Impact of NO2 horizontal heterogeneity on tropospheric NO2

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Conclusions 1 Introduction
In Canada, nitrogen oxides (NO x = NO + NO 2 ) are classified as Criteria Air Contaminants due to their adverse effects on human health and the environment (Environment Canada, 2006).Anthropogenic production of NO x is largely attributed to fossil fuel combustion, with the transportation sector accounting for 68 % of NO x emissions in Ontario, Canada during 2008(MOE, 2011).NO x emissions predominantly consist of NO, which Introduction

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Full is oxidized to NO 2 by species such as O 3 , HO 2 , and RO 2 .Daytime cycling between NO, NO 2 , and O 3 occurs according to the photostationary state relationship, which illustrates that the steady-state concentration of O 3 is proportional to the steady-state ratio of NO 2 to NO.In the presence of volatile organic compounds (VOCs), the ratio of NO 2 to NO increases, and O 3 production is enhanced, thereby contributing to the formation of photochemical smog.O 3 production is also dependent the on ratio of NO x present in the atmosphere relative to HO 2 .Geddes et al. (2009) demonstrated that Toronto, Ontario's (43.66 • N, 79.39 • W) airshed exists in the NO x -saturated regime, and as a result, O 3 production decreases with increasing NO x concentration, since HO x (OH + HO 2 ) cycling is inhibited by the chain-terminating reaction of OH with NO 2 to yield nitric acid (HNO 3 ) (Seinfeld and Pandis, 2006).At night, the dominant removal mechanism of NO x is the oxidation of NO 2 by O 3 to produce the nitrate radical (NO 3 ).The majority of NO 3 exists as dinitrogen pentoxide (N 2 O 5 ), which is formed via the reaction of NO 2 and NO 3 .Heterogeneous reactions of N 2 O 5 with aerosol to produce nitrate-bound particles serve as a major night time NO x sink (Seinfeld and Pandis, 2006).
Previous studies investigating the spatial distribution of NO x have focused on nearroad environments as a means to assess human exposure to traffic-related air pollution (TRAP), characterize TRAP dilution and chemical evolution, inform urban infrastructure planning and policy, and validate dispersion models (Villena et al., 2011;McAdam et al., 2011;Y. Wang et al., 2011;Clements et al., 2009;Beckerman et al., 2008).These studies suggest that variability in traffic volume and fleet characteristics, as well as local topography and meteorological conditions impact the NO x concentration (distancedecay) gradients observed amongst different near-road environments.Negative health effects associated with exposure to NO 2 when used as a marker for TRAP include the exacerbation of asthma symptoms, the increased risk of developing cardiovascular and lung diseases, and increased mortality rates.(Andersen et al., 2011;Valari et al., 2011;Pereira et al., 2010;Jerrett et al., 2009;Salam et al., 2008 and references therein;Nafstad et al., 2003).Therefore, an effective means of monitoring the spatiotemporal Introduction

