S. Venkataramani

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.

Influences of the springtime northern Indian biomass burning over the central Himalayas

The influences of the springtime northern Indian biomass burning are shown for the first time over the central Himalayas by using three years (2007-2009) of surface and space based observations along with a radiative transfer model. Near-surface ozone, black carbon (BC), spectral aerosol optical depths (AODs) and the meteorological parameters are measured at a high altitude site Nainital (29.37 degrees N, 79.45 degrees E, 1958 m amsl) located in the central Himalayas. The satellite observations include the MODIS derived fire counts and AOD (0.55 mu m), and OMI derived tropospheric column NO(2), ultraviolet aerosol index and single scattering albedo. MODIS fire counts and BC observations are used to identify the fire-impacted periods (372 h during 2007-2009) and hence the induced enhancements in surface BC, AOD (0.5 mu m) and ozone are estimated to be 1802 ng m(-3) (similar to 145%), 0.3 (similar to 150%) and 19 ppbv (similar to 34%) respectively. Large enhancements (53-100%) are also seen in the satellite derived parameters over a 2 degrees x 2 degrees region around Nainital. The present analysis highlights the northern Indian biomass burning induced cooling at the surface (-27 W m(-2)) and top of the atmosphere (-8 W m(-2)) in the lesser polluted high altitude regions of the central Himalayas. This cooling leads to an additional atmospheric warming of 19 W m(-2) and increases the lower atmospheric heating rate by 0.8 K day(-1). These biomass burning induced changes over the central Himalayan atmosphere during spring may also lead to enhanced short-wave absorption above clouds and might have an impact on the monsoonal rainfall.

Temporal variations in surface ozone at Thumba (8.6°N, 77°E)-a tropical coastal site in India

Abstract Surface measurements of ozone and meteorological parameters are made at a tropical coastal site, Thumba (8.6°N, 77°E, 2xa0m) in India from April 1997 to March 1998. Ozone shows a diurnal variation with daytime higher levels and a sharp change in its values during evening time. The evening time change in ozone values with a secondary peak is found to be due to change in wind pattern from sea-breeze to land-breeze at this site. This secondary peak in ozone is weakest during monsoon period. A detailed study of the meteorological parameters shows that during nighttime, polluted air from land side moves to the nearby marine region relatively increasing the levels of ozone and precursor gases. Observations show that the onset time of daytime ozone increase and its rates are related with each other. If onset time of ozone increase is early, its increase rate is slower and vice versa. Maximum ozone levels are observed to be during March, probably due to intense photochemical production. However, this is different when compared to other Indian site like Ahmedabad, where maximum ozone levels are observed during late autumn and early winter. Monthly average ozone levels are observed to be very low (in the range of 13–22xa0ppbv) at Thumba.

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 (57.5u2009±u200912.6 ppbv), CO (215.2u2009±u2009147 ppbv), and NOy (1918u2009±u20091769.3 parts per trillion by volume (pptv)) have been attributed mainly to regional pollution supplemented with northern Indian biomass burning. However, lower levels of ozone (34.4u2009±u200918.9 ppbv), CO (146.6u2009±u200971 ppbv), and NOy (1128.6u2009±u20091035 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 (57u2009±u200914 ppbv), CO (206u2009±u2009125 ppbv), and NOy (1856u2009±u20091596 pptv) in the continental air masses when compared with their respective values (28u2009±u200913 ppbv, 142u2009±u200947 ppbv, and 226u2009±u2009165 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.

Impact of transport from the surrounding continental regions on the distributions of ozone and related trace gases over the Bay of Bengal during February 2003

