This study reports day-night and seasonal variations of aqueous brown carbon (BrC_aq) and constituent humic-like substances (HULIS) (neutral and acidic HULIS: HULIS-n and HULIS-a) from the eastern Indo-Gangetic Plain (IGP) of India during 2019–2020. This is followed by the application of the receptor model positive matrix factorization (PMF) for optical source apportionment of BrC_aq and the use of stable isotopic ratios (δ¹³C and δ¹⁵N) to understand atmospheric processing. Nighttime BrC_aq absorption and mass absorption efficiencies (MAE) were enhanced by 40–150 % and 50–190 %, respectively, compared to the daytime across seasons, possibly as a combined effect from daytime photobleaching, dark-phase secondary formation, and increased nighttime emissions. MAE_{250 nm}/MAE_{365 nm} (i.e., E₂/E₃) ratios and Angstrom Exponents revealed that BrC_aq and HULIS-n were relatively more aromatic and conjugated during the biomass burning-dominated periods while BrC_aq and HULIS-a were comprised mostly of non-conjugated aliphatic structures from secondary processes during the photochemistry-dominated summer. The relative radiative forcing of BrC_aq with respect to elemental carbon (EC) was 10–12 % in the post-monsoon and winter in the 300–400 nm range. Optical source apportionment using PMF revealed that BrC_aq absorption at 300, 365 and 420 nm wavelengths in the eastern IGP is mostly from biomass burning (60–75 %), followed by combined marine and fossil fuel-derived sources (24–31 %), and secondary processes (up to 10 %). Source-specific MAEs at 365 nm were estimated to be the highest for the combined marine and fossil fuel source (1.34 m² g⁻¹) followed by biomass burning (0.78 m² g⁻¹) and secondary processing (0.13 m² g⁻¹). Finally, δ¹³C and δ¹⁵N isotopic analysis confirmed the importance of summertime photochemistry and wintertime NO₃⁻-dominated chemistry in constraining BrC characteristics. Overall, the quantitative apportionment of BrC_aq sources and processing reported here can be expected to lead to targeted source-specific measurements and a better understanding of BrC climate forcing in the future.

• Optical source apportionment of diurnal BrCaq carried out using PMF.
• MAE of BrCaq increased by 50–190 % during nighttime compared to daytime.
• Biomass burning (BB) was the major source of BrCaq (60–75 %) in the eastern IGP.
• Source-specific MAE for marine and fossil fuel emission was twice that of BB.
• δ¹³C and δ¹⁵N signatures provided insights into the atmospheric processing of BrCaq.