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Full behaviour of NO 2 could do much to support the development of appropriate pollution mitigation strategies.Satellite measurements of tropospheric NO 2 vertical column densities (VCDs) can provide long-term spatiotemporal trends on a global-to-regional scale.The Ozone Monitoring Instrument (OMI) was launched onboard NASA's EOS-Aura satellite on 15 July 2004 and provides daily tropospheric NO 2 column measurements in the uv-vis spectral range of 405-465 nm with the finest resolution of 13 × 24 km 2 at nadir and a local ascending equatorial crossing time of 13:45 (Levelt et al., 2006).Previous studies have extended the utility of OMI tropospheric NO 2 column measurements to validate and improve chemical transport models (e.g. S. Wang et al., 2011;X. Wang et al., 2011;Hains et al., 2010;Huijnen et al., 2010), provide top-down emission estimates of NO x and validate bottom-up emission inventories (Shaiganfar et al., 2011;Lamsal et al., 2010), quantify long-term trends in NO x emissions over continental source regions (Boersma et al., 2008), separate anthropogenic NO 2 emissions from biomass burning events (Mei et al., 2010), estimate the lifetime of tropospheric NO x and account for observed seasonal patterns (Lamsal et al., 2010), infer NO 2 surface concentrations and associated long-term trends (Lee et al., 2011;Lamsal et al., 2008), and analyze the impact of precursor species on surface O 3 formation (Duncan et al., 2010).Although OMI tropospheric NO 2 columns have been applied to inform a variety of objectives, there is still a strong need for independent validation measurements (Vlemmix et al., 2010;Celarier et al., 2008;Irie et al., 2008), especially since previous validation studies have demonstrated that the relationship between OMI and field measurements varies from region-to-region (Hains et al., 2010).On a regional and short-term scale, Ontario's Air Quality Index informs the public about their potential to experience adverse health effects from outdoor air, relying on hourly-average in situ measurements of pollutants such as NO 2 , PM 2. environment.Previous studies have assessed the relationship between ground-based and satellite tropospheric NO 2 VCDs, and established that VCDs derived from localised-point measurements can directly capture near-source emissions of NO 2 , while the large spatial footprint (≥ 312 km 2 ) of satellite measurements make it challenging to retrieve this information.Petritoli et al. (2004) derived tropospheric NO 2 VCDs from ground-based in situ measurements conducted at a Po Valley background site in Gherardi, Italy during 2000-2001 using the assumption of a well-mixed planetary boundary layer, and compared these measurements to tropospheric NO 2 VCDs from the Global Ozone Monitoring Experiment.The relationship between ground-based and satellite columns exhibited seasonal differences with satellite/in situ slope ranging from 0.25 in August to 1.2 in November and December.Slopes less than 1 were shown to be dependent on the fraction of the satellite pixel's coverage of NO 2 source areas.
Ordonez et al. ( 2006) compared GOME satellite measurements of tropospheric NO 2 to ground-based in situ measurements of NO 2 in Lombardy, northern Italy during 1996-2002.NO 2 VCDs were derived using in situ data by using a modelled seasonal tropospheric vertical profile (using MOZART-2; spatial resolution 2.8 × 2.8 • ).A strong agreement was seen between the measurements in relatively unpolluted areas; orthogonal regression yielded a slope near one and a correlation coefficient (R) of 0.78.However, weaker agreements were observed over polluted sites, since the horizontal resolution of GOME (320 × 40 km 2 ) could not isolate regions with high NO 2 concentrations.Multi-axis differential optical absorption spectroscopy (MAX-DOAS) is a relatively new technique used to retrieve tropospheric NO 2 columns that is well-suited for the validation of coincident OMI measurements (Halla et al., 2011;Vlemmix et al., 2010).MAX-DOAS employs intensity measurements of scattered sunlight at a series of elevation angles, relying on the narrowband absorbance structures of NO 2 within the uv/visible wavelength range, to obtain tropospheric vertical column densities (VCDs) of NO 2 .Knowledge of average photon trajectories is essential for resolving NO 2 VCDs from MAX-DOAS spectra, but is an involved task, having previously been accomplished with radiative transfer modeling or trigonometric approximations.MAX-DOAS draws upon the advantages of both localised-point and satellite-based measurements, offering average pollutant concentrations covering a horizontal scale which has been reported to vary from 3 to 11 km (λ = 357 nm, aerosol extinction coefficient within 1 km above ground level varied from 1.02 to 0.05 km −1 ) with a time resolution on the scale of a few minutes (Irie et al., 2011).Previous studies have investigated the relationship between OMI and MAX-DOAS tropospheric NO 2 VCDs (Shaiganfar et al., 2011;Halla et al., 2011;Wagner et al., 2010;Vlemmix et al., 2010;Kramer et al., 2008;Brinksma et al., 2008;Celarier et al., 2008;Irie et al., 2008), and demonstrated that a fair agreement was generally observed.Differences amongst the retrieved tropospheric NO 2 VCDs have been attributed to differences in the spatial resolution of ground and satellite remote sensing techniques, in addition to the vertical sensitivity of each instrument, and the NO 2 VCD retrieval algorithm employed.
The majority of studies suggest that OMI exhibits a positive bias over rural (unpolluted) MAX-DOAS measurement sites, and a negative bias over urban (polluted) MAX-DOAS measurement sites (Shaiganfar et al., 2011;Halla et al., 2011;Kramer et al., 2008;Brinksma et al., 2008;Celarier et al., 2008), despite the use of different OMI products (Standard versus DOMINO Product and versions of these products), MAX-DOAS instrumentation, and NO 2 VCD retrieval algorithms.While the OMI pixel size is ≥ 312 km 2 , the horizontal resolution of the MAX-DOAS instrument decreases Introduction

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Full   Irie et al. (2008) saw an improved agreement between OMI (SP v 3) and MAX-DOAS measurements (N = 4; mean OMI bias +20 ± 8 %, but within the reported OMI uncertainty) at an urban site (Tai'an, China) when the OMI pixel centre was within 0.1 × 0.1 • of the MAX-DOAS measurement site, while a poor agreement was observed when the pixel centre was extended to 0.3 × 0.3 • (N = 10; mean OMI bias +45 ± 38 %).Shaiganfar et al. (2011) performed orthogonal regression using OMI (DP v 1.02) and mobile MAX-DOAS measurements in Delhi, India.When OMI pixels having MAX-DOAS coverage ≥ 50 % were considered, the coefficient of determination (R 2 ) increased from 0.48 to 0.79, but OMI pixels still exhibited a consistent negative bias.This study examines the influence of NO 2 horizontal heterogeneity in an urban en- through comparison with in situ tropospheric NO 2 VCDs, based on data collected in the downtown core of Toronto, Ontario during select periods in 2006-2010.Tropospheric NO 2 VCDs were derived using data from in situ (chemiluminescence) monitors situated near-roadside at 0.01 and 0.5 km above ground level.This is the first study that uses stationary in situ measurements of NO 2 collected near ground level to derive tropospheric NO 2 columns in an urban environment for comparison with remotely-sensed data.
MAX-DOAS NO 2 differential slant column densities (∆SCDs) were converted to tropospheric vertical column densities (VCDs) using the geometric Air Mass Factor (AMF) approximation (H önninger et al., 2004) in conjunction with the single-scattering validation criteria discussed by Halla et al. (2011) andBrinksma et al. (2008).The impact of NO 2 horizontal heterogeneity on the remotely-sensed VCDs was assessed by comparing MAX-DOAS and OMI tropospheric NO 2 VCDs to those derived in situ (and near-road).
This paper is structured as follows: Sect. 2 describes the methodology employed, providing an overview of the downtown Toronto measurement site, along with a description of the in situ, MAX-DOAS, and OMI instruments, and the respective NO 2 VCD retrieval algorithms.Section 3 presents the in situ-derived NO 2 VCD's seasonal trends, and the comparison between point-source and remotely sensed tropospheric NO 2 columns.Conclusions are derived based on the findings from Sect. 3, and presented in Sect. 4. downtown Toronto Site (DT), located 10 m above ground level (MOE, 2011b).A second set of NO x and O 3 monitors (Thermo 49C and 42C) situated at the CN Tower (CN) sampled ambient air at 445 m above ground level and 2.3 km SSE of DT.The MOE's monitoring sites are characterized in Table 1.O 3 data was used to correct NO 2 measurements for interference from NO z , using the procedure reported by Boersma et al. (2009), which is discussed in Sect.2.2.MAX-DOAS measurements were conducted at the University of Toronto's Wallberg and McLennan Physics buildings, and at Toronto's Centre Island, as summarized in Table 2. Azimuth angles were chosen based on the practical constraints of each monitoring site's topography to ensure buildings and trees did not obstruct the MAX-DOAS instrument's field of view.Both MAX-DOAS and in situ measurements of NO 2 provided the opportunity to observe the impact of diurnal traffic patterns and associated meteorology on the spatio-temporal distribution of NO 2 in an urban environment.

