Fig. 1: Ambient observations at Cape D’ Aguilar, Hong Kong, from 31 August to 9 October 2018.
From: Photodissociation of particulate nitrate as a source of daytime tropospheric Cl2
Back trajectories of air mass from (A) continental region (5 September - 9 October) and (B) the South China Sea (31 August - 4 September). Contour represents the number of trajectories in each 0.1-degree latitude × 0.1-degree longitude grid. The diurnal profiles of Cl2 and O3 (C) in the air mass from the continental region (5 September - 9 October); (D) in the air mass from the South China Sea (31 August - 4 September). The dashed line represents the detection limit of the CIMS instrument. The blue line is the 10-min average of Cl2, and the blue shade represents the 25 percentile and 75 percentile values. The red line is the 10-min average of O3. (E) The scatter plot of the production rate of Cl2 (\({P}_{{{Cl}}_{2}}\)) and the product of the solar actinic flux (W m−2), the aerosol surface area density (Sa, μm2 cm−3), and O3 mixing ratio (ppbv) from 08:00 to 18:00 in the continental air mass. (F) The scatter plot of the production rate of Cl2 (\({P}_{{{Cl}}_{2}}\)) and the product of the solar actinic flux, the aerosol surface area density (Sa, μm2 cm−3), and nitrate concentration in PM10 (μg m−3) from 08:00 to 18:00 in the continental air mass. The \({P}_{{{Cl}}_{2}}\)equals the photolysis rate of Cl2 (\({J}_{{{Cl}}_{2}}\)× measured Cl2 concentration) as Cl2 was near a photo stationary state. \({J}_{{{Cl}}_{2}}\) was calculated from the TUV model (http://cprm.acom.ucar.edu/Models/TUV/Interactive_TUV) under clear sky conditions and then scaled to the solar radiation derived JNO2 (see Methods section “Chemical box model”).
