长三角地区雾-霾天气条件下大气黒碳气溶胶混合状态特征的观测研究

Fog-haze has been a common phenomenon that occurs frequently and extensively in China. The physical, chemical and optical properties will be changed during fog-haze period, which will also modify the aerosol climate effect. The black carbon is the essential light absorbing material in the atmosphere. The mixing state of black carbon can introduce great uncertainties in estimates of aerosol direct radiative forcing. The main uncertainty is mainly due to the scarce knowledge about the mixing state of black carbon and mechanism. Thus, the study of mixing state could improve the modeling work with more accurate assumption of aerosol radiative properties and mechanism. In this work, the fall and winter will be chosen as the studying period due to the high occurrence frequency of fog-haze. The measurement of particle number size distribution, optical properties and chemical component will be conduct at a regional background station in Yangtze River Delta, Lin’an Atmosphere Background Station. Then we use Mie model to simulate the optical properties, which should be in closure with the measured values with a reasonable uncertainty. Based on the optical closure study, the mixing state of black carbon can be retrieved. The evolution of mixing state of black carbon and the related meteorological factors will also be focused on. The study of mixing state of black carbon will help to know the evolution of aerosol characteristics and also provide scientific information to validate the role of aerosol in the regional climate impact.

雾-霾现象已经成为我国区域空气污染首要面对的问题。雾-霾天气条件下，气溶胶理化及光学性质的改变也将影响其气候效应。黑碳作为最主要的光吸收物质，它的混合状态会为评估气溶胶辐射强迫带来很大的不确定性。这种不确定性主要来自黑碳气溶胶混合状态的变化和机制尚不明确。因此，对混合状态的研究可以减少模式中对气溶胶辐射特性和相关机制所做假设的不确定性。本研究拟将选择长三角地区临安区域大气本底站作为观测点，选择雾-霾发生频率较高的秋冬季节开展大气气溶胶数谱分布、光学特性和化学组分的观测。利用Mie模型，将模拟得到的光学特性和观测值进行闭合实验，并在二者闭合的基础上反演黑碳气溶胶的混合状态，揭示混合状态的变化及受气象条件的影响。对黑碳气溶胶混合状态的观测研究，有助于深入了解雾-霾条件下气溶胶特性的演变过程，并为准确评估该区域气溶胶的气候效应提供科学依据。

长三角地区是我国经济最发达的地区之一，同时也面对着严峻的空气质量恶化问题。2013年冬季我国发生的一次持续数日、涉及范围较广的雾-霾事件，使得对雾-霾发生发展机制的研究更为迫切。本研究选择在长三角本底站临安大气成分站开展了长达两年的气溶胶数谱分布和光学特性观测，并且结合Mie模型反演散射系数，与实际观测有较好的闭合结果。利用光学闭合结果推算出黑碳气溶胶的混合状态。获得的主要研究结果包括：（1）观测期间，临安大气气溶胶数浓度平均值（13±6.2）×103 cm-3，其中核模态（3–25 nm）数浓度平均值4120 cm-3，爱根核模态（25–100 nm）数浓度平均值为4820 cm-3，积聚模态（100–1000 nm）为3100 cm-3。PM2.5质量浓度平均值78±22 μg/m3。（2）根据能见度、相对湿度和PM2.5的关系，将天气类型分为雾、霾和晴天，获得了不同天气类型的气溶胶数谱分布及参数化结果。（3）利用Mie模型，将气溶胶数谱和化学组分作为输入数据，计算得到散射系数，并与实际观测的散射系数有较好的一致性。（4）通过光学闭合结果反演出黑碳气溶胶的混合状态，结果表明黑碳外混比例在0.4左右。（5）混合状态表现出一定的日变化，上午9时较高，下午及夜间降低，主要受新鲜排放和气溶胶老化过程影响。混合状态受气团影响，来自长三角区域传输的气团，气溶胶相对以内混状态为主。本研究不仅获得了长达两年的气溶胶数谱及光学特性观测数据，数谱数据为中国气象局环境数值预报模式CUACE提供了验证，并改善了模式中成核和凝结过程的物理机制，提高了模式对PM2.5质量浓度预报的精确性。