Evidence for renoxification in the tropical marine boundary layer

TY - JOUR

T1 - Evidence for renoxification in the tropical marine boundary layer

AU - Reed, Chris

AU - Evans, Mathew J.

AU - Crilley, Leigh R.

AU - Bloss, William J.

AU - Sherwen, Tomás

AU - Read, Katie A.

AU - Lee, James D.

AU - Carpenter, Lucy J.

N1 - © Author(s) 2017

PY - 2017/3/27

Y1 - 2017/3/27

N2 - We present 2 years of NOx observations from the Cape Verde Atmospheric Observatory located in the tropical Atlantic boundary layer. We find that NOx mixing ratios peak around solar noon (at 20-30pptV depending on season), which is counter to box model simulations that show a midday minimum due to OH conversion of NO2 to HNO3. Production of NOx via decomposition of organic nitrogen species and the photolysis of HNO3 appear insufficient to provide the observed noontime maximum. A rapid photolysis of nitrate aerosol to produce HONO and NO2, however, is able to simulate the observed diurnal cycle. This would make it the dominant source of NOx at this remote marine boundary layer site, overturning the previous paradigm according to which the transport of organic nitrogen species, such as PAN, is the dominant source. We show that observed mixing ratios (November-December 2015) of HONO at Cape Verde (∼ 3.5pptV peak at solar noon) are consistent with this route for NOx production. Reactions between the nitrate radical and halogen hydroxides which have been postulated in the literature appear to improve the box model simulation of NOx. This rapid conversion of aerosol phase nitrate to NOx changes our perspective of the NOx cycling chemistry in the tropical marine boundary layer, suggesting a more chemically complex environment than previously thought.

AB - We present 2 years of NOx observations from the Cape Verde Atmospheric Observatory located in the tropical Atlantic boundary layer. We find that NOx mixing ratios peak around solar noon (at 20-30pptV depending on season), which is counter to box model simulations that show a midday minimum due to OH conversion of NO2 to HNO3. Production of NOx via decomposition of organic nitrogen species and the photolysis of HNO3 appear insufficient to provide the observed noontime maximum. A rapid photolysis of nitrate aerosol to produce HONO and NO2, however, is able to simulate the observed diurnal cycle. This would make it the dominant source of NOx at this remote marine boundary layer site, overturning the previous paradigm according to which the transport of organic nitrogen species, such as PAN, is the dominant source. We show that observed mixing ratios (November-December 2015) of HONO at Cape Verde (∼ 3.5pptV peak at solar noon) are consistent with this route for NOx production. Reactions between the nitrate radical and halogen hydroxides which have been postulated in the literature appear to improve the box model simulation of NOx. This rapid conversion of aerosol phase nitrate to NOx changes our perspective of the NOx cycling chemistry in the tropical marine boundary layer, suggesting a more chemically complex environment than previously thought.

KW - Aerosol Nitrate

KW - halogen nitrate

KW - MBL

KW - renoxification

UR - http://www.scopus.com/inward/record.url?scp=85016288800&partnerID=8YFLogxK

U2 - 10.5194/acp-17-4081-2017

DO - 10.5194/acp-17-4081-2017

M3 - Article

SN - 1680-7316

VL - 17

SP - 4081

EP - 4092

JO - Atmospheric Chemistry and Physics

JF - Atmospheric Chemistry and Physics

IS - 6

ER -