The present study is organized in two sections. First, we discuss the meteorology behind the three-dimensional transport within the time scales of UFP lifetime. Second, we describe selected experimental data confirming the horizontal and vertical aerosol patterns.

In our studies, based on detailed airborne observations using small and slow-flying instrumented aircraft, we were able to characterize major UFP sources, their quantitative contribution to the total budget of UFPs, and how local nucleation mode particle appearance may be linked to elevated particle sources via meteorological processes. We hypothesize that particles attributable to flue-gas-cleaning efforts, established since the late 1980s, have resulted in a substantial increase in primary nucleation size mode particles and are now a major contribution to the anthropogenic budget of UFPs. This change in emissions may also be the relevant process to explain the worldwide increase of nucleation mode particles.

Ultrafine particles (UFPs) are distributed highly unevenly in the lower troposphere. Although these UFPs are positively detectable and have been studied for more than a century, their three-dimensional distribution, formation, and budget in the atmosphere remain largely uncertain, despite their obvious climate relevance. This is due to their short lifetime and the fact that they are invisible to the human eye and to remote sensing techniques. From the moment of their emission or generation, their spatial distribution is a result of meteorological processes, regional-scale transport, local thermal convection, and rapid loss by interaction with clouds as cloud condensation nuclei. Here, we report about three-dimensional airborne in situ studies aimed at investigating UFP sources, distribution, and behavior on different spatial and temporal scales. We identified fossil fuel–burning power stations, refineries, and smelters as major anthropogenic UFP sources. On a regional scale, their emissions are significantly higher than urban emissions. Particle emissions from such power stations are released typically at altitudes between 200 and 300 m AGL. Detailed in situ measurements of particle concentration and related parameters, together with meteorological measurements and analyses, enable reliable source attribution even over several hundred kilometers downwind from the emitter. Comprehensive meteorological analysis is required to understand the highly variable 3D concentration patterns generated by advective transport and thermal convection. Knowledge of primary emission strength, together with size distributions and atmospheric 3D transport of UFPs derived from airborne measurements, makes it possible to estimate the aerosols’ impact on meteorology, hydrological cycles, and climate.

The observation that UFP burst events at field sites occur more frequently in spring than in summer ( Dal Maso et al. 2005 ; Petäjä et al. 2016 ) is well in line with the more pronounced differences between air and surface temperatures in springtime, which offer more favorable conditions for thermal convective activity. During the summer months, the nocturnal inversion could be less pronounced because of the higher nighttime surface temperatures, but final mixing-layer depth and thus vertical dilution may be higher ( Fig. 1 ).

Temporal development of the planetary boundary layer (PBL) in summer [Jun–Aug (JJA)] and spring [Mar–May (MAM)]. PBL data taken from the ARM Southern Great Plains (SGP) site, Oklahoma ( www.arm.gov/capabilities/observatories/sgp ), adapted from Liu and Liang (2010) . Background picture: Buschhaus power station, Germany. UFP plumes emitted into a ∼300-m RL with ∼60,000 cm –3 and approximated ground concentrations for hourly intervals between 1000 and 1400 local time. UFPs injected into the well-mixed PBL during the day reach the ground after ∼15 min; later in the day, full PBL mixing prevents high ground concentrations. More detailed description in the text.

Temporal development of the planetary boundary layer (PBL) in summer [Jun–Aug (JJA)] and spring [Mar–May (MAM)]. PBL data taken from the ARM Southern Great Plains (SGP) site, Oklahoma ( www.arm.gov/capabilities/observatories/sgp ), adapted from Liu and Liang (2010) . Background picture: Buschhaus power station, Germany. UFP plumes emitted into a ∼300-m RL with ∼60,000 cm –3 and approximated ground concentrations for hourly intervals between 1000 and 1400 local time. UFPs injected into the well-mixed PBL during the day reach the ground after ∼15 min; later in the day, full PBL mixing prevents high ground concentrations. More detailed description in the text.

