Department of Chemistry, Oklahoma State University, Stillwater, OK 74078.
Department of Chemistry, University of British Columbia, Vancouver V6T 1Z1, Canada.
Proc Natl Acad Sci U S A. 2022 Sep 20;119(38):e2205610119. doi: 10.1073/pnas.2205610119. Epub 2022 Sep 12.
Biomass burning organic aerosol (BBOA) in the atmosphere contains many compounds that absorb solar radiation, called brown carbon (BrC). While BBOA is in the atmosphere, BrC can undergo reactions with oxidants such as ozone which decrease absorbance, or whiten. The effect of temperature and relative humidity (RH) on whitening has not been well constrained, leading to uncertainties when predicting the direct radiative effect of BrC on climate. Using an aerosol flow-tube reactor, we show that the whitening of BBOA by oxidation with ozone is strongly dependent on RH and temperature. Using a poke-flow technique, we show that the viscosity of BBOA also depends strongly on these conditions. The measured whitening rate of BrC is described well with the viscosity data, assuming that the whitening is due to oxidation occurring in the bulk of the BBOA, within a thin shell beneath the surface. Using our combined datasets, we developed a kinetic model of this whitening process, and we show that the lifetime of BrC is 1 d or less below ∼1 km in altitude in the atmosphere but is often much longer than 1 d above this altitude. Including this altitude dependence of the whitening rate in a chemical transport model causes a large change in the predicted warming effect of BBOA on climate. Overall, the results illustrate that RH and temperature need to be considered to understand the role of BBOA in the atmosphere.
大气中的生物质燃烧有机气溶胶（BBOA）含有许多吸收太阳辐射的化合物，称为棕色碳（BrC）。当 BBOA 在大气中时，BrC 可以与氧化剂（如臭氧）发生反应，从而降低吸收率或使物质变白。温度和相对湿度（RH）对变白的影响尚未得到很好的限制，这导致在预测 BrC 对气候的直接辐射效应时存在不确定性。使用气溶胶流动管反应器，我们表明，臭氧氧化对 BBOA 的变白强烈依赖于 RH 和温度。使用 poke-flow 技术，我们表明 BBOA 的粘度也强烈依赖于这些条件。通过假设变白是由于在 BBOA 的主体内部、在表面下的一个薄壳内发生的氧化反应，用粘度数据很好地描述了 BrC 的变白速率。使用我们的综合数据集，我们开发了这种变白过程的动力学模型，并表明 BrC 的寿命在大气中约 1 公里以下的高度为 1 天或更短，但在该高度以上通常长于 1 天。在化学传输模型中包含这种变白速率的高度依赖性会导致对 BBOA 对气候变暖影响的预测发生很大变化。总体而言，结果表明需要考虑 RH 和温度，以了解 BBOA 在大气中的作用。