In situ NO 2 VCD retrieval
In situ-derived tropospheric NO 2 VCDs (NO 2 VCD in situ ) were calculated assuming NO 2 exhibits an exponentially decaying vertical profile, as shown in Eq. ( 1) where z = 12 km.The value selected to describe the height of the troposphere (z) made no difference to the magnitude of the VCD as z >> H NO 2 .The characteristic height of NO 2 (H NO 2 ) was derived using the in situ NO 2 concentration at DT ([NO 2 ] 10 m ) and at CN ([NO 2 ] 445 m ), as shown in Eq. ( 2), where z = 435 m.An increase in characteristic height indicates that NO 2 occupies a greater vertical fraction of the atmosphere.NO 2 VCD in situ were calculated on a daily basis using data averaged from 12:00-14:00 EST to coincide with the OMI overpass time, and hourly-averaged NO 2 VCD in situ were also compared to coincident MAX-DOAS measurements.

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Full Previous studies have investigated the positive bias associated with the CL detection of NO 2 due to the reduction of other oxidized nitrogen species (NO z = nitric acid: HNO 3 + peroxy-acetyl nitrate: C 2 H 3 NO 5 + peroxyacyl nitrates: C x H y O 3 NO 2 + alkyl nitrates: RONO 2 + nitrous acid: HONO) by the CL monitor's molybdenum (Mo) catalyst, which is unspecific to the reduction of NO 2 -to-NO.The detection of NO z is dependent on the concentration of reactive nitrogen species at the monitoring site, the relative location of emission sources, meteorology, the conversion efficiency of NO z species by the heated molybdenum surface, and their respective line-losses (Lamsal et al., 2008 and references therein).The positive bias associated with the detection of NO z exhibits seasonal and diurnal trends, reaching a maximum in the summer and during the afternoon when NO z constitutes a larger fraction of NO y (NO z + NO x ).These trends have been associated with the photochemical production of reactive NO z species (such as HNO 3 and PAN) alongside O 3 (Lee et al., 2011;Lamsal et al., 2008;Dunlea et al., 2007;Steinbacher et al., 2007).It was hypothesised that the bias would also vary with elevation, and thus that this positive bias should be accounted for.The procedure reported by Boersma et al. (2009), which was based on measurements performed by Dunlea et al. (2007), was implemented to remove the influence of NO z from CL NO 2 measurements ([NO 2 ] CL ) after 10:00 EST, as shown in Eq. ( 3), where [NO 2 ] refers to the corrected measurement.The OMI subscript refers to the average concentration during 12:00-14:00. where

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Full   2, the CN site experienced 24 h average weekday vehicle counts that were over 5 times greater than the DT site.Therefore, an effort was made to filter data by removing hours that were strongly influenced by horizontal NO 2 gradients, and those that did not follow a vertically decaying vertical profile.Overall, 654 of 1426 days (46 %) were maintained for further analysis.

MAX-DOAS instrument and NO 2 VCD retrieval algorithm
Measurements were conducted using a commercially available Mini-MAX-DOAS instrument developed by the Institute of Environmental Physics at the University of Heidelberg in collaboration with Hoffmann Messtechnik GmbH (Bobrowski et al., 2005).This compact unit is comprised of entrance optics, a quartz fibre bundle, a spectrograph, and Peltier cooler enclosed in an airtight metal case.The elevation angle surveyed is controlled by a stepper motor attached to the enclosed unit.The instrument is powered by a rechargeable 12 V battery, and controlled by a laptop via a USB connection.Introduction

