Aerosol characterization over the southeastern United States using high-resolution aerosol mass spectrometry: spatial and seasonal variation of aerosol composition and sources with a focus on organic nitrates
Aerosol characterization over the southeastern United States using high-resolution aerosol mass spectrometry: spatial and seasonal variation of aerosol composition and sources with a focus on organic nitrates
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DOI：
10.5194/acp-15-7307-2015
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发表时间：
2015-07
影响因子：
6.3
通讯作者：
Lu Xu;Sriram Suresh;Hongyu Guo;R. Weber;N. Ng
中科院分区：
文献类型：
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作者：
Lu Xu;Sriram Suresh;Hongyu Guo;R. Weber;N. Ng
摘要。我们部署了高分辨率飞行时间气溶胶质谱仪（HR-ToF-AMS）和气溶胶化学形态监测仪（ACSM）来表征美国东南部亚微米非难熔颗粒物（NR-PM$_{1}$）的化学成分。作为东南空气污染和流行病学研究中心（SCAPE）和南部氧化剂和气溶胶研究中心（SOAS）的一部分，在佐治亚州（GA）大亚特兰大地区和阿拉巴马州（AL）的农村和城市站点进行了大约1年的测量。无论采样地点和季节如何，有机气溶胶（OA）占NR-PM1质量浓度的一半以上。正矩阵分解（PMF）分析的HR-ToF-AMS测量确定了不同的OA来源，取决于地点和季节。类碳氢化合物OA （HOA）和烹饪OA （COA）对城市站点的OA总量有重要的贡献，但不是主要的贡献（根据站点和季节的不同，占OA总量的21 - 38%）。生物质燃烧OA （BBOA）浓度具有明显的季节变化特征，冬季的增强幅度大于夏季。我们发现BBOA与棕色碳之间存在良好的相关性，表明生物质燃烧是棕色碳的重要来源，尽管在约克维尔农村地区可能存在另外一种未知的棕色碳来源。异戊二烯衍生OA因子（异戊二烯-OA）仅在温暖月份反卷积，占OA总量的18 - 36%。城市站点中异戊二烯- oa因子的存在更有可能是在存在氮氧化物的情况下由当地产生的，而不是来自农村站点的运输。多氧化和少氧化的有机气溶胶（分别为MO-OOA和LO-OOA）是OA的优势组分（47 - 79%）。MO-OOA与臭氧的相关性在夏季较好，而在冬季不明显，表明MO-OOA的来源可能随季节变化。LO-OOA在夜间达到每日最大值，与有机硝酸盐的估计硝酸盐功能比与总硝酸盐的估计功能更相关。根据HR-ToF-AMS测量结果，我们估计有机硝酸盐的硝酸盐功能占夏季总硝酸盐测量值的63 - 100%。此外，有机硝酸盐对夏季总OA的贡献估计为5 - 12%，表明有机硝酸盐是美国东南部大气气溶胶的重要组成部分。通过比较两个不同采样点同时进行的HR-ToF-AMS测量和ACSM测量，研究了OA的空间分布。夏季OA在空间上是均匀的，这可能是由于美国东南部的停滞气团和区域二次有机气溶胶（SOA）占主导地位。冬季均匀性较差，这可能与一次排放的空间差异有关。我们观察到OA浓度的季节性表现出明显的城市/农村对比。OA在城市站点表现出较弱的季节变化，而在农村站点，其浓度在夏季高于冬季。我们一年的观测结果与东南气溶胶研究和表征（SEARCH）网络14年的有机碳（OC）数据一致。用先进仪器进行的短期测量与基本空气质量指标的长期测量之间的比较，不仅检验了短期测量结果的稳健性，而且为解释长期测量结果提供了见解。我们发现，从HR-ToF-AMS测量的PMF分析中得到的OA因子具有明显不同的日变化。不同日变化趋势对OA因子的补偿可能是美国东南部反复观测到相对平坦的OA日剖面的原因之一。此外，长期观测分析表明，夏季OC与硫酸盐的相关性明显强于冬季。这种季节性可能部分是由于硫酸盐对异戊二烯SOA形成的影响，正如短期强化测量所揭示的那样。
Abstract. We deployed a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) and an Aerosol Chemical Speciation Monitor (ACSM) to characterize the chemical composition of submicron non-refractory particulate matter (NR-PM$_{1}$) in the southeastern USA. Measurements were performed in both rural and urban sites in the greater Atlanta area, Georgia (GA), and Centreville, Alabama (AL), for approximately 1 year as part of Southeastern Center for Air Pollution and Epidemiology study (SCAPE) and Southern Oxidant and Aerosol Study (SOAS). Organic aerosol (OA) accounts for more than half of NR-PM1 mass concentration regardless of sampling sites and seasons. Positive matrix factorization (PMF) analysis of HR-ToF-AMS measurements identified various OA sources, depending on location and season. Hydrocarbon-like OA (HOA) and cooking OA (COA) have important, but not dominant, contributions to total OA in urban sites (i.e., 21–38 % of total OA depending on site and season). Biomass burning OA (BBOA) concentration shows a distinct seasonal variation with a larger enhancement in winter than summer. We find a good correlation between BBOA and brown carbon, indicating biomass burning is an important source for brown carbon, although an additional, unidentified brown carbon source is likely present at the rural Yorkville site. Isoprene-derived OA factor (isoprene-OA) is only deconvolved in warmer months and contributes 18–36 % of total OA. The presence of isoprene-OA factor in urban sites is more likely from local production in the presence of NOx than transport from rural sites. More-oxidized and less-oxidized oxygenated organic aerosol (MO-OOA and LO-OOA, respectively) are dominant fractions (47–79 %) of OA in all sites. MO-OOA correlates well with ozone in summer but not in winter, indicating MO-OOA sources may vary with seasons. LO-OOA, which reaches a daily maximum at night, correlates better with estimated nitrate functionality from organic nitrates than total nitrates. Based on the HR-ToF-AMS measurements, we estimate that the nitrate functionality from organic nitrates contributes 63–100 % to the total measured nitrates in summer. Furthermore, the contribution of organic nitrates to total OA is estimated to be 5–12 % in summer, suggesting that organic nitrates are important components in the ambient aerosol in the southeastern USA. The spatial distribution of OA is investigated by comparing simultaneous HR-ToF-AMS measurements with ACSM measurements at two different sampling sites. OA is found to be spatially homogeneous in summer due possibly to stagnant air mass and a dominant amount of regional secondary organic aerosol (SOA) in the southeastern USA. The homogeneity is less in winter, which is likely due to spatial variation of primary emissions. We observe that the seasonality of OA concentration shows a clear urban/rural contrast. While OA exhibits weak seasonal variation in the urban sites, its concentration is higher in summer than winter for rural sites. This observation from our year-long measurements is consistent with 14 years of organic carbon (OC) data from the SouthEastern Aerosol Research and Characterization (SEARCH) network. The comparison between short-term measurements with advanced instruments and long-term measurements of basic air quality indicators not only tests the robustness of the short-term measurements but also provides insights in interpreting long-term measurements. We find that OA factors resolved from PMF analysis on HR-ToF-AMS measurements have distinctly different diurnal variations. The compensation of OA factors with different diurnal trends is one possible reason for the repeatedly observed, relatively flat OA diurnal profile in the southeastern USA. In addition, analysis of long-term measurements shows that the correlation between OC and sulfate is substantially stronger in summer than winter. This seasonality could be partly due to the effects of sulfate on isoprene SOA formation as revealed by the short-term intensive measurements.