Apparent PM2.5 Decay Following Delhi's October 2025 Cloud Seeding Operation: A Detection-Limit Analysis

Abstract

On 28 October 2025, the Indian Institute of Tropical Meteorology seeded clouds over north Delhi - the country's first such operation in more than fifty years, according to officials. A Cessna 206H aircraft released silver iodide flares and hygroscopic sodium chloride particles along a corridor between Rohini and Burari. We examined hourly PM2.5 readings from the four Delhi monitoring stations lying within 6.87 km of the flight path and fitted a simple exponential decay model, C(t) = C₀ × exp(−Λₐₚₚ × t), to the 16:00–22:00 Indian Standard Time window. The fitted apparent decay rate Λₐₚₚ ranged from 5.39 × 10⁻⁴ hr⁻¹ (Wazirpur; R² = 0.149) to 5.60 × 10⁻³ hr⁻¹ (Jahangirpuri; R² = 0.651), matching observed declines of 1.6–6.0%. Λₐₚₚ deserves a precise reading: it is a total decay rate. It folds in wet scavenging, but also boundary-layer dilution, wind-driven transport and changes in emissions. By itself, it says nothing about scavenging. For the recorded rainfall of 0.1–0.2 mm hr⁻¹, the standard below-cloud scavenging formula (Λ = 8 × 10⁻⁴ × R⁰·⁷⁹) predicts a six-hour removal of only 0.08–0.13% - one to two orders of magnitude smaller than the declines we observed. We also set the seeding afternoon against five dry afternoons at the same stations (6, 24, 26 October and 2, 9 November 2025). None of the four seeding-day Λₐₚₚ values differed significantly from the dry-day values: the z-scores were −0.63, −0.85, −0.94 and −1.13, all well inside ±1.96 and all negative. PM2.5, in other words, dropped more slowly - not faster - on the seeding afternoon (Bhandari et al., 2020; Ojha et al., 2020). We conclude that the small declines carry no detectable scavenging signal. They match the normal afternoon behaviour of the Delhi boundary layer and a detectable signal would have needed rainfall of roughly 9 mm hr⁻¹ - about 60 times what was recorded. We explain how we derived this detection threshold and what it implies for the design of any future urban seeding experiment.