Seasonal and Diurnal Variation of Formaldehyde and Its Meteorological Drivers at the GAW Site Zugspitze

Seasonal and Diurnal Variation of Formaldehyde and its Meteorological Drivers at the GAW Site Zugspitze

Michael Leuchner1,3*, Homa Ghasemifard1, Marvin Lüpke1, Ludwig Ries2, Christian Schunk1, Annette Menzel1,3

1 Professur für Ökoklimatologie, Technische Universität München, Hans-Carl-von-Carlowitz-Platz 2, D-85354 Freising, Germany 2 GAW Global Observatory Zugspitze/Hohenpeissenberg, Umweltbundesamt, II 4.5, Messnetz, Zugspitze 5, D-82475 Zugspitze, Germany 3 Institute for Advanced Study, Technische Universität München, Lichtenbergstraße 2a, D-85748 Garching, Germany

ABSTRACT

Continuous formaldehyde measurements were performed at the high-altitude GAW site Environmental Research Station Schneefernerhaus for more than one year. This unique dataset was analyzed for daily and seasonal variation and for the influence of large-scale synoptic conditions and air-mass origin on the observed concentrations. The average daily course exhibited maxima in the afternoon and minima at night, however differing between seasons. The general strong seasonal variation with average values for winter, spring, summer, and fall of 0.350, 0.529, 0.986, 0.429 ppbv, respectively, could be well explained by secondary production following photochemical activity. The large variability of formaldehyde mixing ratios within the seasons was shown to be influenced by different factors in this complex topography such as mixing of air masses from the planetary boundary layer and the free troposphere, advection of differently aged air from various source regions, and local meteorological conditions. An analysis of the impact of large-scale weather types, cyclonality, and flow directions revealed that the cleanest air masses were advected from westerly directions in particular under cyclonic conditions while southerly cyclonic and northerly/northwesterly anticyclonic conditions led to the highest formaldehyde levels.

Keywords: Formaldehyde; Remote site; Trajectories; Large-scale weather types.

INTRODUCTION An HCHO net flux from snowpack during sunlight in arctic climates was reported by Dassau et al. (2002), Sumner Formaldehyde (HCHO) is a key substance in atmospheric et al. (2002) and Hutterli et al. (2004). The largest fraction chemistry, an important radical source in the remote free of atmospheric HCHO results from secondary sources troposphere (FT), precursor of ozone (O3), and an important through photochemical production from other directly indicator of atmospheric photochemical activity. It is also emitted volatile organic compounds (VOC). Most important known as a human and animal carcinogen. HCHO is emitted secondary formation processes are the oxidation of methane, directly by incomplete combustion and evaporation from ethene, propene, and other alkenes such as 1-butene, 1,3- anthropogenic processes (e.g., road traffic especially diesel butadiene, and isoprene with additional contribution from exhaust, natural gas, biofuels, biomass burning, chemical and much slower oxidation of alkanes and aromatic compounds petrochemical industry, industrial production of plastics, as well as oxygenated VOCs such as methanol (Arlander et solvents and paint, waste water treatment) (Garcia et al., al., 1995; Still et al., 2006; Parrish et al., 2012). Besides a 1992; Grosjean et al., 1996; Smidt et al., 2005; Balzani Lööv, broad range of anthropogenic VOCs that contribute to HCHO 2007; Herndon et al., 2007) as well as from natural and production, biogenic sources (e.g., ethene, isoprene) play biogenic sources (e.g., living and decaying plants, biomass an important role especially in rural and remote settings burning, seawater) (Guo et al., 2009; Luecken et al., 2012). during the vegetation period (Duane et al., 2002; Millet et al., 2006; Guo et al., 2009; Luecken et al., 2012). The apportionment to primary and secondary origin is of interest and has been addressed in several studies (e.g., * Corresponding author. Solberg et al., 2001; Luecken et al., 2006; 2012; Parrish et Tel.: +49-8161-714745; Fax: +49-8161-714753 al., 2012) reporting that most of the atmospheric HCHO E-mail address: [email protected] results from photochemical production with large variability

