Interactive comment on “ Evidence for an unidentified ground-level source of formaldehyde in the Po Valley with potential implications for ozone production ”

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Introduction
Stagnant air masses, abundant solar radiation, and high anthropogenic emissions make Northern Italy's Po Valley one of Europe's most polluted regions.Previous measurements have shown that the regional O 3 background can reach as high as 90 ppb (Liu et al., 2007).Photochemical ozone production is tied to the reactions of NO x (NO + NO 2 ), HO x (OH + HO 2 ), and volatile organic compounds (VOCs).In the troposphere, NO 2 photodissociates to form oxygen atoms (Reaction R1), which then react Figures

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Full with molecular oxygen to generate O 3 (Reaction R2).The partitioning of NO x between NO and NO 2 determines the production rate of O 3 .The hydroxyl-radical (OH) initiated oxidation of VOCs creates peroxy radicals (XO 2 = HO 2 + RO 2 ) (Reaction R3).These radicals shift the partitioning of NO x radicals towards NO 2 (Reaction R4), thus increasing the net ozone production rate.
In this analysis, we define the net ozone production rate (P(O 3 )) as the calculated difference between the NO 2 photolysis rate (Reaction R1) and the rate of NO to NO 2 conversion by O 3 .While measuring all VOCs and their oxidation products is non-trivial, formaldehyde (HCHO) is formed in the oxidation of nearly every VOC and thus provides a downstream constraint on this chemistry.In addition, photolysis of HCHO constitutes an important source of HO 2 radicals without consuming OH, effectively accelerating O 3 production via Reaction (R4) followed by Reactions (R1) and (R2).
In 2002-2003, the FORMAT (FORMaldehyde as A Tracer of oxidation in the troposphere) campaign aimed to use HCHO to trace the effect of VOC oxidation on ozone production in the Po Valley.Though modeling efforts focused primarily on the Milan urban plume, an agricultural region upwind of the city was also investigated in the 2003 FORMAT study (Liu et al., 2007).There, HCHO mixing ratios were up to two times higher than those predicted by regional chemistry transport models.Primary emissions were estimated to be a minor source of HCHO in the agricultural region (∼ 10 %), and OH-initiated oxidation of underrepresented local biogenic or anthropogenic VOC emissions was cited as the likely cause of underpredicted HCHO.Because the morning increase in HCHO was not well represented, and because the regional background Figures

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Full was not well understood, the effect of anthropogenic emissions on the diurnal cycle under polluted conditions could not be reproduced by the model (Junkermann, 2009).
While HCHO measurements provide a product-based view of VOC oxidation, direct measurements of OH reactivity, the inverse lifetime of OH, can provide further insight into the instantaneous VOC oxidation rate.OH reactivity is calculated as where k X i +OH are the rate coefficients for the reaction of all species X with OH.Field measurements of OH reactivity have been available since 2001, and Edwards et al. (2013) and Lou et al. (2010) provide summaries of recent comparisons of modeled and measured reactivity in a variety of environments.Notably, measurements in Paris demonstrated that more than half of the measured reactivity in highly aged continental air masses could not be explained by available measurements (Dolgorouky et al., 2012).The authors concluded the missing OH sink was likely (multi)oxidized compounds from processed anthropogenic emissions.Previous work has examined the effect of discrepancies between modeled and measured OH reactivity on calculated O 3 production potentials (Sadanaga et al., 2005;Yoshino et al., 2012).In one study in Tokyo, including unmeasured VOC precursors indicated by OH reactivity measurements increased the calculated ozone production potential by as much as 8 ppb h trace gas measurements were acquired (Li et al., 2014).The Zeppelin's unique flight abilities enabled vertical profiling flight tracks from as low as 50 m up to an altitude of ∼ 750 m, making possible assessment of the role of exchange between the nocturnal boundary layer, residual layer, and growing mixed layer.In this study we focus the analysis on one flight for which a clear delineation between those layers occurred (Li et al., 2014).Using a 1-D chemical transport model, we examine the structure and chemical evolution of HCHO vertical profiles.By combining measurements of OH reactivity and VOC precursors, we investigate sources of HCHO in the agricultural regions of the Po-Valley.Finally, we discuss the effects of HCHO sources on calculated ozone production rates as a function of time and altitude.

