Characterization of nitrated aromatic compounds in fine particles from Nanjing, China: Optical properties, source allocation, and secondary processes

Characteristics and source apportionment of water-soluble organic nitrogen (WSON) in PM2.5 in Hong Kong: With focus on amines, urea, and nitroaromatic compounds

Water-soluble organic nitrogen (WSON) is ubiquitous in fine particulate matter (PM2.5) and poses health and environmental risks. However, there is limited knowledge regarding its comprehensive speciation and source-specific contributions. Here, we conducted chemical characterization and source apportionment of WSON in 65 PM2.5 samples collected in Hong Kong during a 1-yr period. Using various mass-spectrometry-based techniques, we quantified 22 nitrogen-containing organic compounds (NOCs), including 17 nitroaromatics (NACs), four amines, and urea. The most abundant amine and NACs were dimethylamine and 4-nitrocatechol, respectively. Two secondary (i.e., secondary formation and secondary nitrate) and five primary sources (i.e., sea salt, fugitive dust, marine vessels, vehicle exhaust, and biomass burning) of WSON and these three categories of NOCs were identified. Throughout the year, secondary sources dominated WSON formation (69.0%), while primary emissions had significant contributions to NACs (77.1%), amines (75.9%), and urea (83.7%). Fugitive dust was the leading source of amines and urea, while biomass burning was the main source of NACs. Our multi-linear regression analysis revealed the significant role of sulfate, NO3, nitrate, liquid water content, and particle pH on WSON formation, highlighting the importance of nighttime NO3 processing and heterogeneous and aqueous-phase formation of NOCs in the Hong Kong atmosphere.

The role of nitroaromatic compounds (NACs) in constraining BrC absorption in the Indo-Gangetic Plain (IGP)

We present here the first measurements of nitroaromatic compounds (NACs) including nitrophenols (NPs), nitrocatechols (NCs) and nitrosalicylic acids (NSAs) from the Indian subcontinent and their role in constraining brown carbon (BrC) absorption. NACs at a rural receptor site in the eastern Indo-Gangetic Plain (IGP) (annual average: 185 ± 94 ng m-3) was dominated by NSAs (135 ± 77 ng m-3), followed by NPs (29 ± 11 ng m-3) and NCs (17 ± 16 ng m-3), with notable enrichments during nighttime and during the biomass burning seasons. An equilibrium absorption partitioning model estimated that >90 % of NSAs and NCs were in the particle-phase, suggesting lower degradation rates via oxidation and photolysis potentially due to year-round high relative humidity. While the contribution of NACs to organic aerosol mass was only 0.42 ± 0.23 %, their contribution to BrC absorption in the 300-450 nm range was higher by an order of magnitude (8 ± 4 %), with NCs and NSAs contributing almost equally in the low-visible (400-450 nm) range as at 365 nm. Despite having mass concentrations lower than NPs by factors of ∼2, contribution of NCs to BrC absorption at λ ≥ 400 nm was comparable to that by NPs, indicating the importance of the absorption efficiency of chromophores. The receptor model positive matrix factorization (PMF) quantified three major NAC sources: fossil fuel combustion (49 ± 15 %; annual average), secondary formation (40 ± 12 %), and biomass burning (11 ± 9 %), with variable contributions on seasonal and day-night bases. In summary, the study uncovered the significant role of NACs in constraining BrC absorption in the IGP, which stresses the importance for molecular-level characterization of BrC chromophores.

Effects of aerosol water content and acidity on the light absorption of atmospheric humic-like substances in winter

Atmospheric humic-like substances (HULIS) could affect regional climate due to their strong light-absorbing capacity. Daily fine particulate matter (PM2.5) samples were collected from December 18, 2016 to January 8, 2017 at an urban site in Chongqing, Southwest China. The mean concentration of HULIS in terms of carbon (HULIS-C) was 6.4 ± 3.4 μg m-3, accounting for 72% of water-soluble organic carbon. The mass absorption efficiency at 365 nm (MAE365) and absorption Ångström index (AAE) of atmospheric HULIS were 2.8 ± 0.30 m2 g-1 C and 4.6 ± 0.37, respectively. Good correlations between the light absorption coefficients of HULIS at 365 nm (Abs365) and the concentrations of K+, elemental carbon, NO3-, and NH4+ were observed, with correlation coefficients higher than 0.83, indicating that biomass burning and secondary formation were potential sources of light-absorbing HULIS, as evidenced by abundant fluorescent components related to less-oxygenated HULIS. Comparing the changes in Abs365 values, concentrations of major water-soluble inorganic ions and carbonaceous compounds in PM2.5, and environmental factors during the clean and pollution periods, we found that extensive biomass burning during the pollution period contributed significantly to the increase of Abs365 values. Moreover, the aerosol pH during the pollution period was close to 4, and NO2 concentration and aerosol water content were about 1.6 and 2.7 times higher than those during the clean period, respectively, which were favorable to form secondary HULIS through aqueous phase reactions in the presence of high NOx, resulting in an evident increase in its light absorption. Knowledge generated from this study is critical for evaluating the regional radiative forcing of brown carbon in southwest China.