Manish Naja

Variations in surface ozone at Nainital: A high-altitude site in the central Himalayas

Surface ozone measurements have been made for the first time at Nainital (29.37 degrees N, 79.45 degrees E, 1958 m amsl), a high-altitude site in the central Himalayas, between October 2006 and December 2008. Diurnal variations in ozone do not show the daytime photochemical build-up typical of urban or rural sites. The seasonal variation shows a distinct ozone maximum in late spring (May; 67.2 +/- 14.2 ppbv) with values sometimes exceeding 100 ppbv and a minimum in the summer/monsoon season (August; 24.9 +/- 8.4 ppbv). Springtime ozone values in the central Himalayas are significantly higher than those at another high-altitude site (Mt. Abu) in the western part of India. Seasonal variations in ozone and the processes responsible for the springtime peak are studied using meteorological parameters, insolation, spatial and temporal classifications of air mass trajectories, fire counts, and simulations with a chemical transport model. Net ozone production over the Northern Indian Subcontinent in regionally polluted air masses is estimated to be 3.2 ppbv/day in spring but no clear build-up is seen at other times of year. Annual average ozone values in regionally polluted air masses (47.1 +/- 16.7 ppbv) and on high insolation days (46.8 +/- 17.3 ppbv) are similar. Background ozone levels are estimated to be 30-35 ppbv. Regional pollution is shown to have maximum contribution (16.5 ppbv) to ozone levels during May-June and is about 7 ppbv on an annual basis, while the contribution of long-range transport is greatest during January-March (8-11 ppbv). The modeled stratospheric ozone contribution is 2-16 ppbv. Both the trajectory analysis and the model suggest that the stratospheric contribution is 4-6 ppbv greater than the contribution from regional pollution. Differences in the seasonal variation of ozone over high-altitude sites in the central Himalayas (Nainital) and western India (Mt. Abu) suggest diverse regional emission sources in India and highlight the large spatial and temporal variability in ozone over the Indian region.

Ozone in the marine boundary layer over the tropical Indian Ocean

Surface measurements of ozone, aerosols (in 10 different size ranges), carbon monoxide, and methane are made during a ship cruise over the Indian Ocean from January 5 to February 3, 1996. These results have been used to study the variabilities in their distributions and mixing of the continental and pristine air masses over the tropical oceanic region. Concentrations of ozone, aerosols, and carbon monoxide show decreasing trends from coastal region to the open ocean region. During this cruise, influence of the continental polluted air from the South Asian region was observed, resulting in high levels of these species in the open ocean. Incidence of encountering extremely low levels of ozone has been observed near the equator. These low and high values of ozone, aerosols etc. are supported by the back trajectory analyses made using European Centre For Medium-Range Weather Forecasts (ECMWF) meteorological parameters. These results confirm the transport of the continental air mass down to the equator over the Indian Ocean. A simple photochemical box model has been used to estimate the ozone production potential (OPP). The discrepancies in the observed and estimated ozone diurnal variation suggest a need for better understanding of the processes.

Changes in surface ozone amount and its diurnal and seasonal patterns, from 1954–55 to 1991–93, measured at Ahmedabad (23 N), India

A comparative study of surface ozone observed at Ahmedabad during 1954–55 and 1991–93 periods has been made. The 1954–55 measurements were made at 0600, 1000, 1400, 1800, and 2100 hours (IST) by an electro-chemical method, whereas 1991–93 measurements are made at every 15 minutes interval by an UV absorption technique. Higher ozone concentrations including larger amplitudes of diurnal and seasonal variations have been observed during 1991–93 as compared to the measurements made during 1954–55. The observed seasonal maximum during 1954–55 is in spring months whereas during 1991–93, it is in winter months. Increase in the concentration of ozone from 0600 hrs in the morning to 1400 hrs in the noon time is higher in summer and October–November months for both the periods and lowest during monsoon season. Furthermore, this change is higher during 1991–93 period as compared to 1954–55 suggesting higher levels of ozone precursors. The average ozone concentration increased from 14.7 ppbv during 1954–55 to 25.3 ppbv during 1991–93, resulting in a linear increase of 1.45% per year. The increase in background ozone concentrations, for 2100 and 0600 hrs during monsoon months when there is low photochemical ozone production even during day time, is 0.49% per year.

First simultaneous measurements of ozone, CO, and NOy at a high‐altitude regional representative site in the central Himalayas

