The Himalayas and the Tibetan Plateau region (HTP), despite being a remote and sparsely populated area, is regularly exposed to polluted air masses with significant amounts of aerosols including black carbon. These dark, light-absorbing particles are known to exert a great melting potential on mountain cryospheric reservoirs through albedo reduction and radiative forcing. This study combines ground-based and satellite remote sensing data to identify a severe aerosol pollution episode observed simultaneously in central Tibet and on the southern side of the Himalayas during 13–19 March 2009 (pre-monsoon). Trajectory calculations based on the high-resolution numerical weather prediction model COSMO are used to locate the source regions and study the mechanisms of pollution transport in the complex topography of the HTP. We detail how polluted air masses from an atmospheric brown cloud (ABC) over South Asia reach the Tibetan Plateau within a few days. Lifting and advection of polluted air masses over the great mountain range is enabled by a combination of synoptic-scale and local meteorological processes. During the days prior to the event, winds over the Indo-Gangetic Plain (IGP) are generally weak at lower levels, allowing for accumulation of pollutants and thus the formation of ABCs. The subsequent passing of synoptic-scale troughs leads to southwesterly flow in the middle troposphere over northern and central India, carrying the polluted air masses across the Himalayas. As the IGP is known to be a hotspot of ABCs, the cross-Himalayan transport of polluted air masses may have serious implications for the cryosphere in the HTP and impact climate on regional to global scales. Since the current study focuses on one particularly strong pollution episode, quantifying the frequency and magnitude of similar events in a climatological study is required to assess the total impact.
The Himalayas and Tibetan Plateau region (HTP), sometimes called the “third pole”, contains the largest volume of ice outside the polar regions and impacts radiative budgets and climate (Ye and Wu, 1998; Ma et al., 2009). Recently, a growing body of research has demonstrated that the atmosphere and cryosphere in the HTP are undergoing extraordinary changes, including atmospheric warming (Gautam et al., 2010; Thompson et al., 2000; Kang et al., 2010) and in many parts rapid glacier melting (Bolch et al., 2012; Yao et al., 2012). Consequently, the seasonal water availability of important Asian river systems are very likely to be affected (Immerzeel et al., 2010; Kehrwald et al., 2008). In addition to greenhouse gases, increasing ambient concentrations of black carbon (BC) appear to be an anthropogenic driving force of the observed changes in these remote regions (Lau et al., 2010; Ramanathan and Carmichael, 2008). Light-absorbing aerosol particles such as mineral dust and BC contribute to the atmospheric heating and the albedo reduction once deposited on glaciers. Albeit only contributing a few percent to the total aerosol mass, BC exerts major radiative effects (Bond et al., 2013; Jacobson, 2001), especially over the HTP during pre-monsoon seasons when the solar radiative flux at the surface is very high (Flanner et al., 2007).
Even though background air pollution levels in the HTP are very
low, recurring pre-monsoonal BC peaks have been documented at high
altitudes of the south-facing Himalayan slopes (e.g., Marinoni et al.,
2010, 2013; Decesari et al., 2010) which are
sometimes directly exposed to atmospheric brown clouds (ABC)
(Ramanathan et al., 2007b; Bonasoni et al., 2010). Brown clouds have
been defined as “huge blankets or layers of
Recent GEOS-Chem and HYSPLIT model calculations of BC advection to the TP suggest that the dominant source regions depend on season and receptor location, but South and East Asia show the highest overall contribution (Kopacz et al., 2011; Lu et al., 2012). However, to date, the mechanisms of pollutant transport from the ABC hotspot in the IGP and from the foothills of the Himalayas to the TP have not been investigated in detail. One reason for the limited knowledge about aerosol pollution on the HTP is the small number of long-term in situ observations. In addition, the results from chemistry transport models still have large uncertainties due to the complex terrain and the specific meteorological conditions, as do the emission data sets that are used for modeling (Fleming et al., 2012). A widespread hypothesis (see e.g., Cao et al., 2011) suggests that the high altitude of the Himalayas acts as a physical barrier inhibiting the transport of BC across the mountains onto the TP with the exception of deep river valleys that cut across the mountain range and provide a pathway for pollutant transport.