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Full Standalone operation of the MAX-DOAS instrument was achieved with the assistance of DOASIS software (Kraus, 2003), and entailed adjusting the appropriate jscript routine by specifying the desired series of elevation and/or azimuth angles to be surveyed.For each direction scanned, the entrance optics (a quartz lens with a field of view of 0.6 • and focal length of 40 mm) collects and focuses scattered sunlight onto the attached quartz fibre bundle.Sunlight is transferred to the entrance slit of a commercial crossed Czerny-Turner Spectrograph (OceanOptics USB2000) with a linear resolution of 2048 pixels covering a wavelength range of 290-433 nm.The optical signal produced by the spectrograph is converted into a digital one via a Sony ILX511 charged-coupled device (CCD).The digital signal consists of light intensity as a function of channel (pixel) for each spectrum.This information is transferred to a laptop for future evaluation via a USB connection.
The exposure time for each spectrum was calculated by the designated jscript routine such that the CCD detector reached 80 % of its saturation level.An integrated average of 1000 spectra was used to create the final spectrum.During the hours of 09:00-16:00, the temporal resolution of measurements varied from approximately 1 to 1.5 min, mainly as a result of solar zenith angle.Around solar noon, a sequence of 6 elevation angles took approximately 8 min to survey.
"Dark current" and "offset" spectra were taken each time the instrument was set up for monitoring and subtracted from recorded spectra during analysis.The spectrometer was stabilized at 5 • C for all measurements, except those conducted during the winter when the spectrometer was maintained at −5 • C to minimize the spectrometer's exposure to temperature instability.MAX-DOAS spectra were processed with DOASIS (Kraus, 2003) and QDOAS (Fayt et al., 2011) software to retrieve NO 2 ∆SCDs.Each spectrum was corrected for dark current and offset using DOASIS.The pixel-to-wavelength calibration was initially performing using a spectrum from a mercury lamp with known emission peaks.A calibration polynomial was generated in DOASIS based on the wavelength-to-pixel mapping of these peaks.QDOAS was then used to refine the wavelength calibration of each Introduction

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Full spectrum while simultaneously characterizing the resolution (expressed as the Full-Width-Half-Maximum; FWHM) of the CCD.Each spectrum was calibrated in QDOAS by performing a non-linear least-squares fit to align the structures of the measured spectrum to those of a high-resolution solar reference spectrum (0.01 nm) (Kurucz et al., 1984) degraded to the resolution of the instrument.The resolution of the CCD (FWHM) was 0.6 nm at wavelengths greater than 380 nm.NO 2 tropospheric ∆SCDs were retrieved from calibrated spectra by applying the DOAS technique (Platt and Stutz, 2008;H önninger et al., 2004;Platt, 1994) using QDOAS (Fayt et al., 2011).The ∆SCD refers to the difference between the average concentration of a trace gas of interest (C) integrated along the average path length (L) traversed by photons prior to entering the spectrometer at elevation angle θ , (CL) θ or SCD θ , and the corresponding observation at an elevation angle of 90 • within a measurement cycle, (CL) 90 • or SCD 90 • , defined in Eqs. ( 3) and ( 4).Since the zenith spectrum within each measurement sequence was utilized as the Fraunhofer Reference Spectrum (FRS), the influence of stratospheric absorption was removed from the fit.
Broadband absorption was accounted for by including a DOAS polynomial of order 3.A first-order offset polynomial was also used in each fit to account for stray light in the 837 Introduction

