National Public Radio: If you’re an environmentally conscious traveler, you may be feeling a little bit guilty about flying these days. Airplanes spew harmful greenhouse gases into the air. San Francisco International Airport is taking a step to help ease these concerns: It’s the first airport to install self-serve kiosks where passengers can purchase carbon offsets for their flights. The airport has partnered with a private company to provide the offsets. But carbon offsets for travel are …