The present work addresses the mechanisms and pathways of cross-Himalayan
pollution transport that occurs over large areas under specific
meteorological conditions. We base the analysis on a pollution event in
March 2009 that can be identified from ground-based and space-borne remote
sensing data (Sect.
Overall, this study seeks to improve the understanding of the intensely discussed cross-Himalayan pollution transport and its effect on cryosphere and climate in the context of raising anthropogenic emissions in Asia.
We describe the data obtained from ground-based and satellite remote sensing
measurements in Sect.
The identification of aerosol pollution events in the
HTP is based on quality-assured data from the Aerosol Robotic
Network (AERONET) (Holben et al., 1998). AERONET provides globally
distributed observations of spectral AOD,
inversion products and the amount of precipitable water. Complete
descriptions of the sun–sky scanning spectral radiometer instruments,
measurement sequences, uncertainties and cloud screening procedures
can be found in the literature (e.g., Eck et al., 1999; Dubovik and
King, 2000; Smirnov et al., 2000). In this study, only data sets that
have the lowest possible uncertainties in unpolluted regions are
used. We therefore do not include spectral refractive indices and
single-scattering albedo from almucantar scans with AOD
(440
Here, FMF is taken as a proxy for polluted air since anthropogenic
aerosol optical thickness is dominated by fine aerosols, while natural
aerosols contain a substantial fraction of coarse aerosols. Typically,
the radius of these coarse particles such as mineral dust and sea salt
is
The two main HTP reference stations used in this study are Nam Co Monitoring and Research Station for
Multisphere Interactions (Nam Co), situated in central Tibet
(30.77
Topography of the Indian subcontinent and the Tibetan Plateau in COSMO. Blue triangles indicate the AERONET stations used in this study and the blue square shows the observed location of a pollution plume.
Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) is a two-wavelength
polarization-sensitive lidar on board the Cloud-Aerosol Lidar and Infrared
Pathfinder Satellite Observation (CALIPSO) satellite. Its data sets provide
high-resolution vertical profiles of aerosols and clouds at a 16-day
revisiting time. Nighttime profiles show lower background noise, which is an
advantage when analyzing pollution aerosols in remote areas such as the HTP;
as for most lidars, daylight acts as a disturbance to the signal returns and
hence reduces the signal-to-noise ratio, with the consequence that
CALIPSO's nighttime data have a superior quality to the daytime data (Marenco
et al., 2014). Detailed information about the instrument, data resolution,
measurement techniques and final products are given by Winker et al. (2009).
For this study, we selected nighttime profiles upwind of the HTP AERONET
stations during the pollution event in order to analyze the vertical
extension of polluted air masses that are transported towards the HTP. The
products used in this study include the following data sets: level 1B total
(parallel and perpendicular) attenuated backscatter (
AAOD is calculated with the near-UV
OMAERUV algorithm for measurements by the Ozone Monitoring Instrument
(OMI) aboard the Aura satellite (Torres et al., 2007). This AAOD
500
AERONET AOD time series. Red bars indicate the fine-mode
fraction (FMF) of the total aerosol optical depth (AOD) at
500
Due to the complex topography of the HTP, a detailed analysis of the history
of polluted air masses requires high-resolution meteorological fields. To
this end, we performed a 7-day simulation with the non-hydrostatic numerical
weather prediction model COSMO (Baldauf et al., 2011) from 00:00 UTC
on
10 March 2009 to 00:00 UTC on 17 March 2009. The model was run at a horizontal
resolution of 7
Kinematic backward and forward trajectories were calculated using the
Lagrangian Analysis Tool LAGRANTO (Wernli and Davies, 1997), which applies an
iterative Eulerian integration scheme (three steps) to the hourly wind fields
from COSMO. Two ensembles of trajectories were started in this study: the
first one (48
In addition to the ECMWF analyses, we use the reanalysis data set
ERA-Interim (Dee et al., 2011) for the averages presented in
Sect.