The emission of UFPs into the lower atmosphere from flue gas stacks ( Junkermann et al. 2011a ) typically takes place at heights of 200–400 m AGL, depending on stack height and excess flue gas temperatures. In daytime conditions, this release height puts the emissions well into the mid-PBL, with convective turbulence distributing the UFPs in both directions, down to the ground and up toward the cloud base. Elevated emission altitude with rapid vertical mixing ( Buzorius et al. 2001 ; Wehner et al. 2010 ) typically leads to plumes reaching the ground at most by about ∼2–5 km downwind of the stack, with UFP concentrations subsequently diminished by dispersion and deposition losses. At nighttime, while wind speed usually decreases markedly in the surface layer, values of 5–10 m s –1 are still typical above the nocturnal inversion and can reach considerably higher values over large flat landscapes. Thus, during the night hours a high concentration layer of particles can form within the less turbulent residual layer (RL), containing remaining PBL air and fresh emissions injected above the nocturnal inversion layer of ∼100–150 m. Such conditions may extend over hundreds of kilometers, with little vertical mixing ( Ayers et al. 1979 ; Junkermann et al. 2011b ). Under clear or only low-cloud-cover conditions such nocturnal layering is broken up the next morning by thermal convection, and UFPs from far-distant elevated sources can reach the ground by fumigation. The highest concentrations close to the ground are thus often found about 30 min after thermal convection reaches the altitude of the UFP layer, and their location is strongly dependent on wind direction. It is not clear in what manner particles age during nighttime transport in the RL. However, as they are decoupled from fresh biogenic emissions and this layer is often drier than the PBL, growth would not always be expected and even shrinking may occur. This means that number concentrations are likely affected by coagulation and diffusion only. In either case, UFP transport to the ground depends on the strength of the thermal convection, which, in turn, depends on the energy input by solar radiation since sunrise, the strength of the inversion layer, and/or a preexisting haze or fine particle load within the surface layer that could reduce or suppress thermal convection. Convective dilution to higher altitudes, with increasing PBL height later in the day, accounts for a further decrease of particle number concentrations in the whole increasing volume of the PBL ( Fig. 1 ). Once mixed with biogenic emissions, growth occurs as a result of agglomeration of volatile organic compounds (VOC; or ELVOC) ( Ehn et al. 2014 ).

Meteorological processes play a major role in the interpretation of the observed temporal and spatial patterns of UFPs. The typical emissions from elevated smoke stacks that form plumes with high UFP number concentrations offer opportunities for detailed case studies on the impact of meteorological transport phenomena on the three-dimensional distribution of UFPs. Both the lifetime of UFPs and their growth processes take place at similar time scales as regional-scale horizontal transport (i.e., between one and a few days). In contrast thereto, convective vertical transport occurs at time scales of less than 1 h and is rapid compared to aerosol aging processes ( Georgii 1956 ; Stull 1988 ; Kulmala et al. 2013 ). Physical properties of the aerosol are thus preserved during vertical transport by thermal convection ( Bigg et al. 1978 ) but may change during horizontal transport. Horizontal transport typically dominates during stable stratification of the lower atmosphere, at night or under daytime overcast conditions.

From an operational perspective, and to adjust and optimize flight strategy and flight patterns to the actual meteorological and environmental conditions, the real-time data display in the aircraft and visual clues observed by the scientist/pilots were essential decision-making tools. Having the freedom to fly the research aircraft mostly in noncontrolled airspace under visual flight rules (VFR) enabled rapid changes of the flight strategy in response to real-time observations. This flexibility, and the comparatively low flying altitude above ground, is usually not feasible for larger aircraft ( Hamburger et al. 2012 ).

Long-distance survey flights in 2012 over Australia (distances of >3,500 km) and in 2012/14 over Germany (>2,000 km) were used to investigate the relative frequency of occurrence of anthropogenic particles, as well as any indications of potential contributions of biogenic-particle-related processes. Biogenic particles from atmospheric gas to particle conversion (nucleation) would be expected, for example, at low to midelevations of the PBL over VOC-emitting forest areas and most likely under sunny conditions (i.e., between midmorning to early afternoon hours). A search for biogenic UFPs would thus require flights between 1000 and 1500 local time and under at least partially sunny conditions ( Baranizadeh et al. 2014 ). In contrast, anthropogenic primary emissions from continuously operating aerosol sources should be independent of the time of day. This notion was confirmed in Junkermann and Hacker (2015) for flights downwind of the 750-MW Kogan Creek power station in Queensland, Australia, over 1 h around sunset (SS) (takeoff 30 min before SS and landing 30 min after SS) and in the late morning the next day, with particle emissions of 3 × 10 18 particles s –1 in both cases ( Junkermann and Hacker 2015 ).

