Major increase in winter and spring precipitation during the Little Ice Age in the westerly dominated northern Qinghai-Tibetan Plateau

There have been numerous attempts to use long-chain alkenones (LCAs) in saline lakes for paleotemperature and paleosalinity reconstructions. However, LCAs in saline lakes are often produced by multiple haptophyte species, which may confound data interpretations. Here we analyzed LCAs in a finely laminated, high sedimentation rate core from the hypersaline Lake Gahai in the northern Qinghai-Tibetan Plateau and compared our results with regional instrumental records. We find that LCA unsaturation ratios display a stepwise jump during the instrumental period, most likely originating from a sudden shift in the dominant alkenone producers. In contrast, the percentage of the C37:4 alkenone (%C37:4) displays strong correlations with spring and combined winter-spring precipitation amount (R2 = 0.83 and R2 = 0.81, respectively). We hypothesize that high winter-spring precipitation leads to greater freshening of lake surface water immediately after spring melting, promoting greater production of LCAs with relatively high %C37:4 values by the early blooming haptophyte species. Extending the instrumental calibration downcore allows us to quantitatively reconstruct regional spring and winter-spring precipitation for the past millennium. We find a major increase in winter-spring precipitation during the Little Ice Age (LIA), especially during the phases of solar minima. Our finding provides novel quantitative support to the previous studies suggesting relatively wet conditions during the LIA in the westerly dominated regions of central Asia and northern Tibetan Plateau and infers a substantial increase in regional winter-spring precipitation should the predicted grand solar minimum in the forthcoming decades become a reality.</p

Seasonal characteristics of oxalic acid and related SOA in the free troposphere of Mt. Hua, central China: Implications for sources and formation mechanisms

PM10 aerosols from the summit of Mt. Hua (2060 m a.s.l) in central China during the winter and summer of 2009 were analyzed for dicarboxylic acids, ketocarboxylic acids and alpha-dicarbonyls. Molecular composition of dicarboxylic acids (C-2-C-11) in the free tropospheric aerosols reveals that oxalic acid (C-2, 399 +/- 261 ng m(-3) in winter and 522 +/- 261 ng m(-3) in summer) is the most abundant species in both seasons, followed by malonic (C-3) and succinic (C-4) acids, being consistent with that on ground levels. Most of the diacids are more abundant in summer than in winter, but adipic (C-6) and phthalic (Ph) acids are twice lower in summer, suggesting more significant impact of anthropogenic pollution on the wintertime alpine atmosphere. Moreover, glyoxal (Gly) and methylglyoxal (mGly) are also lower in summer (12 +/- 6.1 ng m(-3)) than in winter (22 +/- 13 ng m(-3)). As both dicarbonyls are a major precursor of C-2, their seasonal variation patterns, which are opposite to those of the diacids, indicate that the mountain troposphere is more oxidative in summer. C-2 showed strong linear correlations with levoglucosan in winter and oxidation products of isoprene and monoterpane in summer. PCA analysis further suggested that the wintertime C-2 and related SOA in the Mt Hua troposphere mostly originate from photochemical oxidations of anthropogenic pollutants emitted from biofuel and coal combustion in lowland regions. On contrast, the summertime C-2 and related SOA mostly originate from further oxidation of the mountainous isoprene and monoterpene oxidation products. The AIM model calculation results showed that oxalic acid concentration well correlated with particle acidity (R-2 = 0.60) but not correlated with particle liquid water content, indicating that particle acidity favors the organic acid formation because aqueous-phase C-2 production is the primary mechanism of C-2 formation in ambient aerosols and is driven by acid-catalyzed oxidation.</p