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Full spectrometer.The Ring Effect (Grainger and Ring, 1962) was accounted for by fitting measured spectra to a synthetic Ring spectrum generated in QDOAS by convolving a high-resolution solar spectrum (0.05 nm) (Kurucz et al., 1984b) with rotational Raman scattering cross-sections (Fayt and van Roozendael, 2001).Table 4 summarizes the fitting parameters used to retrieve the NO 2 ∆SCD.
Fitting errors associated with the MAX-DOAS ∆SCD are indicative of the systematic and random errors associated with the QDOAS retrieval.Systematic errors include incorrect spectral calibration, dark current and offset correction, and slit function characterization, as well as errors in the reference spectra, while random errors may be due to the presence of unknown spectral absorbance structures, and a low signal-to-noise ratio (Vlemmix et al., 2010;Fraser et al., 2009).The QDOAS fitting error generally increased as the magnitude of the ∆SCD decreased.With respect to NO 2 , a retrieved ∆SCD of 1 • 10 17 molec cm −2 had a relative fitting error ≤ ± 2 %, while a retrieved ∆SCD of 2 ± 10 15 molec cm −2 had a relative fitting error ≤ ± 20 %.
Systematic errors associated with the uncertainty, and temperature-dependence of the NO 2 absorbance cross-section have been addressed in previous studies.The measurement uncertainty associated with the NO 2 (294 K) absorption cross-section is ≤ 10 % (Vandaele et al., 1998) < 5 • C. Thus this temperature sensitivity was not accounted for since, based on previous sensitivity studies, it would only have caused on average, an error of a few percent.Certainly, some measurements were collected in March where the average temperature was closer to 273 K and these MAX-DOAS VCDs.may be overestimated by up to 10 %.
The total uncertainty associated with the NO 2 ∆SCD was calculated as the rootsum-of-squares of the QDOAS fit error, absorbance cross-section accuracy, and uncertainty associated with the temperature dependence of the differential cross-section.The total relative uncertainty of the NO 2 ∆SCD varied from 15 % to 25 %.Hourly averaged NO 2 ∆SCDs were calculated to convert MAX-DOAS measurements to the same timescale as in situ measurements.
Hourly averaged geometric NO 2 VCDs were determined by applying the single scattering approximation proposed by H önninger et al. ( 2004) to MAX-DOAS NO 2 ∆SCDs.The NO 2 differential AMF (∆AMF = ∆SCD/VCD) was calculated as shown in Eq. ( 5), and verified by using the criteria NO 2 VCD 10 • = VCD 20 • (or 30 • ) ± 15 % (Halla et al. 2011;Brinksma et al., 2008) to ensure tropospheric photon scattering occurred above the NO 2 column.Using this criterion resulted in 113 of the 169 available hourly MAX-DOAS NO 2 ∆SCDs being excluded (Table 5).Thus, 56 h (33 %) of MAX-DOAS NO 2 ∆SCDs were converted to geometric VCDs and from these time periods, 37 corresponding hours of in situ data were available for comparison.
Previous studies have compared NO 2 VCDs retrieved using radiative transfer modeling to those calculated using the geometric ∆AMF (Wagner et al., 2010(Wagner et al., , 2011;;Halla et al. 2011;Shaiganfar et al., 2011;Vlemmix et al., 2010).(R 2 = 0.88), 0.96 (R 2 = 0.96), and 0.92 (R 2 = 0.86), respectively.Although no relationship between the geometric NO 2 VCD's negative bias and aerosol optical depth (AOD) was found, this bias exhibited a systematic dependency on the vertical distribution of NO 2 .This systematic dependency is anticipated since the geometric ∆AMF assumes that the tropospheric NO 2 column is contained below the last scattering altitude of photons.Wagner et al. (2011) predicted that for a NO 2 column below 1000 m, the error of the geometric NO 2 VCD is typically within 20 %.Shaiganfar et al. (2011) demonstrated that geometric NO 2 VCDs at elevation angles of 22 • and 30 • differed from those determined using McArtim by ± 20 % when assuming a 500 m vertical NO 2 box profile (AOD < 1).The geometric NO 2 VCD also exhibited a systematic dependency on relative azimuth angle (difference between the solar azimuth angle and MAX-DOAS viewing direction): the geometric VCD underestimated the true VCD at low relative azimuth angles (∼ 0 • ) and overestimated the VCD at higher relative azimuth angles (≥ 90 • ).Vlemmix et al. (2010) demonstrated that, for relative azimuth angles greater than 30 • , the difference between the geometric VCD at an elevation angle of 30 (NO 2 VCD 10 • = VCD 20 • (or 30 • ) ± 15 %), it is estimated that the uncertainty associated with the geometric NO 2 ∆AMF is ≤ 15 %.The total relative uncertainty associated with the NO 2 VCD was calculated as the root-sum-of-squares of the random and systematic uncertainties associated with the NO 2 ∆SCD and the geometric NO 2 ∆AMF.The total relative uncertainty of the MAX-DOAS NO 2 VCD retrieval ranged from 20 to 29 %.

OMI tropospheric NO 2 columns
Dutch OMI NO 2 (DOMINO Collection 3 version 1.02) tropospheric columns were obtained from the Tropospheric Emissions Monitoring Internet Service (TEMIS) (Boersma et al., 2007) for the period of March 2006-March 2010.The DOMINO Product (DP) employs the DOAS algorithm (Platt, 1994) to retrieve a NO 2 slant column density (SCD) from a measured spectrum of backscattered solar radiation for the 405-465 nm wavelength range (Bucsela et al., 2006).The stratospheric NO 2 SCD is determined by assimilating the total NO 2 SCD into the TM4 global chemistry and transport model (CTM) (Dirksen et al., 2011).The stratospheric NO 2 SCD is then subtracted from the total NO 2 SCD to determine the tropospheric SCD.The AMF is obtained from the DAK radiative transfer model (Stammes, 2001), and relies on the a-priori tropospheric NO 2 profile simulated in TM4, in addition to parameters including cloud radiance fraction, cloud pressure, surface albedo, and satellite viewing geometry.The tropospheric AMF permits the conversion of the tropospheric NO 2 SCD to the VCD (Boersma et al., 2007).Under polluted conditions, the tropospheric SCD has an absolute uncertainty of approximately 1 × 10 15 molec cm −2 (Boersma et al., 2007(Boersma et al., , 2009)), while the AMF has a relative uncertainty ranging from 10-40 % (Hains et al., 2010;Boersma et al., 2009).The coincidence criteria used to pair OMI tropospheric NO 2 columns with independent measurements was informed by previous studies, which have constrained the sample size according to OMI cloud radiance fraction, pixel size, proximity of pixel centre to independent monitoring site, and overlap of the independent data's averaging interval with OMI overpass time (Lee et al., 2011;Hains et al., 2010;Kramer et al., 2008;Irie et al., 2008).Table 5 summarizes the criteria and coincident hours of in situ Introduction

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Full data that were used in this study.Results are presented in this study for data that obeyed criteria 2 to 3 and 2 to 4. Overall, of the 55 OMI overpasses with a cloud radiance fraction ≤ 0.3 and pixel centre within 0.1 × 0.1 • of the MAX-DOAS measurement site, 9 coincided with MAX-DOAS NO 2 VCD measurements within ± 3 h.

