Ground-based in-situ observations of oxides of nitrogen: experience from the Swiss National Air Pollution Monitoring Network

Transcription

1 Ground-based in-situ observations of oxides of nitrogen: experience from the Swiss National Air Pollution Monitoring Network Basel Härkingen Lägeren Zürich Tänikon Dübendorf Chaumont Bern Payerne Lausanne Sion Jungfraujoch Rigi Magadino Davos urban, kerbside urban, in park at motorway suburban rural, below 1000 m asl rural, above 1000 m asl forest high-altitude 50km Lugano N M. Steinbacher Empa, Laboratory for Air Pollution / Environmental Technology Experts Workshop on Oxides of Nitrogen, Hohenpeissenberg, October 2009

2 NOx measurements within NABEL instruments with photolytic converters

3 Scale, calibration, precision scale standards traced back to NIST SRM, since 2001 to NMI PRM standards calibration NO working standards (5 to 20ppm in N 2 ) from Messer Griesheim / Messer Schweiz, diluted with zero air using a dilution unit (MKAL, Breitfuss Messtechnik GmbH, Germany) automatic zero and span checks every 25 hrs.; manual zero and span checks every 2 weeks calibration of conversion efficiency for NO 2 Mo instruments: yearly check of converter efficiency using gas-phase titration measurement precisions Mo instruments: 4.3 to 6.7% (NO) and from 6.1 to 9.4% (NO 2 ) Ecophysics: Overall uncertainties for 1h-averages are estimated to be ±5% for NO and ±10% for NO 2 at ambient levels of 500 ppt monitors with BLC: NO precision as for Mo instruments, slightly worse for NO 2

4 Photolytic converters: conversion efficiency 70 Ecophysics, PLC % TEI 42C with BLC PLC conversion efficiency [%] time BLC conversion NO 2 - Wirkungsgrad efficiency 55% 50% 45% 40% 1. Dez Jun Dez Jun Dez Jun Dez 07

5 Case study, fair weather period at Taenikon NO 2 [ppb] (NO 2 ) m [ppb] NO 2 [ppb] (NO2) m / NO Jul 9 Jul 10 Jul 11 Jul 12 Jul 13 Jul 14 Jul 15 Jul 16 Jul 17 Jul PAN [ppb] O 3 [ppb] temp. [ o C] global rad. [W m -2 ] Jul 9 Jul 10 Jul 11 Jul 12 Jul 13 Jul 14 Jul 15 Jul 16 Jul 17 Jul Steinbacher et al., 2007, JGR

6 Results NO 2 [ppb] Taenikon a) NO 2 (NO 2 ) m NO 2 PAN NO 2, PAN [ppb] (NO 2 ) m /NO 2 [-] a) Taenikon summer fall winter spring J F M A M J J A S O N D Month time of day NO 2 [ppb] Rigi b) NO 2 (NO 2 ) m NO NO 2 [ppb] (NO 2 ) m /NO 2 [-] b) Rigi summer fall winter spring J F M A M J J A S O N D Month time of day Steinbacher et al., 2007, JGR

7 Parameterization of the overestimation Attempt to keep the multiple linear regression model as simple as possible that a correction should be also applicable for other routinely equipped monitoring sites Using a multiple linear regression approach, i.e. NO 2 = a (NO 2 ) m + b O 3 + c month + d (day / light) + e + ε with the variables (NO 2 ) m, O 3, month, (day/night) and the coefficients of the variables a to d, the intercept e and a residual ε (day / night) 1 0 for for globalradiation > 5 W m globalradiation < 5 W m -2-2

8 Application to other stations Taenikon -> Payerne monthly means NO2 [ppb] (NO 2 ) m (NO 2 ) BLC (NO 2 ) calc 2006/ / / /01 time (yyyy/mm)

9 Application to other stations Taenikon -> Payerne monthly means NO2 [ppb] (NO 2 ) m (NO 2 ) BLC (NO 2 ) calc 2006/ / / /01 time (yyyy/mm)

10 Application to other stations Taenikon -> Payerne NO2 [ppb] (NO 2 ) m (NO 2 ) BLC (NO 2 ) calc case study 07/15 07/16 07/17 07/18 07/19 07/20 time in 2006 (mm/dd)

11 Application to other stations Taenikon -> Payerne NO2 [ppb] (NO 2 ) m (NO 2 ) BLC (NO 2 ) calc case study 07/15 07/16 07/17 07/18 07/19 07/20 time in 2006 (mm/dd)

12 NO2 comparison, PLC BLC daily averages at Rigi, year NO 2 (BLC) [ppb] y = 0.985x R 2 = NO2 BLC NO 2 Cranox [ppb] NO 2 (PLC) [ppb]

13 NO2 comparison, PLC BLC daily averages at Rigi, year NO 2 (BLC), NO 2 (Mo) [ppb] NO 2 (BLC) resp. (Mo) [ppb] y = 1.219x R 2 = y = 0.985x R 2 = NO2 BLC NO2 (Mo) NO 2 Cranox [ppb] NO 2 (PLC) [ppb]

14 NOx and NOy at Jungfraujoch O3 [ppb] NOy, NOx [ppb] NOy NOx NOy is measured with CLD89p (since 2007; CLD770 before) as NO after conversion on a heated gold catalyst (300 C) with 2% CO as a reducing agent

15 Conclusions long-term collocated ambient air measurements of NO 2 proved that NO 2 instruments equipped with surface (molybdenum) converters are significantly overestimating the real NO 2 concentrations the degree of overestimation can vary within the range of hours to days largest discrepancies appeared in winter when NO 2 concentrations are highest and in spring when NO z species are known to be enhanced largest differences on a relative basis were observed in spring and summer with mean (NO 2 ) m /NO 2 ratios about 1.5 at Taenikon and up to 3.5 at Rigi the overestimation in winter might be due to particulate nitrate collected at the inlet filter and subsequent evaporation interferences have to be taken into account for monitoring and regulatory issues and have to be considered when using these data for ground-truthing of satellite data or for validation of CTMs a multiple linear model approach allows a reasonable correction, also applicable to other (similar) sampling sites

16 Acknowledgements the team of the Laboratory for Air Pollution/Environmental Technology Laboratory at Empa Swiss Federal Office for the Environment