atmosphere

Communication A New Type of Haze? The December 2015 Purple (Magenta) Haze Event in , China

Duanyang Liu 1,2,*, Xuejun Liu 3, Hongbin Wang 1, Yi Li 4, Zhiming Kang 2, Lu Cao 2, Xingna Yu 5 and Hao Chen 2

1 Key Laboratory of Transportation Meteorology, China Meteorological Administration, Nanjing 210008, China; [email protected] 2 Meteorological Observatory, Nanjing 210008, China; [email protected] (Z.K.); [email protected] (L.C.); [email protected] (H.C.) 3 College of Resources and Environmental Sciences, China Agricultural University, Beijing 100193, China; [email protected] 4 Department of Atmospheric Science, Colorado State University, Fort Collins, CO 80523, USA; [email protected] 5 Key Laboratory for Aerosol-Cloud-Precipitation of China Meteorological Administration, of Information Science and Technology, Nanjing 210044, China; [email protected] * Correspondence: [email protected]; Tel.: +86-25-8328-7206

Academic Editors: Yuxuan Wang and Robert W. Talbot Received: 25 December 2016; Accepted: 6 April 2017; Published: 14 April 2017

Abstract: A special and unusual purple (magenta) haze episode was observed in Nanjing, China, at 17:00 on 22 December 2015. Many local and national news outlets reported this event. Based on an analysis of the pollution features and meteorological factors, including boundary layer characteristics, we concluded that this haze event was similar in most respects to other local haze episodes. We discuss the reasons and the possibilities about this rare color haze at the end of the paper. One way to attain a combination of blue and red light is to have the green wavelengths selectively absorbed, and this seems unlikely for typical atmospheric constituents. Another way involves pollution gases or particles together with small liquid-water drops, which need further confirmation. A third possibility is that the combination of transmitted red light from the sun and scattered blue light from above could produce a purple/magenta color in the sky. In general, further studies are required to assess the physical, chemical, and optical features of this purple haze in order to explain and predict this phenomenon in the future.

Keywords: air pollution; cloud; East China; inversion; purple haze; rare atmospheric phenomenon

1. Introduction In many polluted areas, a layer of haze is formed; usually, the haze has white, red, reddish, yellow, brownish, or brown colors, and different researchers [1–6] have discussed the formation mechanism of these haze colors. Smog particles are well known for creating red skies and brilliant horizons [4,5]. In heavily polluted air, aerosol particles scatter the sun’s blue, green, and red light, providing a source of white light to a viewer from all points along the horizon [4,5]. By analyzing the solar radiation, Delusis et al. (1977) suggested that the absorption rate of the polluted air is about four times that for natural aerosols, and some gases such as NO2,O3, and H2O might increase the absorption [7]. Early research has suggested that reddish and brown colors in smog could be due to many factors. One is the preferential absorption of blue and green light by NO2, which allows most of green and red light to be transmitted, giving smog with NO2 a yellow, brown, or reddish-green color [4,5,8,9]. However, some studies conclude that light absorption by NO2 is only a minor contributor to the

Atmosphere 2017, 8, 76; doi:10.3390/atmos8040076 www.mdpi.com/journal/atmosphere Atmosphere 2017, 8, 76 2 of 10

