New Particle Formation and Growth of Climate-Relevant Aerosols at Two Key Himalayan Sites in Uttarakhand, India

In this study, the observation site Himalayan Cloud Observatory is located at the high-altitude location (30.34 N, 78.40 E, 1706 m above mean sea level) and established at Swami Ram Tirth Campus, Badshahithaul, Tehri Garhwal, Uttarakhand in the western Himalaya. We have identified and characterized the new particle formation events for 12-months period (January to December, 2021) of continuous monitoring of the aerosol size distribution using NanoScan Scanning Mobility Particle Sizer. Another year-long observation was carried out at the Himalayan Atmospheric and Space Physics Research Laboratory (HASPRL), Department of Physics, Hemvati Nandan Bahuguna Garhwal University (A Central University), Srinagar Garhwal, Uttarakhand, in the Alaknanda Valley (30°13'37.3"N, 78°48'14.2"E, 640 m AMSL), from January 1, 2023, to December 31, 2023.

We have observed 51 new particle formation events out of 278 days of observations having 14% frequency of new particle formation occurrence. New particle formation events were most frequent in March-April-May (pre-monsoon) and least frequent in June-July-August-September (monsoon). This trend is linked to high temperatures, strong solar radiation, and low relative humidity in pre-monsoon, which enhance the formation of low-volatility organic compounds, while in monsoon, wet scavenging reduces aerosol precursor gases. The seasonal mean of growth rate (GR_{11.5-27.4 nm}), formation rate (J_11.5), coagulation sink (CoagS_11.5-27.4) and condensation sink (CS_TOT, 11.5-154 nm) during the study period were 1.27±0.23 nm h^-1, 0.12±0.08 cm^-3 s^-1, 2.92±1.65×10^-5 s^-1 and 9.91±3.13×10^-3 s^-1 respectively. Seasonal distributions show particles within 11.5–100 nm predominantly originate from secondary emissions, while particles 100–154 nm result from both direct and nucleated process, highlighting the seasonal sources of particles at Himalayan Cloud Observatory. A significant reduction (by 25%) found in incoming solar radiation on non-event days limits the oxidation of precursor gases, thereby inhibiting particle formation. Polar bivariate analysis reveals that winter airmasses, transported via mountain winds from the southwest and northeast, introduce mixed particle sizes. In contrast, the localized concentration of particles with elevated GR_{11.5-27.4 nm} and J_11.5 during pre-monsoon highlights the role of aerosol precursors, condensable vapors, and favorable meteorological conditions, emphasizing new particle formation as the dominant particle source. Comparison with prior cloud condensation nuclei study at Himalayan Cloud Observatory reveals that new particle formation significantly supplements cloud condensation nuclei production beyond primary emissions, especially in pre-monsoon. The satellite-based observation of sulfur dioxide and formaldehyde complement and support the condensable vapours during event days at Himalayan Cloud Observatory. At HASPRL, smaller-sized aerosol particles showed an increase in the morning, likely due to emissions from anthropogenic sources originating from the southwest. In the evening, larger-sized aerosol particles were observed to increase, possibly resulting from various human activities such as vehicular emissions, transportation of sand and stone, and other anthropogenic emissions from the southeast. At HCO, particle transport is likely influenced by movement from valley and forested regions, with air masses traveling from the southeast to the southwest.

In summary, this research offers fresh perspectives on the characterization of new particle formation events in the Himalayan region of Uttarakhand. These insights are crucial for comprehending secondary aerosol formation processes worldwide, particularly at the process level.