印度西高止山脉高海拔站点挥发性有机化合物的季节性变化：生物源、人为源以及远距离传输的影响

Seasonal variability of volatile organic compounds (VOCs) at a high-altitude station in the Western Ghats, India: Influence of biogenic, anthropogenic emissions and long-range transport

Subrata Mukherjee, G. Pandithurai*, Vinayak Waghmare, Anoop S. Mahajan, Liselotte Tinel, M.Y. Aslam, G.S. Meena, Sachin Patil, Pallavi Buchunde, Anil Kumar

亮点

l  首次报道了在印度西高止山脉高海拔地区观测到的1年VOCs。

l  除季风期外，站点主要接受大陆老化气团。

l  生物源排放在季风前期占主导地位。

l  异戊二烯是季风前期排放的最大生物源VOC ( 1.01 ppb )。

l  观察到的单萜烯浓度具有生物源和人为源的混合来源。

l  局部交通源的贡献在站点的西南部得到了证实。

l  PMF分析表明，生物质燃烧和老化气团是主要贡献源。

Highlights

l  VOCs observed during 1 year over a high-altitude site in the Western Ghats, India were reported for the first time.

l  The site receives predominately continental aged air masses except during monsoon.

l  Biogenic emissions were dominant during pre-monsoon season.

l  Isoprene was the largest emitted biogenic VOC in pre-monsoon (1.01 ppb).

l  Observed monoterpene concentration had mixed biogenic and anthropogenic origin.

l  Contribution of local traffic sources was evidenced at the SW of observation site.

l  PMF analysis showed main contributions from biomass burning and aged air masses.

摘要

在印度西高止山高海拔（海拔高度1380米）的森林站点，基于为期一年的观测（2019年6月-2020年5月），研究了挥发性有机化合物（VOCs）的季节性变化。研究发现，异戊二烯这种典型的生物源VOCs在季风前期达到峰值。温度压力和更高的光合有效辐射（PAR）是周围森林区域异戊二烯排放的驱动因素。在夏季，异戊二烯的白天平均混合比为1.92 ± 1.51 ppb，而夜间平均为0.02 ± 0.01 ppb。研究还得出异戊二烯混合比与环境温度之间存在强相关性。大多数人为源VOCs如乙腈、苯、甲苯和二甲苯在冬季的浓度较高。这些人为源VOCs大多在下午至晚间达到峰值，表明除了交通排放外，生物燃料等其他燃烧源以及周围山谷地区的污染物垂直上升也对其有贡献。在季风季节的下午和晚间，甲苯与苯的比值（T/B）大于1，而在其他季节则以T/B比值较低（<0.7）的老化气团为主。极坐标双变量图显示，无论季节如何，甲苯、二甲苯和三甲基苯（TMB）大多来自测量站点的西南方，表明全年存在显著的本地交通相关排放源。对两个选定的芳香族VOCs（甲苯和二甲苯）的本地排放和区域背景贡献进行了估算，结果显示在季风和季风后期，本地人为源占主导地位。正矩阵因子分解揭示了所有季节中的三个源因子，并在冬季和季风前期出现了一个额外的因子。第一个因子由甲苯、苯、二甲苯和TMB组成，T/B比值约为1.2，表明其来源于交通/化石燃料。第二个“生物质燃烧”因子则由乙腈、乙醛、丙酮和苯主导，T/B比值为0.45。通过与使用不同燃烧源和溶剂进行的烟雾箱实验对比，进一步验证了因子一和二。第三个因子由异戊二烯和单萜烯主导，归因于生物源排放。第四个因子主要由乙腈、乙醛和丙酮组成，表明由于远距离传输、光化学生产和生物质燃烧的共同作用，形成了老化气团。