In this section, detailed analysis is presented for the pollution episode
during 13–19 March 2009, which was identified as the most severe event in
the data set, showing the highest amounts of anthropogenic aerosol at Nam Co
in 2009 and simultaneously very high pollution levels at EvK2. This episode
has been identified by analyzing AERONET time series and comparing them to BC
observational data from sites in the HTP (Sect.
As Fig. 2 but for AERONET stations in the IGP and Thailand.
Complete 24 months of AERONET L2 measurements at Nam Co (cf. Fig. 1) are only
available for the years 2009 and 2010. These measurements show episodes of
significant fine-mode aerosol contribution to total AOD 500
The AERONET fine-mode AOD (500
The maximum FMF (500
OMI AAOD (500
In order to further support the association of FMF with pollution aerosols and in particular with absorbing aerosols, we compare the AERONET data with available in situ BC concentrations. The BC data were retrieved from Multi-Angle Absorption Photometer measurements and from filter analyses at EvK2 and Nam Co, respectively. The circles in Fig. 2 show a high temporal correlation of BC concentrations with the FMF at EvK2. Elevated concentrations of BC and FMF at EvK2 were retrieved during the 13–19 March event and also during the second half of April 2009. It is interesting to note that the BC-to-FMF ratios differ between these events, possibly caused by varying aerosol type contributions to the polluted air masses reaching the measurement site. Continuous BC measurements at Nam Co station are only available since late 2010 (M. Jing, personal communication, 20 November 2013) but a recent study on total suspended particulate matter by Zhao et al. (2013) points out that a sudden increase of carbonaceous aerosols, i.e., organic carbon and black carbon, also occurred in mid-March 2009 (not shown). These in situ data suggest that this pre-monsoon aerosol event consisted of significant pollution which reached central Tibet not only at elevated levels but also at the surface.
A significant build-up of air pollution during the 13–19 March 2009 event
was also observed at stations in the IGP and in northern Thailand. Measurements of AERONET
sites with available L2 data during the case study are shown in Fig. 3 (see
Fig. 1 for the location of the stations). Interestingly, compared to data from EvK2 shown in Fig. 2, the AERONET measurements from the IGP show a relatively
large contribution of coarse-mode particles to the total AOD 500
Overall, the AERONET data sets in Figs. 2 and 3 suggest the occurrence
of an ABC over South Asia that extends as far as the HTP. This is
supported by the significant (nearly total) FMF contribution to the
total AOD 500
The high contribution of absorbing aerosols to the total AOD during the
13–19 March 2009 pollution event is shown in Fig. 4a (OMI AAOD
500
The combination of ground- and space-based remote sensing data suggests the
presence of an ABC with AERONET AOD
Several CALIOP transects that were retrieved over the past years were found to show significant extensions of pollution plumes “coating” the HTP. This indicates that the polluted air masses do not only accumulate in the valleys but can also cover large areas in this usually pristine region.
In Fig. 5 we show the CALIOP transect retrieved upstream of Nam Co station at
20:35 UTC on 13 March 2009 during the onset of the pollution event analyzed
in this study. Thin cirrus clouds can be seen at about 10
In this section, we identify the source regions and involved transport
mechanisms of two major pollution features observed over the TP during the
March 2009 event: (1) the pollution “plume” retrieved by CALIOP (cf.
Sect.
For both cases, we identify three transport pathways of air (hereafter referred to as “air streams”) arriving at the plume region and Nam Co from trajectories started near these locations.