Atmosphere 2017, 7, 76 2 of 10 reduction of the visual range [10] and that particle absorption (29%) makes a greater contribution thanthe does reduction NO2 absorption of the visual (7%) range [11 [10]], whichand that may part beicle due absorption to the minor(29%) makes NO2 acontent. greater contribution Waggoner et al. (1983)than found does aerosol NO2 absorption extinction, (7%) compared [11], which to may extinction be due to due theto minor NO 2NO, more2 content. variable Waggoner over the et al. period of measurement(1983) found aerosol [12]. A extinction, second possible compared causal to extinction factor isdue the to preferentialNO2, more variable absorption over the of period blue light of by nitroaromaticsmeasurement and [12]. polycyclic A second aromatic possible causal hydrocarbons factor is the (PAHs) preferential in aerosol absorption particles; of blue such light absorption by can alsonitroaromatics contribute and a reddish polycyclic or aromatic brownish hydrocarbons appearance (PAHs) to the in atmosphere aerosol particles; [4,5 ].such A thirdabsorption possibility can is that,also when contribute soil dust a reddish concentrations or brownish are appearance high, air canto the appear atmosphere reddish [4,5]. or A brown third possibility because soil is that, particles preferentiallywhen soil absorb dust concentrations blue and green are light high, over air redcan light.appear In reddish the absence or brown of a dustbecause storm soil and particles away from preferentially absorb blue and green light over red light. In the absence of a dust storm and away desert regions, however, the concentration of soil-dust particles is generally low, and the effect of soil from desert regions, however, the concentration of soil-dust particles is generally low, and the effect dustof on soil atmospheric dust on atmospheric optics is limited.optics is Alimited. thin layer A thin of layer predominantly of predominantly monodisperse monodisperse droplets droplets has also beenhas observed also been to cause observed brown to hazecause atbrown some haze distance at some above distance ground above by selective ground lightby selective scattering light [11 ,13]. scatteringFor particles [11,13]. smaller than the wavelengths of visible light, a preferential scattering of shorter, blue wavelengthsFor particles can lend smaller a blue than appearance the wavelengths to haze of [ 14visi,15ble]. light, Blue a haze preferential has been scattering studied of over shorter, forested areasblue [16 ],wavelengths where small can particles lend a formedblue appearance by interactions to haze between[14,15]. Blue biogenic haze has organic been acidsstudied and over sulfuric acid scatterforested light. areas Blue[16], where skies havesmallalso particles been formed investigated by interactions in a volcanic between eruption biogenic area organic [4,5]; acids after and a strong volcanicsulfuric eruption, acid scatter volcanic light. emissionBlue skies ofhave sulfur also dioxidebeen investigated into the stratospherein a volcanic eruption can lead area to formation[4,5]; after a strong volcanic eruption, volcanic emission of sulfur dioxide into the stratosphere can lead to of small sulfuric acid particles, many of which reach the stratosphere. Such particles scatter light formation of small sulfuric acid particles, many of which reach the stratosphere. Such particles through the stratospheric ozone layer, and ozone weakly absorbs green and some red wavelengths, scatter light through the stratospheric ozone layer, and ozone weakly absorbs green and some red transmittingwavelengths, blue transmitting and some red, blue which and some combine red, which to form combine purple to form [4,5]. purple [4,5]. A purpleA purple (magenta) (magenta) haze haze episode episode was was observedobserved in in Nanjing, Nanjing, China, China, on on22 December 22 December 2015. 2015. Many Many newsnews outlets outlets reported reported this this event event [17 ].[17]. Based Based on on Newton’s Newton’s primary primary colors colors (see(see FigureFigure 11),), wewe cancan see that, if greenthat, light if green is removed, light is removed, the resulting the colorresulting is magenta color is magenta or purple. or Onepurple. way One to attainway to a combinationattain a of bluecombination and red lightof blue is and to have red light the greenis to have wavelengths the green wavelengths selectively selectively absorbed. absorbed. This seems This unlikely seems for typicalunlikely atmospheric for typical constituents. atmospheric The constituents. formation The of purpleformation haze of ispurple difficult haze to is be difficult explained to be by the phenomenaexplained described by the phenomena above. The described aim of this above. short The communication aim of this short is tocommunication describe the purpleis to describe haze event the purple haze event in Nanjing, China, examine the pollution characteristics and meteorological in Nanjing, China, examine the pollution characteristics and meteorological conditions, and discuss conditions, and discuss the possible mechanisms for this rare color haze. The formation mechanisms the possible mechanisms for this rare color haze. The formation mechanisms need further observation need further observation and analysis. and analysis.

FigureFigure 1. 1. Newton’sNewton’s primary primary colors. colors.

2. Observation and Analysis

2.1. Observation Sites and Data The haze was observed on 22 December 2015 at different locations in Nanjing (Figure2). During this period, the visibility was about 1–2 km, and the relative humidity (RH) was lower than 85%, Atmosphere 2017, 7, 76 3 of 10

2. Observation and Analysis

2.1. Observation Sites and Data Atmosphere 2017, 8, 76 3 of 10 The haze was observed on 22 December 2015 at different locations in Nanjing (Figure 2). During this period, the visibility was about 1–2 km, and the relative humidity (RH) was lower than 85%, according to the China Meteorolog Meteorologicalical Administration observation standard, standard, this this was was a a heavy heavy haze. haze. This weather phenomenon was observed by different citizens during the same time at different positions inin NanjingNanjing fromfrom 16:0016:00 toto 17:3017:30 (the (the sunset sunset time time was was 17:05). 17:05). The The duration duration was was about about 1.5–2.0 1.5–2.0 h. Ith. wasIt was also also recorded recorded by aby meteorological a meteorological observatory. observatory. In order In order to show to thisshow phenomenon this phenomenon more clearly, more weclearly, chose we pictures chose frompictures citizens from from citizens different from positions. different Accordingpositions. toAccording the meteorological to the meteorological observatory record,observatory this kindrecord, of weather this kind phenomenon of weather hadphenomenon not been observed had not inbeen the observed previous 50in years.the previous The rarity 50 justifiesyears. The our rarity discussion. justifies our discussion.

Figure 2. Map of China showing the location of Nanjin Nanjingg and observation sites sites for for the the purple purple haze. haze. The suburbansuburban observationobservation locations locations were were at (A)at (A) Liuhe; Liuhe; (B) Pukou;(B) Pukou; and (C)and Jiangpu. (C) Jiangpu. The arrows The arrows denote directionsdenote directions of photographs of photographs taken at taken these at locations. these locations.

Figure 2 shows three points where different citizens watched this phenomenon, took pictures, Figure2 shows three points where different citizens watched this phenomenon, took pictures, and uploaded them to the web immediately. All pictures were taken in the northern suburbs of and uploaded them to the web immediately. All pictures were taken in the northern suburbs of Nanjing at 17:00 local time. The photographs were taken in different directions: the observer at Point Nanjing at 17:00 local time. The photographs were taken in different directions: the observer at A took pictures facing north at Liuhe (Figure 3A); most of the sky was purple or magenta, and this Point A took pictures facing north at Liuhe (Figure3A); most of the sky was purple or magenta, and phenomenon lasted more than 2 h. The observer at Point B took photos facing west at Pukou (Figure this phenomenon lasted more than 2 h. The observer at Point B took photos facing west at Pukou 3B); most of the sky was purple or magenta, and the phenomenon lasted more than 2 h. The observer (Figure3B); most of the sky was purple or magenta, and the phenomenon lasted more than 2 h. at Point C took photographs facing south at Jiangpu (Figure 3C); the sky was purple gray, and the The observer at Point C took photographs facing south at Jiangpu (Figure3C); the sky was purple gray, phenomenon persisted for more than 1 h. and the phenomenon persisted for more than 1 h. The meteorological conditions from Liuhe, Pukou, and Jiangning are examined here, including wind speed, wind direction, air temperature 2 m above ground, surface air temperature, dew point temperature, RH (relative humidity), visibility, and total and low cloud cover. Atmosphere 2017, 7, 76 4 of 10

