ratios. This is the first estimate of PO2
Delhi is one of the most populated metropolis in the world witha population of about 16.7 million at a density of 11,297 persons perkm2 in 2011. Along with the surrounding urban region, known asthe National Capital Region (NCR), it is India's largest and theworld's second largest urban agglomeration with a population ofabout 22.2 million (Census of India, 2011). Delhi has a decadalpopulation growth rate of 20.96%. Delhi had approximately 5.1million motor vehicles in 2007 (Department of Transport, Delhi,
ropmet.res.in (D.M. Chate).
2007), and more than 100,000 petrol and diesel vehicles weresince then added annually. Delhi has seen rapid urbanizationassociated with the industrialization where growing demands forenergy over the last two decades, which has caused an increase inthe number of vehicles, industrial and power sectors in the NCR. Asa result, Delhi continually suffers from serious air pollution prob-lems, with high levels of nitrogen oxides (NOx ¼ NO þ NO2), ozone(O3), black carbon (BC) and particulate matter (PM) (MoEFF, 1997;Gurjar and Lelieveld, 2005; Gurjar et al., 2008; Jain et al., 2005;Ghude et al., 2009). Vehicular exhaust in and around Delhi is thedominant contributor to the photochemistry of NOx (precursors ofozone).
In the past, photochemistry of NOx within the planetaryboundary layer has been investigated in detail in different regions
D.M. Chate et al. / Atmospheric Environment 96 (2014) 353e358354
of theworld (Trebs et al., 2012 and references therein). It is alsowellknown that NOx acts as a key catalyst in the formation of tropo-spheric ozone (e.g. Crutzen and Lelieveld, 2001). When othercompeting reactions are absent in the troposphere, a dynamicequilibrium between NO, NO2, and O3 is established during thedaytime (Seinfeld and Pandis, 2006). NO in the troposphere isconverted to NO2 by reaction with O3 and this NO2 is photolysedback to NO and O(3P), which in turn reacts with background oxygento give ozonee resulting in an equilibrium that can be described as:
NO þ O3 / NO2 þ O2 (1)
NO2 þ hn / NO þ O(3P) (for l < 420 nm) (2)
O(3P) þ O2 þ M / O3 þ M (3)
This equilibrium, limited by reactions (1) and (2), is establishedon a timescale of a few hundred seconds depending on the envi-ronmental conditions and is known as the Photo Stationary State(PSS) (Leighton, 1961). The equilibrium is sensitive to the flux ofphotons associated with radiation of the appropriate wavelength tophotolyse NO2 and fresh emissions of NO from sources such asmotorised vehicles. The net result of these reactions is a null cycleand when PSS is assumed, the O3 concentration can be predictedby:
½O3� ¼jNO2
½NO2�k1½NO�
(4)
where, jNO2(s�1) is the photolysis rate of NO2, and k1
(cm3 molecule�1 s�1) is the temperature dependent rate constantfor the reaction between NO and O3 (1.4 � 10�12 exp (�1310/T))(Atkinson et al., 2004). The Leighton ratio, F (Leighton, 1961) canthen be calculated as:
f ¼ jNO2½NO2�
k1½NO�½O3�(5)
Numerous studies have investigated adherence to/deviationfrom F approximately equal to unity (e.g. Carpenter et al., 1998;Thornton et al., 2002; Volz-Thomas et al., 2003; Griffin et al.,2007). In general, it has been shown that in areas with high NOx
levels, F values are consistently close to unity (Calvert andStockwell, 1983; Shetter et al., 1983; Parrish et al., 1994; Yanget al., 2004). Over some rural locations with high photolytic activ-ity, the F value ranges from 1.2 to approximately 3.0 (Ridley et al.,1992; Cantrell et al., 1997; Hauglustaine et al., 1996; Rohrer et al.,1998, Hosaynali et al., 2011). Values less than one are seen inareas of fresh NO emissions or times when there are rapid changesin jNO2
, suggesting that the PSS is not achieved.Satellite observations have shown an increasing trend of
tropospheric column NO2 over Delhi and surrounding industrialregion (Ghude et al., 2008, 2009; Hilboll et al., 2013). Increasingtrend in column amount of NO2 over this region was mainly due tothe increasing vehicle numbers and industrialisation in and aroundand cities (Ghude et al., 2013). Ground-basedmeasurements withinurban areas in India have shown higher O3 values during photo-chemically active seasons (Naja and Lal, 2002; Jain et al., 2005;Ghude et al., 2006). On one hand, ozone production rates can beorders of magnitude higher within the megacities compared to thesemi-polluted air in the outskirts, but on the other hand, there arealso effective removal processes such as titration of O3 with freshlyemitted NO. In Delhi, urban centres (road junctions, the traffic in-sertions or signalized roadways) create urban emission hotspotsbecause vehicles spend more time at these locations (Kanlindkar,
2007). Emissions of NOx and other pollutants are higher at thesehotspots compared to other areas in Delhi (Sahu et al., 2011). Higheremissions at these urban centres are thought to have significantinfluence on the photochemistry and hence on the O3, NO and NO2levels, PSS and F values.