Flight patterns include initial vertical profiles extending from near the ground into the FT to define the height of the PBL, to detect UFPs in the lower FT, and to confirm that the PBL is well mixed and regional-scale (>300 km) horizontal patterns to identify and trace possible sources. Lagrangian flight patterns across plumes from several individual anthropogenic sources at different downwind distances were used to derive particle budgets and to investigate aerosol aging.

(top) UFP plumes at the German–Polish border originating from power stations Spremberg, Boxberg, and Jänschwalde, Germany, and Mělník, Czech Republic, under well-mixed moderately polluted and stable stratified heavily polluted conditions, 8 (yellow and brown) and 10 Jun (blue and gray) 2014 (campaign 12). Conditions on 8 Jun included southwesterly winds and well-mixed PBL up to >1,000 m AGL; main image shows particle number concentrations along flight path, max of 85,000 cm –3 with PM10 3–4 µg m –3 (green; not to scale) BC 300–500 ng m –3 (not shown). Yellow and light brown colors for individual plume cross sections and corresponding size distributions in the plumes of Spremberg and Boxberg (corresponding colors). On 10 Jun, there were stable conditions with clean RL (light blue) above a hazy and polluted PBL (dark gray) and southerly to southeasterly winds. Above, in the RL: (PM10; dark blue; 2–4 µg m –3 ; not to scale) low BC (300–500 ng m –3 ) but high nucleation mode UFP concentrations (max of 65,000 cm –3 ). PBL high fine particulate concentrations (dark blue; PM10 10–25 µg m –3 ; not to scale), visibility reduction to <15 km and high BC (up to 2000 ng m –3 ) but low UFP concentrations (max of 12,000 cm –1 ) in the Turow power station plume as listed in Table 3 . (bottom) Plumes from power stations Turow and Melnik and 15-h HYSPLIT back trajectories for 10 Jun, PBL air (<450 m AGL; blue) and RL (>550 m AGL; white). Insert shows temp, dewpoint (DP), and spread (temp minus DP) profiles for 10 Jun illustrate the stable stratification and the dry RL.

(top) UFP plumes at the German–Polish border originating from power stations Spremberg, Boxberg, and Jänschwalde, Germany, and Mělník, Czech Republic, under well-mixed moderately polluted and stable stratified heavily polluted conditions, 8 (yellow and brown) and 10 Jun (blue and gray) 2014 (campaign 12). Conditions on 8 Jun included southwesterly winds and well-mixed PBL up to >1,000 m AGL; main image shows particle number concentrations along flight path, max of 85,000 cm –3 with PM10 3–4 µg m –3 (green; not to scale) BC 300–500 ng m –3 (not shown). Yellow and light brown colors for individual plume cross sections and corresponding size distributions in the plumes of Spremberg and Boxberg (corresponding colors). On 10 Jun, there were stable conditions with clean RL (light blue) above a hazy and polluted PBL (dark gray) and southerly to southeasterly winds. Above, in the RL: (PM10; dark blue; 2–4 µg m –3 ; not to scale) low BC (300–500 ng m –3 ) but high nucleation mode UFP concentrations (max of 65,000 cm –3 ). PBL high fine particulate concentrations (dark blue; PM10 10–25 µg m –3 ; not to scale), visibility reduction to <15 km and high BC (up to 2000 ng m –3 ) but low UFP concentrations (max of 12,000 cm –1 ) in the Turow power station plume as listed in Table 3 . (bottom) Plumes from power stations Turow and Melnik and 15-h HYSPLIT back trajectories for 10 Jun, PBL air (<450 m AGL; blue) and RL (>550 m AGL; white). Insert shows temp, dewpoint (DP), and spread (temp minus DP) profiles for 10 Jun illustrate the stable stratification and the dry RL.