802 Leuchner et al., Aerosol and Air Quality Research, 16: 801–815, 2016 due to season, climate, and vegetation cover. Nevertheless, phenomenon in summer when PBL air can reach high in urban areas in winter, direct emissions can contribute up mountain sites (Baltensperger et al., 1997; Nyeki et al., 1998; to 50% (Luecken et al., 2012). Forrer et al., 2000; Zellweger et al., 2000; Winkler et. al., HCHO has an average atmospheric lifetime up to two 2006). Effects such as Foehn, deep convection, stratospheric days; however, in the sunlit atmosphere in summer it is intrusions, and synoptic lifting also influence receptor sites reduced to several hours (Lowe and Schmidt, 1983; Seinfeld at high altitudes alternately and thus the transport of HCHO or and Pandis, 1998). Thus, long-range transport cannot be the its precursors to the sites (Zanis et al., 1999; Seibert et al., main contributor to ambient levels at remote sites but rather 2000; Balzani Lööv et al., 2008; Legreid et al., 2008). Effects the frequency of air arriving from the planetary boundary of dilution and loss by deposition that can be particularly layer (PBL) or in-situ photochemical production from important for water-soluble HCHO also play an important methane, peroxy radicals and other precursors (Solberg et role. However, these effects are neglected if only multivariate al., 2001; Balzani Lööv et al., 2008). However, precursor correlation approaches between different gases are compounds can exhibit much larger lifetimes and thus considered (Parrish et al., 2012). Parrish et al. (2012) also long-range transport of those can add to local or regional stressed the fact that simple correlations between HCHO photochemical production of HCHO. Solberg et al. (2001) and other compounds might lead to wrong conclusions modeled the memory effect of an emission pulse of NOx about primary or secondary origin. Thus, any correlation (NO + NO2), VOCs, and in particular isoprene. The study with other substances has to be viewed with care and under showed that these emissions could be seen in the HCHO consideration of confounding factors, especially if primary concentrations as long as 48 hours. Due to its short lifetime emissions are deducted from correlations with SO2, NOx, the contribution of primary HCHO sources at remote sites or CO, although HCHO might have been produced in summer is reduced and that of secondary production is photochemically during transport from VOC precursors increased (Solberg et al., 2001), although increasing mixing that were co-emitted during combustion processes with the layer height in summer is observed (Birmili et al., 2009). other substances (Parrish et al., 2012). The most important loss processes of HCHO are washout For the interpretation of the observed atmospheric in winter and OH radical reaction in summer, photolysis composition the origin of air masses is crucial and can be during the entire year with maximum in summer, and dry assessed in many different ways. Fleming et al. (2012) deposition with a higher contribution in winter (Solberg et summarized different methods applied such as wind sector al., 2001). analysis, trajectory models, and dispersion models and Most studies of HCHO were conducted in urban explored advantages and limitations. Trajectory clusters environments (e.g., Friedfeld et al., 2002; Possanzini et al., are among the more powerful tools for air-mass history 2002; Rappenglück et al., 2010; Parrish et al., 2012) but only and have been successfully applied to HCHO data from a few at rural, forested, or remote sites (e.g., Solberg et al., Jungfraujoch by Balzani Lööv et al. (2008). 2001; Sumner et al., 2001; Hellen et al., 2004; Villanueva- In this paper, a dataset for more than one year of Fierro et al., 2004; Müller et al., 2006; Khwaja and Narang, continuous HCHO measurements at the high-altitude GAW 2008; Choi et al., 2010; DiGangi et al., 2011, 2012). HCHO site Zugspitze/Schneefernerhaus is presented and analyzed. mixing ratios have also been reported from several remote First statistical analyses with regard to seasonal distribution ground sites and during flight and oceanic campaigns, are shown and trajectory clusters as well as large-scale however, for high altitude sites only measurements at Mauna weather patterns are examined for meteorological influences Loa, Jungfraujoch, and Whiteface mountain exist (Heikes, on HCHO levels at this background site. Two case studies 1992; Arlander et al., 1995; Zhou et al., 1996; Heikes et of the highest values of HCHO in winter and summer, al., 2001; Kormann et al., 2003; Li et al., 2004; Singh et respectively, are presented in the supplement. al., 2004; Zhou et al., 2007; Balzani Lööv et al., 2008; Fried et al., 2008; Khwaja and Narang, 2008; Legreid et METHODS al., 2008; Fried et al., 2011; Klippel et al., 2011), but mainly in field campaigns of a few weeks to months. An overview Experimental Setup of the measured mixing ratio ranges of different campaigns The remote Environmental Research Station from high-mountain and global-remote sites was given by Schneefernerhaus (UFS) is located in the northern limestone Balzani Lööv (2007) and from several campaigns in Alps at the border of Germany and Austria (47°25’00” Austria by Smidt et al. (2005). The lowest HCHO levels North, 10°58’46” East) about 90 km southwest of Munich. It were measured in Antarctica with values as low as 35 ppt is situated at an altitude of 2650 m a.s.l. on the southern slope (Sumner et al., 2002). However, no long-term time series of Zugspitze mountain, the highest mountain of Germany, of continuous HCHO measurements at high mountain sites around 300 m below its summit. It is one of the global has been reported so far. stations of the Global Atmosphere Watch (GAW) program Meteorological conditions at mountain sites are quite of the World Meteorological Organization (WMO). The diverse due to topography and local wind fields. Air masses site generally receives FT air but is frequently exposed to can be transported from long distances without having PBL air due to thermally induced flow systems especially touched the planetary boundary layer (PBL), but can also during summer time (Reiter et al., 1987; Gantner et al., be uplifted from the valleys. Mountain venting by thermally 2003; Pandey Deolal et al., 2014). Thus, air masses with induced flows during fair weather days is a common very different origin and history are measured and the site

