Characteristics of peroxyacetyl nitrate pollution during a 2015 winter haze episode in Beijing

Peroxyacetyl nitrate (PAN) are effective indicators of photochemical pollution, and also play an important role in regional oxidant balance. Surprisingly, in recent years, PAN have also been detected under conditions that do not favor the photochemical processes. To obtain a better understanding of the mechanisms of formation of atmospheric compound pollution, this study examined the relationships between concentrations of PAN and other pollutants (e.g., ozone [O₃] and PM_2.5) during a winter haze episode. The observation periods were from December 31, 2015, to February 2, 2016, and from February 19, 2016, to March 4, 2016. The maximum daily concentration of PAN during haze episodes was 4-10 times higher than that during non-haze episodes. The continuous cumulative increase in PAN concentrations was the result of a combination of photochemical production during the daytime and production based on free radical chemical reactions during the nighttime. During the haze episode, the correlation between concentrations of PAN and O₃ was weak, while a significant correlation was observed between PAN and PM_2.5 concentrations (R² = 0.82). This may have been due to higher concentrations of particulate matter impairing illumination, which can then inhibit the photochemical reactions that produce PAN and O₃. OH radicals can replace the role of light in PAN formation, which can cause concentrations of PAN and O₃ to vary independently. During the haze episode, the ratio of PAN/O₃ was around 0.3, which was much higher than that during the clean period.

Theoretical investigation of the NO₃radical addition to double bonds of limonene

The addition reactions of NO(3) to limonene have been investigated using ab initio methods. Six different possibilities for NO(3) addition to the double bonds, which correspond to the two C-C double bonds (endocyclic or exocyclic) have been considered. The negative activation energies for the addition of NO(3) to limonene are calculated and the energies of NO(3)-limonene radical adducts are found to be 14.55 to 20.17 kcal mol-1 more stable than the separated NO(3) and limonene at the CCSD(T)/6-31G(d) + CF level. The results also indicate that the endocyclic addition reaction is more energetically favorable than the exocyclic one.

Carbonyls and non-methane hydrocarbons at a rural mountain site in northeastern United States

Measurements of carbonyls and C(2)-C(6) non-methane hydrocarbons (NMHCs) were made in ambient air at a rural site at the summit of Whiteface Mountain (WFM) in New York State. Alkanes dominated in the samples, with ethane and propane making up about 55% of the total on a carbon-atom basis. Ethane, the longest-lived of the NMHCs, showed a mixing ratio in the range of 0.86-2.1 ppbv. Photochemical ageing analysis indicated an anthropogenic influence on the NMHC levels. The photochemical reactivity of the hydrocarbons, calculated in terms of propylene-equivalent concentration, was dominated by alkenes (propene and ethene), which accounted for 74% of the total NMHC sum. Air mass back-trajectories have been used to investigate the origin of the observed NMHCs and carbonyls. Higher concentrations were found when air masses arrived from the midwestern US corridor. Acetone was the most abundant species, comprising from 31% to 53% of the total detected carbonyls, followed by MEK (15-53%), HCHO (7-39%), and CH(3)CHO (7-19%). Average concentrations were determined to be 1.61 ppbv for CH(3)C(O)CH(3), 1.40 ppbv for MEK, 1.16 ppbv for HCHO, and 0.49 ppbv for CH(3)CHO. The variations in carbonyl concentrations were observed to follow patterns similar to variations in O(3) concentrations, typical of secondary products. Correlations and statistical analysis of the carbonyls and NMHCs were performed, and showed that most of the compounds derived from mixing and photochemical transformation of long-range transported pollutants from the major source areas. Ranking of the carbonyls with respect to removal of the OH radical showed HCHO to be the most important species, followed by CH(3)CHO, MEK, and CH(3)C(O)CH(3).