AtmosphereThe2017 meteorological, 8, 76 conditions from Liuhe, Pukou, and Jiangning are examined here, including4 of 10 wind speed, wind direction, air temperature 2 m above ground, surface air temperature, dew point temperature, RH (relative humidity), visibility, and total and low cloud cover. The concentrations of six major pollutants, including PM2.5, PM10, SO2, NO2,O3, and CO at nine The concentrations of six major pollutants, including PM2.5, PM10, SO2, NO2, O3, and CO at nine officialofficial monitoring monitoring sites sites (Olympic (Olympic center, center, Caochangmen, Caochangmen, Maigaoqiao, Maigaoqiao, Pukou, Pukou, Ruijin Ruijin road, road, Shanxi Shanxi road, Xianlin,road, Xianlin, Xuanwu , and Lake, Zhonghuamen) and Zhonghuamen) are also are summarized.also summarized. TheThe concentrations concentrations of of PM PM2.52.5 at 13 13 different different cities cities are are also also examined examined here. here.

(A)

(B)

(C)

FigureFigure 3. 3.The The purple purple haze haze eventevent in the winter winter of of Na Nanjing,njing, China China at at17:00 17:00 22 December 22 December 2015 2015 local local time time (locations for photography: (A), Liuhe; (B), Pukou; (C), Jiangpu). (locations for photography: (A), Liuhe; (B), Pukou; (C), Jiangpu). 2.2. Trajectory Analysis 2.2. Trajectory Analysis The NOAA HYSPLIT4.8 (Hybrid Single Particle Lagrangian Integrated Trajectory) model is usedThe to NOAA calculate HYSPLIT4.8 backward trajectories (Hybrid Single for Pukou Particle at Lagrangian17:00 22 December Integrated 2015 Trajectory) local time, modelusing the is used toUnited calculate States backward National trajectories Center for for Environmental Pukou at 17:00 Prediction 22 December (NCEP) 2015 reanalysis local time, of meteorological using the United Statesdata National [18,19]. Seventy-two Center for Environmental hour backward Predictiontrajectories (NCEP) were calculated reanalysis at offour meteorological heights: 50 m, data 150 m, [18 ,19]. Seventy-two300 m, and 500 hour m. backward trajectories were calculated at four heights: 50 m, 150 m, 300 m, and 500 m.

2.3. Satellite Image The Himawari-8 satellite image was used during the haze episode to show the cloud conditions. We choose Band 13: 10.4 µm—the image from 08:00 to 09:30 UTC 22 December 2015. Atmosphere 2017, 8, 76 5 of 10

3. Results

3.1. Pollution Features and Meteorological Elements Table1 presents surface pollution concentrations at 17:00 on 22 December 2015 (local time) for nine stations in Nanjing, China. During the purple haze, the surface PM2.5 concentrations in Nanjing 3 3 were all greater than 180 µg/m , with some exceeding 250 µg/m . PM10 concentrations were all 3 3 greater than 230 µg/m , with some exceeding 300 µg/m . The SO2 concentrations were mostly lower 3 3 than 30 µg/m . Most NO2 concentrations exceeded 100 µg/m . CO concentrations were all larger than 1.0 mg/m3, with concentrations at the downtown stations (Caochangmen, Maigaoqiao, Ruijinlu, Xianlin, and Xuanwu Lake) all exceeding 2 mg/m3.

Table 1. Surface pollution concentrations at 17:00 on 22 December 2015 (local time) in Nanjing, China.

Stations PM2.5 PM10 SO2 NO2 O3 CO Unit µg/m3 mg/m3 Olympic Center 205 246 20 107 11 1.7 Caochangmen 192 293 26 107 3 2.1 Maigaoqiao 251 304 29 61 1 2.3 Pukou 199 274 22 85 16 1.6 Ruijin road 253 299 31 121 6 2.5 Shanxi road 251 309 21 129 16 1.0 Xianlin 220 302 26 125 44 2.4 Xuanwu lake 207 277 19 106 13 2.0 Zhonghuamen 188 239 26 130 4 1.5

From the pollution concentration changes of 9 stations (Figure4), we can see that the PM 2.5, PM10, CO, and NO2 all showed an increasing trend as of 19 December. PM2.5 and PM10 reached the first peak during 20:00–23:00 on 21 December, and then declined. There was an increasing trend after 10:00 on 22 December and reached a peak again at 04:00 on 23 December. The purple haze appeared during these two peaks. NO2 also peaked twice: the first one was from 19:00 to 21:00 on 21 December, and the second one was from 16:00 to 18:00 on 22 December. 3 When the second peak occurred, most NO2 concentrations exceeded 100 µg/m , and this was when the purple haze occurred. O3 concentration also peaked twice: once from 13:00 to 17:00 on 21 December, and once from 14:00 to 16:00 on 22 December, respectively, and the second peak appeared just before the purple haze. In Figure5 and Table2, it can be seen that, during the purple haze period, surface winds came from the northwest with speeds below 2.5 m/s. The temperatures were between 8.5 and 9.6 ◦C, RH values were in the range 75–82%, and visibilities were between 1.1 and 1.6 km (Table2).

