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Introduction
The Ganges Valley region is one of the most rapidly developing and densely populated regions in the northern part of the Indian subcontinent.In recent decades, this region has experienced increasing emissions of aerosols arising from fossil fuel, biofuel and biomass burning, besides mineral dust from the Thar Desert (Chinnam et al., 2006).Atmospheric aerosols scatter and absorb solar radiation and have the ability to modify regional and, to some extent, global climate forcing (e.g., Lau and Kim, 2006;Ramanathan et al., 2005;Gautam et al., 2007).Several observational and modeling studies have addressed the properties of aerosols in this region and their impact on monsoon rainfall in southern Asia (Bahadur et al., 2012;Costabile et al., 2012;Russell et al., 2010;Bergstrom et al., 2002;Dubovik et al., 2002).To investigate the regional im-Introduction

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Full pact of aerosols on radiative transfer and cloud processes, the Ganges Valley Aerosol Experiment (GVAX) was conducted from June 2011 to March 2012 at Manora Peak, Nainital, India (29 • 21 N, 79 • 27 E).
The GVAX project was a joint study conducted by the US Department of Energy, Atmospheric Radiation Measurement Program, and the Indian Institute of Science, Bangalore, India.During this study, instruments for ground-level measurements of various atmospheric dynamics, cloud properties and radiative transfer were deployed at the Aryabhatta Research Institute of Observational Sciences, Nainital, approximately 280 km northeast of New Delhi.The site is situated at an altitude of roughly 1980 m a.m.s.l.(above the mean sea level) in the foothills of the Himalayan mountain range, far from major pollution sources such as industries and large metropolitan areas.Regions to the south and southwest of the observation site have low elevation (< 200 m a.m.s.l.) and merge with the vast Ganges basin.Additional details of the project are in Kotamarthi (2011).

Aerosol absorption and scattering measurements
In situ aerosol properties were measured continuously at the research institute at Nainital.A three-wavelength particle soot absorption photometer (470, 528 and 660 nm) was used to measure the particle absorption coefficient (σ abs ), and a three-wavelength nephelometer (450, 550 and 700 nm) measured the total particle scattering coefficient (σ scat ) and hemispheric back-scattering coefficient.The photometer and nephelometer switched between aerosol particles of diameter < 10 µm and < 1 µm (D 10 µm and D 1 µm , respectively) every 30 min.Measuring in two aerosol size ranges allowed optical measurement of both fine-and coarse-mode aerosols, which often have different sources.The details of instrument operation, associated uncertainty limits and corrections applied can be found at http://www.arm.gov/sites/amf/pgh/.Introduction

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Full 3 Observations and analysis

Absorption properties of aerosols
The daily variations of aerosol absorption coefficients measured by the photometer at three wavelength bands (470, 528 and 660 nm) for D 1 µm and D 10 µm aerosol particles during the entire observation period are shown in Fig. 1.Absorption shows dependence on wavelength -decreasing absorption with increasing wavelength (Bergstrom et al., 2007).During the Indian summer monsoon of July-September 2011 (days 22-83 in Fig. 1) a significant decrease in aerosol absorption and scattering was observed throughout the 24 h day, with an average absorption coefficient of approximately 7 M m −1 at the three wavelengths in visible spectrum.This is due to the aerosols washout from the atmosphere by continuous rainfall.However, during the post-monsoon season, absorption increased.Averaged over the nine months of observations, the mean value of the aerosol absorption coefficient was roughly 18 M m −1 .
Another distinctive feature indicates that for approximately 10 h between 16:00 and 02:00 UTC (21:30 to 07:30 LT); the absorption was low on most of the days after monsoon.The exception was during the last week of October and the first two weeks in November 2011 (days 140-160), when a fair amount of high-intensity absorption was observed throughout the day.The absorption was highest for the D 10 µm particles (see Supplement Fig. S1 and Supplement Note A).This period (days 140-160) coincides with the beginning of post-harvest biomass burning over northwestern India (Sahai et al., 2012).

Absorption coefficient spectral index
The time series of daily averaged absorption coefficient values at the 470 nm and 660 nm wavelength for D 1 µm and D 10 µm particles are shown in Fig. 2a.The measurements made at 660 nm for D 1 µm particles and at 470 nm for D 10 µm particles represent the lowest and highest levels of absorption, respectively.However, the difference be-Introduction

