I'll have to let the locals chime in on Sequoia and Kings Canyon in terms of summer vs. spring, I've only been there a couple of times, I liked the spring better because the snow made it a bit cooler for hiking. However, since it doesn't have the same waterfall vistas as Yosemite, the season you visit may not be as critical ... I'm just not certain if the views are impacted by air quality in the different seasons.
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