Zeppelin NT payload and 12 July flight
The Zeppelin NT platform, its scientific payload, and the 12 July flight have been described previously (Li et al., 2014) and are described briefly here.Between 05:30 LT (Local Time = UTC + 2 h) and 10:45 LT, the airship performed a series of near-surface vertical spirals starting at 50 m and reaching ∼ 750 m a.s.l.(Fig. 1).The airship spiraled upward for ∼ 15 min and then returned to lower altitudes within 5 min.The spirals were performed near a ground-based field site at San Pietro Capofiume (SPC, 44 The instrumentation, time resolution, accuracy, and precision of the measurements are fully described in Li et al. (2014) and are summarized here (Table 1).Specifically, HCHO was measured at the Zeppelin nose boom using Fiber Laser-Induced Figures

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Full Fluorescence (FILIF) (Hottle et al., 2009;DiGangi et al., 2011;Kaiser et al., 2014).The time resolution, precision, and accuracy of the measurement are 1 s, 20-200 ppt, and 15 %, respectively.OH reactivity was measured from a platform on top of the Zeppelin by flash photolysis of ozone combined with time resolved OH detection in a flow tube.The instrument is an improved, more compact version of the instrument described by Lou et al. (2010).The accuracy of the OH reactivity data is 10 %, with ±0.5 s −1 systematic error of the zero-air decay rate coefficients (Gomm, 2014).Speciated C4-C11 VOCs, acetonitrile, and select oxygenated VOCs were measured by a fast gas chromatograph/mass spectrometer system with a time resolution of 180 s and 1σ precision between 3 and 10 % (Jäger, 2014).In addition, OH, HO 2 , NO, NO 2 , O 3 , CO, HONO, particle concentration/size distribution, solar actinic flux densities, temperature, pressure, relative humidity, and 3-D wind were measured simultaneously.

Model simulations
The Chemistry of Atmosphere-Forest Exchange (CAFE) model is a 1-D chemical transport model which has previously been used in steady-state analysis of trace gas fluxes above a pine forest (Wolfe and Thornton, 2011;Wolfe et al., 2011).For this study, the CAFE framework has been adapted to run in a time-dependent manner, and as the region is not forested, no canopy structure is included.The chemical mechanism generated by the Master Chemical Mechanism (MCM) v3.2 (Saunders et al., 2003;Jenkin et al., 1997)  In the base case scenario, the model is constrained to all measurements with the exception of HCHO.Given the extensive constraints, deposition, emission, and advection are not treated explicitly.Because deposition can be a non-negligible sink for many oxidized species (including HCHO), model results represent an upper limit on calculated mixing ratios.Turbulent diffusion is represented using K-theory, where diffusion coefficients are calculated using the Single Column chemistry and climate Model ECHAM4(SCM) (Ganzeveld et al., 2002).Further discussion of the eddy diffusion coefficient, uncertainty associated with turbulent diffusion, and the potential influence of deposition is available in the Supplement.
To initialize non-measured species (e.g.speciated RO 2 and organic nitrates), a "spinup" 0-D diurnal model run was performed constraining all species to a combination of the lowest altitude Zeppelin measurements and available measurements from the nearby SPC ground site acquired between 12 June to 10 July 2012.This includes average diurnal profiles of CO, O 3 , benzene, toluene, NO x , and relevant meteorological parameters.The global background of 1760 ppb methane was assumed.Anthropogenic VOCs were scaled to ground benzene measurements according their observed relationship with benzene measurements acquired on the Zeppelin.To mimic the temperature dependence of isoprene emission rates, isoprene mixing ratios were assumed to be proportional to the cosine of the solar zenith angle and scaled to available Zeppelin measurements.As isoprene concentrations are small (< 100 ppt), the diurnal cycle has a negligible impact on modeled results.For 1-D model simulations, non-measured species are initialized to the output of the spin-up model at 06:00 LT on the 4th day scaled to HCHO measured on the Zeppelin as a function of altitude.Introduction