关键词：挥发性有机物、生物源排放、长距离传输、光化学过程、源解析PMF分析

Abstract

Seasonal variability of volatile organic compounds (VOCs) was studied using year-long observations (June 2019–May 2020) over a high-altitude (1380m AMSL) forested site in the Western Ghats of India. Isoprene, a well-known biogenic VOC, peaked during pre-monsoon. Temperature stress and higher photo synthetically active radiation (PAR) were the drivers behind isoprene emissions from the surrounding forested regions. The daytime average isoprene mixing ratio was 1.92 ± 1.51 ppb during summer and the night time average was 0.02 ± 0.01 ppb. A strong relationship between isoprene mixing ratio and ambient temperature was derived. Most of the anthropogenic VOCs like acetonitrile, benzene, toluene and xylene were found to be higher during winter. These anthropogenic VOCs mostly exhibited afternoon to evening peaks, which suggests the contribution by traffic emissions along with other combustion sources (biofuels) and vertical updraft of pollutants from the surrounding valley region. The toluene to benzene ratio (T/B) was >1 during the afternoon and evening hours in monsoon, whereas, aged air masses with lower T/B ratios (<0.7) prevailed during other seasons. Polar bi-variate plots reveal that toluene, xylene and tri-methyl benzene (TMB) were mostly emitted at the south-west of the measurement site, irrespective of the season, indicating a dominant local traffic related source throughout the year. The contribution of local emissions and regional background on two selected aromatic VOCs, toluene and xylene was estimated, revealing the dominance of local anthropogenic sources during monsoon and post monsoon. Positive matrix factorization revealed three source factors in all the seasons with one additional factor during winter and pre-monsoon. The first factor was constituted of toluene, benzene, xylene and TMB, and the T/B ratio was ~1.2, indicating traffic/fossil fuel contribution. A second ‘biomass burning’ factor was dominated by acetonitrile, ethanal, acetone, and benzene in which the T/B ratio was 0.45. Factors one & two were further validated by comparison with chamber experiments conducted using different combustion sources and solvents. The third factor was dominated by isoprene and monoterpenes which were attributed to biogenic emissions. The fourth factor comprised mainly of acetonitrile, ethanal, and acetone indicating aged air masses due to long-range transport with contributions from photochemical production and biomass burning.

Keywords: Volatile organic compounds, Biogenic emission, Long-range transport, Photochemical processing, Source apportionment PMF analysis

Fig. 1. The satellite map of (a) the observation site, (b) zoomed satellite view depicting the land use, land cover around the observation site. Panels (c) and (d) are the pictures from the observation site towards east and south.

Fig. 2. Diurnal variation of (a) isoprene, and (b) monoterpenes average mixing ratios during monsoon (black), post-monsoon (red), winter (blue), pre-monsoon (pink) over HACPL, Mahabaleshwar.

Fig. 3. Relationship of isoprene mixing ratio with atmospheric temperature during monsoon (a), post-monsoon (b), winter (c) and pre-monsoon (d) season at HACPL, Mahabaleshwar.

Fig. 4. Diurnal variation of measured VOCs i.e., acetonitrile (a), acetone (b), MEK (c), benzene (d), toluene (e), xylene (f), TMB (g) and T/B ratio (h) during monsoon (black), post-monsoon (red), winter (blue), and pre-monsoon (pink) over HACPL, Mahabaleshwar.

Fig. 5. the scatted plot of ln(Xylene/Benzene) vs ln(Toluene/Benzene) during (a) monsoon, (b) post-monsoon, (c) winter, and (d) pre-monsoon season over HACPL, Mahabaleshwar. The different colored points depict literature reported ratios for urban and rural sites.

Fig. 6. Source profiles of different factors as obtained utilizing Positive Matrix Factorization analysis with VOCs mixing ratios during monsoon (left panel) and post-monsoon (right panel) season over HACPL, Mahabaleshwar. The red dot indicates the percent of VOCs explained by the respective factors.

Fig. 7. Source profiles of different factors as obtained utilizing Positive Matrix Factorization analysis with VOCs mixing ratios during winter (left panel) and pre-monsoon (right panel) season over HACPL, Mahabaleshwar. The red dot indicates the percent of VOCs explained by the respective factors.

Fig. 8. Percentage contribution of source factors to the total measured VOCs during monsoon (a), post-monsoon (b), winter (c), and pre-monsoon season (d) over HACPL, Mahabaleshwar.