Leuchner et al., Aerosol and Air Quality Research, 16: 801–815, 2016 803 is particularly suitable for the detection of northern advection a zero trap at midnight (CET), additional zeroing at noon (Kaiser et al., 2007). Some influence of surface emissions (CET). The zero-trap consisted of a filter cartridge containing on trace gas observations have been shown and thus Henne a Hopcalite catalyst (Dräger, Lübeck, Germany) scrubbing et al. (2010) categorized the site as “weakly influenced”, HCHO out of ambient air. Further details of the calibration when analyzing remote air quality sampling sites in Europe. and zeroing procedure are described in Junkermann and This is due to anthropogenic activities in the direct vicinity Burger (2006). of UFS from the adjacent ski and hiking areas on the 14.6% of the data were missing due to instrumental Schneeferner glacier as well as on the Zugspitze summit failures, power outages, stabilizing times after re- and the affiliated tourist services. Detailed site information initialization, and rejection by the plausibility control. can be found in the GAW station information system Another 8.3% of the data were missing due to the twice (GAWSIS; http://gaw.empa.ch/gawsis). daily calibration and zeroing procedures, respectively. This Air for analysis of atmospheric formaldehyde (HCHO) resulted in coverage of 77.1% of valid data over the entire was continuously sampled from Dec 6th 2012 to Dec. 15th measuring period. 2013. The 5 m inlet line (OD ¼”) consisted of Sulfinert© Continuous ozone (O3) measurements were performed by coated stainless steel. The inlet air flow was set to 1 L min–1, a Thermo Scientific TS49i UV-photometric ozone analyzer the inlet line was heated to an outside temperature of 80°C with a lower detection limit of 0.5 ppbv, CO with an to prevent build-up of condensate in the line as well as AeroLaser AL5002 continuous UV-fluorescence monitor freezing processes on the outside under the extreme with a detection limit of 0.8 ppbv (referred to an integration conditions especially during the wintertime. time of 10 s). NO, NO2, and NOx were measured in parallel HCHO was measured continuously with an AeroLaser with an Eco Physics CRANOXII chemiluminescence detector formaldehyde monitor (model AL4021, www.aerolaser.com) consisting of a two-channel system to achieve a detection using the fluorometric Hantzsch reaction (FHR) described limit of 25 pptv and a Thermo Scientific TS42i trace level in detail by Junkermann and Burger (2006). It requires the chemiluminescence monitor to ensure continuous monitoring transfer of HCHO from the gas into the liquid phase by of low mixing ratios. NO2 is measured as NO-equivalent stripping HCHO from air in a stripping coil at constant from the output of a photolysis converter. O3, CO, and temperature of 10°C to ensure quantitative sampling even NOx were sampled with a stainless-steel inlet tube with an at low air pressure present at the UFS. The detection of inner borosilicate glass tube with a diameter of 10 cm. The HCHO employs the fluorescence of 3,5-diacetyl-1,4- inlet flow was highly turbulent with a rate of 500 L min–1. dihydrolutidine (DDL) at 510 nm, which is produced from Calibration and quality assurance for these compounds the reaction of aqueous HCHO with an acetylacetone and followed the standard operating procedures of the German ammonium acetate solution, with an excitation wavelength of Environmental Protection Agency (UBA) in accordance 412 nm (Hak et al., 2005). FHR-based instrumentation have with GAW quality standards. been extensively field-used, compared to other measuring Data for NMHCs were obtained by using automated gas techniques (e.g., spectroscopic, chromatographic), and chromatography-flame ionization detection with a Perkin- validated (e.g., Gilpin et al., 1997; Cárdenas et al., 2000; Elmer system consisting of a Clarus 500 GC equipped with Klemp et al., 2004; Hak et al., 2005; Apel et al., 2008; two FIDs and a Turbomatrix 650 Automatic Thermal Wisthaler et al., 2008). Desorber unit. Air was sampled through a 5 m heated inlet The temporal resolution of the data acquisition was a 10 line (OD 1/8”) of Sulfinert© coated stainless steel with an s mean that in the further analysis was averaged to 30 min air flow of 15 mL min–1. Details on the performance of the mean values. Depending on the time of the gaseous-liquid GC system in an intercomparison experiment can be found reactions and the flow-rate settings the delay time of in Hoerger et al. (2014). measurement was approximately 4 min (Hak et al., 2005). The determined gas-phase detection limit with a relatively Trajectories and Data Analysis low liquid flow of 0.32 mL min–1 was 24 pptv (3σ of Backward trajectories, spatiotemporal pathways of an air instrumental noise) and close to the determined detection parcel, were calculated with HYSPLIT 4 (Hybrid Single limit of 30 pptv reported by Junkermann and Burger (2006) Particle Lagrangian Integrated Trajectory Model; version and of 22 pptv reported by Balzani Lööv (2007). October 2014) (Draxler et al., 1998) from the meteorological Calibration of the instrument consisted of a liquid and a model GDAS (Global Data Assimilation System; gas-phase calibration. Absolute calibration of the desired https://www.ncdc.noaa.gov/data-access/model-data/model- measuring range was performed with aqueous standards datasets/global-data-assimilation-system-gdas) with a 0.5° (accuracy 0.5%; Bernd Kraft GmbH, Duisburg, Germany) resolution. For each day within the entire period eight approximately every third week when changing the solutions. backward trajectories with a 3-hour interval and a runtime Gas-calibration standard was provided by an internal of 96 hours were calculated, starting each day at 02:00 h permeation source (Kintek wafer, heated to 45°C) delivering CET (UTC+1). The selected interval and starting times were approx. 8 ng min–1 of HCHO that was diluted with 100–150 necessary to avoid calibration times of the instrument at noon mL min–1 of zero air. The permeation rate was measured and midnight for which data were missing. Trajectories were and compared to the liquid standard with every calibration. started at the coordinates of UFS at an altitude of 1500 m Daily automatic calibration and zeroing of the instrument a.g.l. which corresponded well to the average air pressure were performed with this on-board permeation source and of 733 hPa at UFS. Each trajectory calculation included

