ALBERT

Description: 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°N, 79.45°E, 1958 m amsl) located in the central Himalayas. The satellite observations include the MODIS derived fire counts and AOD (0.55 μm), and OMI derived tropospheric column NO2, 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 μm) and ozone are estimated to be 1802 ng m−3 (∼145%), 0.3 (∼150%) and 19 ppbv (∼34%) respectively. Large enhancements (53–100%) are also seen in the satellite derived parameters over a 2° × 2° 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.

Description: The Indo-Gangetic Plain (IGP) region is one of the most densely populated regions in the World, but ground-based observations of air pollutants are highly limited in this region. Here, surface ozone observations made during March 2009–June 2011 at a semi-urban site (Pantnagar; 29.0°N, 79.5°E, 231 m amsl) in the IGP region are presented. Ozone mixing ratios show a daytime photochemical buildup with ozone levels sometimes as high as 100 ppbv. Seasonal variation in 24-h average ozone shows a distinct spring maximum (39.3 ± 18.9 ppbv in May) while daytime (1130–1630 h) average ozone shows an additional peak during autumn (48.7 ± 13.8 ppbv in November). The daytime, but not daily average, observed ozone seasonality is in agreement with the space-borne observations of OMI tropospheric column NO2, TES CO (681 hPa), surface ozone observations at a nearby high altitude site (Nainital) in the central Himalayas and to an extent with results from a global chemistry transport model (MATCH-MPIC). It is suggested that spring and autumn ozone maximum are mainly due to photochemistry, involving local pollutants and small-scale dynamical processes. Biomass burning activity over the northern Indian region could act as an additional source of ozone precursors during spring. The seasonal ozone photochemical buildup is estimated to be 32–41 ppbv during spring and autumn and 9–14 ppbv during August–September. A correlation analysis between ozone levels at Pantnagar and Nainital along with the mixing depth data suggests that emissions and photochemical processes in the IGP region influence the air quality of pristine Himalayan region, particularly during midday hours of spring. The evening rate of change (8.5 ppbv hr−1) is higher than the morning rate of change, which is dissimilar to those at other urban or rural sites. Ozone seasonality over the IGP region is different than that over southern India. Results from the MATCH-MPIC model capture observed ozone seasonality but overestimate ozone levels. Model simulated daytime ratios of H2O2/HNO3 are higher and suggesting that this region is in a NOx-limited regime. A chemical box model (NACR Master Mechanism) is used to further corroborate this using a set of sensitivity simulations, and to estimate the integrated net ozone production in a day (72.9 ppbv) at this site.

Description: [1]  Simultaneous in-situ measurements of ozone, CO, and NO y have been made for the first time at an high altitude site Nainital (29.37 o N, 79.45 o E, 1958 m amsl) in the central Himalayas during 2009-2011. CO and NO y 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 NO y (1918 ± 1769.3 pptv) have been attributed mainly to the regional pollution supplemented with northern Indian biomass burning. However, lower levels of ozone (34.4 ± 18.9 ppbv), CO (146.6 ± 71 ppbv), and NO y (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, the altitude variations along trajectory and the boundary layer shows higher levels of ozone (57 ± 14 ppbv), CO (206 ± 125 ppbv), and NO y (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 inter-species correlations are observed which suggest the transport of air-mass from common source regions (except during winter). Ozone–CO and ozone-NO y slope values are found to be lower in comparison to those at other global sites, which clearly indicates the incomplete in-situ photochemistry and greater role of transport processes in this region. The higher CO/NO y value also confirms minimal influence of the fresh emissions at the site. Enhancements in ozone, CO, and NO y during high fire activity period are estimated to be 4-18%, 15-76%, and 35-51% respectively. Despite higher CO and NO y concentrations at Nainital, ozone levels are nearly similar to those at other global high altitude sites.

Description: [1]  This study presents a CO source contribution analysis for the atmosphere of South Asia during January-February 2008. The approach includes into the Weather Research and Forecasting Model with Chemistry (WRF-Chem) eleven CO tracers, which track CO from different source types and regions. The comparison of model results with MOPITT CO retrievals shows that the model reproduces the spatial, vertical and temporal distributions of MOPITT retrievals fairly well, but generally overestimates CO retrievals in the lower troposphere. CO mixing ratios averaged over the model domain at the surface, in the planetary boundary layer and the free troposphere are estimated as 321 ± 291 ppbv, 280 ± 208 ppbv and 125 ± 27 ppbv, respectively. Model results show that wintertime CO in the boundary layer and free troposphere over India is mostly due to anthropogenic emissions and to CO inflow. In the boundary layer, the contribution from anthropogenic sources dominates (40-90%), while in the free troposphere the main contribution is due to CO inflow from the lateral boundaries (50-90%). Over the Arabian Sea and the Bay of Bengal, 43-51% of surface CO mixing ratios come from the Indian subcontinent and 49-57% from regions outside of South Asia. The anthropogenic sources in the Indo-Gangetic Plain region are found to contribute, on average, 42% and 76% to anthropogenic surface CO over the Arabian Sea and the Bay of Bengal, respectively. The anthropogenic emissions from western and southern India contribute 49% to anthropogenic surface CO over the Arabian Sea. Anthropogenic emissions contribute only up to 40% over Burma where biomass burning plays a more important role. Regional transport contributes significantly to total anthropogenic CO over southern India (41%), Burma (49%) and even exceeds the contribution from local sources in western India (58%).

Description: 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 (~2 km) in the Indo-Gangetic Plains were examined using nearly year-round measurements (June 2011 to March 2012) at the Atmospheric Radiation Measurement (ARM) mobile facility as part of the RAWEX-GVAX 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 (CN) concentrations during the dry autumn and winter months. The CCN concentration (at a supersaturation of 0.46) was higher during periods of high aerosol absorption (single-scattering albedo (SSA) 〈 0.80) than during periods of high aerosol scattering (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.