On the role of monoterpene chemistry in the remote continental boundary layer

Abstract

The formation of organic nitrates (RONO[subscript 2]) represents an important NO[subscript x] (NO[subscript x] = NO + NO[subscript 2]) sink in the remote and rural continental atmosphere, thus impacting ozone production and secondary organic aerosol (SOA) formation. In these remote and rural environments, the organic nitrates are primarily derived from biogenic volatile organic compounds (BVOCs) such as isoprene and monoterpenes. Although there are numerous studies investigating the formation of SOA from monoterpenes, there are few studies investigating monoterpene gas-phase chemistry. Using a regional chemical transport model with an extended representation of organic nitrate chemistry, we investigate the processes controlling the production and fate of monoterpene nitrates (MTNs) over the boreal forest of Canada. MTNs account for 5–12% of total oxidized nitrogen over the boreal forest, and production via NO[subscript 3] chemistry is more important than production via OH when the NO[subscript x] mixing ratio is greater than 75 pptv. The regional responses are investigated for two oxidation pathways of MTNs: one that returns NO[subscript x] to the atmosphere and one that converts MTNs into a nitrate that behaves like HNO[subscript 3]. The likely situation is in between, and these two assumptions bracket the uncertainty about this chemistry. In the case where the MTNs return NO[subscript x] after oxidation, their formation represents a net chemical NO[subscript x] loss that exceeds the net loss to peroxy nitrate formation. When oxidation of MTNs produces a molecule that behaves like HNO[subscript 3], HNO[subscript 3] and MTNs are nearly equal chemical sinks for NO[subscript x]. This uncertainty in the oxidative fate of MTNs results in changes in NO[subscript x] of 8–14%, in O[subscript 3] of up to 3%, and in OH of 3–6% between the two model simulations.