804 Leuchner et al., Aerosol and Air Quality Research, 16: 801–815, 2016 hourly coordinates, trajectory, and mixing layer height. 2.24 ppbv on Jan. 25th, 2013) occurred. These two examples Further analysis of the trajectories was performed with of the winter and the overall maximum are discussed in the the open-air package (Carslaw and Ropkins, 2012) of the case studies in the supplement. The descriptive statistics of statistical program R (R Core Team, 2014). Trajectory the HCHO dataset, also for the different seasons, is given clusters were calculated from all trajectories by using an in Table 1. The other gases also show distinct seasonal angle-based distance matrix method applying the k-medoids patterns and other variations throughout the year dependent algorithm (Sirois and Bottenheim, 1995). The criterion that on emissions, meteorology and photochemistry (Fig. 1) which at least 10% of the trajectories needed to be affiliated with will be discussed later. The presented HCHO values with a cluster resulted in a total number of five clusters. In order to 0.429 ppbv (fall mean), 0.350 ppbv (winter mean), 0.529 derive clearer information on possible background air masses ppbv (spring mean), and 0.986 ppbv (summer mean) exhibit a each trajectory was split into two categories: (1) PBL clear seasonal pattern. In general, the annual variation of influenced, if the trajectory was running below the mixing HCHO background is primarily driven by photochemistry layer height over a land mass at least in one time step and that is mainly dependent on solar radiation, producing the (2) FT, if the trajectory always remained above the mixing highest amounts of secondary HCHO from methane and layer height within the free troposphere. Group comparisons other precursors during the summer months (Solberg et al., were performed with a two-sided Student’s t-test, 2001). In addition, biogenic precursors such as isoprene dependencies of two compounds with a linear regression are abundant in much higher amounts in summer and also model. All analyses were done in R (R Core Team, 2014). enhance HCHO levels (Luecken et al., 2012). However, at high-mountain sites such as UFS, which are alternately RESULTS AND DISCUSSION exposed to aged air masses from the FT and uplifted air from the PBL or the residual layer, synoptic parameters, Seasonal Variability of HCHO Mixing Ratios local meteorological conditions, and topography also play The entire time series of HCHO, O3, CO, NO, and NO2 a crucial role. This could also be seen in the monthly from Dec 6th 2012 to Dec 15th 2013 are displayed in Fig. 1. maximum that occurred in July and the absolute maximum A clear annual course of HCHO mixing ratios is revealed beginning of August in contrast to June, the month with with minimal values during wintertime (minimum: 0.05 the highest solar irradiance. During July, the UFS site was ppbv on Dec. 9th, 2013, 30 min mean) and maximal values also under the influence of long-range transport from arctic during the summer (maximum: 3.96 ppbv on Aug. 3rd, 2013). North America where a very high activity of boreal forest However, also in winter, episodes with high HCHO (up to fires was reported (Trickl et al., 2015). The impact could

