Episodes of enhanced condensation nuclei concentrations during the GVAX campaign at ARIES, Nainital

Description

Simultaneous measurements of condensation particles (NCN) and cloud condensation nuclei (NCCN) concentrations were made along with the measurements of solar radiation and meteorological parameters during the Atmospheric Radiation Measurement Mobile Facility (AMF1) deployment at a high-altitude site—ARIES, Nainital (29.4°N, 79.5°E; 1950 m amsl)—in central Himalaya as part of the Ganges Valley Aerosol Experiment (GVAX). The preliminary analysis of the data obtained during June 15–September 30, 2011, shows significant signatures of transport of atmospheric particles to the observation site with prevailing and episodic winds over the region. While the nominal concentrations of NCN, with some occasional spikes, were in the range of 500–2000 cm-3, their concentration increased to about four times with a change in surface wind direction. A similar trend in concentrations of NCCN with respect to the change in wind direction was also observed at all seven air supersaturations between -0.01 to + 0.75%. The absolute increase in NCCN during the episodes of reversal of surface winds was more in higher supersaturations (> 0.31 %). The wind-rose diagram clearly shows two prominent wind direction sections, i.e., easterly/southeasterly sector and westerly/northwesterly sector with frequency of occurrence ~30% and ~4 % of total winds, respectively. The wind speed versus wind direction scatter diagram suggests stronger winds of the order of 6–12 ms-1 associated with the westerly sector. The pronounced signatures of transport of aerosol particles with prevailing winds are clear from these observations and suggest different origins for the air masses arriving at observation site. The 5-day backward trajectories derived from the HYSPLIT trajectory model also indicate different pathways of transport of air parcels with changes in wind directions during the southwest monsoon period. In addition to scavenging of aerosols due to precipitation, the events of new particle formation and role of aerosols in modifying cloud properties will be studied from the GVAX data. Detail analysis of data from additional instruments operated in GVAX is planned to understand the physical properties of aerosols in the Ganges valley and their effect on monsoons and climate of the region.