Long-term Observations of Aerosol Optical Properties Over a high-altitude Station in the Western Ghats, India

Analysing temporal variability of particulate matter and possible contributing factors over Mahabaleshwar, a high-altitude station in Western Ghats, India

This study investigates chemical composition of rainwater (RW) and its contribution from different sources collected over the period of two years (2016 and 2017) at a high altitude location (1380 m above mean sea level) located at Mahabaleshwar situated in the Western Ghats in Peninsular India. The volume weighted mean pH of RW was found to vary between 4.57 and 7.51 (average 5.95) indicating overall alkaline nature of the RW. Prominent ionic species in the RW were Ca2+ (25%), Na+ (19%), Cl−(23%), SO42− (10%), and Mg2+ (9%) with NH4+, NO3− and K+ together forming about 8% of ionic composition. Moreover, ample presence of dust source (Ca2+) was found that acted as a major neutraliser to the acidic ions. The order of Neutralisation Factor of ions was Ca2+ > Mg2+ > NH4+. In addition, a strong correlation between Na+ and Cl− (r ≈ 0.99) further suggested substantial supplement of marine (NaCl) component to the RW. The impact of local anthropogenic activities such as fossil fuel/biomass burning was observed apart from some contribution from the long-range transport. The high contribution of non-sea salt fractions to Ca2+, SO42−, Mg2+ and K+ showed a substantial effect of crustal and continental air masses. Results of source apportionment for the RW composition by using the Positive Matrix Factorization technique indicated four factors i.e. Marine and long range transport (Na+, Cl−), crustal (Ca2+, Mg2+), emissions from the fossil fuel and biomass burning (NO3−, SO42−) and the agriculture/farming activities (NH4+).

This article reports the results of a study related to variations in columnar aerosol optical depth (AOD), total column ozone (TCO), and precipitable water content (PWC) over a high-altitude station, Sinhagad (18° 21′ N, 73° 45′ E, 1450 m above mean sea level (AMSL)), employing a microprocessor-based total ozone portable spectrometer, MICROTOPS-II, comprising both a sun photometer and ozonometer, during November 2009–April 2010. The aerosol optical depth at 500 nm (AOD500 nm) portrayed seasonal variation with higher values (0.39) in summer and lower values (0.15) in winter. The TCO and PWC also exhibited lower values in winter and started increasing by the pre-monsoon season. The Ångström wavelength exponent, α, was found to be high (1.79) during February, indicating the relative dominance of accumulation-mode particles. During the summer season, the lower value (0.94) of the Ångström wavelength exponent indicates the relative dominance of coarse-mode particles. The ground-based observations from MICROTOPS-II revealed good correlation with satellite observations of the Moderate Resolution Imaging Spectroradiometer (MODIS) and Ozone Monitoring Instrument (OMI). The observed short-wave solar flux at the bottom of the atmosphere decreased due to aerosol extinction and was found to be 19 and 78 W m–2 for the winter and pre-monsoon seasons, respectively. This implies that greater concentrations of accumulation-mode particles – which are due to local anthropogenic sources – affected the down-welling radiation than those from natural sources – which are due to long-range transport processes – over the experimental location.

Seasonal variability of volatile organic compounds (VOCs) at a high-altitude station in the Western Ghats, India: Influence of biogenic, anthropogenic emissions and long-range transport

Atmospheric ice nuclei concentration measurements over a high altitude-station in the Western Ghats, India

Abstract. High altitude stations are often emphasized as free tropospheric measuring sites but they remain influenced by atmospheric boundary layer (ABL) air masses due to convective transport processes. The local and meso-scale topographical features around the station are involved in the convective boundary layer development and in the formation of thermally induced winds leading to ABL air lifting. The station altitude alone is not a sufficient parameter to characterize the ABL influence. In this study, a topography analysis is performed allowing calculation of a newly defined index called ABL-TopoIndex. The ABL-TopoIndex is constructed in order to correlate with the ABL influence at the high altitude stations and long-term aerosol time series are used to assess its validity. Topography data from the global digital elevation model GTopo30 were used to calculate five parameters for 43 high and 3 middle altitude stations situated on five continents. The geometric mean of these five parameters determines a topography based index called ABL-TopoIndex, which can be used to rank the high altitude stations as a function of the ABL influence. To construct the ABL-TopoIndex, we rely on the criteria that the ABL influence will be low if the station is one of the highest points in the mountainous massif, if there is a large altitude difference between the station and the valleys or high plains, if the slopes around the station are steep, and finally if the inverse drainage basin potentially reflecting the source area for thermally lifted pollutants to reach the site is small. All stations on volcanic islands exhibit a low ABL-TopoIndex, whereas stations in the Himalayas and the Tibetan Plateau have high ABL-TopoIndex values. Spearman's rank correlation between aerosol optical properties and number concentration from 28 stations and the ABL-TopoIndex, the altitude and the latitude are used to validate this topographical approach. Statistically significant (SS) correlations are found between the 5th and 50th percentiles of all aerosol parameters and the ABL-TopoIndex, whereas no SS correlation is found with the station altitude. The diurnal cycles of aerosol parameters seem to be best explained by the station latitude although a SS correlation is found between the amplitude of the diurnal cycles of the absorption coefficient and the ABL-TopoIndex.