Fig. 1. Time series of half-hourly mean values of HCHO, O3, CO, NO2, and NO for the period Dec 6th 2012 to Dec 15th 2013. The two highlighted areas mark the winter and the total maximum during the period selected for the case studies.

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Table 1. Descriptive statistics of the HCHO mixing ratios (in ppbv) for the entire period and different seasons (half-hourly mean values). Mixing ratio (ppbv) Mean Median S.D. Min. Max. All data 0.58 0.46 0.39 0.05 3.96 Winter (DJF) 0.35 0.29 0.23 0.05 2.24 Spring (MAM) 0.53 0.49 0.22 0.13 1.98 Summer (JJA) 0.98 0.90 0.46 0.22 3.96 Fall (SON) 0.43 0.38 0.23 0.09 1.64 be seen in elevated CO values. Those air masses have also background levels. contributed to the elevated O3 in July observed at UFS as a result of build-up during transport and possibly contributed Diurnal Variation of HCHO Mixing Ratios to some extent to precursors for HCHO formation. Fig. 2 shows the diurnal variation of HCHO during the The HCHO mixing ratios represent a mixture of (diluted) four seasons. For all seasons a clear daily pattern with PBL and FT air, thus, the average concentration of the short- highest values in the afternoon is revealed. In summer the lived HCHO depends mainly on the frequency of transport average maximum reached almost 1.2 ppbv with a nightly from the PBL (Balzani Lööv, 2007). Birmili et al. (2009) minimum still above 0.8 ppbv. The amplitude was highest showed that aerosol concentrations at UFS were directly in summer and lowest in winter where only a slight diurnal related to mixing layer height that reached a maximum profile could be identified. Average HCHO values during altitude during June with the largest influence of PBL air at winter ranged between 0.3 and 0.4 ppbv during the 24-hour the site. During winter the site was mostly exposed to FT air period. This shows on the one hand the photochemical origin while in summer transport of PBL air occurred frequently of HCHO that increases after sunrise and follows the according to increasing mixing layer height during afternoons radiation input. On the other hand, especially in summer, in summer. However, during nighttime in summer, UFS transport of PBL air from the valleys by mountain venting was also exposed to FT air most of the time. possibly also contributed to the afternoon maximum. This The mean HCHO values of 0.577 ppbv (mean of all statement is supported by the high standard deviation in the data) as well as the seasonal means are in similar ranges as afternoon during summertime indicating higher variability in studies from other remote or rural sites summarized in and episodes of high HCHO during that time. This was Smidt et al. (2005), Balzani Lööv (2007), and Khwaja and shown by Birmili et al. (2009) for aerosol concentrations Narang (2008). However, UFS values are somewhat higher at UFS that were dependent on the mixing layer height than reported for the high mountain site Jungfraujoch, with maxima in the afternoon where mountain venting was Switzerland (3580 m a.s.l.), with 0.303 ppbv (mean fall most abundant. At the very remote site Mauna Loa, Zhou campaign), 0.362 ppbv (mean winter campaign), 0.397 ppbv et al. (1996) reported diurnal maxima of up to 0.9 ppbv (mean spring campaign) and 0.505 ppbv (mean summer with upslope winds during summer. campaign), respectively (Legreid et al., 2008). This can be explained by a lower altitude of UFS and thus a higher Photochemical Influence on HCHO and O3 impact of the PBL. The low lying Whiteface Mountain, USA, A way to assess the secondary origin of substances is a (1483 m a.s.l.) even exhibits a much higher impact of the comparison with O3 that, apart from stratospheric intrusion PBL (Zhou et al., 2007; Khwaja and Narang, 2008). The episodes, is only produced photochemically (Khwaja and HCHO data from six rural sites in Europe, ranging from sea Narang 2008). In the seasonal distribution, O3 also exhibited a level (Mace Head, Ireland) to 775 m a.s.l. (Donon, France), maximum abundance in summer and a minimum in winter, presented by Solberg et al. (2001) showed similar annual and was generally following HCHO in summer (Fig. 1), cycles with summer maxima and winter minima with indicating secondary production of both species. However, different concentration ranges, mostly above the values at differences could be observed during several periods in UFS. Data from Mauna Loa (Zhou et al., 1996) were well winter, when low O3 was accompanied by high HCHO below the UFS values in the range between 0.149 ppbv (e.g., 25th Jan. 2013) and vice versa. This phenomenon was (winter seasonal mean) and 0.211 ppbv (summer seasonal investigated in detail in a case study in the supplement. mean) which can be explained by the very remote setting Equally, mixing ratios of both compounds were not on an island in the Pacific without major anthropogenic concurrent during spring, when O3 usually exhibits its spring area sources of a highly populated continent. UFS values maximum due to the impact of different precursors such as were in the range of observations for the respective altitudes peroxyacetic nitric anhydrides (PAN) that accumulate during and latitudes during flights across Europe reported by winter and peak in early spring at mid and high latitudes, Klippel et al. (2011). providing NOx for O3 production (Penkett and Brice, 1986; In summary, the seasonal HCHO levels found at UFS Monks, 2000; Zellweger et al., 2002; Stroud et al., 2003; were in good agreement to other mountain or rural sites as Pandey Deolal et al., 2014). In addition, biogenic emissions well as to aircraft data from the Northern hemispheric that are contributing an important fraction of HCHO troposphere and show despite temporary influence of the precursors are more abundant in summer than in spring. PBL that data can be used for the determination of HCHO Detailed monthly correlations of HCHO with O3 are