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Full 3 Results and discussion

Observed HCHO and OH reactivity
In the following section, we present observations of HCHO, VOCs, and OH reactivity acquired on the 12 July flight in the context of previous Po Valley measurements.
A detailed presentation of additional trace gas measurements (notably HONO, NO x , O 3 , HO 2 , and H 2 O) as well as discussion of the delineation between residual layer, nocturnal boundary layer, and mixed layer can be found in Li et al. (2014).
Figure 2 shows measured HCHO, OH reactivity, and selected VOCs as a function of time and altitude.Primary biogenic VOC concentrations were low throughout the entire flight (isoprene < 60 ppt), which is consistent with previous measurements at Verzago, an agriculture site downwind of Milan (Steinbacher et al., 2005a).Anthropogenic VOCs such as toluene and benzene were around an order of magnitude lower than at Verzago in 2003 (Steinbacher et al., 2005b).In contrast to primary biogenic and anthropogenic VOCs, oxidized VOCs were abundant (reaching HCHO > 3.8 ppb, acetaldehyde > 1.0 ppb).The overall magnitude and morning rise of HCHO observed were similar to those observed previously in Spessa in 2002 (Junkermann, 2009).
Before sunrise, elevated levels of toluene and other anthropogenic VOCs were observed in the residual layer compared to lower altitudes.Accumulation of VOC oxidation products including HCHO, methacrolein, and acetaldehyde was observed in the nocturnal boundary and in the residual layer.These oxidation products are either built up overnight or remain elevated from the previous day.After sunrise (05:45 LT), both biogenic and anthropogenic primary VOC increase in the developing mixed layer.The observed increase in HCHO mixing ratios lags that of primary VOCs, so that higher HCHO concentrations were observed ∼

Base scenario modelled OH reactivity and HCHO
The top panel of Fig. 3 shows the measured and modeled OH reactivity as a function of time and altitude.Overall, the magnitude and vertical structure is well captured by measured VOCs and their oxidation products.Where underestimated, the average discrepancy is less than 10 %, with larger discrepancies at lower altitudes.Speciated model contributions to OH reactivity are shown in Fig. 4, calculated with all species (including HCHO) constrained to observations.Largely, OH reactivity was dominated by NO x and CO, with VOCs contributing more significantly in the mixed layer.Of all VOCs, HCHO consistently contributes the largest portion of calculated OH reactivity (HCHO reactivity ∼ 0.2 s −1 ppb −1 ).
Figure 3 also shows measured and modeled HCHO.Before 09:00 LT, base case modeled HCHO matches measurements quite well.This is expected, as the model is initialized to measured HCHO mixing ratios, and low OH concentrations as well as lack of photolysis lead to very little change.Model/measurement discrepancy grows with time and is largest at low altitudes.Between 06:32 LT and 10:06 LT, HCHO increases by as much as 1.3 ppb, while the model predicts no net increase.HCHO loss terms are unlikely to be overestimated as they are constrained by measured OH and measured HCHO photolysis frequencies, and could potentially be underestimated by neglecting deposition (see Supplement).This finding implies that the model is missing either chemical HCHO production, advection of HCHO, or a local source of direct HCHO emissions.

Potential sources of HCHO
The oxidation of additional non-measured VOCs is often cited as a possible source of missing HCHO production in models (compare to FORMAT study, Junkermann, 2009).Using OH reactivity measurements, one can place an upper bound on the overall VOC oxidation rate in the atmosphere.As discussed above, measured VOCs and their modeled oxidation products explain the majority of observed OH reactivity, though a small Figures

