I have given some thought to our discussion on China’s “gray haze”. You may be right in blaming sulfates instead of photochemical smog caused by cars. I have experienced many times photochemical smog in my life, especially in Los Angeles and Mexico City. The photochemical smog is gray bur with some brownish, yellowish components and produces some irritation in  the eyes. In Shanghai I have never experienced eye irritation even though the gray haze was often present. Sulfate PM2.5 will produce a gray haze even with relatively low concentrations. Also, sulfates do not irritate the eye.

我们讨论的要多些，机动车尾气自然也是灰霾重要贡献者。