In our study, two aircraft were used, the Karlsruhe Institute of Technology (KIT) microlight ( Junkermann 2001 ) and the ARA motorized glider ( Junkermann and Hacker 2015 ). Typical cruising speed is 25 m s –1 for the microlight and 40 m s –1 for the motorized glider, resulting in a horizontal resolution of 1.5–2.4 km of sampling length for a complete UFP size distribution (2 min) and a 20–40-m resolution for the particle number concentration. Vertically, a 1-s resolution corresponds to less than 5-m altitude change. A real-time display of the sensor values in the cockpit allows immediate adjustment of the flight procedure. For instance, in case of a rapidly changing CPC signal, the aircraft can be held at constant altitude and approximate location until a full size distribution is completed. Overall, the instrumentation ensured a lower detection limit of below 5 nm, several size bins resolving the nucleation mode (<10 nm), and sufficient sensitivity to measure size distributions even in pristine environments.

UFP burst observed on the morning of 3 Jul 2009 over the grasslands of Inner Mongolia (campaign C9); local flight at altitude I500 m AGL; local time is UTC plus 8 h. (top left) Forward view from the aircraft. (top center) Time series of UFP > 10 nm. (top right) Surface temperature rising from 24° to 39°C (red); air temperature at 500 m AGL remains near constant at ∼19°C (blue). Temperatures indicate fully mixed boundary layer with depth >2,000 m (from HYSPLIT). The 3-h back trajectories indicate changing wind direction from the Xilinhot area power station to the east; lines show HYSPLIT trajectories at 2-h intervals, before (green), at maximum (blue), and after plume passage (red). Flight track of the aircraft is shown in yellow.

UFP burst observed on the morning of 3 Jul 2009 over the grasslands of Inner Mongolia (campaign C9); local flight at altitude I500 m AGL; local time is UTC plus 8 h. (top left) Forward view from the aircraft. (top center) Time series of UFP > 10 nm. (top right) Surface temperature rising from 24° to 39°C (red); air temperature at 500 m AGL remains near constant at ∼19°C (blue). Temperatures indicate fully mixed boundary layer with depth >2,000 m (from HYSPLIT). The 3-h back trajectories indicate changing wind direction from the Xilinhot area power station to the east; lines show HYSPLIT trajectories at 2-h intervals, before (green), at maximum (blue), and after plume passage (red). Flight track of the aircraft is shown in yellow.

Two aircraft, one set of aerosol instruments. (top) The KIT microlight aircraft with aerosol pod (15 kg) mounted at the left-hand side of the fuselage, also showing in the insert the in situ display in the cockpit and the aerosol and cloud droplet instrumentation [CPC, Optical Particle Spectrometer (OPC), SMPS, Forward Scattering Spectrometer Probe (FSSP); Table 1 ]. (bottom) ARA (left) motorized glider and (right) instrumentation pod. The KIT aerosol pod fits into one of the motorized glider’s underwing pods, shown here with the additional FSSP-100 for cloud droplet size distribution measurements during campaign C8 in 2006/07. Radiation sensors on the microlight are mounted on gimballed ±0.2° platforms above the wing.

Two aircraft, one set of aerosol instruments. (top) The KIT microlight aircraft with aerosol pod (15 kg) mounted at the left-hand side of the fuselage, also showing in the insert the in situ display in the cockpit and the aerosol and cloud droplet instrumentation [CPC, Optical Particle Spectrometer (OPC), SMPS, Forward Scattering Spectrometer Probe (FSSP); Table 1 ]. (bottom) ARA (left) motorized glider and (right) instrumentation pod. The KIT aerosol pod fits into one of the motorized glider’s underwing pods, shown here with the additional FSSP-100 for cloud droplet size distribution measurements during campaign C8 in 2006/07. Radiation sensors on the microlight are mounted on gimballed ±0.2° platforms above the wing.

As the emission and modification of anthropogenically generated UFPs takes place in either the PBL or the lower free troposphere (FT) at local to regional scales, we used a highly versatile flying laboratory, with miniaturized state-of-the-art aerosol, air chemistry, and meteorology sensors ( Table 1 ) mounted in instrumentation pods carried by aircraft capable of flying low and slow but also capable of climbing rapidly into the FT. The instruments, covering aerosols, air chemistry, and meteorology are listed in Table 2 . For studies in Australia, the aerosol package was installed in the underwing pods of the Airborne Research Australia (ARA) motorized glider, which is already instrumented for (micro)meteorological measurements ( Hacker and Crawford 1999 ; Hacker et al. 2016 ) ( Fig. 2 ). Using slow-flying aircraft proved to be a well-matched approach to capturing the spatial scale of UFP events, as well as to achieve the desired spatial resolution given by the temporal resolution of the instrumentation. The comparatively slow-flying speed and simple installation in instrument pods with extremely short inlet lines (between 5 and 30 cm for the aerosol instruments) also helped to avoid sampling problems that are common in larger, faster, and often pressurized aircraft, where samples tend to be constrained to sizes >20 nm, owing to loss processes in the inlet lines ( Andreae et al. 2018 ).