Table 2. Meteorological conditions at 17:00 on 22 December 2015 in Nanjing, China.

Meteorological Total Cloud Low Cloud WS WD T T Td RH Visibility Elements 2m surface Cover Cover Unit m/s ◦ ◦C % % km Jiangning 0.3 315.0 10.0 9.3 6.5 79 10 0 1.3 Liuhe 1.3 292.5 9.2 8.5 6.3 82 10 0 1.1 Pukou 2.3 337.5 10.2 9.6 6.0 75 10 0 1.6 WS: wind speed; WD: wind direction; Td: dew-point temperature; RH: relative humidity. Atmosphere 2017, 8, 76 6 of 10 AtmosphereAtmosphere 2017, 7 2017, 76, 7, 76 6 of 10 6 of 10

400 400 140 Olympic Olympic Centre Centre Caochangmen Caochangmen Maigaoqiao Maigaoqiao Olympic Centre Olympic CentreCaohangmen Caohangmen Maigaoqiao Maigaoqiao PM10 140 Pukou Ruijin road Shanxi road NO2 350 Pukou Ruijin road Shanxi road PM10 Pukou Ruijin road Shanxi road NO2 350 Pukou Ruijin road Shanxi road Xianlin Xuanwu Lake Zhonghuamen Xianlin Xuanwu Lake Zhonghuamen 120 Xianlin Xuanwu Lake Zhonghuamen 300 Xianlin Xuanwu Lake Zhonghuamen 120 300 100 250 100 250 80 N_PM10 200 N_NO2 6080 N_PM10 200 150 N_NO2 4060 150 100 2040 100 160 Olympic Centre Caochangmen 300 Olympic Centre Caochangmen Maigaoqiao PM2.5 20 Maigaoqiao Pukou O3 Pukou Ruijin road Shanxi road 120 Ruijin road Shanxi road 250 Xianlin Xuanwu Lake Zhonghuamen 7580160 Xianlin Olympic Centre Xuanwu Lake Caochangmen 300 Olympic Centre Caochangmen Maigaoqiao PM2.5 120 Zhonghuamen Maigaoqiao Pukou O3 200Pukou Ruijin road Shanxi road 60 Ruijin road Shanxi road 250 Xianlin Xuanwu Lake Zhonghuamen 7580 Xianlin Xuanwu Lake N_O3 45 Zhonghuamen N_PM25 150 200 3060 100

15N_O3 45 N_PM25 150 50 3.5 70030 100 Olympic Centre Caochangmen Maigaoqiao CO 3.0 Pukou Ruijin road Shanxi road) 6015 Olympic Centre Caochangmen Maigaoqiao SO2 Xianlin Xuanwu Lake Zhonghuamen Pukou Ruijin road Shanxi road 50 2.5 50 Xianlin Xuanwu Lake Zhonghuamen 3.5 700 Olympic2.0 Centre Caochangmen Maigaoqiao CO 40 3.0 Pukou Ruijin road Shanxi road) 60 Olympic Centre Caochangmen Maigaoqiao SO2

1.5 N_SO2 N_CO 30 Xianlin Xuanwu Lake Zhonghuamen Pukou Ruijin road Shanxi road 2.5 1.0 2050 Xianlin Xuanwu Lake Zhonghuamen 2.0 0.5 1040 0.0 0