[1]xa0Simultaneous surface level measurements of O3, CO, methane, and light nonmethane hydrocarbons (NMHCs) were made over the Bay of Bengal during a cruise campaign between 19 February and 28 February 2003. The mixing ratios of O3, CO, methane, ethane and acetylene were observed in the ranges of 20–52 ppbv, 126–293 ppbv, 1.65–1.85 ppmv, 622–2088 pptv and 134–1388 pptv, respectively, during the campaign period. Ratios of some measured trace gases have been used to estimate the level of photochemical processes and transport times of air parcels. Three types of air masses have been identified on the basis of source regions and transport pathways. The study region is frequently affected by transport of pollutants from the nearby continental emission sources, but the strongest pollution event during this period was due to long-range transport from extratropical Northern Hemisphere. These higher mixing ratios of trace gases are due to faster transport of air parcels from the source regions via free troposphere. On the basis of C2H2/CO ratio, it is also observed that this pollution plume was photochemically fresh. Latitudinal distributions of all the measured trace gases show significant north-south decreasing trends. In particular, gradients in the mixing ratios of NMHCs over the Bay of Bengal compare 2–3 times higher than those observed over the Arabian Sea, the Indian Ocean and the Pacific Ocean. Comparisons with previous measurements over the Arabian Sea and the Indian Ocean show significantly higher levels of these trace gases over the Bay of Bengal.

Vertical distribution of ozone in the lower troposphere over the Bay of Bengal and the Arabian Sea during ICARB‐2006: Effects of continental outflow

[1]xa0Measurements of vertical distributions of ozone and meteorological parameters were made over the Bay of Bengal (BOB) and Arabian Sea (AS) from ocean research vessel Sagar Kanya during the period of 18 March to 10 May 2006 as a part of the Integrated Campaign for Aerosols, Gases and Radiation Budget (ICARB). The observations showed a highly polluted layer (average ozone ∼68.0 ± 10.1 ppbv) over the northern BOB as compared to the northern AS (51.5 ± 7.6 ppbv), southern BOB (42.7 ± 12.8 ppbv), and southern AS (40.9 ± 9.5 ppbv) in the altitude range of 1–3 km. In this altitude range, specific humidity was lower by about 2–6 g/kg, and temperature was higher by 1°C–3°C over the northern BOB and northern AS as compared to their southern counterparts. This comparison and the total potential source contribution function analysis indicate that the observations over the northern BOB and the northern AS were influenced by the transport of continental air masses. The outflow from the polluted atmosphere over northern India, particularly over the Indo-Gangetic Plain, resulted in higher mixing ratios of ozone over the northern BOB. The air masses from the northern Indian region contributed an enhancement of about 13 ± 6 ppbv to the mixing ratio of ozone over the BOB in the altitude range of 0.75–3.0 km. The mixing ratios of ozone in the unperturbed marine air masses were found to be 11 ± 6 and 5 ± 2 ppbv lower than the average ozone over the BOB and AS, respectively. These results clearly show the outflow of continental air containing high ozone and possibly other pollutants over the northern BOB, which can have significant implications in the chemistry and climate of relatively cleaner regions.

Impact of monsoon circulations on oceanic emissions of light alkenes over Bay of Bengal

[1]xa0Surface level measurements of ethene (C2H4) and propene (C3H6) were made in the marine boundary layer (MBL) of Bay of Bengal (BOB) during the summer and winter monsoon campaigns. The time series trends in the mixing ratios of alkenes were similar to that of wind speed, while no clear relations were observed with other meteorological parameters. The diurnal variations of ethene and propene show correlations with the intensity of solar flux as their daytime mixing ratios were ∼45% higher than the nighttime measurements in summer. While winter measurements of alkenes do not show any local time dependencies. The mixing ratios of alkenes were particularly elevated during the episodes of cyclones and convective activities in the summer season over BOB. The uptake rate of nutrients shows a similar trend to those of mixing ratios of alkenes in summer, and the measurements of phytoplankton indicate high primary production during the episodes of elevated alkenes over central BOB. The mixing ratios of alkenes showing minima in winter and maxima in summer are similar to the seasonal patterns reported for global oceans; however, their variability over BOB was less pronounced compared to the extratropical oceans. The ratios of ethene/propene were comparable to a mean ratio of 2.3 pptv/pptv derived from the database for global oceans confirming the fresh oceanic emissions of alkenes over BOB. The emissions of alkenes were mainly controlled by the distribution of dissolved organic carbon (DOC) in seawaters and actions of wind speed in the presence of sunlight. This study is an important step to understand the processes controlling the emissions of alkenes from equatorial oceans and also to improve their global budget estimates.