Impact of NO z interference on chemiluminescence measurement of NO 2
Table 6 illustrates the impact of removing the NO z interference from the CL measurement of NO 2 .This correction reduced the CL NO 2 concentration at DT by 8 ± 1 % and at CN by 12 ± 1 %, suggesting, but not proving, that the need to correct for the NO z interference associated with the CL detection of NO 2 did vary with elevation.Further, the larger NO z interference correction at the higher elevation suggested that the correction is greater for aged air masses, consistent with observations by Boersma et al. (2009) and Steinbacher et al. (2007).The larger NO z interference correction at the higher elevation suggests that there is a greater concentration of reactive nitrogen species at this altitude, as Dunlea et al. (2007) demonstrated that the positive and linear relationship with [NO z ] and [O 3 ] is due to the photochemical production of reactive nitrogen species alongside O 3 .
The absolute decrease in the NO 2 concentration after accounting for the NO z interference is less than 1 ppb at both DT and CN, which is similar to observations in rural Southwestern Ontario by Lee et al. (2011), who demonstrated that the median difference between the CL and true NO 2 was only 0.9 ppb.Overall, the NO 2 characteristic height decreased by an average of 30 m, and the NO 2 VCD in situ decreased by less than 1 × 10 15 molec cm −2 .Introduction

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Full The seasonal variation of the in situ derived tropospheric NO 2 VCDs is shown in Fig. 1.The monthly averaged tropospheric NO 2 VCD reaches a maximum during winter (January, February, and December), and a minimum during summer (June, July, and August).This seasonal trend is attributed to the increased photolysis during the summer.
The photolysis of NO 2 decreases the concentration of NO 2 relative to NO (Seinfeld and Pandis, 2006).The increased photolysis of hydroxyl radical (OH) precursor species (such as O 3 ) also decreases the summertime NO 2 concentration via its reaction with OH to yield nitric acid (HNO 3 ) (Seinfeld and Pandis, 2006;Jacob, 1999).Furthermore, an increase in anthropogenic NO x emissions during winter months due to residential heating may also increase the wintertime NO 2 VCD.These results are consistent with observations of seasonal NO 2 VCD trends in Lombardy, northern Italy by Ordonez et al. (2006), who combined daily ground-based in situ NO 2 concentrations with modelled tropospheric vertical profiles (using MOZART-2; spatial resolution 2.8 × 2.8 • ).
The seasonal variation of the in situ derived NO 2 characteristic height is shown in Fig. 2.These characteristic heights for NO 2 are below the expected midday mixing height, which may reach 2 km in the summer.Overall, the NO 2 characteristic height varies insignificantly within a given season.However statistically significant differences are seen when comparing winter (colder) months to warmer months.January and February show the greatest NO 2 characteristic heights of 0.71 ± 0.08 km and 0.78 ± 0.08 km, respectively (average temperature −2.3 ± 1.8 • C and −3.9 ± 1.

Comparison between in situ and remotely-sensed NO 2 VCDs
Figure 3a shows the linear regression of the OMI versus in situ tropospheric NO 2 VCD.
Fig. 3b only considers a subset of these data, overpasses when the OMI pixel area was ≤ 600 km 2 .In both figures, OMI exhibits a negative bias when compared to the in situ NO 2 VCD, however this bias decreases from 36 % to 21 %, and the Pearson R increases from 0.64 to 0.77 when a smaller OMI pixel area is considered.These results are consistent with the spatial foot print represented by these two types of measurements.The in situ monitors measure a point-location "near-roadside" NO 2 column, while the OMI NO 2 VCD reflects an average over the spatially heterogeneous Greater Toronto Area, consisting of roads, residential neighbourhoods, and Lake Ontario.Using a stricter coincidence criterion yielded an improved agreement, since a larger fraction of the OMI pixel was able to capture the downtown Toronto core.
Although version 1.02 of the DOMINO tropospheric NO 2 VCD was used for this analysis, the conclusions are anticipated to be applicable with the recently released version 2.0.Over polluted sites, DOMINO v 2.0 tropospheric NO 2 VCDs are reported to be 20 % less than the v 1.02 values during the winter, and 10 % less during the summer (Boersma et al., 2011).Of the 55 days of coincident OMI and in situ NO 2 VCDs plotted in Fig. 3b, 13 measurements are during the summer, and 15 are during the winter.The linear regression of these 28 days yield y = 0.83x and R = 0.79, which indicates that OMI has a negative bias of 17 %, when averaged over all seasons.By scaling the winter v 1.02 NO 2 VCDs by a factor of 0.8 and the summer data by a factor of 0.9, the resulting regression is y = 0.67x, while the Pearson R remains unchanged.Therefore, the satellite measurement's negative bias with respect to the in-situ measurements may be larger than that suggested by Fig. 3b  accurate than v1.02 values.These results would still be consistent with the spatial foot print represented by the OMI and in-situ measurements.
Figure 4 shows the linear regression results of the MAX-DOAS versus in situ tropospheric NO 2 VCD, and although a good agreement is seen (Pearson R = 0.79), the MAX-DOAS NO 2 VCD is only 45 % of the corresponding value determined using the in situ monitors.The difference between the MAX-DOAS versus in situ derived NO 2 VCD indicates that differences in the geographic footprint surveyed by each instrument impacted the results.As summarized in Table 2, much of the MAX-DOAS path length was over Lake Ontario during campaigns A, D and E, facing east for campaign C, and was located 65 m above ground-level in campaigns C and D. During all campaigns, the MAX-DOAS instrument's path length was also influenced by the NO 2 concentration in residential areas in the downtown core.
The spatial heterogeneity of the NO 2 concentration across Toronto was evaluated by Jerrett et al. (2007Jerrett et al. ( , 2009)).Passive sampling measurements of NO 2 concentration at 143 sites in the early fall of 2002 and spring of 2004 were used in conjunction with land-use regression modeling to derive a NO 2 surface concentration map.Despite changes in absolute NO 2 concentration between seasons, overall spatial patterns remained similar: NO 2 exhibited higher concentrations in the west (> 20 ppb), and lower concentrations in the east (10-15 ppb).The downtown core and areas near major highways also exhibited high NO 2 concentrations (> 30 ppb).The MAX-DOAS path length over mainland Toronto consists of a combination of urban background and roadside NO 2 concentrations, and the MAX-DOAS NO 2 VCD is dependent on these gradients.
Figure 5 shows the linear regression results of OMI versus MAX-DOAS tropospheric NO 2 VCDs.A good agreement is seen between the measurement techniques, and the correlation coefficient is R = 0.85.The slope of 1.12 suggests that the OMI tropospheric NO 2 VCD is biased high when compared to MAX-DOAS measurements.However, this slope is not significantly different than 1.0 indicating that the OMI and MAX DOAS VCD do not differ beyond the uncertainties of these data.This slope suggests that the MAX DOAS and OMI are both "looking at" similar mixes of the spatial heterogeneity in NO 2

