Tropospheric HONO distribution and chemistry in the southeastern US

Abstract

Abstract. Here we report the measurement results of nitrous acid (HONO) and a suite of relevant parameters on the NCAR C-130 research aircraft in the southeastern US during the NOMADSS 2013 summer field study. The daytime HONO concentration ranged from low parts per trillion by volume (pptv) in the free troposphere (FT) to mostly within 5–15 pptv in the background planetary boundary layer (PBL). There was no discernible vertical HONO gradient above the lower flight altitude of 300 m in the PBL, and the transport of ground surface HONO was not found to be a significant contributor to the tropospheric HONO budget. The total in situ HONO source mean (±1 SD) was calculated as 53 (±21) pptv h−1 during the day. The upper-limit contribution from NOx-related reactions was 10 (±5) pptv h−1, and the contribution from photolysis of particulate nitrate (pNO3) was 38 (±23) pptv h−1, based on the measured pNO3 concentrations and the median pNO3 photolysis rate constant of 2.0 × 10−4 s−1 determined in the laboratory using ambient aerosol samples. The photolysis of HONO contributed to less than 10 % of the primary OH source. However, a recycling NOx source via pNO3 photolysis was equivalent to ∼ 2.3 × 10−6 mol m−2 h−1 in the air column within the PBL, a considerable supplementary NOx source in the low-NOx background area. Up to several tens of parts per trillion by volume of HONO were observed in power plant and urban plumes during the day, mostly produced in situ from precursors including NOx and pNO3. Finally, there was no observable accumulation of HONO in the nocturnal residual layer and the nocturnal FT in the background southeastern US, with an increase in the HONO ∕ NOx ratio of ≤ 3 × 10−4 h−1 after sunset.

Sources of nitrous acid (HONO) in the upper boundary layer and lower free troposphere of the North China Plain: insights from the Mount Tai Observatory

“…First, it can be easily expected that the HONO concentrations should be rather low in the upper PBL and lower FT, given the short lifetime of HONO under sunlight. This has been confirmed by the limited measurement studies at mountaintops and aboard aircraft (Acker et al, 2006;Zhou et al, 2007;Ye et al, 2016Ye et al, , 2018. Field observations at high-elevation sites could provide direct constraints for better understanding the vertical distribution and transport processes of HONO.…”

“…Another important finding from the comparison is that the HONO concentrations at Mt. Tai are substantially higher than those measured from the high-elevation atmosphere, e.g., Whiteface Mountain, USA (Zhou et al, 2007); Summit, Greenland (Dibb et al, 2002); Hohenpeissenberg, Germany (Acker et al, 2006); Brocken, Germany (Acker et al, 2001); and aboard research aircraft over the North Atlantic Ocean (Ye et al, 2016), northern Michigan (Zhang et al, 2009), and the southeastern USA (Ye et al, 2018). This demonstrates the higher pollution levels of HONO in the upper PBL and lower FT in the NCP region, which may further lead to a stronger supply of OH radicals and higher atmospheric oxidation capacity in the highelevation atmosphere.…”

“…The almost coincident noontime concentration peak of CO (e.g., ∼ 12:00-15:00 LT) confirmed the upslope transport of boundary layer air to the mountaintop. We also examined the diurnal variations of HONO/NO 2 , the ratio commonly used to indicate the efficiency of HONO formation from the reactions involving NO 2 (Yu et al, 2009). From Fig.…”

Global inorganic nitrate production mechanisms: comparison of a global model with nitrate isotope observations

“…Anthropogenic NO x emissions have been increasing in China and decreasing in the US and Europe (Richter et al, 2005;Hoesly et al, 2018), with implications for the relative importance of inorganic and organic nitrate formation as a sink for NO x (Zare et al, 2018). To examine the impacts of recent changes in anthropogenic NO x emissions for nitrate formation pathways, we run the standard model using the year 2000 emissions and meteorology after a 1year model spin-up and compare the results to the standard model simulation run in the year 2015.…”

“…B. Alexander et al: Global inorganic nitrate isotopes emissions have declined (Zare et al, 2018). Uncertainties in the rate of oxidation of NO x to nitrate have been shown to represent a significant source of uncertainty for ozone and OH formation in models (e.g., Newsome and Evans, 2017), with implications for our understanding of the atmospheric lifetime of species such as methane, whose main sink is reaction with OH.…”