ASSESSMENT OF AMBIENT PARTICULATE MATTER (PM10, PM2.5, PM1) AND PARTICLE NUMBER CONCENTRATION DURING DIWALI AT A RESIDENTIAL SITE IN DELHI

Abstract

Delhi is known for its bad air quality and the ambient air quality gets worse during the post-monsoon season, and the burdens of Diwali festival are heavy pollution stress on an already stressed Delhi air quality. No previous study had measured PM mass fractions (PM₁₀, PM₂.₅, PM₁) and PNC at a less-trafficked residential location in Delhi, which was necessary to isolate the firework signal from vehicular emissions. This study addresses this by conducting a continuous monitoring campaign at Palam, south west Delhi, for 6 days between 19–25 October 2025, using a GRIMM Aerosol OPC 11D at 1 min resolution, which produced 8,555 quality-controlled data for four phases: Pre-Diwali, Diwali Day, Post-Diwali 1 and Post-Diwali 2. On all monitoring days, the Indian NAAQS 24-hour guideline was violated; while the pre-festival baseline was exceeded by 2.37× the PM₂.₅ and 3.61× the PM₁₀ limits. The Diwali evening episode (20 October, 18:00–24:00 h) produced the campaign's peak aerosol loading: PM₁₀ = 1,542.7 µg/m³, PM₂.₅ = 834.7 µg/m³, PM₁ = 531.4 µg/m³, and PNC = 9.661 × 10⁶ particles/cm³. The evening 6-hour mean PM₂.₅ was 405.6 µg/m³, which was 6.8 times the NAAQS limit and 27 times the WHO guideline for 24 hours. Most importantly, the post-Diwali episode of smog (21 October) had the highest mean of PM₂.₅ during the study period (264.7 µg/m³; EF = 4.41) and the highest excess exposure (2,945 µg·h/m³). The Mann–Whitney U test showed no significant difference in distribution between Diwali Day and Post-Diwali 1 (p = 0.261), indicating that the hazardous air quality event persisted for 3 days. The Pearson correlations were very high (r ≥ 0.997) between PM₁, PM₂.₅ and PNC and the Fine Fraction Ratios were consistently above 0.7, reflecting the dominance of sub-micron combustion aerosols throughout. Excess PM₂.₅ above background was found to be 35-46% pyrotechnic activity. During Diwali week, all the analysed elements recorded significantly higher levels than the national and international standards. The results could help policymakers and regulatory bodies assess the effectiveness of existing regulations for festivals on the ground and enact measures to limit air pollution from firecrackers and their impact on public health.