As a part of the System of Air Quality Forecasting And Research(SAFAR, http://safar.tropmet.res.in/) project, a network of AirQuality Monitoring Stations (AQMS) and Automatic Weather Sta-tions (AWS) was established in 2010 in and around Delhi. The airquality and weather data collected from these sites offer us aunique opportunity to examine the difference in the PSS and F athigh spatial resolutionwithin the city of Delhi. Here, we investigatethrough observations, for the first time, the role of urban centresandmeteorological conditions on the variability in the area-specificO3, NO and NO2 levels, adherence to/deviation from F equal tounity based on observed and calculated O3 levels in Delhi, bycomparing different sites within the same city.
2. Measurements
Observations of O3, NO and NO2 made during 1e31 December2012 at various strategic locations in Delhi-NCR covering2500 km2 with 12 AQMS were used in this study. More details onthe SAFAR system and air-quality and weather monitoring setupcan be found at http://safar.tropmet.res.in/. Data related to policyinduced air quality control has been already presented elsewhere(Beig et al., 2013). In this study, we focus on six stations: CRRI -Central Road Research Institute, CVR e Sir CV Raman IndustrialTraining Institute, Dheerpur, DU e Delhi University, IITM-D e
Indian Institute of Tropical Meteorology e Delhi, IMD-LD e IndianMeteorological Department, Lodhi Road and IGI-A e IndiraGhandi International Airport, most of which were in almostcontinuous operation during the study period (see Fig. 1). One ofthe reasons for selecting these stations is the site specific differ-ence with respect of NOx sources. For example, DU is located about500 m away from the main traffic road, while IITM-D is locatedaway from the large traffic insertion.
Each of the AQMS is comprised of US Environmental ProtectionAgency (US-EPA) approved analysers housed inside temperaturecontrolled shelters. Ozone was measured with a photometric UVanalyzer (Thermo-49i, precision ~1 ppbv) and NO and NO2 withchemiluminescence NOx analysers (Thermo-42i, precision~0.4 ppbv). Calibration of the O3 analyzer was done on everyalternate day using an inbuilt O3 calibrator, whereas NO2 cali-brationwas performed with multipoint calibration technique. Theinlets for NO2 and O3 analysers were located about 3 m above thesurface at all the stations. BC was measured using the MageeScientific black carbon aethalometer (Model AE31), while VOCs,including benzene, were measured with VOC analyser (ModelA73022). The particulate matter was measured using a suspendedparticulate matter analyser with a detection limit of 1 mg m�3 andan inbuilt calibration unit. Meteorological parameters e globalradiation, wind speed, wind direction, temperature, relative hu-midity, pressure and rainfall were recorded with an AWS at eachof the AQMS locations. The raw data were collected at a resolutionof 5 min at the sites and then underwent a quality check beforefurther analysis and comparisons. It is important to note thatjNO2
was not measured directly during the study, but was calcu-lated using the Tropospheric Ultraviolet and Visible RadiationModel (TUV). The model was tuned with the measured globalradiation at each of AQMS-AWS locations and the satellitemeasured overhead ozone columns from ozone monitoring in-strument (OMI) onboard NASA's Earth Observing System's (EOS)Aura satellite.
