Brown carbon (BrC) absorbs radiation in the near-UV and visible ranges, affecting atmospheric photochemistry andradiative forcing. Our understanding on the photochemicaltransformation of BrC is still limited, especially when mixed withthe abundant and photochemically labile inorganic salt, nitrate.Herein, we investigate the photochemical reactions of four BrCchromophores, including two methoxyphenols and two nitro-phenols. Experiments were conducted in the absence and presenceof different concentrations of H2O2and nitrate with lights of 254and 313 nm. The results show that the pseudo-first-order decayrate constants (k) of these four BrC compounds at 313 nmillumination were approximately 10 times lower than those at 254nm, demonstrating longer lifetimes of these BrC chromophoresunder tropospherically relevant irradiation. Photo-enhancement in the visible range was observed in most experiments, with thoseunder 313 nm illumination lasting longer, indicating the prolonged effects of nascent and transformed BrC chromophores onradiative forcing. Methoxyphenols had higher averagedkvalues than nitrophenols during direct photolysis with 254 or 313 nmlights, but thekvalues for nitrophenols under high-nitrate (or high-H2O2) conditions approached those of methoxyphenols. Thephoto-enhancement in the visible range for methoxyphenols in the presence of nitrate was substantially contributed by nitroproducts, while that for nitrophenols was mainly contributed by hydroxylated and/or dimerized products. Our results reveal thesimilarity and difference between the photolysis of methoxyphenols and nitrophenols, which may help better understand the aging ofdifferent types of BrC for better model representation of their effects on radiative forcing.

Brown carbon (BrC) absorbs radiation in the near-UV and visible ranges, affecting atmospheric photochemistry and radiative forcing. Our understanding on the photochemical transformation of BrC is still limited, especially when mixed with the abundant and photochemically labile inorganic salt, nitrate. Herein, we investigate the photochemical reactions of four BrC chromophores, including two methoxyphenols and two nitrophenols. Experiments were conducted in the absence and presence of different concentrations of H2O2 and nitrate with lights of 254 and 313 nm. The results show that the pseudo-first-order decay rate constants (k) of these four BrC compounds at 313 nm illumination were approximately 10 times lower than those at 254 nm, demonstrating longer lifetimes of these BrC chromophores under tropospherically relevant irradiation. Photo-enhancement in the visible range was observed in most experiments, with those under 313 nm illumination lasting longer, indicating the prolonged effects of nascent and transformed BrC chromophores on radiative forcing. Methoxyphenols had higher averaged k values than nitrophenols during direct photolysis with 254 or 313 nm lights, but the k values for nitrophenols under high-nitrate (or high-H2O2) conditions approached those of methoxyphenols. The photo-enhancement in the visible range for methoxyphenols in the presence of nitrate was substantially contributed by nitro products, while that for nitrophenols was mainly contributed by hydroxylated and/or dimerized products. Our results reveal the similarity and difference between the photolysis of methoxyphenols and nitrophenols, which may help better understand the aging of different types of BrC for better model representation of their effects on radiative forcing.

Water soluble organic carbon (WSOC) can significantly influence the aerosol optical properties and the aqueous phase chemistry in cloudwater, fogwater and aerosol liquid water. Here, we examine how the changing pH (in acidic range) affects the absorption spectra of aqueous extracts from field biomass burning aerosols, under dark conditions and in presence of simulated sunlight illumination. The observation under dark conditions indicates that pH variation from 2 to 5 induces significantly enhanced light absorbance in the wavelength ranges of 235-270 nm and 300-550 nm, whereas the light absorbance decreased in the range of 270-300 nm, which might be partially ascribed to the deprotonation of carboxylic acids and phenols. During the extract photolysis, light absorption exhibits photo-bleaching below 380 nm and photo-enhancement above 380 nm, indicating that at acidic levels (pH = 2-5), the particle extracts could undergo a significant composition evolution leading to a modification of absorptive properties. Meanwhile, after 12 h-photolysis, the acidity ([H+]) normalized by WSOC concentration in aqueous extracts ([WSOCae]) increased with a variation of Delta[11(+)]/[WSOCae] = (3.7 +/- 0.7) x 10(-7) mol mgC(-1) (mean +/- standard deviation), suggesting the formation of new acidic substances. Although these findings were acquired in aqueous solutions more relevant to cloud and fog water, the similar evolution likely occurs in wetted aerosols. This calls more attention to the effect of acidity on the wetted aerosols in order to better estimate the aerosol radiative forcing.