PEM Tropics A DC-8 Photolysis Frequencies (J-Values)

PEM-Tropics-A_jValue_AircraftInSitu_DC8_Data is the photolysis frequencies (j-values) measured along the DC-8 flight during the Pacific Exploratory Mission (PEM) Tropics A suborbital campaign. Data collection for this product is complete.

From 1983-2001, NASA conducted a collection of field campaigns as part of the Global Tropospheric Experiment (GTE). Among those was PEM, which intended to improve the scientific understanding of human influence on tropospheric chemistry. Part of the PEM field campaigns were focused on the tropical Pacific region (PEM-Tropics) which was recognized as a “very large chemical vessel.” The overarching science objective was to assess the anthropogenic impact on tropospheric oxidizing power. A secondary objective was to investigate the impact of atmospheric sulfur chemistry, including oxidation of marine biogenic emission of dimethyl sulfide (DMS) on aerosol loading and radiative effect, which is of critical importance in the assessment of global climate change. The PEM-Tropics mission was conducted in two phases to contrast the influence of biomass burning in the dry season and the “relatively clean” wet season. The first, PEM-Tropics A, was carried out during the end of the dry season (August-September 1996), and the second, PEM-Topics B, was conducted during the wet season (March-April 1999).
To accomplish its objectives, PEM-Tropics enlisted the NASA DC-8 and P-3B aircrafts to carry out longitudinal and latitudinal surveys at various altitudes as well as vertical profile sampling across the Pacific basin. Both aircrafts were equipped with in-situ instruments measuring hydroperoxyl radicals (HOx), ozone (O3), photochemical precursors (including, reactive nitrogen species and non-methane hydrocarbon species), and intermediate products (e.g., hydrogen peroxide (H2O2), formaldehyde (CH2O), and acetic acid (CH3OOH). The P3-B in-situ instrument payload also included a direct measurement of hydroxyl (OH) for both missions, while the OH and hydroperoxyl radical (HO2) measurements were added to DC-8 aircraft for PEM-Tropics B. Taking advantage of its excellent low altitude capability, the P-3B was instrumented with a comprehensive sulfur measurement package and conducted pseudo-Lagragian sampling for evaluating DMS oxidation chemistry, including measurements of DMS, sulfur dioxide (SO2), sulfuric acid (H2SO4), and methylsulfonic acid (MSA) as well as the first airborne measurement of dimethyl sulfoxide (DMSO) during PEM-Tropics B. More importantly, it was the first time that DMS (the source), OH and O3 (primary oxidants), and products (DMSO, MSA, H2SO4, SO2) were measured simultaneously aboard an aircraft in the tropical pacific. These observations, specifically DMSO, presented a substantial challenge to the DMS oxidation kinetics to this day. The DC-8 aircraft was equipped with the Differential Absoprtion Lidar (DIAL) during PEM-Tropics A, and the differential absorption lidars DIAL and LASE during PEM-Tropics B. These lidars provided real-time information for fine tuning the flight tracks to capture sampling opportunities. The lidar data products themselves provide valuable information of vertical profiles of ozone as well as aerosol and water vapor in tropical Pacific Furthermore, both aircrafts were fitted with instruments for aerosol composition and microphysical property measurements. Detailed description related to the motivation, implementation, and instrument payloads are available in the PEM-Tropics A overview paper and the PEM-Tropics B overview paper. Most of the publications based on PEM-Tropics A and B observations are available in the Journal of Geophysical Research special issues: Pacific Exploratory Mission-Tropics A and NASA Global Tropospheric Experiment Pacific Exploratory Mission in the Tropics Phase B: Measurement and Analyses (PEM-Tropics B), while other publications such as Nowak et al. (2001) were published prior to the special issues.

PEM-Tropics-A_jValue_AircraftInSitu_DC8_Data 是太平洋探索任务（Pacific Exploratory Mission, PEM）热带A次轨道任务期间，沿DC-8飞机飞行路径测得的光解频率（j-values）数据集，该数据集的数据采集工作已全部完成。

1983年至2001年间，美国国家航空航天局（National Aeronautics and Space Administration, NASA）作为全球对流层实验（Global Tropospheric Experiment, GTE）的一部分，开展了一系列野外考察任务，其中便包含PEM计划，其核心目标是深化对人类活动对流层化学影响的科学认知。PEM野外考察的一部分聚焦于热带太平洋区域（即PEM-Tropics），该区域被视作“巨型化学容器”。本次任务的核心科学目标是评估人为活动对对流层氧化能力的影响；次要目标则是探究大气硫化学的影响，包括海洋生源排放的二甲基硫（dimethyl sulfide, DMS）的氧化过程对气溶胶载荷与辐射效应的作用——这一研究对全球气候变化评估具有关键意义。

PEM-Tropics任务分为两个阶段，以对比旱季与“相对清洁”湿季的生物质燃烧影响：第一阶段PEM-Tropics A于1996年8-9月的旱季末期开展，第二阶段PEM-Tropics B则在1999年3-4月的湿季进行。

为达成任务目标，PEM-Tropics动用了NASA的DC-8与P-3B两架飞机，在太平洋盆地上空开展多高度层的经纬度扫描与垂直剖面采样任务。两架飞机均搭载了原位测量仪器，可检测过氧自由基（hydroperoxyl radicals, HOx）、臭氧（ozone, O₃）、光化学前体物（包括活性氮物种与非甲烷烃类）以及中间产物（如过氧化氢（hydrogen peroxide, H₂O₂）、甲醛（formaldehyde, CH₂O）与乙酸（acetic acid, CH₃OOH））。

两次任务中，P-3B的原位仪器载荷均包含羟基（hydroxyl, OH）的直接测量；而在PEM-Tropics B任务中，DC-8飞机新增了羟基（OH）与过氧自由基（HO₂）的测量模块。凭借出色的低空作业能力，P-3B搭载了一套完整的硫测量组件，并开展了伪拉格朗日采样，以评估DMS氧化化学过程，其中涵盖了DMS、二氧化硫（sulfur dioxide, SO₂）、硫酸（sulfuric acid, H₂SO₄）与甲基磺酸（methylsulfonic acid, MSA）的测量，同时在PEM-Tropics B任务中完成了全球首次机载二甲基亚砜（dimethyl sulfoxide, DMSO）测量。

更为重要的是，这是热带太平洋区域首次实现机载同步测量DMS（排放源）、OH与O₃（初级氧化剂）以及各类产物（DMSO、MSA、H₂SO₄、SO₂）。这些观测结果——尤其是DMSO数据——至今仍对DMS氧化动力学研究构成了显著挑战。

在PEM-Tropics A任务中，DC-8飞机搭载了差分吸收激光雷达（Differential Absorption Lidar, DIAL）；在PEM-Tropics B任务中，则搭载了DIAL与LASE两款差分吸收激光雷达。这些激光雷达可为飞行轨迹的实时微调提供数据支撑，以捕捉最佳采样时机。激光雷达数据产品本身也可提供热带太平洋区域臭氧、气溶胶与水汽的垂直剖面信息。此外，两架飞机均配备了用于气溶胶成分与微观物理特性测量的仪器。

与任务动机、实施流程及仪器载荷相关的详细说明，可参阅PEM-Tropics A与PEM-Tropics B的综述论文。绝大多数基于PEM-Tropics A与B观测结果的研究成果发表于《地球物理研究杂志》（*Journal of Geophysical Research*）的特刊：《太平洋探索任务——热带A》与《NASA全球对流层实验热带太平洋探索任务B阶段：测量与分析（PEM-Tropics B）》；另有部分成果如Nowak等人（2001）的研究则早于上述特刊发表。