Vehicular volatile organic compounds (VOCs)-NO<sub><i>x</i></sub>-CO emissions in a tunnel study in northern China: emission factors, profiles, and source apportionment

Abstract

<p><strong>Abstract.</strong> Vehicular emission is a key contributor to ambient volatile organic compounds (VOCs) and NO<sub><i>x</i></sub> in Chinese megacities. However, the information of real-world emission factors (EFs) for a typical urban fleet is still limited, hindering the development of a more reliable emission inventory in China. Based on a more-than-two-week (August 8–24, 2017) tunnel test in urban Tianjin in northern China, and on the use of a statistical regression model, the Positive Matrix Factorization (PMF) receptor model, and the Calculate Emissions from Road Transport (COPERT) IV model, characteristics of vehicular VOCs-NO<sub><i>x</i></sub>-CO emissions were analyzed systematically. The fleet-average EFs (pollutant: downslope, upslope, and overall in mg<span class="thinspace"></span>km<sup>−1</sup><span class="thinspace"></span>veh<sup>−1</sup>) were estimated respectively as follows: (NO: 61.92<span class="thinspace"></span>±<span class="thinspace"></span>72.46, 158.58<span class="thinspace"></span>±<span class="thinspace"></span>73.48, 97.52<span class="thinspace"></span>±<span class="thinspace"></span>69.84), (NO<sub>2</sub>: 16.52<span class="thinspace"></span>±<span class="thinspace"></span>11.49, 23.98<span class="thinspace"></span>±<span class="thinspace"></span>20.14, 15.86<span class="thinspace"></span>±<span class="thinspace"></span>9.38), (NO<sub><i>x</i></sub>: 79.45<span class="thinspace"></span>±<span class="thinspace"></span>78.43, 181.22<span class="thinspace"></span>±<span class="thinspace"></span>88.29, 116.56<span class="thinspace"></span>±<span class="thinspace"></span>77.61), and (CO: 269.96<span class="thinspace"></span>±<span class="thinspace"></span>342.38, 577.76<span class="thinspace"></span>±<span class="thinspace"></span>382.22, 344.67<span class="thinspace"></span>±<span class="thinspace"></span>250.01). The EFs of NO-NO<sub>2</sub>-NO<sub><i>x</i></sub> and CO from heavy-duty vehicles (or diesel vehicles) were differentiated from light-duty vehicles (or gasoline vehicles). The ratios (v<span class="thinspace"></span>/<span class="thinspace"></span>v) of NO<sub>2</sub> to NO<sub><i>x</i></sub> in the primary vehicular exhaust were approximately 0.18<span class="thinspace"></span>±<span class="thinspace"></span>0.09, 0.10<span class="thinspace"></span>±<span class="thinspace"></span>0.22 and 0.10<span class="thinspace"></span>±<span class="thinspace"></span>0.05 for downslope, upslope, and the entire tunnel, respectively. The fleet-average EF of the 99-target non-methane VOCs (NMVOCs) was 40.56<span class="thinspace"></span>±<span class="thinspace"></span>12.18<span class="thinspace"></span>mg<span class="thinspace"></span>km<sup>−1</sup><span class="thinspace"></span>veh<sup>−1</sup>, lower than the previous studies in China. The BTEX (benzene, toluene, ethylbenzene, <i>p</i>-xylene, <i>m</i>-xylene and <i>o</i>-xylene) levels decreased by approximately 79<span class="thinspace"></span>% when emission standards increased from China I to China V. The source profiles of NMVOCs from the tailpipe and evaporative emissions were resolved by the PMF model. The evaporative emissions accounted for nearly one-half of the total vehicular VOC emissions, indicating that evaporative and tailpipe emissions contributed equally to VOC emissions. The relative contributions of evaporative NMVOC emissions to total vehicular NMVOC emissions are temperature-dependent with the average increasing ratio of 7.55<span class="thinspace"></span>%<span class="thinspace"></span>°C<sup>−1</sup>. The primary emission ratio (ER, m<span class="thinspace"></span>/<span class="thinspace"></span>m) of VOCs<span class="thinspace"></span>/<span class="thinspace"></span>NO<sub><i>x</i></sub> was approximately 2.04, suggesting that vehicular NO<sub><i>x</i></sub> and VOCs can be co-emitted with a proper ER. According to the vehicular ERs of VOCs<span class="thinspace"></span>/<span class="thinspace"></span>NO<sub><i>x</i></sub> in Tianjin (2000–2016) and China (2010–2030), as even more stringent emission standards are implemented in the future, the O<sub>3</sub> chemical regimes were likely to be VOCs-limited (i.e., 8<span class="thinspace"></span>:<span class="thinspace"></span>1 threshold) for cities or regions where VOCs and NO<sub><i>x</i></sub> emissions are dominated by vehicular exhaust. Our study enriched the database on the fleet-average emission factors of on-road vehicles for emission inventory, air quality modeling, and health effects studies, provided implications for following O<sub>3</sub> control in China from the view of primary emission, and highlighted the importance of further control of evaporative emissions.</p>

Ozone Formation Potential of Ambient Volatile Organic Compounds at Roadside in Bangkok, Thailand

“…The T/B ratios is high due to range of toluene concentration (0.00-12.79 ppb) higher than range of benzene (0.00-1.18 ppb). In another emission profile study, the T/B ratio was 0.61 and 1.61 for diesel and gasoline exhausts, respectively [43], and 1.56 in urban Tianjin in China [44]. m/p-X/B ratios indicated the possibility of air mass transported [45].…”