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Fig. 2. Diurnal variation of the hourly arithmetic mean and standard deviation of HCHO mixing ratios for different seasons (winter = December, January, February; spring = March, April, May; summer = June, July, August; fall = September, October, November). displayed in Fig. 3. A clear shift in the relationship between the following weather types in decreasing order (Fig. 4): the two substances occurred throughout the year. In summer trough over Western Europe (TRW), northwesterly higher HCHO values were accompanied by higher O3 anticyclonic conditions (NWA), high over Fennoscandia levels indicating common photochemical origins (Balzani (anticyclonic over Central Europe) (HFA) that occurred Lööv, 2007). Daily HCHO means also showed a significant quite rarely on only three days, and southerly cyclonic (SZ). dependency on daily solar radiation values in June, August, Both TRW and SZ belong to the flow direction/cyclonality and September (Jun.: R2 = 0.70, p < 0.001; Aug.: R2 = 0.38, category southerly cyclonic (SC) that in total exhibited the p < 0.001; Sep.: R2 = 0.21, p < 0.05) as well as for the highest HCHO values (Fig. 5). High HCHO mixing ratios entire dataset (R2 = 0.39, p < 0.001) but not for the rest of were also observed under northwesterly/northerly the months. This indicates secondary production as the main anticyclonic conditions (NA), especially under high over source of HCHO in summer. No significant relationship Iceland (anticyclonic over Central Europe) (HNA). The between HCHO and O3 was detected for February and lowest HCHO mixing ratios were found under high over March. In the winter months the relationship between HCHO Central Europe (HM) and southeasterly anticyclonic and O3 was inverted resulting from a higher impact of the conditions (SEA), both very rare weather types with only PBL in winter with a possibly much higher fraction of five and four occurrences, respectively. Frequent weather primarily emitted HCHO from combustion processes. This is types with very low HCHO were maritime westerly (WW) supported additionally by a comparison with CO and NOx and westerly cyclonic (WZ) both in the flow direction/ (see Fig. 1) that correlated well with HCHO in winter but cyclonality category westerly cyclonic (WC). On the one not in summer (not shown here), but secondary production hand cyclonic conditions with rising air may transport cannot be ruled out completely. HCHO or precursor enriched PBL air to the site, but it is also linked to higher cloudiness with less photochemical Synoptic Influence on HCHO Mixing Ratios potential. On the other hand anticyclonic conditions with The origin of air masses transported to a receptor site subsiding air, but higher solar radiation and thus more plays a crucial role on HCHO levels. Thus, the dependence potential for photochemical build-up can equally lead to of HCHO mixing ratios at UFS on large-scale weather high HCHO values. This shows that high or low HCHO patterns and different trajectory clusters was investigated. values are not only dependent on the cyclonality, but mainly Fig. 4 gives an overview of the distribution of HCHO on the position of the respective high or low and thus on mixing ratios classified into daily large-scale weather types the resulting flow direction and origin of the air mass. The (Hess and Brezowsky, 1952; Werner and Gerstengarbe, 2010) large range of the quantiles of several classes also indicated and in Fig. 5 classified into cyclonality and flow direction that other influences such as frequency of injection of PBL (following Philipp et al., 2010; Wastl et al., 2013). air, season, and local or regional factors play an important The highest HCHO values on average were found under role and can interfere with the large-scale synoptic patterns.

