Investigating atmospheric nitrate sources and formation pathways between heating and non-heating seasons in urban North China

Abstract

Abstract In urban North China, nitrate ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mtext>NO</mml:mtext> </mml:mrow> <mml:mn>3</mml:mn> <mml:mo>−</mml:mo> </mml:msubsup> </mml:math> ) is a primary contributor to haze formation. So far, the production processes and source apportionments of atmospheric <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mtext>NO</mml:mtext> </mml:mrow> <mml:mn>3</mml:mn> <mml:mo>−</mml:mo> </mml:msubsup> </mml:math> during the heating season (i.e. the wintertime) have not yet been well understood. This study determined δ 15 N– <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mtext>NO</mml:mtext> </mml:mrow> <mml:mn>3</mml:mn> <mml:mo>−</mml:mo> </mml:msubsup> </mml:math> , δ 18 O– <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mtext>NO</mml:mtext> </mml:mrow> <mml:mn>3</mml:mn> <mml:mo>−</mml:mo> </mml:msubsup> </mml:math> , and Δ 17 O– <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mtext>NO</mml:mtext> </mml:mrow> <mml:mn>3</mml:mn> <mml:mo>−</mml:mo> </mml:msubsup> </mml:math> of aerosol samples to compare the potential sources and formation pathways of atmospheric <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mtext>NO</mml:mtext> </mml:mrow> <mml:mn>3</mml:mn> <mml:mo>−</mml:mo> </mml:msubsup> </mml:math> during heating (November to March) and non-heating (April to May) seasons. Combining stable isotope composition with the MixSIAR model based on Δ 17 O– <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mtext>NO</mml:mtext> </mml:mrow> <mml:mn>3</mml:mn> <mml:mo>−</mml:mo> </mml:msubsup> </mml:math> showed that NO 3 + DMS/HC (dimethyl sulfate/hydrocarbon) pathway was the dominant process of atmospheric nitrate formation during the heating season (mean = 52.88 ± 16.11%). During the non-heating season, the contributions of NO 3 + DMS/HC (mean = 37.89 ± 13.57%) and N 2 O 5 + H 2 O (mean = 35.24 ± 3.75%) pathways were comparable. We found that Δ 17 O– <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mtext>NO</mml:mtext> </mml:mrow> <mml:mn>3</mml:mn> <mml:mo>−</mml:mo> </mml:msubsup> </mml:math> was negatively correlated with wind speed and positively correlated with relative humidity during the heating season, possibly associated with the sources and production of atmospheric <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mtext>NO</mml:mtext> </mml:mrow> <mml:mn>3</mml:mn> <mml:mo>−</mml:mo> </mml:msubsup> </mml:math> . In specific, in a dust storm event, the very low Δ 17 O– <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mtext>NO</mml:mtext> </mml:mrow> <mml:mn>3</mml:mn> <mml:mo>−</mml:mo> </mml:msubsup> </mml:math> is likely associated with particles from land surface. Under the premise of considering 15 N fractionation, the constraint-based on δ 15 N– <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mtext>NO</mml:mtext> </mml:mrow> <mml:mn>3</mml:mn> <mml:mo>−</mml:mo> </mml:msubsup> </mml:math> illustrated that coal combustion was the major source of NO x emission during the heating season, and the relative contribution of coal combustion decreased rapidly from the heating season (mean = 42.56 ± 15.50%) to the non-heating season (mean = 21.86 ± 4.91%). Conversely, the proportion of NO x emitted by soil microbes rose significantly from the heating (mean = 9.67 ± 5.99%) to non-heating season (mean = 24.02 ± 11.65%). This study revealed differences in the sources and formation processes of atmospheric <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mtext>NO</mml:mtext> </mml:mrow> <mml:mn>3</mml:mn> <mml:mo>−</mml:mo

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