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Full discrepancy is occasionally observed.To investigate the possibility of non-measured HCHO precursors, an additional model scenario is constructed in which the missing OH reactivity is assumed to be comprised entirely of ethene (C 2 H 4 ).C 2 H 4 was chosen as a surrogate species because it produces HCHO from OH and O 3 oxidation with respective yields of 160 % (Niki et al., 1981) and 154 % (Alam et al., 2011).Thus, the increase in modeled HCHO per increase in calculated OH reactivity is maximized.
Figure 3 shows the effect of increasing C 2 H 4 on HCHO and OH reactivity.While measured mixed-layer HCHO increases by as much as 1.3 ppb between 06:30 and 10:00 LT, model HCHO increases by only 330 ppt.In order to generate the required HCHO, modeled C 2 H 4 would need to be increased such that calculated OH reactivity is up to 45 % greater than the measurements.While the model vertical profile at 10:40 LT is in better agreement with measurements, at the 09:24 LT vertical profile additional VOC precursors can explain no more than 0.42 ppb, or 33 %, of the missing HCHO budget.We therefore conclude that non-measured VOCs cannot explain the discrepancy in measured and modeled HCHO.
Another possible source of HCHO is transport from nearby urban centers.In the early morning, the average wind speed was less than 1.2 m s −1 , and the average HCHO lifetime was ∼ 3.5 h.Between 06:00 and 10:30 LT, the bottom most layer in contact with the surface grows from less than 50 m to more than 600 m in height.Accounting for this dilution, and the HCHO lifetime and wind speed, and assuming a nighttime concentration in Bologna of 6 ppb (near the maximum nocturnal concentration reported in Milan, Junkermann, 2009), the amount of HCHO advected could be no more than 90 ppt, or 7 % of the missing HCHO budget.Additionally, no other long-lived tracers of anthropogenic influence (i.e.CO, xylenes) show a rise in the late morning.Finally, the vertical profile of the missing HCHO suggests a strong source near the ground which is convectively incorporated into the growing mixed layer.As advection of HCHO, e.g., from Bologna, would more likely affect the mixed layer as a whole, transport is an unlikely source of HCHO.Introduction

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Full As the air aloft initially has slightly elevated levels of HCHO, entrainment of air from the residual layer into the mixed layer is an addition potential source of HCHO.Using ECHAM4(SCM) to investigate observations from the October 2005 field campaign over the Atlantic Ocean, French Guyana and Suriname, Ganzeveld et al. (2008) demonstrated the assessment of daytime HCHO requires a thorough evaluation of the morning turbulent transport.The model predicted entrainment of HCHO would affect the daytime radical budget and resulting oxidative chemistry; however, limited observations in the residual layer did not allow for comparing SCM simulations with measurements.If entrainment was the primary cause of measurement and model discrepancy, the missing HCHO would be larger near the top of the boundary layer and when HCHO concentrations aloft are the highest.In this study, the largest discrepancies occur at the lowest altitudes and later in the morning.The highly resolved vertical measurements enabled by the Zeppelin aircraft demonstrate that for this study, entrainment is unlikely to be the primary cause of model/measurement discrepancies at low altitudes.
An additional potential source of HCHO is local direct emission from biomass burning or other anthropogenic activities.Aircraft measurements in 2003 showed evidence of biomass burning contribution to elevated HCHO in the agricultural regions of the Po Valley (Junkermann, 2009); however, these measurements were in September and October after the harvesting of the rice fields, and we did not see such strong local sources during the flight.Acetonitrile, a tracer of biomass burning, remains at a background levels of < 250 ppt.As CO is a product of incomplete fuel combustion, it can be used to trace the influence of local traffic.CO does not increase significantly during the time HCHO increases in the mixed layer (Fig. 5).Using an emission ratio of 3.14 g HCHO (kg CO) −1 observed at a highway junction in Houston, Texas (Rappenglueck et al., 2013), the increase of 19 ppb in CO between 06:20 and 10:00 LT, if wholly from traffic emissions, could account for only 57 ppt (4 %) of the observed increase in HCHO.
We therefore conclude neither biomass burning nor traffic can account for the relatively high levels of observed HCHO.Introduction