Fig. 1. Network of air quality and meteorology monitoring stations discussed in detail in this study are shown in blue (CRRI e Central Road Research Institute, CVR e Sir CV RamanIndustrial Training Institute, Dheerpur, DU e Delhi University, IITM-D e Indian Institute of Tropical Meteorology e Delhi, IMD-LD e Indian Meteorological Department, Lodhi Roadand IGI-A e Indira Gandi International Airport). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The NOx chemiluminescence instruments with a molybdenumbased thermal convertor are known to have interference issues forNO2 from NOy species such as HNO3 and PAN (e.g. Dunlea et al.,2007). This is due to the non selective dissociation for the detec-tion of NO2. To check whether this was the case for this study, weused an instrument with a photolytic convertor next to one of theoriginal instruments with molybdenum based convertors. A com-parison plot between NO2 from the two instruments is shown inFig. 2, and it can be seen that the interference due to NOy specieswas not large. Thus these observations allow a direct examination
Fig. 2. Comparison of two instruments measuring NO2 in Delhi. The blue line showsthe chemiluminescence instrument with a molybdenum based convertor, while thered line shows the tunable infrared laser differential absorption spectrometer (TILDAS)instrument, which were placed next to each other for inter-comparison and did notshow any considerable difference. (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)
of the variations of all the measured parameters within Delhi andalso to quantify the changes in the PSS.
3. Results and discussions
Fig. 3 shows the time series data of the three chemical speciesmeasured during the length of the study at the six measurement lo-cations chosen here. A large variation was observed in the NO(<1 ppbv to a peak of 295 ppbv), NO2 (<2 ppbve47 ppbv) and O3(4 ppbve95 ppbv) mixing ratios, which showed clear diurnal cyclesfor most of the days during the study period. Stations close to majorroads and traffic junctions show higher mixing ratios and largervariation compared to the stations which are relatively further away;suggesting that vehicular traffic is themain source of theNOx, and thesecondary pollutant O3. The diurnal profiles of NO and NO2 show aclear signature of vehicular traffic, showing the well known doublepeakmatching thepeak traffic times. TheO3mixing ratios by contrastare governed by the photochemistry, peaking shortly after noon,suggesting photochemical production, as expected. Non zero valuesof NO at night show that there was fresh emission even during thenight time (for e.g. emission mostly from on road vehicle in night).
The calculated F values at all the stations are shown in Fig. 4(panels a, b and c). Only data for jNO2
>0:001was used to computetheF values, thus ignoring the earlymorning and late evening data.The F values showed negative deviations for most of study period,the reasons for which, can be either fresh emissions of NO or fastchanges in jNO2
. The jNO2however, was stable with little variation
indicating that fresh NO emissions were the main reason why thePSS is not achieved at the measurement sites. No clear dependenceof the F values on wind direction or speed was observable, indi-cating that the locations were surrounded by NOx sources, as wouldbe expected in a large city. Natural candidates to exert a chemicalcontrol on the F are the constituents of NOx themselves. Fig. 4(d)presents the inverse relationship between the calculated F valuesand the measured total NOx. Larger mixing ratios of total NOx
clearly show lower values of F with characteristic negative
Fig. 3. Time series observations of the key species measured during the period reported in this study are shown here. The variation in (a) NO, (b) NO2 and (c) O3 is shown at thethree locations considered in this study.
D.M. Chate et al. / Atmospheric Environment 96 (2014) 353e358356
deviations, while smaller values (generally less than 10 ppbv) tendto show positive deviations. Urban centres, such as road junctions,the traffic intersections and signalized roadways are the majoremission hotspots and could be the main reason for the negativedeviations due to the fresh emissions close to the measurementsites.
F deviates positively from unity when some chemical processother than the reaction between NO and O3 converts NO to NO2.Few studies (Volz-Thomas et al., 2003; Mannschreck et al., 2004)have suggested that an unknown oxidation processes may cause anadditional conversion of NO to NO2 leading to values of F > 1. One
Fig. 4. (aec) The variation in the Leighton ratio over the study period at different loc
major pathway for the positive deviations in F is the reaction be-tween NO and a peroxy radical (PO2), either organic (RO2) or hydro(HO2)(PO2 ¼ RO2 þ HO2) (reaction (6)).
NO þ PO2 / NO2 þ PO (6)
RO2 and HO2 generally result from the atmospheric oxidation ofvolatile organic compounds (VOCs) and/or carbon monoxide (CO).Such reactions also lead to the formation of tropospheric O3(Seinfeld and Pandis, 2006). An additional pathway by which NOcan be converted to NO2 is via halogen monoxide (XO, where X
ations in Delhi, (d) along with its dependence on the absolute NOx mixing ratios.