Simultaneous in situ measurements of ozone, CO, and NOy have been made for the first time at a high altitude site Nainital (29.37°N, 79.45°E, 1958 m above mean sea level) in the central Himalayas during 2009–2011. CO and NOy levels discern slight enhancements during the daytime, unlike in ozone. The diurnal patterns are attributed mainly to the dynamical processes including vertical winds and the boundary layer evolution. Springtime higher levels of ozone (57.5 ± 12.6 ppbv), CO (215.2 ± 147 ppbv), and NOy (1918 ± 1769.3 parts per trillion by volume (pptv)) have been attributed mainly to regional pollution supplemented with northern Indian biomass burning. However, lower levels of ozone (34.4 ± 18.9 ppbv), CO (146.6 ± 71 ppbv), and NOy (1128.6 ± 1035 pptv) during summer monsoon are shown to be associated with the arrival of air mass originated from marine regions. Downward transport from higher altitudes is estimated to enhance surface ozone levels over Nainital by 6.1–18.8 ppbv. The classification based on air mass residence time, altitude variations along trajectory, and boundary layer shows higher levels of ozone (57 ± 14 ppbv), CO (206 ± 125 ppbv), and NOy (1856 ± 1596 pptv) in the continental air masses when compared with their respective values (28 ± 13 ppbv, 142 ± 47 ppbv, and 226 ± 165 pptv) in the regional background air masses. In general, positive interspecies correlations are observed which suggest the transport of air mass from common source regions (except during winter). Ozone-CO and ozone-NOy slope values are found to be lower in comparison to those at other global sites, which clearly indicates incomplete in situ photochemistry and greater role of transport processes in this region. The higher CO/NOy value also confirms minimal influence of fresh emissions at the site. Enhancements in ozone, CO, and NOy during high fire activity period are estimated to be 4–18%, 15–76%, and 35–51%, respectively. Despite higher CO and NOy concentrations at Nainital, ozone levels are nearly similar to those at other global high-altitude sites.

The influence of a south Asian dust storm on aerosol radiative forcing at a high-altitude station in central Himalayas

The impact of long-range transported dust aerosols, originating from the Thar Desert region, to a high-altitude station in the central Himalayas was studied with the help of micro-pulse lidar (MPL) observations. A drastic change in lidar back-scatter profile was observed on a dust day as compared with that on a pre-dust day. The back-scatter coefficient on a dust day revealed that the dust layer peaked at an altitude ∼1300 m above ground level (AGL) and extended up to ∼3000 m AGL, with maximum value ∼3 × 10–5 m–1 sr–1. Aerosol Index (AI) and air mass back-trajectory analysis substantiate the transport of dust aerosols from the far-off Thar Desert region to the experimental site. A significant effect of dust aerosols was also observed over the station on the spectral aerosol optical depths (AODs), measured using a Microtops-II Sunphotometer. It showed significantly different spectral behaviour of AOD on a dust day as compared with that on a pre-dust day. The Ångström exponent (α) showed a marked decrease from 0.42 to 0.04 from the pre-dust day to the dust day. The aerosol radiative forcing estimated using the Santa Barbara DISORT (discrete ordinate radiative transfer) atmospheric radiative transfer (SBDART) model, in conjunction with the optical properties of aerosol and cloud (OPAC) model, showed values of about –30, –45 and +15 W m–2, respectively, at top-of-atmosphere (TOA), surface and in the atmosphere on the dust day. The positive atmosphere forcing caused an estimated heating of the lower atmosphere by ∼0.4 K day–1.

Transport effects on the vertical distribution of tropospheric ozone over western India

In situ tropospheric ozone measurements by balloon-borne electrochemical concentration cell (ECC) sensors above Ahmedabad in western India from May 2003 to July 2007 are presented, along with an analysis of the transport processes responsible for the observed vertical ozone distribution. This analysis is supported by 12 day back trajectory calculations using the FLEXPART Lagrangian particle dispersion model. Lowest ozone (~20 ppbv) is observed near the surface during September at the end of the Asian summer monsoon season. Average midtropospheric (5–10 km above sea level) ozone is greatest (70–75 ppbv) during April–June and lowest (40–50 ppbv) during winter. Ozone variability is greatest in the upper troposphere with higher ozone during March–May. The FLEXPART retroplume results show that the free tropospheric vertical ozone distribution above this location is affected by long-range transport from the direction of North Africa and North America. Ozone levels are also affected by transport from the stratosphere particularly during March–April. The lower tropospheric (<3 km) ozone distribution during the Asian summer monsoon is affected by transport from the Indian Ocean via the east coast of Africa and the Arabian Sea. Influence from deep convection in the upper troposphere confined over central Asia has been simulated by FLEXPART. Lower ozone levels are observed during August–November than in any other season at 10–14 km above sea level. These in situ observations are in contrast to other studies based on satellite data which show that the lowest ozone values at these altitudes occur during the Asian summer monsoon.

Optical properties and CCN activity of aerosols in a high‐altitude Himalayan environment: Results from RAWEX‐GVAX

The seasonality and mutual dependence of aerosol optical properties and cloud condensation nuclei (CCN) activity under varying meteorological conditions at the high-altitude Nainital site (2km) in the Indo-Gangetic Plains were examined using nearly year-round measurements (June 2011 to March 2012) at the Atmospheric Radiation Measurement mobile facility as part of the Regional Aerosol Warming Experiment-Ganges Valley Aerosol Experiment of the Indian Space Research Organization and the U.S. Department of Energy. The results from collocated measurements provided enhanced aerosol scattering and absorption coefficients, CCN concentrations, and total condensation nuclei concentrations during the dry autumn and winter months. The CCN concentration (at a supersaturation of 0.46) was higher during the periods of high aerosol absorption (single scattering albedo (SSA) 0.85), indicating that the aerosol composition seasonally changes and influences the CCN activity. The monthly mean CCN activation ratio (at a supersaturation of 0.46) was highest (>0.7) in late autumn (November); this finding is attributed to the contribution of biomass-burning aerosols to CCN formation at high supersaturation conditions.