Results from multiple airborne campaigns flown since 1998 over various European countries as well as Australia and China (see Table 1) form the basis for the current study. The following paragraphs present key findings from these campaigns that are then combined to form a novel overall interpretation and explanation of the spatial and temporal UFP occurrence within the lower troposphere. For the campaigns C1 to C6 in Table 1, a twin CPC approach with different size cutoffs, 3 and 10 nm, was used to trace UFPs over regional-scale ranges of up to 150 km by 150 km and in the vertical from ∼10 to 3,000 m AGL. Local-scale (<5 km) UFP events were observed over individual coastal basins at the Irish west coast at Mace Head, Ireland (campaign C2), most likely attributable to biogenic sources (O’Dowd et al. 2007). High UFP concentrations over the boreal forests at Hyytiälä, Finland, in 2003 (campaign C4) and concurrent low concentrations over the embedded (ice covered) lakes in the area (O’Dowd et al. 2009) suggested a link between land surface properties, albedo, surface roughness, local biogenic emissions, and UFP spatial distribution. However, a clear source attribution was not successful with the limited instrumentation applied in these campaigns (O’Dowd et al. 2007, 2009; Laaksonen et al. 2008) (campaigns C2, C4, and C5). Anthropogenic sources of UFPs within the nucleation mode (4–10 nm) from fossil fuel burning were clearly identified for the first time as major emission in 2007 in a plume study downwind of the city of Karlsruhe, Germany. A coal-fired power station and a large refinery (REF) there dominated the city plume (Junkermann et al. 2011a; campaign C7). Similar industrial installations are located in the Italian Po Valley, a region that we investigated already in 2004 (campaign C5) without being able to identify the UFP sources. The conditions for atmospheric secondary particle formation were quite different from the boreal forest in Finland, with higher pollution levels and higher UV radiation but most likely less biogenic emissions over the agricultural land in early spring season. Also, the onset of UFP events was delayed until about noon with respect to Finland, despite the higher UV radiation levels (Hamed et al. 2007).

Using a more detailed Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT; Draxler and Rolph 2013) analysis for the case study of Laaksonen et al. (2005), the UFP events could be reassigned to a plume originating from the Mestre industrial complex in Italy about 6 h upwind under changing wind conditions.

In 2006/07 we were able to identify, for the first (and so far only) time, a local clearly non–fossil fuel source of nucleation mode particles in Western Australia (WA). Interestingly, these UFPs were found not above the forest but over the adjacent agricultural area with embedded salt lakes. These lakes were identified as the particle sources and attributable to a change in lake chemistry following deforestation decades ago (Junkermann et al. 2009; Kamilli et al. 2016) (campaigns C9 and C10). This remained the only case in about 1,200 flight hours spanning more than 15 years of our airborne research where a natural source significantly contributed to the regional budget of UFPs at a similar magnitude as anthropogenic sources. Similar to the missing nucleation mode particles in (above canopy) measurements over the Amazon (Fan et al. 2018; Andreae et al. 2018), we never found any signatures of atmospheric nucleation over the forested areas of Australia (Junkermann et al. 2009; Junkermann and Hacker 2015).

Enhanced UFP number concentrations were observed several times over the grasslands of Inner Mongolia (China) during a research project focusing on turbulent fluxes of energy and water vapor (campaign C8; Junkermann et al. 2011b). Because of legal limitations to import a radioactive source for the SMPS into China, it was only possible to use one CPC combined with sophisticated turbulence and meteorology instrumentation (Metzger et al. 2013). In one instance, while flying a 3 km by 5 km rectangular pattern at 500 m AGL, a local UFP event was observed under changing wind direction in a well-mixed boundary layer (up to 2,000 m AGL). A HYSPLIT analysis clearly related the origin of the plume to a power station at Xilinhot, China (see Fig. 3), 65 km to the north of the flight location. Over a 3-h period, the particle number concentration changed in response to this plume sweeping over the area because of a gradually backing wind direction. It increased from 1,000 cm–3 background to 40,000 cm–3 concurrently with a marked increase of surface temperature from 25° up to 45°C, well above the boundary layer temperature at 500 m of nearly constant ∼19°–20°C. The particle number concentration peaked after ∼90 min and then decreased again to background, displaying a signature very similar to UFP events observed in ground-based events elsewhere.

