Seasonal variations of water-soluble organic carbon, dicarboxylic acids, ketocarboxylic acids, and α-dicarbonyls in Central Himalayan aerosols

Aerosol samples were collected from a high elevation mountain site (Nainital, India; 1958 m a.s.l.) in the central Himalayas, a location that provides an isolated platform above the planetary boundary layer to better understand the composition of the remote continental troposphere. The samples were analyzed for water-soluble dicarboxylic acids (C2-C12) and related compounds (ketocarboxylic acids and α-dicarbonyls), as well as organic carbon, elemental carbon and water soluble organic carbon. The contributions of total dicarboxylic acids to total aerosol carbon during wintertime were 1.7% and 1.8%, for day and night, respectively whereas they were significantly smaller during summer. Molecular distributions of diacids revealed that oxalic (C2) acid was the most abundant species followed by succinic (C4) and malonic (C3) acids. The average concentrations of total diacids (433±108 ng m−3), ketoacids (48±23 ng m−3), and α-dicarbonyls (9±4 ng m−3) were similar to those from large Asian cities such as Tokyo, Beijing and Hong Kong. During summer most of the organic species were several times more abundant than in winter. Phthalic acid, which originates from oxidation of polycyclic aromatic hydrocarbons such as naphthalene, was found to be 7 times higher in summer than winter. This feature has not been reported before in atmospheric aerosols. Based on molecular distributions and air mass backward trajectories, we conclude that dicarboxylic acids and related compounds in Himalayan aerosols are derived from anthropogenic activities in the highly populated Indo-Gangetic plain areas.

Seasonal variations of water-soluble organic carbon, dicarboxylic acids, ketoacids, and α-dicarbonyls in the central Himalayan aerosols

Aerosol samples were collected from a high elevation mountain site (Nainital, India; 1958 m a.s.l.) in the central Himalayas, which provide an isolated platform above the planetary boundary layer to better understand the composition of the remote continental troposphere. The samples were analyzed for water-soluble dicarboxylic acids (C2–C12) and related compounds (ketocarboxylic acids and α-dicarbonyls), as well as organic carbon, elemental carbon and water soluble organic carbon. The contributions of total dicarboxylic acids to total aerosol carbon during wintertime were 1.7 and 1.8%, for day and night, respectively whereas they significantly reduced during summer. Molecular distributions of diacids demonstrated that oxalic (C2) acid was the most abundant species followed by C4 and C3 diacids. The average concentrations of total diacids (433 ± 108 ng m−3), ketoacids (48 ± 23 ng m−3), and α-dicarbonyls (9 ± 4 ng m−3) were similar to those from Asian cities such as Tokyo, Beijing and Hong Kong. During summer season most of the organic species were several times more abundant than in winter. Phthalic acid, which originates from oxidation of polycyclic aromatic hydrocarbons such as naphthalene, was found to be 7 times higher in summer than winter. This feature has not been reported in atmospheric aerosols. Based on molecular distributions and air mass backward trajectories, we report that dicarboxylic acids and related compounds in Himalayan aerosols are influenced by the anthropogenic activities from highly populated Indo-Gangetic plain areas.

New particle formation infrequently observed in Himalayan foothills – why?

A fraction of the Himalayan aerosols originate from secondary sources, which are currently poorly quantified. To clarify the climatic importance of regional secondary particle formation in the Himalayas, data from 2005 to 2010 of continuous aerosol measurements at a high-altitude (2180 m) Indian Himalayan site, Mukteshwar, were analyzed. For this period, the days were classified, and the particle formation and growth rates were calculated for clear new particle formation (NPF) event days. The NPF events showed a pronounced seasonal cycle. The frequency of the events peaked in spring, when the ratio between event and non-event days was 53 %, whereas the events were truly sporadic on any other seasons. The annual mean particle formation and growth rates were 0.40 cm−3 s−1 and 2.43 nm h−1, respectively. The clear annual cycle was found to be mainly controlled by the seasonal evolution of the Planetary Boundary Layer (PBL) height together with local meteorological conditions. Spring NPF events were connected with increased PBL height, and therefore characterised as boundary layer events, while the rare events in other seasons represented lower free tropospheric particle formation. This provides insight on the vertical extent of NPF in the atmosphere.

New particle formation infrequently observed in Himalayan foothills – why?

A fraction of the Himalayan aerosols originate from secondary sources, which are currently poorly quantified. To clarify the climatic importance of regional secondary particle formation at Himalayas, data from 2005 to 2010 of continuous aerosol measurements at a high-altitude (2180 m) Indian Himalayan site, Mukteshwar, were analyzed. For this period, the days were classified, and the particle formation and growth rates were calculated for clear new particle formation (NPF) event days. The NPF events showed a pronounced seasonal cycle. The frequency of the events peaked in spring, when the ratio between event and non-event days was 53 %, whereas the events were truly sporadic on any other seasons. The annual mean particle formation and growth rates were 0.40 cm−3 s−1 and 2.43 nm h−1, respectively. The clear annual cycle was found to be mainly controlled by the seasonal evolution of the Planetary Boundary Layer (PBL) height together with local meteorological conditions. Spring NPF events were connected with increased PBL height, and therefore characterised as boundary layer events, while the rare events in other seasons represented lower free tropospheric particle formation.