Search

The Tibetan Plateau (TP) is one of the world's most sensitive areas for climate change, but the lack of information on light-absorption by aerosols limits the understanding of climate forcing feedbacks. Here, the contributions of black carbon (BC) and brown carbon (BrC) to light absorption and radiative forcing were investigated. Absorption Ångström exponents (α), mass absorption cross sections (MAC), and absorption coefficients (b abs) for selected wavelengths were measured for a year of aerosol samples collected at Lulang on the southeast TP. Aerosol absorption at all wavelengths was strongest in the pre-monsoon when levoglucosan, a biomass burning indicator, was elevated. The contributions of BC, BrC, and dust to b abs were decoupled. Results showed that dust contributed 8.5% to the total light absorption at 405 nm and 3.9% 808 nm. A two-component model indicated that BC and BrC contributed 48.7% and 44.0% to total b abs at 405 nm but BrC had a smaller effect at middle-visible wavelengths. Elevated b abs,non-dust,BC and b abs,non-dust,BrC and a high α BrC but low α aerosol values in the pre-monsoon were attributed to biomass burning, which produces not only BrC but also BC which has a much lower α value. Average non-dust MACs for BC and BrC at 405 nm were 6.1 ± 2.8 and 0.72 ± 0.55 m2 g−1, respectively. Nonparametric statistical tests showed that the MAC non-dust,BC was relatively constant but MAC non-dust, BrC was more variable. In addition, BrC was correlated with non-dust b abs,BC and MAC non-dust,BC in winter and the pre-monsoon, implying BrC and BC shared sources in those two seasons, but lower correlations in the monsoon and post-monsoon suggest that a mixture of sources impacted BrC (e.g., biogenic emission, secondary formation, etc.). Finally, the relative contributions of BrC to BC for radiative forcing from 405 to 808 nm were 29.4 ± 9.5% with no remarkable seasonal differences, confirming the importance of BrC to light absorption in the near UV throughout the year. As a result, the BrC absorption is an important additional factor which needs to be considered in atmospheric models, although the atmospheric heating by BC seems to be a larger climate forcer in this region. • Filter-based measurement of light absorption from southeast TP were carried. • Dust contributed 8.5% to total light absorption at 405 nm and 3.9% at 808 nm. • BrC shared the same primary sources with BC in pre-monsoon and winter. • MAC non-dust,BC was relatively constant while that for BrC was more variable. • Averaged contribution of direct radiative forcing of BrC to that of BC was 29.4%. [ABSTRACT FROM AUTHOR]

Stable carbon isotopes provide information on aerosol sources, but no extensive long-term studies of these isotopes have been conducted in China, and they have mainly been used for qualitative rather than quantitative purposes. Here, 24 h PM 2.5 samples (n = 58) were collected from July 2008 to June 2009 at Xi'an, China. The concentrations of organic and elemental carbon (OC and EC), water-soluble OC, and the stable carbon isotope abundances of OC and EC were determined. In spring, summer, autumn and winter, the mean stable carbon isotope in OC (δ 13 C OC ) were −26.4 ± 0.6, −25.8 ± 0.7, −25.0 ± 0.6 and −24.4 ± 0.8‰, respectively, and the corresponding δ 13 C EC values were −25.5 ± 0.4, −25.5 ± 0.8, −25.2 ± 0.7 and −23.7 ± 0.6‰. Large δ 13 C EC and δ 13 C OC values in winter can be linked to the burning coal for residential heating. Less biomass is burned during spring and summer than winter or fall (manifested in the levels of levoglucosan, i.e., 178, 85, 370, 935 ng m −3 in spring, summer, autumn, and winter), and the more negative δ 13 C OC in the warmer months can be explained by the formation of secondary organic aerosols. A levoglucosan tracer method combined with an isotope mass balance analysis indicated that biomass burning accounted for 1.6–29.0% of the EC, and the mean value in winter (14.9 ± 7.5%) was 7 times higher than summer (2.1 ± 0.4%), with intermediate values of 6.1 ± 5.6 and 4.5 ± 2.4% in autumn and spring. Coal combustion accounted for 45.9 ± 23.1% of the EC overall, and the percentages were 63.0, 37.2, 36.7, and 33.7% in winter, autumn, summer and spring respectively. Motor vehicles accounted for 46.6 ± 26.5% of the annual EC, and these contributed over half (56.7–61.8%) of the EC in all seasons except winter. Correlations between motor vehicle-EC and coal combustion-EC with established source indicators (B(ghi)P and As) support the source apportionment results. This paper describes a simple and accurate method for apportioning the sources of EC, and the results may be beneficial for developing model simulations as well as controlling strategies in future. [ABSTRACT FROM AUTHOR]

Highly time-resolved measurements of nitrous acid (HONO) were carried out with a highly sensitive long path absorption photometer (LOPAP) at an urban site of Xi'an in Western China from 24 July to 6 August 2015 to investigate the atmospheric variations, sources, and formation pathways of HONO. The concentrations of HONO vary from 0.02 to 4.3 ppbv with an average of 1.12 ppbv for the entire measurement period. The variation trends of HONO and NO 2 are very similar and positively correlated which, together with the similar diurnal profiles of HONO/NO 2 ratio and HONO, suggest the importance of heterogeneous conversion of HONO from NO 2 . The nocturnal HONO level is governed by heterogeneous formation from NO 2 , followed by homogeneous formation of NO with OH and then by direct emissions. Further, it is found that the heterogeneous formation of HONO is largely affected by relative humidity and aerosol surface. Daytime HONO budget analysis indicates that an additional unknown source with HONO production rate of 0.75 ppbv h − 1 is required to explain the observed HONO concentration, which contributes 60.8% of the observed daytime HONO. [ABSTRACT FROM AUTHOR]

PM 2.5 samples were collected in Sanya, China in summer and winter in 2012/2013. Organic carbon (OC), elemental carbon (EC), and non-polar organic compounds including n -alkanes ( n -C 14 - n -C 40 ) and polycyclic aromatic hydrocarbons (PAHs) were quantified. The concentrations of these carbonaceous matters were generally higher in winter than summer. The estimated secondary organic carbon (OC sec ) accounted for 38% and 54% of the total organic carbon (TOC) in winter and summer, respectively. The higher value of OC sec in addition to the presences of photochemically-produced PAHs in summer supports that photochemical conversions of organics are much active at the higher air temperatures and with stronger intense solar radiation. Carbon preference index (CPI) and percent contribution of wax n -alkanes suggest that anthropogenic sources were more dominant than derivation from terrestrial plants in Sanya. Diagnostic ratios of atmospheric PAHs further indicate that there was a wide mix of pollution sources in winter while fossil fuel combustion was the most dominant in summer. Positive Matrix Factorization (PMF) analysis with 18 PAHs in the winter samples found that motor vehicle emissions and biomass burning were the two main pollution sources, contributing 37.5% and 24.6% of the total quantified PAHs, respectively. [ABSTRACT FROM AUTHOR]