The most important overall finding for a better understanding of the observed patterns in UFP distributions was that, with the exception of Mace Head (O’Dowd et al. 2007) and Western Australia (Junkermann et al. 2009), all of our pronounced UFP event observations could without doubt be traced back to a limited number of large, modern, so-called clean, fossil fuel–burning sources, even in the presence of big-city plumes (Bonn et al. 2016). Most importantly, the magnitudes of these UFP events could not be explained plausibly without recognizing atmospheric advection to be the essential mechanism. It was possible to confirm this for every single case studied.

The location and technical description of most of these anthropogenic sources is well documented worldwide (http://endcoal.org/tracker). With a major nucleation mode between 5 and 15 nm and only minor particle concentrations in the lowest size bins, the size spectrum of the emitted particles matches the size distributions initially observed during ground-based UFP events well (Junkermann et al. 2011a, 2016; Junkermann and Hacker 2015).

Airborne aerosol studies in the seventies followed visible plumes for several hundreds of kilometers (Whitby et al. 1978; Pueschel and Van Valin 1978; Ayers et al. 1979; Hobbs et al. 1980). These plumes contained both fine and ultrafine particles and were thus simple to track. The invisible plumes of today, however, which lack significant number concentrations of visible fine particles but contain larger concentrations of UFPs, are more difficult to track but cover similar distances. In a similar approach to our investigations, but without using a size distribution measurement, Bigg and Turvey (1978) found enhanced particle number concentrations 160 km downwind of the city of Perth, speculatively at the time thought to be associated with urban and port emissions. In a later publication they recognized that the coal-fired Muja power station in Australia ∼180 km farther to the south may have contributed as a possible source to their measurements (Bigg et al. 2015). In the present study, the relevance of the Muja power station was confirmed for this case by HYSPLIT (not shown).

Figure 4 (top panel) shows two contrasting cases of power station emissions, observed in 2014 close to the Polish border (campaign C12), illustrating the impact of daytime versus nighttime meteorology and of clean versus polluted (hazy) PBL conditions, and the effects of horizontal transport and convective vertical mixing. The 8 Jun flight (yellow and brown bars), discussed in more detail in Junkermann et al. (2016), was flown in thermal convective conditions and moderate pollution. The 8 Jun data show two transects (south–north and north–south) under southwesterly winds and ∼1-h downwind advection time from the power stations along the Polish border, with emissions into a well-mixed turbulent boundary layer (up to 1,100 m AGL) and surface temperatures well above the threshold for thermal convection. A source strength of 1.5–2.5 × 1018 particles per second for both of the southern power stations was estimated, similar to the emission rates of the Karlsruhe power station (Junkermann et al. 2011a) or Kogan Creek, Queensland (Junkermann and Hacker 2015). On 10 Jun (gray and blue bars), under stable stratified conditions, the high PBL particle load suppressed most of the solar radiation reaching the ground, and surface temperatures accordingly stayed well below the threshold required for initiation of convection (Table 3). In the bottom panel of Fig. 4, the 10 Jun data are combined with 15-h HYSPLIT back trajectories for the planetary boundary and the early morning RL above 550 m between Dresden and Berlin, Germany. The flight altitude was changed several times to cover 1) a highly polluted hazy surface layer and 2) an approximately 400-m-deep clear RL above, separated by a strong (∼5°C) inversion at ∼500 m AGL (Fig. 4), with a further climb into the FT at the end of the flight pattern. Airmass characteristics for the PBL and the RL and FT, respectively, are summarized for both days in Table 3. In situ winds from the aircraft on 10 Jun were southeasterly but changed significantly within the few hours before the flight (see also the HYSPLIT trajectories). HYSPLIT analysis suggests that the high particle number concentrations in the upper layer are due to emissions into the evening or nocturnal RL (∼100 m AGL) from the Czech “clean” power station Melnik located 120 km upwind. The lower surface layer below 500 m AGL in Fig. 4 was, according to HYSPLIT, affected by the “dirty” power station Turow on the Polish side of the border, featuring the typical composition of unfiltered emissions with high concentrations of particulate matter with diameters of 2.5 and 10 µm or less (PM 2.5 and PM 10 , respectively) and of black carbon (BC), as well as low ozone values (Table 3). The flight results were not conducive to derive particle budgets similar to the previous days, as the plumes were not covered by the flight to their full extent. However, they resulted in a case study of the vertical stratiform layers in the early morning lower troposphere and confirmed nighttime transport in the RL.

