Fig. 1.
Sulfate production during pollution episodes in Xi’an and Beijing. (A–D) Measurements in Xi’an from 17 November to 12 December 2012, and the particle properties correspond to those in PM2.5. (E–H) Measurements in Beijing from 21 January to February 4, 2015, and the particle properties correspond to those in PM1 (particles smaller than 1 μm). In A and E, the dates on the x axis correspond to midnight local time. B and F show the mass fractions of the five main nonrefractory constituents from 5 to 12 December 2012 in Xi’an and from 21 January to 4 February 2015 in Beijing, respectively. The lines in C and G represent the exponential fits through the data, i.e., y = 0.07 + 1.0 × 10−4exp (x/11) with R2 = 0.60 in Xi’an and y = 0.05 + 7.0 × 10−3exp (x/15) with R2 = 0.88 in Beijing. Except for the colors in B and F depicting the aerosol compositions, the blue, orange, and black colors correspond to the SO42− mass concentrations of less than 10 μg m−3 (clean), 10–20 μg m−3 (transition), and greater than 20 μg m−3 (polluted), respectively. The top and bottom of the vertical line for each box in D and H correspond to the 95th and 5th percentiles, respectively, and the top, middle, and bottom horizontal lines of the box mark the 75th, 50th, and 25th percentiles of the data range. The white dot in each box represents the mean value.