1.5 N_SO2 N_CO 30 1.0 20 0.5 10 0.0 0 2015/12/19 00:00 2015/12/19 04:00 2015/12/19 08:00 2015/12/19 12:00 2015/12/19 16:00 2015/12/19 20:00 2015/12/19 24:00 2015/12/19 04:00 2015/12/20 08:00 2015/12/20 12:00 2015/12/20 16:00 2015/12/20 20:00 2015/12/20 24:00 2015/12/20 04:00 2015/12/21 08:00 2015/12/21 12:00 2015/12/21 16:00 2015/12/21 20:00 2015/12/21 24:00 2015/12/21 04:00 2015/12/22 08:00 2015/12/22 12:00 2015/12/22 16:00 2015/12/22 20:00 2015/12/22 24:00 2015/12/22 04:00 2015/12/23 08:00 2015/12/23 12:00 2015/12/23 16:00 2015/12/23 20:00 2015/12/23 24:00 2015/12/23 2015/12/19 00:00 2015/12/19 04:00 2015/12/19 08:00 2015/12/19 12:00 2015/12/19 16:00 2015/12/19 20:00 2015/12/19 24:00 2015/12/19 04:00 2015/12/20 08:00 2015/12/20 12:00 2015/12/20 16:00 2015/12/20 20:00 2015/12/20 24:00 2015/12/20 04:00 2015/12/21 08:00 2015/12/21 12:00 2015/12/21 16:00 2015/12/21 20:00 2015/12/21 24:00 2015/12/21 04:00 2015/12/22 08:00 2015/12/22 12:00 2015/12/22 16:00 2015/12/22 20:00 2015/12/22 24:00 2015/12/22 04:00 2015/12/23 08:00 2015/12/23 12:00 2015/12/23 16:00 2015/12/23 20:00 2015/12/23 24:00 2015/12/23 Figure 4. Surface pollution concentrations from 19 to 23 December 2015 (local time) in Nanjing (9 stations: Olympic Center, Caochangmen, Maigaoqiao, Pukou, Ruijin Road, Shanxi Road, Xianlin, 2015/12/19 00:00 2015/12/19 04:00 2015/12/19 08:00 2015/12/19 12:00 2015/12/19 16:00 2015/12/19 20:00 2015/12/19 24:00 2015/12/19 04:00 2015/12/20 08:00 2015/12/20 12:00 2015/12/20 16:00 2015/12/20 20:00 2015/12/20 24:00 2015/12/20 04:00 2015/12/21 08:00 2015/12/21 12:00 2015/12/21 16:00 2015/12/21 20:00 2015/12/21 24:00 2015/12/21 04:00 2015/12/22 08:00 2015/12/22 12:00 2015/12/22 16:00 2015/12/22 20:00 2015/12/22 24:00 2015/12/22 04:00 2015/12/23 08:00 2015/12/23 12:00 2015/12/23 16:00 2015/12/23 20:00 2015/12/23 24:00 2015/12/23 Xuanwu Lake, Zhonghuamen), China. 00:00 2015/12/19 04:00 2015/12/19 08:00 2015/12/19 12:00 2015/12/19 16:00 2015/12/19 20:00 2015/12/19 24:00 2015/12/19 04:00 2015/12/20 08:00 2015/12/20 12:00 2015/12/20 16:00 2015/12/20 20:00 2015/12/20 24:00 2015/12/20 04:00 2015/12/21 08:00 2015/12/21 12:00 2015/12/21 16:00 2015/12/21 20:00 2015/12/21 24:00 2015/12/21 04:00 2015/12/22 08:00 2015/12/22 12:00 2015/12/22 16:00 2015/12/22 20:00 2015/12/22 24:00 2015/12/22 04:00 2015/12/23 08:00 2015/12/23 12:00 2015/12/23 16:00 2015/12/23 20:00 2015/12/23 24:00 2015/12/23 Figure 4. Surface pollution concentrations from 19 to 23 December 2015 (local time) in Nanjing FigureIn 4. Figure Surface 5 and pollution Table 2,concentrations it can be seen that,from du 19ring to the23 Decemberpurple haze 2015 period, (local surface time) winds in Nanjing came (9 (9 stations: Olympic Center, Caochangmen, Maigaoqiao, Pukou, Ruijin Road, Shanxi Road, Xianlin, stations:from the Olympic northwest Center, with speeds Caocha belowngmen, 2.5 m/s.Maigaoqiao, The temperatures Pukou, Ruijwerein between Road, Shanxi8.5 and Road,9.6 °C, Xianlin,RH XuanwuXuanwuvalues Lake,were Lake, in Zhonghuamen), Zhonghuamen), the range 75–82%, China. China. and visibilities were between 1.1 and 1.6 km (Table 2).

In Figure 5 and Table10 2, it can be seen that, during the purple haze period, surface winds came from the northwest with1 speeds below 2.5 m/s. The temperatures were between 8.5 and 9.6 °C, RH Visibility values were in the rangeVis (m) 0.1 75–82%, and visibilities were between 1.1 and 1.6 km (Table 2). 100 90 RH 10 80 70 60 1RH (%) 50 40 20 Visibility Vis (m) 0.1 15 Tsur 10 100 5 90 RH 80 T (℃) 0 -5 70 8 60 RH (%) 50 6 WS 40 4 20 2 Tsur

15 WS (m/s) 0 10 5

T (℃) 0 -5 8

6 2015/12/19 00:00 2015/12/19 04:00 2015/12/19 08:00 2015/12/19 12:00 2015/12/19 16:00 2015/12/19 20:00 2015/12/19 24:00 2015/12/20 04:00 2015/12/20 08:00 2015/12/20 12:00 2015/12/20 16:00 2015/12/20 20:00 2015/12/20 24:00 2015/12/21 04:00 2015/12/21 08:00 2015/12/21 12:00 2015/12/21 16:00 2015/12/21 20:00 2015/12/21 24:00 2015/12/22 04:00 2015/12/22 08:00 2015/12/22 12:00 2015/12/22 16:00 2015/12/22 20:00 2015/12/22 24:00 2015/12/23 04:00 2015/12/23 08:00 2015/12/23 12:00 2015/12/23 16:00 2015/12/23 20:00 WS2015/12/23 24:00

4 FigureFigure 5. Meteorological 5. Meteorological2 conditions conditions from from 1919 to 23 23 December December 2015 2015 (local (local time) time) in Nanjing, in Nanjing, China. China.