To illustrate the impact emissions can have on meteorological parameters, Fig. 5 shows the plume characteristics of two large UFP sources located north of Adelaide, South Australia (SA), in October 2014: the smelter (SME) at Port Pirie and the Port Augusta power station (campaign C13). At a distance of 10 km from the stacks the two plumes contained more than 180,000 and 130,000 particles per cubic centimeter, respectively. The satellite image insert (from 1997; Rosenfeld 2000) shows for the same southwesterly wind direction the cloud modification with reduced droplet sizes in stratiform clouds over distances of >1,000 km. The second insert shows SMPS data from the two UFP sources measured along a 40-km road at a distance of ∼70–100 km, after approximately 2–3-h advection time downwind.

Fig . 5. View largeDownload slide Particle number concentrations (yellow) and fine particles >300 nm (green) within the PBL 400–600 m AGL north of Adelaide, Australia, 14 Oct 2014 (campaign 13) with south to southwesterly winds. Graph insert: particle size distributions within (yellow) and outside (red) the plume of Port Pirie from ground-based (vehicle based) measurements 70 km downwind. The satellite picture insert shows the corresponding plume of cloud droplets with decreased droplet sizes, plume width about 60 km after 300 km of transport. [Insert courtesy of Daniel Rosenfeld, Hebrew University, Tel Aviv, Israel; see also Rosenfeld (2000).] Fig . 5. View largeDownload slide Particle number concentrations (yellow) and fine particles >300 nm (green) within the PBL 400–600 m AGL north of Adelaide, Australia, 14 Oct 2014 (campaign 13) with south to southwesterly winds. Graph insert: particle size distributions within (yellow) and outside (red) the plume of Port Pirie from ground-based (vehicle based) measurements 70 km downwind. The satellite picture insert shows the corresponding plume of cloud droplets with decreased droplet sizes, plume width about 60 km after 300 km of transport. [Insert courtesy of Daniel Rosenfeld, Hebrew University, Tel Aviv, Israel; see also Rosenfeld (2000).]

It was possible to track these plumes over sparsely populated inland areas of Australia over 3 days and more than 1,200 km during a long-distance survey from Adelaide to Chinchilla, Queensland (Junkermann and Hacker 2015). Secondary GPC changed the size distribution within this plume during days 2 and 3. While aging shifted the initial aerosol mode to larger sizes, fresh particles from GPC in the sulfur-containing plume (Mohnen and Lodge 1969; Kiang et al. 1973) refilled the smallest-size bins but without any known sulfur emitter less than 400 km upwind. All back trajectories indicate Port Pirie and Port Augusta as the most likely sulfur sources.

Figure 6 shows such power-station-emission-related continental cloud modification, as detected from satellite for an area of the Czech and German power stations (cf. also Fig. 4).

Fig . 6. View largeDownload slide Power station “tracks” showing brighter clouds observed from space within a stratiform cloud deck over Germany, Poland, and the Czech Republic. The plumes under northerly to northeasterly winds, less clear compared to ship tracks over a dark sea surface but marked with dotted lines, originate from the same power stations investigated in more detail in Jun 2012 and 2014 (Fig. 4; campaign 12). Individual power stations can be identified. (Image courtesy of Daniel Rosenfeld, Hebrew University, Tel Aviv, Israel.) Fig . 6. View largeDownload slide Power station “tracks” showing brighter clouds observed from space within a stratiform cloud deck over Germany, Poland, and the Czech Republic. The plumes under northerly to northeasterly winds, less clear compared to ship tracks over a dark sea surface but marked with dotted lines, originate from the same power stations investigated in more detail in Jun 2012 and 2014 (Fig. 4; campaign 12). Individual power stations can be identified. (Image courtesy of Daniel Rosenfeld, Hebrew University, Tel Aviv, Israel.)