基于多轴差分吸收光谱技术测量对流层HCHO垂直分布

甲醛（HCHO）在大气光化学反应中扮演着重要的角色，是一种重要的气溶胶前体物和光化学氧化指示剂。大气中HCHO的来源主要是直接排放和光化学反应生成。大气光化学反应与太阳辐射强度密切相关，一般来说，太阳辐射强度越强，大气光化学反应越剧烈，HCHO的二次来源产率也就越高。故针对HCHO的研究成为当今大气环境研究的一个重要课题。介绍了基于多轴差分吸收光谱技术 (MAX-DOAS) 获取对流层HCHO垂直柱浓度（VCD）及垂直廓线的反演算法。该方法是基于非线性最优估算法的两步反演方法，首先反演气溶胶垂直廓线, 然后在此基础上反演HCHO垂直廓线。其中第二步气体廓线反演时，气溶胶廓线线型会影响气体廓线反演的权重函数从而影响气体垂直廓线反演的精度, 为此, 研究了三种不同气溶胶廓线类型（指数型、高斯型和玻尔兹曼型）对HCHO垂直廓线反演的影响。结果表明，在三种气溶胶廓线类型条件下，当气溶胶光学厚度(AOD)为0.1时，气体反演的总误差、平均核的包络线、灵敏高度上限、自由度以及HCHO垂直廓线结果都比较接近，即气溶胶廓线类型对HCHO垂直廓线反演的影响很小。而对于200 m以下（含200 m）的近地面，通过指数型、高斯型和玻尔兹曼型气溶胶廓线获取的HCHO体积混合比（VMR）与真实HCHO VMR的差异分别为36.89%，-0.04%和23.30%, 表明使用指数型和玻尔兹曼型气溶胶廓线类型反演HCHO垂直廓线会高估近地面HCHO浓度，而高斯型气溶胶廓线类型则正好相反。此外，还反演了北京国科大站点一次污染过程中HCHO的垂直廓线，分析了污染过程中HCHO的垂直分布特征。结果表明，HCHO主要集中在1.0 km以下且一天中高值出现在午后，主要来自于本地产生，即西南风将污染的VOCs气团带到观测点，经过本地的光化学反应产生HCHO而积累，造成了此次HCHO浓度升高。结合气流后向轨迹分析，来自站点西南方向的输送是引起HCHO污染的重要原因。故观测站点的HCHO主要受污染输送和二次氧化的影响。最后对比了此次污染过程中不同气溶胶条件对HCHO廓线反演的误差影响。结果显示，气溶胶浓度高时，反演的灵敏高度和自由度下降，反演的高度分辨率下降，且反演总误差增加。

Retrieving Tropospheric Vertical Distribution in HCHO by Multi-Axis Differential Optical Absorption Spectroscopy

Formaldehyde (HCHO) plays an important role in atmospheric photochemical reactions. Besides, it’s an important aerosol precursor and photochemical oxidation indicator. The sources of HCHO in the atmosphere are mainly from primary direct emissions and photochemical reactions. Atmospheric photochemical reaction is closely related to the intensity of solar radiation. In general, the stronger the intensity of solar radiation is, the more active atmospheric photochemistry reaction will be so that the secondary sources of HCHO are higher. Thus, the research on HCHO has become an important topic in today’s atmospheric environment research. This paper introduces a method for retrieving tropospheric vertical column density (VCD) and vertical profile in HCHO based on multi-axis differential absorption spectroscopy (MAX-DOAS). This method is a two-step inversion method based on nonlinear optimal estimation method. Firstly, the vertical profile of aerosol is retrieved. Secondly, the vertical distribution of HCHO is retrieved based on the retrieved aerosol profile. In the second step of the gas profile inversion, the aerosol information affects the inversion accuracy of the gas vertical profile by influencing the weight function. Therefore, the effects of three different aerosol profile types (exponential, Gaussian and Boltzmann) on HCHO vertical profile inversion were studied. The results show that the total gas inversion error, the envelope curve of the average kernel, the limit of sensitivity height, the degree of freedom, and the vertical profile of HCHO retrieved in the three aerosol types are similar, which means that the aerosol profile type has little effect on the HCHO vertical profile inversion. For the near-surface below 200 m (including 200 m), the differences between the real HCHO volume mixing ratios (VMRs) with the HCHO VMRs obtained by the aerosol profile in exponential, Gaussian and Boltzmann shapes are 36.89%, -0.04%, and 23.3%, respectively. It is shown that the retrieved HCHO vertical profiles using the priori aerosol types in exponential and Boltzmann overestimates near-surface HCHO VMRs, while it is just the opposite in Gaussian shape. Furthermore, the vertical profile of HCHO in one polluted episode in the University of Chinese Academy of Science in Huairou District (UCAS) of Beijing is studied to obtain the vertical distribution in HCHO. The results indicate that the high value in HCHO mainly concentrates below 1.0 km from the vertical profile in HCHO and the high value in HCHO diurnal variations appears in the early afternoon, which indicates that the HCHO is mainly from local photochemical reaction. In a word, the southwest wind brings the polluted VOCs gas mass to the UCAS, and then HCHO generates and accumulates in local photolysis by VOC, which causes this HCHO polluted episode. Combined with backward trajectories determined (HYSPLIT) model, the transport from the southwest of the UCAS is an important cause of this polluted episode in HCHO. Therefore, HCHO at the observation site is mainly affected by pollution transport and secondary oxidation. Finally, the influence of different aerosol conditions on the HCHO profile inversion during this pollution episode was compared. The results show that when the aerosol optical depth increases, the retrieved limit of sensitivity height, the degree of freedom and the height resolution decreases, and the retrieved total error increases.