WS (m/s) 0 3.2. The Mixing Boundary Layer Features The wind, RH, and temperature profiles of Nanjing at 20:00 on 22 December 2015, 3 h after this purple haze, are displayed in Figure6. From these profiles, we can see that there were three cloud layers: the first between2015/12/19 00:00 2502015/12/19 04:00 2015/12/19 08:00 and2015/12/19 12:00 2015/12/19 16:00 7002015/12/19 20:00 2015/12/19 24:00 m,2015/12/20 04:00 the2015/12/20 08:00 2015/12/20 12:00 second2015/12/20 16:00 2015/12/20 20:00 2015/12/20 24:00 2015/12/21 04:00 between2015/12/21 08:00 2015/12/21 12:00 2015/12/21 16:00 2015/12/21 20:00 15002015/12/21 24:00 2015/12/22 04:00 2015/12/22 08:00 and2015/12/22 12:00 2015/12/22 16:00 16002015/12/22 20:00 2015/12/22 24:00 m,2015/12/23 04:00 2015/12/23 08:00 and2015/12/23 12:00 2015/12/23 16:00 the2015/12/23 20:00 2015/12/23 24:00 third between

2000 and 2200 m. These three cloud layers were associated with three temperature inversions. The first inversionFigure was 5. between Meteorological 400 and conditions 760 m, the from second 19 to 23 inversion December between 2015 (loc 1500al time) and in 1600 Nanjing, m, and China. the third inversion between 2000 and 2250 m. Atmosphere 2017, 7, 76 7 of 10

Table 2. Meteorological conditions at 17:00 on 22 December 2015 in Nanjing, China.

Meteorological WS WD T2m Tsurface Td RH Total Cloud Cover Low Cloud Cover Visibility Elements Unit m/s ° °C % % km Atmosphere 2017Jiangning, 7, 76 0.3 315.0 10.0 9.3 6.5 79 10 0 1.3 7 of 10 Liuhe 1.3 292.5 9.2 8.5 6.3 82 10 0 1.1 PukouTable 2. Meteorological 2.3 337.5 10.2 conditions 9.6 6.0 at 17:0075 on 22 December 10 2015 in Nanjing, 0 China. 1.6 WS: wind speed; WD: wind direction; Td: dew-point temperature; RH: relative humidity. Meteorological WS WD T2m Tsurface Td RH Total Cloud Cover Low Cloud Cover Visibility 3.2.Elements The Mixing Boundary Layer Features Unit m/s ° °C % % km JiangningThe wind, RH, 0.3 and315.0 temperature 10.0 9.3 profiles 6.5 of Nanjin79 g at 20:00 10 on 22 December 02015, 3 h after this 1.3 purpleLiuhe haze, are1.3 displayed 292.5 in9.2 Figure 8.5 6. From6.3 thes82e profiles, we 10 can see that there 0 were three cloud 1.1 layers:Pukou the first between 2.3 337.5 250 and10.2 700 m,9.6 the 6.0second 75 between 1500 10 and 1600 m, and 0the third between 1.6 2000 andWS: 2200 wind m. speed; These WD: three wind cloud direction; layers were Td: dew-point associated temperature; with three temperature RH: relative inversions. humidity. The first inversion was between 400 and 760 m, the second inversion between 1500 and 1600 m, and the 3.2. Thethird Mixing inversion Boundary between Layer 2000 Featuresand 2250 m. The wind direction was westerly or southwesterly above 600 m, and easterly below 600 m. The windThe wind,speed wasRH, lower and temperature than 4 m/s below profiles 660 m, of and Nanjin lowerg atthan 20:00 2 m/s on below 22 December 100 m and 2015, between 3 h after350 this purpleand haze, 550 m are (Figure displayed 6). in Figure 6. From these profiles, we can see that there were three cloud layers: theFrom first the between satellite 250image and (Figure 700 m, 7), the it can second be seen between that, from 1500 08:00 and to 160009:30 m,UTC and on the22 December third between 20002015, and there2200 wasm. These a cloud three layer cloud covered layers Nanjing were from associated 16:20–17:30 with on three 22 December temperature 2015 (local inversions. time), The and the cloud moved from the southwest to the northeast, which was consistent with the second Atmospherefirst inversion2017, 8 ,was 76 between 400 and 760 m, the second inversion between 1500 and 1600 m, and7 ofthe 10 thirdand inversion the third between cloud layers 2000 (Figure and 2250 6). m. The wind direction was westerly or southwesterly above 600 m, and easterly below 600 m. The -2 0 2 4 6 8 10 12 3000 3000 wind speed was lower than 4 m/s below 660 m, and lowerT (℃ ) than 2 m/s below 100 m and between 350 and 550 m (Figure 6). 2500 2500 From the satellite image (Figure 7), it can be seen that, from 08:00 to 09:30 UTC on 22 December

2015, there was a cloud layer2000 covered Nanjing from 16:20–17:30 on 200022 December 2015 (local time), and the cloud moved from the southwest to the Wind northeast, which was consistent with the second 1500 RH 1500 and the third cloud layers (Figure 6). Temperature Height (m) Height Height (m)

1000 1000 -2 0 2 4 6 8 10 12 3000 3000 ℃ 500 700m T ( ) 500

2500 2500 0 0 wind profile 65 70 75 80 85 90 95 2000 RH (%) 2000 Wind Figure 6. The profiles of the wind, RH, and temperature in Nanjing at 20:00 on 22 December 2015. Figure 6. The profiles of1500 the wind, RH, and temperature RH in Nanjing at 20:001500 on 22 December 2015.