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Full Finally, the soil, decaying plant matter from harvesting, or wheat or other crops in the region of the Zeppelin spirals may be a source of local direct HCHO emission.Measurements of oxygenated VOCs (OVOCs) from agricultural crops are limited.Konig et al. (1995) reported total OVOC emission rates from wheat were of 10.9 ng g −1 h −1 , though speciated measurements of formaldehyde were not available.
Dry weight HCHO emission rates from tree species in Italy are much higher, ranging from 382 to 590 ng g −1 h −1 (Kesselmeier et al., 1997).Oxygenated VOC emissions are expected to respond differently to light and temperature than terpenoids (Rinne et al., 2007), nevertheless the classic terpenoid exponential model is often extended to OVOC emissions.For example, for ground emissions of HCHO in a ponderosa pine for- emission rate for all oxygenated VOCs independent of land use or plant type (Karl et al., 2009).To realize the full potential of these oxidized VOCs as tracers of the photochemistry that forms secondary pollutants, and to understand their effects on such chemistry, thorough studies of direct emission are needed.

Implications for ozone production
An additional HCHO source, regardless of the type, will have a direct impact on calculated ozone production rates.The in-situ ozone production can be calculated as where NO 2 is assumed to photodissociate leading to immediate ozone production (Reactions R1 and R2).In our MCM-based calculations, the formation of organonitrates is accounted for in the RO 2 + NO reaction rates.As direct measurements of RO 2 were not available on the Zeppelin, the analysis presented here relies on speciated modeled RO 2 concentrations and reaction rates.Typical model RO 2 concentrations are between 10 and 30 % of the sum of modeled RO 2 and measured HO 2 , such that HO 2 accounts for the majority of the modeled NO to NO 2 conversion.Because HCHO photolysis and oxidation accounts for as much as 45 % model HO 2 production, failing to include all sources of HCHO has significant effects on calculated HO 2 concentrations.Though not probable in this analysis, if oxidation of unmeasured VOCs contributes significantly to the HCHO budget, RO 2 concentrations would likely be underestimated.
Because measured HO 2 had large uncertainty and because the effects of transport on O 3 may be large, we do not explicitly compare measured and modeled HO 2 or O 3 in this study.Instead, two model scenarios were constructed to estimate the impact of missing HCHO on HO 2 mixing ratios and therefore ozone production.Both simulations were carried out with HO 2 unconstrained, and HCHO was either fixed to observations or calculated by the model.While both model runs produced HO 2 concentrations within the uncertainty of measurements, differences in HO 2 mixing ratios in the two model Introduction

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Full scenarios were observed (up to 2 ppt).At the observed mixed layer NO concentrations of ∼ 1 ppb and an rate constant of k HO 2 +NO = 8.6 × 10 −12 cm 3 molec −1 s −1 , an increase of just 1 ppt HO 2 corresponds to an additional 0.7 ppb h −1 (10 %) of ozone production.
Figure 6 shows the difference in (PO 3 ) driven by differences in calculated HO 2 concentrations.Assuming the trend in the discrepancy in HCHO continues to increase throughout the day, an increasing under-prediction of local ozone production rate is expected for this agricultural region.

Conclusions
Using a near-explicit 1-D model and a comprehensive set of trace gas measurements acquired from a Zeppelin airship, we have examined VOC oxidation and its relationship to ozone production in the Po Valley.As in previous work in the region, our model was largely unable to reproduce the morning rise and high levels of observed HCHO.Measured OH reactivity, however, was explained by measured VOCs and their calculated oxidation products.The most probable source of missing HCHO is direct emission from the soil and plant matter beneath the Zeppelin.As a result of the underestimate in HCHO, model ozone production rates based on HO 2 concentrations are underestimated by as much as 10 % before noon, and the underestimate is expected to increase.When considering photochemical models of O 3 production, even small underestimates in HCHO can lead to large underestimates of local ozone production rates.For that reason, and considering the large portion of land used globally for similar agricultural purposes, direct measurements of OH reactivity and HCHO as well as improved OVOC emission inventories would aid in the prediction of high ozone events.

