华北农村夏季大气甲醛浓度变化特征

为研究华北农村大气污染状况,于2013年夏季和2014年夏季先后在华北两个农村站点——固城站、饶阳站,采用基于汉栖(Hantzsch)反应的在线甲醛荧光分析仪开展大气甲醛在线观测,结合同期φ(O₃)、φ(PAN)(PAN为过氧乙酰硝酸酯)、φ(CO)和光解速率系数等数据,分析φ(甲醛)日变化及其光解速率等特征. 结果表明:华北农村夏季大气甲醛污染较重. 固城站和饶阳站φ(甲醛)分钟均值范围分别为0.05×10-9 ~59.18 ×10-9和1.66×10-9 ~ 46.83×10-9,平均值分别为12.82×10-9±7.59×10-9和13.73×10-9±5.82×10-9,高于文献报道的国内外其他地区的观测值,与同期观测的φ(NO_x)相当. 两个站点φ(甲醛)具有典型的日变化特征,φ(甲醛)小时均值峰值(固城站为17.43×10-9,饶阳站为17.57×10-9)均出现在上午10:00左右,与φ(O₃)和φ(PAN)的日变化趋势相似,但峰值早1 h出现. 光解速率系数观测结果表明,两个站点甲醛光解过程主要发生在06:00—18:00,固城站甲醛小时光解量峰值(2.59×10-9)出现在11:00,饶阳站甲醛小时光解量于正午12:00达到峰值(3.00×10-9),甲醛光解成稳定分子的速率是光解成自由基速率的1.2~1.7倍. 研究表明,两个站点φ(甲醛)的变化主要受局地过程的影响,光化学生成是甲醛的重要来源. 

Characteristics of Ambient Formaldehyde at Two Rural Sites in the North China Plain in Summer

As a partial study on gaseous trace pollutants in the North China Plain, ambient formaldehyde (HCHO), a dominant carbonyl compound in the atmosphere and an indicator of photochemical pollution, was measured at two rural sites,Gucheng and Raoyang located in the middle of the North China Plain in summer of 2013 and 2014, respectively, using an in-situ HCHO analyzer (AL4021) with online fluorescence method based on Hantzsch reaction. The variation and distribution characteristics of HCHO, along with ozone (O₃), peroxyacetyl nitrate (PAN) and carbon monoxide (CO), were obtained and discussed. Photolysis rate coefficients were simultaneously observed and used for the calculations of photolysis rates of HCHO. The results revealed that photochemical processes played an important role in the variation of HCHO at the two sites. The minutely mean mixing ratios of formaldehyde at the two sites were (12.82×10-9± 7.59 × 10-9) and (13.73×10-9± 5.82×10-9), with a range of 0.05×10-9-59.18×10-9 at Gucheng and of 1.66×10-9-46.83×10-9 at Raoyang, respectively, which were higher than those reported from other Chinese or overseas sites and comparable with those of nitric oxides (NO_x) measured at the same sites. Formaldehyde displayed a similar diurnal variation as O₃ and PAN for both sites, but peaked (hourly mean mixing ratio of 17.43×10-9 for Gucheng and 17.57×10-9 for Raoyang) earlier at 10:00 am. According to the observation of photolysis rate coefficients, the photolysis process of formaldehyde happened mainly during daytime from 06:00 to 18:00, with the photolysis amount per hour reaching its peak at 11:00 and at 12:00 at Gucheng (2.59×10-9) and Raoyang (3.00×10-9), respectively. The photolysis rate for yielding hydrogen and CO was faster than that for yielding H and HCO radicals, with the ratio between both varying from 1.2 to 1.7 during daytime. The results suggest that the mixing ratio of formaldehyde was mainly influenced by local processes and photochemical formation and decomposition played an important role at both sites.