Temperature Height (m) Height Height (m) The wind direction was1000 westerly or southwesterly above 600 m,1000 and easterly below 600 m. The wind speed was lower than 4 m/s below 660 m, and lower than 2 m/s below 100 m and between 500 700m 500 350 and 550 m (Figure6). From the satellite image (Figure7), it can be seen that, from 08:00 to 09:30 UTC on 22 December 0 0 2015, there was a cloud layer coveredwind Nanjingprofile 65 from 70 16:20–17:30 75 80 85 on 90 22 95December 2015 (local time), and the cloud moved from the southwest to the northeast, whichRH (%) was consistent with the second and the third cloudFigure layers 6. The (Figureprofiles6 of). the wind, RH, and temperature in Nanjing at 20:00 on 22 December 2015.

Atmosphere 2017, 7, 76 8 of 10

Figure 7. The Himawari-8 satellite image (Band 13: 10.4 μm—from 08:00 to 09:30 UTC on 22 Figure 7. The Himawari-8 satellite image (Band 13: 10.4 µm—from 08:00 to 09:30 UTC on December 2015). 22 December 2015).

3.3. Regional Transport Process 3.3. Regional Transport Process In order to analyze the pollution transportation of this purple haze, the 72 h backward In order to analyze the pollution transportation of this purple haze, the 72 h backward trajectories trajectories during the purple haze were calculated (Figure 8). The length of cluster-mean trajectories during the purple haze were calculated (Figure8). The length of cluster-mean trajectories determines determines the transport speed of the air masses. A longer pathway equates to faster transport. From 72 h backward trajectories, we can see that all of the air parcels were recirculated within a 300 km area, the air parcels at 50 m, 150 m, and 300 m height are all circled in Jiangsu province. Figure 9 shows the surface PM2.5 concentrations from 02:00 to 17:00 on 22 December 2015 (local time) in 13 cities of Jiangsu, and it can be seen that the pollutions were all greater than 100 μg/m3 in the south of Jiangsu. The upstream transportation caused the PM2.5 concentrations to increase to a high level. In Figure 6, it can be seen that the first inversion was between 400 and 760 m, and based on Figures 8 and 9, we can estimate that the pollution concentrations would have similar concentrations below 400 m.

Figure 8. Backward trajectories ending at 17:00 BST on 22 December 2015 for Nanjing (height: red, 50 m; blue, 150 m; cyan, 300 m; green, 500 m). Atmosphere 2017, 7, 76 8 of 10

Figure 7. The Himawari-8 satellite image (Band 13: 10.4 μm—from 08:00 to 09:30 UTC on 22 December 2015).

3.3. Regional Transport Process Atmosphere 2017 8 In order, ,to 76 analyze the pollution transportation of this purple haze, the 72 h backward8 of 10 trajectories during the purple haze were calculated (Figure 8). The length of cluster-mean trajectories thedetermines transport the speed transport of the speed air masses.of the air Amasses. longer A pathway longer pathway equates equates to faster to transport.faster transport. From From 72 h backward72 h backward trajectories, trajectories, we can we seecan that see allthat of all the of air the parcels air parcels were were recirculated recirculated within within a 300 a km 300 area, km thearea, air the parcels air parcels at 50 m,at 50 150 m, m, 150 and m, 300 and m 300 height m height are all circledare all circled in Jiangsu in Jiangsu province. province. Figure 9Figure shows 9 2.5 theshows surface the surface PM2.5 concentrations PM concentrations from 02:00 from to 02:00 17:00 to on 17:00 22 December on 22 December 2015 (local 2015 time) (local in time) 13 cities in 13 of Jiangsu,cities of andJiangsu, it can and be seenit can that be theseen pollutions that the pollutions were all greater were all than greater 100 µ g/mthan3 100in the μg/m south3 in ofthe Jiangsu. south 2.5 Theof Jiangsu. upstream The transportation upstream transportation caused the PMcaused2.5 concentrations the PM concentrations to increase to increase a high level. to a high level. In FigureFigure6 6,, it it can can be be seen seen that that the the first first inversion inversion was was between between 400 400 and and 760 760 m,m, andand basedbased onon Figures8 and8 and9, we 9, can we estimate can estimate that the pollutionthat the concentrationspollution concentrations would have similarwould concentrations have similar belowconcentrations 400 m. below 400 m.

Figure 8. Backward trajectories ending at 17:00 BST on 22 December 2015 for Nanjing (height: red, 50 Figure 8. Backward trajectories ending at 17:00 BST on 22 December 2015 for Nanjing (height: red, m; blue, 150 m; cyan, 300 m; green, 500 m). Atmosphere50 m; 2017 blue,, 7, 76 150 m; cyan, 300 m; green, 500 m). 9 of 10

FigureFigure 9. 9. SurfaceSurface PM PM2.5 2.5concentrationsconcentrations from from 02:00 02:00 to to17:00 17:00 on on22 December 22 December 2015 2015 (local (local time) time) in 13 in cities13 cities (Nanjing, (Nanjing, Xuzhou, Xuzhou, Suqian, Suqian, Lianyungang, Lianyungang, Huaian, Huaian, Yancheng, Yancheng, Yangzhou, Yangzhou, Taizhou, Taizhou, Zhenjiang, Zhenjiang, Changzhou,Changzhou, Wuxi, Wuxi, Suzhou, Suzhou, an andd Nantong), Nantong), China. China. Unit: Unit: μµg/mg/m3. 3.