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Full tween these two extreme levels of absorption increases and decreases at different times of the year.For a given size of the particle (1 or 10 µm), on the log-log scale, the coefficient of absorption decreases nearly linearly with increasing of wavelength and the slope of the best fit straight line provides the absorption spectral index.A clear view of absorption characteristics is obtained from the daily average spectral index estimates across the three wavelengths (Fig. 2b).The absorption spectral index (ASI) is a slope determined separately for D 1 µm and D 10 µm (ASI 1 µm and ASI 10 µm , respectively) measurements by fitting the best straight line to the corresponding logarithmic daily averages of absorption coefficients at the three wavelengths.The wavelength dependence of the absorption coefficient is often defined as , where σ absλ1 and σ absλ2 are the absorption coefficients at two wavelength bands λ 1 and λ 2 , and ASI is the negative slope of absorption vs. wavelength in a log-log plot.
Figure 2b shows that the daily averages of ASI 1 µm and ASI 10 µm take similar shapes during the entire period of observation, though their magnitudes are not always the same.The periods when the magnitudes of ASI 1 µm and ASI 10 µm are very different are indicated with yellow shading.These differences in the magnitudes of ASI 1 µm and ASI 10 µm follow a pattern similar to the observed differences in absorption coefficient (Fig. 2a).Furthermore, all of the shaded regions in Fig. 2b arise when ASI 10 µm > ASI 1 µm , indicating a flattening of the ASI 10 µm spectrum.This, in turn, implies that the ASI 10 µm spectrum is becoming much less dependent on the wavelength.
A flatter ASI 10 µm spectrum for D 10 µm particles can result because of (1) a reduction in absorption at 470 nm with respect to 660 nm and/or (2) enhanced absorption at 660 nm with respect to 470 nm and/or (3) a relatively high absorption at all wavelengths, causing less difference between the values at 470 nm and 660 nm.Overall, this distinct feature indicates the presence of larger particles.Thus, when ASI 10 µm > ASI 1 µm , super-micron particles are in abundance.Conversely, when ASI 10 µm ≤ ASI 1 µm , submicron particles are the dominant component.These results suggest that significant amounts of super-micron-sized particles were loaded continuously into the ambient aerosol, causing a steady increase in absorption.Eck et al. (2010)  the possibility of anomalously strong absorption in coarse mode aerosols in observations made at two AERONET sites (Beijing and Xianghe) located in the eastern China.These two sites are in or near downwind of the heavy metropolitan pollution, whereas our area of study, Nainital is away from major cities and at high elevation.The aerosols discussed here are likely from local sources as discussed later.
The average values of ASI 1 µm and ASI 10 µm are, respectively, −0.85 and −0.80.The average values of ASI before and after monsoon (before and after day 100) were −0.70 and −0.95, respectively, for both D 1 µm and D 10 µm , indicating steepening of the absorption spectrum after the monsoon period.In Fig. 2b, the gap between ASI 1 µm and ASI 10 µm stays nearly constant between days 140 and 160, corresponding to the three-week period in October-November 2011 when increased absorption was also observed throughout the day (see Supplement Fig. S1 and Supplement Note A).
The plot of ASI 1 µm vs. ASI 10 µm (Fig. 2c) illustrates an overall correlation of approximately 90 %, with most of the data points lying close to the one-to-one line.The lower absorption by D 10 µm vs. D 1 µm particles is associated mostly with regions in Fig. 2b having near-constant values over multi-day periods (shaded yellow areas corresponding to the presence of super-micron-sized aerosol particles in the atmosphere).

Scattering properties of aerosols
The daily variations of aerosol scattering coefficients observed by the nephelometer for the D 1 µm and D 10 µm particles in wavelength bands at 450, 550 and 700 nm over the entire observation period (see Supplement Fig. S2 and Supplement Note B) demonstrate that scattering intensity decreases with wavelength.Moreover, these diurnal variations in scattering coefficient resemble the absorption coefficient variations shown in Fig. 1.As mentioned earlier, due to washout of aerosols by continuous rainfall, low aerosol scattering was observed during monsoon (July-September).After monsoon, aerosol scattering increased, with a peak centered at 06:30 UTC (local noon) daily.Scattering reached a maximum during the first week of November (roughly day 150).Introduction

