Brown carbon (BrC) represents not only a major component of haze pollution but also a non-negligible contributor to positive radiative forcing, making it a key species for coordinating air quality and climate policies. In China, field observations on BrC remain limited given the highly variable emission sources and meteorological conditions across different regions. Here we focused on the optical properties of BrC in a distinct but rarely studied megacity in Northeast China, which is within a major agricultural region and experiences extremely cold winter. Agricultural fires were evident in April of 2021 and the fall of 2020, although open burning was strictly prohibited. Such emissions enhanced BrC's mass absorption efficiency at 365 nm (MAE365), more efficiently by the fall fires which were inferred to have relatively high combustion efficiencies (CE). After taking CE into consideration, the relationships between MAE365 and the levoglucosan to organic carbon ratio (a measure of the significance of agricultural fire influence) roughly converged for the fire episodes in different seasons, including those identified in February and March of 2019 by a previous campaign. Agricultural fires also influenced the determination of absorption & ANGS;ngstrom exponent (AAE), by resulting in non-linearity for BrC's absorption spectra shown on ln-ln scale. Based on three indicators developed by this study, the non-linearity was inferred to be caused by similar chromophores although the fires were characterized by various CE levels in different seasons. In addition, for the samples without significant influence of open burning, coal combustion emissions were identified as the dominant influencing factor for MAE365, whereas none solid link was found between the solution-based AAE and aerosol source.

Agricultural open burning, that is, the practice of burning crop residue in harvested fields to prepare land for sowing a new crop, is well recognized as a significant contributor to CO2 and black-carbon emissions, and long-term climate change. Low-soiltillage practices using an agricultural machine called the Happy Seeder, which can sow the new seed without removing the previous crop residue, have emerged as the most effective alternative to open burning. However, given the limited supply of Happy Seeders from the government, and the fact that farmers incur a significant yield loss if they delay sowing the new crop, farmers are often unwilling to wait to be processed by the Happy Seeder and, instead, burn their crop residue. We study how the government can use effective information-disclosure policies in the operation of Happy Seeders to minimize open burning. A Happy Seeder is assigned to process a group of farms. The government knows, but does not necessarily disclose, the Happy Seeder's schedule at the start of the sowing season. We propose a class of information-disclosure policies, called dilatory policies, that provide no information to the farmers about the schedule until a prespecified switch period and then reveal the entire schedule afterward. We show that an optimal dilatory policy can significantly lower the number of farms burnt compared with that under the full-disclosure and the no-disclosure policies. Using data from the rice-wheat crop system in northwestern India, we demonstrate that the optimal dilatory policy can reduce CO2 and black-carbon emissions by 17% on average. We also examine the impact of the government's policy on the trade-off between environmental damage and farmers' welfare.