Seasonal abundances of primary and secondary carbonaceous aerosols at a high-altitude station in the Western Ghat Mountains, India

Analysing Seasonal Variations in the Tropical Tropopause and the Impact of Deep Convection on the Tropopause Structure Over a High-Altitude Station in the Western Ghats

Compensatory effect of biomass burning on black carbon concentrations during COVID-19 lockdown at a high-altitude station in SW India

The rainfall characteristics (orographic and convective) over Mahabaleshwar (MBL), a Western Ghat hill station in India, has been studied in this work. In the first part, the role of orography and convective instability in quantifying rainfall over the station has been examined. In the second part, the variability of rainfall over MBL has been analyzed. In order to estimate orographic rainfall using a 3-D dynamical model, a high-resolution ECMWF reanalysis data was utilized to construct wind and thermal fields at a far upstream location. Whereas, for estimating convective rainfall, the ECMWF reanalysis data and in situ radiosonde data over MBL are used. It is seen that the estimated rainfall using the 3-D dynamical model for orographic rainfall is in good agreement with the pattern of observed daily rainfall as compared to the estimated rainfall using the convective model. Also, the estimated rainfall was able to capture the daily fluctuations in the rainfall. However, the rainfall estimated using the convective model shows much difference in the pattern and magnitude compared to the observed rainfall.

The structure of clouds during the convective and stratiform precipitation over the high-altitude tropical station, Mahabaleshwar, has been studied in the present paper by using Doppler weather Radar and several other in-situ measurements. The station, receiving an average annual rainfall of 5000 mm represents one of the important points of heavy precipitation belt of Western Ghats orographic region of India. It has been found that the region experiences a shallow convection with the cloud top height merely exceeding 6 km. The drop size of diameter 1–2 mm contributes more in the span of rain rate range of 2–4 mm/hr compared to other rain rate classes. Correspondingly, drops of larger diameter dominate the convective rain with respect to the stratiform one.

The aerosol optical properties have been investigated using the Raman lidar system for the month of November 2018 at the western Himalayan station of Palampur. Before deriving the optical properties, the lidar data has been applied with initial pre-processing such as Dead time correction, atmospheric noise correction, temporal and spatial averaging, range correction, gluing etc. The optical properties such as backscatter coefficient, extinction coefficient and linear depolarization ratio have been derived by using the inversion algorithm proposed by Fernald. The results show that the backscatter coefficient was found in the range from 9.00E-9 m−1sr−1 to 4.97E-6 m−1sr−1 and the extinction coefficient was found in the range from 3.16E-7m-1 to 1.74E-4m-1. The Linear depolarization ratio was in the range from 0.0179 to 0.621 with lower values at near heights suggesting the dominance of spherical particles at the lower heights. We have also observed a cloud layer at a height of 9.5 km to 12.1 km with high depolarization ratio during the observation period on 22/11/2018.

Abstract. First long-term observations of South American biomass burning aerosol within the tropical lower free troposphere are presented. The observations were conducted between 2007 and 2009 at a high altitude station (4765 m a.s.l.) on the Pico Espejo, Venezuela. Sub-micron particle volume, number concentrations of primary particles and particle absorption were observed. Orographic lifting and shallow convection leads to a distinct diurnal cycle at the station. It enables measurements within the lower free troposphere during night-time and observations of boundary layer air masses during daytime and at their transitional regions. The seasonal cycle is defined by a wet rainy season and a dry biomass burning season. The particle load of biomass burning aerosol is dominated by fires in the Venezuelan savannah. Increases of aerosol concentrations could not be linked to long-range transport of biomass burning plumes from the Amazon basin or Africa due to effective wet scavenging of particles. Highest particle concentrations were observed within boundary layer air masses during the dry season. Ambient sub-micron particle volume reached 1.4±1.3 μm3 cm−3, refractory particle number concentrations (at 300 °C) 510±420 cm−3 and the absorption coefficient 0.91±1.2 Mm−1. The respective concentrations were lowest within the lower free troposphere during the wet season and averaged at 0.19±0.25 μm3 cm−3, 150±94 cm−3 and 0.15±0.26 Mm−1. A decrease of particle concentrations during the dry seasons from 2007–2009 could be connected to a decrease in fire activity in the wider region of Venezuela using MODIS satellite observations. The variability of biomass burning is most likely linked to the El Niño–Southern Oscillation (ENSO). Low biomass burning activity in the Venezuelan savannah was observed to follow La Niña conditions, high biomass burning activity followed El Niño conditions.

Investigation of physical and optical properties of aerosol over high altitude stations along the sub-Himalayan region of North-Eastern India

Dust storm events are annual phenomena observed over the Indo-Gangetic plain (IGP) during the pre-monsoon period (May–June). These dust storms affect the air quality, weather conditions and radiation budget of the region. In this paper we characterize the aerosol optical parameters associated with a rare dust storm event that hit the IGP during early April 2005. This event was considered rare as it occurred much earlier than the general occurrence of dust storms in India (May–June), and in the year 2005, the warmest year in the span of the previous hundred years. In this study we considered the optical aerosol parameters for two places in the IGP: Delhi (28.5° N, 77.2° E, 325 m asl) and the high altitude station, Manora Peak (29.4° N, 79.5° E, 1958 m asl). Of the two selected stations, Delhi represents a highly populated and polluted location whereas Manora Peak represents a cleaner location in the central Himalayan region. During this dust storm event, the aerosol optical depth (AOD) was observed to increase considerably. The increment was 2.6–4.6 times over Delhi and 1.6–3.2 times over Manora Peak at wavelengths 380 and 1020 nm, respectively, with respect to the background values, whereas the Ångström exponent (α) for both the stations remained close to zero during the event. The effect shows a considerable increase in direct dust radiative forcing in terms of a reduction in the broadband global irradiance for Delhi as well as for Manora Peak stations. The direct aerosol radiative forcing thus obtained was about 34% in the 400–1100 nm wavelength band at Manora Peak.