**Krzysztof J. Rudzi ´nski \* and Rafał Szmigielski**

Institute of Physical Chemistry of the PAS, Kasprzaka 44/52, 01-224 Warsaw, Poland; rafal.szmigielski@gmail.com

**\*** Correspondence: kjrudz@ichf.edu.pl; Tel.: +48-22-343-3402

Received: 14 November 2019; Accepted: 4 December 2019; Published: 9 December 2019

**Abstract:** Nitrophenols, hazardous environmental pollutants, react promptly with atmospheric oxidants such as hydroxyl or nitrate radicals. This work aimed to estimate how fast nitrophenols are removed from the atmosphere by the aqueous-phase reactions with sulfate radical-anions. The reversed-rates method was applied to determine the relative rate constants for reactions of 2-nitrophenol, 3-nitrophenol, 4-nitrophenol, 2,4-dinitrophenol, and 2,4,6-trinitrophenol with sulfate radical-anions generated by the autoxidation of sodium sulfite catalyzed by iron(III) cations at ~298 K. The constants determined were: 9.08 <sup>×</sup> <sup>10</sup><sup>8</sup> , 1.72 <sup>×</sup> <sup>10</sup><sup>9</sup> , 6.60 <sup>×</sup> <sup>10</sup><sup>8</sup> , 2.86 <sup>×</sup> <sup>10</sup><sup>8</sup> , and 7.10 <sup>×</sup> <sup>10</sup><sup>7</sup> <sup>M</sup>−<sup>1</sup> s −1 , respectively. These values correlated linearly with the sums of Brown substituent coefficients and with the relative strength of the O–H bond of the respective nitrophenols. Rough estimation showed that the gas-phase reactions of 2-nitrophenol with hydroxyl or nitrate radicals dominated over the aqueous-phase reaction with sulfate radical-anions in deliquescent aerosol and haze water. In clouds, rains, and haze water, the aqueous-phase reaction of 2-nitrophenol with sulfate radical-anions dominated, provided the concentration of the radical-anions was not smaller than that of the hydroxyl or nitrate radicals. The results presented may be also interesting for designers of advanced oxidation processes for the removal of nitrophenol.

**Keywords:** atmospheric processes; secondary organic aerosol; rate constants; atmospheric pollutants; advanced oxidation processes