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Scattering coefficient spectral index
Time series of daily averaged scattering coefficient values in the 450 nm and 700 nm wavelength bands for D 1 µm and D 10 µm particles are shown in Fig. 3a.The measurements at 700 nm for D 1 µm particles represent the lower side of the scattering, while the highest values are observed at 450 nm for D 10 µm particles.As for absorption, the wavelength dependence of the scattering coefficient was used to compute scattering coefficient spectral index (SSI) for D 1 µm and D 10 µm particles (SSI 1 µm and SSI 10 µm , respectively).During monsoon (before day 100) the scattering was low, as expected, averaging roughly 25 M m −1 for both D 1 µm and D 10 µm particles.After the monsoon period (after approximately day 100), the scattering level and the SSI increased, peaking at about day 140.These results are consistent with those for absorption.However, as Fig. 3b shows, the daily average SSI 1 µm and SSI 10 µm values differ remarkably from the averages for the absorption spectra.Also, irrespective of season, the difference between SSI 10 µm and SSI 1 µm does not change significantly as indicated by the gap between SSI 10 µm and SSI 1 µm .The averages of SSI 1 µm and SSI 10 µm are, respectively, −0.90 and −0.54.A nearly constant ratio of approximately 0.6 between these spectra indicates that the scattering is high for D 10 µm particles at all wavelength bands, leading to the flatness in the spectrum.
The plot of SSI 1 µm vs. SSI 10 µm yields a correlation coefficient of about 83 % (Fig. 3c).As the ratio of spectral indices reveals, the correlation shows an offset of roughly 0.6 with respect to the one-to-one correlation line.Further, during most of the observation period, SSI 10 µm spectra are flatter than SSI 1 µm spectra.As indicated by the daily average scattering coefficient plot (Fig. 3a and b), these periods correspond to a higher level of scattering.

Emission source location analysis
Measurements of surface wind from GVAX (Supplement Fig. S3 and Supplement Note C) indicate that the wind direction at Nainital was commonly from northwest and Introduction

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Full southeast.Figure 4 shows HYSPLIT five-day back-trajectory analyses ending at 500 m and 2500 m a.g.l.over Nainital during October 2011.Trajectories at the 500 m level illustrate that the air masses most commonly originated in the nearby valley.This observation indicates that the strongly absorbing D 10 µm particles observed during GVAX were likely from local sources.In contrast, for the back-trajectories ending at the 2500 m level over Nainital, air masses generally originated from far northwestern and southcentral India and remained above the boundary layer (altitude > 2500 m a.m.s.l.).This latter type of air mass is likely to carry mixtures of biomass burning aerosols above the boundary layer from the post-harvest agricultural burning in the northwestern states of Punjab and Haryana.

Implications of super-micron-sized particles for direct radiative forcing
Here, we explore the significance of the present findings for aerosol direct radiative forcing.A single-column Monte Carlo radiative transfer model (Kim and Ramanathan, 2008) was used to estimate forcing due to the abundance and increased absorption of super-micron-sized particles during the three-week period in October-November 2011 when a large fraction of total aerosol absorption near the surface was observed to be contributed by super-micron-sized particles (i.e., about 20 % more at 525 nm and even more at smaller wavelengths -about 30 % more at 470 nm).The radiation model adopts a surface albedo of 0.18 from the European Centre for Medium-Range Weather Forecasts surface solar radiation reanalysis over Nainital.
The GVAX surface measurements of aerosol extinction and absorption coefficients and the derived spectral indices were used to calculate aerosol optical depth and single scattering albedo for D 1 µm and D 10 µm particles.We assumed that the aerosols over Nainital were concentrated in the surface layer, up to 500 m a.g.l.The calculated column-integrated aerosol optical depth for aerosol particles was similar to the values retrieved from multifilter rotating shadowband radiometer measurements (0.1±0.05) at 550 nm.Introduction

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Full The estimated clear-sky direct radiative forcing of aerosol particles for this period was −5.2 W m −2 at the top of the atmosphere, with roughly 44 % of the total aerosol forcing attributed to super-micron-sized particles.Because of the strong absorption in the shortwave range, the atmospheric absorption and the heating effect due to supermicron aerosols could be comparable to that of sub-micron aerosols.For example, on 8 November 2011 (day 152), the calculated aerosol heating effect increased by nearly 70 %, from 0.6 K day −1 to 1 K day −1 , when we accounted for absorption due to supermicron particles.At the surface, aerosol forcing increased from −6.8 W m −2 for D 1 µm particles only to −10.5 W m −2 for all aerosols.

Conclusions
We have analyzed the aerosol absorption and scattering coefficients for D 1 µm and D 10 µm particles for data collected over a period of 10 months during the GVAX campaign in Nainital, India.The primary conclusions drawn from this study are as follows: 1. Large particles (D 10 µm ) exhibited markedly higher measured absorption values (by roughly 30 %) than smaller particles (D 1 µm ) during several segments of the post-monsoon period.
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2.Figure 1 -
Figure 1 -Temporal and spectral variation of aerosol absorption.Daily variations of aerosol absorption coefficients in three wavelength bands -470, 528 and 660 nmfor D 1μm and D 10μm particles from June 9, 2011, to March 31, 2012.Data are missing in the white areas.

Fig. 1 .Fig. 2 .
Fig. 1.Temporal and spectral variations of aerosol absorption.Daily variations of aerosol absorption coefficients in three wavelength bands -470, 528 and 660 nm -for D 1 µm and D 10 µm particles from 9 June 2011 to 31 March 2012.Data are missing in the white areas.