[论文解读] El Roque de Los Muchachos Site Characteristics. III. Analysis of Atmospheric Dust and Aerosol Extinction

G. Lombardi, V. Zitelli+3 more|ArXiv.org|Feb 27, 2008

Astrophysics and Star Formation Studies参考文献 20被引用 5

一句话总结

本研究利用位于泰莱斯科皮奥·纳索纳莱伽利望远镜（Telescopio Nazionale Galileo）的多通道尘埃监测仪五年数据，分析了洛斯穆恰科斯天文台的大气尘埃与气溶胶消光。研究发现，夏季尘暴会使颗粒物浓度最高提升至1,000倍，消光峰值达1.0 mag airmass⁻¹，主要由1.0–10.0 µm颗粒主导，且来自西南方向的局地风场驱动了显著的地面尘埃再悬浮，而不仅限于撒哈拉沙尘输送。

ABSTRACT

Canary Islands are normally interested by dominant North-East winds that, in some meteorological conditions, can transport sand at high altitude from the Sahara desert. The dust may affect the efficiency of the telescopes and decreases the transparency of the sky. In order to maximize the scientific return of the telescopes located at the ORM, we present an analysis of the atmospheric dust content and its effects on astronomical observations. B, V and I dust aerosol astronomical extinction are derived. Using a 5 years series database of data taken from the four channel TNG dust monitor, we compute a mean hourly and daily values of the dust content. We have detected particles having size 0.3, 0.5, 1.0 and 5.0 um. Using a power law we have derived the content of 10.0 um particles. We found a typical local dust concentration ranging from 3x10^6 particles per cubic meter at 0.3 um, to 10^3 at 5.0 um and 10 at 10.0 um, increasing up to 3 order of magnitudes during the dust storms, with a relative higher increase of 1.0, 5.0 and 10.0 um particles. The number of local dust storm events is the same in winter- and summertime, but, the average background and storm-related increases in the dust concentration in summer are significantly higher than in winter. In a uniform approximation, during the dust storms, an average height of the dust layer of 2.5 km above the telescope is inferred. During the sand storms La Palma Island is affected by an almost uniform layer extending up to 5 km above the sea level, down, at least the height of the telescope. The visible extinction is dominated by particles at 1.0, 5.0 and 10.0 um. In agreement with the results from Carlsberg Automatic Meridian Circle (CAMC) we find a typical extinction during dust storms of about 0.2 mag/airmass.

研究动机与目标

• 量化洛斯穆恰科斯天文台（ORM），拉帕尔马，的大气尘埃浓度及其对天文观测的影响。
• 确定0.3–10.0 µm颗粒物的季节性与昼夜变化规律，及其与风向和消光的相关性。
• 评估局地尘埃再悬浮与高层撒哈拉沙尘输送对大气消光的相对贡献。
• 评估尘埃对望远镜性能的影响，包括镜面反射率下降与信噪比退化。
• 验证地面尘埃监测仪与卫星数据（TOMS）结合使用在综合站点测试中的有效性。

提出的方法

• 利用TNG穹顶的四通道尘埃监测仪，实时测量0.3、0.5、1.0和5.0 µm粒径的颗粒物数量。
• 采用幂律分布从实测数据外推估算10.0 µm颗粒物的浓度。
• 基于五年数据集计算平均小时与日尘埃浓度值，以识别趋势与风暴事件。
• 将尘埃峰值事件与风向和风速相关联，以区分局地再悬浮与长距离撒哈拉输送。
• 将地面尘埃数据与TOMS气溶胶指数及CAMC天文消光数据对比，验证相关性。
• 假设尘埃层分布均匀，利用消光与颗粒物浓度数据估算尘埃层的垂直范围。

实验结果

研究问题

• RQ1 在TNG站点，0.3–10.0 µm粒径的大气尘埃浓度的季节性与昼夜变化如何，尤其在该范围内？
• RQ2 风向与风速如何与尘暴事件相关联？这揭示了局地与远距离尘埃来源的何种信息？
• RQ3 局地尘埃再悬浮事件在多大程度上独立于撒哈拉“卡利马”（Calima）事件，对天文消光产生贡献？
• RQ4 风暴事件期间尘埃层的典型垂直范围是多少？这如何影响与空气质量相关的消光？
• RQ5 地面尘埃监测仪测量值与卫星反演气溶胶指数及天文消光数据的相关性如何？

主要发现

• 典型背景尘埃浓度在0.3 µm时为3×10⁶ particles/m³，5.0 µm时为10³，10.0 µm时为10，尘暴期间最高可提升1,000倍。
• 尽管事件频率相似，夏季尘暴的背景浓度更高（高3–4倍），且颗粒物数量增幅也大于冬季事件。
• 尘暴通常持续3–4天，罕见事件可持续5–6天；持续时间≤2天的短时风暴与高TOMS气溶胶指数无显著相关性，表明其更可能源于局地再悬浮。
• 在多尘天气中，≥1.0 µm颗粒物主导消光（贡献超过70%），而在晴朗天气中，0.3 µm颗粒物占主导（约70%）。
• 尘暴期间平均天文消光约为0.2 mag airmass⁻¹，极端事件中可升至约1.0 mag airmass⁻¹，与CAMC数据一致（Spearman相关性>0.8）。
• 推断出均匀分布的尘埃层垂直范围约延伸至望远镜上方2.5 km（海拔5 km），表明0.3–10.0 µm颗粒物在观测路径中沉降极少。