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2 Fig. 3. Monthly linear regressions of half-hourly mean mixing ratios of HCHO and O3. Coefficient of determination R and slope a are given for each plot.

Fig. 4. Boxplots of HCHO mixing ratios categorized into daily weather types (after Hess and Brezowsky, 1952; Werner and Gerstengarbe, 2010). The boxplots are ordered by the respective HCHO arithmetic mean (not displayed); the width of the boxplots indicates the number of occurrences. The markers indicate the minimum and maximum values excluding outliers (lower and upper bonds), 25th- and 75th-percentiles (lower and upper boundaries of boxes), and medians (horizontal lines inside boxes) are given. Abbreviations are listed in Table 2.

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Fig. 5. Boxplots of HCHO mixing ratios categorized into cyclonality and flow direction (Philipp et al., 2010; Wastl et al., 2013). The boxplots are ordered by the arithmetic mean (not displayed); the width of the boxplots indicates the number of occurrences. The markers indicate the minimum and maximum values excluding outliers (lower and upper bonds), 25th- and 75th-percentiles (lower and upper boundaries of boxes), and medians (horizontal lines inside boxes) are given. Abbreviations are listed in Table 2.

Table 2. Daily weather types of Hess and Brezowsky (1952) classified after cyclonality and flow direction. CE is the abbreviation for Central Europe, FS for Fennoscandia (after Wastl et al., 2013). Flow direction Cyclonality Abbreviation Original definition NEA Northeasterly anticyclonic Anticyclonic EA HFA High FS, CE anticyclonic HNFA High FS-Iceland, CE anticyclonic Northeasterly, Easterly NEZ Northeasterly cyclonic Cyclonic EC HFZ High FS, CE cyclonic HNFZ High FS-Iceland, CE cyclonic SWA Southwesterly anticyclonic Anticyclonic SA SA Southerly anticyclonic SEA Southeasterly anticyclonic SWZ Southwesterly cyclonic Southerly SZ Southerly cyclonic Cyclonic SC TB Low British Isles TRW Trough Western Europe SEZ Southeasterly cyclonic Anticyclonic WA WA Westerly anticyclonic Westerly WZ Westerly cyclonic Cyclonic WC WW Maritime westerly NWA Northwesterly anticyclonic NA Northerly anticyclonic Anticyclonic NA HNA High Iceland, CE anticyclonic HB High British Isles NWZ Northwesterly cyclonic NZ Northerly cyclonic Northwesterly, Northerly Cyclonic NC HNZ High Iceland, CE cyclonic TRM Trough CE High H HM High CE L TM Low CE, Cut off Low BM Ridge CE T Ü Transient

