atmosphere Communication A New Type of Haze? The December 2015 Purple (Magenta) Haze Event in Nanjing, 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 Jiangsu 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, Nanjing University 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 toand 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.