Abstract
Ambient non-methane hydrocarbons (NMHCs) are important precursors of ozone (O3) and secondary organic aerosol (SOA). Online and offline measurements of NMHCs were conducted during September 2015 in Baoding, Hebei province of China, in order to investigate their sources and roles in ozone formation. Average levels of total NMHCs online measured at the urban site were 44.5 ± 26.7 ppb. Aromatics was the largest contributor to NMHCs levels and OH reactivity, with fraction of 27.1% and 35.9%, respectively. Based on offline measurements at eight sites, we found that toluene, ethylbenzene, and m,p-xylene displayed the highest level at the site close to automobile manufacturing factories, followed by downwind receptor sites and other sites. Positive matrix factorization (PMF) model was then used to analyze NMHCs sources. Four factors were identified, including traffic-related emission, automobile manufacturing coating, biogenic emission, and NG/LPG usage and background. Average relative contribution of automobile manufacturing coating to NMHCs levels during the entire online measurement period was 33.4%, and this value increased to 42% during two O3 pollution days. Sensitivity of O3 formation to NMHCs and NOX during an O3 pollution episode were analyzed using a box model based on observations. Relative incremental reactivity (RIR) results suggested that O3 formation was in NOx-titration regime (i.e., highly NMHCs-limited regime). Further scenario analyses on relationship of O3 formation with reduction of NOx and anthropogenic NMHCs (AHC) indicated that AHC and NOx should be reduced by a ratio greater than two and three to achieve 5% and 10% O3 control objectives, respectively. The largest RIR value for anthropogenic NMHC species was from xylenes, which were also an important contributor to SOA formation and dominantly from sources related to automobile manufacturing coating and traffic emission. This means reducing NMHCs emission from automobile manufacturing coating and traffic emission should be given priority for synergetic control of O3 and PM2.5.
Highlights
The Beijing-Tianjin-Hebei (BTH) region is one of the fast-developing city clusters in China
High non-methane hydrocarbons (NMHCs) levels frequently appeared at night and morning with low wind speeds, while low NMHCs levels tended to occur at noon and early afternoon
Ambient NMHCs were online measured by a cryogen-free automatic GC-MS/flame ionization detector (FID) system at urban site in Baoding during September 2015
Summary
The Beijing-Tianjin-Hebei (BTH) region is one of the fast-developing city clusters in China. A lot of control measures have been taken by the government to improve air quality, and ambient levels of primary pollutants have shown a decreasing trend. Environmental Situation Communique (http://www.mee.gov.cn/hjzl/zghjzkgb/lnzghjzkgb/) showed that average concentrations of ambient sulfur dioxide (SO2 ), PM10 , PM2.5 , nitrogen dioxide (NO2 ), and carbon monoxide (CO) in 13 important cities of BTH region decreased by 71%, 40%, 43%, 16%, and. Ground-level O3 in urban area is mainly from photochemical oxidation of volatile organic compounds (VOCs) and nitrogen oxides (NOX ) [1]. Understanding roles of VOCs and NOX in O3 formation and sources of VOCs is necessary for establishment of effective O3 control measures [2,3]. A few of studies on sensitivity of O3 to VOCs and NOX have been conducted in Beijing
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