The sources and concentrations of ice‐nucleating particles (INPs) over India are not well known. Here, INP concentrations in rainwater from Northern India and a dust sample from the Thar Desert are characterized. Rainwater INP concentrations ranged between 104 and 3 × 107 L−1 water, spanning temperatures between −4 and −28 °C. During the monsoon season, INP concentrations were low and approached those in remote marine air mass. During the winter season, INPs active between −4 to −10 °C were occasionally observed. An increase in INP activity sometimes occurred after the initial onset of rain. The onset freezing temperature of samples active at warmer temperatures was shifted to colder temperature after heat treatment, suggesting that the INP activity stemmed from biological influence. Plating was used to isolate and sequence INP active bacterial strains from some of the rainwater samples, specifically strains of close taxonomic affiliation with the ice nucleating genera Pantoea. The size‐resolved ice nucleation active site density for 200–600‐nm particles of Thar Desert Dust ranged between 107 and 109 m−2 at −20 °C, values similar to dusts from other regions of the world. The data reported herein may help constrain models that seek to predict the impact of INP on the properties of mixed‐phased clouds over the Indian subcontinent.

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Picophytoplankton (PicoP) are increasingly recognized as significant contributors to primary productivity and phytoplankton biomass in coastal and estuarine systems. Remarkably though, PicoP composition is unknown or not well-resolved in several large estuaries including the semi-lagoonal Neuse River Estuary (NRE), a tributary of the second largest estuary-system in the lower USA, the Pamlico-Albemarle Sound. The NRE is impacted by extreme weather events, including recent increases in precipitation and flooding associated with tropical cyclones. Here we examined the impacts of moderate to extreme (Hurricane Florence, September 2018) precipitation events on NRE PicoP abundances and composition using flow cytometry, over a 1.5 year period. Phycocyanin-rich Synechococcus-like cells were the most dominant PicoP, reaching ~ 106 cells mL−1, which highlights their importance as key primary producers in this relatively long residence-time estuary. Ephemeral “blooms” of picoeukaryotic phytoplankton (PEUK) during spring and after spikes in river flow were also detected, making PEUK periodically major contributors to PicoP biomass (up to ~ 80%). About half of the variation in PicoP abundance was explained by measured environmental variables. Temperature explained the most variation (24.5%). Change in total dissolved nitrogen concentration, an indication of increased river discharge, explained the second-most variation in PicoP abundance (15.9%). The short-term impacts of extreme river discharge from Hurricane Florence were particularly evident as PicoP biomass was reduced by ~ 100-fold for more than 2 weeks. We conclude that precipitation is a highly influential factor on estuarine PicoP biomass and composition, and show how ‘wetter’ future climate conditions will have ecosystem impacts down to the smallest of phytoplankton.

Aerosols play an important role in modulating climate from the local to the global scale. The ice nucleating properties of atmospheric aerosols affect cloud microphysics, precipitation efficiency and the hydrological cycle (Kanji et al., 2017). In the atmosphere, ice formation at temperatures >−38°C is assisted by ice nucleating particles (INPs). Heterogeneous nucleation by INP is the first step for precipitation initiation in clouds that do not produce rain through collision/coalescence alone (Kreidenweis et al., 2019). These INPs are injected into the atmosphere from terrestrial and water sources. Important compositional classes of INPs are mineral dust, sea salt, combustion derived soot, organic species and bioaerosols (Hoose & Möhler, 2012). The INPs that initiate ice at temperatures ≥−10°C are generally associated with biological material (Bowers et al., 2009). Bioaerosols are associated with living and dead biological material including bacteria, fungi, algae, lichen, molds, archaea, viruses, vegetation, and animal debris (Després et al., 2012). The concentration, composition and distribution of bioaerosols in atmosphere is governed by factors including land use pattern, local source emissions, windblown long-range transport from distant sources and seasonal variations in meteorological parameters (