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Full Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | )(Yoshino et al., 2012).Measurements of OH reactivity provide an upper bound on total VOC oxidation, and in conjunction with measurements of OH concentration, the total RO 2 production rate.Similarly, as HCHO is a major source of HO 2 , measurements of HCHO place a lower bound on calculated HO 2 production rates.Correcting for any missing OH reactivity or missing HCHO can increase model XO 2 production rates, in turn increasing (PO 3 ).Here, we provide an analysis of HCHO, OH reactivity, and O 3 production using an extensive set of measurements acquired onboard a Zeppelin airship during the Pan-European Gas-AeroSOls Climate Interaction Study (PEGASOS) in the Po-Valley region.Through the Zeppelin's slow flight speed, highly spatially and temporally resolved Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | contains near-explicit degradation schemes for all constrained VOCs as well as all relevant inorganic chemistry.The model was run with 7 evenly spaced altitude bins, with altitudes from 50-150 m for the lowest box and 650-750 m for the highest box.Measurements acquired during Zeppelin ascents were averaged into these 100 m altitude bins.Where instrument time resolution limits data availability, concentrations are interpolated from data at surrounding altitude and time bins.In all model scenarios, measured photolysis frequencies are used where available.Otherwise, MCM calculated values are scaled according to the ratio of the calculated and measured photolysis rate of NO 2 Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 4 h after sunrise.The general vertical structure of the observed OH reactivity tracks well with HCHO, with elevated values in nocturnal boundary and growing mixed layers.Based on the vertical structure of the observed HCHO and other trace gasses, potential sources of HCHO are discussed further in Sect.3.3Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | est,DiGangi et al. (2011) applied an emission algorithm of E HCHO = A • exp(βT ), where A = 740 ng m −2 h −1 and β = 0.07 • C −1 .The emissions were scaled by photosynthetically active radiation, with night time emissions fixed to 15 % of daytime.A final model scenario was constructed which incorporates direct emissions of HCHO according to the sunlight-weighted exponential emission function similar to Di-Gangi et al., employing a much smaller prefactor of A = 375 ng m −2 h −1 to best capture the observed HCHO mixing ratios.These emissions are added as a direct HCHO source for the model's surface layer (0-50 m), with all other surface layer concentrations constrained to their lowest altitude measurement.The results are shown in Fig.3.The vertical profile is mostly consistent with measurements, with possible discrepancies arising from uncertainty in eddy diffusion constants (see Supplement).Due to the good agreement of this model result and the improbability of other HCHO sources, we conclude local direct emissions from agricultural land are the most likely source of additional HCHO.The finding that direct biogenic emission could account for a large percentage of the observed increase in HCHO mixing ratio is in contrast with the Liu et al. (2007) assumption that direct emissions accounts for only ∼ 10 % of HCHO source in the agricultural Po Valley.Due to scarce data availability, limited information on chemical speciation, and only rough estimation of emission rates, models often assume a default Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Wolfe, G. M. and Thornton, J. A.: The Chemistry of Atmosphere-Forest Exchange (CAFE) Model -Part 1: Model description and characterization, Atmos.Chem.Phys., 11, 77-101, doi:10.5194/acp-11-77-2011,2011.Wolfe, G. M., Thornton, J. A., Bouvier-Brown, N. C., Goldstein, A. H., Park, J.-H., McKay, M., Matross, D. M., Mao, J., Brune, W. H., LaFranchi, B. W., Browne, E. C., Min, K.-E., Wooldridge, P. J., Cohen, R. C., Crounse, J. D., Faloona, I. C., Gilman, J. B., Kuster, W. C., de Gouw, J. A., Huisman, A., and Keutsch, F. N.: The Chemistry of Atmosphere-Forest Exchange (CAFE) Model -Part 2: Application to BEARPEX-2007 observations, Atmos.Chem.
Figure 1.(a) Po Basin, with 12 July 2012 flight track shown in the box and enlarged in (b).(c) Zeppelin altitude during flight.
38 E), which is a background urban site according to the European Monitoring and Evaluation Programme (EMEP) criteria (http://www.nilu.no/projects/ccc/manual/). The nearest urban areas include Bologna 25 km to the southwest and Ferrara 20 km to the north.The more immediate region consists primarily of wheat and corn fields which experienced intense harvesting activities during the campaign.