Fig. 5. Mixing ratios of PO2 (ppbv) that would force F values to be equal to 1.0 isshown. These values are calculated according the details given in the text only for F
represents a halogen atom) reactions. However, these are known toplay a major role mostly in the marine boundary layer. Delhi issituated in the northern plain of India and is far away from themarine region (more than 1000 km). Therefore, halogen com-pounds are not thought to play a major role in the control of the F
values.If it is assumed that PO2 is the sole contributor to the observed
deviations from F ¼ 1, an estimate of the ambient PO2 mixing ratiocan be made by adding a second term (k2[NO][PO2]) to the de-nominator of equation (5), forcing F equal to 1.0, and solving for[PO2]:
½PO2� ¼jNO2
½NO2�k2½NO�
þ k1½O3�k2
(7)
where, k2 is the temperature-dependent rate coefficient for thereaction between HO2 and NO (k2 ¼ 3.45 � 10�12 exp(270/T))(Atkinson et al., 2004). For simplicity, it is assumed that the ratecoefficients for reactions between individual RO2molecules and NOare equivalent to k2 (DeMore et al., 1997). In this scenario, it is thePO2 reactionwith NO that converts NO to NO2 in addition to the NOand O3 reaction. On the basis of equation (7), the required PO2 levelfor those times when the F is greater than 1, ranges from approx-imately 4 � 10�4 ppbv to 0.17 ppbv, with an average of0.025 ± 0.021 (SD) ppbv for the entire data set. The time series ofrequired PO2 needed to bring F values equal to 1.0 is shown inFig. 5. It can be seen that, the temporal profile of PO2 follows that ofthe F values presented in Fig. 4. This value is the first estimate ofPO2 reported for the city of Delhi and compares well with mea-surements and estimates in other parts of the world.
Numerous studies in past have measured PO2 mixing ratiosdirectly or have used the method described here to quantify them,or applied radical budget or photochemical modelling to estimatethem. The variety of locations and seasons for which these mea-surements and calculations have been performed make a directcomparison difficult and hence it can only be compared qualita-tively. Measurements of PO2 using chemical amplification andother techniques have shown average mixing ratios on the order oftens of pptv in locations ranging from Hawaii to the Canary Islandsto MaceHead (Ireland) to rural Germany (Hauglustaine et al., 1996;Zenker et al., 1998; Carslaw et al., 1999; Mihelcic et al., 2003).During summer 1990, Cantrell et al. (1997) measured PO2 mixingratios as high as approximately 300 pptv in rural Alabama. Radicalbudget and photochemical steady state models predict PO2 mixingratios that are generally on the same order of magnitude (Kleinmanet al., 1995; Frost et al., 1998). Calculations of PO2 from expressionssimilar to equation (7) result in estimates of the PO2 mixing ratiothat are close to those presented here, despite differences in loca-tion and season (Ridley et al., 1992; Kleinman et al., 1995; Carpenteret al., 1998; Frost et al., 1998; Rohrer et al., 1998; Baumann et al.,2000; Duderstadt et al., 1998; Volz-Thomas et al., 2003; Yanget al., 2004; Griffin et al., 2007).
4. Conclusions
Measurements of O3, NO, NO2 and meteorological parameterswere made across Delhi as a part of the SAFAR project. The mea-surements indicate significant deviations in calculated Leightonratios, F from a Photo Stationary State (PSS), where F ¼ 1. Most ofthe sites show F values of less than 1, indicating fresh emissions ofNO close to the measurement sites and that the PSS was not ach-ieved. The PO2 concentration were calculated for data where F > 1.These estimates indicate a large presence of PO2 in Delhi, similar tolevels reported elsewhere in the world, with an average mixingratio of 0.025 ± 0.021 ppbv (ranging between 4 � 10�4 to0.17 ppbv). Direct measurements of PO2 need to be done in Delhi tovalidate the estimations from this study.
Acknowledgements
Indian Institute of Tropical Meteorology, Pune is supported bythe Ministry of Earth Sciences (MoES), Government of India, NewDelhi. Authors acknowledge the support of Prof. B. N. Goswami,Director IITM Pune. Authors appreciate the efforts of entire teaminvolved in SAFAR project under the MoES.
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