title: Revisiting nitrous acid (HONO) emission from on-road vehicles: A tunnel study with a mixed fleet.
abstract: Nitrous acid (HONO) is an important precursor of OH radicals in the atmosphere. In urban areas, emissions from vehicles are the main source of air pollutants, including reactive nitrogen. Previously reported emission ratios of HONO (HONO/NOx) from vehicles were measured in the late 1990s and need to be updated due to the significant changes in emission control technologies. We measured the emission ratio of a fleet of vehicles (38% diesel on average) from March 11 to 21, 2015, in a road tunnel in Hong Kong. The emission ratio of 1.24% (±0.35%) obtained is greater than the commonly adopted 0.8% or 0.3%. The elevated emission ratio is found to be related to the presence of vehicles equipped with diesel particle filters (DPFs). Positive correlation between HONO and black carbon (BC) shows that HONO and BC were emitted together, while the lack of correlation or even anticorrelation between HONO/NOxand BC indicates that the BC-mediated conversion of NO2to HONO in the dark was insignificant in the immediate vicinity of the emission sources.
  
  
  
  Vehicular emission is a key source for HONO in the urban atmosphere. However, the most commonly used emission ratio HONO/NOxin modeling studies was measured more than 15 years ago. Our tunnel study suggests that a mixed fleet nowadays has a higher emission ratio, possibly because of the diesel particle filter (DPF) retrofit program and the growing share of Euro IV or more advanced diesel vehicles. Our study also provides new insight into the role of black carbon in HONO formation from vehicles.
authors.affiliation: a Department of Civil and Environmental Engineering , The Hong Kong Polytechnic University , Hong Kong , People's Republic of China.
authors.forename: Yutong
authors.initials: Y
authors.lastname: Liang
authors.affiliation: a Department of Civil and Environmental Engineering , The Hong Kong Polytechnic University , Hong Kong , People's Republic of China.
authors.forename: Qiaozhi
authors.initials: Q
authors.lastname: Zha
authors.affiliation: a Department of Civil and Environmental Engineering , The Hong Kong Polytechnic University , Hong Kong , People's Republic of China.
authors.forename: Weihao
authors.initials: W
authors.lastname: Wang
authors.affiliation: a Department of Civil and Environmental Engineering , The Hong Kong Polytechnic University , Hong Kong , People's Republic of China.
authors.forename: Long
authors.initials: L
authors.lastname: Cui
authors.affiliation: b Jockey Club School of Public Health and Primary Care , The Chinese University of Hong Kong , Hong Kong , People's Republic of China.
authors.forename: Ka Hei
authors.initials: KH
authors.lastname: Lui
authors.affiliation: b Jockey Club School of Public Health and Primary Care , The Chinese University of Hong Kong , Hong Kong , People's Republic of China.
authors.forename: Kin Fai
authors.initials: KF
authors.lastname: Ho
authors.affiliation: a Department of Civil and Environmental Engineering , The Hong Kong Polytechnic University , Hong Kong , People's Republic of China.
authors.forename: Zhe
authors.initials: Z
authors.lastname: Wang
authors.affiliation: a Department of Civil and Environmental Engineering , The Hong Kong Polytechnic University , Hong Kong , People's Republic of China.
authors.forename: Shun-Cheng
authors.initials: SC
authors.lastname: Lee
authors.affiliation: a Department of Civil and Environmental Engineering , The Hong Kong Polytechnic University , Hong Kong , People's Republic of China.
authors.forename: Tao
authors.initials: T
authors.lastname: Wang
mesh_terms: D000393:Air Pollutants / Q000032:analysis* / Q000737:chemistry; D001272:Atmosphere / Q000737:chemistry; D004784:Environmental Monitoring; D006723:Hong Kong; D018986:Motor Vehicles; D009608:Nitrous Acid / Q000032:analysis* / Q000737:chemistry; D053260:Soot / Q000032:analysis / Q000737:chemistry; D001335:Vehicle Emissions / Q000032:analysis*
chemical_list: D000393:Air Pollutants; D053260:Soot; D001335:Vehicle Emissions; D009608:Nitrous Acid