In order to investigate how this plume (cf. Fig. 5) reached the TP and where
the polluted air was advected to, we calculate 48
Trajectories (48
Three major source regions can be identified (see Fig. 6b). A large fraction
of the trajectories is advected over the Himalayas from the southwest
and stays at approximately constant altitude. We refer to these
trajectories as “air stream 1” (blue lines in Fig. 6b) if they stay
above 5500
Temporal evolution of height, water vapor (
The temporal evolution of height, water vapor (
Geopotential height at 500
All trajectories enter the PBL over the TP in the afternoon of 13 March
(Fig. 7). As a consequence, water vapor content rises abruptly to values
around 1
Air stream 3 descends from the TP on 12 March on the western side of the
passing trough (see Figs. 6 and 8) to the southern side of the Himalayas.
There, the air is first advected eastwards in the afternoon of 12 March
before returning to west Nepal during the following night. This is consistent
with the zonal winds along the southern face of the Himalayas at
850
Geopotential height at 500
With the Himalayan flanks heating up during daytime on 13 March, air stream 3
rises approximately 2000
The high PBL and the strong updrafts near the surface associated with the
heated mountain flanks can be seen in a cross section along the main flow
direction (thick black line segment in Fig. 6b) at 09:00 UTC on 13 March 2009
(Fig. 10). While air streams 1 and 2 (blue and green squares, respectively)
cross the Himalayas quasi-isentropically at approximately 500
Vertical cross sections along the mean direction of transport
(thick black line segment in Fig. 6b) at 07:00, 09:00, 11:00 and
13:00 UTC on 13 March 2009. Colored contours show vertical wind speed
(
We further analyzed the mean atmospheric stability calculated as the vertical
gradient in equivalent potential temperature
To summarize, the analysis of the backward trajectories presented in
this section suggests that the source region of the plume is located
southwest of the Himalayas and that the transport occurs predominantly
along a relatively narrow band without much vertical motion. Some of
the backward trajectories, however, also originate from lower
altitudes (about 2000
From the plume location, the polluted air is transported eastwards, which
fits well to the remarkably high levels of fine-mode aerosols measured at Nam
Co on the following day (14 March, see Fig. 2, data available from 01:00 to
07:00 UTC). This suggests that there is a direct connection between the
CALIOP aerosol-rich air (plume) and the AERONET high FMF at Nam Co. Note the
wide spread of heights of the trajectories in the vicinity of Nam Co, showing
that the pollution is distributed throughout the whole troposphere, reaching
levels of up to 10 000
Also for the 72
Trajectories (72
The basic motion of air parcels in air streams 1 and 2 can again be explained by the synoptic situation. The ridge present on 13 March weakens on 14 March (see Figs. 8 and 12) and a trough associated with the inflow of upper atmospheric cold air from Central Asia forms on the western side of the TP. This leads to westerly and southwesterly winds over northern (air stream 2) and central India (air stream 1), respectively. As the trough moves eastward on 14 March, the winds over much of India turn to a southwesterly direction such that both air streams 1 and 2 are now transported towards the Himalayas (see Figs. 11 and 12).
As Fig. 8 but at 11:00 UTC on 14 March 2009 (left panel) and 11:00 UTC on 15 March 2009 (right panel).
As Fig. 7 but for the 72
Parts of air stream 3 are advected from the east while other parts are
trapped in the northern IGP and the adjacent valleys (Fig. 11b). We find that
most air masses in this air stream reach the plateau via two main crossings:
one is the Chumbi Valley between Sikkim, Bhutan and Tibet and the other one
lies further to the east over central and eastern Bhutan. Since the adjacent
southern regions are highly polluted (cf. Fig. 4a) and winds are generally
weak at lower levels during this period, this indicates an effective
transport pathway for pollution including absorbing aerosols. It is
interesting to note that similar cross-mountain pathways were also observed
in a study on trans-Himalayan flights of bar-headed geese (Hawkes
et al., 2011). On 15 March,
the trough deepens and continues to move eastward, such that the winds at
500
From Fig. 13 it can be seen that air stream 3 is in the PBL most of the time
(e.g., from the diurnal temperature variations and high humidity) whereas air
streams 1 and 2 are only affected by slow cooling (
For both discussed sets of trajectories (plume and Nam Co), a significant
amount air mass transport occurs at mid-tropospheric levels (around
500
The combination of ground- and space-based remote sensing data together with in situ observations and high-resolution trajectory calculations enables a comprehensive assessment of possible transport pathways of polluted air masses found in ABCs into the remote and sensitive HTP area.