AMTD Introduction
Full concentrations created by the unpolluted regions over the lake and polluted regions over the city.Thus this finding is indirectly consistent with the majority of previous studies which suggest that OMI exhibits a positive bias over rural (unpolluted) MAX-DOAS measurement sites, and a negative bias over urban (polluted) MAX-DOAS measurement sites (Shaiganfar et al., 2011;Halla et al., 2011;Wagner et al., 2010;Kramer et al., 2008;Brinksma et al., 2008;Celarier et al., 2008).In the heterogeneous polluted and unpolluted environment of the current study, the two remote sensing methods yield similar vertical columns.More importantly, when these ground and satellite based measurement methods "look at" the same mixture of environments they agree, providing reassurance in the validity of the measurements produced by each method.
The conclusions resolved from Fig. 5 should still be applicable if DOMINO version 2.0.were used instead to obtain NO 2 VCDs.Repeating the correlation shown in Fig. 5 using only the summer and winter data yields a slope of y = 1.06x (n = 8), and a correlation coefficient of R = 0.88.When the winter scaling factor of 0.8 and the summer scaling factor of 0.9 (Boersma et al., 2011) are applied to the v 1.02 OMI data, the regression slope of the OMI and MAX-DOAS measurements decreases to y = 0.88, and the correlation coefficient is R = 0.84.This suggests that using an improved satellite data product may result in OMI measurements being biased low when compared to MAX-DOAS tropospheric NO 2 VCD observations.However, as discussed, in all cases the slopes are not significantly different from 1 when the uncertainties in these measurements are considered.More generally, these results are in agreement with the analysis by Irie et al. (2012), who demonstrated that the DOMINO v 2.0 tropospheric NO 2 VCD product exhibits a negative bias of 8 ± 14 % with respect to MAX-DOAS measurements, and concluded this bias to be insignificant.

Conclusions
In situ measurements of NO 2 in an urban environment were compared with remotely sensed satellite, and multi-axis differential optical absorption spectroscopy Introduction

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Full (MAX-DOAS), tropospheric NO 2 vertical column densities (VCDs).The chemiluminescence measurements were first corrected for the influence of NO z , which reduced the NO 2 concentrations at the near ground level and 445 m by 8 ± 1 % and 12 ± 1 %, respectively.The absolute decrease in the chemiluminescence NO 2 measurements as a result of this correction was less than 1 ppb.
Good agreement was observed between the remotely sensed and in situ NO 2 VCDs (Pearson R ranging from 0.64 to 0.79).However, the in situ VCDs were 27 % to 55 % greater than the remotely-sensed columns due to horizontal spatial heterogeneity.The in situ NO 2 VCD were representative of a local NO 2 column in a polluted near-road environment, while the remotely-sensed (MAX-DOAS and OMI) VCDs were representative of a spatial heterogeneous region, which included the downtown city core, residential neighbourhoods, and Lake Ontario.Overall the reasonable agreement between the VCD values determined by the three distinct methods increased confidence in the validity of the values provided by each of the methods.Introduction