Enhanced SO2 concentrations observed over northern India: role of long-range transport

The combustion of fossil fuels (coal and petroleum products) constitutes a source of continuous release of anthropogenic SO2 into the atmosphere. Furthermore, natural sources such as volcanoes can inject large amounts of SO2 directly into the troposphere and sometimes even into the stratosphere. These event-based volcanic eruptions provide solitary opportunities to study the transport and transformation of atmospheric constituents. In this study, we present an episode of high SO2 concentration over northern India as a result of long-range transport from Africa using multiple satellite observations. Monthly averaged column SO2 values over the Indo-Gangetic Plain (IGP) were observed in the range of 0.6–0.9 Dobson units (DU) during November 2008 using observations from the Ozone Monitoring Instrument (OMI). These concentrations were conspicuously higher than the background concentrations (<0.3 DU) observed during 2005–2010 over this region. The columnar SO2 loadings were highest on 6 November over most of the IGP region and even exceeded 6 DU, a factor of 10–20 higher than background levels in some places. These enhanced SO2 levels were not reciprocated in satellite-derived NO2 or CO columns, indicating transport from a non-anthropogenic SO2 source. As most of the local aerosols over the IGP region occur below 3 km, a well-separated layer at 4–5 km was observed from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite. Wind fields and back-trajectory analysis revealed a strong flow originating from the Dalaffilla volcanic eruption in Ethiopia during 4–6 November 2008. Although volcanic SO2 plumes have been extensively studied over many parts of Asia, Europe, and the USA, analysis of such events for the IGP region is being reported for the first time in this study.

Aerosol vertical profiles strongly affect their radiative forcing uncertainties: study by using ground-based lidar and other measurements

In the present study, we illustrate for the first time that direct aerosol radiative forcing has large uncertainty due to diversity in simulated vertical profile of aerosol over Manora Peak, Nainital (considered to be free troposheric site). In order to have a comprehensive picture, we choose March and October months as representative months of high and low aerosol mass loading over the site, respectively. Monthly averaged aerosol optical depths (AODs) at 0.5 μm are ∼0.30 (±0.02) and 0.13 (±0.01), respectively, during the above months. The derived aerosol extinction profile showed an elevated aerosol layer with maximum extinction of ∼0.10 ± 0.01 km−1 (0.08 ± 0.02 km−1) at ∼1.12 km (0.75 km) during March (October) month. The elevated aerosol layer contributed 44% and 68% to the total AOD during March and October months, respectively. The observed AODs at different wavelengths and black carbon (BC) measurements were used to estimate the other aerosol optical parameters, which are crucial in aerosol radiative forcing. The derived aerosol extinction profile has been used in radiative transfer (RT) model in addition to the standard aerosol extinction profile of RT model along with the aerosol optical properties. Our results indicate that there is an increment in surface radiative forcing, which is ∼10% (25%) due to the insertion of derived aerosol extinction profile for the same columnar properties of aerosols during March (October). Moreover, we found that higher the aerosol layer contribution to the total AOD, the more the uncertainty in aerosol radiative forcing. Apart from this, significant differences were also found in atmospheric forcing at each altitude due to variation in vertical profile of aerosol extinction, which leads the modification of the thermal structure of the atmosphere. Hence, our study has emphasized the importance of proper selection of aerosol vertical profile to obtain more realistic values of radiative forcing.

Variability of Ozone and Related Trace Gases Over India

Ozone plays key roles in atmospheric chemistry, radiation balance, and air quality, including human health. In this chapter, an analysis was made to document the variability and changes in ozone and related trace gases over India. The analysis of the total ozone data up to 1996 at six sites including Ahmedabad shows increasing trend at all the sites except at Varanasi, where a decreasing trend was found (Chakrabarty et al. in J. Geophys. Res, 103:19245–19249, 1998). The rates of increase in ozone during this period were found to be 1.98, 2.33, 1.85, and 0.68 % per decade at Kodaikanal, Ahmedabad, New Delhi, and Srinagar, respectively, while rate of decrease in ozone at Varanasi is 1.02 % per decade. Ozonesonde data (1972–2001), Pune and Trivandrum, show statistically insignificant trends over Trivandrum. But the trend over Pune is close to statistical significance at 9.7 ± 6.1 % per year in the planetary boundary layer and somewhat insignificant above 600 hPa. A comparison of surface ozone observations at Ahmedabad during 1954–55 and 1991–93 show increase of 1.45 % year−1. These observations also show change in seasonal variations with maximum ozone in winter–spring during the 1950s, while maximum ozone has been in autumn–winter during the 1990s. Surface ozone observations made in Delhi show positive trends in daily maximum and daytime (1000–1700 h) ozone levels with a rate of about 1.7 (±0.7) and 1.3 (±0.6) ppbv year−1, during 1997–2004, respectively.