The Sichuan Basin is a low visibility area in southwest China, where the hilly and basin topography, plus humid and stagnant weather, lead to unique pollution patterns. To identify the characteristics and sources of carbonaceous aerosols, one-year record of 24-h PM2.5samples were analyzed for organic carbon (OC) and elemental carbon (EC) content following the thermal/optical transmission protocol at three cities (Chengdu (CD), Neijiang (NJ), and Chongqing (CQ)) in the region during May 2012 to April 2013. The annual average concentrations were 19.0 ± 13.3 μg OC m−3 and 4.6 ± 2.6 μg EC m−3 in CD, 18.3 ± 8.4 μg OC m−3 and 4.1 ± 1.8 μg EC m−3 in NJ, and 15.2 ± 8.4 μg OC m−3 and 4.0 ± 1.6 μg EC m−3 in CQ, respectively. Organic matter (1.6OC) plus EC contributed about 40% of PM2.5 mass and displayed weak regional uniformity. Relatively high ratios of OC to EC were observed in the region with 4.3 for CD, 4.6 for NJ, and 3.8 for CQ, respectively. OC and EC pollution in the region exhibited interesting season-dependent characteristics with the lowest concentrations and OC/EC ratios in summer, but higher levels in other seasons. Higher OC/EC ratios in spring and autumn resulted from biomass burning, and in winter were from the enhanced secondary organic aerosol formation under favorable conditions. The exceptionally high OC and EC levels in May and October, mostly notable in CD, resulted from the burning of agricultural residues during harvest period. The high K+concentrations and the high Kexcess/EC ratios implied the persistent influence of biomass burning throughout the year. Using a novel technique combing the EC tracer method and potassium mass balance in the aerosols, a K/EC ratio of 1.22 was used to retrieve the OC from biomass burning and the estimated contributions were 30.8%, 28.3%, and 21.9% in CD, NJ, and CQ, respectively, while secondary OC contributions to OC were 26.7%, 24.6%, and 25.7% in CD, NJ, and CQ, respectively.

To evaluate the variations in temporal and spatial distribution of biogenic volatile organic compound (BVOC) emissions in China, historical BVOC emission inventories at a spatial resolution of 36 km × 36 km for the period of 1981–2003 were developed firstly. Based on the time-varying statistical data and Vegetation Atlas of China (1:1,000,000), emissions of isoprene, 37 monoterpenes, 32 sesquiterpenes, and other volatile organic compounds (OVOCs) were estimated using MEGANv2.1 driven by WRF model. Results show China's BVOC emissions had increased by 28.01% at an annual average rate of 1.27% from 37.89 Tg in 1981 to 48.50 Tg in 2003. Emissions of isoprene, monoterpenes, sesquiterpenes, and OVOCs had increased by 41.60%, 34.78%, 41.05%, and 4.89%, respectively. With fixed meteorological variables, the estimated BVOC emissions would increase by 19.25%, resulting from the increasing of vegetation biomass during the last 23 years. On average, isoprene, monoterpenes, sesquiterpenes, and OVOCs were responsible for 52.40%, 12.73%, 2.58%, and 32.29% of the national BVOC emissions, respectively. β-pinene and α-pinene, farnesene and caryophyllene were the largest contributors to the total monoterpene and sesquiterpene emissions, respectively. The highest emissions were found over northeastern, southeastern, southwestern China, Qinling Mountain, and Hainan and Taiwan provinces. The regions with high emissions had been expanding over the years, especially in the Changbai Mountain, southern China, and southwestern forest regions. The lowest emissions in southern China occurred in 1984–1988. Almost all the provinces had experienced increasing emissions, but their contributions to the national emissions differed significantly over the past 23 years. Yunnan, Guangxi, Heilongjiang, Jiangxi, Fujian, Guangdong, and Sichuan provinces always dominated the national BVOC emissions, excluding in 1977–1981, when the three northeastern provinces had relatively lower emissions.

To track the chemical characteristics and formation mechanism of biomass burning pollution, the hourly variations of meteorological factors and pollutant concentrations during a heavy pollution on 18–21 May, 2012 in Chengdu are presented in this study. The episode was the heaviest and most long-lasting pollution event in the historical record of Chengdu caused by a combination of stagnant dispersion conditions and enhanced PM2.5 emission from intensive biomass burning, with peak values surpassing 500 μg m− 3. The event was characterized by three nighttime peaks, relating to the burning practice and decreased boundary layer height at night. The prevailing northeasterly wind during nighttime preferentially brought more pollutants to the urban regions from northern suburbs of Chengdu, where dense fire spots were observed. Due to the obstruction of hilly topography and weak wind speed, minor regional features were reflected from the PM10 variations in nearby cities, whereas the long-distance transport of the plume impacted extensive regions in northern and eastern China. Carbon monoxide (CO) concentrations increased by more than 200%, while exceptionally high PM2.5 levels of 190.1 and 268.4 μg m− 3 on 17 May and 18 May, were observed and showed high correlation with CO (r = 0.75). The relative contribution of biomass burning smoke to organic carbon was estimated from OC/EC ratios (organic carbon/elemental carbon) and elevated to 81.3% during the episode, indicating a significant impact on urban aerosol levels. The occurrence of high PM2.5/PM10 ratios (> 0.80) and K+/EC ratios (> 1.0), along with the increased carbonaceous concentrations and their fraction in PM2.5 (> 40%) and high OC/EC ratios (about 8), could be used as immediate indicators for biomass burning pollution in cities. In addition, the heavy pollution involved a mixture of anthropogenic sources, reflected from the high SOR and NOR values and increases in the EFs (enrichment factors) of Mo, Zn, Cd, and Pb.

Ambient volatile organic compounds (VOCs) were measured intensively using an online gas chromatography–mass spectrometry/flame ionization detector (GC–MS/FID) at Ziyang in the Chengdu–Chongqing Region (CCR) from 6 December 2012 to 4 January 2013. Alkanes contributed the most (59%) to mixing ratios of measured non-methane hydrocarbons (NMHCs), while aromatics contributed the least (7%). Methanol was the most abundant oxygenated VOC (OVOC), contributing 42% to the total amount of OVOCs. Significantly elevated VOC levels occurred during three pollution events, but the chemical composition of VOCs did not differ between polluted and clean days. The OH loss rates of VOCs were calculated to estimate their chemical reactivity. Alkenes played a predominant role in VOC reactivity, among which ethylene and propene were the largest contributors; the contributions of formaldehyde and acetaldehyde were also considerable. Biomass burning had a significant influence on ambient VOCs during our study. We chose acetonitrile as a tracer and used enhancement ratio to estimate the contribution of biomass burning to ambient VOCs. Biomass burning contributed 9.4%–36.8% to the mixing ratios of selected VOC species, and contributed most (>30% each) to aromatics, formaldehyde, and acetaldehyde.