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Full  3). 2  3). 5 Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 5 and O 3 (MOE, 2010).In downtown Toronto, NO x monitoring stations situated at 0.01 and 0.5 km above ground level (43.663 • N, −79.388 • W and 43.642 • N, 79.387 • W) provide the opportunity to derive tropospheric vertical profiles of NO 2 in a near-road Discussion Paper | Discussion Paper | Discussion Paper | Boersma et al. (2009) compared satellite measurements of tropospheric NO 2 from OMI to in situ-derived VCDs from 8 cities in Israel during 2006 assuming NO 2 was wellmixed in the boundary layer and negligible above this height.In situ measurement sites directly influenced by local pollutant sources showed poorest agreements with OMI observations (OMI VCD/in situ measurement orthogonal regression slopes of 0.30 and 0.59 × 10 molec cm 2 ppb −1 , and correlation coefficients of 0.65 and 0.54, respectively).Improved agreements were observed between OMI and in situ-derived VCDs from the 6 less-polluted sites (slope = 0.93; R = 0.63).Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | vironment on VCDs determined by three methods of differing spatial scales.The goal of this study was to evaluate the response of remotely-sensed NO 2 measurements to roadside emissions of NO 2 .Specifically remotely sensed tropospheric NO 2 VCD measurements from OMI (DOMINO Product, version 1.02) and MAX-DOAS were evaluated Discussion Paper | Discussion Paper | Discussion Paper | of downtown Toronto measurement sites In situ pollution data was obtained for the period of March 2006-March 2010 from NO x chemiluminescence (CL), and O 3 ultraviolet photometry monitors (Thermo 42i and 49i) situated at the Ontario Ministry of the Environment (MOE) Air Quality Network's Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | . With respect to NO 2 ,Vandaele et al. (1998) observed a 20 % decrease in retrieved stratospheric NO 2 slant columns when using a NO 2 absorption cross-section measured at 220 K instead of 294 K.Vlemmix et al. (2011) demonstrated that NO 2 ∆SCDs measured using a NO 2 absorbance cross-section at 295 K should be corrected by a factor of 0.92 for an effective atmospheric temperature of 283 K (determined by considering the vertical temperature profile and the vertical distribution of NO 2 in the lower troposphere).Irie et al. (2012) reported an 11 % decrease in the NO 2 VCD by scaling it with a vertical temperature profile that decreased to 260 K at 2 km.The majority of the MAX-DOAS measurements in the current study were collected in the summer, where the characteristic height of NO 2 was typically only 500 m.Any vertical variation in the NO 2 absorbance cross-section within this region would have been small, as the relevant vertical variation of temperature was presumably Discussion Paper | Discussion Paper | Discussion Paper | Wagner et al. (2011) converted NO 2 ∆SCDs measured at an elevation angle of 18 • in south, north, and west viewing directions to NO 2 VCDs using the geometric approximation (VCD geo ) and the radiative transfer model McArtim (VCD RTM ; λ = 360 nm).The VCD geo /VCD RTM slopes were 0.88 Discussion Paper | Discussion Paper | Discussion Paper | = 428 nm) reaches a maximum of 25 %, and this offset is strongly a function of solar position relative to the MAX-DOAS instrument, as well as AOD.The above studies did not consider the use of validation criteria in conjunction with the geometric ∆AMF to ensure the NO 2 vertical column was contained below the last scattering altitude of photons into the MAX-DOAS detector.Halla et al. (2011) compared geometrically approximated NO 2 VCDs that met the criteria VCD 10 • = VCD 30 • (± 15 %) with those determined using the radiative transfer model McArtim (λ = 413 nm; assuming a vertical NO 2 box profile), and demonstrated geometric VCDs underestimated modeled VCDs by 8-12 %.Wagner et al. (2010) suggest that the strong agreement observed between geometrically-derived NO 2 VCDs retrieved at multiple elevation angles (22 • and 40 • ) renders the uncertainty involved in using geometrically approximated VCDs associated with aerosol loading < 15 %.Since a similar criterion for employing the geometric ∆AMF was used in this study AMTD Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 4 • C), while minimum values are observed during the spring and summer (i.e. the characteristic height during July is 0.51 ± 0.08 km and the temperature is 24.7 ± 0.9 • C).The increased NO 2 characteristic height during colder months indicates that [NO 2 ] (445 m) /[NO 2 ] (10 m) is greater during the winter than the summer, consistent with the increased lifetime of NO 2 during winter months, resulting in increased vertical homogeneity.For example, the monthly average ratio of the NO 2 concentration at 445 m to the surface, varied from 0.52 ± 0.04 during the coldest month, February Discussion Paper | Discussion Paper | Discussion Paper | to 0.34 ± 0.04 during the warmest month, July.More generally, the monthly average [NO 2 ] (445 m) /[NO 2 ] (10 m) exhibited a negative linear dependence on the monthly average temperature, with Pearson R = 0.83).
if DOMINO v 2.0 NO 2 VCDs are more Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |
Boersma et al. (2009)applied Eq. (3) to correct CL NO 2 measurements at 8 cities inIsrael (2006)during the OMI overpass time(13:45 LT).This reduced the CL NO 2 measurement by an average of 8 % (0.7 ppb) during 2006.This approach was validated by using the CHIMERE chemistry-transport model to determine the NO 2 -to-NO z ratio over Europe, which was typically greater than 90 % for cities exhibiting a similar NO 2 concentration as those in Israel (0-25 ppb).Boersma et al. (2009)demonstrated the NO 2 -to-NO z ratio using Eq.(3) for January, April, July, and October was 0.99, 0.98, 0.95, and 0.98, respectively, which was in excellent agreement with CHIMERE NO 2 -to-NO z ratios of 0.98, 0.96, 0.92, and 0.97.Table3summarizes the inclusion criteria applied to the in situ NO 2 data during March 2006-March 2010, and corresponding sample size for measurements coinciding with the OMI overpass time.Since the in situ monitors were located approximately 2.3 km apart from one another, differences in the NO x emission characteristics at each measurement site may have influenced the observed NO 2 concentration at 10 m and 0.5 km.As shown in Table