We investigate a severe pollution episode that occurred on the TP during
13–19 March 2009. This event was selected on the base of AERONET L2 data
showing the maximum yearly AOD 500
Further AERONET sites located south of the Himalayas and in Thailand
also retrieved very high fine-mode AOD 500
Air mass trajectories are analyzed for severe pollution features observed on the TP during the case study using wind fields from the high-resolution numerical weather prediction model COSMO. The transport of pollution is found to occur along several effective pathways from the IGP and the foothills of the Himalayas to all tropospheric levels over the TP. Approximately two-thirds of the analyzed air masses are advected to the TP quasi-isentropically at mid-tropospheric levels and one-third is lifted from the PBL south of the Himalayas.
The cross-Himalayan transport of pollution during this case study is
enabled by the combination of local meteorological patterns such as
updrafts near heated mountain flanks, valley wind systems, and large-scale
forcing at mid-tropospheric levels. Weak zonal winds over the IGP and
the Himalayan foothills support the accumulation of pollution in this
region and relatively low stability above the PBL over the IGP permits for
vertical spreading of air pollutants up to the middle troposphere.
Finally, the inflow of cold air from higher latitudes leads to the
formation of a trough, adding a southerly component to the winds at
500
Overall, the analyses presented in this study provide an in-depth understanding of cross-Himalayan pollution advection reported in previous works. In particular, we find that ABCs and other polluted air masses from the southern side of the Himalayas can traverse the high mountain range not only through the major north–south river valleys but also by being lifted and advected over the Himalayas. Furthermore, we quantify the relative contribution of advection from mid-tropospheric levels over India and the PBL over the Himalayan foothills.
The findings from the meteorological analysis could be used to put the
presented case study into a climatological perspective, i.e., to quantify the
frequency of such events, which is required to quantify the
total impact of cross-Himalayan pollution transport on the HTP. Such an
analysis could include the quantification of periods with the following
characteristics: weak zonal winds at the southern side of the Himalayas and
low stability above the PBL (up to about 500
Moreover, chemical transport models could be used to determine the contribution of specific sources to polluted air masses that cross the Himalayas. Further observational and modeling studies are thus urgently needed to identify the chemical speciation and the spatiotemporal distribution of pollutants in the HTP in order to investigate their climatic and environmental implications.
Because of the rapid and effective transport pathway onto the HTP, a reduction in anthropogenic aerosols such as BC would not only be beneficial for human health near the emission sources but it would also help to protect the sensitive environments on the Tibetan Plateau.
This study was supported by the National Natural Science Foundation of China (41121001, 41225002), the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (XDB03030504) and the project “A Sustainable Atmosphere for the Kathmandu Valley” SusKat at the Institute for Advanced Sustainability Studies (IASS), Germany. Our appreciation goes to the many colleagues who have shared their interest and constructive comments for this study, with our special thanks to A. Miltenberger. We thank B. Holben, G. P Gobbi, S. N. Tripathi, G. Leeuw, S. Verma, S. Janjai and their staff for establishing and maintaining the AERONET sites (Nam Co, EVK2-CNR, Kanpur, Pantnagar, Gual_Pahari, Kolkata and Chiang_Mai_Met_Sta). The CALIOP and OMI data were available from the Atmospheric Science Data Center. BC data from the Ev-K2-CNR SHARE station were generously provided by A. Marinoni. S.-W. Kim was supported by KMA R&D (CATER 2012–3020) and by the Korean Ministry of Environment as “Climate Change Correspondence”.Edited by: Y. Cheng