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HCHO Mixing Ratios by Back-Trajectory Clusters 750 (24.9%) trajectories only had contact with the European Fig. 6 illustrates five calculated trajectory clusters with PBL in 40% and 35% of the cases, respectively. This is main directions from west (W), northwest (NW), northeast also reflected in the low values of HCHO and the other gases, (NE), southwest (SW), and southeast (SE). Fig. 7 gives an which can be explained by long-range transport from rather overview of chemical and meteorological properties of air maritime and FT air masses to the receptor site. The other masses in the five clusters, also classifying whether the air trajectory categories (NE, SW, SE) stayed longer over the mass was in contact with the European boundary layer European continent and also mixed more frequently with (PBL) or not (FT). In almost all cases mixing ratios were the PBL leading to higher values of HCHO and other significantly higher (p < 0.01) when trajectories passed pollutants. Especially relative humidity of the air masses, through the European PBL with the exception of lower O3 to a smaller extent air temperature (not significant for SW in the PBL of cluster NW. No significant differences were and SE), clearly distinguishes PBL and FT air masses. observed for O3 in clusters W and NE, CO in cluster NE, The HCHO mixing ratios of the cluster fractions without and NOx in clusters NE, SW, and SE. Highest HCHO (mean: PBL influence were substantially higher than modeled 0.75 ppbv; PBL influence: 0.83 ppbv; FT: 0.59 ppbv), O3 background values of 0.29 ppbv for four seasonal campaigns (mean: 51.6 ppbv), CO (140.6 ppbv), and NOx (0.58 ppbv) throughout the year for Jungfraujoch, Switzerland, by Balzani mixing ratios were found in cluster NE followed by cluster Lööv et al. (2008). Cluster W and SE exhibited the lowest SW for HCHO (0.65 ppbv; PBL influence: 0.72 ppbv; FT: FT values with 0.43 and 0.41 ppbv, respectively. The 25th- 0.50 ppbv) and O3 (49.0 ppbv), but not for CO (123.6 percentile values of all clusters (range: 0.25–0.32 ppbv) ppbv) and NOx (0.20 ppbv) that exhibited quite low values were very close to the calculated value of 0.29 ppbv by compared to the other clusters. The lowest HCHO (0.52 Balzani Lööv et al. (2008), but for a more quantitative ppbv; 0.64 ppbv; 0.44 ppbv), O3 (47.1 ppbv), intermediate determination of background concentrations, more elaborate CO (128.0 ppbv) and very low NOx (0.22 ppbv) values methods are needed. were linked to cluster W. The three shorter clusters NE, SW and SE had the lowest CONCLUSIONS frequencies with 317 (10.5%), 507 (16.8%), and 361 (12.0%) trajectories assigned to the cluster, respectively, but the One-year data of HCHO mixing ratios showed a high highest frequencies of contact with the European PBL with variability diurnally and seasonally. In general the observed 63%, 65%, and 70% of all trajectories, respectively. The patterns resemble very well expected courses from secondary more frequent clusters W and NW with 1080 (35.8%) and production following the solar radiation input and thus

Fig. 6. Five trajectory clusters for the entire period Dec 6th 2012 to Dec 15th 2013.

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Fig. 7. Boxplots of mixing ratios of HCHO, O3, CO, and NOx as well as air temperature and relative humidity for the five trajectory clusters of Fig. 6 for the entire period. The clusters are also differentiated if the air mass was in contact with the European boundary layer (PBL) or not (FT). The markers indicate the minimum and maximum values excluding outliers (lower and upper bonds), 25th- and 75th-percentiles (lower and upper boundaries of boxes), and medians (horizontal lines inside boxes). Significance levels of the two-sided Student’s t-test between PBL influence and FT are: p < 0.01 (***), p < 0.05 (**), p < 0.1 (*), not significant (n.s.).

Leuchner et al., Aerosol and Air Quality Research, 16: 801–815, 2016 811 photochemical decay of precursor compounds. This could to the observed patterns and locate emissions and origin of the be seen by the diurnal variation with maxima in the afternoon trace gases, e.g., by inverse modeling. Thus, apportionment and minima at night. During summer, HCHO build-up in into different photochemical production and loss mechanisms, the morning starts earlier than in winter following the into biogenic and anthropogenic, or into primary and lengths of days. The diurnal amplitude is also much larger secondary origin is needed and requires a modeling approach during summer. The importance of secondary production that can also capture the different influencing factors that are is supported by significant positive correlations of HCHO more numerous in mountain areas. with O3 and solar radiation during summer months. In winter, the exact opposite could be observed with negative ACKNOWLEDGMENTS correlations to ozone and solar radiation; spring and fall do not show any consistent patterns. This indicates that We gratefully acknowledge the support by the crew photochemistry had a much lower impact on the observed from UFS Schneefernerhaus. The research leading to these HCHO levels in winter. Possible explanations could be a results has received funding from the European Research larger influence of air masses from the PBL with a higher Council under the European Union’s Seventh Framework fraction of primary formaldehyde from combustion processes Programme (FP7/2007-2013)/ERC grant agreement no typical for the winter season, such as wood burning and [282250]. Funding for the HCHO instrument was provided by residential heating, although the frequency of uplift of the the Bavarian State Ministry for Environment and Consumer mixing layer during winter is reduced (Birmili et al., 2009). Protection. Funding for the HCHO instrument was provided Local sources from the skiing area might also contribute to within the EPP initiative of the Bavarian State Ministry for some extent. In comparison to summer these emissions are Environment and Consumer Protection. With the support of less mixed in a shallower PBL and transport to the site can the Technische Universität München – Institute for Advanced lead to high HCHO values as seen in a case study for the Study, funded by the German Excellence Initiative. winter maximum in January (see supplement). However, during summer months, local effects such as thermally SUPPLEMENTARY MATERIALS induced mountain venting occur more frequently than in winter. Supplementary data associated with this article can be The results show that large-scale synoptic patterns have found in the online version at http://www.aaqr.org. a great influence on the observed HCHO mixing ratios. 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