4. Questions, Possibilities and Discussion During sunset, with such typical pollution conditions, a brown haze is expected. Pollution levels, meteorological conditions, and transport patterns were not unusual relative to those commonly seen during other Nanjing haze events [20–24]. Why might the haze have presented such a distinctive purple color? Where did the blue light come from? Why was the blue light not absorbed? Why was the green light absorbed, the blue and red light transmitted, and what pollution absorbed the green light? Was it O3 or other polluting gases or particles? The scattering of sunlight by atmospheric gas molecules tends to make the sky look blue, since short wavelengths are scattered more effectively by these small objects compared to longer ones. The shortest wavelengths in the visible spectrum are actually violet/purple, but the human eye does not detect these as well, so the sky generally appears blue to a human observer. At sunset, as the sun’s light travels through a longer atmospheric path before reaching the observer, the sky tends to appear red/orange, because the shorter blue wavelengths are scattered out of the line of sight. Jacobson (2012) [5] points out that, under severe polluted conditions, sunset hazes also generally tend toward red or brown due to the absorption of blue and green light by NO2, with a more effective transmission of longer wavelengths. A second possibility not only includes other pollution gas or particle influences, but also small liquid-water drops, or pollution together with small liquid-water drops. However, this requires further physical, chemical, and optical observation and explanation. Another possibility is that red light, typically associated with sunsets, is transported toward the observer. The addition of a shorter-wavelength blue light scatters off small particles in the atmosphere above the observer. This scattering could be enhanced by sunlight reflected off of overhead clouds still illuminated by the sun. The combination of transmitted red light from the sun and scattered blue light from above could produce a purple/magenta color in the sky. There are three cloud layers and inversions below 2500 m. There is a great amount of purple sunset glow in the sky in many different places, and a purple sky is usually influenced by clouds during the sunrise of sunset. Thus, the third possibility is that this event is a combination of sunset glow and haze. Atmosphere 2017, 8, 76 9 of 10

4. Questions, Possibilities and Discussion During sunset, with such typical pollution conditions, a brown haze is expected. Pollution levels, meteorological conditions, and transport patterns were not unusual relative to those commonly seen during other Nanjing haze events [20–24]. Why might the haze have presented such a distinctive purple color? Where did the blue light come from? Why was the blue light not absorbed? Why was the green light absorbed, the blue and red light transmitted, and what pollution absorbed the green light? Was it O3 or other polluting gases or particles? The scattering of sunlight by atmospheric gas molecules tends to make the sky look blue, since short wavelengths are scattered more effectively by these small objects compared to longer ones. The shortest wavelengths in the visible spectrum are actually violet/purple, but the human eye does not detect these as well, so the sky generally appears blue to a human observer. At sunset, as the sun’s light travels through a longer atmospheric path before reaching the observer, the sky tends to appear red/orange, because the shorter blue wavelengths are scattered out of the line of sight. Jacobson (2012) [5] points out that, under severe polluted conditions, sunset hazes also generally tend toward red or brown due to the absorption of blue and green light by NO2, with a more effective transmission of longer wavelengths. A second possibility not only includes other pollution gas or particle influences, but also small liquid-water drops, or pollution together with small liquid-water drops. However, this requires further physical, chemical, and optical observation and explanation. Another possibility is that red light, typically associated with sunsets, is transported toward the observer. The addition of a shorter-wavelength blue light scatters off small particles in the atmosphere above the observer. This scattering could be enhanced by sunlight reflected off of overhead clouds still illuminated by the sun. The combination of transmitted red light from the sun and scattered blue light from above could produce a purple/magenta color in the sky. There are three cloud layers and inversions below 2500 m. There is a great amount of purple sunset glow in the sky in many different places, and a purple sky is usually influenced by clouds during the sunrise of sunset. Thus, the third possibility is that this event is a combination of sunset glow and haze.

5. Conclusions If this third explanation is correct, the appearance of purple haze is not a new phenomenon—just an unusual one. It is simply a combination of typical haze and cloud layers arranged in such a way such that a low sun angle results in an observer’s witnessing a combination of transmitted (red) and scattered (blue) light that combine to make the sky appear purple.

Acknowledgments: Funding for this work was jointly provided by the National Key Project of MOST (JFYS2016ZY01002213-03), the Natural Science Foundation of Jiangsu Province (Grant No. BK20130111, BK20161073), the National Natural Science Foundation of China (Grant Nos. D0511/41275151, D0512/91544229 and D0512/91544231), the Key Projects of Jiangsu Meteorological Bureau (Grant NO. KZ201405), the National Key Technology Support Program (2014BAC22B04), and the Qing Lan Project. The authors would also like to thank William C. Malm from Cooperative Institute for Research in the Atmosphere (CIRA), Colorado State University for his advice on text revision. Author Contributions: This study was completed with cooperation among all authors: Duanyang Liu conceived and designed the research topic. Duanyang Liu and Yi Li conducted the research and wrote the manuscript. Duanyang Liu, Xuejun Liu, Xingna Yu, Zhiming Kang, Hongbin Wang, Hao Chen, and Lu Cao collaborated in discussing the results and providing editorial advice. Conflicts of Interest: The authors declare no conflict of interest.

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